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The deformation behavior of hot-roll-bonded Mg/Al laminated metal composites (LMCs) is investigated, focusing on the crack formation around the interface. The material demonstrates a positive strain rate sensitivity mainly arising from the Mg matrix. A diffusion layer was formed at the Mg/Al interface with a serrated morphology during the two-pass rolling process, which consists of the Al3Mg2 and Mg17Al12 phases. The mechanical responses of the LMCs are evaluated using uniaxial tension, interfacial debonding tests (in normal and shear modes), and microstructure characterization techniques, which clarify the interfacial heterogeneity. The fracture constants of the interface were calibrated and incorporated in the finite element models to predict the failure modes of heterogeneous Mg/Al interface under three-point bending. With the increased thickness ratio of the Mg layer, the damage accumulation rate decreases at the interface and increases in the Mg matrix. As a result, the failure mode transferred from interface failure to matrix fracture. The findings of the present work provide new insight into the failure mechanism in laminated metal composites under bending conditions and can provide theoretical guidance for the plastic forming of such materials.

In this paper, a new resistance rivet welding process (RRW) is used to achieve the joining of Mg alloy and steel. A rivet is utilized to pierce the upper Mg plate during the welding process and then welded to the lower steel plate. It realizes a metallurgical bond between the rivet and the steel plate, and a hybrid rivet-welded joining between the steel plate and the Mg plate. The joint morphology, forming mechanism, microstructure, and mechanical performance of RRW joints were investigated. And the role of welding current variations on the failure mode and mechanical properties of the joint was studied. The results showed that at welding currents between 11 kA and 13 kA, the main bearing area of the joint was primarily the Mg plate base material. As the welding current increased, the bonding location between the Mg refilled into the rivet cavity and the Mg plate could also be subject to more load, significantly affecting the mechanical properties and failure mode of the joint. The RRW joints achieved a tensile-shear load of 6.08 kN and an energy absorption value of 8.95 J at the optimum parameters.

Among recently developed joining technologies, resistance element welding (REW) is an effective process for joining hybrid materials. In this study, Al alloy/Q235 steel joints were produced via a flat REW (FREW) technique. The FREW joint was lighter than the traditional REW joint, and no bulges existed on the surface of the specimen. The process was performed using two methods with different positions of sheets and rivets: upward FREW (UFREW) and downward FREW (DFREW). The microstructure and tensile shear performance of the joints obtained via FREW were investigated. The maximum peak tensile shear loads of the UFREW and DFREW joints were 5537 N and 5656 N, respectively; and their maximum energy absorption values were 20.98 J and 16.55 J, respectively. The failure mode of the FREW joints changed from interface failure, to pull-out, and then to base material pull-out modes with an increase in welding current. [ABSTRACT FROM AUTHOR]

• An extrapolation method is proposed to assess the DFT of the ideal cracks. • The K plane is used to illustrate the comprehensive sensitivity of the DFT to the two factors. • The influence of notch width on DFT is more pronounced than that of loading rate. • The propagation of cracks is governed by interactions between dynamic shear fracture and ASB. Existing work on size effects in the measurement of dynamic fracture toughness (DFT) has focused mainly on the overall geometric dimensions of the test specimens, while the effects of notch width and loading rate were not explored. In this paper, DFT experiments were conducted using Ti-6Al-4V alloy to study the effects of notch width effect and loading rate effect on its dynamic mode II fracture toughness. An extrapolation method is proposed to estimate the DFT of an ideal sharp crack, and the relationship between DFT, loading rate, and notch width is captured with a 3D spatial plane. Results revealed that the DFT decreases with a reduction of the notch width and increases with loading rate. The effects of notch width on DFT will be shown to be more sensitive compared to loading rate. The failure mode transitional behavior from ductile fracture to adiabatic shearing will be revealed through fractographic analysis of the microstructural evolution, which will elucidate the effects of loading rate on the failure mechanism of the material. [ABSTRACT FROM AUTHOR]

