Showing papers on "Deformation (engineering) published in 1981"

Finite element analysis of deformation of strain‐softening materials

Abstract: A finite element analysis of strain-softening materials is presented in which the shear band of prescribed thickness is assumed to exist within elements where maximal stress intensity is reached. The incremental stiffness matrix of the element is derived including shear band deformation. Examples presented in the Paper demonstrate that the load-displacement curve and the displacement field are not sensitive to the mesh size used in the solution.

Failure of materials in mechanical design : analysis, prediction, prevention

Abstract: The Role of Failure Prevention Analysis in Mechanical Design. Modes of Mechanical Failure. Strength and Deformation of Engineering Metals. State of Stress. Relationships Between Stress and Strain. Combined Stress Theories of Failure and Their Use in Design. High-Cycle Fatigue. Concepts of Cumulative Damage, Life Prediction, and Fracture Control. Use of Statistics in Fatigue Analysis. Fatigue Testing Procedures and Statistical Interpretations of Data. Low-Cycle Fatigue. Stress Concentration. Creep, Stress Rupture, and Fatigue. Fretting, Fretting Fatigue, and Fretting Wear. Shock and Impact. Buckling and Instability. Wear, Corrosion, and Other Important Failure Modes. Index.

Elastic recovery at hardness indentations

Abstract: The mechanics of hardness indentation are considered. On the basis of a cycle in which the loading is elastic-plastic and the unloading (and subsequent reloading) elastic, an expression is derived for the relative depth recovery of the impression as a function of hardness/modulus,H/E. Experimental observations on indented surfaces of selected materials, mostly ceramics, using a tilting procedure in the scanning electron microscope to measure the residual depths, confirm the predicted trends. The analysis offers a simple means of characterizing the deformation properties of materials and should provide a basis for evaluating a range of contact-related properties, particularly surface damage phenomena in sharp-particle impact.

Effects of morphology on the mechanical behavior of a dual phase Fe/2Si/0.1C steel

Abstract: A study has been made on the effects of morphology on the mechanical behavior of a dual phase Fe/2Si/0.1C steel. The coarse dual phase structure obtained by continuously annealing in the two phase region directly from the austenite region results in poor elongation ductility with relatively high strength. However, upon obtaining a fine fibrous or fine globular dual phase structure by following different transformation paths, significant improvements occur in elongation ductility without much sacrifice in strength. The poor elongation ductility of the coarse dual phase structure is due to the initiation of cleavage cracks in the ferrite region where maximum localized stress concentration took place. But, in steels with both fine fibrous or globular morphologies, fracture occurred by void nucleation and coalescence after large amounts of plastic deformation.

Cyclic deformation and fatigue behaviour of α-iron mono-and polycrystals

Abstract: The reported studies are based on a series of cyclic deformation tests that were conducted at room temperature on decarburized high-purity α-iron specimens in mono-and polycrystalline form. The experimental data cover plastic strain ranges Δe pl in the regime 10−4 ≲ Δe pl ≲ 10−2 and variations in cyclic plastic strain rates έ pl between ∼10-5 and ∼10−2 s−1. In the case of single crystals, the effect of solute carbon (∼30 wt. ppm) was investigated as well. The mechanical data were supplemented by detailed studies of the dislocation arrangements by transmission electron microscopy and of the surface patterns by scanning electron and optical microscopy. Detailed accounts are given of the following topics: cyclic hardening and saturation, dislocation mechanisms, shape changes due to asymmetric slip of serew dislocations, cyclic stress-strain response and fatigue crack initiation. Under conventional conditions of “high” έ pl (≲10−4 s−1) the fatigue behaviour of α-iron at room temperature reflects the low mobility of the screw dislocations which is characteristic of the lowttemperature mode of deformation of body-centred cubic (b.c.c.) metals. As a consequence the behaviour exhibits significant differences with respect to that of fatigued face-centred cubic (f.c.c.) metals such as: strongly impeded dislocation multiplication below Δe pl ∼ 5 × 10−4, appreciable secondary slip at higher Δe pl leading to a cell structure (persistent slip bands do not form), shape changes due to asymmetric slip of screw dislocations and a relatively high effective stress level. The reduction of έ pl and the presence of solute carbon atoms modify this behaviour significantly, making it more similar to that of f.c.c. metals. In all cases it was found that only the athermal component of the peak (saturation) stress but not the latter itself represents a suitable measure of the properties of the dislocation substructure. On the basis of the cyclic deformation behaviour and of observations of trans-and intergranular fatigue crack initiation it was concluded that the fatigue limit of α-iron is an intrinsic property of the b.c.c. structure whose characteristics, however, are affected sensitively by interstitial impurity content and by the strain rate of the fatigue test.

