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125-530: The National Physical Laboratory (NPL) is the national measurement standards laboratory of the United Kingdom. It sets and maintains physical standards for British industry. Founded in 1900, the NPL is one of the oldest metrology institutes in the world. Research and development work at the laboratory has contributed to the advancement of many disciplines of science, including the development of early computers in

250-416: A caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen and Jack Parry in 1955 at NPL. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET). This led to the internationally agreed definition of the latest SI second being based on atomic time. NPL has undertaken computer research since

375-416: A crack growth equation by summing up the width of each increment of crack growth for each loading cycle. Safety or scatter factors are applied to the calculated life to account for any uncertainty and variability associated with fatigue. The rate of growth used in crack growth predictions is typically measured by applying thousands of constant amplitude cycles to a coupon and measuring the rate of growth from

500-679: A gateway . Concurrently, the NPL connection to the Post Office Experimental Packet Switched Service used a common host protocol in both networks. NPL research confirmed establishing a common host protocol would be more reliable and efficient. The EIN protocol helped to launch the proposed INWG standard. Bob Kahn and Vint Cerf acknowledged Davies and Scantlebury in their 1974 paper " A Protocol for Packet Network Intercommunication ". Derek Barber proposed an electronic mail protocol in 1979 in INWG 192 and implemented it on

625-482: A 95% confidence that the measured value will fall inside the uncertainty interval. Other values of k can be used to indicate a greater or lower confidence on the interval, for example k = 1 and k = 3 generally indicate 66% and 99.7% confidence respectively. The uncertainty value is determined through a combination of statistical analysis of the calibration and uncertainty contribution from other errors in measurement process, which can be evaluated from sources such as

750-652: A CIPM report and endorse new developments in the SI as advised by the CIPM. The last meeting was held on 13–16 November 2018. On the last day of this conference there was vote on the redefinition of four base units, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year. The new definitions came into force on 20 May 2019. The International Committee for Weights and Measures ( French : Comité international des poids et mesures , or CIPM)

875-444: A Government Owned Contractor Operated (GOCO) model, via a new operating company, NPL Management Ltd. Serco won the bid and all staff transferred to their employment. Under this regime, overhead costs halved, third-party revenues grew by 16% per annum, and the number of peer-reviewed research papers published doubled. NPL procured a large state-of-the-art laboratory under a Private Finance Initiative contract in 1998. The construction

1000-402: A National Communications Service for On-line Data Processing . Subsequently, the NPL team, led by Roger Scantlebury , were the first to implement packet switching in the local-area NPL network in early 1969, which operated until 1986. They carried out work to analyse and simulate the performance of a wide-area packet-switched network capable of providing data communications facilities to most of

1125-422: A community of bright people that were interested in new things, they were good fodder for a system like Scrapbook" and "When we had more than one Scrapbook system, hyperlinks could go across the network without the user knowing what was happening". It was decided that any commercial development of Scrapbook should be left to industry and it was licensed to Triad and then to BT who marketed it as Milepost and developed

1250-476: A conference in 1977 to develop international cooperation for accredited testing and calibration results to facilitate trade. In 2000, 36 members signed the ILAC mutual recognition agreement (MRA), allowing members work to be automatically accepted by other signatories, and in 2012 was expanded to include accreditation of inspection bodies. Through this standardisation, work done in laboratories accredited by signatories

1375-575: A country's accreditation body must comply with international requirements and is generally the product of international and regional cooperation. A laboratory is evaluated according to international standards such as ISO/IEC 17025 general requirements for the competence of testing and calibration laboratories. To ensure objective and technically-credible accreditation, the bodies are independent of other national measurement system institutions. The National Association of Testing Authorities in Australia and

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1500-489: A country's measurement infrastructure. The NMS sets measurement standards, ensuring the accuracy, consistency, comparability, and reliability of measurements made in the country. The measurements of member countries of the CIPM Mutual Recognition Arrangement (CIPM MRA), an agreement of national metrology institutes, are recognized by other member countries. As of March 2018, there are 102 signatories of

1625-469: A coupon or a full-scale test article to determine: These tests may form part of the certification process such as for airworthiness certification . Composite materials can offer excellent resistance to fatigue loading. In general, composites exhibit good fracture toughness and, unlike metals, increase fracture toughness with increasing strength. The critical damage size in composites is also greater than that for metals. The primary mode of damage in

1750-467: A failure condition. It plots stress amplitude against mean stress with the fatigue limit and the ultimate tensile strength of the material as the two extremes. Alternative failure criteria include Soderberg and Gerber. As coupons sampled from a homogeneous frame will display a variation in their number of cycles to failure, the S-N curve should more properly be a Stress-Cycle-Probability (S-N-P) curve to capture

1875-633: A kilogram without an artefact the value of the Planck constant must be known to twenty parts per billion. Scientific metrology, through the development of the Kibble balance and the Avogadro project , has produced a value of Planck constant with low enough uncertainty to allow for a redefinition of the kilogram. Applied, technical or industrial metrology is concerned with the application of measurement to manufacturing and other processes and their use in society, ensuring

