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30-434: There are two types of zwieback. One type is made by pinching round pieces of dough, placing one piece on top of another, pressing them together by pushing a finger down through both pieces. It is then baked and served as warm soft rolls. This type is identified with Mennonites. The other type is a bread sliced before it is baked a second time, which produces crisp, brittle slices that closely resemble melba toast . Zwieback

60-593: A crystal relative to another part along crystallographic planes and directions. Slip occurs by the passage of dislocations on close/packed planes, which are planes containing the greatest number of atoms per area and in close-packed directions (most atoms per length). Close-packed planes are known as slip or glide planes . A slip system describes the set of symmetrically identical slip planes and associated family of slip directions for which dislocation motion can easily occur and lead to plastic deformation . The magnitude and direction of slip are represented by

90-416: A sample is stressed, a good example being high-impact polystyrene or HIPS. The least brittle structural ceramics are silicon carbide (mainly by virtue of its high strength) and transformation-toughened zirconia . A different philosophy is used in composite materials , where brittle glass fibers , for example, are embedded in a ductile matrix such as polyester resin . When strained, cracks are formed at

120-454: A slip direction of <11 2 0>. This creates a total of three slip systems, depending on orientation. Other combinations are also possible. There are two types of dislocations in crystals that can induce slip - edge dislocations and screw dislocations. Edge dislocations have the direction of the Burgers vector perpendicular to the dislocation line, while screw dislocations have the direction of

150-414: A stress concentration. Typically, slip bands induce surface steps (i.e. roughness due persistent slip bands during fatigue ) and a stress concentration which can be a crack nucleation site. Slip bands extend until impinged by a boundary, and the generated stress from dislocation pile-up against that boundary will either stop or transmit the operating slip. Formation of slip bands under cyclic conditions

180-401: A total of 48). Although the number of possible slip systems is much higher in bcc crystals than fcc crystals, the ductility is not necessarily higher due to increased lattice friction stresses . While the {123} and {112} planes are not exactly identical in activation energy to {110}, they are so close in energy that for all intents and purposes they can be treated as identical. In the diagram on

210-471: Is prepečenec which would imply baked over ordinary or overbaked. The Serbo-Croatian name is dvopek which, again, is literally twice ( dvo ) baked ( pek ). Russian Mennonite zwieback, called Tweebak in Plautdietsch , is a yeast bread roll formed from two pieces of dough that are pulled apart when eaten. Placing the two balls of dough one on top of the other so that the top one does not fall off during

240-480: Is addressed as persistent slip bands (PSBs) where formation under monotonic condition is addressed as dislocation planar arrays (or simply slip-bands). Slip-bands can be simply viewed as boundary sliding due to dislocation glide that lacks (the complexity of ) PSBs high plastic deformation localisation manifested by tongue- and ribbon-like extrusion. And, where PSBs normally studied with (effective) Burger’s vector aligned with extrusion plane because PSB extends across

270-453: Is commonly used to feed teething babies and as the first solid food for patients with an upset stomach. The name comes from German zwei ("two") or zwie ("twi-"), and backen , meaning "to bake". Zwieback hence literally translates to "twice-baked". The French and Italian names, respectively, biscotte and fette biscottate have the same origin, biscotto (biscuit), which also means twice ("bis-") baked (-"cotto"). The Slovene name

300-581: Is crack motion faster than the speed of sound in a brittle material. This phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research in Stuttgart ( Markus J. Buehler and Huajian Gao ) and IBM Almaden Research Center in San Jose , California ( Farid F. Abraham ). Slip (materials science) In materials science , slip is the large displacement of one part of

330-455: Is extremely brittle at temperature 4˚C, but experiences increased ductility with increased temperature. Amorphous polymers are polymers that can behave differently at different temperatures. They may behave like a glass at low temperatures (the glassy region), a rubbery solid at intermediate temperatures (the leathery or glass transition region), and a viscous liquid at higher temperatures (the rubbery flow and viscous flow region). This behavior

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360-456: Is known as viscoelastic behavior . In the glassy region, the amorphous polymer will be rigid and brittle. With increasing temperature, the polymer will become less brittle. Some metals show brittle characteristics due to their slip systems. The more slip systems a metal has, the less brittle it is, because plastic deformation can occur along many of these slip systems. Conversely, with fewer slip systems, less plastic deformation can occur, and

390-451: Is often accompanied by a sharp snapping sound. When used in materials science , it is generally applied to materials that fail when there is little or no plastic deformation before failure. One proof is to match the broken halves, which should fit exactly since no plastic deformation has occurred. Mechanical characteristics of polymers can be sensitive to temperature changes near room temperatures. For example, poly(methyl methacrylate)

420-506: Is taken to an extreme, fracture becomes the more likely outcome, and the material can become brittle. Improving material toughness is, therefore, a balancing act. Naturally brittle materials, such as glass , are not difficult to toughen effectively. Most such techniques involve one of two mechanisms : to deflect or absorb the tip of a propagating crack or to create carefully controlled residual stresses so that cracks from certain predictable sources will be forced closed. The first principle

450-424: Is used in laminated glass where two sheets of glass are separated by an interlayer of polyvinyl butyral . The polyvinyl butyral, as a viscoelastic polymer, absorbs the growing crack. The second method is used in toughened glass and pre-stressed concrete . A demonstration of glass toughening is provided by Prince Rupert's Drop . Brittle polymers can be toughened by using metal particles to initiate crazes when

