Fracture Initiation Sites Research Articles

Sequential dual-scale approach for microstructure-informed ductile fracture prediction

This study presents a novel dual-scale finite element method to establish a microstructure-informed ductile fracture criterion for ferrite–martensite dual-phase (DP) steel. At the macroscale, an anisotropic plasticity model with rate-dependent hardening was employed to simulate the material's deformation history. Simultaneously, at the microscale, a dislocation density-based crystal plasticity model was utilized to simulate deformation within a representative volume element (RVE) of the dual-phase steel, constructed using tomography aided by a plasma-focused ion beam/electron backscatter diffraction system.The material properties of the ferrite and martensite phases were determined through X-ray diffraction (XRD) analysis and load-displacement measurements obtained via nanoindentation for each phase. The RVE simulation results were validated against experimentally measured mechanical properties and microstructural changes. The local deformation history at the fracture initiation site, extracted from the macroscale model, was used as boundary conditions for the microscale RVE simulation; sequential dual-scale approach. The models were applied to specimens with varying notch radii, generating different local stress triaxialities and accumulated shear strains at fracture onset. This process allowed the establishment of a ductile fracture criterion, which was further tested in a hole expansion experiment, demonstrating close alignment with experimental data.This sequential dual-scale analysis effectively predicts the deformation behavior of multiphase metallic materials by incorporating realistic microstructures while minimizing computational costs. Consequently, the proposed ductile fracture prediction technique offers a robust method with broad applicability across various metallic materials.

A local surface roughness mapping method for post-failure interpretation of brittle fracture propagation

Tensile fractures in brittle materials may exhibit striation features running along the surface that indicate the trajectory of fracture propagation. Because of the directional persistence of these features, directional roughness characterization methods can be used on subsampled fracture surface data to map localized roughness features that follow these striations. These maps can aid in the interpretation of fracture initiation sites and crack front positions with the use of post-processing visualization techniques such as vector maps and streamlines. Tensile fractures in rock and synthetic rock analogues are used to demonstrate the various fracture propagation patterns that emerge following this analysis. This mapping method and streamline analysis can be further used to analyze elevation profiles that follow fracture propagation along non-linear paths which may inform about the mechanisms that can associate surface features with the fracture process.

Microstructural characterisation of brittle fracture initiation sites in reactor pressure vessel steels

Neutron embrittlement of reactor pressure vessel (RPV) steels potentially limits the safe operational time of nuclear reactors. In tested RPV steel samples microcracks are known to originate from cleaved precipitates, inclusions or grain boundaries, which eventually lead to brittle fracture. In most cases, these precipitates are reported as carbides. Latest research indicates the critical fracture event to be the crossing of grain boundary instead of the particle–matrix interface by the microcrack. Very few works have performed detailed fractography, which revealed the size and type of the feature found in the crack nucleation centre, together with metallographic bulk investigations. Additionally, the effect of subsequent neutron irradiation on microcrack nucleation is unknown. In the present work, detailed fractography and metallography is performed on a RPV steel in both its unirradiated as well as neutron irradiated state. A particle in the centre of the initiation site has been identified in a majority of the cases, while no specific type of particles were involved in more than 1/3 of all samples. Those particles are either Mo-rich carbides or Al-rich inclusions. In comparison, the metallographic analysis of the bulk steel reveals Mo-rich and Mn-rich carbides. Mn-rich carbides constitute the majority of all carbides in the steel but are never found to be responsible for brittle fracture initiation. Only a small fraction of observable carbides is Mo-rich which are responsible for fracture. Thus, the present work demonstrates a discrepancy between particles found in brittle fracture initiation sites and particles that are directly observable in bulk steel. Some of the initiating particles are extremely rare in the bulk. Furthermore, it is revealed that the local fracture stress does not depend on the type or size of particle or grain involved at the initiation site. In conclusion, the fracture stress strongly depends on the type of the grain boundary, rather than on a stress based upon a Griffith criterion, which considers particle or grain size alone. The brittle fracture mechanism was found to be unaffected by neutron irradiation. These findings could help refine modelling of the critical brittle fracture stress and fracture toughness using microstructural parameters as input.

