Failure Mode Transition Research Articles

EFFECT OF ELECTRODE FORCE ON FRACTURE TYPE OF DQSK STEEL RESISTANCE SPOT WELDS

Failure mode is a qualitative measure of resistance spot weld performance. To ensure reliability of resistance spot welds during vehicle lifetime, process parameters should be adjusted so that the pullout failure mode is guaranteed. In this paper, the effect of electrode force on the failure mode of drawing quality specially killed (DQSK) steel resistance spot welds during the quasistatic tensile-shear test is investigated. Results showed that there is a minimum weld nugget size to ensure pullout failure mode. It was found that increasing electrode force decreases fusion zone size and therefore leads to transition of failure mode from pullout to interfacial mode.

On the critical failure mode transition depth for rock cutting

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.

Shift in failure modes in foam core sandwich composites subject to repeated slamming on water

AbstractA test program designed and carried out to mimic the repeated impact of the bow section of fast-moving small boats on the ocean surface provided some unique observations in terms of failure mode transition. Damage progression and modes of failure were evaluated for two types of sandwich composites with comparable global strength and stiffness but different foam density and facesheet strength. Testing was performed on flat rectangular specimens that contained symmetric semi-elliptical edge flaws produced near the end of the specimen held by the rotating cam. Type 1 specimens (softer core/stronger facesheet) consistently failed by interface and through-the-thickness core shear, independent of the flaw size. In contrast, a gradual decrease in flaw size in Type 2 specimens (denser core/weaker facesheet) produced a striking transition in the mode of failure from local buckling in the vicinity of the flaw site along with exponentially increasing lifetime, to interface shear failure at the free end accompanied by a dramatic drop in lifetime. The lifetime of Type 2 specimens was more than two orders of magnitude greater than that of Type 1 specimens.

Novel method for determination of critical fiber length in short fiber carbon/carbon composites by double lap joint

A novel approach is introduced for the experimental determination of critical fiber length in carbon fiber reinforced carbon (CFRC) composites. Critical fiber length is investigated using double lap joint samples. The transition of failure mode from bonding failure to fiber fraction with increasing overlap length correlates with the critical fiber length. Tested overlap lengths were in the range of 4–100mm. For CFRC at hand, failure mode changes at an overlap length of 26±2mm. Hence critical fiber length is derived as lc=52±4mm.

Correlation between microstructure evolution and mechanical strength in the Sn–3.0Ag–0.5Cu/ENEPIG solder joint

This study investigated the interfacial reaction and mechanical strength of Sn–3.0Ag–0.5Cu solder jointed with electroless Ni–P/immersion Au (ENIG) and electroless Ni–P/electroless Pd/immersion Au (ENEPIG) surface finishes under various reflow times. The morphology of interfacial (Cu,Ni,Pd)6Sn5 was refined to needle-like by the Pd insertion in the ENEPIG surface finish. The impact energy of solder joints with Pd doped declined slower than that without Pd-doped after prolonged reflow. By inspecting the fracture surface, the cleavage of interfacial (Cu,Ni)6Sn5 was frequently observed in the ENIG joints after prolonged soldering, whereas ductile dimple-like failure dominated the fracture surface of ENEPIG joints. The enhanced impact strength and the transition of failure mode in the ENEPIG joints was attributed to the needle-like morphology of (Cu,Ni,Pd)6Sn5. The detailed mechanism of improved mechanical strength for solder joints with Pd dissolved will be deliberately addressed and discussed regarding the distinct microstructural evolution in the ENEPIG joint.

Failure mode transitions in RC beams: A cohesive/overlapping crack model application

The analysis of reinforced concrete beams in flexure taking into account the nonlinear behaviour of concrete is addressed by a numerical approach based on the Cohesive-Overlapping Crack Model. An extensive experimental research has been proposed by Bosco and Carpinteri (Scale effects and transitional phenomena of reinforced concrete beams in flexure. ESIS Technical Committee 9 Round Robin proposal, 1993), Bosco et al. (Scale effects and transitional failure phenomena of reinforced concrete beams in flexure. Report to ESIS Technical Committee 9, 1996) and El-Khatieb (Transizione di scala duttile-fragile per le travi in calcestruzzo armato. PhD Thesis, 1997) in order to obtain a rational explanation for failure transitional phenomena of RC beams by varying steel percentage and/or beam slenderness and/or beam size-scale. In the present paper, collapse mechanisms due to concrete tensile cracking, concrete compressive crushing and steel yielding and/or slippage are analysed and a numerical vs. experimental comparison is presented in order to validate the proposed model.

