Fracture Initiation Sites Research Articles

Effect of Ti(C, N) Particle on the Impact Toughness of B-Microalloyed Steel

Simultaneously improving the toughness and strength of B-microalloyed steel by adding microalloying elements (Nb, V, Ti) has been an extensively usedmethod for researchers. However, coarse Ti(C, N) particle will precipitate during solidification with inappropriate Ti content addition, resulting in poor impact toughness. The effect of the size, number density, and location of Ti(C, N) particle on the impact toughness of B-microalloyed steel with various Ti/N ratios was investigated. Coarse Ti(C, N) particles were investigated to act as the cleavage fracture initiation sites, by using scanning electron microscopy (SEM) analysis. When more coarse Ti(C, N) inclusions were located in ferrite instead of pearlite, the impact toughness of steel with ferrite–pearlite microstructure was lower. Meanwhile, when the size or the number density of Ti(C, N) inclusions was larger, the impact toughness was adversely affected. Normalizing treatment helps to improve the impact property of B-microalloyed steel, owing to the location of Ti(C, N) particles being partly changed from ferrite to pearlite. The formation mechanism of coarse Ti(C, N) particles was calculated by the thermodynamic software Factsage 7.1 and Thermo-Calc. The Ti(C, N) particles formed during the solidification of molten steel, and the N-rich Ti(C, N) phase precipitated first and, then, followed by the C-rich Ti(C, N) phase. Decreasing the Ti and N content is an effective way to inhibit the formation of coarse Ti(C, N) inclusions.

Experimental evaluation of effective surface energy for cleavage microcrack propagation across grain boundary in steels

Fracture tests and finite element analysis were conducted to evaluate the temperature dependence of the effective surface energy γmm corresponding to microcrack propagation across the grain boundary of steels. Steels containing coarse cementite particles were prepared to enable γmm evaluation. The critical fracture stress of microcrack propagation across the grain boundary is considered larger than the critical fracture stress of microcrack propagation across the interface between a cementite particle and a ferrite grain in these steels because of the large cementite particles; therefore, local fracture stress estimated by finite element analysis and fracture initiation site obtained by fractography reflect the critical fracture stress of microcrack propagation across the grain boundary. Thus, the accuracy of γmm calculated from local fracture stress is observed to be enhanced by employing the steels prepared for the present study. In addition, the effect of nickel on γmm was studied using steels containing different concentrations of nickel. Values of γmm were evaluated considering both the direction of a neighboring facet relative to the crack initiation facet and the mixed mode stress intensity factors. The authors proposed a new temperature dependence ofγmm. Additionally, the negligible effect of nickel addition on γmm was observed. The results of the present study can be used effectively to develop a probabilistic cleavage fracture model based on microstructural information.

Effects of untransformed ferrite on Charpy impact toughness in 1.8-GPa-grade hot-press-forming steel sheets

Hot press forming (HPF) steel sheets are austenitized, press-formed, and rapidly cooled to obtain a martensitic microstructure with an ultra-high strength. When they are insufficiently austenitized, their microstructures might contain a small amount of untransformed ferrite, which can deteriorate impact toughness as well as strength, but its causes and relevant fracture mechanisms have not been clearly verified yet. In this study, thus, 1.8-GPa-grade HPF sheets were austenitized at various temperature and time, and their tensile and Charpy impact test results were analyzed in relation with untransformed ferrite and its effect on fracture mechanisms. In the HPF sheets containing the untransformed ferrite, voids were formed mostly at ferrite/martensite interfaces, and were grown and propagated linearly to form a cleavage crack, whereas deformation bands were well developed without voids or cracks in the non-ferrite-containing sheets. The highly localized strains accommodated in the soft ferrite made ferrite/martensite interfaces or ferrite itself work as fracture initiation sites, which led to the brittle fracture and consequently to the deterioration of impact energy. This result can provide an important idea for optimization of austenitization conditions demanded for ultra-high strength and excellent impact toughness in HPF applications.