Fracture of Contemporary Femoral Stems: Common Trends in This Rare Occurrence

BackgroundFracture of contemporary femoral stems is a rare occurrence in total hip arthroplasty. A knowledge gap remains regarding manufacturing, patient, and surgeon factors that may contribute to the increased risk of this complication. MethodsWe analyzed 13 contemporary fractured porous-coated femoral stems of various designs to determine cause and contributing factors of mechanical failure. Cases included 12 men and 1 woman who had an average age at index surgery of 53 years (range, 34 to 76 years). There were 10 of 13 patients who had a body mass index more than 30 (obese); 3 of the 10 had a body mass index more than 40. The mean time to fracture was 7.6 years (range, 7 months to 12 years). ResultsThere were 4 titanium alloy stems that fractured an average of 3.6 years postrevision surgery for head/cup exchange and had associated iatrogenic mechanical and electrocautery damage to the femoral neck at fracture initiation sites. There were 6 modular stems that failed at the stem-sleeve or stem-neck interfaces with evidence of fretting corrosion. For 2 stem-neck fractures, mismatched head/stem combinations from different manufacturers resulted in untested mechanical offsets and loading. There were 2 proximal neck fractures and 1 mid-shaft fracture of coated cobalt-chromium alloy stems that occurred in 3 obese men. The neck fractures (10 to 12 years) were well-fixed stems. Lack of proximal fixation contributed to the mid-shaft fracture (7 months). ConclusionWhile rare, femoral stem fractures pose catastrophic outcomes in primary and revision total hip arthroplasty. Manufacturing, patient, and surgical factors contributing to stem failures were identified, including patient obesity, heat-treatment reduction of mechanical properties, iatrogenic implant damage, and mixing of different vendor stems and heads.

Strain ageing embrittlement behaviour of X80 self-shielded flux-cored girth weld metal

ABSTRACT Strain ageing embrittlement behaviour of X80 self-shielded flux-cored arc welding (FCAW) girth weld metal at different strain ageing conditions (1% prestrain, and aged at 150°C/100°C for 1 –6 h, and 80°C for 1–30 days) was studied. Microstructure of FCAW girth weld generally consisted of coarse granular bainite and lath bainite. After strain ageing, toughness notably decreased, and higher ageing temperature or longer ageing induced lower toughness, until ageing is fully yielded. At initial stage of strain ageing, more dimples were found accompanying with cleavage facet. With the progress of strain ageing, the embrittlement was more obvious, while larger cleavage facet and less dimples were found. Meanwhile, the critical size for cleavage fracture initiation site became smaller. Martensite–austenite constituent (1–2 μm) was found to initiate cleavage fracture when partially strain aged, while submicron inclusions could initiate cleavage facet when fully strain aged.

Anisotropic tensile behavior of laser powder-bed fusion made Ti5553 parts

Directional heating during metal additive manufacturing promotes the growth of a columnar-grained microstructure along the building direction with a preferred crystal orientation, which results in anisotropy of properties. In this work, the anisotropy of laser powder bed fused Ti-5553 metastable β titanium alloy components are investigated. Samples printed in the normal (vertically printed), and parallel (horizontally printed) orientations to the building direction were subjected to quasi-static uniaxial tensile tests. In comparison, samples printed parallel to the building direction exhibit a significantly higher ultimate strength of 846 ± 6 MPa, whereas the samples printed normal to the building direction reached 780 ± 10 MPa. Fracture initiation and propagation were investigated using interrupted tensile testing. The fracture path of partially fractured specimens reveals the development of slip bands oriented at a 45 ± 5° angle with respect to the loading direction, which promoted fracture by coalescence of aligned pores. Electron backscatter diffraction results indicate that the grain boundaries act as favorable nucleation sites for fracture initiation, particularly when they align perpendicular to the loading direction. Conversely, fracture in samples printed parallel to the building direction was predominantly transgranular, which is expected to be a major contributing factor to the higher strength observed.

