Analysis of the Transformation-induced Plasticity Effect during the Dynamic Deformation of High-manganese Steel

The transformation-induced plasticity(TRIP) effect and resistance characteristics to adiabatic shear failure at high strain rates of high-manganese steel were investigated by using scanning electron microscopy and electron backscattering diffraction. Results showed that the high-manganese steel exhibited excellent strain hardening effect and resistance to adiabatic shear failure because of the TRIP effect. The TRIP effect occurred during dynamic deformation and showed two distinct stages,namely,the smooth TRIP process before the formation of adiabatic shear band(ASB) and the inhibited TRIP process during further deformation. In the first stage,the martensitic transformation showed slight orientation dependence and weak variant selection,which promoted the TRIP effect. In the second stage,reverse martensitic transformation occurred. Adiabatic shear bands(ASBs) developed typical shear microtextures {111}<110>. In microtextures,two groups of fine grains are in a twin relationship and uniform distribution,which restrained the formation of holes and cracks within the ASBs and enhanced damage resistance after ASB formation.     

Thermo-mechanical aspects of adiabatic shear failure of AM50 and Ti6Al4V alloys

The thermo-mechanical aspects of adiabatic shear band (ASB) formation are studied for two commercial alloys: Mg AM50 and Ti6Al4V. Tests are carried out on shear compression specimens (SCS). The evolution of the temperature in the deforming gauge section is monitored in real-time, using an array of high-speed infrared detectors synchronized with a Kolsky apparatus (split Hopkinson pressure bar). The evolution of the gage temperature is found to comprise three basic stages, in agreement with Marchand and Duffy’s simultaneous observations of mechanical data and gauge deformation patterns (1988). The onset and full formation stages of ASB are identified by combining the collected thermal and mechanical data. Full development of the ASB is identified as the point at which the measured and calculated temperature curves intersect and diverge thereon. At that stage, the homogeneous strain assumption used in calculating the maximum temperature rise is no longer valid.     

Microstructural characterization and evolution mechanism of adiabatic shear band in a near beta-Ti alloy

The adiabatic shear band (ASB) was obtained by split Hopkinson pressure bar (SHPB) technique in the hat-shaped specimen of a near beta-Ti alloy. The microstructure and the phase transformation within the ASB were investigated by means of TEM. The results show that the elongated subgrains with the width of 0.2-0.4 μm have been observed in the shear band boundary, while the microstructure inside the ASB consists of fine equiaxed subgrains that are three orders of magnitude smaller than the grains in the matrix. The β → ω_(althermal) phase transformation has been observed in the ASB, and further analysis indicates that the shear band offers thermodynamic and kinetic conditions for the ω_(althermal) phase formation and the high alloying of this alloy is another essential factor for this transformation to take place. The thermo-mechanical history during the shear localization is calculated. The rotational dynamic recrystallization (RDR) mechanism is used to explain the microstructure evolution mechanism in the shear band. Kinetic calculations indicate that the recrystallized fine subgrains are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

The use of hat-shaped specimens to study the high strain rate shear behaviour of Ti–6Al–4V

To study the high strain rate shear behaviour of Ti–6Al–4V, hat-shaped specimens have been used in a compression split Hopkinson bar set-up. With this technique, highly concentrated shear strains are obtained which eventually cause strain localization and adiabatic shear bands (ASB). Because of the complex stress distribution in the specimen, interpretation of the experimental results is not straightforward. In this paper, results of a comprehensive experimental and numerical study are presented, aiming at a more judicious use of hat-shaped specimens and a fundamental understanding of the obtained results. Specimens with different dimensions are considered. It is found that the width of the shear region and the radius of the corners are the most important parameters. The first mainly affects the homogeneity of stresses and deformations in the shear zone and the presence of a hydrostatic stress next to the shear stress, while the latter primarily governs the initiation of the ASB. The relation between the global measured response and the local material behaviour is studied. It is shown that, within certain limits, the shear stress in the shear region can be extracted from the measured force. Several experiments which have been interrupted at a certain level of deformation have been carried out. The microstructure could thus be observed at different stages: onset of strain localization, formation of ASBs, initiation and propagation of micro-cracks.     