2000-1020: A lead NMI and several decentralised institutes specialising in specific national standards. Some examples of NMI's are the National Institute of Standards and Technology (NIST) in the United States, the National Research Council (NRC) in Canada, the Physikalisch-Technische Bundesanstalt (PTB) in Germany, and the National Physical Laboratory (United Kingdom) (NPL). Calibration laboratories are generally responsible for calibrations of industrial instrumentation. Calibration laboratories are accredited and provide calibration services to industry firms, which provides

2125-480: A metal structure is cracking. For metal, cracks propagate in a relatively well-defined manner with respect to the applied stress, and the critical crack size and rate of crack propagation can be related to specimen data through analytical fracture mechanics. However, with composite structures, there is no single damage mode which dominates. Matrix cracking, delamination, debonding, voids, fiber fracture, and composite cracking can all occur separately and in combination, and

2250-503: A mutual acceptance arrangement (MAA) for measuring instruments that are subject to legal metrological control, which upon approval allows the evaluation and test reports of the instrument to be accepted in all participating countries. Issuing participants in the agreement issue MAA Type Evaluation Reports of MAA Certificates upon demonstration of compliance with ISO/IEC 17065 and a peer evaluation system to determine competency. This ensures that certification of measuring devices in one country

2375-460: A national measurement system to be recognized internationally by the CIPM Mutual Recognition Arrangement, an NMI must participate in international comparisons of its measurement capabilities. BIPM maintains a comparison database and a list of calibration and measurement capabilities (CMCs) of the countries participating in the CIPM MRA. Not all countries have a centralised metrology institute; some have

2500-430: A number of methods to determine the fatigue life of a material: Whether using stress/strain-life approach or using crack growth approach, complex or variable amplitude loading is reduced to a series of fatigue equivalent simple cyclic loadings using a technique such as the rainflow-counting algorithm . A mechanical part is often exposed to a complex, often random , sequence of loads, large and small. In order to assess

2625-524: A positive impact on GDP . In the United Kingdom, an estimated 28.4 per cent of GDP growth from 1921 to 2013 was the result of standardisation; in Canada between 1981 and 2004 an estimated nine per cent of GDP growth was standardisation-related, and in Germany the annual economic benefit of standardisation is an estimated 0.72% of GDP. Legal metrology has reduced accidental deaths and injuries with measuring devices, such as radar guns and breathalyzers , by improving their efficiency and reliability. Measuring

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2750-464: A result of plasticity at the crack tip. When the stress intensity exceeds a critical value known as the fracture toughness, unsustainable fast fracture will occur, usually by a process of microvoid coalescence . Prior to final fracture, the fracture surface may contain a mixture of areas of fatigue and fast fracture. The following effects change the rate of growth: The American Society for Testing and Materials defines fatigue life , N f , as

2875-556: A rule that had first been proposed by Arvid Palmgren in 1924. The rule, variously called Miner's rule or the Palmgren–Miner linear damage hypothesis , states that where there are k different stress magnitudes in a spectrum, S i (1 ≤ i ≤ k ), each contributing n i ( S i ) cycles, then if N i ( S i ) is the number of cycles to failure of a constant stress reversal S i (determined by uni-axial fatigue tests), failure occurs when: Usually, for design purposes, C

3000-623: A small fraction of the applied stress . And the fibers in the wake of the crack experience fatigue damage. In many cases, the damage rate is accelerated by deleterious interactions with the environment like oxidation or corrosion of fibers. Following the King Louis-Philippe I 's celebrations at the Palace of Versailles , a train returning to Paris crashed in May 1842 at Meudon after the leading locomotive broke an axle. The carriages behind piled into

3125-414: A technical infrastructure and tools that can then be used to pursue further innovation. By providing a technical platform which new ideas can be built upon, easily demonstrated, and shared, measurement standards allow new ideas to be explored and expanded upon. Fatigue (material) Fatigue has traditionally been associated with the failure of metal components which led to the term metal fatigue . In

3250-402: A traceability link back to the national metrology institute. Since the calibration laboratories are accredited, they give companies a traceability link to national metrology standards. An organisation is accredited when an authoritative body determines, by assessing the organisation's personnel and management systems, that it is competent to provide its services. For international recognition,

3375-883: A transaction processor as an additional feature. Various implementations were marketed on DEC , IBM and ITL machines. All NPL implementations of Scrapbook were closed down in 1984. In the early 1990s, the NPL developed three formal specifications of the MAA : one in Z , one in LOTOS , and one in VDM . The VDM specification became part of the 1992 revision of the International Standard 8731–2, and three implementations in C , Miranda , and Modula-2 . A 2020 study by researchers from Queen Mary University of London and NPL successfully used microwaves to measure blood-based molecules known to be influenced by dehydration. The National Physical Laboratory

3500-481: A water tank. After the equivalent of 3,000 flights, investigators at the Royal Aircraft Establishment (RAE) were able to conclude that the crash had been due to failure of the pressure cabin at the forward Automatic Direction Finder window in the roof. This 'window' was in fact one of two apertures for the aerials of an electronic navigation system in which opaque fibreglass panels took the place of

3625-570: Is a collaboration of eight partner organisations: The JCGM has two working groups: JCGM-WG1 and JCGM-WG2. JCGM-WG1 is responsible for the GUM, and JCGM-WG2 for the VIM. Each member organization appoints one representative and up to two experts to attend each meeting, and may appoint up to three experts for each working group. A national measurement system (NMS) is a network of laboratories, calibration facilities and accreditation bodies which implement and maintain