480-457: The Burgers vector , b . An external force makes parts of the crystal lattice glide along each other, changing the material's geometry. A critical resolved shear stress is required to initiate a slip. Slip in face centered cubic (fcc) crystals occurs along the close packed plane . Specifically, the slip plane is of type {111} , and the direction is of type < 1 10>. In the diagram on

510-412: The Burgers vector parallel to the dislocation line. The type of dislocations generated largely depends on the direction of the applied stress, temperature, and other factors. Screw dislocations can easily cross slip from one plane to another if the other slip plane contains the direction of the Burgers vector. Formation of slip bands indicates a concentrated unidirectional slip on certain planes causing

540-615: The Netherlands, who settled in around Danzig in West Prussia continued this practice and brought it to Russia, when they migrated to new colonies in what is today Ukraine. Traditionally, zwieback are made using lard instead of butter or a mixture of the two. As such, zwieback is very rich and does not need butter when being eaten, although jelly or jam both go quite well. Many zwieback recipes do not use modern units of measurement, instead relying on anecdotal measurements handed down through

570-400: The baking process is part of the art and challenge that must be mastered by the baker. Traditionally, this type of zwieback is baked Saturday and eaten Sunday morning and for afternoon Faspa (Standard German: "Vesper"), a light meal. This zwieback originated in the port cities of the Netherlands or Danzig , where toasted , dried buns were used to provision ships. Mennonite immigrants from

600-412: The basal planes, for arbitrary plastic deformation additional slip or twin systems needs to be activated. This typically requires a much higher resolved shear stress and can result in the brittle behavior of some hcp polycrystals. However, other hcp materials such as pure titanium show large amounts of ductility. Cadmium , zinc , magnesium , titanium , and beryllium have a slip plane at {0001} and

630-400: The generations. This German cuisine -related article is a stub . You can help Misplaced Pages by expanding it . Brittle A material is brittle if, when subjected to stress , it fractures with little elastic deformation and without significant plastic deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength . Breaking

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660-585: The glass–matrix interface, but so many are formed that much energy is absorbed and the material is thereby toughened. The same principle is used in creating metal matrix composites . Generally, the brittle strength of a material can be increased by pressure . This happens as an example in the brittle–ductile transition zone at an approximate depth of 10 kilometres (6.2 mi) in the Earth's crust , at which rock becomes less likely to fracture, and more likely to deform ductilely (see rheid ). Supersonic fracture

690-572: The grain and exacerbate during fatigue; monotonic slip-band has a Burger’s vector for propagation and another for plane extrusions both controlled by the conditions at the tip. The main methods to identify the active slip system involve either slip trace analysis of single crystals or polycrystals , using diffraction techniques such as neutron diffraction and high angular resolution electron backscatter diffraction elastic strain analysis, or Transmission electron microscopy diffraction imaging of dislocations . In slip trace analysis, only

720-469: The ions’ electric charge and their repulsion of like-charged ions, slip is further restricted. Materials can be changed to become more brittle or less brittle. When a material has reached the limit of its strength, it usually has the option of either deformation or fracture. A naturally malleable metal can be made stronger by impeding the mechanisms of plastic deformation (reducing grain size , precipitation hardening , work hardening , etc.), but if this

750-458: The metal will be more brittle. For example, HCP (hexagonal close packed ) metals have few active slip systems, and are typically brittle. Ceramics are generally brittle due to the difficulty of dislocation motion, or slip. There are few slip systems in crystalline ceramics that a dislocation is able to move along, which makes deformation difficult and makes the ceramic more brittle. Ceramic materials generally exhibit ionic bonding . Because of

780-443: The plane of shortest Burgers vector as well; however, unlike fcc, there are no truly close-packed planes in the bcc crystal structure. Thus, a slip system in bcc requires heat to activate. Some bcc materials (e.g. α-Fe) can contain up to 48 slip systems. There are six slip planes of type {110}, each with two <111> directions (12 systems). There are 24 {123} and 12 {112} planes each with one <111> direction (36 systems, for

810-679: The residual dislocation content instead of the slipped dislocations, which is only a good approximation for systems that accumulate networks of geometrically necessary dislocations , such as Face-centred cubic polycrystals. In low-symmetry crystals such as hexagonal zirconium , there could be regions of the predominantly single slip where geometrically necessary dislocations may not necessarily accumulate. Residual dislocation content does not distinguish between glissile and sessile dislocations. Glissile dislocations contribute to slip and hardening , but sessile dislocations contribute only to latent hardening. Diffraction methods cannot generally resolve

840-454: The right the specific slip plane and direction are (110) and [ 1 11], respectively. Slip in hexagonal close packed (hcp) metals is much more limited than in bcc and fcc crystal structures. Usually, hcp crystal structures allow slip on the densely packed basal {0001} planes along the <11 2 0> directions. The activation of other slip planes depends on various parameters, e.g. the c/a ratio. Since there are only 2 independent slip systems on

870-404: The right, the specific plane and direction are (111) and [ 1 10], respectively. Given the permutations of the slip plane types and direction types, fcc crystals have 12 slip systems. In the fcc lattice, the norm of the Burgers vector, b, can be calculated using the following equation: Where a is the lattice constant of the unit cell. Slip in body-centered cubic (bcc) crystals occurs along

900-422: The slip plane is measured, and the slip direction is inferred. In zirconium, for example, this enables the identification of slip activity on a basal, prism, or 1st/2nd order pyramidal plane. In the case of a 1st-order pyramidal plane trace, the slip could be in either ⟨𝑎⟩ or ⟨𝑐 + 𝑎⟩ directions; slip trace analysis cannot discriminate between these. Diffraction -based studies measure

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