Two-dimensional discrete element simulations of the fracture behaviour of slate

The fracture behaviour of slate, which is a transversely isotropic rock with a (extreme) large strength anisotropy, is investigated. Two-dimensional discrete element simulations are conducted, and compared to earlier laboratory experiments of uniaxial loading of rectangular samples and diametrical loading of disks. The observed fracture patterns in the experiments, as well as in the simulations, are a complex combination of fractures in the schistosity direction and/or in other directions. A conceptual model is put forward, whereby the strength anisotropy on the element scale is related to the strength anisotropy on the sample scale and to the deformation behaviour. It is observed that the load configuration (i.e. load direction relative to the schistosity layers) determines whether fractures grow in the layer direction only, or in both layer and non-layer directions. Moreover, the transition of failure mode (shear or tensile) as a function of the load configuration is captured. Finally, the non-linear behaviour on sample scale is explained by micro-fracturing in the schistosity direction as a means of dissipating energy.

Statistical Characterization of Reinforced Concrete Beams Strengthened with FRP Sheets

This paper presents a comprehensive overview of the characteristics of reinforced concrete beams strengthened with fiber-reinforced polymer (FRP) sheets. A large database consisting of 230 test beams is constructed and analyzed using advanced statistical approaches such as multivariate analysis of variance, parameter estimation analysis, and discriminant analysis. Pearson correlation is conducted to identify linear dependency between test parameters. Parameters influencing debonding of the FRP are determined at a confidence level of 95%: concrete strength, steel reinforcement ratio, FRP thickness, and beam geometry. Transition of failure modes between FRP debonding and cover delamination is affected by FRP rupture strain and a steel reinforcement ratio. Discriminant functions are developed to infer the failure mode of FRP-strengthened beams. Existing design proposals for FRP debonding are evaluated using the test data. A new statistics-oriented equation is proposed to predict the effective strain of FRP-concrete interface on the basis of Monte-Carlo simulations associated with random sampling of contributing parameters.

RKPM MESHLESS ANALYSIS OF BRITTLE-TO-DUCTILE FAILURE

We study the failure mode transition in double-notched and single-notched 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. We employ the meshless reproducing Kernel particle method (RKPM) that is well suited for problems involving large deformation and localization. The key feature of the proposed methodology is that it (1) treats brittle and ductile failure in the same manner and (2) employs the same constitutive model and failure criterion for both brittle and ductile failure. Therefore, cracks and shear bands are both treated as strong discontinuity in the context of the RKPM. Material stability analysis performed at every sampling point determines automatically the failure mode. We demonstrate the validity of the method by comparison to experimental data.

A Phase Field Model for Ductile to Brittle Failure Mode Transition

AbstractThe computational modeling of failure mechanisms in solids due to fracture based on sharp crack discontinuities suffers in dynamic problems with complex crack topologies. This can be overcome by a diffusive crack modeling based on the introduction of a crack phase field. We outline a conceptual framework for phase field models of crack propagation in brittle elastic and ductile elastic‐plastic solids under dynamic loading and investigate the ductile to brittle failure mode transition observed in the experiment performed by Kalthoff and Winkeler [3]. We develop incremental variational principles and consider their numerical implementations by multi‐field finite element methods. To this end, we define energy storage and dissipation functions for the plastic flow including the fracture phase field. The introduction of local history fields that drive the evolution of the crack phase field inspires the construction of robust operator split schemes. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation of temperature dependences of macro- and micro-interfacial properties in carbon fiber/epoxy resin composite

The short beam shear test and single fiber fragmentation test with an energy-based model were conducted to characterize the interfacial properties of two types of carbon fiber–reinforced epoxy resin composites. The temperature dependences of interlaminar shear strength and interfacial fracture energy ( Γi) from room temperature to 130°C were investigated. The correlation between interlaminar shear strength and the micro-interfacial property at elevated temperature was discussed. The scanning electron microscope images of fractographic cross-sections of the composite laminates were used to analyze the failure mode transition as the temperature rising. The results show that the interfacial properties of the two carbon fiber composites with the same matrix are obviously different, especially at high temperature. The changes of interlaminar shear strength and interfacial fracture energy are different at elevated temperature, mainly ascribed to the different treating ways of thermal residual stress and matrix property in the two methods. The similar temperature sensitivities in the changes of Γi and the amount of matrix adhering on carbon fibers in the short beam shear failure images suggest the variations of the micro-interfacial properties in the short beam test as increasing temperature.