Fatigue testing of 2198 T8 FSW aluminum alloy with and without LoP defect

PurposeThe Lack of Penetration (LoP) defect is one of the flaws that can be generated during the friction stir welding (FSW) process. Depending on the size, the depth and the severity of the LoP defect, it is possible that it is hardly detectable by optical inspection or other NDT methods. Whether detectable or not, this defect may lead to a significant degradation of the fatigue properties of the welded material, as the improperly welded zone can act as a fracture initiation site. The paper aims to discuss these issues.Design/methodology/approachIn this experimental investigation, an attempt is made to assess and compare the fatigue behavior of FSW aluminum joints with and without the LoP defect.FindingsIt was found that the LoP defect affects the fatigue behavior of the welded material at high stress levels whereas the effect diminishes with decreasing stress levels.Originality/valueDepending on the design stress levels, the LoP defect may dominate fracture and thus, the welding parameters should be carefully selected so as to avoid such defect.

Investigation of cleavage fracture under dynamic loading conditions: Part I fractographic analysis

The fracture toughness of ferritic steels under dynamic loading conditions on the one hand shows decreasing values with elevated loading rates but on the other hand the shape of the temperature dependent fracture toughness curve has turned out to be different from the static Master Curve according to ASTM E 1921. This difference is often explained by adiabatic heating in the crack tip region, yet it is not clear if there are other additional mechanisms under dynamic loading conditions that contribute to these changes. This work is dedicated to systematically identifying and quantifying additional mechanisms regarding cleavage fracture under dynamic loading conditions. In part I of this study an extensive fractographic analysis of the fracture surfaces was conducted for various crack tip loading rates and testing temperatures. The primary fracture-inducing mechanism was found to be identical to the dominant one under quasi-static conditions (carbide cracking). Yet, the dynamic loading conditions appear to change the origin of fracture, promote local crack arrest, and cause multiple fracture initiation sites that lead to global failure. These results also question the reliability of current local approach concepts if used to assess fracture probability at elevated loading rates. The fractographic results are used in, and complemented by, part II of this study which deals with the numerical analysis of other additional mechanisms such as inertia.

Effect of microstructure on fatigue behaviour of advanced high strength ductile cast iron produced by quenching and partitioning process

The fatigue behaviour of high strength ductile cast iron produced by a quenching and partitioning process (Q&P) is evaluated. The Q&P process is receiving increased attention as a new way to produce ultra-high strength steels with multiphase microstructures composed of a martensite matrix and substantial amounts of carbon-stabilized retained austenite. Currently, the mechanical properties arising from applying the Q&P process in ductile cast irons have not been investigated. Thermal treatments consisted of heating the material to 880°C for a 2h soaking time followed by quenching in oil at 140°C and 170°C, intermediate temperatures between Ms and Mf allowing the formation of a controlled amount of athermal martensite. The material was reheated to 300°C and 375°C (the partition treatment) for different times between 15 and 120min and subsequently air-cooled to room temperature. Microstructural evolution and carbon partitioning was investigated by in situ synchrotron X-ray diffraction. With a microstructure composed of tempered martensite, bainitic ferrite and carbon-enriched stabilized austenite, a new class of properties was obtained, with an enhanced strength when compared to ADI (austempered ductile iron) while still maintaining reasonable elongation. Fatigue testing was undertaken using polished plain bend bars (no stress concentration features) assessed under four-point bending. Uninterrupted tests at varying loads show that the higher partitioning temperature is beneficial for fatigue life. Fracture initiation sites are primarily from pores and a number of decohered graphite nodules. A strong influence of the microstructure on subsequent fatigue crack growth is observed from interrupted testing with replica records and SEM examination of tested samples, with cracks exhibiting significant tortuosity, at times even appearing to grow approximately parallel to the tensile axis. Using these results, the effects of the Q&P treatment, particularly the role of the partitioning time and temperature, on the fatigue properties of ductile cast iron are assessed.

