Showing papers on "Deformation (engineering) published in 1998"

Plastic deformation and fracture behaviour of Ti–6Al–4V alloy loaded with high strain rate under various temperatures

Abstract: This study investigates the plastic deformation and fracture behaviour of titanium alloy (Ti–6Al–4V) under high strain rates and various temperature conditions. Mechanical tests are performed at constant strain rates ranging from 5×102 to 3×103 s−1 at temperatures ranging from room temperature to 1100°C by means of the compressive split-Hopkinson bar technique. The material's dynamic stress–strain response, strain rate, temperature effects and possible deformation mechanisms are discussed. Furthermore, the plastic flow response of this material is described by a deformation constitutive equation incorporating the effects of temperature, strain rate, strain and work hardening rate. The simulated results based on this constitutive equation are verified. The fracture behaviour and variations of adiabatic shear band produced by deformation at each test condition are investigated with optical microscopy and scanning electron microscopy. The results show that the flow stress of Ti–6Al–4V alloy is sensitive to both temperature and strain rate. Nevertheless, the effect on flow stress of temperature is greater than that of strain rate. Fracture observations reveal that adiabatic shear banding turns out to be the major fracture mode when the material is deformed to large plastic strain at high temperature and high strain rate.

Connecting atomistic and mesoscale simulations of crystal plasticity

Abstract: A quantitative description of plastic deformation in crystalline solids requires a knowledge of how an assembly of dislocations — the defects responsible for crystal plasticity — evolves under stress1. In this context, molecular-dynamics simulations have been used to elucidate interatomic processes on microscopic (∼10−10 m) scales2, whereas ‘dislocation-dynamics’ simulations have explored the long-range elastic interactions between dislocations on mesoscopic (∼10−6 m) scales3. But a quantitative connection between interatomic processes and behaviour on mesoscopic scales has hitherto been lacking. Here we show how such a connection can be made using large-scale (100 million atoms) molecular-dynamics simulations to establish the local rules for mesoscopic simulations of interacting dislocations. In our molecular-dynamics simulations, we observe directly the formation and subsequent destruction of a junction (a Lomer–Cottrell lock) between two dislocations in the plastic zone near a crack tip: the formation of such junctions is an essential process in plastic deformation, as they act as an obstacle to dislocation motion. The force required to destroy this junction is then used to formulate the critical condition for junction destruction in a dislocation-dynamics simulation, the results of which compare well with previous deformation experiments4.

Nanomechanical properties of Au (111), (001), and (110) surfaces

Abstract: Using the interfacial force microscope in an indentation mode, we have quantitatively investigated the mechanical properties for the (111), (001), and (110) surfaces of Au single crystals. Nanoscale indentations of wide, atomically flat terraces provide a measure of the nanomechanical properties of Au in the absence of bulk and surface defects. The elastic indentation modulus for the (111) surface was found to be 36% greater than for the (001) and 3% greater than for the (110) surfaces. These results are compared to earlier theoretical predictions of the effect of anisotropy on indentation based on continuum mechanics and atomistic simulations. Additionally, we have quantified the yield point of the three crystal orientations by measuring the stress at which initial plastic deformation occurs. By resolving the applied stresses on {111} slip planes, we have estimated maximum shear stresses at the yield point. For each orientation, plastic deformation occurred when the maximum resolved shear stress reached approximately 1.8 GPa on all {111} planes that appeared to contribute to deformation. Based on this estimate, we propose that the critical resolved shear stress for plastic indentation of Au is 1.8 GPa and that the yield criterion is that this stress be attained on all {111} slip planes noncoplanar with the surface.

Self-monitoring structural materials

Abstract: Self-monitoring (or intrinsically smart) structural materials, including concrete containing short carbon fibers, and polymer-matrix and carbon-matrix composites containing continuous carbon fibers, were reviewed. Each material is capable of monitoring its own reversible strain and damage through the effects of these on the electrical resistance of the material. This capability is valuable for structural control and structural health monitoring. Among these three materials, the concrete gives the highest strain sensitivity or gage factor (up to 700), while the carbon-carbon composite gives the highest damage sensitivity (i.e., sensitivity even to the damage after the first cycle of tensile loading within the elastic regime). The origin of the self-monitoring ability differs among the three materials. For the concrete, it is related to slight fiber pull-out during strain and fiber and matrix fracture during damage. For the polymer-matrix composite, it is related to the increase in the degree of fiber alignment and reduction of fiber pre-stress during tension in the fiber direction and to fiber fracture and delamination during fatigue. For the carbon-carbon composite, it is related to dimensional changes during strain and fiber and matrix fracture during damage. © 1998 Elsevier Science S.A.

Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiCp composite

Abstract: The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S’ precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.

Adiabatic shear bands in a TI-6Al-4V titanium alloy

Abstract: Dynamic (γ≈103⧹sec) torsional experiments were performed to investigate the process of initiation and formation of adiabatic shear bands in Ti-6Al-4V alloy. In this study, thin-wall tubular specimens were deformed dynamically in a torsional Kolsky bar (torsional split Hopkinson bar) . Through high-speed photography of a grid pattern previously printed on the specimens outer surface, the local strain and the local strain rate were found to be in the range of 75%–350% and 8.0×104⧹sec, respectively. The width of the shear bands ranged from 12–55 μm. In addition, an array of infrared detectors was employed to measure the local temperature rise during the deformation process. A peak temperature of 440–550°C was found in the various tests. The fracture surface of the shear band material was characterized by (1) regions of elongated dimples within which no second phase particles were observed, and (2) regions with a relatively flat and smeared appearance. There was no clear evidence based either on the appearance of the shear band in SEM or the measured temperature rise to suggest that the material within the shear band had undergone a phase transformation.

