Showing papers on "Deformation (engineering) published in 1968"

Behavior of Granular Materials Under High Stresses

Abstract: Triaxial tests at mean normal stresses up to 1,200 kg per sq cm on a medium-grained, uniform quartz sand show that the nature of sand deformation varies with pressure. As mean normal stress increases, crushing becomes more pronounced and dilatancy effects gradually disappear. Beyond breakdown pressure, at which all effects of initial void ratio disappear, sand behaves essentially as a linearly deformable solid with modulus of deformation E proportional to mean normal stress q. In the same range, sand strength is characterized by a constant angle of internal friction, φ, equal to the angle of interparticle friction from the low pressure tests. Below breakdown pressure, E increases as a power function of q; however the exponent of this function varies with pressure and, to a certain degree, with the initial void ratio of sand. The strength envelopes in this range are all curved and depend on the initial void ratio of sand. Relative compressibility of granular materials generally increases with pressure and may be responsible for scale effects in a number of passive pressure phenomena.

Mechanisms of plastic deformation of olivine

Abstract: Olivine deformed experimentally at 5-kb confining pressure undergoes plastic flow by slip on several systems. The active slip system is determined from microscopic study of slip bands, deformation lamellas and kink bands. At temperatures less than 1000°C the slip systems are {110} [001], (100) [001], and (100) [010]. At 1000°C, slip on {0kl} [100], pencil glide in [100], the system shown here to be predominant in naturally deformed terrestrial olivine, occurs in addition to the system {110} [001] and becomes the predominant mechanism with decreasing strain rate. Dependence of glide mechanism on temperature and strain rate is also observed in diopside. Data of the sort acquired in these experiments may be used to place limits on the possible temperatures and strain rates of naturally deformed rocks.

Phase Transformation and Deformation Processes in Oriented Polyethylene

Abstract: Plastic deformation processes in oriented polyethylene are investigated by the X-ray method, and changes in texture produced by transverse compression are interpreted in terms of twinning, slip and phase transformation. The crystal structure of the new form produced by the phase transformation is determined, and the unit cell is a monoclinic one with a=8.09, b=2.53, c=4.79 A and β=107.9°. The phase transformation is considered to be of diffusion-less type and a transition mechanism similar to that of twinning is proposed.

The Mechanical Behaviour of the Intermediate Phase NiAl

Abstract: The deformation behaviour of polycrystalline NiAl of various compositions, and of stoichiometric single crystals with different orientations, has been studied in compression in the temperature range 77–1300° K. All alloys except the aluminium-rich composition (53 at.-% aluminium) were deformable down to 77°K. Plastic deformation of single crystals was strongly anisotropic. All polycrystals and single crystals exhibited the three-stage temperature-dependence of the yield stress typical of b.c.c. alloys. The main effect of non-stoichiometry was to increase the athermal contribution to the yield stress; the strengthening produced by vacancies (aluminium-rich alloys) was greater than that produced by substitutional atoms (nickel-rich alloys).

Plastic Deformation Bands in Glassy Polystyrene

Abstract: Plastic deformation bands nearly parallel to planes of maximum shear stress have been observed in unoriented glassy polystryene in compression and in preoriented polystyrene in tension and torsion. In the temperature range 77°–375°K, the shear stress for homogeneous nucleation of deformation bands is of the order of a few percent of the shear modulus, and while this shear stress is only moderately dependent on temperature and independent of strain rate, it shows considerable pressure dependence compared to metals. The shear strain of order unity inside the bands, the positive dilatation observed during deformation, and the stress and temperature conditions of band nucleation and propagation suggest a model in which ``rough'' molecules slide through each other in an initially unordered but closely packed molecular tangle.

Energy absorber by means of plastic deformation

Abstract: This invention relates to a device for absorbing energy by means of plastic deformation which, when compressed by an external force or forces, exhibits a certain deformative resistance and, without being fractured, reduces the shock due to the external impact force through continuous and permanent plastic deformation, and more particularly to a machine element buffer as an energy absorber in which a predetermined amount of energy may be absorbed under a certain load with at least possible reduction of length and with no danger of springback after the deformation.

