Dominant Fracture Mechanism Research Articles

Effect of misorientation evolution on the fracture mechanism

This communication identifies the fracture mechanism for Ti-6Al-4V alloy during superplastic forming, which is established based on the grain size evolutions. Particularly, the dominant fracture mechanism is identified by misorientation evolutions. In total, the grain-growth-dependent fracture mechanism is proposed, potentially useful for forming limit determination.

Effect of mixed-mode loading on surface crack propagation in steels

To understand the effect of mixed-mode loading on surface crack propagation in structural alloy steels, fatigue crack growth tests were performed on surface crack tension (SCT) specimens with initial semi-elliptical surface cracks. SCT specimens with cracks normal to the load axis were used for the mode I tests. Experiments with mixed-mode loading were conducted on SCT specimens with inclined surface cracks. A specific feature of the surface crack propagation, in the segmentation of the crack front, the occurrence of «factory roof» patterns, and deviations in the crack front from a planar shape were established under mixed-mode loading. A reverse engineering technique was applied in a numerical study to reproduce a complex fracture surface under mixed-mode loading. The equivalent stress intensity factor (SIF) formulation was used to interpret the fatigue-crack growth rate. A significant reduction in the fatigue crack growth rate was established for mixed-mode loading compared with pure mode I conditions. Several fracture mechanisms were identified along the surface crack front under mixed-mode loading. The areas corresponding to changes in the dominant fracture mechanism were identified using detailed fractography of the fracture surfaces.

The Effect of Intrafibrillar Post-Yield Behavior on Fracture of Mineralized Collagen Fibril Arrays

Mineralized collagen fibrils (MCFs) are important building blocks of bone at the submicroscale, and the mechanical performance of MCF arrays has a great influence on fracture resistance of bone at large length scales. In this study, we carry out the analyses of fracture process in MCF arrays under tensile loading. The plastic deformation of extrafibrillar matrix (EFM), post-yield behavior of MCFs, MCF breakage and debonding of the MCF-EFM interface are accounted for in the calculations. It is found that the fracture mechanisms of MCF arrays depend on the post-yield characteristics of MCFs. Shear-band-induced cracking of MCFs is the dominant fracture mechanism in the case of strain softening of MCFs, while strain hardening of MCFs promotes the MCF-EFM interfacial debonding, which controls fracture of MCF arrays. In addition, we reveal that plastic energy dissipation of MCFs and EFM provides major contribution to toughness of MCF arrays. Compared with the case of strain softening of MCFs, the MCFs exhibiting post-yield strain hardening can give rise to larger plastic deformation zone in MCFs and activate higher levels of plastic strain of EFM, enhancing plastic energy dissipation and thereby amplifying toughness of MCF arrays. The findings of this study shed new light on the fracture mechanisms of bone associated with alterations in submicroscale structure and composition.

Fracture toughness of PA6/POE‐g‐MA/TiO2 ternary nanocomposites according to the essential work fracture method

AbstractPolymer‐based nanocomposites can be used in a wide variety of applications in the industrial, electronics, and energy segments. In order to attain the application‐specific properties that are desired, good mechanical and fracture efficiency is frequently required. Polyamide 6 (PA6)/polyethylene octene grafted with maleic anhydride (POE‐g‐MA)/titanium dioxide (TiO2) nanocomposites' fracture characteristics were investigated utilizing the essential work of fracture (EWF) approach in this work. Four levels of POE‐g‐MA (0, 10, 20, and 30 wt%) and three levels of TiO2 (0, 2, and 4 wt%) are therefore utilized. The reliability of the EWF theory is demonstrated via the self‐similarity of the force‐displacement curve and Hill's analysis. Results showed that EWF and non‐essential work of fracture (non‐EWF) were improved by 73% and 54%, respectively, by increasing POE‐g‐MA up to 30 wt%. The improvement of EWF value confirmed the role of POE‐g‐MA as an impact modifier. Nevertheless, by increasing TiO2 up to 4 wt%, EWF, and non‐EWF decrease by 20% and 25%, respectively. Adding 30 wt% POE‐g‐MA reduced tensile strength and enhanced strain at the break by 45% and 109%, respectively. Moreover, the tensile strength was enhanced up to 10% by adding 4 wt% of TiO2 content. However, the strain at break was decreased by 44% by increasing 4 wt% of TiO2 nanoparticle. In addition, the dominant fracture mechanism in polyamide‐based blends and nanocomposites is shear‐yielding and fibrillation structures.