Compressive failure analysis of in-situ Al-Mg2Si composites: Experiment and finite element modelling

Al-Mg2Si composites have been developed through gravity die-casting techniques. The influence of Mg2Si reinforcement concentration on strength, micromechanics and failure initiation of Al-Mg2Si composites subjected to compression has been studied. The compressive strength and brittle fracture of the Al-Mg2Si composite increase and strain decrease with an increase in Mg2Si concentration. The eutectic and primary Mg2Si phase sharp corners and narrow sections of Mg2Si are fracture initiation sites, and the Al phase nearby the eutectic and Mg2Si phase sharp corners and narrow Al phase regions between two Mg2Si islands are void initiation sites in the composites subjected to compression.

Computer-aided High-efficiency Detection of Fracture Initiation Sites in Steel

Computer-aided High-efficiency Detection of Fracture Initiation Sites in Steel

Combining fractal and topological analyses to quantify fracture surfaces in additively manufactured Ti-6Al-4V

Quantitative fractography has been hindered by the lack of tools capable of accurately characterizing fracture modes and crack initiation locations. An easily implementable, non-destructive image analysis-based tool utilizing fractal and topological techniques was developed to overcome these traditional shortcomings. With this tool, feature complexity, heterogeneity, and connectivity are quantified through the calculation of fractal dimensions and lacunarity and topology measurements, respectively. Validation of this tool was performed on complex fatigue fractures in additively manufactured Ti-6Al-4V, with fracture initiation sites at near surface, sub-surface, and internal defects and fatigue fracture modes being easily differentiated.

Fracture analysis of one/two-piece clinically failed zirconia dental implants

ObjectivesAnalyzing factors that may have led to fracture of zirconia implants by macro/micro-fractography. MethodsSix one-piece and ten two-piece full-ceramic zirconia implants from two manufacturers, Z-Systems and CeraRoot, were retrieved after clinical failure. The time-to-failure ranged from 3 to 49 months. Optical and scanning electron microscopy (SEM) were used to analyze the fracture planes at the macro- and microscopic level. Treatment planning, surgical protocol, fracture-origin location and characteristic fracture features were assessed. ResultsThe fracture of all implants seemed to have been primarily due to overload in bending mode, while the fracture-initiation sites varied for the one- and two-piece implants. The fracture of all one-piece implants originated in the constriction region between two threads in the endosseous implant part. For two-piece implants, the abutment neck, internal abutment-implant connections and inner threads were found to be the main fracture-initiation sites. Surface defects at the root area for one-piece implants and damages at the abutment surface for two-piece implants were connected to the fracture origins. Importantly, the clinical failures of implants were often found to result from combined effects related to patient aspects, treatment planning/protocols, a high bending moment at the weakest link, implant-surface conditions and specific implant designs. SignificanceThis study provided information to be considered for future optimization of treatment planning and the surgical protocol for zirconia implants. Optimization of the surface conditions and the zirconia-starting powder were also suggested.

Bending fatigue strength of aerospace quality gear steels at ambient and elevated temperatures

Gear bending fatigue testing was conducted on six commercial aerospace quality steels, in the carburised, shot-peened and super finished condition, establishing S-N curves at ambient and elevated temperatures, providing evidence of higher than predicted strength established in accordance with ISO 6336:2006. At elevated temperatures, a systematic drop in mean strength was experienced for all candidate steels. Detailed fractography established the fracture initiation site, unique to each steel, illustrating a propensity for surface initiation in the finite life region and subsurface beyond the knee point, whilst certain grades displayed a distinct change in the location of fracture initiation at elevated temperatures.