A non-equilibrium thermodynamics model of viscoplasticity coupled with damage is presented. Keeping in view the experimentally observed failure mode transitions, e.g. brittle to ductile, under dynamic loading condition, the emphasis of the present work has been to endow the formulation with capability of modelling such transitions in a physically consistent manner. Within a framework of internal variables, the current formulation tracks the effect of isotropic viscoplasticity with accumulated plastic strain, and a scalar phase field variable traces the degradation of the material caused by either tensile cracking or shear induced failure. An explicit-implicit strategy is adopted for numerical implementation of the nonlinear formulation. A series of numerical simulations has been carried out on notched metallic specimen to demonstrate the predictive ability of the proposed model and to investigate the influence of several parameters in governing failure mode transition. [ABSTRACT FROM AUTHOR]

This paper presents the extension of some finite elements with embedded strong discontinuities to the fully transient range with the focus on dynamic fracture. Cracks and shear bands are modeled in this setting as discontinuities of the displacement field, the so-called strong discontinuities, propagating through the continuum. These discontinuities are embedded into the finite elements through the proper enhancement of the discrete strain field of the element. General elements, like displacement or assumed strain based elements, can be considered in this framework, capturing sharply the kinematics of the discontinuity for all these cases. The local character of the enhancement (local in the sense of defined at the element level, independently for each element) allows the static condensation of the different local parameters considered in the definition of the displacement jumps. All these features lead to an efficient formulation for the modeling of fracture in solids, very easily incorporated in an existing general finite element code due to its modularity. We investigate in this paper the use of this finite element formulation for the special challenges that the dynamic range leads to. Specifically, we consider the modeling of failure mode transitions in ductile materials and crack branching in brittle solids. To illustrate the performance of the proposed formulation, we present a series of numerical simulations of these cases with detailed comparisons with experimental and other numerical results reported in the literature. We conclude that these finite element methods handle well these dynamic problems, still maintaining the aforementioned features of computational efficiency and modularity.

This study focuses on the forming mechanism of resistance riveting welding joints of 6061 Al alloy and DP590 steel plate and the effect of adding Zn interlayer on the mechanical properties of the joints. During the piercing stage, the rivet pierces through the Al plate, the Al plate beneath the rivet melts and reacts with the steel, with intermetallic compounds (IMC) generated. Some of the melted Al and IMCs are squeezed onto the side of the rivet. In the process of adding the Zn interlayer, the Zn is gradually incorporated into the melted Al near the rivet during the piercing process. At the welding stage, as the current is increased, the nugget diameter, peak joint load, and energy absorption values gradually increase. The failure mode of the joint changes from an interfacial fracture to pull-out fracture, eventually changing to base material pull-off, as the welding current increases. Compared to joints without Zn addition, joints with Zn addition exhibit a smaller nugget diameter, and narrower IMC layers at the interface. At a welding current of 13 kA, because the failure mode was a shear fracture of the nuggets, the peak joint load of the joints with zinc added decreased by 215 N compared to the unadded ones. On the contrary, when the current increased to 14 kA, the peak joint load of the joints was elevated by 639 N, because the IMC layer and the aluminum plate became the main load-bearing locations. [ABSTRACT FROM AUTHOR]

Highlights • An approach to model failure mode switching and shear band propagation is developed. • A combined peridynamic nonlocal and classical local damage model is used. • The classical constitutive relation for viscous fluid is adapted for use in peridynamics. • The approach is validated by modeling asymmetric impact on a pre-notched plate. • Dynamic failure mode switching and shear band propagation are accurately captured. Abstract This study presents an approach to model dynamic failure mode switching and shear band propagation using the correspondence framework of state-based peridynamics. To effectively model spontaneous shear-band-to-crack switching phenomenon, which is of intrinsic complexity, a combined peridynamic nonlocal and classical local damage model for failure prediction is proposed. The concept of bond failure, often employed in the bond-based peridynamic modeling of brittle materials, is extended to the state-based peridynamics to model cleavage failure in elasto-viscoplastic materials so that the directional and progressive nature of damage can be readily handled. The classical constitutive relation for viscous fluid is adapted for use in non-ordinary state-based peridynamics to model stress collapsing state in shear bands. To show the effectiveness of the proposed approach, a comprehensive numerical study of a pre-notched plate subjected to asymmetric impact loading is conducted, including the phenomenon of shear-band-to-crack switching under an intermediate impact velocity and ductile failure under high impact velocity. A zero-energy mode suppression with an upper bound to avoid over-correction is introduced. By using the proposed modeling approach, dynamic failure mode switching and shear band propagation path are accurately captured. [ABSTRACT FROM AUTHOR]