Young’s modulus of low-pressure cold sprayed composites: an analysis based on a minimum contact area model

A theoretical and mathematical model based on minimum contact area (MCA) is developed to explain the bonding that takes place in the low-pressure gas dynamic spray (LPGDS) process. It is shown that by normalizing this MCA it is possible to compare the relative elastic modulus as a function of porosity. Theoretical predictions of relative elastic modulus are compared against results obtained through acoustic analysis and it is found that the correlation between is dependent on the porosity. For low porosity, the experimental and theoretical results differ substantially, while for higher porosity there seems to be good agreement between the two. To explain this behaviour it is theorized that full adiabatic shear bands (ASB) are created between only some of the particles. The higher porosity causes higher strain in the samples and thus more local deformation of the particles. This, in turn, causes more actual ASB formation. Since the theoretical model assumes full ASB formation, only the higher porosities cause enough strain to have a comparable relative elastic modulus. For the lower porosities, the local strain is less, and some of the bonds will not achieve full ASB formation. For these cases, the relative elastic modulus will be lower than that predicted.     

Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s⁻¹ and quasi-static compressive loading at a strain rate of 0.0032 s⁻¹. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.     

An experimental study on the energy absorption capacity of thin-walled castings

The energy absorption potential of high-pressure die cast (HPDC) components made of magnesium alloys AM20, AM50, AM60, AZ91 and the aluminium alloy A356 is investigated using a shear–bolt principle. Both quasi-static and dynamic tests have been performed. In addition, single cast plates of AM60 and A356 alloy with different thickness have been tested in order to investigate the effect of plate thickness on the shear–bolt mechanism. It is found that this deformation principle gives an approximately constant average force during the deformation process. Therefore, thin-walled HPDC components can be suitable as energy absorbing components when using the shear–bolt principle. A simple empirical model for prediction of the average shearing force as a function of plate thickness and bolt diameter is proposed.     

Mechanical behavior of bulk nanocrystalline copper alloys produced by high energy ball milling

Copper alloys with different amounts of zinc were synthesized via high energy ball milling at liquid nitrogen and room temperature. Bulk samples were produced in situ by controlling the milling temperature. It is shown that temperature plays an important role in formation of artifact-free consolidated samples via its effect on defect formation and annihilation during the milling process. The mechanical behavior of Cu–Zn nanocrystalline alloys was examined using Vickers microhardness and tensile tests. The nanostructure of the alloys was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hardness results of processed alloys vary as a function of the alloying elements. Considering typical low ductility of nanocrystalline materials, the improved ductility with the high strength observed in these alloys suggests that they are artifact-free and may have several deformation mechanisms, which may include dislocation activity and nano-twinning.     

Deformation and fracture in Al–Li-base alloys

Abstract

Aluminium–lithium-base alloys are of considerable interest because of their low density and high modulus. However, they have been shown to have low ductility and poor fracture toughness. This has been attributed to a variety of factors, including intense shear band formation, segregation to grain boundaries, and weakened grain boundaries due to precipitation and precipitate-free zones. The authors have investigated the deformation structures observed in binary and more complex commercial alloys. As would be expected, considering the microstructure of the alloys, extensive strain localization and shear band formation occurs in these alloys. However, it is shown that the commercial alloys are less sensitive to strain localization than the model binary alloy systems investigated. The stresss–train behaviour has been investigated. The alloys exhibit jerky flow, which is indicative of negative strain rate sensitivity, and strain rate change tests showed this to be the case. This is consistent with the deformation structures observed. The effect of weakened grain boundaries due to precipitation and precipitate-free zones has been studied by comparing the fracture characteristics of aged and unaged material. It is shown that the mode of failure is identical under appropriate conditions. It is concluded that segregation to grain boundaries is the major cause of the lower ductility and toughness of Al–Li alloys. This possibility has been investigated using in situ fracture surface analysis techniques. Results are presented on grain boundary segregation, and methods of reducing its influence on fracture behaviour are indicated.

