Crack Initiation Research Articles

Heat treatment effects on the shear performance and microstructure evolution of W-core SiC fibers

SiC fiber reinforced titanium alloy matrix composites employed in elevated temperature aeroengine components are subjected to complex loading conditions. Investigating the shear performance of SiC fibers after heat treatment can help ensure the load-carrying capability of composites at service temperatures. This study presents a novel double shear testing method for W-core SiC fibers. The shear strength and stiffness of the SiC fibers in as-received and heat-treated conditions were measured. The morphologies of SiC fibers’ shear fracture surfaces and C coatings were thoroughly analyzed. The evolution of the internal microstructure and the composition of reaction layers after heat treatments were investigated. The shear strength of W-core SiC fibers at 95% survival probability reached 588.64MPa after 200 h of heat treatment at 600°Celsius, representing a 55% improvement over both the as-received and 1100°Celsius heat-treated conditions. After heat treatment at 1100°Celsius, the reaction layer between the W core and the SiC sheath thickened exponentially due to extensive elemental diffusion, resulting in the formation of numerous Kirkendall voids. The voids facilitated crack initiation and propagation, leading to a deterioration in fiber shear performance. The insights gained in this study regarding the shear properties of SiC fibers can provide support for predicting the mechanical performance of composites under complex loading conditions.

Enhancement of creep properties with trace nitrogen in a low-cost nickel-based single crystal superalloy

Nitrogen (N) is an unavoidable element in single-crystal superalloys that combines with other elements to form nitrides. Large-size nitrides often become the sites of crack initiation and propagation, which adversely affect the microstructures and mechanical properties of superalloys. However, in this study, it was found that the addition of trace amounts of N could improve the creep properties of superalloys. The results indicated that the size and number of nitrides gradually increased as the N contents increased from 3 ppm to 25 ppm. In addition, the porosity first decreased and then increased. The cause of the increase and subsequent decrease in creep lives as N contents increased was the lattice expansion caused by the formation of nitrides, which reduced the porosity by compensating for volume contraction. Meanwhile, the small-size nitrides could impede dislocation movement, which benefited the creep life of the specimens, resulting in the best creep life of the specimens containing 12 ppm of N.

Low-cycle fatigue properties and fracture location transition mechanism of dissimilar steel welded joints in towers of wind turbines

This study investigates the low-cycle fatigue (LCF) behavior of two types of dissimilar steel welded joints (DSWJs): AISI 1035 & S550Q and AISI 1020 & S550Q. Digital image correlation (DIC) and finite element analysis (FEA) were used to analyze the heterogeneous strain distribution. The results reveal that the location of maximum strain, which is load-dependent and related to strength mismatch, corresponds to the fracture location of the LCF specimens. Specifically, the crack initiation site shifts from the weld toe to the base metal as strain increases. Moreover, the weld reinforcement of the DSWJs significantly affects the failure site transition, introducing strain concentration at lower stress levels while enhancing deformation resistance at higher stress levels, as evidenced by DIC strain measurements and FEA. These findings highlight the importance of joint reinforcement profile, strength mismatch and external load level in designing DSWJs for wind turbine towers against LCF failure.

Study on the fatigue properties of SPR-bonded aluminum-steel sheet joints

This study deals mainly with the influence of adhesive layer on the fatigue behavior and fretting of self-piercing riveted (SPR) joint joining differing thicknesses of AlSi10MnMg aluminum and DP590 steel sheets in lap shear geometry. Fatigue life, failure modes, crack propagation and fretting behaviors of the SPR and SPR-bonded joints were investigated and compared. The results showed that the static strength and fatigue life of SPR-bonded joints were significantly enhanced than that of SPR joints, but the fatigue failure mode of SPR-bonded and SPR joints differed. The SPR-bonded joints all failed in a rivet tail pull-out mode at all tested loads, which accompanied with small cracking in the aluminum sheet in contact with the rivet tail at lower test load. However, the failure mode of the SPR joints shifted from the aluminum sheet cracking to rivet tail pull-out as the fatigue test load increased. And during the aluminum sheet cracking failure, the fatigue crack originated from the faying interface of aluminum and steel sheet at the vicinity of the rivet shank and propagated along the width direction of aluminum sheet. Fretting wear analysis showed that the overall extent of fretting decreased and the location of fretting areas varied due to the adhesive bonding. Fretting of SPR joints mostly existed at the faying interface between aluminum and steel about 1.5mm away from the rivet shank and resulted in fatigue crack initiation, while the fretting region of SPR-bonded joints shifted to the faying interface between rivet tail and aluminum sheet.

