Failure Mode Transition Research Articles

Mechanism of brittle-to-ductile transition in tungsten under small-punch testing

Exceptional high brittle-to-ductile transition temperature (BDTT) is a critical issue that limits the application of tungsten as structural materials. The controlling factors in BDT have been the focus of debate in past decades. In this work, small-punch testing were performed in a wide temperature range to explore the mechanism of BDT in as-rolled tungsten. A transition in failure mode from cleavage cracking to delamination fracture with increasing temperature marks the BDT of the as-rolled tungsten. Numerous (001) planar triangle-shaped shear tongues are produced on fracture surface. The number density and distribution of shear tongues show a clear temperature and orientation dependence. Analysis of the slip line angle on the shear tongues reveals that profuse dislocation activities on the {110}, {112} and {123} planes at and above BDTT. Temperature and orientation dependent slip activities are decisive factors controlling the BDT. Below BDTT, fewer freely moved edge dislocations are insufficient to blunt the crack tip, thus as-rolled tungsten displays brittleness. Once above the BDTT, the mobility of screw dislocations is increased, and a large fraction of mixed dislocations glide could effectively suppress cracking and make as-rolled tungsten ductile.

In-process reinforcing method: dual 3D printing procedure for ultra-high performance concrete reinforced cementitious composites

The general application of 3D printed concrete (3DP-C) in practical engineering is significantly hindered by the lack of adequate reinforcement procedures, which is an intrinsic limitation of the 3D printing process. To address this limitation, this study proposes an in-process dual-printing reinforcement method, which concurrently and continuously prints ultra-high performance concrete (UHPC) and ordinary 3DP-C. The UHPC, as an integral part of the printed structure, serves as reinforcement for the overall concrete composite. Printing equipment for these dual concrete materials is developed to realize the coordination of the dual-material deposition process. Significant improvement of mechanical properties of 3D printing UHPC (3DP-UHPC) reinforced 3DP-C, which is then abbreviated as 3DP-UHPCRC, is demonstrated through four points bending tests of thin plate specimens. In comparison to plain 3DP-C, the reinforced concrete possesses an 160.5% increase in ultimate bending strength as well as a brittle to ductile bending failure mode transition, thus demonstrating the effectiveness of the proposed reinforcement method.

Shear behaviour of adhesive anchors under different strain rates

Finite element analysis was carried out to investigate the shear behaviour of post-installed adhesive anchors at strain rates ranging from 10-5 s−1 to 103 s−1. The shear capacity and the corresponding dynamic increase factor (DIF) at the different strain rates (ε̇) and for different design parameters were investigated. Failure mode of the adhesive anchor was also examined. The results show that the shear capacity and DIF of the adhesive anchors increased with increase in the strain rate. Anchor diameter and embedment depth were observed to influence the shear capacity of the adhesive anchor. The failure mode was influenced by the strain rate wherein the increase in the strain rate from 10-5 s−1 to 103 s−1 resulted in failure mode transition from pryout failure to concrete spalling, and then to steel anchor fracture failure.

Cyclic deformation and fatigue behavior of 7075-T651 Al alloy with a gradient structure

Fully reversed stress-controlled tension–compression fatigue experiments (R = −1) were conducted to study the cyclic deformation and fatigue behavior of 7075-T651 aluminum alloys with and without a surface mechanical rolling treatment (SMRT). The strain amplitude, ratcheting effect, and asymmetric behavior of base metal (BM) and SMRT specimens were analyzed. SMRT specimens showed more remarkable ratcheting effects and tension–compression asymmetry under high stresses. Fatigue tests showed that the fatigue strengths of 7075 Al alloys were increased significantly by the SMRT. Both BM and SMRT specimens presented a failure-mode transition from the intrusions and extrusions failure modes to the Al18Ti2Mg3 constituent phase. The hysteresis loop was utilized to obtain the plastic-strain amplitude. Furthermore, according to the cyclic-deformation behavior combined with the Manson–Coffin law, strain–life evaluation of BM and SMRT specimens under the stress-control is given.