MST/570     

Nonlinear dynamic response and vibration of imperfect shear deformable functionally graded plates subjected to blast and thermal loads

Based on Reddy's higher-order shear deformation plate theory, this article presents an analysis of the nonlinear dynamic response and vibration of imperfect functionally graded material (FGM) thick plates subjected to blast and thermal loads resting on elastic foundations. The material properties are assumed to be temperature-dependent and graded in the thickness direction according to a simple power-law distribution in terms of the volume fractions of the constituents. Numerical results for the dynamic response and vibration of the FGM plates with two cases of boundary conditions are obtained by the Galerkin method and fourth-order Runge–Kutta method. The results show the effects of geometrical parameters, material properties, imperfections, temperature increment, elastic foundations, and boundary conditions on the nonlinear dynamic response and vibration of FGM plates.     

Effect of imperfections in the bond on the strength of FRP wrapped cylindrical concrete columns

Effect of imperfections at the interface between concrete and FRP on the strength of FRP confined axially loaded cylindrical concrete columns is investigated, experimentally and numerically. It is seen that the presence of imperfections facilitates localization of deformation, adversely affects the confining capacity of FRP, and reduces the failure load. The influence of size, location and orientation of imperfection on failure load is studied: the orientation and location are found to be more important than size. Critical locations and orientations of the imperfection are found and explained in terms of the mechanics of shear banding in pressure-sensitive elasto-plastic materials.     

A simplified beam analysis of the end notched flexure mode II delamination specimen

A modified classical beam theory solution is developed for the end notched flexure (ENF) specimen, one of the most widely used mode II delamination tests for fibre reinforced composite materials. The effect of crack tip deformation is analyzed by assuming that a region of certain length close to the crack tip rests on an elastic shear spring foundation. The mathematical procedure of the present analysis is simple and clear, and the resulting solutions for the compliance and the strain energy release rate of the ENF specimen are highly accurate. Excellent agreement is obtained over a wide range of material and geometrical properties of ENF specimens when the current results are compared with finite element results and other analytical analyses that include the effect of crack tip deformation in their solutions. The success of the present analysis indicates that the effect of crack tip deformation is the most important factor that must be considered when calculating the relationship between G_II and the load in the ENF specimen.     

Experimental Study on Shear Localisation in Granular Materials Within Combined Strain and Stress Field

Abstract: The characteristic features of the shear zones formation in deforming granular materials were investigated using particle image velocimetry (PIV), which was combined with a photo‐elastic study of the stress field. Laboratory tests were performed for an active translation of rigid retaining wall. PIV is an optical technique for measuring displacement fields from successive digital images and was employed to analyse experiments on two different granular materials, composed of (1) sand grains and (2) glass granules. The tests on glass granules were supplemented by taking photo‐elastic images in circularly polarised light to gather information on changes in the average stress field, accompanying the specimen deformation. Attention was focused on the effect of the initial density, grain coarseness and magnitude of wall displacement on shear localisation within a strain field and its geometrical relation to some structures found in the stress field.     

装甲用镁合金抗弹性能表征体系探讨

介绍了高应变率载荷条件下镁合金的吸能特性及变形特征；论述了对镁合金抗弹性能有重要影响的动态强度、高应变率能量吸收率、高应变率变形断裂特征和动态强度等科学问题；就镁合金在装甲领域的应用研究做了初步探讨。     

Observations of common microstructural issues associated with dynamic deformation phenomena: Twins, microbands, grain size effects, shear bands, and dynamic recrystallization