In situ observation on fracture behavior of the three-phase zone of the L415/N08825 bimetallic composite pipe welded joint

In this study, the fracture behavior of the three-phase zone of the L415/N08825 bimetallic composite pipe welded joint was investigated by in-situ tensile test and EBSD observation. According to the results of the tensile tests, the fracture occurs in the heat-affected zone (HAZ). There, the interface between the cladding and the base layer first separated, the N08825 layer fractures, and then the L415 layer finally fractures. The results of the in-situ tensile test show that the crack initiated in the unmixed zone of the interface between the L415 and N08825 layers, and then multiple crack initiation points appeared on the N08825 side, eventually merging and rapidly propagating. The crack propagation path is approximately straight, and the interface has little effect on the propagation path. The dominant fracture mechanism is microporous aggregated ductile fracture. The results of EBSD observation revealed that the high frequency of low angle grain boundaries (LAGBs) concentrated in the unmixed zone originated from the substructure. The CSL grain boundaries of the BCC structure crystal are mainly Ʃ3, Ʃ11, and Ʃ17b, and the CSL grain boundaries of the FCC structure crystal are mainly Ʃ3, Ʃ5, and Ʃ17b, which increase fracture resistance and improve toughness. In addition, the unmixed zone and its interface with L415 have a high level of stress concentration.

Revealing deformation mechanisms in additively manufactured Alloy 718: Cryogenic to elevated temperatures

Tensile behavior of Alloy 718, fabricated through laser powder directed energy deposition, was examined over a wide temperature range (−195, 25, 200, 425, 650, 870, and 980 °C). The tensile properties, deformation mechanisms, and their temperature dependence were specifically investigated. The microstructure and mechanical properties were characterized in fully heat treated condition; i.e., first stress relieved, then homogenized, then solution annealed, and finally 2-step-aged. Tensile results showed thermally stable behavior from room temperature up to 650 °C, similar to other additively manufactured and wrought counterparts. Rapid reductions of strength with increasing test temperature were observed at −195 to 25 °C and 650–870 °C ranges, which were respectively ascribed to a significant reduction in deformation twinning density and the coarsening of γ″-precipitates as confirmed by electron microscopy. The fully heat treated microstructure also contained populous Mo/Nb-rich carbides within grain interior which were observed to be the dominant mechanism of fracture and responsible for the temperature insensitive ductility up to 650 °C. At 870 and 980 °C, sliding between dynamically recrystallized grains became significant resulting in an increased ductility.

Characterisation of fracture evolution of a single cemented brittle grain using in-situ X-ray computed tomography

In this paper, the diametrical fragmentation of a single brittle glass bead cemented by ductile epoxy resin is studied using in-situ X-ray computed tomography, focusing on the fractures within the grain that underpin its crushability. Key quantities of interest include the evolution of fracture regions, paths and orientations, as well as the evolution of fracture statistics within the grain. The complex nature of the fracture process is revealed by qualitative and quantitative analysis based on both continuous X-ray radiography and discrete tomography. A series of image processing techniques are used and developed to obtain the statistical details of fragment volume, surface area, morphological geometry, fracture spatial locality and fracture preferential orientations. The single cemented grain failure consists of an initial splitting fracture process creating large and irregular glass fragments, followed by a further rapid propagation of multiple fractures through the cleavage fragments in the form of severe comminution that tends to generate smaller and more regular fragments. Despite the whole fragmentation process shows a shifting trend of the overall median anisotropy of fragments and a decreasing trend of conversion rate between fracture energy and input energy, it still maintains a constant fragment surface area power law distribution and appears to remain scale-invariant over time. The central region of the glass bead is markedly more susceptible to fracture with a constant preferred fracture orientation along the loading direction, indicating that the tensile-activated fracture is the dominant fracture mechanism during diametrical fragmentation.