3750-401: Is a parameter that scales with tensile strength obtained by fitting experimental data, N f {\displaystyle N_{\text{f}}} is the number of cycles to failure and b {\displaystyle b} is the slope of the log-log curve again determined by curve fitting. In 1954, Coffin and Manson proposed that the fatigue life of a component was related to

3875-497: Is an organisation based in Sèvres, France which has custody of the international prototype of the kilogram , provides metrology services for the CGPM and CIPM, houses the secretariat for the organisations and hosts their meetings. Over the years, prototypes of the metre and of the kilogram have been returned to BIPM headquarters for recalibration. The BIPM director is an ex officio member of

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4000-407: Is assumed to be 1. This can be thought of as assessing what proportion of life is consumed by a linear combination of stress reversals at varying magnitudes. Although Miner's rule may be a useful approximation in many circumstances, it has several major limitations: Materials fatigue performance is commonly characterized by an S-N curve , also known as a Wöhler curve . This is often plotted with

4125-627: Is automatically recognised internationally through the MRA. Other work done by ILAC includes promotion of laboratory and inspection body accreditation, and supporting the development of accreditation systems in developing economies. The Joint Committee for Guides in Metrology (JCGM) is a committee which created and maintains two metrology guides: Guide to the expression of uncertainty in measurement (GUM) and International vocabulary of metrology – basic and general concepts and associated terms (VIM). The JCGM

4250-593: Is compatible with another country's certification process, allowing the trade of the measuring devices and the products that rely on them. WELMEC was established in 1990 to promote cooperation in the field of legal metrology in the European Union and among European Free Trade Association (EFTA) member states. In the United States legal metrology is under the authority of the Office of Weights and Measures of National Institute of Standards and Technology (NIST), enforced by

4375-542: Is compatible with the certification process in other participating countries, allowing the trade of the measuring devices and the products that rely on them. The International Laboratory Accreditation Cooperation (ILAC) is an international organisation for accreditation agencies involved in the certification of conformity-assessment bodies. It standardises accreditation practices and procedures, recognising competent calibration facilities and assisting countries developing their own accreditation bodies. ILAC originally began as

4500-420: Is defined as the "property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty". It permits the comparison of measurements, whether the result is compared to the previous result in the same laboratory, a measurement result a year ago, or to the result of a measurement performed anywhere else in

4625-524: Is defined by the International Bureau of Weights and Measures (BIPM) as "the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology". It establishes a common understanding of units, crucial to human activity. Metrology is a wide reaching field, but can be summarized through three basic activities: the definition of internationally accepted units of measurement,

4750-594: Is involved with new developments in metrology , such as researching metrology for, and standardising, nanotechnology . It is mainly based at the Teddington site, but also has a site in Huddersfield for dimensional metrology and an underwater acoustics facility at Wraysbury Reservoir near Heathrow Airport . Directors of NPL include a number of notable individuals: Managing Directors Chief Executive Officers Measurement standards laboratory Metrology

4875-509: Is made up of eighteen (originally fourteen) individuals from a member state of high scientific standing, nominated by the CGPM to advise the CGPM on administrative and technical matters. It is responsible for ten consultative committees (CCs), each of which investigates a different aspect of metrology; one CC discusses the measurement of temperature, another the measurement of mass, and so forth. The CIPM meets annually in Sèvres to discuss reports from

5000-583: Is not a microstructural change within the material, but rather a propagation of dislocations within the material. Instead of a smooth interface, the intrusions and extrusions will cause the surface of the material to resemble the edge of a deck of cards, where not all cards are perfectly aligned. Slip-induced intrusions and extrusions create extremely fine surface structures on the material. With surface structure size inversely related to stress concentration factors, PSB-induced surface slip can cause fractures to initiate. These steps can also be bypassed entirely if

5125-536: Is operated by NPL Management Ltd, a company owned by the Department for Science, Innovation and Technology , and is one of the most extensive government laboratories in the United Kingdom. In the 19th century, the Kew Observatory was run by self-funded devotees of science. In the early 1850s, the observatory began charging fees for testing meteorological instruments and other scientific equipment. As universities in

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5250-400: Is the fatigue ductility coefficient, c {\displaystyle c} is the fatigue ductility exponent, and N f {\displaystyle N_{f}} is the number of cycles to failure ( 2 N f {\displaystyle 2N_{f}} being the number of reversals to failure). An estimate of the fatigue life of a component can be made using

5375-563: Is the international standards, which beholds the world's standards. The next level is the national Metrology institutes that have primary standards that are traceable to the international standards. The national Metrology institutes standards are used to establish a traceable link to local laboratory standards, these laboratory standards are then used to establish a traceable link to industry and testing laboratories. Through these subsequent calibrations between national metrology institutes, calibration laboratories, and industry and testing laboratories

5500-581: Is the scientific study of measurement . It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution 's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted

5625-427: Is to fall within the uncertainty interval. Uncertainty is generally expressed as follows: Where y is the measurement value and U is the uncertainty value and k is the coverage factor indicates the confidence interval. The upper and lower limit of the uncertainty interval can be determined by adding and subtracting the uncertainty value from the measurement value. The coverage factor of k = 2 generally indicates

