Adiabatic shear band in a Ti-3Al-5Mo-4.5V Titanium alloy

An investigation has been carried out on the adiabatic shear band (ASB) in a Ti-3Al-5Mo-4.5V (TC16) alloy deformed at high strain rate by a split Hopkinson pressure bar (SHPB). ASB in TC16 alloy is a “white” band with a width of about 13 μm. Microhardness of the ASB is larger than that of the matrix. The elongated cell structures of width about 0.2–0.5 μm with thick dislocation exist in the boundary of the shear band. Results suggest that the fine equiaxed grains with α-phase and α″-phase coexist in the shear band. The “white” band is a transformation band. Calculation of the adiabatic temperature rise indicates that the maximum temperature within ASB is about 1,069 K that is above the phase transformation temperature. Finally, formation of an ASB in the TC16 alloy and its microstructure evolution are described.     

Geometrical imperfection and adiabatic shear banding

This work addresses the effect of small geometrical imperfections on adiabatic shear band (ASB) formation. The separate effect of the length and radius of short notches is systematically investigated in AM50 and Ti6Al4V alloys, using shear compression specimens. It is observed that the length of the imperfection does not influence ASB formation in these experiments. By contrast, the notch-root radius appears to be the dominant parameter for the two materials, in perfect agreement with the analytical predictions of Dinzart et al. [The catastrophic development of shear localization in thermoviscoplastic materials. J Phys 1994; IV(C8): 435–40]. The distribution of deformation energy over the gauge length is modeled numerically. The calculated average dynamic deformation energy levels are quite similar to those that are measured for the two investigated alloys. It is concluded that the global measure of the dynamic deformation energy provides valuable information about ASB failure from geometrical imperfections.     

Effect of heat treatment on ballistic impact behavior of Ti–6Al–4V against 7.62 mm deformable projectile

The present study describes the effect of heat treatment on mechanical properties and ballistic impact resistance of Ti–6Al–4V alloy against 7.62 mm deformable lead projectiles. As-received plates were solution treated above and below the β-transus temperature followed by aging. The plates that were solution treated below the β-transus temperature followed by aging exhibited good combination of strength and ductility, and better ballistic impact resistance. Post ballistic microstructural examination showed formation of adiabatic shear bands (ASBs) and adiabatic shear band induced cracks. Both as-received plates and the plates that were solution treated above β-transus temperature followed by aging showed higher number of ASBs and ASB induced cracks compared to plates that were solution treated below β-transus temperature and then aged. Plug formation was observed through the ASB induced shear localization in planes parallel to the direction of projectile impact in all the conditions.     

Microstructural study of adiabatic shear bands formed in serrated chips during high-speed machining of hardened steel

The characterization of the microstructure and phase transformation in adiabatic shear bands (ASB_s) within the serrated chips generated during high-speed machining of hardened 30CrNi₃MoV steel has been performed. The observations showed that the microstructure gradually changes from the center of the ASB to the matrix of the chip, the fine equiaxed grains appear with size of about 0.4–0.6 μm in the center of the ASB_s, the transitional region adjacent to the ASB is characterized by the broken and elongated martensite laths in shear direction. The analysis indicated that the serrated chip formation was likely due to adiabatic shear instability that occurred in the primary shear zones and the transformation to martensite within the ASB. Dynamic recovery and recrystallization are the dominant metallurgical processes during microstructural evolution of ASB.     

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.     

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.     

30MnCrNiMoB钢绝热剪切与动态再结晶

计算了30MnCrNiMoB用正交切削形成绝热剪切带（ASB）的应变和温升，结果表明随着切削速度的增加ASB内应变增大而温度升高．电镜观察发现切削速度小于1．123m·s-1时，ASB内组织严重碎化，而切削速度为1．57m·s－1时，ASB组织为再结晶组织表明在一定条件下绝热剪切过程伴随着动态再结晶，ASB中碳化物起促进再结晶作用．     

Energy-based variational modeling of adiabatic shear bands structure evolution

A novel energy-based variational approach is proposed for modeling adiabatic shear band (ASB) structure evolution, including elasticity, work hardening, and heat conduction. Conservation laws are formulated as a mathematical optimization problem with respect to a limited set of scalars. Consequently, by means of canonical expressions of displacement and temperature, the bandwidth and the central temperature can be accurately computed as internal variables. Based on this thermo-mechanical coupled variational framework, we can verify the generality of the proposed analytical approach with respect to constitutive models, as illustrated through various thermal softening laws such as power laws or the popular Johnson–Cook model. In addition, accounting for work hardening and elasticity, we propose an effective (or macroscopic) thermo-elasto-viscoplastic model of the shear localization zone in transient regime. A new loading/unloading condition, stemming as a Kuhn–Tucker relation, is introduced for this variational model. The stress evolution and the capacity of the approach to handle cyclic loading are analyzed, presenting a very good correspondence with ASB simulations by finite element method.     