Plane-wave shock deformation has been shown to produce deformation twins or twin-faults in essentially all metal and alloys. In FCC metals and alloys twinning depends upon stacking-fault free energy (SFE) and a critical twinning pressure; which increases with increasing SFE. For impact cratering where the shock wave is spherical and a prominent deviatoric (shear) stress is involved, metals and alloys with high SFE form microbands coincident with {111} plane traces while low SFE metals and alloys either form mixtures of twins and microbands or microtwins. Oblique shock loading of copper also produces mixtures of twins and microbands. Both microtwins and microbands increase in volume fraction with increasing grain size. BCC iron is observed to twin in both shock loading and as a result of impact cratering. Impact craters, shaped charges, and other examples of extreme deformation and flow at high strain rates exhibit various regimes of shear bands and dynamic recrystallization as a mechanism for solid-state flow. Deformation twins and microbands are also often precursors to this process as well. Examples of these phenomena in FCC materials such as Al, Ni, Cu, stainless steel and brass, and BCC materials such as Fe, W, Mo, W-Ta, and Ta are presented; with emphasis on optical metallography and transmission electron microscopy.     

镁合金高速冲击载荷下的变形行为研究进展

概述了国内外对镁合金在高应变率加载条件下的力学行为及其高速变形时组织中绝热剪切带形成机制的研究现状,归纳了在镁合金动态力学行为及微观变形机制方面以及镁合金中形成绝热剪切带带内组织成因研究方面存在的不足。基于此,提出了镁合金动态力学行为及其在动态变形过程中微观组织演变机制的研究重点,并指出了进一步尚需开展的工作。     

Factors affecting dynamic recrystallization of metals and alloys

Abstract

The high-temperature mechanical behaviour of copper, Cu–Al alloys, and nickel has been examined using torsional testing with hollow testpieces in conjunction with microstructural observations on deformed and quenched specimens using both optical and electron microscopy. Dynamic recrystallization occurred in these materials as the restoration process during high-temperature deformation. The factors influencing dynamic recrystallization have been considered, including materials of high stacking fault energy. It was found that the regime of dynamic recrystallization and the transition in flow stress behaviour could be reasonably represented in terms of the Zener–Hollomon parameter. In Cu–Al solid solution alloys, although the addition of the solute aluminium into copper lowered the stacking fault energy, dynamic recrystallization was retarded to higher strains due to the reduced mobility of the grain boundary. By mechanical and microstructural analysis of the behaviour of various single phase metals and alloys during dynamic recrystallization, the factors influencing the behaviour (i.e. stacking fault energy (solute elements), Zener–Hollomon parameter (deformation condition), and strain) can be summarized on a three dimensional schematic.

MST/587     

Crazing and shear deformation in glass bead-filled glassy polymers

The competition between craze formation and shear band formation at small glass beads embedded in matrices of glassy polymers has been investigated. This has been done by performing constant strain rate tensile tests over a wide range of strain rates and temperatures, and examining the deformation pattern formed at the beads with a light microscope. The glassy polymers under investigation were polystyrene, polycarbonate, and two types of styrene—acrylonitrile copolymer. It was found that besides matrix properties, strain rate and temperature, the degree of interfacial adhesion between the glass beads and the matrix also has a profound effect on the competition between craze and shear band formation: at excellently adhering beads craze formation is favoured, whereas at poorly adhering beads shear band formation is favoured. This effect is caused by the difference in local stress situation, craze formation being favoured under a triaxial stress state and shear band formation under a biaxial stress state. The kinetics of crazing and shear deformation have also been studied, using a simple model and Eyring's rate theory of plastic deformation. The results suggest that chain scission may be the rate-determining step in crazing but not in shear deformation.     

Relation between shear banding and penetration characteristics of conventional tungsten alloys

The dynamic compression failure and ballistic penetration characteristics of conventional tungsten alloys similar in strength were investigated. Dynamic compression failure properties were generated with a symmetric Taylor test technique and penetration characteristics were obtained with 44 mm kinetic penetrators against an 300 HB hardness steel target at 1400 m/s. From shear crack length data generated with Taylor specimens impacted at different impact speeds a critical speed characterizing shear band initiation was deduced. The critical equivalent plastic strain at shear band initiation sites, obtained from the numerical simulation of the Taylor test at the critical impact speed, was found to decrease with the increase of the penetration performance. These results reinforce the argument that shear band formation is a failure mechanism associated with the erosion process for conventional tungsten alloys.