Fracture Toughness Behavior Research Articles

Newly revealed features of fracture toughness behavior of spot welded dual phase steel sheets for automotive bodies

Abstract Fracture toughness is one of the parameters, which are used to estimate the fatigue life of resistance spot-welded (RSW) joints. A spot-welded pair is affected by the shear stress of the weld zone when it is exposed to tensile load. Repetitive loads reduce the fatigue life of the spot weld, and the material splits at the spot-welded region. This study investigates the effect of welding current, weld time and nucleus size ratios on the fracture toughness of RSW of galvanized DP450 steels having 1.0 mm thickness. The specimens were joined by spot welding at different welding currents and times. Welding processes were carried out using 3, 5, 7 and 9 kA welding current and 10, 20, 30 and 40 cycles (1 cycle = 0.02 s) weld time and the electrode pressure was fixed at 600 MPa. All series of specimens were exposed to tensile shear test in order to determine the fracture toughness. The fracture toughnesses for all series of RSW joints were calculated by using the formula given in the literature. The nugget diameters, core sizes and their heights were measured via an optical microscope. The Vickers microhardness measurement was carried out on the weld nugget, heat affected zone (HAZ) and base metal. Nucleus size ratios were calculated. The results of the study demonstrated that the fracture toughness of RSW depended not only on the nugget diameter D, but also sheet thickness t, tensile rupture force F, hardness H and nucleus size ratios hn/dn.

Flexural strength and reliability of monolithic and trilayer ceramic structures obtained by the CAD-on technique

ObjectiveTo evaluate the flexural strength, Weibull modulus, fracture toughness, and failure behavior of ceramic structures obtained by the CAD-on technique, testing the null hypothesis that trilayer structures show similar properties to monolithic structures. MethodsBar-shaped (1.8mm×4mm×16mm) monolithic specimens of zirconia (IPS e.max ZirCAD – Ivoclar Vivadent) and trilayer specimens of zirconia/fusion ceramic/lithium dissilicate (IPS e.max ZirCAD/IPS e.max CAD Crystall./Connect/IPS e.max CAD, Ivoclar Vivadent) were fabricated (n=30). Specimens were tested in flexure in 37°C deionized water using a universal testing machine at a crosshead speed of 0.5mm/min. Failure loads were recorded, and the flexural strength values were calculated. Fractography principles were used to examine the fracture surfaces under optical and scanning electron microscopy. Data were statistically analyzed using Student's t-test and Weibull statistics (α=0.05). ResultsMonolithic and trilayer specimens showed similar mean flexural strengths, characteristic strengths, and Weibull moduli. Trilayer structures showed greater mean critical flaw and fracture toughness values than monolithic specimens (p<0.001). Most critical flaws in the trilayer groups were located on the Y-TZP surface subjected to tension and propagated catastrophically. Trilayer structures showed no flaw deflection at the interface. SignificanceConsidering the CAD-on technique, the trilayer structures showed greater fracture toughness than the monolithic zirconia specimens.

Effect of inhomogeneous distribution of non-metallic inclusions on crack path deflection in G42CrMo4 steel at different loading rates

An inhomogeneous distribution of non-metallic inclusions can result from the steel casting process. The aim of the present study was to investigate the damaging effect of an inhomogeneous distribution of nonmetallic inclusions on the crack extension behavior. To this end, the fracture toughness behavior in terms of quasi-static J-?a curves was determined at room temperature. Additionally, dynamic fracture mechanics tests in an instrumented Charpy impact-testing machine were performed. The fracture surface of fracture mechanics specimens was analyzed by means of scanning electron microscopy. It was shown that an inhomogeneous distribution significantly affected the path and, therefore, the plane of crack growth. Especially clusters of non-metallic inclusions with a size of up to 200 ?m exhibited a very low crack growth resistance. Due to the damaging effect of the clusters, the growing crack was strongly deflected towards the cluster. Furthermore, crack tip blunting was completely inhibited when inclusions were located at the fatigue precrack tip. Due to the large size of the non-metallic inclusion clusters, the height difference introduced by crack path deflection was significantly larger than the stretch zone height due to the crack tip blunting. However, the crack path deflection introduced by a cluster was not associated with a toughness increasing mechanism. The dynamic loading ( 1 0.5 5 s MPam 10 ? ? K? ) did not result in a transition from ductile fracture to brittle fracture. However, the crack growth resistance decreased with increased loading rate. This was attributed to the higher portion of relatively flat regions where the dimples were less distinct.