A Workability Criterion for the Transformed Adiabatic Shear Band Phenomena during Cold Heading of 1038 Steel

A criterion for predicting the workability limits for internal britde failure was developed for cold heading of 1038 steel. The criterion considers internal defects caused by microstructural changes generated by adiabatic shear. This transformation is termed the transformed adiabatic shear band (TASB) phenomena. The defect that develops is the formation ofbritde martensite as a result ofthe temperature rise and fall inside the adiabatic shear band (ASB). In this work, the material is considered to have a TASB defect when the temperature inside the ASB exceeds the phase transformation temperature (A_C3). The empirical formulas provided by Andrews^[1] were used to determine transformation temperatures. Microhardness testing and etching with 2% Nital and Le Pera etchants were performed on the sectioned specimens to locate and study the TASB. In order to simulate the cold heading process, a drop weight compression test was used and modeled with finite-element analysis (implemented within ABAQUS/Explicit).     

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.     

Numerical and experimental investigations in electromagnetic riveting with different rivet dies

In this work, the numerical simulations and electromagnetic riveting (EMR) experiments were conducted to investigate microstructure evolution and the forming mechanism of adiabatic shear bands (ASBs). And the effects of rivet dies on microstructure distributions in formed heads and mechanical properties of riveted structures were systematically explored. The impact velocity and deformation distribution results demonstrated that the proposed numerical method was accurate and reliable. The simulation results showed the slope angle of rivet dies notably affected the plastic flow of materials, and then determined the microstructure distribution in formed heads. The combined effects of inhomogeneous plastic flow and thermal softening were accounted for the forming of ASBs. The formed heads had two obvious ASBs (upper and lower ASB) for the 40° rivet die and flat rivet die. The formed heads only had the lower ASB and no clear upper for the 60° rivet die and 80° rivet die. The pull-out test results showed that the specific rivet die could improve the mechanical properties of the EMR joints, which contribute to the engineering applications of EMR riveted structures.     

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.     

Filling behavior,morphology evolution and crystallization behavior of microinjection molded poly(lactic acid)/hydroxyapatite nanocomposites

In this paper, the filling behavior, morphology evolution, crystallization behavior, thermal stability and mechanical property of poly(lactic acid) (PLA)/hydroxyapatite (HA) nanocomposite under microinjection molding conditions were systematically investigated. The comparison between micropart and macropart of PLA/HA nanocomposite was also conducted. Results showed that in the four stages occurring in the microinjection molding process, the mold cavity filling stage is an extremely rapid process and injection speed influences the filling behavior much more significantly than mold temperature. The remarkably enhanced shear force field generated under microinjection molding conditions proves to be beneficial to formation of highly oriented PLA matrix self-fibrillating structure, improvement of HA filler dispersion and enhancement of interfacial combination. Formation of such a highly oriented structure could lead to the remarkable difference in both crystallization behavior and mechanical property between micropart and macropart. The PLA/HA nanocomposite micropart possessed a significantly enhanced mechanical property and showed a good application prospect.     

Adiabatic Shear Bands in 30CrNi3MoV Structural Steel Induced during High Speed Cutting

The width and spacing of adiabatic shear bands (ASBs) in the serrated chips generated during high speed orthogonal cutting of 30CrNi3MoV structurai steel were measured by opticai microscopy (OM), the temperature rise in the shear band was estimated. The microstructures of the ASBs were also characterized by SEM and TEM. The results show that the width and spacing of ASBs decrease with the increase of the cutting speed. The further observations show that the microstructure between the matrix and the center of the ASB gradually changes, and that the martensitic phase transformation, carbide precipitation and recrystallization may occur in the ASB.     

Adiabatic Shear Bands in 30CrNi¬3MoV Structural Steel Induced during High Speed Cutting

The width and spacing of adiabatic shear bands (ASBs) in the serrated chips generated during high speed orthogonal cutting of 30CrNi¬3MoV structural steel were measured by optical microscopy (OM), the temperature rise in the shear band was estimated. The microstructures of the ASBs were also characterized by SEM and TEM. The results show that the width and spacing of ASBs decrease with the increase of the cutting speed. The further observations show that the microstructure between the matrix and the center of the ASB gradually changes, and that the martensitic phase transformation, carbide precipitation and recrystallization may occur in the ASB.     

Constitutive and damage modelling of H11 subjected to low‐cycle fatigue at high temperature

Hot‐work tool steel H11 is extensively applied in extrusion industries as extrusion tools. The understanding of its mechanical properties and damage evolution as well as failure is crucial for its implementation. In this paper, a finite element (FE) model employing Chaboche unified constitutive model and ductile damage rule is proposed to simulate the mechanical responses of H11 subjected to low‐cycle fatigue (LCF). Accumulated inelastic hysteresis energy is adopted to demonstrate the impact on damage initiation and evolution rules. A series of tension and LCF experiments were conducted to investigate H11's mechanical properties and its deterioration processes. In addition, to deeply understand the deformation and damage mechanism, scanning electron microscope (SEM) investigations were performed on the fracture section of gauge‐length part of the specimen after failure. Furthermore, the parameters in both constitutive model and damage rule are identified based on experimental data. The comparison of the hysteresis loop of the first cycle and stable cycle with different strain amplitudes demonstrates that the Chaboche constitutive model provides high precision to predict the evolution of mechanical properties. Based on the reliable achieved constitutive model, LCF behaviour prediction with damage rule was executed successfully using FE model and gains a good agreement with the experiments. It is believed that the proposed FE method lays the foundation of structure analysis and rapid design optimization in further applications.     

