Fracture Mechanism Transition Research Articles

Crack initiation and propagation in composite microstructure of dentin

Dentin possesses unique hierarchical structure through long-term natural selection, which imparts exceptional mechanical properties to this hard tissue. In this study, the combined effects of ductile fracture of intertubular dentin (ITD) and brittle cracking of peritubular dentin (PTD) on the crack initiation and propagation in the composite microstructure of dentin are explored. A micromechanical model accounting for the unique two-layer microstructure of dentin is developed and numerical simulations of fracture of microstructured dentin are performed. It is found that microcracking of PTD prior to the penetration of main crack into PTD and crack deflection along the interface between PTD and ITD are two major mechanisms involved in the fracture of microstructured dentin, which is in a good agreement with experimental observations. The competition between the two mechanisms is governed by PTD strength. Furthermore, we reveal that the elastic and fracture properties of PTD and strain hardening of ITD greatly affect the fracture behavior of dentin. The decrease in elastic modulus and increase in tensile strength of PTD elevate the notch tip plasticity, increasing the propensity of crack initiation at notch tip. Whereas such variations in elastic modulus and tensile strength of PTD can lead to the transition of fracture mechanism from multiple void interactions to crack-void interaction, thereby enhancing toughness of dentin. Increasing the strain hardening exponent of ITD can also give rise to the amplified toughness.

Ductile to brittle failure transition of HSLA-100 Steel at high strain rates and subzero temperatures

Mechanical properties of HSLA-100 steel at room and subzero temperatures of −40°C, −80°C and −196°C were investigated in this work. Dry ice and liquid nitrogen were used to reach the low temperatures. Also, the material response to different strain rates was studied. Quasi-static tensile test rig and tensile Hopkinson split bar were employed to carry out the experiments. Furthermore, fracture behavior and the transient temperature of this steel was investigated using the curve of stress triaxiality factor (P/Y) against true fracture strain (εf) using Bridgman equation. In addition, fracture mechanisms for this steel at different testing conditions were studied. Finally, SEM observations were performed to back the results found in this research. The results indicated that the material behavior at room temperature to −40°C was quite ductile regardless of the strain rate level. Fracture mechanism transition from ductile to brittle occurred at −80°C. For −196°C, the material behavior was completely brittle for high strain rate and entirely ductile for lower strain rates. It was also found that the yield stress of material progressively increased with the decrease of temperature and the increase of strain rate. The maximum increase was 160% and occurred for −196°C at the strain rate of 10,800s−1.

Correlation between the internal friction and fracture mechanism in quenched and tempered carbon steels

The mechanical behavior of quenched and tempered steels is analyzed by mechanical spectroscopy (internal friction), hardness measurements, Charpy impact and tensile mechanical tests. In quenched steels, the high dislocation density leads to a stress relaxation mainly based on the Snoek-Köster relaxation, and inhibits the plastic deformation, leading to a predominantly intergranular mechanism of fracture. During tempering, the martensite decomposition decreases the dislocation density and hardness. This leads to a stress relaxation based on both dislocation-enhanced Snoek and Snoek-Koster relaxations, and promotes the fracture mechanism transition from brittle to ductile. This suggests that the Snoek-Köster and dislocation-enhanced Snoek relaxations are correlated with the intergranular and ductile fracture mechanisms, respectively. Nevertheless, the tempered martensite embrittlement cannot be detected by means of internal friction since the strain rate associated to this technique is more similar to the strain rate observed in tensile mechanical test than that observed in Charpy impact test. On the other hand, the risk of embrittlement is increased when the retained austenite is thermally instable.

Brittle versus ductile fracture mechanism transition in amorphous lithiated silicon: From intrinsic nanoscale cavitation to shear banding

Large-scale atomistic simulations were performed to investigate the fracture behaviors of amorphous lithiated silicon. The simulation results revealed that as the lithium concentration increases, there exists a transition in fracture mechanism from intrinsic nanoscale cavitation to extensive shear banding ahead of the crack tip. It is shown that the observed fracture-mechanism transition can be understood from the changing ratio between critical stresses for cavitation and plastic yield under increasing lithium content. Furthermore, we investigated the mechanistic details of cavitation (i.e. growth of nanovoids) in amorphous lithiated silicon using fully three-dimensional atomistic simulations. It was revealed that in a low lithium concentration environment, an initial void grows heterogeneously by merging with neighboring nucleated voids. However, at high lithium concentrations, the initial void continues to grow in a homogeneous mode. These atomistic mechanisms provide a fundamental understanding of how silicon anodes in lithium-ion batteries fracture during lithiation.