5750-532: Is used. Each coupon or component test generates a point on the plot though in some cases there is a runout where the time to failure exceeds that available for the test (see censoring ). Analysis of fatigue data requires techniques from statistics , especially survival analysis and linear regression . The progression of the S-N curve can be influenced by many factors such as stress ratio (mean stress), loading frequency, temperature , corrosion , residual stresses, and

5875-592: The International Network Working Group (INWG). Connecting heterogeneous computer networks creates a "basic dilemma" since a common host protocol would require restructuring the existing networks. NPL connected with the European Informatics Network (Barber directed the project and Scantlebury led the UK technical contribution) by translating between two different host protocols; that is, using

6000-545: The Planck constant ( h ), the elementary electric charge ( e ), the Boltzmann constant ( k ), and the Avogadro constant ( N A ), respectively. The second , metre , and candela have previously been defined by physical constants (the caesium standard (Δ ν Cs ), the speed of light ( c ), and the luminous efficacy of 540 × 10  Hz visible light radiation ( K cd )), subject to correction to their present definitions. The new definitions aim to improve

6125-586: The Second World War to decipher German encrypted messages, worked at the National Physical Laboratory from 1945 to 1947. He designed there the ACE (Automatic Computing Engine), which was one of the first designs for a stored-program computer. Clifford Hodge also worked there and was engaged in research on semiconductors. Others who have spent time at NPL include Robert Watson-Watt , generally considered

6250-658: The United Kingdom Accreditation Service are examples of accreditation bodies. Metrology has wide-ranging impacts on a number of sectors, including economics, energy, the environment, health, manufacturing, industry, and consumer confidence. The effects of metrology on trade and the economy are two of its most-apparent societal impacts. To facilitate fair and accurate trade between countries, there must be an agreed-upon system of measurement. Accurate measurement and regulation of water, fuel, food, and electricity are critical for consumer protection and promote

6375-459: The quantum Hall effect for the ohm ), and the use of a material object as the measurement standard. A standard (or etalon) is an object, system, or experiment with a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures by realising, preserving, or reproducing a unit against which measuring devices can be compared. There are three levels of standards in

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6500-500: The steelyard balance and other tools. Other civilizations produced generally accepted measurement standards, with Roman and Greek architecture based on distinct systems of measurement. The collapse of the empires and the Dark Ages that followed lost much measurement knowledge and standardisation. Although local systems of measurement were common, comparability was difficult since many local systems were incompatible. England established

6625-469: The threshold or after the application of an overload , and the increased rate of crack growth associated with short cracks or after the application of an underload . If the loads are above a certain threshold, microscopic cracks will begin to initiate at stress concentrations such as holes, persistent slip bands (PSBs), composite interfaces or grain boundaries in metals. The stress values that cause fatigue damage are typically much less than

6750-399: The yield strength of the material. Historically, fatigue has been separated into regions of high cycle fatigue that require more than 10 cycles to failure where stress is low and primarily elastic and low cycle fatigue where there is significant plasticity. Experiments have shown that low cycle fatigue is also crack growth. Fatigue failures, both for high and low cycles, all follow

6875-517: The "Dam Busters" wartime raids; H. J. Gough , one of the pioneers of research into metal fatigue , who worked at NPL for 19 years from 1914 to 1938; and Sydney Goldstein and Sir James Lighthill who worked in NPL's aerodynamics division during World War II researching boundary layer theory and supersonic aerodynamics respectively. Alan Turing , known for his work at the Government Code and Cypher School (GC&CS) at Bletchley Park during

7000-448: The Assize of Measures to create standards for length measurements in 1196, and the 1215 Magna Carta included a section for the measurement of wine and beer. Modern metrology has its roots in the French Revolution . With a political motivation to harmonise units throughout France, a length standard based on a natural source was proposed. In March 1791, the metre was defined. This led to

7125-463: The BIPM to complete the task of defining all SI base units in terms of physical constants . By defining SI base units with respect to physical constants, and not artefacts or specific substances, they are realisable with a higher level of precision and reproducibility. As of the revision of the SI on 20 May 2019 the kilogram , ampere , kelvin , and mole are defined by setting exact numerical values for

7250-503: The CCs, to submit an annual report to the governments of member states concerning the administration and finances of the BIPM and to advise the CGPM on technical matters as needed. Each member of the CIPM is from a different member state, with France (in recognition of its role in establishing the convention) always having one seat. The International Bureau of Weights and Measures ( French : Bureau international des poids et mesures , or BIPM)

7375-402: The CIPM MRA, consisting of 58 member states, 40 associate states, and 4 international organizations. A national metrology institute's (NMI) role in a country's measurement system is to conduct scientific metrology, realise base units, and maintain primary national standards. An NMI provides traceability to international standards for a country, anchoring its national calibration hierarchy. For

7500-485: The CIPM and a member of all consultative committees. The International Organization of Legal Metrology ( French : Organisation Internationale de Métrologie Légale , or OIML), is an intergovernmental organization created in 1955 to promote the global harmonisation of the legal metrology procedures facilitating international trade. This harmonisation of technical requirements, test procedures and test-report formats ensure confidence in measurements for trade and reduces

7625-585: The EIN. This was referenced by Jon Postel in his early work on Internet email, published in the Internet Experiment Note series. Scrapbook was an information storage and retrieval system that went live in mid-1971. It included what would now be called word processing , e-mail and hypertext , anticipating many elements of the World Wide Web . The project was managed by David Yates who said of it "We had