High and very high cycle fatigue behavior of an additive manufactured hot-work tool steel

In the present study, the fatigue response of an additive manufactured H13 type hot-work tool steel is investigated across the High Cycle Fatigue (HCF) and Very High Cycle (VHCF) regimes. The primary focus encompasses the interpretation of fatigue strength models, the defect type analysis along with a detailed examination of crack initiation and growth mechanisms. Despite the tremendous development in AM technology, experimental data regarding advanced mechanical properties, and particularly fatigue behavior, are still limited. Here, microstructural analysis of a modified AMed H13 hot-work tool steel, a combination of HCF and VHCF testing methodologies implemented for the characterization of the fatigue behavior, as well as a thorough fractographic analysis of the fractured surfaces were performed. Results are compared with historical data of a conventionally ingot cast and forged grade to assess the influence of the AM process on the fatigue response of H13 hot-work tool steels. It proves to be comparable to the conventionally manufactured grade, showcasing the potential utilization of AM in the production of components used in high-demanding applications, and in hot work tooling applications. However, the type of critical defects identified in the AM grade was found to be process-induced, emphasizing the need to optimize process parameters to reduce both the number and size of defects and also to ensure component reliability and high performance in various industrial applications.

Laser additive manufacturing of Ti and Ce co-modified 2195 difficult-to-process aluminum alloy: Grain refinement, cracking suppression and enhanced mechanical properties

High cracking susceptibility of Al-Li alloys with Ti/CeB6 addition is thoroughly suppressed in laser powder bed fusion (LPBF) processing of Ti/Ce co-modified 2195 alloys at relatively high scan speeds, while the cracking suppression mechanism and phase formation in these composites are not clarified. In this work, microstructure evolution and mechanical performance of the LPBF-fabricated Ti/Ce co-modified 2195 are investigated to reveal their cracking suppression and strengthening mechanisms. The results show that apparent grain refinement of the composites is ascribed to high supercooling from rapid formation of constitutional supercooling zone in front of solid–liquid interfaces by high-Q-value Ti solute, and heterogeneous nucleation of in situ formed Al3Ti and Al11Ce3 precipitates. Their synergistic interactions promote formation of fine equiaxed grains and thus inhibit crack initiation. The composites exhibit high microhardness of 100 ± 5 HV0.2, nano-hardness of 1.6 ± 0.1 GPa and elastic modulus of 97 ± 3 GPa, where the elastic modulus increases by ∼27% and ∼31% compared to those of LPBF-processed and conventionally manufactured 2195 alloys, respectively. A tensile strength of ∼336 MPa and an elongation of ∼3% are obtained from in-situ synchrotron X-ray diffraction measurement. The improved properties are derived from grain refinement and Orowan strengthening. Based on the optimal processing parameter and composition, a bracket component filled with lattice structures is designed and manufactured with good manufacturing quality and processing accuracy.

Investigation of individual lack-of-fusion defects in the fatigue performance of laser-powder bed fusion Ti6Al4V alloys: A finite element analysis

Laser-Powder Bed Fusion (L-PBF) techniques have revolutionized the production of Ti6Al4V alloys across various industries. However, the widespread adoption of L-PBF Ti6Al4V alloys is impeded by their inadequate fatigue performance, particularly in high cycle regimes. A significant contributing factor to this limitation is the presence of internal defects inherent to the L-PBF process, act as sites for fatigue crack initiation. Previous investigations have focused on the fatigue performance of L-PBF Ti6Al4V alloys with gas porosities, while research on lack-of-fusions (LOFs) which are recognized as the most detrimental defects, remains limited. In order to deepen our understanding of the factors influencing the reduced fatigue performance observed in L-PBF Ti6Al4V alloys associated with inherent LOFs, this study employed three-dimensional (3D) finite element analysis approaches. Such approaches allow to assess fatigue indicator parameters to evaluate fatigue life and predict fatigue crack propagation. A 3D average Smith-Watson-Topper method has been proposed, which is able to rationally estimate the fatigue life of L-PBF Ti6Al4V. In addition, a novel finite element method has been developed to accurately calculate stress intensity factors along irregular shaped crack front of a notch-like feature embedded on a LOF. Additionally, parametric studies were conducted to gain further insights into the influence of LOFs on fatigue performance. The results shown the presence of embedded humps and notch-like features, and their topologies play key roles in the fatigue performance of LOF predominated L-PBF Ti6Al4V alloys.