Effects of curing pressure on mode II fracture toughness of uni-directional GFRP laminates

The purpose of this work is to investigate the effect of curing pressure on the Mode II interlaminar fracture toughness behaviour of uni-directional glass fibre reinforced plastics (GFRP). Investigation was carried out experimentally on uni-directional GFRP specimens using end notched flexure (ENF) tests to find out the changes in the material and geometric properties due to the application of pressure. Stiffness of the different laminates cured under different pressure was studied and it was shown that both material and geometric stiffness accounts for the total stiffness of the laminate, but their contribution to total stiffness varies. It is shown that fracture toughness values, GIIC, remain unaffected by stiffness since it is only related to the new surfaces created due to crack initiation and propagation. It is shown that the thickness of the matrix layer influences all the properties, including the fracture toughness. The laminates cured at 5 Psi, 400 Psi and 800 Psi exhibit different fracture toughness. Optimum thickness of matrix layer offers both adhesive and cohesive resistance to the crack propagation and thus offers high fracture resistance.

Fracture Toughness and Fatigue Crack Growth Behavior of As-Cast High-Entropy Alloys

The fracture toughness and fatigue crack growth behavior of two as-vacuum arc cast high-entropy alloys (HEAs) (Al0.2CrFeNiTi0.2 and AlCrFeNi2Cu) were determined. A microstructure examination of both HEA alloys revealed a two-phase structure consisting of body-centered cubic (bcc) and face-centered cubic (fcc) phases. The notched and fatigue precracked toughness values were in the range of those reported in the literature for two-phase alloys but significantly less than recent reports on a single phase fcc-HEA that was deformation processed. Fatigue crack growth experiments revealed high fatigue thresholds that decreased significantly with an increase in load ratio, while Paris law slopes exhibited metallic-like behavior at low R with significant increases at high R. Fracture surface examinations revealed combinations of brittle and ductile/dimpled regions at overload, with some evidence of fatigue striations in the Paris law regime.

Effect of hybridization on the mode II fracture toughness properties of flax/vinyl ester composites

In this study, flax fiber reinforced and flax/basalt hybridized vinyl ester composites were produced and their interlaminar fracture toughness (mode II) behavior was investigated using the three-point bend end-notched flexural (3ENF) testing. From the results, the average of the maximum values for each group of specimen obtained for critical strain energy release rate GIIC and stress intensity factor KII for flax/vinyl ester specimens were 1,940 J/m2 and 134 kPam0.5. Similarly, GIIC and KII values recorded for hybridized specimens were 2,173 J/m2 and 178 kPam0.5, respectively. The results for the flax/basalt hybridized composites exhibited an improved fracture toughness behavior compared to flax/vinyl ester composites without hybridization. The cohesive zone modeling (CZM) was also used to predict the delamination crack propagation in mode-II in laminated composite structures. After the experimental study, the 3ENF specimens were modeled and simulated using ANSYS. The CZM/FEA results were in reasonable agreement with the experimental results. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

Investigation of meso-failure behaviors of Jinping marble using SEM with bending loading system

