Showing papers on "Deformation (engineering) published in 1982"

Stress-strain behavior of concrete confined by overlapping hoops at low and high strain rates

Abstract: An experimental investigation into the behavior of short reinforced concrete columns is described. Twenty-five concrete units, each 450 mm (17.7 in.) square by 1200 mm (47.2 in.) high, containing either 8 or 12 longitudinal steel bars and different arrangements of square or octagonal steel hoops, were subjected to concentric or eccentric loads to failure at different strain rates. Results presented include an assessment of the effect of eccentricity of load, strain rate, amount and distribution of longitudinal steel, and amount and dis­ tribution of transverse steel. A stress-strain curve for concrete con­ fined by hoop reinforcement and loaded at a high strain rate (com­ parable with seismic loading) is proposed and compared with an existing curve based on previous tests conducted at low strain rates. The available ultimate compressive strain for concrete confined by hoop reinforcement is also discussed.

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic Measurements and Mechanical Behavior

Abstract: The γ→α transformation in 304 stainless steel can be induced by plastic deformation at room temperature. The kinetics of strain-induced transformations have been modeled recently by Olson and Cohen. We used magnetic techniques to monitor the progress of the γ→α transformation in 304 stainless steel sheet loaded in uniaxial and biaxial tension at both low (10-3 per second) and high (103 per second) strain rates. We found that using the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics under general strain states. The principal effect of increased strain rate was observed at strains greater than 0.25. The temperature increase resulting from adiabatic heating was sufficient to suppress the γ→α transformation substantially at high rates. The consequences of the γ→α transformation on mechanical behavior were noted in uniaxial and biaxial tension. Uniaxial tension tests were conducted at temperatures ranging from 50 to -80°C. We found that both the strain hardening and transformation rates increased with decreasing temperature. However, the martensite transformation saturates at ≈85 pct volume fraction α. This can occur at strains less than 0.3 for conditions where the transformation is rapid. Once saturation occurs, the work hardening rate decreases rapidly and premature local plastic instability results. In biaxial tension, the same tendency toward plastic instability associated with high transformation rates provides a rationale for the low biaxial ductility of 304 stainless steel.

Constraints on geological strain rates: arguments from finite strain states of naturally deformed rocks

Abstract: A summary of known finite strain states is presented; longitudinal strains (1+e) as measured in many rocks often range from 1 to 40 and 1 to 0.025. The time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m.y., possibly less than 1 m.y., which constrains conventional strain rates into the range of 10/sup -13/ s/sup -1/ to 10/sup -15/ s/sup -1/. For both pure and simple shear (the most efficient way and much less efficient way to accumulate incremental strains, respectively) the ellipticity of the finite strain ellipse increases in a nonlinear manner. Finite strain variations in adjacent layers, which give rise to features such as cleavage refraction, arise with only slight differences in the strain rates within these layers.

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part II. Microstructural Study

Abstract: The γ→α transformation in 304 stainless steel was induced by plastic deformation under various conditions of strain, strain state, and strain rate, and the transformation microstructures were examined by transmission electron microscopy (TEM). The nucleation of α martensite embryos was always confined to microscopic shear band (faults, twins, and e-martensite) intersections. In cases where shear bands consisted of bundles of intermixed faults, twins, and e-martensite, α nucleated preferentially only within specific portions of the intersection volume. At sufficiently large strains α appeared to grow into polyhedral shapes. We postulate that growth occurs by repeated nucleation of new α embryos and coalescence of such embryos into polyhedral shapes. These shapes can grow either within an active slip plane or out of it, depending on how many shear band intersections are produced during deformation. Actual measurements of the number of intersections indicated that more intersections are formed in biaxial tension per unit effective strain than in uniaxial tension. This accounts for the more irregular, blocky α morphology observed in biaxial tension. At high strain rates we also found an increase in the number of intersections. However, adiabatic heating at large strains and high rates restricts repeated nucleation and coalescence and limits the amount of α transformation product.

Stress Analysis for Anisotropic Hardening in Finite-Deformation Plasticity

Abstract: : Kinematic hardening represents the anisotropic component of strain hardening by a shift of the center of the yield surface in stress space. The current approach in stress analysis at finite deformation includes rotational effects by using the Jaumann derivatives of the shift and stress tensors. This procedure generates the unexpected result that oscillatory shear stress is predicated for monotonically increasing simple shear strain. A theory is proposed which calls for a modified Jaumann derivative based on the spin of specific material directions associated with the kinematic hardening. This eliminates the spurious oscillation. General anisotropic hardening is shown to require a similar approach. (Author)

Plastic deformation mechanisms in poly(acrylonitrile-butadiene styrene) [ABS]