Electron microscope observations on the dislocation arrangement in deformed copper single crystals in the stress-applied state

Abstract: The dislocation arrangement in the stress-applied condition of copper single crystals deformed in tension at 78°K was pinned by irradiating with a dose of 1018 n/cm2 fast neutrons at 4·2°K or 20°K Transmission electron microscope studies of the primary glide plane showed that significant differences exist between the dislocation arrangements in the stress-applied and in the stress-removed state of the crystals, indicating considerable dislocation rearrangement during unloading In particular, dislocation groups and pile-ups consisting of 10–20 dislocations of the same sign were observed in stage II in the stress-applied state

The Effect of Hydrogen on the Plastic Deformation of Nickel Single Crystals

Abstract: Shear stress/shear strain curves for nickel single crystals containing 9 ppm hydrogen (0.045 at.-%) have been compared with those of hydrogen-free crystals over the range −196 to 100°C. The presence of hydrogen causes serrated yielding of the nickel and an increase in the shear stress τIII between − 125 and 0° C, but has no effect on the critical resolved shear stress (C.R.S.S.) at any temperature. These observations are explained in terms of the interaction between dislocations and dissolved hydrogen which is able to diffuse during deformation. It is suggested that hydrogen can lower the stacking-fault energy of nickel from 240 to 155ergs.cm−2.

Pressure Dependence of the Plastic Flow Stress of Alkali Halide Single Crystals

Abstract: The change in the plastic yield stress of alkali halide single crystals due to applied hydrostatic pressure has been measured. The pressure dependence of the yield stress (δσ/σ) ranges from ∼9×10−2/kbar in soft RbI to ∼0 in LiF. The experimental results suggest that in soft crystals point‐defect generation and/or diffusion is effective in limiting dislocation mobility in the early stages of deformation, whereas in crystals hardened by irradiation or cold work, elastic interactions are rate limiting.

Incremental growth due to creep and plastic yielding of thin tubes subjected to internal pressure and cyclic thermal stresses.

Abstract: A theoretical investigation is made of incremental growth in a thin pressurized tube when the tube wall is subjected to large cyclic variations of temperature and temperature gradient, the mechanisms of deformation being plastic yielding during application and removal of the temperature gradient and creep during the high temperature part of the cycle. In a previous paper a simple model of the tube was developed and used to determine approximately when the mode of incremental growth known as ratcheting occurs and to estimate the ratchet strain per cycle, both when creep is ignored and when it causes complete relaxation of internal stresses. The above work is extended in the present paper to cases where only partial relaxation of internal stresses occurs and a method of evaluating the ratchet strain per cycle is presented. In addition, the method allows the increased creep strain due to non-linear interaction between internal stresses and the pressure stress to be calculated. This accompanies ratche...