Deformation and unstable flow in hot forging of Ti-6Ai-2Sn-4Zr-2Mo-0.1Si

Abstract: The stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s-1 in order to establish its hot forging characteristics. In hot, isothermal compression, Ti-6242 with an equiaxed a structure deforms stably and has a flow stress which decreases with straining due to adiabatic heating. With a transformed-β microstructure, unstable flow in hot compression is observed and concluded to arise from large degrees of flow softening caused by microstructural modification during deformation and, to a small extent, by adiabatic heating. Both microstructures have a sharp dependence of flow stress on temperature. Using the concepts of thermally-activated processes, it was shown analytically that this dependence is related to the large strain-rate sensitivity of the flow stress exhibited by the alloy. From lateral sidepressing results, the large dependence of flow stress on temperature was surmised to be a major factor leading to the shear bands occurring in nonisothermal forging of the alloy. Shear bands were also observed in isothermal forging. A model was developed to define the effect of material properties such as flow softening rate and strain-rate sensitivity on shear band development and was applied successfully to predict the occurrence of shear bands in isothermal forging.

The plastic deformation of silicon between 300°C and 600°C

Abstract: Constant strain-rate compression of silicon single crystals has been performed under a hydrostatic pressure of 1500 MPa. The crystals were deformed at temperatures as low as 300°C and had resolved flow stress values of up to 1000 MPa. The variation of yield strength with temperature suggests a transition of the mechanism controlling the deformation near 600°C.

Influence of the martensitic transformation on the deformation behaviour of an AISI 316 stainless steel at low temperatures

Abstract: The deformation behaviour of an AISI 316 stainless steel under uniaxial tension was examined at 25, − 70 and − 196° C. The flow curves exhibited peculiar shapes and the work hardening rates were found to increase with strain beyond certain values of plastic strain. X-ray diffraction analyses showed that the transformation toα-martensite commenced at these values of plastic strain and thereafter the volume fraction ofα increased steadily with strain. On the other hand, the amount of the∈-martensite was found to increase with plastic strain initially, reach a maximum and then decrease gradually. The contribution of theα-phase to the flow stress of the alloy was found to be directly proportional to the volume fraction ofα. It is shown that the analysis of the flow curves provides a simple method of detecting the onset of the strain-induced martensitic transformation as well as estimating the amount of this martensite during further deformation.

Atomistic process of plastic deformation in a model amorphous metal

Abstract: Elementary process of plastic deformation in a computer simulated metallic amorphous model which was subjected to shear deformation has been investigated by analysing spatial distributions of atomic displacement, atomic strain, atomic stresses, atomic shear modulus and atomic coordination expressed by Voronoi polyhedra. Plastic shear deformation took place at localized sites in a spherical shape with a diameter of 3–4 atomic distances. The deformation sites are characterized by an atomic pressure distribution around thc site which is arranged so as to assist the local deformation, and at the same time by a local mechanical instability expressed by a low atomic shear modulus which in turn is related to the Voronoi polyhedra type. It was also observed that adjacent deformation sites were transformed sequentially in the manner of a chain reaction, resulting in macroscopic slip.

The experimental ductile deformation of polycrystalline and single crystal pyrite

Abstract: Polycrystalline pyrite, deformed in triaxial compression tests at a confining pressure of 300 MPa, strain rates of 10 (super -4) sec (super -1) to 10 (super -5) sec (super -1) , and temperatures above about 450 degrees C, shortens by dislocation flow mechanisms. Above 500 degrees C dynamic recovery and recrystallization are important, and steady state flow is attained after 10 to 20 percent shortening. The dynamically recrystallized grain size (d) (in mu m) is related to the flow stress (Sigma ) by the relationSigma / G = k (d/b) (super -0.9) ,where k = 16.2, G = 7.8 X 10 4 MPa, and b = 5.42 X 10 (super -4) mu m. Below about 450 degrees C polycrystalline pyrite deforms by brittle mechanisms after less than a few percent strain by dislocation flow.The strength of polycrystalline pyrite decreases significantly with increasing temperature and decreasing strain rate. Over the temperature interval 500 degrees to 700 degrees C, at strain rates of 2 X 10 (super -5) sec (super -1) , the steady state flow stress drops from about 500 to 70 MPa.Pyrite single crystals shortened in the and orientations at 600 degrees C, about 10 (super -5) sec (super -1) , and 300 MPa confining pressure exhibit three-stage work-hardening behavior in which the stage 2 work-hardening rates are nearly as high as the elastic modulus. For shortening the yield stress (280 MPa) and the differential stress at the commencement of stage 3 (800 MPa) are several times larger than for shortening. The strength during shortening is comparable to that of the polycrystalline pyrite tested.The single crystal strength data, structural considerations, and transmission electron microscopy suggest that {100} and possibly {100} are major slip systems. The {110} dislocation glide may also be important but has a critical resolved shear stress several times higher than {100} glide.The present data indicate that pyrite can be considerably weaker and more ductile than indicated by previous studies, and that pyrite is likely to be deformed by dislocation flow mechanisms under a range of geologically realistic conditions.