Experimental investigation into the seismic behavior of squat reinforced concrete walls subjected to acid rain erosion

Acid rain erosion will continuously deteriorate the physical and mechanical properties of materials and finally degrade the seismic performance of reinforced concrete (RC) structures. Nevertheless, the effect of acid deposition on squat RC walls, an important part of lateral force-resisting systems, has not been reported in existing research. Thus, in this paper, an artificial climate simulation method was used to accelerate the acidic attack process on four squat RC wall specimens with an aspect ratio of 1.0. Then quasi-static loading tests were conducted to observe their cyclic behavior under different acid rain spraying cycles (ARSCs). The results show that as ARSCs increase, the degree of degradation of concrete strength and the corrosion weight loss of steel bars have a rising trend, and the bearing capacity and deformation capacity of the squat RC walls deteriorate gradually. Simultaneously, the ratio of shear displacement to total lateral displacement increases, indicating a more noticeable shear failure characteristic with the addition of ARSCs. Moreover, the failure mode shifts from the mixed flexure and diagonal compression mode to the diagonal tension mode with a significant decrease in ductility and energy-dissipation capacity. It is also found that the degradation rate of the shear strength of squat walls under acid rain erosion is larger than that of the flexural strength, which may lead to the transition of failure mode.

A rate-dependent phase-field framework for the dynamic failure of quasi-brittle materials

In recent years, phase-field theory has become an efficient approach for predicting damage and fracture of engineering materials. However, the rate-dependence of quasi-brittle materials is not considered in most exiting models. To address this issue, we develop a new phase-field damage model for the dynamic failure of quasi-brittle solids based on the microforce balance law, within which a linear viscoelastic constitutive relation in effective stress space and a hyperbolic phase-field evolution equation are incorporated. Then several representative numerical examples are presented to demonstrate the ability of the proposed model in characterizing the dynamic failure process of quasi-brittle solids. Good agreements are achieved between numerical predictions and theoretical and experimental results. In particular, the increase of tensile strength and the transition of failure modes of concrete-like materials with the increase of loading rates can be well reproduced.

Random transition of failure modes in concrete shear walls with constitutive parameters involving spatial variation

There are two types of probabilistic dependence between the damage constitutive parameters of concrete, i.e., the probabilistic dependence between the parameters at the same position and the spatial dependence of a parameter at different positions. In this paper, a newly proposed multi-variate random field model by synthesizing the spatial correlation function with the multivariate copula function (SCFVCF) is adopted to generate spatially variant constitutive relationship of concrete in concrete shear walls. Then the failure modes and hysteresis performance of an I-type reinforced concrete shear wall with spatially variant concrete constitutive relationship under compressive and cycling horizontal loading are studied. The failure modes of the shear wall are random in nature due to the spatial randomness involved in the constitutive relationship, e.g., the damage position of the shear wall may transfer to the weaker area at the upper elements for some samples. By observation, the failure modes can be classified into four types. Particularly significant is that random transition of failure modes will occur when the spatial correlation length increases, and consequently the probability of occurrence of different failure modes changes against the spatial correlation length. This means that if the two types of dependence between the material constitutive parameters are not reasonably taken into account, the results of failure analysis of reinforced concrete shear walls might be misleading.

Effect of saw-tooth ply drops on the mechanical performance of tapered composite laminates

Automated Fibre Placement (AFP) is a manufacturing technique to produce large, high quality composite parts, where preimpregnated carbon fibre tapes are laid side-by-side to generate the composite preform. Thickness changes within a component are realised through internal ply terminations, with the tapes being cut perpendicular to the fibre direction. In plies laid up at an angle to the taper direction, the AFP tape cuts create saw-tooth shaped ply drop tips. This increases the size of resin rich zones and enlarges the region where stress concentrations and cracks can develop, which amplifies the risk of failure. This study investigates the effect of simulated AFP saw-tooth ply drop tips, created by hand layup, on the mechanical properties and failure behaviour of carbon fibre/epoxy composites and compares its results to reference broad goods layups. The stiffness was found to be unaffected and the strength was reduced by ~ 10%. The failure mode was governed by delamination for the saw-tooth ply drop tips, whereas the reference specimens failed by rupture of the unidirectional fibres. A finite element modelling technique was used to select the layup to be tested. The models were then refined in the light of experimental results, to accurately predict the failure and explain the failure mode transition.

