Showing papers on "Deformation (engineering) published in 1974"

The plastic deformation of TiAl

Abstract: The deformation substructure of TiAl (Llo type ordered lattice) tested in compression, and the factors determining it were investigated. Two types of dislocations take part in the plas-tic deformation, namely a/2 [110] and a/2 [Oil]. The latter type will disorder the Llo super-lattice and therefore would be expected to move in pairs as superdislocations. Some obser-vations are essentially in agreement with the predictions, however the large proportion and morphology of a/2 [O1l] dislocations observed was unexpected. Twins of the [112] (111) type play an important role in the deformation of the alloy, and the early stages of their formation have been recorded. Finally, equi-Schmid factor lines have been constructed in an attempt to evaluate the importance of the sense of the applied stress on the deformation capability of the alloy.

Elastic Properties of Metals and Alloys. II. Copper

Abstract: The elastic properties of copper have been compiled and reviewed. Polycrystalline elastic constants included are: Young's modulus, the shear modulus, the bulk modulus, and Poisson's ratio. Single‐crystal constants of second‐, third‐, and fourth‐order are included. Over 200 references to the experimental literature are given. A few theoretical numbers are included. When sufficient data exist, best values are recommended together with their standard errors. Effects on the elastic constants of temperature, pressure, and mechanical (plastic) deformation are included. The Cauchy (central‐force) relationships and the single‐crystal—polycrystal relationship are also discussed.

A molecular model for yield and flow in amorphous glassy polymers making use of a dislocation analogue

Abstract: A model has been proposed to account for the yield behaviour of amorphous glassy polymers. The critical step in the yield process is envisaged as being the nucleation under stress of small disc-shaped sheared regions (the strain fields of which are analogous to those of dislocation loops) that form with the aid of thermal fluctuations. The model explains quantitatively the variation of the yield stress with temperature, strain rate and hydrostatic pressure using only two parameters, the shear modulus of the material which must be determined experimentally and the Burgers vector of the sheared region which is a constant related to the dimensions of the molecular chains making up the polymer. The model is also capable of accounting qualitatively for the marked strain softening that occurs in many glassy polymers after the yield point and for the observed variation of the degree of softening with temperature, strain rate and thermal treatment. Finally it is shown that the visco-elastic deformation a...

Quasi-static extension of a tensile crack contained in a viscoelastic-plastic solid

Abstract: Final stretch criterion of failure is applied to the problem of quasi-static extension of a crack embedded in an elastic-plastic or viscoelastic-plastic matrix. The slow growth under subcritical conditions in a rate-sensitive Tresca solid is shown to be a superposition of creep rupture and McClintock's ductile growth. This type of growth occurs at subcritical magnitude of the imposed K-factor and can be accounted for only through a recognition of inelastic properties of solids. In the subcritical range there is no unique value for K sub c independent of geometrical configuration and flaw size. Not only the produced states of stress and strain are dependent on the loading path, but also the material resistance to fracture turns out to be a function of the history of loading that precedes catastrophic failure. A nonlinear integro-differential equation of motion is derived for a crack progressing through a viscoelastic medium with some limited ability to plastic flow. Examples of numerical integration are given incorporating both monotonic and cyclic loading programs.

Superdrawn crystalline polymers: A new class of high‐strength fiber

Abstract: A new kind of acetal fiber has been discovered which has a tensile strength of 1.7 GPa (250,000 psi) and an elastic modulus of 35 GPa (5 × 106 psi). This fiber is produced by a special two-stage drawing process in the solid state which requires careful control of deformation rate and temperature. Previously known drawn fibers are reported to consist of folded-chain blocks joined by a limited number of tie-molecules. It is hypothesized that the second stage of the novel drawing process eliminates the lamella (block) surfaces which act as strength-limiting stress concentrators. A new type of fiber is created in which any remaining chain-folds are distributed as defects in a continuous crystal matrix. It is the continuity of the crystal matrix which is believed responsible for the remarkable properties of the fiber.

Cyclic deformation and fracture of polymers

Abstract: The cyclic stress-strain behaviour of a wide variety of rigid polymers has been studied Three classes of fatigue response can be defined, each class displaying a characteristic evolutionary pattern in the stress-strain relation as deformation proceeds from the initial fatigue cycle to fatigue-crack propagation Ductile polymers undergo a marked decrease in deformation resistance prior to crack formation; the detailed mechanism by which this “softening” develops can be related to the material microstructure and thermomechanical history Amorphous polymers with a moderate degree of ductility soften slightly; in these materials crazing plays a dominant role in both the cyclic stress-strain response and the structural fatigue resistance Brittle and nearly-brittle polymers are essentially stable in cyclic deformation; the fatigue resistance of these materials is very sensitive to strain amplitude in cyclic deformation

Some problems of static deformation of the earth

Abstract: The static deformation of gravitating elastic earth model is studied in some detail. It is found that the problem becomes well posed if we discard the stress-strain relation in the liquid core. A convenient formulation, in which derivative of density does not appear, is proposed in the liquid core by introducing a new variable. Static solutions obtained by the present theory are compared with dynamic solutions in which the liquid core causes no trouble. The differences between them are within 0.2%. The elastic deformation due to earth's rotation and the deformation of degree 1 modes induced by surface loads are also computed.