Effects of PWHT on the microstructure and mechanical properties of ERNiCrFe-7 all-weld metal

The effects on the microstructure and room temperature mechanical properties of the nickel-based filler metal ERNiCrFe-7 due to a post weld heat treatment (PWHT) at 610 °C have been studied. It was shown that PWHT caused an increase in yield strength and ultimate tensile strength, but at the cost of reduced ductility and impact toughness. Crack tip opening displacement (CTOD) fracture toughness tests revealed an initial increase in maximum CTOD when a short PWHT was given, but declined with prolonged PWHT. The PWHT also caused a transition in fracture mechanism from microvoid coalescence to quasi-cleavage in room temperature air. Chromium-rich M23C6 type grain boundary carbides were present already in the as-welded weld metal, and were observed to coarsen during the PWHT. Intragranular M23C6 carbides were not observed in the as-welded state, but were precipitated during the 610 °C PWHT. Precipitation of these intragranular carbides is believed to be the main reason for the changes in mechanical properties.

In situ bending of layered compounds: The role of anisotropy in Ti2AlC microcantilevers

The paper concentrates on in situ bending tests and analysis of the failure of materials exhibiting strong anisotropy. As a typical example, Ti2AlC microcantilevers (MCs) were scrutinized. The anisotropic flexural strength of Ti2AlC MCs was measured, and the strength of the strong orientation was found to be two and half times as high as that of the weak orientation. The transition of fracture strength and mechanisms could be evaluated. Our findings have generic implications for future study on the fracture behavior of layered materials. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Crack Growth Features in Hydrogenating High-Strength Steel AISI 4340 under Cycling

There are various types of structural components, which are subjected to hydrogen aggressive environment and cycling. Their unexpected failure occurs suddenly and may entail serious consequences such as emergency stop of equipment, failure of the structure etc. The special engineering models are developed to predict the crack kinetics in structure components, which are subjected both to hydrogen environment and to cycling simultaneously. These models enable estimation of structure component life. The paper considers some features of such empirically specified special model giving emphasis to the one of them, namely to the prevailing fracture mechanism transition from fatigue to hydrogen embrittlement and back in crack kinetics process.

Fracture Mode Transition for Explosively Loaded GB/JB 20 Steel Containment Vessels

A rupture experiment was conducted on cylindrical explosion containment vessels (ECVs), where the fracture mode transition was observed. Microstructure examinations indicate the material GB/JB20 (AISI 1020) experienced a fibrous-to-cleavage fracture mechanism transition with increment of loading rate. Different from fracture mechanics method, a rate-dependent failure criterion is proposed to account for the dynamic fracture behavior, which is compatible with experimental observation that the material fails at low effective plastic strain when at high strain rates. A finite element analysis of a cylindrical containment vessel with different sizes of initial cracks was performed, where the overpressure caused by detonation was calculated, and the dynamic crack propagation and fracture mode transition were reproduced. In addition, a failure assessment including the estimation of limiting crack sizes corresponding to impulsive loading was conducted. It was found that a small variation of initial crack size has minor influence on the final fracture mode and profile, which is mainly dependent upon the intensity of impulsive load as well as the loading rate. The results also indicate that the crack propagates with strongly nonlinear speeding, most cracking length developed during the first structural vibration cycle.

Dislocation Forces on an Edge Dislocation Near Crack Tip in Ferroelectric Materials

As dislocation emission is regarded as the critical point of brittle-to-ductile transition of fracture mechanism and the defects interact with domain walls motions due to their self-stress fields, the investigations on dislocation forces, especially the interaction force and the reaction between domain switching and dislocations, are the most important fundamentals of dislocation slip in the brittle-to-ductile transition and the pinning effect on domain wall motion mechanisms. In this paper, an innovative method base on the strain nucleus simulated by an assembly of four dislocations and the Green’s function integration is used to solve generalized stress field arising from domain switching. Then, the accurate expressions of the interaction forces, pinning forces and the image forces on dislocation are obtained and analyzed. At last, the lattice resistance is also discussed. The numerical results show that the dislocation emission is possible when temperature and load are proper, and the domain switching interaction force is the main driving force of dislocation slip in ferroelectric material under negative electric field load. The curve and equation of the lattice resistance correlated with temperature can also be fitted by the results of the proposed analytical method and corresponding experiments.