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7750-662: The International Committee for Weights and Measures (CIPM), was an advisory committee of metrologists of high standing. The third, the International Bureau of Weights and Measures (BIPM), provided secretarial and laboratory facilities for the CGPM and CIPM. The General Conference on Weights and Measures ( French : Conférence générale des poids et mesures , or CGPM) is the convention's principal decision-making body, consisting of delegates from member states and non-voting observers from associate states. The conference usually meets every four to six years to receive and discuss

7875-415: The SI without changing the size of any units, thus ensuring continuity with existing measurements. The realisation of a unit of measure is its conversion into reality. Three possible methods of realisation are defined by the international vocabulary of metrology (VIM): a physical realisation of the unit from its definition, a highly-reproducible measurement as a reproduction of the definition (such as

8000-589: The U.K. Their research and practice influenced the ARPANET in the United States, the forerunner of the Internet , and other researchers in the UK and Europe, including Louis Pouzin . NPL sponsors a gallery, opened in 2009, about the development of packet switching and "Technology of the Internet" at The National Museum of Computing . NPL internetworking research was led by Davies, Barber and Scantlebury, who were members of

8125-428: The United Kingdom created and expanded physics departments, the governing committee of the observatory became increasingly dominated by paid university physicists in the last two decades of the nineteenth century. By this time, instrument-testing was the observatory's main role. Physicists sought the establishment of a state-funded scientific institution for testing electrical standards. The National Physical Laboratory

8250-581: The advantage that they can predict the intermediate size of cracks. This information can be used to schedule inspections on a structure to ensure safety whereas strain/life methods only give a life until failure. Dependable design against fatigue-failure requires thorough education and supervised experience in structural engineering , mechanical engineering , or materials science . There are at least five principal approaches to life assurance for mechanical parts that display increasing degrees of sophistication: Fatigue testing can be used for components such as

8375-436: The applied load. This causes the amplitude of the applied stress to increase given the new restraints on strain. These newly formed cell structures will eventually break down with the formation of persistent slip bands (PSBs). Slip in the material is localized at these PSBs, and the exaggerated slip can now serve as a stress concentrator for a crack to form. Nucleation and growth of a crack to a detectable size accounts for most of

8500-407: The base units is to make the entire system derivable from physical constants , which required the removal of the prototype kilogram as it is the last artefact the unit definitions depend on. Scientific metrology plays an important role in this redefinition of the units as precise measurements of the physical constants is required to have accurate definitions of the base units. To redefine the value of

8625-499: The change in compliance of the coupon or by measuring the growth of the crack on the surface of the coupon. Standard methods for measuring the rate of growth have been developed by ASTM International. Crack growth equations such as the Paris–Erdoğan equation are used to predict the life of a component. They can be used to predict the growth of a crack from 10 um to failure. For normal manufacturing finishes this may cover most of

8750-468: The costs of discrepancies and measurement duplication. The OIML publishes a number of international reports in four categories: Although the OIML has no legal authority to impose its recommendations and guidelines on its member countries, it provides a standardised legal framework for those countries to assist the development of appropriate, harmonised legislation for certification and calibration. OIML provides

8875-413: The cracking process. It is for this reason that cyclic fatigue failures seem to occur so suddenly where the bulk of the changes in the material are not visible without destructive testing. Even in normally ductile materials, fatigue failures will resemble sudden brittle failures. PSB-induced slip planes result in intrusions and extrusions along the surface of a material, often occurring in pairs. This slip

9000-429: The cracks form at a pre-existing stress concentrator such as from an inclusion in the material or from a geometric stress concentrator caused by a sharp internal corner or fillet. Most of the fatigue life is generally consumed in the crack growth phase. The rate of growth is primarily driven by the range of cyclic loading although additional factors such as mean stress, environment, overloads and underloads can also affect

9125-457: The cracks reach a critical size they propagate quickly during stage II crack growth in a direction perpendicular to the applied force. These cracks can eventually lead to the ultimate failure of the material, often in a brittle catastrophic fashion. The formation of initial cracks preceding fatigue failure is a separate process consisting of four discrete steps in metallic samples. The material will develop cell structures and harden in response to

9250-538: The creation of the decimal-based metric system in 1795, establishing standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure international conformity, the International Bureau of Weights and Measures ( French : Bureau International des Poids et Mesures , or BIPM) was formed by the Metre Convention . Although the BIPM's original mission

9375-435: The cyclic stress ( S ) against the cycles to failure ( N ) on a logarithmic scale . S-N curves are derived from tests on samples of the material to be characterized (often called coupons or specimens) where a regular sinusoidal stress is applied by a testing machine which also counts the number of cycles to failure. This process is sometimes known as coupon testing . For greater accuracy but lower generality component testing

9500-460: The development of new measurement methods, the realisation of measurement standards, and the transfer of traceability from these standards to users in a society. This type of metrology is considered the top level of metrology which strives for the highest degree of accuracy. BIPM maintains a database of the metrological calibration and measurement capabilities of institutes around the world. These institutes, whose activities are peer-reviewed, provide

9625-470: The easiest-observed societal impacts. To facilitate fair trade, there must be an agreed-upon system of measurement. The ability to measure alone is insufficient; standardisation is crucial for measurements to be meaningful. The first record of a permanent standard was in 2900 BC, when the royal Egyptian cubit was carved from black granite . The cubit was decreed to be the length of the Pharaoh's forearm plus