Abstract: Thin films of two poly (acrylonitrile-butadiene-styrene) [ABS] resins have been strained in tension, and the ensuing deformation has been characterized by transmission electron microscopy. To enhance contrast of the rubber particles, some of the specimens were stained with OsO4. Films containing only solid rubber particles 0.1 μm in diameter show little tendency for crazing. Instead, cavitation of the rubber particles occurs, together with localized shear deformation between the particles along a direction nearly normal to the tensile axis. For specimens containing a mixture of the same small particles plus larger (1.5μm diameter) particles containing glassy occlusions, some crazing does occur. Crazes tend to nucleate at the larger particles only. When crazes encounter the smaller particles these cavitate without appearing to impede or otherwise affect the craze growth. The occluded particles also show significant cavitation, with voids forming at their centres at sufficiently high levels of strain. These voids do not seem to lead to rapid craze break-down and crack propagation. In commercial ABS, which typically has both large and small rubber particles, both crazing, nucleated by the large particles, and shear deformation, encouraged by the cavitation of small rubber particles, can be expected to make important contributions to the toughness of the polymer.

Effect of Strain Rate on Deformation Behavior of Semi-Solid Dendritic Alloys

Abstract: The behavior of semi-solid Sn-Pb alloys was studied in compression between two parallel plates Small dendritic samples were deformed at cross-head speeds leading to initial strain rates ranging from 13 × 10-3 s-1 to 12 × 103 s-1 in the semi-solid state at a temperature just above the eutectic At the lower rates of deformation, breakdown of the dendrite structure occurs, at strains of 02 to 04, and a high degree of segregation of the liquid phase occurs For higher rates the segregation no longer occurs to such a great extent and the alloy deforms more homogeneously Some related experiments involving compression over a filter are presented to obtain stress-strain relations in bulk compression for later analysis The behavior in compression of alloys in the semi-solid state may be used as a refining process in the low strain-rate range where segregation of the liquid is large It may also prove useful in the high strain-rate range as a forming method

Direct Verification of Hydrogen Termination of the Semiconducting Diamond (111) Surface

Abstract: Low‐energy, high‐resolution electron energy loss spectroscopy has been used to identify the vibrational modes of hydrogen on the semiconducting diamond surface providing the first direct evidence that the (111) 1×1 surface is terminated by hydrogen. The vibrational loss spectrum from the ’’as‐polished’’ surface shows two major losses near 160 meV (CH3 deformation), a major loss at 360 meV (CH3 stretch), and two minor losses at 520 and 720 meV (combinations and overtones). All of these losses disappear from the spectrum after heating the sample to ∠1000 °C (which has been established by other experiments to be sufficient to reconstruct the surface to 2×2/2×1). The loss spectrum for the reconstructed surface is indicative of a two‐dimensional metallic state of the dangling‐bond surface states for clean diamond. Exposure of this reconstructed surface to atomic hydrogen results in a loss spectrum which is essentially identical to that for the as‐polished surface. Further verification that the loss spectrum re...

Mixed‐mode fatigue thresholds

Abstract: — Near threshold fatigue crack growth under mixed-mode loading and elastic plane-strain conditions has been studied in 316 stainless steel in laboratory air at room temperature. Particular emphasis was placed on the influence of the mode II component. Crack growth from the starter crack, although initially coplanar, branches to be perpendicular to the maximum normal stress. However the threshold for the branched crack growth is controlled not only by mode I displacement, but also by the mode II component. Upper and lower bound curves are obtained for the threshold condition and discussed in terms of crack tip reversed plastic deformation, crack surface rubbing and oxide-induced closure. A theoretical method for predicting the lower bound curve is proposed and compared with the maximum normal stress and strain energy density criteria. The new theory shows the best agreement with experimental results, giving a safe prediction for design purposes.

The Deformation and Fracture of β-HMX

Abstract: A study has been made of the mechanical deformation properties of $\beta$-HMX, an important secondary explosive. It is shown that under compressive loading twinning takes place on the (101)-plane. At low loads, this twinning is elastic and usually precedes fracture. Cleavage in $\beta$-HMX takes place on the {011}-planes. The fracture surface energy of 0.06 J m$^{-2}$ has been determined by a micro-indentation technique. This compares with a value of 0.045 J m$^{-2}$ obtained for the thermodynamic surface energy from contact-angle measurements. The values suggest that there is relatively little energy loss by plastic deformation associated with crack propagation in HMX compared with, for example, the secondary explosives PETN and RDX. Despite this brittleness the twin deformation allows $\beta$-HMX to undergo large changes of shape: the significance of this in plastic-bonded explosives is commented on.