Investigation on micromechanism and stress state effects on cleavage fracture of ferritic-pearlitic steel at −196 °C

Mechanical tests of a ferritic-pearlitic steel were conducted at −196°C with various specimen geometries resulting in different stress states. Elastic-plastic finite element method (FEM) simulations were used to analyze the local stress and strain distributions of the studied steel. It is found that cementite lamellae are the initiation sites for cleavage fracture. In addition, the cleavage fracture critical stage and the local equivalent plastic strain at initiation site varies with stress triaxiality and Lode angle.

3D Observation of Micro-cracks as Cleavage Fracture Initiation Site in Ferrite-pearlite Steel

3D Observation of Micro-cracks as Cleavage Fracture Initiation Site in Ferrite-pearlite Steel

Study on coexistence of brittle and ductile fractures in nano reinforcement composites under different loading conditions

Experimental study on high volume fraction of metallic matrix nano composites (MMNCs) was conducted, including uniaxial tension, uniaxial compression, and three-point bending. The example materials were two magnesium matrix composites reinforced with 10 and 15% vol. SiC particles (50 nm size). Brittle fracture mode was exhibited under uniaxial tension and three-point bending, while shear dominated ductile fracture mode (up to 12% fracture strain) was observed under uniaxial compression. The original Modified Mohr–Coulomb (MMC) fracture model (Bai and Wierzbicki in Int J Fract 161:1–20, 2010; in a mixed space of stress invariants and equivalent strain) was transferred into a stress based MMC (sMMC) model. This model was demonstrated to be capable of predicting the coexistence of brittle and ductile fracture modes under different loading conditions for MMNCs. A material post-failure softening model was postulated along the damage accumulation to capture the above two different failure modes. This model was implemented to the Abaqus/Explicit as a material subroutine. Numerical simulations using finite element method well duplicated the material strength, fracture initiation sites and crack propagation modes of the Mg/SiC nano composites with a good accuracy. The proposed model has a good potential to predict fracture for a wide range of material with strength asymmetry and coexistence of brittle and ductile fractures modes.

Relationship Between Ulnar Variance, Cortical Bone Density, and Load to Failure in the Distal Radius at the Typical Site of Fracture Initiation

Increased ulnar variance has been shown to lead to diminished load borne by the distal radius. The purpose of this study was to determine the correlations among ulnar variance, bone mineral density, and load to failure at the distal radius. Posteroanterior radiographs and computed tomographic scans were taken of 12 cadaveric forearms in neutral rotation. Ulnar variance was measured for each wrist by the method of perpendiculars. Measurements of cortical, trabecular, and combined bone density were made at the distal radius. We performed linear regression analysis and correlation analysis to determine the relationship between bone densities and ulnar variance measurements. Next, we loaded the 12 cadaveric radii to failure under axial compression. Linear regression analysis and correlation analysis were then performed to determine the relationship between load to failure and both ulnar variance and cortical density. Increased ulnar variance was significantly correlated with decreased cortical bone density at the distal radius and both were correlated with decreased load to failure. We found no correlation between ulnar variance and trabecular density or combined trabecular and cortical bone density at the distal radius. Our study found that increased ulnar variance and decreased cortical bone mineral density correlates with decreased load to failure under axial compression. Ulnar variance is linked to both bone quality and load to failure at the distal radius.