Effect of heat input of TIG repair welding on microstructure and mechanical properties of cast AZ91 magnesium alloy

In the present study, the effect of heat input of gas tungsten arc welding on the microstructure and mechanical properties of AZ91 magnesium alloy was investigated. The microstructural studies were performed by using an optical microscope and scanning electron microscope equipped with energy dispersive X-ray spectroscopy, and tensile test was used to evaluate the mechanical properties. It was shown that, comparing to the base metal, a fine microstructure of α(Mg) and Mg17Al12 phase is formed in the weld metal and the size of α(Mg) phase slightly increased with heat input during welding process. The results also indicated that the thickness of partially melted zone increased with heat input. Tensile strength of welded samples was lower than that of the base metal and it decreases with increasing heat input. Failure of the welded specimens accrues in the partially melted zone. Studying of fracture surface revealed that the interface of α(Mg) and Mg17Al12 compound acts as crack initiation and growth path during the tensile test. Formation of cleavage steps in the fracture surface was the dominant fracture mechanism during tensile tests.

Fatigue characterization of polyethylene under mixed mode I/III conditions

Mixed mode I/III fatigue fracture behaviour of bulk polymers is scarcely investigated. Thus, the aim of this work is to focus on the comparison of pure mode I and mixed mode I/III fatigue loading of polyethylene. During mixed mode I/III testing, both increases and decreases of lifetime were observed, depending on the levels of applied mode I and mode III loading. Additionally, the critical influence of friction and the accompanying increase of local temperature between the shear loaded crack flanks in mode III, as well as the dominant fracture mechanisms were discussed based on the fracture surface morphology.

Acoustic properties of fracture of dental restorative materials and endocrown restorations under quasi-static loading

Objectives(1) To investigate the properties of acoustic emission (AE) signals from fracture of dental materials and endocrown restorations during quasi-static compression using the continuous wavelet transform analysis. (2) To determine by the parameters of AE signals in which structural element of tooth-endocrown system the fracture starts. MethodsFive restorative materials (zirconium dioxide (Prettau zirconia), ceramics (IPS e.max Press), metal ceramics (GC Initial MC+Nicrallium N2 BCS), composite resin (Nano Q), luting cement (Relyx™ U200)) were manufactured into discs, 13mm diameter, 5mm thick, which were divided into 5 groups (Group B, C, D, E, F, respectively) and included into Type 1. Twenty-five extracted human molars were divided into 5 groups and included into Type 2: Group A (control, no restoration); BE (tooth restored by zirconium dioxide endocrowns); CE (restored by ceramic endocrowns); DE (restored by metal ceramic endocrowns); EE (restored by composite resin endocrowns). An increasing load was applied to the center of the samples with a hard steel ball until a fracture occurred. The loading rate was 0.12mm/min. Two-channel AE detection system was used to record the AE activity during testing. The parameters including the peak frequency, bandwidth, duration time and energy of AE signals were analyzed. ResultsThe restorative materials (Type 1) differ from each other by acoustic properties such as the peak frequency main bands and energy of AE signals. The detected AE signals correspond to the different fracture types. The dominant fracture mechanism of Group C, E and F specimens was the macrocracking (brittle fracture), and of Group D specimens was the microcracking, which alternated with the plastic deformation and macrocracking. For the number of events corresponding to the brittle fracture, statistically significant differences (p<0.05) were found between all group pairs. AE signals were not released during compression test of the zirconium dioxide samples, thus the fracture was not detected.All the AE signals detected during compression test of the different endocrown systems (Type 2) had two peak frequency bands. The fracture of Groups A, BE, CE and DE samples characterized by alternating the micro- and macrocracking with plastic deformation, and the brittle fracture was the dominant fracture mechanism. The restorations of Group EE fractured without plastic deformation. For the number of events corresponding to the brittle fracture, statistically insignificant differences (p>0.05) were found between Groups A vs CE and A vs EE, and statistically significant differences (p<0.05) were found between Groups A vs BE and A vs DE. SignificanceThe initiation of fracture in the restorations with endocrowns made from different materials begins with the tooth damage. The metal ceramic endocrown restoration showed the highest fracture resistance, what is especially important for clinical use.