9750-401: The fatigue life of a component where growth can start from the first cycle. The conditions at the crack tip of a component are usually related to the conditions of test coupon using a characterising parameter such as the stress intensity, J-integral or crack tip opening displacement . All these techniques aim to match the crack tip conditions on the component to that of test coupons which give

9875-403: The flow of goods and services between trading partners. A common measurement system and quality standards benefit consumer and producer; production at a common standard reduces cost and consumer risk, ensuring that the product meets consumer needs. Transaction costs are reduced through an increased economy of scale . Several studies have indicated that increased standardisation in measurement has

10000-407: The fundamental reference points for metrological traceability. In the area of measurement, BIPM has identified nine metrology areas, which are acoustics, electricity and magnetism, length, mass and related quantities, photometry and radiometry, ionizing radiation, time and frequency, thermometry, and chemistry. As of May 2019 no physical objects define the base units. The motivation in the change of

10125-411: The hierarchy of metrology: primary, secondary, and working standards. Primary standards (the highest quality) do not reference any other standards. Secondary standards are calibrated with reference to a primary standard. Working standards, used to calibrate (or check) measuring instruments or other material measures, are calibrated with respect to secondary standards. The hierarchy preserves the quality of

10250-524: The higher standards. An example of a standard would be gauge blocks for length. A gauge block is a block of metal or ceramic with two opposing faces ground precisely flat and parallel, a precise distance apart. The length of the path of light in vacuum during a time interval of 1/299,792,458 of a second is embodied in an artefact standard such as a gauge block; this gauge block is then a primary standard which can be used to calibrate secondary standards through mechanical comparators. Metrological traceability

10375-409: The hole created by punch riveting caused manufacturing defect cracks which may have caused the start of fatigue cracks around the rivet. The Comet's pressure cabin had been designed to a safety factor comfortably in excess of that required by British Civil Airworthiness Requirements (2.5 times the cabin proof test pressure as opposed to the requirement of 1.33 times and an ultimate load of 2.0 times

10500-429: The human body is challenging, with poor repeatability and reproducibility , and advances in metrology help develop new techniques to improve health care and reduce costs. Environmental policy is based on research data, and accurate measurements are important for assessing climate change and environmental regulation. Aside from regulation, metrology is essential in supporting innovation, the ability to measure provides

10625-426: The idea that the metal had somehow "crystallized". The notion was based on the crystalline appearance of the fast fracture region of the crack surface, but ignored the fact that the metal was already highly crystalline. Two de Havilland Comet passenger jets broke up in mid-air and crashed within a few months of each other in 1954. As a result, systematic tests were conducted on a fuselage immersed and pressurised in

10750-419: The individual states. The International System of Units (SI) defines seven base units: length , mass , time , electric current , thermodynamic temperature , amount of substance , and luminous intensity . By convention, each of these units are considered to be mutually independent and can be constructed directly from their defining constants. All other SI units are constructed as products of powers of

10875-479: The instrument history, manufacturer's specifications, or published information. Several international organizations maintain and standardise metrology. The Metre Convention created three main international organizations to facilitate standardisation of weights and measures. The first, the General Conference on Weights and Measures (CGPM), provided a forum for representatives of member states. The second,

11000-567: The inventor of radar, Oswald Kubaschewski, the father of computational materials thermodynamics and the numerical analyst James Wilkinson . Metallurgist Walter Rosenhain appointed the NPL's first female scientific staff members in 1915, Marie Laura Violet Gayler and Isabel Hadfield . NPL research has contributed to physical science , materials science , computing , and bioscience . Applications have been found in ship design , aircraft development , radar , computer networking , and global positioning . The first accurate atomic clock,

11125-506: The late 1940s and 1950s, construction of the first accurate atomic clock in 1955, and the invention and first implementation of packet switching in the 1960s, which is today one of the fundamental technologies of the Internet . The former heads of NPL include many individuals who were pillars of the British scientific establishment. NPL is based at Bushy Park in Teddington , west London. It

11250-418: The measurement value and uncertainty of the device that is being calibrated (the comparator) and create a traceability link to the measurement standard. The four primary reasons for calibrations are to provide traceability, to ensure that the instrument (or standard) is consistent with other measurements, to determine accuracy, and to establish reliability. Traceability works as a pyramid, at the top level there

11375-609: The measurement. Recognition of the metrological competence in industry can be achieved through mutual recognition agreements, accreditation, or peer review. Industrial metrology is important to a country's economic and industrial development, and the condition of a country's industrial-metrology program can indicate its economic status. Legal metrology "concerns activities which result from statutory requirements and concern measurement, units of measurement , measuring instruments and methods of measurement and which are performed by competent bodies". Such statutory requirements may arise from

11500-603: The metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention . This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960. Metrology is divided into three basic overlapping activities: These overlapping activities are used in varying degrees by

11625-511: The mid-1940s. From 1945, Alan Turing led the design of the Automatic Computing Engine (ACE) computer. The ACE project was overambitious and floundered, leading to Turing's departure. Donald Davies took the project over and concentrated on delivering the less ambitious Pilot ACE computer, which first worked in May 1950. Among those who worked on the project was American computer pioneer Harry Huskey . A commercial spin-off, DEUCE