On failure mode of resistance spot welded DP980 advanced high strength steel

This paper aims at investigating the correlation between metallurgical and mechanical characteristics of DP980 dual phase steel resistance spot welds with attention focused on the transition in failure mode from interfacial to pullout mode during the tensile shear test. It was studied whether the conventional/industrial weld size criteria can produce pullout failure modes for DP980 spot welds. Based on theoretical analysis and experimental results, there is a minimum fusion zone size which beyond that the interfacial failure mode is avoided. Results showed that by increasing welding current, pullout failure mode was promoted due to increasing the fusion zone size and encouraging martensite tempering in subcritical heat affected zone (HAZ). Fusion zone size was proved to be key factor controlling mechanical properties of DP980 welds in terms of peak load, ductility and failure energy.Ce document vise à examiner la corrélation entre les caractéristiques métallurgiques et mécaniques de soudures par points par résistance de l’acier biphasé DP980, en mettant l’accent sur la transition du mode de défaillance de mode d’interface à mode par arrachement lors de l’essai de traction-cisaillement. On voulait savoir si les critères conventionnels ou industriels de taille de la soudure pouvaient produire des modes de défaillance par arrachement des soudures par points de DP980. L’analyse théorique et les résultats expérimentaux indiquent qu’il y a une taille minimale de la zone de fusion au-delà de laquelle on évite le mode de défaillance d’interface. Les résultats ont montré qu’en augmentant le courant de soudage, le mode de défaillance par arrachement était favorisé, à cause de l’augmentation de la taille de la zone de fusion et de l’encouragement du revenu de la martensite dans la HAZ sous critique. On a démontré que la taille de la zone de fusion constituait une composante clé contrôlant les propriétés mécaniques des soudures de DP980 en ce qui a trait à la charge de pointe, à la ductilité et à l’énergie de défaillance.

In situ mechanical characterization during deformation of PVC polymeric foams using ultrasonics and digital image correlation

Cellular solids such as polymeric foams are finding increasing applications including its use as a core material for sandwich structures. In this study, a novel method is used to measure both the longitudinal and shear wave speeds of a material simultaneously while applying a compressive load in four different densities of polymeric foams made from the same base polymer, polyvinyl chloride (PVC). The study showed that there was a significant difference in evolution of the wave speeds and hence in the apparent modulus during deformation of the lower density foams in comparison to the higher density foams. The non-contact full-field method of digital image correlation (DIC) is used to gain insights into the failure modes during deformation. The lower density foams undergo heterogeneous deformation and failed due to buckling of cell walls. In contrast, the higher density foams undergo nominally homogeneous deformation due to plastic collapse. The failure mode transition is shown to be governed by the relative density of the foams and the mechanical properties of the polymer.

Discrete element modeling of tool‐rock interaction I: rock cutting

SUMMARYTool‐rock interaction processes can be classified as indentation or cutting depending on the direction of motion of the tool with respect to the rock surface. The modes of failure induced in the rock by an indenting or a cutting tool can be ductile and/or brittle. The ductile mode is associated with the development of a damage zone, whereas the brittle mode involves the growth of macrocracks. This is the first part of a series of two papers concerned with an analysis of the cutting and the indentation processes based on using the discrete element method. In this paper, numerical simulations of the cutting process are conducted to reproduce the transition from a ductile to a brittle failure mode with increasing depth of cut, which is observed in experiments. The numerical results provide evidence that the critical depth of cut d * controlling the failure mode transition is related to the characteristic length ℓ = (KIc ∕ σc)2 with KIc denoting the material toughness and σc its unconfined compressive strength. The nature of frictional contact between the cutter face and the rock in the ductile failure mode is also examined. It is shown that the inclination of the total cutting force is controlled by a multi‐directional flow mechanism ahead of the cutter that is related to the formation of a wedge of failed material, intermittently adhering to the cutter. As a result, the inclination of the total cutting force varies with the rake angle of the cutter and cannot be considered an intrinsic measure of the interfacial friction between the cutter and the rock. Copyright © 2012 John Wiley & Sons, Ltd.

Experimental investigation and modeling of mean load effect on fatigue behavior of adhesively-bonded pultruded GFRP joints

The effect of mean load on the fatigue behavior of adhesively-bonded pultruded GFRP double-lap joints under constant amplitude loading was experimentally investigated. The joints were examined under nine different stress ratios (R) representing pure tension, compression, and combined tension–compression fatigue loading. A transition of the failure mode, from tensile to compressive, was observed as the mean load decreased from positive to negative values. The slope of the S–N curves derived for R-ratios with positive or negative mean load consistently decreased with increasing mean load. The highest load amplitude corresponded to the R-ratio where the transition of the fatigue failure mode occurred (R=−2). The characteristics of the constant life diagram developed for the examined bonded joints are thoroughly discussed. A phenomenological formulation was proposed and its accuracy evaluated by comparisons with the derived experimental data. The comparison of the new formulation with models commonly used for composite materials proved its higher accuracy that is achieved with less implementation effort.