In this study, the meso-failure mechanism and fracture surface of Jinping marble were investigated by means of scanning electron microscope (SEM) with bending loading system and laser-scanner equipment. The Yantang and Baishan marbles specimens from Jinping II hydropower station were used. Test results show that the fracture toughness and mechanical behaviors of Yantang marble were basically higher than those of Baishan marble. This is mainly due to the fact that Baishan marble contains a large percentage of dolomite and minor mica. Crack propagation path and fracture morphology indicated that the direction of tensile stress has a significant effect on the mechanical behaviors and fracture toughness of Baishan marble. For Yantang and Baishan marbles, a large number of microcracks around the main crack tip were observed when the direction of tensile stress was parallel to the bedding plane. Conversely, few microcracks occurred when the direction of tensile stress was perpendicular to the bedding plane. The presence of a large number of microcracks at the main crack tip decreased the global fracture toughness of marble. The results of three-point bending tests showed that the average bearing capacity of intact marble is 3.4 times the notched marble, but the ductility property of the defective marble after peak load is better than that of the intact marble. Hence, large deformation may be generated before failure of intact marbles at Jinping II hydropower station. The fractal dimension of fracture surface was also calculated by the cube covering method. Observational result showed that the largest fractal dimension of Yantang marble is captured when the direction of tensile stress is parallel to the bedding plane. However, the fractal dimension of fracture surface of Yantang and Baishan marbles with tensile stress vertical to the bedding plane is relatively small. The fractal dimension can also be used to characterize the roughness of fracture surface of rock materials.

Microstructure, mechanical properties and oxidation resistance of Nb silicide based ultrahigh temperature alloys with Hf addition

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–3Cr–3Al–2B–xHf (x=0, 2, 4 and 8) (at%) were prepared by vacuum non-consumable arc melting. The effects of Hf content on the phase selection, microstructure, room-temperature fracture toughness, compressive performance and isothermal oxidation behavior at 1250°C of the alloys have been investigated. The results show that the formation of α(Nb,X)5Si3 is suppressed and instead the formation of γ(Nb,X)5Si3 is promoted by Hf addition. The Hf addition (especially 8at%) improves the room-temperature fracture toughness of the alloy. The hardness of γ(Nb,X)5Si3 increases with its Hf concentration. The compression strength of the alloy is the highest at 4at% Hf, and then obviously reduces at 8at% Hf due to embrittlement of the alloy (especially that of γ(Nb,X)5Si3). The oxidation resistance of the alloy has been firstly ameliorated by the lower Hf addition (2at%) and then degraded by the higher Hf addition (especially 8at%). The details have been discussed.

Effect of the content of B4C on microstructural evolution and wear behaviors of the laser-clad coatings fabricated on Ti6Al4V

TiNi/Ti2Ni-based composite coatings reinforced by TiC and TiB2 were produced on Ti6Al4V by laser cladding the mixture of a Ni-based alloy and different contents of B4C (0wt%, 5wt%, 15wt%, and 25wt%). The macromorphologies and microstructures of the coatings were examined through optical microscopy, X-ray diffractometry, scanning electron microscopy, and energy dispersive spectrometry. The microhardness, fracture toughness, and wear behaviors of the coatings were also investigated by using a microhardness tester and an ultra-functional wear testing machine. Results showed that the coatings were mainly composed of TiNi/Ti2Ni and TiC/TiB2 as the matrix and reinforcement particles, respectively. The phase constituents of the coatings were not influenced by addition of different contents of B4C. The microstructure of the reinforcements in the coatings presented the following evolution: hypereutectic consisting of blocky (TiC+TiB2)e eutectic and primary TiCp cellular dendrites (0wt% B4C), mixture of hypereutectic and willow-shaped (TiB2+TiC)p pseudoeutectic (5wt% B4C), and pseudoeutectic (15 and 25wt% B4C). With increasing B4C content, the volume fraction and size of the pseudoeutectic structures as well as the average microhardness of the coatings (850, 889, 969, and 1002HV0.2) were increased. By contrast, the average fracture toughness of the coatings was gradually decreased (4.47, 4.21, 4.06, and 3.85Mpam1/2) along with their wear volumes (0wt%, 5wt%, and 15wt% B4C). The increase in B4C content to 25wt% did not further reduce wear loss. The wear mechanism transformed from micro-cutting (0wt% B4C) into a combination of micro-cutting and brittle debonding (5wt% B4C) and finally led to brittle debonding (15wt% and 25wt% B4C). Coatings with suitable contents of B4C (less than 15wt%) showed excellent comprehensive mechanical properties.