Microscopic plasticity and damage in two-phase steels: On the competing role of crystallography and phase contrast

This paper unravels micromechanical aspects of metallic materials whose microstructure comprises grains of two or more phases. The local plastic response is determined by (i) the relative misorientation of the slip systems of individual grains, and (ii) the different mechanical properties of the phases. The relative importance of these two mechanisms at the meso-scale is unclear: is the plastic response dominated by the grain’s anisotropy, or is this effect overwhelmed by the mechanical contrast between the two phases? The answer impacts the modeling of such a material at the meso-scale, but also gives insights in the resulting fracture mechanisms at that length-scale. Until now, this question has been addressed only for particular crystallographies and mechanical properties. In contrast, this paper studies the issue systematically using a large set of phase distributions, crystallographies, and material parameters. It is found that the macroscopic and the mesoscopic (grain-averaged) plastic response of the two extreme modeling choices (crystal plasticity or isotropic plasticity) converge with increasing phase contrast. The effect of the crystallography is completely overwhelmed by the phase contrast when the yield stress of the hard phase is a factor of four higher compared to the soft phase. When this ratio is lower than two, its influence may not be neglected. However, even in this regime, fracture initiation is controlled by the local arrangement of the phases. The latter is quantified in this paper through the average arrangement of the phases around fracture initiation sites.

Effects of Low Temperature on Hydrogen-Assisted Crack Growth in Forged 304L Austenitic Stainless Steel

The objective of this study was to evaluate effects of low temperature on hydrogen-assisted crack propagation in forged 304L austenitic stainless steel. Fracture initiation toughness and crack-growth resistance curves were measured using fracture mechanics specimens that were thermally precharged with 140 wppm hydrogen and tested at 293 K or 223 K (20 °C or −50 °C). Fracture initiation toughness for hydrogen-precharged forgings decreased by at least 50 to 80 pct relative to non-charged forgings. With hydrogen, low-temperature fracture initiation toughness decreased by 35 to 50 pct relative to room-temperature toughness. Crack growth without hydrogen at both temperatures was microstructure-independent and indistinguishable from blunting, while with hydrogen microcracks formed by growth and coalescence of microvoids. Initiation of microvoids in the presence of hydrogen occurred where localized deformation bands intersected grain boundaries and other deformation bands. Low temperature additionally promoted fracture initiation at annealing twin boundaries in the presence of hydrogen, which competed with deformation band intersections and grain boundaries as sites of microvoid formation and fracture initiation. A common ingredient for fracture initiation was stress concentration that arose from the intersection of deformation bands with these microstructural obstacles. The localized deformation responsible for producing stress concentrations at obstacles was intensified by low temperature and hydrogen. Crack orientation and forging strength were found to have a minor effect on fracture initiation toughness of hydrogen-supersaturated 304L forgings.

Competing damage mechanisms in a two-phase microstructure: How microstructure and loading conditions determine the onset of fracture

This paper studies the competition of fracture initiation in the ductile soft phase and in the comparatively brittle hard phase in the microstructure of a two-phase material. A simple microstructural model is used to predict macroscopic fracture initiation. The simplicity of the model ensures highly efficient computations, enabling a comprehensive study: a large range of hard phase volume fractions and yield stress ratios, for wide range of applied stress states. Each combination of these parameters is analyzed using a large set of (random) microstructures. It is observed that only one of the phases dominates macroscopic fracture initiation: at low stress triaxiality the soft phase is dominant, but above a critical triaxiality the hard phase takes over resulting in a strong decrease in ductility. This transition is strongly dependent on microstructural parameters. If the hard phase volume fraction is small, the fracture initiation is dominated by the soft phase even at high phase contrast. At higher hard phase volume fraction, the hard phase dominates already at low phase contrast. This simple model thereby reconciles experimental observations from the literature for a specific combination of parameters, which may have triggered contradictory statements in the past. A microscopic analysis reveals that the average phase distribution around fracture initiation sites is nearly the same for the two failure mechanisms. Along the tensile direction, regions of the hard phase are found directly next to the fracture initiation site. This ‘band’ of hard phase is intersected through the fracture initiation site by ‘bands’ of the soft phase aligned with shear. Clearly, the local mechanical incompatibility is dominant for the initiation of fracture, regardless whether fracture initiates in the soft or in the hard phase.