Tensile behavior and failure characteristics of asphalt cements in thin films

Abstract: THE REPORTED STUDY WAS CONDUCTED TO EXTEND THE KNOWLEDGE OF THE BEHAVIOR OF ASPHALT CEMENT IN THIN FILM SUBJECTED TO TENSILE STRESSES. THE TENSILE BEHAVIOR AND FAILURE CHARACTERISTICS OF 11 ASPHALT CEMENTS IN THIN FILMS WERE STUDIED. VARIABLES IN THE STUDY INCLUDED: (1) FILM THICKNESS, (2) RATE OF DEFORMATION, (3) TEMPERATURE, (4) CONSISTENCY, AND (5) SOURCE OF THE BITUMINOUS MATERIAL. A DEFINITE RELATIONSHIP WAS FOUND TO EXIST BETWEEN THE TENSILE PROPERTIES OF A GIVEN ASPHALT CEMENT AND THE FILM THICKNESS OF THE SPECIMEN. THE TENSILE STRENGTH OF ASPHALT CEMENTS IN THIN FILMS IS SIGNIFICANTLY INFLUENCED BY THE TEST RATE OF DEFORMATION AND/OR THE TEST TEMPERATURE. THE CONSISTENCY OF THE ASPHALT CEMENT HAS A SIGNIFICANT INFLUENCE ON TENSILE STRENGTH. THE SOURCE OF THE ASPHALT CEMENT MAY INFLUENCE THE TENSILE STRENGTH OF THE MATERIALS. IT APPEARS THAT THE ASPHALTENE CONTENT OF ASPHALT CEMENTS OBTAINED FROM A GIVEN SOURCE MAY HAVE A SIGNIFICANT INFLUENCE ON THE CONSISTENCY OF THE MATERIALS AND THUS, ON THE TENSILE STRENGTH. THREE MODES OF FAILURES WERE OBSERVED IN THE ASPHALT SPECIMENS. THE RANGE OF FILM THICKNESS EXHIBITING EACH FAILURE MODE WAS DEPENDENT UPON SPECIFIC COMBINATIONS OF: (1) ASPHALT SOURCE AND CONSISTENCY, (2) RATE OF DEFORMATION, AND (3) TEMPERATURE. THE OBSERVED MODES OF FAILURE WERE REFERRED TO AS: BRITTLE FRACTURE, FLOW FAILURE AND INTERMEDIATE FAILURE OR TENSILE RUPTURE. SPECIMENS WHICH EXHIBITED THE BRITTLE MODE HAD FAILURE SURFACES WHICH WERE SMOOTH AND GLASSY IN APPEARANCE. SPECIMENS WHICH EXHIBITED FLOW FAILURE USUALLY EXHIBITED A FAILURE SURFACE FORMED BY EXCESSIVE NECKING AND INTERNAL FLOW OF THE SPECIMEN. THE INTERMEDIATE MODE FAILURE SURFACE EXHIBITED A COMBINATION OF MULTIPLE DEPRESSIONS AND MULTIPLE RIDGES. THE LIMIT OF BRITTLE FRACTURE AND THE LIMIT OF FLOW FAILURE ARE: FUNCTION OF TEMPERATURE, RATE OF DEFORMATION, AND ASPHALT CONSISTENCY. THE TYPE OF FAILURE THAT OCCURS IN THIN FILMS OF BITUMINOUS MATERIAL SUBJECTED TO TENSILE STRESSES CAN GENERALLY BE CLASSIFIED AS A COHESIVE FAILURE RATHER THAN AN ADHESIVE FAILURE. A CONSTANT RATE OF LOADING TEST APPEARS TO BE A MORE DESIRABLE TEST METHOD THAN A CONSTANT RATE OF DEFORMATION TEST FOR DETERMINING ASPHALT CEMENT BEHAVIOR CHARACTERISTICS. BITUMINOUS MATERIALS IN THIN FILMS WHEN SUBJECTED TO TENSILE STRESSES EXHIBIT A CAVITATION PHENOMENA AND THE VOLUME OF THE SPECIMEN APPEARS TO INCREASE. THE AMOUNT OF DEFORMATION TO FAILURE APPEARS TO BE PARTIALLY DEPENDENT UPON FILM THICKNESS.

Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

Abstract: The true stress-true strain behavior of polyoxymethylene, n(-CH2O), as an example of a bulk semi-crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure. In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure-induced increase in the β-transition temperature which points to the potential usefulness of the concept of pressure-temperature super-position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb-Navier criterion, are not valid for general deformation (non-plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi-crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.

Pressure Effects on the Friction Coefficient of Thin‐Film Solid Lubricants

Abstract: Thin solid films on harder backings are now widely used as dry‐film lubricants. An early extension by us of the Bowen‐Tabor adhesion theory of friction showed that the coefficient of friction of a coated backing was equal to only a fraction of that of the coating material, and this fraction was simply the ratio of the mean yield pressure of the coating material to that of the backing. However, we pointed out that the effect of pressure on the shear strength of the coating material was neglected. We have since analyzed and applied to this problem the data on the effect of pressure on shear strength of paraffin, gold, and molybdenum disulfide, using the experimental data of Bridgman as well as of Boyd and Robertson. The principal complication in applying these data to the frictional problem is to allow properly for the elastic (or plastic) deformation of the two sliding solids in calculating the pressure exerted on the coating material during sliding. Our calculated coefficients of friction are in good agreement with the recent measurements of Takagi and Liu on gold‐coated hard steel and the earlier data by Haltner and Oliver on molybdenum‐disulfide‐coated steel. Recently we measured coefficients of friction of thin coatings of paraffin on steel and obtained results which were in good agreement with our calculated values. It is concluded that a sound basis now exists for treating the subject of dry film lubricants. Further research on such systems requires more experimental data on the effect of pressure on shear strength in a variety of indicated polymers and inorganic solids.