Moving toward a more realistic material model of a ductile material with failure mode transition

AbstractIn this paper a universal mathematical model using fully coupled thermal‐structural finite element analysis capable of predicting ductile‐to‐brittle failure mode transition at high strain rates is presented. Appropriate equivalent strain measures describing the onset and the growth of ductile and total damage and heat generation rate per unit volume for dissipation‐induced heating have been proposed. The model was implemented into a finite element code using the updated Lagrangian formulation for finite strains, the von Mises material model with isotropic hardening and the Jaumann rate, in the form of the Green‐Naghdi rate, in the co‐rotational Cauchy's stress update calculation. Plastic bending of a cantilever was studied, applying constant pressure as stepped load to 1/3 of the upper surface of the beam. The analysis result shows that at higher strain rates the ductile damage value is significantly lower than the value of the total damage, while at low strain rates their values are approximately identical, thus the model is likely to open new perspectives in the study and numerical simulation of the behaviour of ductile materials.

Similar and dissimilar resistance spot welding of martensitic advanced high strength steel and low carbon steel: metallurgical characteristics and failure mode transition

Safety, emissions and fuel economy of vehicles have always been of great importance to automakers. Advanced high strength steels (AHSSs) have proved to be effective in fulfilling these needs of the industry, and this has led to great opportunities for increasing usage of AHSSs. The present work aimed to investigate the resistance spot welding characteristics of martensitic steel (M130) having the thickness of 2 mm, utilising metallographic observation, microhardness and tensile–shear testing. The results were compared for different combinations of martensitic steel and low carbon steel (LCS). In the study of interfacial to pull-out failure mode transition, dissimilar M130/LCS welds exhibited the lowest tendency to fail in the interfacial failure mode. Weld nugget diameter is proposed as a main variable controlling the peak load and energy absorption of the welds along with other factors such as electrode indentation and hardness/strength of the failure location.

Impact simulation of different mode failure

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.

The role of delamination in failure of fibre-reinforced composites

The mechanisms by which delamination contributes to the failure of fibre-reinforced composites are reviewed. Through-thickness failure owing to interlaminar stresses is considered first, and the effect of delamination in impact and compression after impact. The way in which in-plane failure can occur by delamination and matrix cracks joining up to produce a fracture surface without the need to break fibres is considered next. Examples of quasi-isotropic laminates loaded at different off-axis angles, and with different numbers and thicknesses of ply blocks show large differences in unnotched tensile strength controlled by delamination from the free edge. Similar mechanisms determine the strength of notched specimens and give rise to the hole size effect, whereby tensile strength increases with decreasing hole diameter owing to increased delamination and splitting. Open hole tension and over-height compact tension tests with constant in-plane dimensions show a transition in failure mode with increasing ply block thickness from fibre-dominated fracture to complete delamination. In all these cases, the critical factor controlling strength is the relative propensity to delaminate.

Susceptibility to interfacial failure mode in similar and dissimilar resistance spot welds of DP600 dual phase steel and low carbon steel during cross-tension and tensile-shear loading conditions

Abstract The paper investigates the failure mode transition from interfacial to pullout in similar and dissimilar combinations of DP600 dual phase steel and low carbon steel (LCS) under tensile-shear (TS) and cross-tension (CT) loading conditions. In both CT and TS loading conditions, a transition in the failure mode from interfacial to pullout was observed with increasing fusion zone size beyond a critical value (DC). The tendency to fail in interfacial mode during the CT loading was increased in the order of LCS/LCS, DP600/LCS and DP600/DP600. It was shown that interfacial to pullout failure mode transition during the CT test is governed by the fracture toughness of the fusion zone and strength of the pullout failure location (i.e. heat affected zone). It was shown that increasing carbon equivalent of the fusion zone promoted interfacial failure mode in CT loading condition. The high carbon equivalent of DP600 steel led to formation of hard and brittle martensite, which in turn promotes crack propagation through fusion zone. In DP600/LCS combination, decreased carbon equivalent and fusion zone hardness through dilution with the LCS promotes pullout failure at smaller weld sizes. DC during the TS loading is increased in the order of DP600/LCS, LCS/LCS and DP600/DP600. No correlation between fusion zone carbon equivalent and the tendency to fail in IF mode during TS loading was found. Failure mode transition during the TS loading is controlled by hardness of fusion zone and stiffness of the joint. The lowest DC for DP600/LCS is a function of its high fusion zone hardness (in comparison to LCS/LCS combination) and its low stiffness (in comparison to DP600/DP600 combination).