Large strain multidirectional deformation of 1100 aluminum at 300 K

Abstract: Multidirectional compression testing of 1100 aluminum cubes at room temperature in three orthogonal directions developed saturation flow stresses at very large accumulated strains. The saturation stress was found to be a function of the strain increment used, and followed a power-law relationship. The results correlated well with fatigue tests of aluminum and alpha iron. Copper data showed a similar but more pronounced behavior. The presence of dislocation cells, subgrains and dislocation tangles dominated the microstructure as observed by transmission electron microscopy. The microstructure changed in a systematic manner with accumulated straining. Significant differences in sizes and concentrations of cells and subgrains were found for unidirectional compared with multidirectional straining. These features were correlated using generally accepted relationships between individual substructural configurations and flow stress.

Elastic–Plastic Crack Growth

Abstract: Summary The asymptotic structure of near-tip stress and deformation fields is analyzed for cracks growing in elastic–perfectly plastic solids. A general formulation is presented for materials of arbitrary yield condition and associated flow rule, including anisotropic response, although detailed results are presented only for isotropic materials of the Huber–Mises type. Centered fan sectors of singular straining at a crack tip are shown to be general, independent of details of material response, as are also the types of plastic strain singularities associated with stationary and growing cracks. Previously results for the Huber–Mises material are recovered by specialization from the general formulation, some corrections are made, and recent work on using results of such a near-tip analysis as a basis for predicting plane strain stable crack growth is reviewed briefly.

A Study of the Deformation and Fracture of a Dual-Phase Steel

Abstract: A metallographic study of the tensile deformation and fracture of a commercial dual-phase steel shows that extensive plastic deformation of the martensite occurs mainly in the neck of the tensile specimen. The average load-transfer stress in martensite was roughly calculated from a shear-lag model, and it was found that it exceeds the estimated martensite yield stress value at strains corresponding to the ultimate uniform elongation, and beyond. Ductile fracture begins with void formation at martensite-ferrite interfaces, most frequently at the poles of closely spaced martensite particles situated on ferrite grain boundaries. Large inclusions and martensite banding also affect the fracture process.

The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

Abstract: Two high strength P/M nickel-base superalloys, AF-115 and AF2-1DA, with different defect populations, were tested to determine the effect of preexisting defects on the fatigue crack initiation process. Strain controlled continuous cycle fatigue tests were performed at room and at elevated temperature; these were followed by fractographic examination to characterize both the location and character of the fatigue origins. In most cases, particularly at elevated temperature, the initiation process was associated with a large pre-existing defect, either a pore or a nonmetallic inclusion. There was also a change in the location of the crack that caused failure as the strain range varied: at high strain ranges initiation occurred at or near the specimen’s surface, while at the lower strain ranges the failure originated in the specimen’s interior. The initiation mode for both alloys at room temperature was different than at elevated temperature. At room temperature, Stage I crystallographic cracking at or near the surface dominated the process in all strain range regimes. This difference was attributed, in part, to the differences in deformation mode for nickel-base superalloys at room and elevated temperature.

Mechanical Properties of Polycrystalline TiC

Abstract: The mechanical properties of fine-grained polycrystalline TiC were studied using both four-point bending and compression tests The ductile-brittle transition (D-B) temperature in compression was determined to be =800°C and was found to depend on grain size Yield-point behavior was observed for the first time in fine-grained TiC deformed in compression and was found to depend on grain size and test temperature The yield stress as a function of grain size can be described by a Hall-Petch type of relation, ie yield stress α (grain size)-1/2 The dislocations resulting from deformation in compression at lower temperatures were predominately screw in character, with edge dipoles and dislocation loops being present As the temperature of deformation was increased, the dipoles and loops were gradually annihilated by climb and the dislocations were observed in the form of hexagonal networks with a much-reduced dislocation density A plot of log yield stress vs 1/T showed a change in slope, which suggests that two rate-controlling mechanisms are in operation during deformation at different test temperatures Thermal activation analysis at T = 1050° to 1500°C suggested that the rate controlling mechanism during deformation in this temperature range is associated with cross slip

Separation of Cavitation‐Strain and Creep‐Strain During Deformation

Abstract: A significant fraction of the strain in high-temperature deformation may arise from cavitation. A methodology is described for separating the creep strain from the cavitation strain. Such a distinction is necessary because cavitation and creep are mechanistically different processes.

Optical Measurement of the Plastic Strain Concentration at a Crack Tip in a Ductile Steel Plate

Abstract: The shadow spot method has been established as a valuable experimental procedure for measuring elastic stress intensity factors in planar fracture specimens. It is noted here that if the crack-tip deformation field in specimens of ductile materials can be characterized by means of a single plastic intensity factor, analogous to the stress intensity factor in linear elastic fracture mechanics, then the shadow spot method has potential for use in measuring this intensity factor. The value of the J-integral is adopted as the plastic strain intensity factor, and the lateral contraction of an elastic-ideally plastic planar specimen is calculated in terms of J from the nonhardening limit of the HRR asymptotic field of elastic-plastic fracture mechanics. The theoretical caustic curve which would be generated by geometrical reflection of normally incident parallel light from points of the deformed specimen surface lying well within the plastic zone is determined, and it is shown that the value of J is proportional to the maximum transverse diameter of the shadow spot to the third power. Results of preliminary experiments, in which values of J for a given single edge notched steel plate specimen are inferred from measurements peformed separately from the elastically and plastically deforming parts of the specimen, are also reported.