Application of the indirect tensile test to stabilized materials

Abstract: THE IMPORTANCE OF THE TENSILE CHARACTERISTICS OF THE SUBBASE OF RIGID PAVEMENTS CAN BE DEMONSTRATED BOTH FROM THEORETICAL CONSIDERATIONS AND FROM FIELD OBSERVATIONS INFORMATION ON THE TENSILE BEHAVIOR AND PROPERTIES OF TREATED AND UNTREATED SUBBASE MATERIAL IS LIMITED PRIMARILY BECAUSE OF THE LACK OF A SATISFACTORY TENSILE TEST ON THE BASIS OF A LITERATURE REVIEW CONCERNED WITH TENSILE TESTING, IT WAS CONCLUDED THAT OF THE CURRENTLY AVAILABLE TENSILE TESTS THE DIRECT TENSILE TEST HAS THE GREATEST POTENTIAL FOR THE EVALUATION OF THE TENSILE PROPERTIES OF HIGHWAY MATERIALS THIS PAPER DISCUSSES TENSILE TESTING, THEORY OF THE INDIRECT TENSILE TEST, AND FACTORS AFFECTING THE TEST IN ADDITION, THE RESULTS OF A LIMITED TESTING PROGRAM TO EVALUATE THE EFFECT OF SUCH FACTORS AS COMPOSITION AND WIDTH OF LOADING STRIP, TESTING TEMPERATURE, AND LOADING RATE ON THE INDIRECT TENSILE TEST PARAMETERS OF STRENGTH, VERTICAL FAILURE DEFORMATION, AND A LOAD VERTICAL FAILURE DEFORMATION MODULUS FOR ASPHALT-STABILIZED AND CEMENT-TREATED MATERIALS ARE INCLUDED ON THE BASIS OF THE LITERATURE REVIEW AND EXPERIMENTAL INVESTIGATION, IT IS RECOMMENDED THAT THE INDIRECT TENSILE TEST BE USED FOR EVALUATING THE TENSILE PROPERTIES OF STABILIZED MATERIALS AND THAT THE TEST BE CONDUCTED UTILIZING A 10-IN WIDE STAINLESS STEEL LOADING STRIP, A LOADING RATE OF 2 IN/MIN, AND A TESTING TEMPERATURE OF 77F /AUTHOR/

Plastic deformation of tantalum single crystals: II. The orientation dependence of yield

Abstract: The plastic deformation phenomena associated with the yielding of single crystals of tantalum are described as a function of orientation at 273 °K and 77 °K in tension and at 293 °K and 77 °K in compression. The orientation dependence of the yield stress in tension was found to be consistent with a critical resolved shear stress law for slip on a {110}〈111〉 system for the whole orientation range at 273 °K and part of the orientation range at 77 °K. The results are interpreted in terms of the Chen and Maddin model of composite slip on non-parallel {110} planes sharing the same slip direction. The anomalies in the orientation-dependence of the yield stress in tension at 77 °K are discussed in terms of the asymmetry of slip on {112} planes, and the reduced mobility of jogs in certain orientations. The orientation dependence of the yield stress in compression was not consistent with any critical resolved shear stress law.

Thermally activated deformation in dilute zirconium--oxygen alloys.

Abstract: The effect of temperature and strain-rate on the yield and flow stress of zirconium, containing oxygen in the range of 0.09 to about 2.5 at. % has been examined at temperatures between 77 and 700° K. The above measurements were coupled with strain rate change tests and stress-relaxation experiments, so that all the deformation parameters associated with thermal activation could be evaluated. The results of these measurements showed that the thermal component of flow stress increases with increasing oxygen content of the alloy. Two stages of activation were found to occur for all these alloys. The first stage was associated with the unpinning of dislocation loops from impurity pinning points. At higher temperatures, because of the unpinning of many impurity pinning points, a second stage of activation associated with a different mechanism was found to occur. The variation of certain parameters associated with this stage of deformation strongly suggests the Peierls-Nabarro force as the controlling criterion.

High‐Temperature‐Creep Mechanism of TiNi

Abstract: Polycrystalline specimens of the intermetallic compound TiNi were tested in tension in the temperature range 700°–1000°C. The specimens were observed to deform uniformly over the gage length until the very end of deformation, when it failed by necking. It was noted that the steady‐state flow stress was independent of the strain and was a function only of the temperature and strain rate. The shear strain rate γ was given by γ∝τn where n=3.0±0.2, r was the shear stress, and n was independent of temperature. The apparent activation energy for creep was found to be 60.0±3.0 kcal/mole and was independent of stress. The result strongly suggests that a viscous creep process, as first formulated by Weertman, is probably the rate‐controlling mechanism for this compound in the stress and temperature range investigated.