Deformation twinning in silver-and copper-alloy crystals

Abstract: Deformation twinning in f.o.c. crystals has been investigated by vising silver-and copper-alloy crystals, which were deformed in tension at temperatures down to 4·2°K. Problems mainly concerned are (1) the nucleation process with particular relation to cross-slip and (2) the suppression of the growth of twins in high-concentration alloys. First, a postulate previously made that twinning is nucleated as a stress-relief process complementary to cross-slip has been confirmed. Second, the suppression of the twin growth in high-concentration alloys has been interpreted in terms of the interaction of twinning dislocations with dislocations of the critical system abundantly induced by the Luders deformation.

Instability and fracture in sheet metal

Abstract: The regime of sheet metal deformation extends from pure shear, 1= - 2, through plane strain, 2=0, to balanced biaxial tension, under which 1=2. During deformation a material passes into different states, termed (1) uniform straining, (2) diffuse straining, and (3) localized straining, respectively. Depending on the strain ratio, (1) alone, (1) & (2) & (3), (1) & (3) or (1) & (2) can be experienced by a sheet between zero strain and fracture. In this work the deformation behaviour, in tensile and hydraulic bulge testing, of steel, aluminium and 70-30 brass has been studied. The onset of state (3) has been correlated with strain measurements, taken from a cine film of the deformation, indicated by a grid electrochemically marked on the sheet surface. Various theories predict the major and minor strains at the initiation of these states, and the theoretical and experimental results are compared for the various materials; also the implications with respect to forming-limit diagrams are discussed.

Non-Elastic Deformation of Polycrystals with a Liquid Boundary Phase

Abstract: A conceptual view is taken to understand the deformation behavior of a polycrystal with a liquid (or a quasi-elastic) boundary phase. The analysis is based on the theories of liquid adhesives, the fracture of liquids and the concepts of fracture mechanics. It is shown that boundary separation rather than boundary sliding is the step that controls the deformation rate. Using this principal result, it is shown that the deformation behavior of a polycrystalline material with a viscous boundary phase is controlled by the flow characteristics, the volume content of the boundary phase and the microstructure features of the polycrystal (viz., voids, solid inclusions and cracks). Polycrystalline materials with a viscous boundary phase will exhibit a much greater rate of deformation in tension than in compression.

Viscoelastic Properties of Silicone, Polysulfide, and Polyether Impression Materials

Abstract: The viscoelastic properties of nine silicone-, polysulfide-, and polyether-based impression materials were determined using creep tests. During deformation the materials demonstrated linear viscoelastic behavior. The creep compliance curves, recovery, and percent set were calculated. Permanent deformation in these materials is a result of lack of recovery of elastic deformation as well as viscous flow.

Elastic-plastic analysis of deformation induced by thermal stress in eutectic composites: II. thermal Expansion

Abstract: In part I a theory was developed to analyze the deformation produced in eutectic composites by temperature changes. Experimental determinations of the coefficient of expansion are used here to test the predictions of that theory. Measurements of length versus temperature in the absence of external stresses were made for Al-Al3Ni, Al-CuAl2, and Sb-Cu2Sb eutectic composites, and for monolithic 6061 Al. Hysteresis loops in the graph of composite coefficient of expansion vs temperature are seen when the expansion coefficients of the phases are different and plastic deformation occurs. The nature of the hysteresis is shown to depend upon heating and cooling rates, prior history and the relative values of the matrix and reinforcement expansion coefficients. The theory correctly predicts these qualitative effects. In Al-Al3Ni the available component data justify a quantitative comparison of calculated and experimentally determined coefficient of expansion-temperature curves. The values compare well, further verifying the analytical approach.

Optical signature method and apparatus for structural integrity verification

Abstract: The structural integrity of a load bearing structure is periodically evaluated by recording on the same holographic recording medium two successive holograms of the structure while the latter is in two different stress conditions, respectively, to produce a holographic interferogram which may be reconstructed to create a deformation fringe pattern representing the deformations in the structure resulting from the change in the stress conditions. This deformation pattern is compared with an earlier deformation pattern of the structure resulting from the same stress conditions to determine differences, if any, between the two patterns, such differences being indicative of a reduction in the stiffness and hence structural integrity of the structure due to weakening of the latter by fatigue damage, stress corrosion cracking, and/or other causes.