Shear stress enhanced fatigue damage accumulation in single crystalline silicon under cyclic mechanical loading

In contrast to the commonly accepted hypothesis where surface oxide layer is supposed to play a dominant role on fatigue fracture of silicon, another scenario newly appeared here being supported by a series of evidences which showed crystal slip deformation leading to fatigue damage accumulation. Two different types of specimens with different shear/tensile stress ratios on {111} crystal slip plane were designed and subjected to fatigue loading. Lifetime was eventually longer with smaller shear stress working on this plane. Striation-like fracture surface patterns were also observed, which should have been created before unstable fracture took place. Even dislocation networks were discovered at the fracture initiation site. Silicon may behave in a much more metallic manner even at room temperature than was ever believed before, no wonder finally leading to fracture under cyclic mechanical loading.

Effects of microstructural variation on Charpy impact properties in heavy-section Mn-Mo-Ni low alloy steel for reactor pressure vessel

The effects of microstructural changes in heavy-section Mn-Mo-Ni low alloy steel on Charpy impact properties were investigated using a 210 mm thick reactor pressure vessel. Specimens were sampled from 5 different positions at intervals of 1/4 thickness from the inner surface to the outer surface. A detailed microstructural analysis of impact-fractured specimens showed that coarse carbides along the lath boundaries acted as fracture initiation sites, and cleavage cracks deviated at prior-austenite grain boundaries and bainite lath boundaries. Upper shelf energy was higher and energy transition temperature was lower at the surface positon, where fine bainitic microstructure with homogeneously distributed fine carbides were present. Toward the center, coarse upper bainite and precipitation of coarse inter-lath carbides were observed, which deteriorated impact properties. At the 1/4T position, the Charpy impact properties were worse than those at other positions owing to the combination of elongated-coarse inter-lath carbides and large effective grain size.

Fracture initiation in multi-phase materials: A systematic three-dimensional approach using a FFT-based solver

This paper studies a two-phase material with a microstructure composed of a hard brittle reinforcement phase embedded in a soft ductile matrix. It addresses the full three-dimensional nature of the microstructure and macroscopic deformation. A large ensemble of periodic microstructures is used, whereby the individual grains of the two phases are modeled using equi-sized cubes. A particular solution strategy relying on the Fast Fourier Transform is adopted, which has a high computational efficiency both in terms of speed and memory footprint, thus enabling a statistically meaningful analysis. This solution method naturally accompanies the regular microstructural model, as the Fast Fourier Transform relies on a regular grid.Using the many considered microstructures as an ensemble, the average arrangement of phases around fracture initiation sites is objectively identified by the correlation between microstructure and fracture initiation – in three dimensions. The results show that fracture initiates where regions of the hard phase are interrupted by bands of the soft phase that are aligned with the direction of maximum shear. In such regions, the hard phase is arranged such that the area of the phase boundary perpendicular to the principal strain direction is maximum, leading to high hydrostatic tensile stresses, while not interrupting the shear bands that form in the soft phase. The local incompatibility that is present around the shear bands is responsible for a high plastic strain. By comparing the response to a two-dimensional microstructure it is observed that the response is qualitatively similar (both macroscopically and microscopically). One important difference is that the local strain partitioning between the two phases is over-predicted by the two-dimensional microstructure, leading to an overestimation of damage.

Fracture behavior of inlay and onlay fixed partial dentures – An in-vitro experimental and XFEM modeling study