Influence of in situ postweld heat treatment on microstructure and failure behavior of dual-phase steel resistance spot weld

The present study aims to investigate the effect of in situ postweld heat treatment (PWHT) on microstructure, tensile shear properties, failure mode, and hardness distribution of resistance spot weld of dual-phase (DP590) steel. Tensile shear properties were evaluated in terms of peak load and failure energy. Results showed that welds subjected to high heat input in situ PWHT via double-pulse RSW process exhibit higher peak load and failure energy compared with conventional single-pulse weld. The improvement in mechanical performance is attributed to increased weld nugget size and the transition of failure mode from interfacial fracture mode to pullout fracture mode. Moreover, it was also found that increasing the tensile shear test velocity from 1.6 × 10−4 ms−1 (quasi-static) to 8.3 × 10−3 ms−1 for low-dynamic increased the peak load and failure energy of the weld.

Collapse Performance of Nominally Identical Nonductile Circular Columns Susceptible to Failure-Mode Variability

AbstractRecent analytical studies have highlighted the existence of a failure-mode transition zone within which nominally identical test specimens may experience variability in failure mode between...

Measurement of Local Material Properties and Failure Analysis of Resistance Spot Welds of Advanced High-Strength Steel Sheets

afety evaluation of resistance spot welds necessitates the accurate measurement of local constitutive properties. This study employed miniature mechanical tests to investigate the deformation and failure behaviors of nugget, heat affected zone (HAZ), and corona bond of resistance spot welded JSC980YL steel. A novel mini-peel test was developed to enable local fracture in HAZ for numerical inverse calibration of constitutive parameters. The fracture constants of weld zones calibrated using Cockcroft-Latham ductile failure criterion were incorporated in finite element models to predict the failure modes of spot welds in tensile-shear and cross-tension coupon tests. The result indicates that the ultimate tensile strengths of the nugget and the corona bond were 37.6% higher and 5.8 % lower, respectively, than that of the base material. The nugget and HAZ exhibited ductile fracture, whereas the corona bond was brittle fracture with only 1.2% elongation. In the coupon tests, the increase of nugget diameter slowed down the damage accumulation rate in the nugget and accelerated that in the HAZ, resulting in the transition of failure mode from interfacial to pullout. The failure load of corona bond in coupon tests increased with the increase of nugget diameter while its contribution to the peak load decreased.

Molecular insights into the weakening effect of water on cement/epoxy interface

The strong interfacial integrity of FRP/concrete bonded system is shown to be significantly weakened in wet environment, with the interfacial debonding between concrete and epoxy adhesive as a major failure mode. However, the fundamental mechanism is still largely unknown. Here a molecular model of calcium silicate hydrate (C-S-H)/epoxy interface is constructed to analyze interfacial degradation mechanism as affected by water molecules, by relating with variations in the interfacial behavior and the failure mode. The simulation results show that water molecules in local C-S-H/epoxy interface degrade the interface and lead to a shift in failure mode from cohesive failure of C-S-H gel to interfacial debonding between C-S-H and epoxy, in accord with experimental observation. By simultaneously capturing interfacial degradation and failure mode transition, it is shown that water molecules significantly weaken the interfacial integrity of cement/epoxy interface, and hence the long-term durability of FRP/concrete bonded system.

Experimental Study on the Effect of Brittleness on the Dynamic Mechanical Behaviors of the Coal Measures Sandstone

As one of the most crucial mechanical parameters of the rock materials, the effect of brittleness on the deformation and failure is of great practical significance for geotechnical construction and disaster prevention and mitigation. In this paper, the deformation and failure behaviors of the different brittle samples under dynamic loading were investigated using a split Hopkinson pressure bar (SHPB) experimental system. Besides, scanning electron microscopy (SEM) was also employed to study the relationship between the microscopic failures and rock brittleness and strain rate effects. The results revealed that the brittleness indexes BI3 and BI5 of the samples under uniaxial compression follow a linearly decreasing trend affected by the temperature changes, while the brittleness of the sample shows an increasing trend with the increase of strain rate under the dynamic loading. Also, the decline in the brittleness leads to an increase in the prepeak yield deformation phase of the sample under dynamic loading; after the peak point, the sample failure mode transitions from type I to type II with self-sustaining failure. Moreover, it was found that the dynamic strength increase factor presents a negative correlation with the sample brittleness. Finally, the macroscopic failure mode of the sample changes from split failure with multiple cracks to shear failure with few cracks due to the effect of decreasing brittleness. The failure surface of the sample gradually becomes smooth with the increase of brittleness, which manifests as a decrease in microcracks, and the gradual increase of the strain rate makes the failure surface rough, accompanied by an increase in microcracks.