A microstructure informed and mixed-mode cohesive zone approach to simulating hydrogen embrittlement

Hydrogen induced failure under uniaxial tension is simulated in a duplex stainless steel considering microstructural feature of the material. There are three key ingredients in the modelling approach: image processing and finite element representation of the experimentally observed microstructure, stress driven hydrogen diffusion and diffusion coupled cohesive zone modelling of fracture considering mixed failure mode. The microstructure used as basis for the modelling work is obtained from specimens cut in the transverse and longitudinal directions. It is found that the microstructure significantly influences hydrogen diffusion and fracture. The austenite phase is polygonal and randomly distributed in the transverse direction, where a larger effective hydrogen diffusion coefficient and a lower hydrogen fracture resistance is found, compared to the specimen in the longitudinal direction, where the austenite phase is slender and laminated. This indicates that the proper design and control of the austenite phase help improve hydrogen resistance of duplex stainless steel. The strength of the interface in the shear direction is found to dominate the fracture mode and initiation site, which reveals the importance of considering mixed failure mode and calibrating the hydrogen induced strength reduction in shear.

Hydrogen-Related Fracture Behavior under Constant Loading Tensile Test in As-Quenched Low-Carbon Martensitic Steel

This study investigated the hydrogen-related fracture behavior in as-quenched low-carbon martensitic steel under a constant loading tensile test with various applied stresses. We found that the fracture time in the constant loading tensile test decreased as the applied stress and hydrogen content increased. The fracture surface topography analysis revealed that when the applied stress was low, the intergranular fracture was initiated around the side surface of the specimen and gradually propagated into the inner part of the specimen. In contrast, several intergranular fractures were separately initiated inside the specimen when the applied stress was high. The mode of hydrogen-related fracture was controlled by the fracture stress and not by the global hydrogen content inside the specimen. Increasing the global hydrogen content caused a decrease in the duration required for the accumulation of critical local hydrogen concentration at the fracture initiation site (prior austenite grain boundary). Accordingly, we propose that the local state at the crack initiation site is constant under a given applied stress, even when the global hydrogen content is different.

Failure Analysis of Friction Weld in Rear Axle Casing

In this study, Housing and Spindle failed at Friction weld (FRW) interface and sample submitted was from a Passenger vehicle after covering 18410 Kms was considered to address the repeated weld failures reported in Rear Axle Casing. The Metallurgical examinations revealed, chemical composition and hardness found satisfactory whereas Microstructure reveals presence of MnS (Manganese Sulphide) Inclusions at the friction weld interface resulting in insufficient friction weld fusion.SEM and Metallographic examinations confirmed the Orientation of MnS (Manganese sulphide) along the surface of the Spindle that was subjected to Friction welding (FRW) resulting in insufficient weld fusion. Microstructure observation of Spindle shows inclusions of Type A - Thick 2 against the allowable limits in Raw Material Type A - Thick 1. Also, the elongated MnS inclusions at the Friction weld (FRW) Interface act as initiation sites, "Stress raiser "that is more vulnerable to failure due to comparatively less tensile and impact strength than base metal similar to the effect of flake graphite’s in cast iron. Due to the anisotropy and orientation of the sulfide inclusions there is reduction in transverse ductility in reduced area and not much significant effect in longitudinal direction. Inclusion control by calcium treatment or Electro slag re-melting in spindle raw material will significantly improve the ductility and toughness of friction weldments. This paper aims to study the effect of Sulfide inclusions on weldability, mechanical properties of weld Interface and role as fracture initiation site.

Comparison between the hydrogen embrittlement behavior of an industrial and a lightweight bearing steel