Effect of deformation twin on toughness in magnesium binary alloys

The impact of alloying elements on toughness was investigated using eight kinds of Mg-0.3 at.% X (X = Al, Ag, Ca, Gd, Mn, Pb, Y and Zn) binary alloys with meso-grained structures. These binary alloys had an average grain size of approximately 20 μm. The fracture toughness and crack propagation behaviour were influenced by the alloying elements; the Mg–Ag and Mg–Pb alloys had the highest and the lowest toughness amongst the alloys, respectively, irrespective of presence in their type deformation twins. The twin boundaries affected the crack propagation behaviour in most of the alloys; in contrast, not only was the fracture related to the twin boundaries, but also the intergranular fracture occurred in the alloys that included rare earth elements. The influential factor for toughness in the meso- and the coarse-grained magnesium alloys, which readily formed deformation twins during plastic deformation, was not the change in lattice parameter with chemical composition, but the twin boundary segregation energy.

Effects of CVD SiBCN interphases on mechanical and dielectric properties of SiCf/SiC composites fabricated via a PIP process

SiBCN coatings were successfully prepared on KD-II SiC fibers via a chemical vapor deposition (CVD) process utilizing borazine and liquid polycarbosilane as precursors. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) indicated the coating contained B–N, Si–C, C–C and Si–N bonds. Effects of the CVD SiBCN interphases on mechanical and dielectric properties of SiCf/SiC composites fabricated via a PIP process were investigated. The results showed that the composites with CVD SiBCN interphases exhibited an obvious toughened fracture behavior, and the flexural strength was about 2.25 times than that of the composites without SiBCN interphases. The composites containing SiBCN interphases had a higher strength retention ratio than that containing BN interphases. Furthermore, SiBCN interphases improved the fracture strength evidently, but hardly had influence on the dielectric properties of the SiCf/SiC composites.

Experimental Characterization and Modeling of Residual Stress Gradients across Straight and Bent Seamless Steel Tubes

Abstract. Residual stress gradients across the wall of seamless steel tubes influence decisively the mechanical stability and reliability of automotive and industrial constructions. Irreversible bending moments imposed on the tubes induce gradual and asymmetric elasto-plastic deformation across the tube cross-sections which result in very complex residual stress distributions. The aim of this contribution is to present a novel methodology as well as complementary modeling approach to assess the three-dimensional distribution of triaxial residual stresses in bent steel tubes. The stress characterization was performed using high energy X-ray diffraction at the HEMS beamline of PETRAIII synchrotron source in Hamburg as well as using laboratory Drill-hole method. For the complementary modeling of the stress distribution, a FEM software package DEFORM HT was used. The results reveal that the stress gradients across the tube wall are primarily influenced by the martensite profile predetermined by the parameters for thermo-mechanical treatment of the tubes. The tube bending causes the formation of continually varying compressive and tensile stresses across the tube circumference whereas the stress magnitude across the wall thickness scales again with the martensite appearance. Finally the results document the importance of the cooling process control and the influence of the applied bending radius on the resulting stress distributions as well as related mechanical parameters like fracture toughness and fatigue behavior.