The flow of solid polymers under high pressure

Abstract: The deformation and flow properties of solid polymers under uniaxial tensile and compressive loading have been determined for a variety of both crystalline and amorphous polymers at hydrostatic pressures to about 7 , kb. For some polymers, like polytetrafluoroethylene, cold drawing is inhibited by increasing pressure, and scanning electron micrographs show that local plastic deformation is greatly reduced. However, the true flow stress and the true fracture stress both increase with pressure, as does, also, the elastic modulus. The increase is greatest in the low-pressure range below 2 kb. This behavior is attributed to the pressure-induced shifting of a lowtemperatureβ′-transition. The influence of polymer composition and of the initial elastic constants on the pressure dependence of the elastic modulus and the yield stress is discussed. The observed behavior can be analyzed in terms of finite strain effects and a pressure-dependent yield criterion. It is shown that some amorphous polymers undergo a pressure-induced brittle-ductile transition, and these findings are used to explore the hydrostatic extrusion of polyimide and polysulfone at room temperature.

Fatigue crack propagation and cyclic deformation at a crack tip

Abstract: The fatigue crack propagation relation da/dN = f(R) Delta K squared can be derived with three assumptions: small-scale yielding, material homogeneity, and that crack tip stresses and strains are not strongly affected by plate thickness. The function f(R) is a constant at a given stress ratio, R. The effects of plate thickness and stress ratio on crack tip deformation and fatigue crack growth in 2024-T351 aluminum alloy were studied. High Delta K level in a thin specimen causes crack tip necking. Necking is more pronounced at high stress ratio. Necking causes high maximum strain near a crack tip and fast crack growth rate.

Strain history of the melt in film blowing

Abstract: Techniques have been developed for measuring the strain and thermal histories of fluid elements as they move from the die lips to the freeze line. Motion pictures were analyzed to determine the rates of extension in the machine and transverse directions. A radiation pyrometer was used to measure the temperature of the film. These techniques were used to study the film blowing of polyethylene; a 2.5-in. diameter die was used, and blow-up ratios in the range of 1.8 to 3.4 were employed. Film thickness ranged from 2 to 4 mils. The maximum measured extensional strain rates in both the transverse and machine direction were in the range of 0.15 to 0.6 sec−1. Standard shrinkage and impact tests were performed on the finished films, and an attempt was made to correlate the results with several simple empirical norms of the strain history. No correlation could be discerned. The results of this study are inconsistent with some popular ideas about the origin of orientation in blow films, but they are consistent with some recently published data on the influence of deformation on orientation in melt-drawn capillary extrudate.

Plastic deformation in Fe-Si alloys

Abstract: A detailed study has been made of the compressive stress-strain behavior of polycrystal-line Fe-Si alloys containing from 0 to 25 at. pct Si. It is shown that the alloys containing less than 10 at. pct Si possess only short range atomic order and deform by slip or by twinning, depending upon temperature. At concentrations greater than 10 at. pct Si, the alloys possess long range atomic order. Deformation twinning is suppressed in this region, and the deformation occurs entirely by slip. Deformation by slip may occur either by the motion of perfect superlattice dislocations, or else by the movement of imperfect superlattice dislocations, with the subsequent generation of antiphase boundaries. The type of superlattice dislocation that prevails depends upon a critical combination of both composition and temperature.

Low-temperature deformation and dislocation mobility in pure and Mg-doped LiF crystals

Abstract: Two batches of LiF crystals, one essentially pure, and the other doped with 140 p.p.m. of Mg, were cooled from high temperature at different rates. The mechanical properties of these crystals were investigated in the temperature range 77 to 360 K. The results obtained are analysed and discussed in terms of the theory of interaction between dislocations and tetragonal defects for which improved expressions are derived. Good agreement is obtained between the predictions of the theory and experimental results. It is concluded that the thermally-activated deformation and low-temperature dislocation mobility are controlled by the interaction between dislocations and Mg2+ vacancy dipoles. The largest concentration of isolated Mg2+-vacancy dipoles is retained in the doped fast-cooled crystals which have the largest yield stress at low temperatures. It is estimated that in the doped slowly-cooled crystals one sixth of the total number of Mg2+ ions aggregate into complexes (containing more than 10 ions) w...

Elastic properties of the nematic phase influenced by molecular properties

Abstract: The intrinsic splay, twist, and bend deformations of nematic molecules are discussed. The intrinsic twist deformation leads to the cholesteric phase. On this molecular basis, the helical twisting power of cholesteric mixtures is discussed. The intrinsic splay and bend deformation cannot induce spontaneous deformation of a nematic liquid crystal but do influence the splay and bend elastic constants. Analogy is made between the change of the elastic constants and the electric orientation polarization produced by an electric dipole. The frequency dependence of the elastic constants is therefore similar to those of a dipole in a nematic surrounding. The splay elastic constant k11 has a low frequency relaxation (kHz‐MHz), and the bend elastic constant k33 has a high frequency relaxation (MHz‐GHz). The change of the elastic constants can be estimated for a realistic nematic molecule to be 10−6 dyn. The elastic constants can be described by two terms. The first one has some relation to the elasticity of ideal me...

The mode of action of phalloidin: demonstration of rapid deformation of isolated hepatocytes by scanning electron microscopy.

Abstract: Suspensions of rat liver cells were treated in vitro with 25 μg phalloidin per ml cell suspension. After an incubation time of 10 min marked deformations of cell surfaces were observed in scanning electron micrographs. The effect is discussed within the context of previous morphological and biochemical results.