• Mode II dynamic fracture properties of UHSS are studied by a new experimental method. • The mode II fracture toughness is positively correlated with the loading rate. • The materials show a gradual transition from tensile fracture to adiabatic shearing failure mode. • The failure of the materials under mode II dynamic fracture is dominated by different mechanism. • The rate sensitivity parameter n is proposed to reflect the loading rate effect on DFT. 40Cr and 30CrMnSiNi2A are both ultra-high-strength steel (UHSS) that is usually used in engineering structures such as aircraft landing gear, wing girder, and fastening bolts. Under dynamic loading, fracture properties of such materials are very important for structural design. A novel mode II dynamic fracture testing technique is adopted to study the mode II dynamic fracture characteristics of these two materials under high loading rates. The mode II dynamic fracture specimen is designed for the SHPB equipment, and the stress intensity factor curve at the crack tip is determined by an experimental-numerical method. The crack initiation time of the specimen is determined by the strain gage method. In the end, the mode II dynamic fracture toughness (K IId) of the two materials is obtained, and the loading rate effects are compared and analyzed in detail. The results show that within the loading rate range of this research (1.08 ∼ 7.73 TPa·m1/2/s), the K IId of the two materials has a positive correlation to the loading rate. Both of the materials exhibit a gradual transition process from tensile failure to adiabatic shearing failure mode. According to the fracture morphologies of the two materials, the failure mechanism of the two materials is analyzed for different failure modes. Failure mode transition (FMT) is specially investigated with the increase of the loading rate for both of the materials. [ABSTRACT FROM AUTHOR]

The results of a series of experimental bending tests carried out on reinforced concrete members without transverse reinforcement are reported and illustrated as a contribution to the study of the flexural to shear to crushing failure mode transition. A total of 45 beams were tested to investigate the effects of a wide range of longitudinal reinforcement amounts – 0.25% to 3.39% –, beam depths – 5 to 80 cm – and beam slendernesses – 3 to 24. The analysis of the experimental data encompasses the identification of the overall mechanical response, the assessment of the ultimate load and the detection of the critical crack that leads to the final collapse of the beam. The results are interpreted with reference to a global failure mode transition scheme, which allows to analyze all the failure modes within the same framework. As a result, an effective and synthetic prediction of the failure mode transition is achieved on the basis of dimensionless numbers, namely N P , N C and λ l , defined in the framework of linear elastic fracture mechanics. Besides, strengths and weaknesses of the provisions of some codes of practice are highlighted on the basis of the experimental results. In this context, some drawbacks are reported for what concerns the inclusion of the size effect in the models provided to assess the shear capacity of reinforced concrete beams. • Fracture mechanics is effective in assessing the shear capacity of concrete members. • A transition from flexural to shear to crushing failure mode is observed. • The failure mode depends on the beam's size and span and the reinforcement amount. • Failure mode transitions can be effectively predicted through dimensionless numbers. • Provisions of Standards need to be refined to correctly include the size effects. [ABSTRACT FROM AUTHOR]

Abstract: It has been established that, in rock cutting as well as in indentation, there exist two failure modes. In the case of rock cutting, when the depth of cut is shallow the rock fails in ductile mode, and as the depth of cut increases the failure shifts to brittle mode. The critical depth, at which the failure mode transition sharpens, is proportional to some material characteristic length. This study explored the means by which this critical transition depth could be obtained. Considering the depth of cut as a measure of size and treating rock cutting to be similar in geometry, this study first demonstrated that Bažant's simple size effect equation for quasibrittle material fit rock-cutting data very well. This led to a further exploration of a characteristic length measure that has been widely used in concrete fracture research. With the aid of finite element analysis and few test data points, the critical depth was expressed in terms of the rock characteristic length. Aided by empirical relationships, the critical depth was also shown to be a function of rock uniaxial compressive strength. [Copyright &y& Elsevier]