Determination of mechanical properties of PVC foam using a modified Arcan fixture

The design and development of a modified Arcan fixture (MAF) is described. The purpose of the fixture is to characterise polymer foam materials with respect to their tensile, compressive, shear and bidirectional mechanical properties. The MAF enables the application of pure compression or high compression to shear bidirectional loading conditions that is not possible with conventional Arcan fixtures. The tensile and shear behaviour to failure of a cross-linked Divinycell H100 PVC foam core material are studied using Digital Image Correlation (DIC). A detailed investigation of the parasitic effects of the fixture and misalignment of the fixture and loading machine are discussed. Thermoelastic Stress Analysis (TSA) is used to directly examine and validate the uniformity and symmetry of the stress fields obtained for both tensile and shear specimens. To account for the inhomogeneity of the strain field across the specimen cross sections, a “correction factor” for the measured “gauge section” surface strains has been determined using nonlinear finite element analysis (FEA). The outcome is a set of validated mechanical properties that are in excellent agreement with material property measurements conducted using conventional test tensile and shear test fixtures.     

Investigation on high temperature mechanical fatigue failure behavior of SnAgCu/Cu solder joint

In this paper, high temperature mechanical fatigue tests on SnAgCu/Cu solder joints were carried out under three test temperatures (100, 125, 150 °C). Failure mechanism was analyzed through observation of micro-crack evolution and fracture morphology. The results show that the deformation curve of solder joint under high temperature mechanical fatigue tests can be divided into three stages: strain hardening stage, stable deformation stage and accelerated failure stage, which is similar to the curve under creep test condition. In addition, the cyclic life decreases rapidly with increasing temperature. Deformation field in the solder joint is non-uniform and shear strain concentration occurs in solder close to the intermetallic compound (IMC) layer. Micro-crack initiates at the corner of the solder joint and then tend to propagate along interface between Cu substrate and solder. The fracture morphology under three temperatures all exhibits ductile fracture mode and the failure path transforms from cutting through the top of Cu₆Sn₅ to propagation in solder matrix close to IMC layer with increasing temperature.     

Martensite and Reverse Transformation during Complete Cycle of Simple Shear of NiTi Shape Memory Alloy

Abstract:  Investigation into the quasi-static simple shear of shape memory alloy (SMA) was carried out. A special grip was designed which allowed replacing compression into the shear process on testing machine. At the same time the shear zone temperature was observed by an infrared camera. The mechanical and thermal characteristics of sheet specimens of NiTi subjected to superelastic shear deformation due to reversible stress-induced phase transformation were clarified. By comparison of the data, the processes of martensite and reverse transformations during the shear deformation were analysed. The investigation shows that stages of the phase transformations during the simple shear process are similar like those observed during tension test. Moreover, during the simple shear deformation no uniform temperature distributions were noticed, especially at higher shear rate, manifesting that the stress-induced phase transformation during the shear process is also inhomogeneous. The thermomechanical properties of the NiTi SMA for various shear rates were investigated. It was found that an increase in the strain rate results in an increase in temperature variation, the shear stresses and the magnitude of hysteresis loops between the forward and reverse transformations.     

Dynamic shear response and evolution mechanisms of adiabatic shear band in an ultrafine-grained austenite–ferrite duplex steel

The dynamic properties of an intercritically annealed 0.2C5Mn steel with ultrafine-grained austenite–ferrite duplex structure were studied under dynamic shear loading. The formation and evolution mechanisms of adiabatic shear band in this steel were then investigated using interrupted experiments at five different shear displacements and the subsequent microstructure observations. The dynamic shear plastic deformation of the 0.2C5Mn steel was observed to have three stages: the strong linear hardening stage followed by the plateau stage, and then the strain softening stage associated with the evolution of adiabatic shear band. High impact shear toughness was found in this 0.2C5Mn steel, which is due to the following two aspects: the strong linear strain hardening by martensite transformation at the first stage, and the suppressing for the formation of shear band by the continuous deformation in different phases through the proper stress and strain partitioning at the plateau stage. The evolution of adiabatic shear band was found to be a two-stage process, namely an initiation stage followed by a thickening stage. The shear band consists of two regions at the thickening stage: a core region and two transition layers. When the adjoining matrix is localized into the transition layers, the grains are refined along with increasing fraction of austenite phase by inverse transformation. However, when the transition layers are transformed into the core region, the fraction of austenite phase is decreased and almost disappeared due to martensite transformation again. These interesting observations in the core region and the transition layers should be attributed to the competitions of the microstructure evolutions associated with the non-uniformly distributed shear deformation and the inhomogeneous adiabatic temperature rise in the different region of shear band. The 0.2C5Mn TRIP steel reported here can be considered as an excellent candidate for energy absorbers in the automotive industry.