Strain Rate

A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-contentJ = ∫oσ ε ⋅dσ from the constitutive equation relating the strain rate (ε) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak inJ. It is shown thatJ is related to the strain-rate sensitivity (m) of the material and reaches a maximum value(J max) whenm = 1. The efficiency of the power dissipation(J/J max) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, noa priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+ β andβ preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927 °C (1200 K) and a strain rate of 1CT•3 s•1. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.

A new approach for modelling discrete cracks in meshfree particle methods in three dimensions is described. The cracks can be arbitrarily oriented, but their growth is represented discretely by activation of crack surfaces at individual particles, so no representation of the crack’s topology is needed. The crack is modelled by a local enrichment of the test and trial functions with a sign function (a variant of the Heaviside step function), so that the discontinuities are along the direction of the crack. The discontinuity consists of cylindrical planes centered at the particles. The method is formulated for large deformations and arbitrary nonlinear and rate-dependent materials; cohesive laws govern the traction-crack opening relations. To reduce computational cost and since more accuracy around the crack tip is needed to obtain adequate results, h-adaptivity is incorporated in the method. The model is applied to several three-dimensional problems, some of which are compared to experimental data.

The microstructure of commercially available nanocrystalline (nc) electroplated Ni foils is studied by means of X-ray diffraction and transmission electron microscopy. It is shown that the microstructure is inhomogeneous and batch-dependent. Tensile properties at strain rates between 10−5 and 103 s−1 are studied and compared with the results of coarse-grained Ni. Data on strength, strain-rate sensitivity and work hardening are presented. At the highest strain rates, shear banding with local grain growth is observed in the nc structure. It is also suggested that the differences found in nc Ni for 3 and 20 mm tensile specimens are the size effects related to the inhomogeneous microstructure.

The mechanical properties of biodegradable polymers and composites proposed for use in internal fixation (in place of stainless steel) are crucial to the performance of devices made from them for support of healing bone. To assess the reported range of properties and degradation rates. we searched and reviewed papers and abstracts published in English from 1980 through 1988. Mechanical property data were found for poly(lactic acid), poly (glycolic acid), poly(ϵ-caprolactone), polydioxanone, poly(ortho ester), poly(ethylene oxide), and/or their copolymers. Reports of composites based on several of these materials, reinforced with nondegradable and degradable fibers, were also found. The largest group of studies involved poly(lactic acid). Mechanical test methods varied widely, and studies of the degradation of mechanical properties were performed under a variety of conditions, mostly in vitro rather than in vivo.Compared to annealed stainless steel, unreinforced biodegradable polymers were initially up to 36% as strong in tension and 54% in bending, but only about 3% as stiff in either test mode. With fiber reinforcement, reported highest initial strengths exceeded that of stainless steel. Stiffness reached 62% of stainless steel wiht nondegradable carbon fibers, 15% with degradable inorganic fibers, but only 5% with degradable polymeric fibers.The slowest-degrading unreinforced biodegradable polymers were poly(L-lactic acid) and poly(ortho ester). Biodegradable composites with carbon or inorganic fibers generally lost strength rapidly, with a slower loss of stiffness, suggesting the difficulty of fiber-matrix coupling in these system. The strength of composites reinforced wiht (lower modulus) degradable polymeric fibers decreased more slowly.Low implant stiffness might be expected to allow too much bone motion for satisfactory healing. However, unreinforced or degradable polymeric fiber reinforced materials have been used successfully clinically. The key has been careful selection of applications, plus use of designs and fixation methods distinctly different from those appropriate for stainless steel devices.

Compression measurements were conducted on plastic-bonded explosive PBX 9502 and its binder, Kel-F 800™, as a function of temperature from -55 °C to +55 °C using an improved split Hopkinson pressure bar at high strain rates (~1400 s-1) and at low strain rates (~0.001 to 0.1 s-1) at ambient temperatures. PBX 9502 exhibits lower dynamic compressive strength, but is much less sensitive to strain rate and temperature, than PBX 9501. In contrast, the mechanical response of the Kel-F 800™ binder is stronger than pure (or plasticized) Estane™, but is again less strain rate and temperature dependent. The effects of longitudinal and transverse loading orientations (due to preferred orientation of TATB) and virgin versus recycled TATB on the properties of PBX 9502 are presented. .

The evolution of twinning and texture in two Mg-based (+Al, Mn, Zn) alloys was investigated using uniaxial tension, uniaxial compression and ring hoop tension testing at temperatures from ambient to 250 °C and a strain rate of 0.1 s−1. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and ring hoop tension testing. Schmid factor analysis indicates that only components that are favorably oriented (i.e., with the highest SF values) can undergo rapid and complete twinning. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves and dramatic texture changes.

We propose a mechanism by which massive ignimbrite and layered ignimbrite sequences — the latter liable to have been previously interpreted as multiple flow units-form by progressive aggradation during sustained passage of a single particulate flow. In the case of high-temperature eruptive products the mechanism simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variations in texture, including between non-welded, eutaxitic, rheomorphic (lineated) and lava-like. Agglutination can occur within the basal part of a hot density-stratified flow. During initial incursion of the flow, agglutinate chills and freezes against the ground. During sustained passage of the flow, agglutination continues so that the non-particulate (agglutinate) layer thickens (aggrades) and becomes mobile, susceptible to both gravity-induced motion and traction-shear imparted by the overriding particulate part of the flow. The particulate to non-particulate (P-NP) transition occurs in and just beneath a depositional boundary layer, where disruptive collisions of hot viscous droplets give way, via sticky grain interactions, to fluidal behavior following adhesion. Because they have different rheologies, the particulate and non-particulate flow components travel at different velocities and respond to topography in different ways. This may cause detachment and formation of two independent flows. The P-NP transition is controlled by factors that influence the rheological properties of individual erupted particles (strain rate, temperature, and composition including volatiles), by cooling and volatile exsolution during transport, and by the particle-size population and concentration characteristics of the depositional boundary layer. At any one location along the flow path one or more of these can change through time (unsteady flow). Thus the P-NP transition can develop momentarily or repeatedly during the passage of an unsteady flow, or it can occur continuously during the passage of a quasi-steady flow supplied by a sustained explosive eruption. Vertical facies successions developed in the deposit (high-grade ignimbrite) reflect temporal changes in flow steadiness and in material supplied at source. The P-NP transition is also influenced by factors that affect flow behaviour, such as topography. It may occur at any location laterally between a proximal site of deflation (e.g. a fountain-fed lava) and a flow's distal limit, but it most commonly occurs throughout a considerable length of the flow path. Up-sequence variations in welding-deformation fabric (between oblate uniaxial to triaxial and prolate) reflect evolving characteristics of the depositional boundary layer (e.g. fluctuations from direct suspension-sedimentation to deposition via traction carpets or traction plugs), as well as possible modifications resulting from subsequent, post-depositional hot loading and slumping. Similar processes can also account for lateral lithofacies gradations in conduits and vents filled with welded tuff. Our consideration of high-grade ignimbrites has implications for ignimbrite emplacement in general, and draws attention to the limitations of the widely accepted models of emplacement involving mainly high-concentration non-turbulent transport and en masse ‘freezing’ of high-yield-strength plug flows.