Effects of deformation parameters and stress triaxiality on the fracture behaviors and microstructural evolution of an Al-Zn-Mg-Cu alloy

Abstract Hot tensile tests are performed on an Al-Zn-Mg-Cu alloy at larger temperature and strain rate ranges. The influences of deformation parameters and stress triaxiality on hot tensile fracture characteristics, fracture mechanisms and microstructural evolution are discussed. It is found that the maximum tensile load increases with raising stress triaxiality. The fracture strain rises with increasing strain rate or decreasing stress triaxiality. When the deformation temperature is raised, the fracture strain firstly increases and then decreases. Dynamic recovery (DRV) is the main softening mechanism. At low strain rates or high deformation temperatures, the partial dynamic recrystallization (DRX) behavior occurs, and continuous dynamic recrystallization (CDRX) is the main DRX mode. The dominant fracture mechanism is the coalescence of micro-voids at the tested deformation conditions. Due to the occurrence of DRX, the intergranular fracture also occurs at low strain rates or high deformation temperatures. The low strain rate or high stress triaxiality easily induces the appearance of micro-voids, which accelerate the fracture failure.

Impact of Particle Size Distribution for Variable Mixing Time on Mechanical Properties and Microstructural Evaluation of Al-Cu/B4C Composite

Abstract In the present work the combined effects of particle size and distribution on the mechanical properties of the 20vol.% B4C particle reinforced Al–Cu alloy composites by powder metallurgy is investigated. The aim of this work was to evaluate the effect of mixing time (5h, 15h and 25h) and particle size (23µm and 67µm) of B4C particles on the metallurgical and mechanical properties. It has been found that small ratio between matrix/reinforcement particles sizes resulted in more uniform distribution in the matrix. The particles distributed more uniformly in the matrix with increasing in mixing time. The results also showed that homogenous distribution of the B4C particles resulted in higher hardness, ultimate tensile strength, yield strength and elongation. Fracture surface observations showed that the dominant fracture mechanism of the composites with small B4C particle size (23µm) is ductile fracture of the matrix, accompanied by the “pull-out” of the particles from the matrix, which is attributed to positive effect of reinforcement particles in resistance to the movement of dislocations while the dominant fracture mechanism of the composites with large B4C particle size (67µm) is ductile fracture of the matrix, accompanied by the B4C particle fracture

Thermal cycling effects on the creep-fatigue interaction in type 316LN austenitic stainless steel weld joint

Thermomechanical fatigue (TMF) tests under in-phase temperature-mechanical strain combination were performed on a type 316 LN austenitic stainless steel weld joint using temperature ranges of 573–823 K, 623–873 K and 673–923 K employing a mechanical strain amplitude (Δεmech/2) of ±0.4%. Isothermal low cycle fatigue (designated as IF) tests were also carried out at the maximum temperatures (Tmax) of TMF cycling. The interaction of creep deformation with IF and TMF cycling was assessed by incorporating hold periods of 1 min and 5 min at the maximum tensile strain. TMF cycling resulted in considerably lower lives compared to IF, with the life reduction being more significant under hold conditions. Further, a clear difference in the cyclic stress response (CSR) was observed between creep-TMF and creep-IF cycling, depending on the hold duration and temperature range. The contribution of creep deformation to the overall damage was found to be higher under TMF compared to IF cycling at the Tmax. The observed variations in the cyclic lives under IF and TMF cycling with and without hold times were explained on the basis of the dominant fracture mechanisms identified through detailed microstructural characterization of the crack initiation and propagation modes. The extent of intergranular damage was seen to increase with an increase in the temperature range and the hold period. The role of oxidation in the development of damage was observed to be different under creep-TMF and creep-IF cycling. Numerous grain boundary microvoids and their inter-linkages caused enhanced crack propagation under creep-TMF, compared to creep-IF cycling. A comparison of the deformation behaviour, cyclic life and damage developed in the material under IF and TMF suggested that the performance of the weld joint under TMF cycling cannot be satisfactorily represented through IF tests at the Tmax.

Stress corrosion cracking fracture mechanism of cold-drawn high-carbon cable bolts

Premature failures of cable bolts due to stress corrosion cracking (SCC) in underground reinforcement systems are universal issues with limited practical countermeasures at present. To determine the SCC fracture mechanism which is an essential step to develop the potential solutions, the environment conditions leading to SCC failures were examined using four-point bend tests. Fractographic features of both serviced-failed and laboratory-failed cable bolts were investigated and compared. It was identified that the fracture surfaces of cable wires were characterised by step-shape fracture mode. The highly-oriented wavy lamellae microstructures of cable wires, produced during cold-drawn manufacturing process, have significant effects on the direction and path of crack propagation. Hydrogen-assisted SCC was suggested as the dominant fracture mechanism in cable bolt failures. SCC only occurred in the environmental conditions that promoted the diffusion of hydrogen diffusion into the cable wires, and crack growth rates were determined by the hydrogen diffusion rates. This study provided fundamental knowledge on fracture mechanism of cable bolts which can be applied to further study in identifying potential countermeasures against the SCC of cold-drawn high-carbon cable bolts.