Effect of Sawtooth Angle on Shear Mechanical Behavior of Structural Plane

In order to study the shear characteristics of sawtooth structural planes, shear tests were carried out on structural planes with different sawtooth angles under different normal stresses to study the influence of sawtooth angles on the shear strength of structural planes, and the quantitative relationship between the shear strength parameters and the sawtooth angles of structural planes was obtained. According to the test results, the failure characteristics of the regular sawtooth structure are expounded. The relationship between the shear strength and the angle of the sawtooth structure is analyzed by using More-Coulomb theory, and the empirical formula of the shear strength of the sawtooth structure is given. At the same time, the failure characteristics of the shear plane were obtained by 3D scanning of the shear plane, and the failure law of the structural plane was obtained. The results show that the shear strength of the regular sawtooth structure surface increases with the increase of sawtooth angle, and the failure mode of the structure surface at low angle is climbing slip failure, and the failure mode transitions to climbing shear failure with the increase of sawtooth angle, and the failure mode changes to total shear failure when the sawtooth angle increases further.

Development of a poly-axial platen for testing true-triaxial behavior of rocks

Mining at greater depths can lead to stress-induced failure, especially in areas of high horizontal in situ stress. The induced stresses around the opening are known to be in a poly-axial stress state where, σ1 ≠ σ2 ≠ σ3 with special case of σ3 = 0 and σ1, σ2 ≠ 0 at its boundary, where σ1, σ2, σ3 are major, intermediate, minor principal stress, respectively. The conventional triaxial testing does not represent the actual in situ strength of the rock in regions of high horizontal stress, as it ignores the influence of intermediate principal stress (σ2). The typical poly-axial testing (biaxial and true-triaxial tests) of intact rock mostly requires sophisticated and expensive loading systems. This study investigated the mechanical behavior of intact rock under a poly-axial stress state using a simple and cost-effective design. The apparatus consists of a biaxial frame and a confining device. The biaxial frame has two platens that apply equal stress in both directions (σ1 = σ2) on a 50.8 mm cubical specimen when placed inside the uniaxial loading device. The confining device performed separate biaxial tests under constant intermediate principal stress (σ2 = constant) and true-triaxial tests when used along with the biaxial frame. This study then compared the failure modes and peak strength of Berea sandstone specimens with other biaxial–triaxial devices to validate the design of the poly-axial apparatus. We also performed uniaxial tests on both standard cylindrical samples and prismatic specimens of different slenderness ratios. These tests provided a complete understanding of the failure mode transition from standard uniaxial compressive tests to triaxial stress conditions on cubical specimens. Additionally, this study determined best-fitted strength envelopes for biaxial and triaxial stress state. Based on regression analysis, we found a quadratic polynomial to be a good fit to biaxial strength envelope. For the true-triaxial strength envelope, we found the three-dimensional (3D) failure criterion to be a good fit with R2 of 0.964.

Measurement of local material properties and failure analysis of resistance spot welds of advanced high-strength steel sheets

Safety evaluation of resistance spot welds necessitates the accurate measurement of local constitutive properties. This study employed miniature mechanical tests to investigate the deformation and failure behaviors of nugget, heat affected zone (HAZ), and corona bond of resistance spot welded JSC980YL steel. A novel mini-peel test was developed to enable local fracture in HAZ for numerical inverse calibration of constitutive parameters. The fracture constants of weld zones calibrated using Cockcroft-Latham ductile failure criterion were incorporated in finite element models to predict the failure modes of spot welds in tensile-shear and cross-tension coupon tests. The result indicates that the ultimate tensile strengths of the nugget and the corona bond were 37.6% higher and 5.8% lower, respectively, than that of the base material. The nugget and HAZ exhibited ductile fracture, whereas the corona bond was brittle fracture with only 1.2% elongation. In the coupon tests, the increase of nugget diameter slowed down the damage accumulation rate in the nugget and accelerated that in the HAZ, resulting in the transition of failure mode from interfacial to pullout. The failure load of corona bond in coupon tests increased with the increase of nugget diameter while its contribution to the peak load decreased.