The effects of environments and adhesive layer thickness on the failure modes of composite material bonded joints

In this study, a structural adhesive was used to bond unidirectional prepreg and fiber fabric in a single lap joint. The mechanical properties of the structural adhesive were investigated under room temperature dry state (RTD) and elevated temperature wet state (ETW, 71 ℃/85% RH), and different adhesive layer thicknesses (0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm). The fracture surfaces of the bonded joints were examined using scanning electron microscopy (SEM), and finite element simulations were conducted to observe the failure modes and failure paths. Additionally, the specimens were immersed in water and hydraulic oil, and their tensile shear strength was tested to evaluate their liquid sensitivity. The experimental results indicated that with increasing adhesive layer thickness, the strength of the specimens decreased by 21% in the RTD and by 52% in the ETW. The strength differences between different environments were minimal for adhesive layer thicknesses of 1 mm and 1.5 mm. The shear strength of the specimens decreased after immersion in water and hydraulic oil, with reductions of 43.78% and 39.21%, compared to the room temperature dry respectively. SEM observations of the bonded joint sections revealed that the primary failure modes were adherend failure and adhesive layer failure. Finite element simulations indicated that fiber tearing and crack initiation occurred in stress concentration areas during loading, leading to structural failure.

Vibration fatigue behavior and failure mechanism of Ni-based single-crystal film cooling hole structure under high temperature

Film cooling hole structures significantly influence vibration fatigue performance of Ni-based single crystal turbine blades. This study investigates the vibration fatigue behavior and failure mechanism of film cooling hole structure of Ni-based single crystal superalloy at high temperature by using the plate specimens with film cooling holes. The vibration fatigue cracks are all initiated at the edge of the film cooling hole on specimen surface, and the macroscopic crack path is a straight line path. At the microscopic scale, the crack path at 850 °C is a Zigzag path, but the crack path at 980 °C still shows a straight line path. The crack initiation of the specimen shows the oxidation crack nucleation in the stress concentration area under the coupling effect of high temperature and alternating stress. The macroscopic crack propagation direction at high temperature depends on the stress gradient direction of the resolved shear stress. At the microscopic scale, the crack propagation at 850 °C is the dislocation slip-climb mechanism, and the crack propagation at 980 °C more inclined to produce only the dislocation climb mechanism. The vibration fatigue cracks have the temperature dependence. The high temperature environment promotes the activation of slip system and the enhancement of dislocation mobility, the microscopic raft structure promotes the crack propagation along the γ phase with a large number of dislocations, the oxidation crack promotes the oxygen to enter the alloy matrix, which accelerates the Mode-I crack propagation.

Experimental and modeling investigation of the thermal shock behavior of TiC-based self-healing coatings on AISI 321 stainless steel

The AISI 321 steels of structural components serviced at high temperature are usually subjected to oxidation and thermal shock damage during service. Therefore, to improve their high-temperature oxidation and thermal shock resistance, the self-healing coating consisting of Al2O3-13 %TiO2 layer, TiC layer and NiCrAlY layer was prepared for 321 steels in this work. The experimental and microstructural characterization results showed that the thermal shock resistance of such self-healing coating was remarkably improved as compared to the counterpart double-AT13 coating. This is because the volume increment resulting from the oxidation reaction between the TiC and oxygen decreased the porosity of the self-healing coating and retarded crack growth, which also led to the improved high-temperature oxidation resistance. Further, a thermal shock life model based on the crack growth model of Paris formula were developed. The modeling results not only agreed well with the experimental results but also indicated that thickening of thermal grown oxide (TGO) is the main cause of crack initiation and growth.