11750-404: The need for protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade. The International Organization for Legal Metrology ( OIML ) was established to assist in harmonising regulations across national boundaries to ensure that legal requirements do not inhibit trade. This harmonisation ensures that certification of measuring devices in one country

11875-444: The nineteenth century, the sudden failing of metal railway axles was thought to be caused by the metal crystallising because of the brittle appearance of the fracture surface, but this has since been disproved. Most materials, such as composites, plastics and ceramics, seem to experience some sort of fatigue-related failure. To aid in predicting the fatigue life of a component, fatigue tests are carried out using coupons to measure

12000-499: The number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. For some materials, notably steel and titanium , there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit or endurance limit . However, in practice, several bodies of work done at greater numbers of cycles suggest that fatigue limits do not exist for any metals. Engineers have used

12125-410: The operation of the laboratory transferred back to the Department for Business, Innovation and Skills (now the Department for Science, Innovation and Technology ) on 1 January 2015. Researchers who have worked at NPL include: D. W. Dye who did important work in developing the technology of quartz clocks ; the inventor Sir Barnes Wallis who did early development work on the "Bouncing Bomb" used in

12250-505: The plastic strain amplitude Δ ε p / 2 {\displaystyle \Delta \varepsilon _{\text{p}}/2} and is given by Basquin's equation for the elastic strain amplitude is where E {\displaystyle E} is Young's modulus . The relation for high cycle fatigue can be expressed using the elastic strain amplitude where σ f ′ {\displaystyle \sigma _{\text{f}}^{\prime }}

12375-461: The plastic strain amplitude using Combining the elastic and plastic portions gives the total strain amplitude accounting for both low and high cycle fatigue where σ f ′ {\displaystyle \sigma _{f}'} is the fatigue strength coefficient, b {\displaystyle b} is the fatigue strength exponent, ε f ′ {\displaystyle \varepsilon _{f}'}

12500-404: The predominance of one or more is highly dependent on the laminate orientations and loading conditions. In addition, the unique joints and attachments used for composite structures often introduce modes of failure different from those typified by the laminate itself. The composite damage propagates in a less regular manner and damage modes can change. Experience with composites indicates that

12625-480: The presence of notches. A constant fatigue life (CFL) diagram is useful for the study of stress ratio effect. The Goodman line is a method used to estimate the influence of the mean stress on the fatigue strength . A Constant Fatigue Life (CFL) diagram is useful for stress ratio effect on S-N curve. Also, in the presence of a steady stress superimposed on the cyclic loading, the Goodman relation can be used to estimate

12750-453: The presence of stress concentrations, the total strain can be used instead of stress as a similitude parameter. This is known as the strain-life method. The total strain amplitude Δ ε / 2 {\displaystyle \Delta \varepsilon /2} is the sum of the elastic strain amplitude Δ ε e / 2 {\displaystyle \Delta \varepsilon _{\text{e}}/2} and

12875-533: The probability of failure after a given number of cycles of a certain stress. With body-centered cubic materials (bcc), the Wöhler curve often becomes a horizontal line with decreasing stress amplitude, i.e. there is a fatigue strength that can be assigned to these materials. With face-centered cubic metals (fcc), the Wöhler curve generally drops continuously, so that only a fatigue limit can be assigned to these materials. When strains are no longer elastic, such as in

13000-403: The rate of crack growth by applying constant amplitude cyclic loading and averaging the measured growth of a crack over thousands of cycles. However, there are also a number of special cases that need to be considered where the rate of crack growth is significantly different compared to that obtained from constant amplitude testing, such as the reduced rate of growth that occurs for small loads near

13125-530: The rate of crack growth. Additional models may be necessary to include retardation and acceleration effects associated with overloads or underloads in the loading sequence. In addition, small crack growth data may be needed to match the increased rate of growth seen with small cracks. Typically, a cycle counting technique such as rainflow-cycle counting is used to extract the cycles from a complex sequence. This technique, along with others, has been shown to work with crack growth methods. Crack growth methods have

13250-402: The rate of damage propagation in does not exhibit the two distinct regions of initiation and propagation like metals. The crack initiation range in metals is propagation, and there is a significant quantitative difference in rate while the difference appears to be less apparent with composites. Fatigue cracks of composites may form in the matrix and propagate slowly since the matrix carries such

13375-439: The rate of growth. Crack growth may stop if the loads are small enough to fall below a critical threshold. Fatigue cracks can grow from material or manufacturing defects from as small as 10 μm. When the rate of growth becomes large enough, fatigue striations can be seen on the fracture surface. Striations mark the position of the crack tip and the width of each striation represents the growth from one loading cycle. Striations are

13500-408: The realisation of the unit definition is propagated down through the pyramid. The traceability chain works upwards from the bottom of the pyramid, where measurements done by industry and testing laboratories can be directly related to the unit definition at the top through the traceability chain created by calibration. Measurement uncertainty is a value associated with a measurement which expresses

13625-401: The realisation of these units of measurement in practice, and the application of chains of traceability (linking measurements to reference standards). These concepts apply in different degrees to metrology's three main fields: scientific metrology; applied, technical or industrial metrology, and legal metrology. Scientific metrology is concerned with the establishment of units of measurement,