Resistance Spot Welding Behavior of Automotive Steels

In present-day automobiles, new high-strength steel grades are being extensively utilized, which have different alloy concentration, microstructure and properties, and results in different spot welding behavior. In the present work, the effect of weld current was studied on four widely used automotive grades, such as IF 270, TRIP 690, DP 780 and TRIP 980, in a laboratory welding machine. The welding current was varied between 5 and 15 kA. Weld quality was assessed based on tensile shear, mode of failure, weld nugget diameter, metallographic observations and hardness tests. Experimental results showed that weld strength increased with an increase in base metal tensile strength but with a drop at the point of expulsion. Variation in hardness along the cross section was found to be higher in high-strength steels. The initial microstructure and phase transformation during cooling affected the final properties. Critical currents for peak strength, expulsion, sticking and failure mode transition were determined for each grade for industrial applications. This study helped in determining the criterion-based optimized welding parameters for specific applications of these automotive-grade steels.

Anisotropic thermal-conductivity degradation in the phase-field method accounting for crack directionality

Dynamic loads applied to metals may lead to brittle or ductile fracture depending on the imposed strain rates, material properties, and specimen geometry. With an increase in the applied velocity, brittle to ductile failure mode transition may also be observed. Furthermore, at high strain rates, ductile fracture may be preceded by shear bands, which are narrow bands of intense plastic deformation, typically accompanied by a significant rise in temperature. Reliable models are needed to predict the response of metals subject to dynamic loads. In particular, capturing the interplay between heat conduction and crack propagation using the phase-field fracture method is still an open research field. To accurately capture the heat transfer physics across crack surfaces, damage models degrading thermal-conductivity are necessary. While isotropic thermal-conductivity degradation models were proposed, they may lead to errors as they are indifferent to crack directionality. In our current work, a new anisotropic approach is proposed in which thermal-conductivity, which depends on the phase-field gradient, is degraded solely across the crack. It is shown that this approach improves the near-field approximation of temperature and heat flux compared with isotropic degradation, when taking the discontinuous crack solutions as reference. Additionally, the anisotropic degradation approach is implemented in a unified dynamic fracture model and a couple of examples demonstrate the viability of this approach.

Temperature and radiation effects on brittle versus ductile fracture behavior in miscible phase boundaries: insight from atomistic simulations

Temperature- and irradiation-assisted failure mechanisms in miscible phase boundaries are clarified via atomistic calculations. We first establish the temperature-dependent brittle-to-ductile transition in U–Zr miscible phase boundaries. Our results confirm that these boundaries are mostly brittle at low temperatures and ductile at elevated temperatures. We then investigate the changes induced by irradiation on the fracture mechanisms in such phase boundaries. The irradiation-induced defect accumulation follows a multi-stage process that starts with the accumulation of isolated small dislocation loops before transitioning to the saturation and growth of larger dislocation loops and end up with a reorganization into forest dislocations. The accumulation of loops is the primary feature to participate in the delineation between brittle and ductile interfacial fracture in irradiated phase boundaries. At low damage levels, radiation defect interactions with the crack tip are limited and U–Zr miscible boundaries fail through the emission of dislocations ahead of the crack tip followed by brittle cleavage in agreement with the classical Griffith’s criterion for crack stability. At higher damage levels, the failure mode transitions from brittle crack growth to ductile void growth. In this case, the microcrack tip is blunted by the high density of pre-existing, radiation-induced defects in the vicinity of the crack. This interaction leads to the development and growth of a cavity at the interface as opposed to interfacial cleavage.

Failure Modes of a Laminated Composite With Complaint Interlayers

Abstract Composites comprising a high-volume fraction of stiff reinforcements within a compliant matrix are commonly found in natural materials. The disparate properties of the constituent materials endow resilience to the composite, and here we report an investigation into some of the mechanisms at play. We report experiments and simulations of a prototype laminated composite system comprising silicon layers separated by polymer interlayers, where the only failure mechanism is the tensile fracture of the brittle silicon. Two failure modes are observed for such composites loaded in three-point bending: failure under the central roller in (i) the top ply (in contact with the roller) or (ii) the bottom ply (free surface). The former mode is benign with the beam retaining load carrying capacity, whereas the latter leads to catastrophic beam failure. Finite element (FE) simulations confirm this transition in failure mode and inform the development of a reduced order model. Good agreement is shown between measurements, FE simulations, and reduced order predictions, capturing the effects of material and geometric properties on the flexural rigidity, first ply failure mode, and failure load. A failure mechanism map for this system is reported that can be used to inform the design of such laminated composites.