Hydrogen embrittlement (HE) is a widely occurring problem, also in the automotive industry. Therefore, this study compares the resistance to HE of two types of bearing steels: an industrial 100Cr6 steel and an innovative Fe-8Al-1.1C steel. The innovative character of the Fe-8Al-1.1C steel is found in the fact that a high weight fraction of Al is added to a simplified Fe-1.1C alloy which results in a low-cost martensitic steel with a similar hardness but a lower density, compared to the industrial 100Cr6 steel. The density reduction offers the opportunity to reduce the vehicle weight and as such to reduce the CO2 emission of fuel-based cars or extend the driving range of electrical vehicles. Both alloys are subjected to a similar heat treatment resulting in a martensitic matrix with specially induced carbides. (In-situ) H bending tests and subsequent post-mortem fracture surface analysis are performed to investigate the mechanical degradation of both steels in a H rich environment. For the industrial steel grade, the large Cr-based carbides act as fracture initiation sites for H-assisted cracks. In contrast, the fracture behavior of the Fe-8Al-1.1C material is similar in air and in an H containing environment, indicating that the Fe-8Al-1.1C steel has a better resistance to HE. Although these results are very promising, further optimization of the innovative alloy is needed to improve the mechanical behavior in the absence of H since the presence of microcracks cause premature failure when tested in air.

A Method for Determining the Workability Diagram by Varying Friction Conditions in the Upsetting of a Cylinder between Flat Dies

This paper aims to develop a method for determining the workability diagram by varying frictional conditions in the cylinder upsetting test. The method is based on a known theoretical relationship between the average stress triaxiality ratio and in-surface strains if the initiation of fracture occurs at a traction-free surface. This relationship is valid for any rigid/plastic strain hardening material obeying the Mises-type yield criterion and its associated flow rule, which shows the wide applicability of the method. The experimental input to the method is the strain path at the site of fracture initiation. Neither experimental nor numerical determination of stress components is required at this site, though the general ductile fracture criterion involves the linear and quadratic invariants of the stress tensor. The friction law’s formulation is neither required, though the friction stress is the agent for varying the state of stress and strain at the site of ductile fracture initiation. The upsetting tests are carried out on normalized medium-carbon steel C45E, for which the workability diagram is available from the literature. Comparison of the latter and the diagram found using the new method shows that the new method is reliable for determining a certain portion of the workability diagram.

Fracture modeling of resistance spot welded ultra-high-strength steel considering the effect of liquid metal embrittlement crack

Zinc-coated ultra-high-strength steel (UHSS) sheets are highly susceptible to liquid metal embrittlement (LME) during resistance spot welding. However, systematic understanding on LME cracks in the pullout fracture of UHSS welds is still lacking. This study employs artificially created pre-cracks on bare DP980 steel welds to simulate the LME cracks, allowing accurate control of the crack shape and location. To investigate the fracture initiation and propagation behavior of welds with various pre-crack characteristics, a combined experimental and computational approach, which incorporated tensile tests on newly designed miniature specimens for detailed mechanical property measurement and fracture parameter calibration of different weld zones, was proposed. Results indicate that external pre-cracks in a highly strained region accelerated damage accumulation at the fracture onset region and deflected the fracture propagation path to pass through the pre-crack, causing a 12.1% loss of tensile-shear peak load. Conversely, internal pre-cracks vary the fracture initiation site and exhibit a more detrimental influence, with 24.1% loss of tensile-shear strength. The effects of the pre-crack length, orientation, and depth on the tensile-shear performance were quantified. Results show that the negative effect of the LME crack on the weld performance can be mitigated or avoided by controlling the crack location and geometry.

Fracture Response in Hot-Stamped Tailor-Welded Blanks of Ductibor® 500-AS and Usibor® 1500-AS: Experiments and Modelling