ObjectivesThis study aimed to explore the “sensitivity” of the fracture load and initiation site to loading position on the central occlusal surface of a pontic tooth for both all-ceramic inlay retained and onlay supported partial denture systems. Materials and methodsThree dimensional (3D) finite element (FE) inlay retained and onlay supported partial denture models were established for simulating crack initiation and propagation by using the eXtended Finite Element Method (XFEM). The models were subjected to a mastication force up to 500N on the central fossa of the pontic. The loading position was varied to investigate its influence on fracture load and crack path. ResultsSmall perturbation of the loading position caused the fracture load and crack pattern to vary considerably. For the inlay fixed partial dentures (FPDs), the fracture origins changed from the bucco-gingival aspect of the molar embrasure to the premolar embrasure when the indenter force location is slightly shifted from the mesial to distal side. In contrast, for onlay FPDs, cracking initiated from bucco-gingival aspect of the premolar embrasure when the indenter is slightly shifted to the buccal side and from molar embrasure when the indenter is shifted to the lingual side. ConclusionsThe fracture load and cracking path were found to be very sensitive to loading position in the all-ceramic inlay and onlay FPDs. The study provides a basis for improved understanding on the role of localized contact loading of the cusp surface in all-ceramic FPDs.

Monte Carlo Simulation to Predict Fracture Initiation in Mild Steel

The brittle fracture occurs suddenly and can cause the fatal accident, so it needs to be prevented. The brittle fracture has been studied by a lot of researchers, but the quantitative relationship between fracture toughness and microstructure has not become clear. In this study, we developed the numerical model to predict cleavage fracture toughness of mild steel by integrating the formulation in previous studies. Besides, the proposed model was validated by comparing the predicted results with the experimental results. The three-point bending tests with notched specimen were conducted and the fracture initiation sites were observed by SEM in order to validate the proposed model. The predicted fracture toughness values showed good agreement with those of the experiments. The fracture initiation sites which the model predicted also show good agreement with them. It is found that the proposed model represents the quantitative relationship between fracture toughness and microstructure of mild steel and predicts fracture toughness with high accuracy.

Development of New Formulation and Likelihood Function on Probabilistic Fracture Initiation Model

The authors propose a new model on cleavage fracture toughness of steels. Fracture toughness of steels is controlled by the weakest link mechanism, so it has intrinsic scatter. Beremin proposed Weibull stress through probabilistic fracture initiation model based on the weakest link theory, and it has been widely used for describing fracture toughness scatter. In his model, only propagation of micro crack is considered. However, micro crack nucleation should also be incorporated in order to estimate fracture toughness distribution more accurately. Bordet introduced micro crack initiation to the Beremin model, in which probability of micro crack nucleation is assumed proportional to plastic strain. Some previous studies have shown, however, that micro crack nucleation probability increases non-linearly with plastic strain. Then, we developed a new probabilistic model by introducing micro crack nucleation probability as a non-linear function of plastic strain.Furthermore, the authors developed a new method for obtaining Weibull parameters, in which not only distribution of fracture toughness values but also location of fracture initiation sites are considered through a newly developed likelihood function. We conducted fracture toughness tests with different specimen configurations and carried out convergence calculation for determining Weibull parameters by applying the likelihood function mentioned above. As a result of the calculation, the authors confirmed that the present model can obtain the Weibull parameters less sensitive to the specimen configurations and simulate distribution of fracture toughness more accurately than the previous models.

WES 2808 for Brittle Fracture Assessment of Steel Components under Seismic Conditions – Part VI: Application of WES 2808 to Beam-to-column Connections

This paper describes application of revised fracture assessment method specified in WES 2808 to full-scale models of beam-to-column connections subjected to cyclic and dynamic loading. Fracture tests of full-scale models of beam-to-column connections were carried out (Morita, 1999, and Yoshimura, 2006). Cyclic loading tests were conducted, which included dynamic loading tests. Brittle fracture occurred from weld defects (lack of fusion or weld crack) or artificial defects in beam-to-column connections. In accordance with WES 2808, the fracture strain at the beam end in the connection was evaluated. Fracture toughness of a local area including fracture initiation site was replaced by the static fracture toughness at reference temperature defined by WES 2808. The equivalent CTOD ratio β was employed to correct the constraint loss in structural components on the basis of the Weibull stress criterion. The fracture performance of full-scale models of beam-to-column connections evaluated by the procedure specified in WES 2808 was almost consistent with the experimental results.