Crystal plasticity-driven evaluation of notch fatigue behavior in IN718

The aim of this study was to establish a method for evaluating notch fatigue behavior through crystal plasticity finite element (CPFE) simulations based on the actual microstructure of the nickel-based alloy Inconel 718 (IN718). Initially, the equivalent plastic strain, εeps, which reflects the comprehensive slip at the grain scale, was employed to analyze the fatigue crack initiation mechanism of IN718, revealing that twinning and triple junctions of grain boundaries were high-risk locations for fatigue crack initiation. Next, fatigue simulations were performed on notched specimens using the CPFE model, with a material-level CPFE model particularly employed at the notch root. The increment of εeps, Δεeps, in a stable cycle was used as the fatigue damage control parameter and correlated with the fatigue life, Nf, revealing that the Δεeps-Nf relationship at material-level satisfied the form of the Mason-Coffin model. Finally, fatigue life prediction of IN718 notched specimens was carried out based on the Δεeps-Nf relationship, with the predicted results falling within the 2-fold scatter band, demonstrating good prediction accuracy.

Dynamic behavior of fractured gabbro treated by high temperatures

Rock masses in deep engineering are often subjected to high geothermal effect, and blasting is a key technique for deep rock excavation. It is crucial to study the dynamic characteristics of fractured rock masses under high geothermal conditions and blasting excavation. This paper focuses on fractured gabbro, examining the effects of high temperatures on its surface chromaticity and mass. Dynamic splitting tests were conducted using a Split Hopkinson Pressure Bar (SHPB) apparatus, and the microstructure was analyzed with scanning electron microscopy (SEM). The results indicate that: (1) High temperatures significantly affect the surface chromaticity and mass of gabbro samples. (2) Elevated temperatures lead to the progression from a microstructure with no noticeable microcracks to the development of extensive microcracks and mineral particle fragmentation. (3) The impacts of high temperatures on the splitting strength, peak strain, and dynamic elastic modulus of gabbro samples reach a critical point around 500 °C. At this temperature, the strengthening effect of high temperature is evident, whereas at 800 °C, these properties significantly decrease due to extensive mineral crystal fragmentation and microcrack expansion, weakening the sample's strength, stiffness, and deformation resistance. (4) From 25 °C to 600 °C, the fissure angle significantly influences the splitting strength, peak strain, and dynamic elastic modulus of gabbro. Although the fissure angle still impacts these properties at 800 °C, the overall effect of high temperature is more pronounced. (5) The failure of fractured gabbro samples is primarily characterized by tensile and shear failures, with the crack initiation angle closely related to the fissure angle.

Creep-to-rupture of T91 steel in static liquid lead-bismuth eutectic: Effects of cyclic temperature and oxygen environment

The creep-to-rupture behavior of T91 steel in static liquid lead–bismuth eutectic (LBE) is investigated, focusing on the impacts of cyclic temperature and oxygen condition. The results indicate that thermal cycling, oxygen deficiency, and high applied stress levels significantly accelerate creep deformation and reduce the creep-to-rupture lifetime of T91 steel in LBE. Surface oxide scales on T91 steel and their self-healing mechanisms play a crucial role in enhancing the creep resistance by isolating the LBE contact and delaying crack initiation and propagation. However, the integrity of this oxide scale is compromised under cyclic thermal conditions and low oxygen levels, leading to premature failure. Microstructural examinations reveal the evolution of oxide scale damage and self-healing mechanisms. The findings suggest that oxide scale failure mechanisms should be considered when designing the long-term operational performance of advanced LBE-based reactors.

TiC in-situ strengthening mechanism and crack initiation mechanism of SiC/TC4 composite coatings by laser cladding

SiC/TC4 composite coatings were fabricated successfully on the TC4 surface by varying the laser power, and the effect of TiC in-situ strengthening on the microhardness and wear resistance of coatings was investigated. The results show that SiC is continuously decomposed to generate TiC in situ, and α martensite in the coating is refined at 1000 W, with less TiCm. α martensite in the coating is roughened at 1900 W, with the highest content of TiC dominated by dendritic TiCd and petal-like TiCf, longitudinal cracks emerged from the bonding zone and penetrating through the coating. While the appropriate laser power well balanced the relationship between α-martensite and in situ precipitated phases. The C16 coating has the best performance, with an average microhardness 2.1 times higher than that of the substrate (351.8 HV0.2) and a wear rate of 38.1 % of the substrate, respectively. The wear mechanism of the substrate is abrasive, oxidative and fatigue wear, and the wear mechanism of C16 is mainly abrasive wear. The change of the mechanism is due to the fact that TiC particles play the role of skeleton and support in the coating, which are uniformly distributed around α-martensite, thus inhibiting the plastic deformation of the coating.