Blends of poly(vinyl chloride) with α‐methylstyrene‐acrylonitrile‐butadiene‐styrene copolymer: Thermal properties, mechanical properties, and morphology

Binary blends of poly(vinyl chloride) (PVC) with α‐methylstyrene‐acrylonitrile‐butadiene‐styrene copolymer (AMS‐ABS) were prepared via melt blending. A single glass transition temperature (Tg) was observed by differential scanning calorimetry, thus indicating that PVC is miscible with the α‐methylstyrene‐acrylonitrile‐styrene in AMS‐ABS. The results from attenuated total reflection Fourier transform infrared spectra indicated that specific strong interactions were not available in the blends. With increasing amounts of AMS‐ABS, both heat distortion temperature and thermal stability were increased considerably. With regard to mechanical properties, flexural and tensile properties decreased with increasing AMS‐ABS content. A synergism was observed in impact strength. The morphology of both impact‐fractured and tensile‐fractured surfaces, observed by scanning electron microscopy, correlated well with the mechanical properties. It is suggested that there was a transition of fracture mechanisms with the changing composition of the binary blends—from shear yielding for blends rich in PVC to cavitation for blends rich in AMS‐ABS. J. VINYL ADDIT. TECHNOL., 19:1–10, 2013. © 2013 Society of Plastics Engineers

Tensile fracture dynamics and intrinsic plasticity of metallic glasses

Under tensile tests of metallic glasses (MGs) subjected to annealing below glass transition temperature Tg, ductile-to-brittle transition (DBT) occurs due to structural relaxation, which results in more ordered atomic packing and decrease of glass fraction φ. DBT is observed simultaneously with fracture mechanism transition: shear banding to cracking. All MG samples annealed under different temperature were also restricted to shear banding and cracking separately under small-aspect-ratio compression and compact tension avoiding DBT. Experimental results prove that as annealing temperature increases (or glass fraction φ decreases), strength for shear banding increases, while strength for cracking decreases; as φ becomes less than critical state φDBT, MG samples tend to cracking instead of shear banding. So, φDBT is proposed as an important parameter to characterize the intrinsic plasticity of various MGs and to conform to the previous factors soundly.

Effect of interfacial solute segregation on ductile fracture of Al–Cu–Sc alloys

Three-dimensional atom probe analysis is employed to characterize the Sc segregation at θ′/α-Al interfaces in Al–2.5wt.% Cu–0.3wt.% Sc alloys aged at 473, 523 and 573K, respectively. The interfacial Sc concentration is quantitatively evaluated and the change in interfacial energy caused by Sc segregation is assessed, which is in turn correlated to yield strength and ductility of the alloys. The strongest interfacial Sc segregation is generated in the 523K-aged alloy, resulting in an interfacial Sc concentration about 10 times greater than that in the matrix and a reduction of ∼25% in interfacial energy. Experimental results show that the interfacial Sc segregation promotes θ′ precipitation and enhances the strengthening response. A scaling relationship between the interfacial energy and precipitation strengthening increment is proposed to account for the most notable strengthening effect observed in the 523K-aged alloy, which is ∼2.5 times that in its Sc-free counterpart and ∼1.5 times that in the 473 and 573K-aged Al–Cu–Sc alloys. The interfacial Sc segregation, however, causes a sharp drop in the ductility when the precipitate radius is larger than ∼200nm in the 523K-aged alloy, indicative of a transition in fracture mechanisms. The underlying fracture mechanism for the low ductility regime, revealed by in situ transmission electron microscopy tensile testing, is that interfacial decohesion occurs at the θ′ precipitates ahead of crack tip and favorably aids the crack propagation. A micromechanical model is developed to rationalize the precipitate size-dependent transition in fracture mechanisms by taking into account the competition between interfacial voiding and matrix Al rupture that is tailored by interfacial Sc segregation.