We study the failure mode transition of steel plates under impact loading. For low impact velocities, brittle crack growths is observed while shear bands occur when the speed of the impactor is increased. Meshfree Galerkin method is employed to simulate the failure mode transition behavior. We use Johnson-Cook model for the steel. The cracks and shear bands are treated as strong discontinuities. This equal representation of the kinematics of the crack and shear bands allows for a consistent and effective modelling of these complicated events. Previous models generally used complex constitutive models for shear band modeling included fluid like behavior when the stress collapses. We demonstrate for the well-known Kalthoff problem the efficiency of our method. [ABSTRACT FROM AUTHOR]

Abstract: Dynamic fracture mechanics theory was employed to analyze the crack deflection behavior of dynamic mode-I cracks propagating towards inclined weak planes/interfaces in otherwise homogenous elastic solids. When the incident mode-I crack reached the weak interface, it kinked out of its original plane and continued to propagate along the weak interface. The dynamic stress intensity factors and the non-singular T-stresses of the incident cracks were fitted, and then dynamic fracture mechanics concepts were used to obtain the stress intensity factors of the kinked cracks as functions of kinking angles and crack tip speeds. The T-stress of the incident crack has a small positive value but the crack path was quite stable. In order to validate fracture mechanics predictions, the theoretical photoelasticity fringe patterns of the kinked cracks were compared with the recorded experimental fringes. Moreover, the mode mixity of the kinked crack was found to depend on the kinking angle and the crack tip speed. A weak interface will lead to a high mode-II component and a fast crack tip speed of the kinked mixed-mode crack. [Copyright &y& Elsevier]

Dynamic shear fracture is investigated by a hybrid experimental–numerical approach. The numerical simulations rely on LEFM assumptions and crack-tip stationarity. Violation of these assumptions is related to fracture or adiabatic shear banding and can be used to identify these events in a typical experiment. Firstly, the reliability of numerical model is verified by comparing our results with the analytical results of Lee and Freund [J. Appl. Mech. 57 (1990) 104]. Then a numerical simulation of Guduru et al.''s experiments is compared with their results to assess the onset of adiabatic shear banding in terms of a critical KII value. Finally, dynamic shear experimental results are reported and analyzed using this approach for commercial PMMA and Maraging 250 steel specimens. In these experiments, the stress intensity factors are determined from miniature strain gauge measurements. A very good agreement is obtained between the present and previous values of KII. The numerical exercises and the experimental results validate the use of hybrid experimental–numerical techniques for the identification of the onset of fracture, with a potential application to large-scale testing. [Copyright &y& Elsevier]

We analyze the initiation and propagation of a crack from a point on the surface of a circular notch-tip in an impact loaded prenotched plate. The material of the plate is assumed to exhibit strain hardening, strain-rate hardening, and softening due to the rise in temperature and porosity. The degradation of material parameters due to the evolution of damage in the form of porosity is considered. Brittle failure is assumed to initiate when the maximum tensile principal stress at a point reaches a critical level. Ductile failure is assumed to ensue when the effective plastic strain reaches a critical value. A crack initiating from the node where a failure first occurs is taken to propagate to the adjacent node that has the highest value of the failure parameter (the maximum tensile principal stress or the effective plastic strain). The opening and propagation of a crack are modeled by the node release technique. Surface tractions and the normal component of the heat flux are taken to be null on the newly created crack surfaces. For the brittle failure, the stress field around the crack tip resembles that in mode-I deformations of a prenotched plate loaded in tension. The distribution of the effective plastic strain in a small region around the surface of the notch-tip is not affected much by the initiation of a ductile fracture there except for a shift in the location of the point where the effective plastic strain is maximum. The initiation of the ductile failure is delayed when a crack is opened at the point where the brittle failure ensues. [ABSTRACT FROM AUTHOR]