Influence of loading rate on the mode II fracture characteristics of SCC samples: Experiments and numerical simulations

This paper explores the influence of loading rate on mode II fracture failure in rock. Impact experiments were performed on samples of SCC at five different impact pressures via an SHPB system. This study reveals the correlations among the peak load, fracture toughness, and dynamic elastic modulus of rock mode II fractures under diverse loading rates, as well as the alterations in fracture trajectories. Simultaneously, PFC3D discrete element software has been adopted to numerically simulate the experiments and analysis of the fracture process of rocks and the variations in crack quantity and energy from a microscopic perspective. The results suggest that dynamic mode II fracture tends to increase as the loading rate increases and that the loading rate affects the fracture failure trajectory of a sample. Concurrently, as loading increased, the percentage of shear cracks in the samples gradually decreased, whereas the proportion of tensile cracks gradually increased, indicating that the samples experienced compressive stress after mode II fracture occurred at high loading rates. The proportion of energy absorbed by the samples for crack initiation and development, as well as the kinetic energy of the particles, initially tends to decrease but then increases with increasing loading rate, which is related to whether the loading rate generates secondary cracks. It is hypothesized that there exists a critical loading rate that triggers secondary cracks in SCC samples.

Enhancing corrosion fatigue performance of 17-4 PH stainless steel by simultaneous aging and plasma nitriding

This study investigated the effects of simultaneous aging and low-temperature plasma nitriding on the corrosion fatigue performance of 17-4 PH stainless steel. Plasma nitriding was conducted under two temperatures of 400 and 500 °C and, for comparison, some other specimens were subjected to aging treatment at the same temperatures and times. Microstructural characterization by scanning electron microscope (FESEM) and electron backscatter diffraction (EBSD) confirmed similar structures at the core in the nitrided and aged specimens, thereby, the influence of nitrogen at the surface of 17-4 alloy could be carefully investigated. X-ray diffraction and grazing incidence X-ray diffraction (GIXRD) identified expanded martensite as the matrix within the nitrided layer, accompanied by CrN, Fe3N, and Fe4N phases. X-ray photoelectron spectroscopy (XPS) analysis revealed high nitrogen concentration, in regions very close to the surface, after nitriding at 500 °C. The N1s spectra shifted towards lower binding energies, indicating interaction between nitrogen and the matrix atoms, and the formation of nitrides in the nitrided layer. The nitrided thicknesses experienced close values at 400 °C (10 h) and 500 °C (5 h), provided at 400 °C the layer was mostly a supersaturated solid solution but at 500 °C it was composed of iron nitrides and chromium nitride. High hardness values of above 1600 HV0.1 were obtained after optimum plasma nitriding as the result of combined effects of solution strengthening induced by nitrogen atoms and the presence of dispersed nitride phases. EBSD results indicated residual stress within the nitrided layer, which would affect the corrosion fatigue resistance. Experimental findings demonstrated that both aging and plasma nitriding substantially enhanced the corrosion fatigue resistance of 17-4 PH stainless steel in 3.5 wt% NaCl. Moreover, simultaneous aging and plasma nitriding proved superior behavior in corrosion fatigue, primarily due to delayed crack initiation and improved fatigue resistance, without the risk of overaging which is detrimental to mechanical properties.