In-situ 3D fracture propagation of short carbon fiber reinforced polymer composites

This paper explored the 3D progressive fracture process of a short carbon fiber reinforced polymer composite under uniaxial tensile loading via high-resolution in-situ micro X-ray computed tomography (μXCT). Microstructural features extracted from the μXCT images were analyzed with the Halpin-Tsai model, laminate analogy approach, and shear-lag model to calculate the mechanical properties and interpret the different fracture morphologies and progressions observed in the “skin-core-skin” structure. The in-situ μXCT scanning revealed a clear fracture progression in the skin layer, which was defined by four stages, i.e., nucleation of small pores at fiber ends, coalescence of pores, initiation of cracks, and propagation of cracks until fracture, whereas no significant fracture feature was found in the core layer. To analyze the difference in the fracture mechanisms in the skin and core layers, the maximum fiber normal stress and average interfacial shear stress were calculated from the microstructural features. The results were also confirmed by the 3D strain distribution quantified via a volumetric digital image correlation method. It was determined that the dominant fracture mechanism in the skin layer was fiber pull-out, rather than fiber breakage, as a consequence of pore formation at the fiber ends; its extent mainly depended on the fiber length and orientation angle. Fiber/matrix debonding was the main fracture mechanism in the core layer, which resulted from the propagation of cracks initiated from the skin layer.

Microstructural and Mechanical Behavior of Al6061-Graphene Oxide Nanocomposites

In this paper, the mechanical properties and microstructure of Al6061-graphene oxide (GO) nanocomposites material, produced by stir casting and hot rolling process have been investigated. In order to make composite samples, GO nanoplates with various weight percents of 0.2, 0.5 and 0.8% were added to the molten Al6061 through the stir casting process. Then the cast composite slabs were hot rolled at 530 °C. Tensile and hardness tests were used to study the mechanical properties of nanocomposite specimens. The microstructures of samples were examined by optical and scanning electron microscopes. The scanning electron microscope images and energy dispersive spectroscopy map analyses showed that the GO nanoplates have been uniformly distributed within the Al matrix. The mechanical test results showed considerable improvements in the hardness (by 41%), yield strength (by 43%), tensile strength (by 38%), ductility (by 58%) and toughness (by 116%) of the composite samples with respect to the base alloy. Studying the fracture surface also showed that ductile fracture is the dominant mechanism of fracture in all the specimens.

Investigation of mechanical properties, formability, and anisotropy of dual phase Mg–7Li–1Zn

In this article, the mechanical properties, microstructure, forming limit diagram (FLD) and anisotropy of dual phase Mg-7Li–1Zn alloy have been evaluated by a uniaxial tensile test at three directions, scanning electron microscopy (SEM), optical microscopy (OM) and hemispherical punch test at ambient temperature. The optical microscopy images showed that the microstructure of as-cast LZ71 alloy possesses a dual phase microstructure such as β-Li matrix with body-centered cubic (BCC) structure and a partitioned α-Mg phase with hexagonal closest packed (HCP) structure in lath shape. After rolling process, the α-Mg phase has been elongated and arranged in the rolling direction and anisotropy at different directions increased, but after full annealing, microstructure arranged more uniform and changed from elongated to the uniform structure which this is the factor that reflects the reduction of anisotropy. FLD shows the limiting surface strains that sheet metal can endure before the start of localized necking and fracture at different deformation modes in tension-tension and tension-compression loading paths. The FLD of Mg–7Li–1Zn showed that this material has desirable formability due to the BCC structure with high slip system in the ambient temperature. Also, the results of the tensile test matched with the obtained FLD. Results of SEM revealed many dimples and some flat and cleavage planes. They showed the combination of ductile and brittle fracture, but the ductile fracture is the dominant fracture mechanisms for Mg–7Li–1Zn. In summary, in the dual phase, Mg–Li alloys via controlling the weight percent of lithium and other alloying elements can be achieved the desired mechanical properties and used for various applications.