Toughness of a patterned interface between two elastically dissimilar solids

A computational model is used to study the behavior of a crack that propagates along a sinusoidal cohesive interface between two dissimilar elastic solids under remote mode-I loading. The effective toughness of the interface is computed as a function of material and geometric parameters. For sufficiently large elastic mismatch, it is found that the fracture mechanism transitions at a critical ratio of roughness amplitude to wavelength (A/λ) from continuous propagation of a single crack, to a mechanism involving nucleation and coalescence of daughter cracks ahead of the main crack. This transition substantially reduces the toughening effect of the interface roughness. The results suggest some guidelines for practical design of failure resistant interfaces through appropriate choice of geometric, material, and cohesive parameters.

Identification of fatigue crack growth mechanisms in IN100 superalloy as a function of temperature and frequency

ABSTRACTA study is undertaken to investigate the fatigue crack growth rate properties of polycrystalline IN100 through the identification of crack growth mechanisms as a function of temperature, frequency and ΔK. An additional goal is to determine the stress free activation energy of IN100. Constant amplitude, load controlled tests are performed at room temperature (22 °C), 316 °C, 482 °C and 649 °C under two different loading frequencies of 20 and 0.33 Hz. These specimens are then analysed via scanning electron microscopy (SEM) to determine failure mechanisms. SEM shows that, as temperature increased from room temperature to 649 °C, the fracture mechanism transitions from transgranular to intergranular. The fracture mechanism is shown to transition from intergranular to transgranular at elevated temperatures as da/dN increases as a result of growing ΔK. Scanning electron microscopy shows that, as frequency decreases from 20 to 0.33 Hz at 649 °C, the fracture mechanism transitions from transgranular to intergranular.

Temperature-dependent mechanical and long crack behavior of zirconium diboride–silicon carbide composite

Abstract Hot pressed ZrB2–20 vol.% SiC ultra-high temperature ceramic composites have been prepared for strength and fracture investigations. Two composites fabricated under differing hot pressing temperatures with (ZSB) and without (ZS) B4C sintering aids were selected for room temperature modulus of rupture (MOR) strength and single-edge-notch bend (SENB) fracture toughness experiments. Structure property relationships were examined for both composites. MOR and stiffness temperature dependence was also investigated up to 1500 °C. Long crack propagation studies were conducted up to 1400 °C using the double cantilevered beam geometry with half-chevron-notch initiation zones. Residual Boron-rich carbide maximum particle sizes were found to be strength limiting in ZSB billets while SiC controlled strength in ZS billets. Flexure strength decreased linearly with temperature from 1000 to 1500 °C with no visible plastic deformation prior to fracture. Similar stiffness decreases were observed with a transition temperature range of 1100–1200 °C. Long crack studies produced R -curves that show no significant toughening behavior at room temperature with some modest rising R -curve behavior appearing at higher temperatures. These studies also show the plateau toughness increases with temperature up to 1200 °C. This is supported by an observed transition from primarily transgranular fracture at room temperature to primarily intergranular fracture at high temperatures. Wake zone toughening is evident up to 1000 °C with K R rise from 0.1 to 0.5 MPa√m. Beyond 1000 °C fracture mechanism transitions to include creep zone development ahead of crack tip with wake zone toughening vanishing.

Accelerated corrosion exposure in ultra thin sheets of 2024 aircraft aluminium alloy for GLARE applications

The effect of corrosion exposure on ultra thin (t<0.4mm) 2024-T3 aluminium sheet thickness is investigated. Microstructural analysis showed that for low accelerated corrosion exposure times, no surface deterioration existed and hence corresponding mechanical properties degradation was assumed to be incited by hydrogen diffusion and subsequent embrittlement. Medium exposure times resulted in cross-section reduction due to formation of sub-surface corrosion products, while crevice corrosion was evident for higher exposure times. Tensile specimens were pre-corroded for single or double sides and then mechanically tested. Decrease in mechanical properties was noticed for both L and T sheet rolling directions, even after a few minutes exposure, with transverse rolling direction having higher degradation values for same exposure times. It was noticed that a single sided corroded specimen needs approximately double exposure time to reach the same ductility decrease of a double sided specimen. Quantitative tensile properties degradation results are discussed; the results were correlated to observed corrosion degradation mechanisms. The stepwise ductile-to-brittle transition of tensile fracture mechanism with increasing corrosion exposure time was denoted via analysis of the ‘un-corroded’ region of tensile fracture surface. Single sided specimens exhibited quasi-cleavage fracture surfaces at the region below protected surface to corrosion.