Energy Contribution to Rubber Elasticity

Abstract: Stress‐temperature measurements were performed for natural rubber at both ``infinitesimal'' (<10%) and finite (<100%) strains. Data are used to calculate the internal‐energy contribution to the elastic stress fe by several thermoelastic equations. A unified derivation of these equations is given, which leads naturally to a new equation based on the temperature coefficient of shear modulus of the rubber. It is demonstrated that the dependence of the relative energy contribution fe/f on deformation as previously reported arises because of the difficulty in obtaining extremely accurate data in the low‐strain regions. Calculation of the same data by our new equation shows fe/f to be independent of deformation for the region of strains in which the statistical theory of rubber elasticity applies. Thus the postulate of additivity of configurational free energy, basic to the derivation of the statistical theory, is vindicated. Finally it is shown that the stress‐temperature measurements are sensitive to the samp...

Critical mechanisms in the development of fatigue cracks in 2024-t4 aluminum.

Abstract: : Notched samples of 2024-T4 aluminum were partially fatigued and examined by means of optical and electron microscopy to identify the critical mechanisms for crack nucleation. Under fully reversed axial loading which led to fatigue lives of 10 to the 5th power - 16 to the 6th power cycles, it was found that deformation in the notch began with the formation of fine slip followed by intensification of slip around inclusions. This intensified slip was accompanied by the production of small dislocation loops and dipoles. Fatigue cracks nucleated at large (> 1 micron) impurity inclusion clusters, and no correlation could be found between the nucleation event and the formation of fine slip, concentrated slip, or dislocation dipoles and loops. Surface diffusion layers of Zn and Ag, designed to harden the surface layer and prevent local plastic deformation, were not effective in extending fatigue life. All of the evidence favors the view that fatigue cracks nucleated at the interface between surface inclusions and matrix by a mechanism which is relatively insensitive to local plastic deformation, but whose probability increases with inclusion size. (Author)

Tensile Deformation of Oriented Polyethylene

Abstract: Tensile test specimens have been cut from high‐density polyethylene sheet which had been fully cold‐drawn. The angle between the tensile axis and the draw direction was varied from 30° to 75°. The tensile deformation of such specimens was analyzed with the aid of a grid printing technique and was shown to be predominantly simple shear in the molecular chain direction. However, detailed analysis has shown that associated with the simple shear process there was a reorientation of the material, which is compared with the predictions of an aggregate theory for oriented polyethylene. A yield criterion and the absence of any work hardening for large deformations have been established.

Remanent and transitory effects of elastic deformation of magnetite crystals

Abstract: Single crystals of natural magnetite have been subjected to extended elastic deformation. The irreversible magnetic effects investigated were the acquisition of remanence and reduction of saturation remanence by compression in a null or directed magnetic field. The magnitude of the remanent moment is dependent on a complementary combination of the extent of deformation and magnitude of the field. The strain-induced moment is much larger than the isothermal remanence acquired in the deformation field, and has a significantly greater relative stability. The transitory effects of interest under the influence of applied stress were increase of saturation remanence and changes in microscopic coercivity spectrum. A structure-dependent model has been developed to interpret the behaviour. The magnetomechanical effects may be associated with the response of a multidomain configuration to changes induced in the internal mechanical microstructure. These changes are caused by the stress, and are attributed t...

Deformation and aging of quenched polypropylene

Abstract: The physical properties, i. e., yield stress and stress-strain curve of the quenched (“smectic”) form of polypropylene have been shown to be time dependent at room temperature. The yield stress increases linearly with the logarithm of film age. No changes in the x-ray diffraction pattern, infra-red spectrum, electron diffraction pattern or morphology have been observed corresponding to this change in physical properties. A molecular mechanism is proposed which will explain this again process. Spherulites of the quenched form have been found to deform affinely when drawn. In contrast to this, slow cooled films show complete destruction of internal spherulite (lamellar) order, indicating a complex mechanism of deformation.