Effect of Ferrite/Martensite Phase Size on Tensile Behavior of Dual-Phase Steels with Nano-Precipitation of Vanadium Carbides

This study investigates the effect of ferrite/martensite phase size and dispersion of nano-precipitates in ferrite on strength, ductility, and fracture behavior of dual-phase (DP) steels. Dispersion of nano-precipitates in ferrite improves the strength with keeping sufficient uniform elongation and post-uniform elongation, while decrease in ferrite/martensite phase size has a little effect on the increase in strength in the phase size range investigated. However, the refinement of phase size significantly improves the post-uniform elongation and reduction in area. It is concluded therefore that a simultaneous combination of refinement of ferrite/martensite phase size and dispersion of nano-precipitation in ferrite is effective to significantly improve the strength and strength–post-uniform elongation in DP steels. Quantitative analysis of void formation reveals that fracture by martensite cracking is a primary fracture mechanism in coarse phase-sized DP samples, while ferrite/martensite interface decohesion becomes a dominant fracture mechanism in fine phase-sized DP samples. It is suggested that the refinement of phase size is a promising strategy to change the fracture behavior from brittle to ductile manner, leading to an enhancement of local deformability after the necking.

Effect of δ-Al3CuNi phase and thermal exposure on microstructure and mechanical properties of Al-Si-Cu-Ni alloys

The effect of δ-Al3CuNi phase and thermal exposure on microstructure and mechanical properties of Al-Si-Cu-Ni alloy is investigated by optical microscopy, scanning electron microscope, and tensile test. The alloys with different volume fractions of δ-Al3CuNi phase are designed according to the thermodynamic calculation, which is confirmed by the microstructure characterization. The mean diameter of primary Si increases while the eutectic Si decreases with increasing the volume fraction of δ-Al3CuNi phase. The eutectic Si is coarsening during thermal exposure at 350 °C. Both strip-shape and fishbone-shape δ-Al3CuNi phase are observed in the as-cast alloys. In addition, the fishbone-shape δ-Al3CuNi phase is composed of many independent short rods. Parts of the thin short rods of δ-Al3CuNi phase change into γ-Al7Cu4Ni phase after thermal exposure at 350 °C for 200 h. The ultimate tensile strength and yield strength increase while the elongation decreases with increasing the volume fraction of δ-Al3CuNi phase. Comparing with the as-cast alloys, the ultimate tensile strength and yield strength are reduced after thermal exposure at 350 °C for 200 h, but the elongation of the alloys is increased. The failure mechanism of the alloys tested at 25 °C is quasi-cleavage fracture and ductile fracture is the dominant fracture mechanism tested at 350 °C.

A study of abrasive waterjet multi-pass cutting on kerf quality of carbon fiber-reinforced plastics

Carbon fiber-reinforced plastics (CFRPs) used extensively in the aerospace and automobile industries pose a challenge into machining techniques because of their hard-machine property. Abrasive waterjet (AWJ) as an emerging technology has been certified as the effective technology to machine such materials, especially for AWJ cutting. In order to further improve the cutting kerf quality without sacrifice of efficiency, this paper is aimed to originally investigate CFRP cutting using multi-pass process with changed parameters among different passes, which is indeed absent in previous works. At first, the material removal mechanism by AWJ multi-pass cutting was discussed to explore the delamination mechanism related to removal material behaviors and to certify that multi-pass cutting can be applied into CFRP machining. The micro-machining including microcutting, plow, and brittle fracture of fibers and matrix was prime material removal mechanism, and the inter-laminar delamination occurring at exit zone was related to the dominant removal mechanism of brittle fracture of fibers, which was influenced heavily by parameters selected. Secondly, the kerf quality produced by the multi-pass cutting with changed parameters and constant parameters is comparatively analyzed. The former could further reduce 53% of kerf taper and improve efficiency 13% as appropriate parameters selected without sacrifice of kerf quality. Regarding the surface quality in smooth cutting zone (SCZ), the former did not show an advantage over the later unless highly increasing pressure at second-pass cutting. It was also induced from experimental results that arranging the low traverse speed at first-pass cutting was superior to it at second-pass cutting.