Material removal mechanism of TiCp/Fe composite by multi-diamond-abrasive-grinding considering the random distribution characteristics of particles

The TiC ceramics reinforced Fe matrix composite (TiCp/Fe) exhibits exceptional properties, including high hardness, strength, wear, and heat resistance. This study focuses on investigating the material removal mechanism to achieve high-quality and low-damage surfaces. A three-dimensional particle random distribution algorithm is proposed based on the random distribution characteristics of TiC particles. Furthermore, a multi-diamond-abrasive grinding finite element model is established using the Rayleigh probability distribution model to account for the randomness of undeformed chip thickness during the grinding process. This study combines experimental and simulation analyses to investigate the variations in grinding forces, stress field distributions, and surface and subsurface quality. The results reveal that the material removal process can be categorized into five stages: ploughing of the Fe matrix and TiC particle, TiC particle crack initiation, TiC particle crack extension, and TiC particle fracture. Moreover, the process of removing TiC particles can be further grouped into ductile removal, ductile-brittle removal, and brittle removal, depending on the undeformed chip thickness. This study improves the comprehension of the mechanism of TiCp/Fe composite material and establishes a significant practical guidance for the diamond grinding processing of metal matrix composites.

The Effect of Erosive Media on the Mechanical Properties of CAD/CAM Composite Materials.

This study aimed to investigate the effect of acidic media storage (gastric acid and Coca-Cola) on the mechanical properties of CAD/CAM materials. Three types of materials were tested: a polymer-infiltrated ceramic network (PICN) (Vita Enamic (En), VITA Zahnfabrik, Germany), a resin composite block (RCB) (Cerasmart (Cs), GC Corp, Japan), and a conventional resin-based composite (Gradia direct (Gr), GC Corp, Japan), which was used as a control. Beam-shaped specimens of each material, with dimensions of 16 mm × 4 mm × 1.5 mm, were prepared (90 in total). The specimens were divided into subgroups (10 each) and stored for 96 h in either gastric acid, Coca-Cola, or distilled water. Flexural strength and elastic modulus were evaluated using a three-point flexural strength test with acoustic emission (AE) monitoring. Vickers microhardness was measured before and after storage in gastric acid and Coca-Cola. Data were statistically analysed using two-way and one-way ANOVA, the Tukey's post hoc, and independent t-test at a significance level of 0.05. The results showed that Cs and En maintained their flexural strength and elastic modulus after acidic media exposure, while Gr experienced a significant decrease in flexural strength following gastric acid storage (p < 0.01). Initial crack detection was not possible using the AE system, impacting the determination of flexural strength. Exposure to acidic media decreased all materials' microhardness, with Gr showing the most notable reduction (p < 0.0001). Gastric acid had a greater impact on the microhardness of all tested materials compared to Coca-Cola (p < 0.0001). In conclusion, storage in erosive media did not notably affect the flexural strength or elastic modulus of CAD/CAM composites but it did affect hardness. CAD/CAM composite blocks demonstrated superior mechanical properties compared to the conventional composite.

Dynamic tensile intralaminar fracture and continuum damage evolution of 2D woven composite laminates at high loading rate

The intralaminar tensile failure of 2D woven composites under dynamic tensile load was investigated in this paper. Compact tension samples were tested at high loading rate with an electromagnetic Hopkinson bar system. The strain field was obtained with high-speed imaging and digital image correlation, and the J-integral method was employed to obtain the fracture toughness and corresponding R-curve. The continuum damage evolution of intralaminar failure was then analyzed by tracking the opening near the initial crack tip. It is found that the dynamic intralaminar fracture toughness is decreased by 51% compared to the quasi-static condition, the continuum damage evolution and its dependence on loading rate have been reported as well. The failure mechanisms were studied with thermal imaging and scanned electron microscopy, shorter fibre pull-out length and thinner failure process zone may be responsible for the reduced toughness at high loading rate.

Experimental and numerical studies on the propagation paths of gear root cracks

Gear bending fatigue failure has a significant impact on the operational performance of advanced machinery, such as electric vehicles and wind turbines, potentially leading to failures and catastrophic consequences for transmission systems. Several methods are currently used to predict gear crack propagation; however, a comprehensive comparison of their accuracy and efficiency in predicting crack paths is lacking. In this study, the propagation path of root cracks in commonly used automotive gears was investigated using finite element (FE) analysis and experimental testing. Three mixed crack path prediction criteria were integrated to predict the gear root crack propagation using the commercial software ABAQUS by user-defined Python script. The impacts of gear crack parameters, including the gear initial crack length, on gear root crack propagation behaviour were analysed. The simulations were verified by the gear bending fatigue test using a single tooth bending test device. The results indicate that the simulation outcomes align with the experimental tests, demonstrating that all three criteria are effective in predicting gear root crack propagation behaviours.