Various aspects of the failure behaviour initiated from mode‐II loaded cracks are discussed: validity criteria and minimum size specimen requirements for measuring the fracture toughness K[sub IIc] ; energy balance of the process of initiation of kinked cracks including compressive notch tip stress concentrations; failure mode transition from tensile cracks to adiabatic shear bands at high loading rates; loading rate dependence of the dynamic fracture toughness K[sub IId] in the regime of failure mode transition. Findings of studies on these subjects are presented and implications of the results for practical applications are discussed. The failure behaviour of mode‐II loaded cracks is found very different and far more complicated than of mode‐I loaded cracks, generalizations from mode‐I to mode‐II can therefore be severely misleading. [ABSTRACT FROM AUTHOR]

Results from an experimental investigation on the compression behavior of balsa wood are presented. Specimens with varying densities, ranging from 55 to 380 kg/m3, are loaded in the grain (fiber, cell) direction using a screw-driven material testing system at a strain rate of 10−3 s−1. The results indicate that compressive strength of balsa wood increases with increasing density. Post-test scanning electron microscopy is used to identify the failure modes. The failure of low-density specimens is governed by elastic and/or plastic buckling, while kink band formation and end-cap collapse dominate in higher density balsa specimens. Based on the experimental results and observations, several analytical models are proposed to predict the compressive failure strength of balsa wood under uniaxial loading conditions. [Copyright &y& Elsevier]

A mesh-free Galerkin simulation of dynamic shear band propagation in an impact-loaded pre-notched plate is carried out in both two and three dimensions. The related experimental work was initially reported by , and later re-examined by , and others.The main contributions of this numerical simulation are as follows: (1) The ductile-to-brittle failure mode transition is observed in numerical simulations for the first time; (2) the experimentally observed dynamic shear band, whose character changes with an increase of impact velocity, propagating along curved paths is replicated; (3) the simulation is able to capture the details of the adiabatic shear band to a point where the periodic temperature profile inside shear band at μm scale can clearly be seen; (4) an intense, high strain rate region is observed in front of the shear band tip, which, we believe, is caused by wave trapping at the shear band tip; it in turn causes damage and stress collapse inside the shear band and provides a key link for self-sustained instability. [Copyright &y& Elsevier]

Results from an experimental investigation on the mechanical behavior of a unidirectional fiber reinforced composite with 50% volume fraction E-glass/vinylester and the vinylester matrix under uniaxial and proportional multiaxial compression are presented. The stress-strain curve and the acoustic emission records together with the post-mortem observations on the specimen show the failure mode transition from axial splitting to kink band formation as the loading condition changes from uniaxial to proportional multiaxial compression. Axial splitting and "splitting induced" kink band formation were observed in the uniaxially loaded specimen and the multiaxially loaded specimen showed conjugate kink bands and no axial splitting. [ABSTRACT FROM PUBLISHER]

Resistance element welding (REW) is a recently developed hybrid joining process for aluminum (Al)/steel dissimilar materials. In this study, the microstructure and mechanical performance of the Al/press hardened steel (PHS) joints generated using an integrated REW process were investigated. The microstructure of fusion zone (FZ) between rivet and PHS was lath martensite, which was transformed from the fast cooling of austenite. The Al could be divided into four zones according to the microstructure and microhardness distribution: re-solidified zone (RZ), softening zone (SZ), transition zone (TZ) and base metal (BM). The dissolution and coarsening of the precipitates are responsible for the hardness reduction of Al sheet. The variation of mechanical performance was explained in light of the increasing FZ size and the softening Al sheet as heat input rising. Moreover, four block shear models were introduced to predict the peak load of REW joints using the average hardness of SZ in Al sheet, among which the models from Architectural Institute of Japan (AIJ) provided a relatively good prediction. Considering both the average hardness of SZ and sheet thickness, an analytical model was established to predict the variation of critical nugget sizes and explain the failure mode transition. • The solidification/transformation path of nugget was revealed. • The dissolution and coarsening of precipitates explained the softening Al sheet. • Four block shear models were introduced to predict the peak load. • An analytical model was established to predict the critical nugget size. [ABSTRACT FROM AUTHOR]