Large sized cubic BN reinforced nanocomposite with improved abrasive wear resistance deposited by cold spray

Abstract In this work, large sized cubic BN (cBN) reinforced nanocomposites are prepared by cold spray deposition of blended powder mixtures of mechanically alloyed 40 vol.% cBN–NiCrAl nanocomposite particles and large sized cBN particles. Deposition behavior of the blended powders, microstructure and mechanical properties including hardness, fracture toughness and two-body dry abrasive wear behavior of the sprayed composites, are investigated. Results show that dense composites with large cBN particle content of 13–20 vol.%, depending on composition of the spray powders, can be obtained. Declining deposition efficiency is detected as increasing large cBN content due to the gradually enhanced sand blasting effect. Increasing content of large cBN in spray powder results in an increment in hardness and a decrement in fracture toughness. Abrasive wear resistance of the 40 vol.% cBN–NiCrAl nanocomposite is doubled by incorporating 20 vol.% of the large cBN particles. Worn surface morphology observation indicates that the improvement is due to the shielding effect of the large protruding cBN particles against SiC abrasive. Material removal mechanism of the sprayed composites during abrasive wear test is also studied.

Strength and fracture toughness of earth-based natural fiber-reinforced composites

This paper presents the results of a combined experimental and theoretical study of the strength, fracture toughness, and resistance-curve behavior of natural fiber-reinforced earth-based composite materials. The composites, which consist of mixtures of laterite, clay, and straw, are stabilized with controlled levels of Ordinary Portland cement. The compositional dependence of compressive, flexural/bend strength, and fracture toughness are explored for different proportions of the constituent materials using composites and crack-tip shielding models. The underlying crack-microstructure interactions associated with resistance-curve behavior were also studied using in situ/ex situ optical microscopy. This revealed evidence of crack bridging by the straw fibers. The measured resistance-curve behavior is also shown to be consistent with predictions from small- and large-scale bridging models. The implications of the results are then discussed for potential applications in the design of robust earth-based building materials for sustainable eco-friendly homes.

Determination of reference transition temperature of In-RAFMS in ductile brittle transition regime using numerically corrected Master Curve approach

The structural material for test blanket module of fusion reactor developed in India is called Indian Reduced Activation Ferritic Martensitic Steel (In-RAFMS). In this study fracture toughness behaviour of In-RAFM steel is investigated using 0.2T bend samples in ductile brittle transition regime. The conventional MC estimates a non-conservative reference transition temperature, T0, due to Large Scale Yielding (LSY) in small size specimens. Finite element analysis was used to correct the experimental KJC to equivalent Small Scale Yielding (SSY) values. The numerical approach used in this study is based on critical maximum principal stress (σ∗) – critical stressed volume (V∗) method.

Effects of the thickness of the pre-placed layer on microstructural evolution and mechanical properties of the laser-clad coatings

The coatings with different pre-placed layer thickness values (0.5, 0.8, 1.0, 1.2, 1.5 and 1.8mm) were fabricated on Ti6Al4V substrates by laser cladding NiCrBSi alloy powders. Surface characteristics and inner defects of the coatings were analyzed. The critical value(1.0mm) ofmaximum allowable pre-placed layer thickness was obtained. For the coatings with 0.5, 0.8 and 1.0mm in pre-placed layer thickness, the dilution rate, microstructural evolution, microhardness, fracture toughness and wear behavior were measured and analyzed. The dilution rates of the coatings were reduced with increasing pre-placed layer thickness (73.5%, 54.8% and 46.0% for thickness values of 0.5, 0.8 and 1.0mm, respectively). The corresponding phase constituents of the matrix in the three coatings evolved as follows: α(Ti)+Ti2Ni, TiNi+Ti2Ni and Ni3Ti+γ(Ni). The main reinforcements in the coatings were transformed from TiC+TiB to TiC+TiB2, finally to TiC+TiB2+Cr7C3+CrB. The average microhardness values of the coatings exhibited an increasing trend (817.7, 837.1 and 1078.3HV0.2) with increasing pre-placed layer thickness. Similarly, the average fracture toughness values were also gradually increased (3.019, 3.526 and 5.055MPam1/2). The average friction coefficient of the coating with 1.0mm pre-placed layer thickness was comparatively lower (0.568) and more stable with the change in sliding time compared with two other coatings. This coating also possessed the lowest wear volume (0.2732mm3). Wear mechanism of this coating was microcutting in a particular area but coupled with the formation and destruction of the transfer layer from the counterpart ZrO2. However, wear mechanism of two other coatings was microcutting. The coating with 1.0mm pre-placed layer thickness possessedmore excellent wear resistance because of its higher microhardness (resistance to microcutting), fracture toughness (resistance to brittle debonding) and the protection of the transfer layer.