Role and behaviour of μ phase during deformation of a nickel-based single crystal superalloy

Abstract: The effect of the presence of variable amounts of the topologically closed packed (TCP) μ phase on the mechanical properties of the MC2 nickel-based single crystal superalloy has been investigated. It is shown that moderate amounts of needle-like μ particles do not affect the tensile properties or the impact strength at 25°C. The precipitates behave similarly to brittle short fibres in a ductile matrix, thus internal cracking occurs under stress rather than interfacial decohesion. For a higher amount of μ phase, precipitates are of smaller size and of globular shape, exhibiting a weaker interface with the γ/γ′ matrix, thus they are more prone to decohesion. By separating effects due to the μ phase from those due to the rafted γ / γ ′ microstructure, it is shown that the low cycle fatigue (LCF) properties of MC2 alloy at 650 and 950°C are not affected by the presence of μ phase. Finally, some observations related to the behaviour of μ phase precipitates during high temperature creep are presented.

Different kinds of structure in aerogels: relationships with the mechanical properties

Abstract: The density and the structure (fractal and non-fractal) of aerogels are modified either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process or by an isostatic pressure deformation. These aerogels have porosities ranging from 98% to 0%. The mechanical properties of the different aerogels (elastic modulus and strength) measured by 3 point bending, are dependent on their structure; they vary by five orders of magnitude as a function of density and follow power law evolution. However for the same relative density the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. These structural differences have been observed by SAXS experiments. The effects of the sintering process compared to that of the plastic transformation on the mechanical properties are explained by the associated structural changes. Sintering increases the network connectivity and the densification by compression leads to a new spatial arrangement of the clusters but their internal structure is not affected. In addition, relationships between structural and porous features and the mechanical properties are discussed in terms of percolation theory and the fractal approach. We show that the exponent of the power law does not depend on the fractal feature and percolation is only an approximation which cannot describe results.

Scaling of misorientation angle distributions

Abstract: The measurement of misorientation angle distributions following different amounts of deformation in cold-rolled aluminum and nickel and compressed stainless steel is reported. The scaling of the dislocation cell boundary misorientation angle distributions is studied. Surprisingly, the distributions for the small to large strain regimes for aluminum, 304L stainless steel, nickel, and copper (taken from the literature) appear to be identical. Hence the distributions may be ``universal.'' These results have significant implications for the development of dislocation based deformation models.

A generalized law for brittle deformation of Westerly granite

Abstract: A semiempirical constitutive law is presented for the brittle deformation of intact Westerly granite. The law can be extended to larger displacements, dominated by localized deformation, by including a displacement-weakening break-down region terminating in a frictional sliding regime often described by a rate- and state-dependent constitutive law. The intact deformation law, based on an Arrhenius type rate equation, relates inelastic strain rate to confining pressure Pc, differential stress σΔ, inelastic strain ei and temperature T. The basic form of the law for deformation prior to fault nucleation is lne˙i=c-(E*/RT)+(σΔ/aσo)sin-a(πei/2eo) where σo and eo are normalization constants (dependent on confining pressure), a is rate sensitivity of stress, and α is a shape parameter. At room temperature, eight experimentally determined coefficients are needed to fully describe the stress-strain-strain rate response for Westerly granite from initial loading to failure. Temperature dependence requires apparent activation energy (E* ∼ 90 kJ/mol) and one additional experimentally determined coefficient. The similarity between the prefailure constitutive law for intact rock and the rate- and state-dependent friction laws for frictional sliding on fracture surfaces suggests a close connection between these brittle phenomena.

Creep behavior and deformation microstructures of Mg–Y alloys at 550 K

Abstract: Creep behavior and deformation substructures of Mg–Y binary alloys containing 0.2–2.4 mol% Y have been investigated at 550 K under 50–200 MPa. Creep strength is remarkably improved by a small addition of yttrium as compared with aluminum and manganese. Two stress regions are recognized based on the stress dependence of the minimum creep rate. The lower-stress region, where the stress exponent n is about 5, corresponds to the H region (class M behavior) commonly observed in other solid solution alloys. The concentration dependence of solid-solution Mg–Y alloys is similar to that of Mg–Al solid solution alloys: the apparent concentration exponent m is about −2. The effect of the yttrium concentration increases when the concentration exceeds 1.6 mol%. Transmission electron microscopy reveals that the activation of the non-basal slip systems is enhanced and dynamic precipitation occurs during creep deformation in a concentrated Mg–Y alloy. The high creep resistance can be correlated with two mechanisms, i.e. forest dislocation-hardening (work hardening) and dynamic precipitation of fine pseudo-equilibrium β′ phase on the dislocation lines during creep.

Effects of hydrostatic pressure on mechanical behaviour and deformation processing of materials

Abstract: The processing and subsequent mechanical behaviour of a variety of commercially important materials are affected by the imposed stress state. In this review, the experimentally documented effects of superimposed pressure on deformation under quasistatic conditions are summarised, followed by a presentation of the effects of superimposed pressure on the fracture behaviour of a variety of materials including both ductile and brittle systems. It is shown that the pressure responses of a variety of materials show distinct differences and the potential reasons for these differences are presented. Finally, in the light of all of these observations, the effects of changes in stress state on deformation processing are reviewed. In particular, the evolution of hydrostatic stresses during various forming operations is covered followed by a review of published work and the potential benefits of superimposing pressure during processing of a variety of materials.