The influence of inertia on elastic-plastic antiplane-shear crack growth

Abstract: Results of a study of steady-state antiplane-shear crack growth in an elastic ideally-plastic material are reported. First, an exact expression for the distribution of strain on the crack line within the primary active plastic zone is obtained. It is shown that the expression reduces to the correct asymptotic form for the special case of vanishingly small distance from the crack tip for any nonzero crack growth speed, and it reduces to the correct limit as the crack speed approaches zero for any point on the crack line. Then, the full elastic-plastic deformation field is determined under the conditions of small-scale yielding by means of the numerical finite element method. The computed strain distribution on the crack line is found to compare closely with the analytical result for this distribution. Finally, the analytical and numerical results are combined with the “critical plastic strain at a characteristic distance” crack growth criterion to generate theoretical fracture toughness vs crack speed relationships. The results are similar to experimentally observed fracture toughness vs crack speed relationships. In particular, the results show a strong dependence of fracture toughness on crack speed for even moderate crack growth rates. Because the material response is independent of strain-rate, this suggests that the influence of inertia on the fracture resistance of the material is significant.

Compressive failure and kinking in uniaxially aligned glass-resin composite under superposed hydrostatic pressure

Abstract: The failure process in uniaxially-aligned 60% fibre volume fraction glass fibre-epoxide compressive specimens strained parallel to the fibre axis was investigated at atmospheric and superposed hydrostatic pressures up to 300 MN m−2. The atmospheric strength was about 1.15 GN m−2 (about 20% less than the tensile) and strongly pressure dependent, rising to over 2.2 GN m−2 at 300 MM m−2 pressure, i.e. by about 30% per 100 MN m−2 of superposed pressure. The corresponding figure is 22% if the maximum shear stress and not the maximum principal compressive stress is considered. This is incompatible with atmospheric compressive failure mechanisms controlled by weakly dependent or pressure independent processes, e.g. shear of the fibres. The results also could not be satisfactorily interpreted in terms of microbuckling of individual fibres. Kinking, involving buckling of fibre bundles was proposed as the mechanism of failure propagation, but the critical stage (for this glass reinforced plastic) is suggested as being yielding of the matrix, which initially restrains surface bundles from buckling. A strong pressure dependent failure criterion, about 25% increase per 100 MN m−2, was derived by modifying the Swift-Piggott analysis of deformation of initially curved fibres. It is postulated that it is the axial compression that causes bundle curvature. Other systems, particularly carbon fibre-reinforced plastic, in which there appears to be a transition in the critical stage of failure from bundle buckling to matrix yielding with increasing superposed pressure, are also considered.

TEM observations of dislocation emission at crack tips in aluminium

Abstract: Direct observations were made of the propagation of ductile cracks and associated dislocation behaviour at crack tips in aluminium during tensile deformation in an electron microscope. In the electropolished area, the cracks propagated as a Mode III shear-type by emitting screw dislocations on a plane coplanar to the crack plane. A zone free of dislocations was observed between the crack tip and the plastic zone. As the cracks propagated into thicker areas, the fracture mode changed from Mode III to predominantly Mode I. The crack top of the Mode I cracks was blunted by emitting edge dislocations on planes inclined to the crack plane. The blunted cracks did not propagate until the area ahead of the crack tip was sufficiently thinned by plastic deformation. The cracks then propagated abruptly, apparently without emitting dislocations. The stress intensity factor was measured from the crack tip geometry of Mode III cracks and it was found to be in good agreement with the critical value of the stress intensity factor required for dislocation generation.

Calculation of thermal trapping in shock deformation of aluminum

Abstract: A precipitation hardened aluminum alloy (6061‐T6) has been investigated extensively in terms of shock deformation properties during past years. Recently, advances in time‐resolved measurements of stress waves in solids have assisted in revealing complicating features in the shock deformation process which are not easily incorporated into existing theories of plastic deformation. Specifically, significant changes in the dynamic hardness, post shock hardness, steady wave viscosity, and post shock microstructure have been observed over the shock pressure range of 5–20 GPa. Microscopic heterogeneity in the shock deformation process is one possible cause for the observed effects. Deformation heterogeneities can lead to transient temperature gradients. We have performed calculations which indicate that over the range of strain rates occurring in steady wave shocks in aluminum, a transition from thermal equilibrated to thermal trapping deformation can occur. The shock pressure at which the thermal transition is ...