Conventional sintering of WC with nano-sized Co binder: Characterization and mechanical behavior

In this study, WC particles of 1–3μm were blended with two different sizes of Co particles and sintered conventionally at two different temperatures. Compaction of initial powders was carried out using a relatively new dynamic compaction method called Magnetic Pulsed compaction (MPC). The maximum Vickers hardness for the samples found was around 1353HV (13.27GPa), while the maximum fracture toughness was observed at 4.6MPa·m1/2. The marked changes in density, hardness, fracture toughness and crack behavior observed in the samples indicate strong correlations among particle size, sintering process and mechanical properties of cemented carbides. In addition, the nature of crack initiation and propagation, which is indicative of the trend in fracture toughness of materials, was observed and analyzed.

Modelling of Crack Initiation in a G42CrMo4 Steel With Non‐Metallic Inclusions

The fracture toughness behavior of the cast steel G42CrMo4 (1.7231) with intentionally added non‐metallic inclusions was evaluated at temperatures of –40 and 20 °C under quasi‐static loading. To this end, crack growth resistance curves were determined. Furthermore, the fracture surface was analyzed with respect to the stretch zone formation and the local inclusion distribution. The casts exhibited an inhomogenous inclusion size and distance distribution. This was attributed to clustering and accumulation of the non‐metallic inclusions. It was shown that models describing the crack initiation toughness Ji can be applied if the inclusion distribution is analyzed on a local scale.

Evaluation of Orientation Dependence of Fracture Toughness and Fatigue Crack Propagation Behavior of As-Deposited ARCAM EBM Ti-6Al-4V

This preliminary work documents the effects of test orientation with respect to build and beam raster directions on the fracture toughness and fatigue crack growth behavior of as-deposited EBM Ti-6Al-4V. Although ASTM/ISO standards exist for determining the orientation dependence of various mechanical properties in both cast and wrought materials, these standards are evolving for materials produced via additive manufacturing (AM) techniques. The current work was conducted as part of a larger America Makes funded project to begin to examine the effects of process variables on the microstructure and fracture and fatigue behavior of AM Ti-6Al-4V. In the fatigue crack growth tests, the fatigue threshold, Paris law slope, and overload toughness were determined at different load ratios, R, whereas fatigue precracked samples were tested to determine the fracture toughness. The as-deposited material exhibited a fine-scale basket-weave microstructure throughout the build, and although fracture surface examination revealed the presence of unmelted powders, disbonded regions, and isolated porosity, the resulting mechanical properties were in the range of those reported for cast and wrought Ti-6Al-4V. Remote access and control of testing was also developed at Case Western Reserve University to improve efficiency of fatigue crack growth testing.

Direct fracture toughness determination of a ductile epoxy polymer from digital image correlation measurements on a single edge notched bending sample

This work presents a combined experimental and numerical study on the fracture toughness behaviour of a ductile epoxy resin system. Quasi-static fracture tests using single edge notched bending (SENB) specimens were conducted under room temperature conditions. In addition, the digital image correlation technique was employed to experimentally map the full-field displacements and strains around the notch and crack tip, allowing direct calculation of the J-integral fracture toughness. The magnitude of fracture toughness was found to be 1.52 ± 0.03 kJ/m2, showing good consistency with the results measured according to the standard analytical formulations. A numerical model of the single edge notch bending specimen was built to compute the local strain field around the crack tip, together with the fracture toughness parameter. Good agreement was confirmed for both the experimental J-integral fracture toughness and the local surface strains around the crack-tip from the digital image correlation based optical technique, compared to the results obtained by numerical simulation. The fracture surfaces of the samples were examined using an optical microscope to analyze the failed surface morphology and the corresponding failure mechanisms.