Ideal elastic, anelastic and viscoelastic deformation of a metallic glass

Abstract: The elastic, viscoelastic and anelastic components of the homogeneous strain response of the metallic glass Pd82Si18 to an applied stress have been examined. The elastic response is fully reversible, instantaneous and linear. The measured elastic modulus, E, and temperature dependence, d(ln E)/dT, are 84±8 GPa and (−3.2±0.6) × 10−4 C−1, respectively. The viscoelastic flow is non-recoverable and, if the configuration remains constant, is characterized by a constant strain rate. This strain rate varies linearly with the stress, gtr, in the low stress regime (τ < 300 MPa), becoming non-linear for higher stresses. For isoconfigurational flow, the strain rate has an Arrhenius-type temperature dependence with an activation energy of - 200 ± 15 kJ mol−1, independent of stress and thermal history. The magnitude of the strain rate is strongly dependent on the degree of structural relaxation and therefore on thermal history. During isothermal annealing the viscoelastic strain rate varies inversely with time. The anelastic response is a transient that, at 500 K, contributes to the flow for approximately fifty hours after a stress increase and is fully recovered upon stress reduction. A spectrum of exponential decays is required to model this flow component. The anelastic strain, τA, varies linearly with the magnitude of the stress change, Δτ, over the entire stress range tested: γA/gDAτ=(8.0±0.8)× 10−6 MPa−1.

Dislocation distribution and prediction of fatigue damage

Abstract: The dislocation density and distribution induced by tensile deformation in single crystals of silicon, aluminum and gold and by tension-compression cycling in aluminum single crystals and Al 2024-T3 alloys were studied by X-ray double-crystal diffractometry. The measurements of dislocation density were made at various depths from the surface by removing surface layers incrementally. In this way, a propensity for work hardening in the surface layers compared to the bulk material was demonstrated for both tensiledeformed and fatigue-cycled metals. Analysis of the cycled Al 2024 alloy as a function of the fraction of fatigue life showed that the dislocation density in the surface layer increased rapidly early in the fatigue life and maintained virtually a plateau value from 20 to 90 pct of the life. Beyond 90 pct the dislocation density increased rapidly again to a critical value at failure. Evaluation of the dislocation distribution in depth showed that the excess dislocation density in the bulk material increased more gradually during the life. Using deeply penetrating molybdenumK α radiation, capable of analyzing grains representative of the bulk region, the accrued damage and the onset of fatigue failure could be predicted nondestructively for 2024 Al, cycled with constant stress as well as with variable stress amplitude. The dislocation structure produced in the bulk by prior cycling was unstable when the work-hardened surface layer was removed. It is proposed that the deformation response of the bulk material is controlled by the accumulation of dislocations and associated stresses in the surface layer.

Effect of Grain anisotropy on limit strains in biaxial stretching: part i. influence of sheet thickness and grain size in weakly textured sheets

Abstract: Processes of inhomogeneous deformation which lead to localized necking in biaxial stretching have been investigated in sheets of copper, Cu-30 pct Zn brass and low-carbon steels of good commercial quality without strongly developed preferred orientations. With sheet thickness,to, in the range 0.4 to 1.2 mm, it was found that limit strains in biaxial stretching decreased with decreasingto/do, wheredo is average grain diameter. It was concluded that, whento/do was less than about 20, plastic anistropy of individual grains of the primary phase was the dominant source of the strain inhomogeneities which developed to cause eventual necking failure. Measurement of the rate at which surface roughness developed with increasing strain,dR’/d- ge, indicated that, in the early stages of stretching, the growth of thickness inhomogeneity was close to being proportional todo and -ge and it was insensitive toto and the applied strain ratio, p, but, at an applied strain -gem which depended onto anddo, dR /d- ge started to increase progressively. In this latter phase of the process strain localization developed on a macroscopic scale. It is concluded that the dependence of -gem onto underlies the effects of sheet thickness on biaxial limit strains, also that the influence of p on the rate of growth of thickness inhomogeneities can change progressively during the evolution of strain localization through the microstructural and macroscopic phases.