Effect of Si Content on Fracture Behaviour Change by Strain Rate in Si Steels

Fracture behaviour transitions due to change in the strain rate in steels with various Si content ranging from 2% to 5 wt% were studied. Room-temperature tensile tests were conducted over wide range of strain rates ranging from 10-3 s-1 to 103 s-1. Concerning of the steels with low Si content (no more than 3%), the nominal stress - nominal strain curves represented both uniform and local elongations at all strain rates. On the other hand, in 4% Si steel at a strain rate higher than 101 s-1, the tensile sample broke down without local elongation (necking). The stress at breaking was found to be nearly equal to its work hardening rate. The strain rate at which fracture behaviour transition took place in 5% Si steel (10-1s-1) was lower than that in 4% Si steel. TEM observations clarified the existence of deformation twins in the sample that fractured without necking. These results indicated that Si addition is subject to the brittle fractures and that the fracture mechanism transition is closely related with the deformation twinning behaviour.

Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments

Fatigue crack propagation (FCP) was studied on wrought magnesium alloys, AZ31 and AZ61, in laboratory air, dry air and distilled water. In laboratory air, the FCP rate versus stress intensity factor plots consisted of two sections with different slopes, which was clearly recognized after allowing for crack closure. This was attributed to the transition in fracture mechanisms operated. In distilled water, FCP rates were nearly the same as in laboratory air, while in dry air, an order of magnitude slower than in laboratory air and distilled water. After allowing for crack closure, the environmental effects still existed and FCP rates were the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Fractography revealed that the fracture mechanisms operated in laboratory air and in distilled water were different, possibly hydrogen embrittlement and anodic dissolution, respectively.

Quantification of constraint effects in fracture mechanism transition for cracked structures under mixed mode loading

ABSTRACTThe objective of the study is to evaluate the effects of plastic constraint on transition between tensile‐type and shear‐type fracture. The T‐stress is employed as the quantifying parameter for constraint and is incorporated into the existing theoretical criteria for modelling this transition. It is found that different constraint levels can dramatically alter the transition point. To verify this finding, two sets of mixed mode tests with different constraint levels are carried out. Alongside the theoretical and experimental study, finite element simulation is performed to verify and support these findings. Substantially improved agreement is observed with experimental data if the effect of plastic constraint on transition is included.

Effects of Mn content and texture on fatigue properties of as-cast and extruded AZ61 magnesium alloys

Fatigue behavior of as-cast and extruded AZ61 magnesium alloys in ambient air (20 °C–55%RH) was investigated. It was found that size and distribution of cast defect influenced tensile and fatigue performance of the as-cast alloy. Fatigue limit of the as-cast alloy was significantly low compared to the extruded alloy. The casting defects served as stress concentration sites for fatigue crack nucleation. Fatigue tests were also carried out on a high Mn content alloy. All of the specimens failed from an inclusion near the specimen surface. Fatigue limit of Mg alloy with high Mn content was lower compared to that of the low Mn content alloy. Further, investigation on the effect of texture on fatigue and fatigue crack growth behavior of the extruded AZ61 magnesium alloy plate was carried out. The results showed that fatigue strength in the longitudinal direction to the extruded direction was higher compared to those in the transverse and 45° directions. Significant effect of specimen orientation on fatigue crack growth behavior for both short and long cracks was found near the threshold region. However, regardless of specimen orientation, the d a / d N – Δ K eff curves for all three kinds of specimens were in a narrow band. It is suggested that the difference in the fatigue life among the specimen orientations will be mainly due to the difference in the crack closure behavior. A transition of fracture mechanism was found for a long crack. Slip fracture mechanism was dominant above the transition point, whereas below the transition point, slip fracture mechanism was associated with cleavage fracture.