13750-513: The result of a broken locomotive axle. Rankine's investigation of broken axles in Britain highlighted the importance of stress concentration, and the mechanism of crack growth with repeated loading. His and other papers suggesting a crack growth mechanism through repeated stressing, however, were ignored, and fatigue failures occurred at an ever-increasing rate on the expanding railway system. Other spurious theories seemed to be more acceptable, such as

13875-538: The safe life of such a part using the fatigue damage or stress/strain-life methods the following series of steps is usually performed: Since S-N curves are typically generated for uniaxial loading, some equivalence rule is needed whenever the loading is multiaxial. For simple, proportional loading histories (lateral load in a constant ratio with the axial), Sines rule may be applied. For more complex situations, such as non-proportional loading, critical plane analysis must be applied. In 1945, Milton A. Miner popularised

14000-428: The same basic steps: crack initiation, crack growth stages I and II, and finally ultimate failure. To begin the process, cracks must nucleate within a material. This process can occur either at stress risers in metallic samples or at areas with a high void density in polymer samples. These cracks propagate slowly at first during stage I crack growth along crystallographic planes, where shear stresses are highest. Once

14125-436: The seven base units. Since the base units are the reference points for all measurements taken in SI units, if the reference value changed all prior measurements would be incorrect. Before 2019, if a piece of the international prototype of the kilogram had been snapped off, it would have still been defined as a kilogram; all previous measured values of a kilogram would be heavier. The importance of reproducible SI units has led

14250-411: The spread of possible values associated with the measurand —a quantitative expression of the doubt existing in the measurement. There are two components to the uncertainty of a measurement: the width of the uncertainty interval and the confidence level. The uncertainty interval is a range of values that the measurement value expected to fall within, while the confidence level is how likely the true value

14375-414: The suitability of measurement instruments, their calibration and quality control. Producing good measurements is important in industry as it has an impact on the value and quality of the end product, and a 10–15% impact on production costs. Although the emphasis in this area of metrology is on the measurements themselves, traceability of the measuring- device calibration is necessary to ensure confidence in

14500-562: The three basic sub-fields of metrology: In each country, a national measurement system (NMS) exists as a network of laboratories, calibration facilities and accreditation bodies which implement and maintain its metrology infrastructure. The NMS affects how measurements are made in a country and their recognition by the international community, which has a wide-ranging impact in its society (including economics, energy, environment, health, manufacturing, industry and consumer confidence). The effects of metrology on trade and economy are some of

14625-470: The width of his hand, and replica standards were given to builders. The success of a standardised length for the building of the pyramids is indicated by the lengths of their bases differing by no more than 0.05 per cent. In China weights and measures had a semi religious meaning as it was used in the various crafts by the Artificers and in ritual utensils and is mentioned in the book of rites along with

14750-401: The window 'glass'. The failure was a result of metal fatigue caused by the repeated pressurisation and de-pressurisation of the aircraft cabin. Also, the supports around the windows were riveted, not bonded, as the original specifications for the aircraft had called for. The problem was exacerbated by the punch rivet construction technique employed. Unlike drill riveting, the imperfect nature of

14875-410: The world. The chain of traceability allows any measurement to be referenced to higher levels of measurements back to the original definition of the unit. Traceability is obtained directly through calibration , establishing the relationship between an indication on a standard traceable measuring instrument and the value of the comparator (or comparative measuring instrument). The process will determine

15000-440: The wrecked engines and caught fire. At least 55 passengers were killed trapped in the locked carriages, including the explorer Jules Dumont d'Urville . This accident is known in France as the "Catastrophe ferroviaire de Meudon" . The accident was witnessed by the British locomotive engineer Joseph Locke and widely reported in Britain. It was discussed extensively by engineers, who sought an explanation. The derailment had been

15125-422: Was established in 1900 at Bushy House in Teddington . Its purpose was "for standardising and verifying instruments, for testing materials, and for the determination of physical constants". The laboratory was run by the UK government, with members of staff being part of the civil service . It grew to fill a large selection of buildings on the Teddington site. Administration of NPL was contracted out in 1995 under

15250-528: Was later postponed for a year. The candidates for lead academic partner were the Universities of Edinburgh , Southampton , Strathclyde and Surrey with an alliance of the Universities of Strathclyde and Surrey chosen as preferred partners. Funding was announced in January 2013 for a new £25m Advanced Metrology Laboratory that will be built on the footprint of an existing unused building. NPL Management Ltd and

15375-461: Was manufactured by English Electric Computers and became one of the best-selling machines of the 1950s. Beginning in the mid-1960s, Donald Davies invented and pioneered the implementation of packet switching , now the dominant basis for data communications in computer networks worldwide. Davies designed and proposed a national commercial data network in his 1965 Proposal for the Development of

15500-509: Was to create international standards for units of measurement and relate them to national standards to ensure conformity, its scope has broadened to include electrical and photometric units and ionizing radiation measurement standards. The metric system was modernised in 1960 with the creation of the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures ( French : Conference Generale des Poids et Mesures , or CGPM). Metrology

15625-453: Was undertaken by John Laing . The new laboratory building, which had been maintained by Serco, was transferred back to the DTI in 2004 after the private sector companies involved made losses of over £100m. It was decided in 2012 to change the operating model for NPL from 2014 onwards to include academic partners and to establish a postgraduate teaching institute on site. The date of the changeover

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