Hot-stamped tailor-welded blanks (TWBs) comprised of steel sheets with different chemical compositions are of interest within the automotive sector to enhance the crash performance of vehicles via imparting local ductile regions to ultra-high strength structural components. This study examines the fracture behavior of hot-stamped TWBs composed of mono- and multi-gauge laser-welded sheets of Ductibor® 500-AS and Usibor® 1500-AS, which are under consideration for use in crash-safety members. Microstructural and microhardness investigations were conducted across the weld line of the TWBs to address the variations of mechanical properties. A fracture characterization campaign via a series of coupon tests, including uniaxial tension, equibiaxial tension Nakazima dome, and V-bend, was conducted on specimens with a range of orientations of the weld line with respect to the loading direction. Numerical models of the tensile and Nakazima dome tests with and without considering the weld region in the TWBs were developed using the LS-DYNA finite-element method software based on mapping constitutive properties and fracture-limit curves across the weld and in parent metals as functions of local hardness. The fracture characterization tests revealed that weld fracture occurred only in the V-bend tests for which the weld line was parallel to the punch (0° V-bend), while in all of the other mechanical tests considered, fracture was initiated in one of the parent metals. The Ductibor® 500-AS region was the site of fracture initiation in the transversely-welded tensile and centrally-welded Nakazima dome samples, whereas fracture initiation occurred in the Usibor® 1500-AS region for the longitudinally-welded tensile and 45° and 90° V-bend samples. The predictions revealed that the inclusion of the narrow weld region of the laser-welded blanks within the hardness-mapped TWB models resulted in an improvement of the predictions in terms of the load–displacement response and fracture locations compared to the “without weld” scenario. Therefore, the correlation of variations in the flow and fracture response of the hot-stamped TWBs of Ductibor® 500-AS and Usibor® 1500-AS with the trend of changes in hardness across the weld can be a more suitable strategy for modelling the mechanical behavior of such TWBs in crash-safety simulations.

Full-field characterisation of oxide-oxide ceramic-matrix composites using X-ray computed micro-tomography and digital volume correlation under load at high temperatures

In situ synchrotron X-ray computed micro-tomography and digital volume correlation (DVC) were utilised to understand the failure mechanisms at room temperature and 1050 °C of two Nextel™720/alumina oxide-oxide ceramic-matrix composites (CMCs), termed materials A and B, sintered respectively at 1200 °C and ~1250 °C. At both test temperatures, three-point-bending strengths were ~55–58 MPa for material A and ~94–100 MPa for material B. Damage was associated with three primary types of cracking modes: interfacial delamination, inclined cracks within fibre tows, opening of existing matrix shrinkage cracks. Material A exhibited higher shrinkage cracking, whereas material B displayed more pronounced diagonal matrix microcracking. At 1050 °C, both systems showed less microcracking but more pronounced delamination. Such damage characteristics were rationalised in terms of the corresponding 3D DVC displacement/strain fields. Specifically, global DVC was utilised and maximum principal strain locations prior to failure, which varied from 0.005 to 0.01, correlated well to the fracture initiation sites. Further, abrupt positive to negative transitions of shear strain components were observed and were attributed to the different bonding strengths between 0°/90° fibres and the matrix. The current study demonstrates that in situ high-temperature tomography/DVC is a powerful method for studying the deformation and fracture of oxide-oxide CMCs.

The Effect of Impact Angle and Fall Height on Skull Fracture Patterns in Infants.

Skull fracture is a common finding for both accidental and abusive head trauma in infants and young children, and may provide important clues as to the energy and directionality of the event leading to the skull fracture. However, little is understood regarding the mechanics of skull fracture in the pediatric skull, and how accidental fall parameters contribute to skull fracture patterns. The objectives of this research were to utilize a newly developed linear elastic fracture mechanics finite element model of infant skull fracture to investigate the effect of impact angle and fall height on the predictions of skull fracture patterns in infants. Nine impact angles of right parietal bone impacts were simulated from three different heights onto a rigid plate. The average ± standard deviation of the distance between the impact location and fracture initiation site was 8.0 ± 5.9 mm. Impact angle significantly affected the fracture initiation site (p < 0.0001) and orientation (p < 0.0001). A 15 deg variation in impact angle changed the initiation site up to 47 mm. The orientation of the fracture pattern was dependent on the impact location and ran either horizontal or vertical toward the ossification center of the bone. Fall height significantly affected the fracture length (p = 0.0356). Specifically, at the same impact angle, a 0.3 m increase in fall height increased the skull fracture length by 21.39 ± 34.26 mm. These data indicate that environmental variability needs to be carefully considered when evaluating infant skull fracture patterns from low-height falls.