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In the present study, very high cycle fatigue (VHCF) characteristics and properties of a direct quenched and partitioned (DQ&P) medium carbon (0.4 wt.% C) steel having ultrahigh-strength (UTS > 2 GPa) was investigated using an ultrasonic-fatigue testing technique (∼19 kHz frequency) up to ∼10¹⁰ cycles at the load ratio (R) of −1 (fully reversed tension-compression). The occurrence of crack initiation in the VHCF regime was observed at interior non-metallic inclusions. The microstructural changes following VHCF failure underneath the fracture surfaces near the crack initiation sites were further investigated through transmission electron microscopy. Formation of a clear nanograined subsurface layer adjacent to the crack initiating site at the interior inclusion was noticeably evident. The variation of average grain size (60 − 140 nm) and thickness (300 − 1200 nm) of nanograined layers at different number of cycles to failure were measured in detail. Apparently, localized plastic deformation caused the fragmentation of martensitic laths and hence, the formation of aforesaid ultrafine-grained layers in the microstructures.

High-strength pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 400–700 rpm, and the grain refinement mechanism of the nugget zone (NZ) was determined. The thermo-mechanical process during FSW in the NZ was simulated by multi-pass thermal compression, thereby achieving the austenitic non-recrystallization temperature (Tnr). The austenitic non-recrystallization in the NZ at the lowest rotation rate of 400 rpm caused a significant grain refinement. Furthermore, the reduced rotation rate also resulted in the formation of a high ratio of island-like martensite-austenite (M-A) constituent. The toughness of the NZs was enhanced as the rotation rate decreased, which is attributed to the fine effective grains and homogeneously distributed fine M-A constituents dramatically inhibiting crack initiation and propagation.

In this paper fretting fatigue crack behavior in 1045 steel is studied by in-situ observation and finite element analysis. in-situ fretting fatigue experiments are conducted to capture real-time fretting fatigue crack formation and propagation process. The fretting fatigue tests under different load conditions are carried out, then the lifetime and fracture surface are obtained. The crack propagation rates under different loading conditions are measured by in-situ observations. With in-situ observation, crack initiation location and direction are analyzed. Finite element model is used to calculate J-integral which then is applied to fitting with experimental crack growth rate, and establishing crack growth rate model. From fitted S-N curve, it turns out that smaller load ratio leads to higher lifetime. Crack initiates slightly below the point equivalent to line contact of the contact surface in different test conditions, and crack direction shows no obvious relationship with load parameters. The established crack growth rate model well agrees with the test results.

A new way is proposed to simulate the crack propagation paths of the multi-cracked structure. Firstly, the complete displacement and stress fields of the multi-cracked structure are calculated by the extended boundary element method (XBEM) coupled with characteristic analysis of the crack tip. Secondly, based on the maximum circumferential stress criterion by considering the contribution of the non-singular stress terms, the crack initiation angles are obtained. Thirdly, the cracks propagate forward along the crack initiation angles to form a new multi-cracked structure. During this process, boundary element adaptive mesh technique is established and the XBEM is used to analyze the newly formed multi-cracked structure repeatedly. Finally, the crack propagation paths of the multi-cracked structure are obtained. Numerical examples show that the results obtained by the XBEM are in good agreement with the experimental results.

It is generally considered that hydrogen could accelerate the corrosion fatigue process, however, the effective monitoring approach for the corrosion fatigue development in the hydrogen related environment is still missing. In this work, the hydrogen effect on phase angle shifts in electrochemical impedance spectroscopy during corrosion fatigue crack formation in drill pipe steel was in-situ studied. The results show that phase angle shifts as a function of time could be used to monitor the corrosion fatigue development in the presence of hydrogen. The permeated hydrogen leads to a lower peak intensity and shifts the peak to a higher frequency on the phase angle vs. frequency curve. These differences are due to the formation of a corrosion fatigue crack with a shorter length than in the absence of hydrogen. The phase angle shift reveals that the presence of hydrogen results in shortening crack initiation time and corrosion fatigue life of the drill pipe steel.

In this work, Cu/Al8011/Al1060 trilayered composite with an ultrathin copper layer was fabricated successfully by the hybrid process of cast-rolling and hot-roll-bonding. The effects of heat treatment were investigated on the microstructure and mechanical behavior of the composite. The results showed that annealing temperatures above 300 ​°C led to the formation of new intermetallic compounds that joined their initial counterparts. Also, the interface growth was fast above 340 ​°C. It was found that the annealing temperature of 320 ​°C led to the optimal tensile properties of the composite. Through rolling, hard-brittle IMCs imposed intense plastic strains on the surrounding regions, resulting in further grain refinement of both metals in the vicinity of the interface. Although the kernel average misorientation of the trilayered composite was almost identical with the Cu/Al8011 bimetal, its amount was considerably decreased after heat treatment, as the distorted structure changed into a nearly strain-free one using the restoration phenomenon. The SEM-based in-situ tensile test showed that crack initiation occurred within the IMCs and propagated into the Cu strip, leading to its early fracture. However, the strength-displacement curve during in-situ tensile deformation did not decrease significantly due to the ultrathin copper layer.

Based on analysis of thermo-hydro-mechanical-chemical (THMC) coupling mechanism for brittle rock, THMC coupling indicator in terms of rock porosity was introduced to represent the influencing degree of THMC coupling field on stress field in order to establish THMC coupling fracture criterion. A novel real-time measurement method of permeability (related to porosity) was proposed to determine the THMC coupling indicator, and self-designed THMC coupling tests and scanning electron microscope tests were conducted on pre-cracked red sandstone specimens to study the macroscopic and microscopic fracture mechanism. Research results show that the higher the hydraulic pressure is, the smaller the crack initiation load is and the easier the Mode I fracture occurs. Test results are in good agreement with prediction results (crack initiation load and angle, and fracture mode), which can verify the effectiveness of the newly established THMC coupling fracture criterion. This new fracture criterion can be also further extended to predict THMC coupling fracture of multi-crack problem.

The desiccation cracking of soil occurs when shrinkage is restricted during the drying process and the induced tensile stress equals the tensile strength. Thus far, experimental estimations of the tensile stress of soil have not been realized, although such estimates are important for predicting crack initiation. This study presents the development of a laboratory-based desiccation stress test to measure the tensile stress generated during the drying process until crack initiation. In this proposed desiccation stress test, the tensile stress is induced during the drying process in the longitudinal direction of bar-shaped specimens with fixed ends. Desiccation stress tests were performed on sandy soil with a rich fine fraction, and the results were verified through photographic observations of crack initiation and comparisons with the results of direct tension tests. The results show that the desiccation stress test yields reliable tensile stress until cracking. The application of the desiccation test results is illustrated via the verification of an existing model of crack initiation by desiccation. The results of the desiccation stress tests are useful for determining the model parameters that significantly influence the development of tensile stress and enable its accurate prediction until crack initiation.

Scratch tests are often performed at a speed that is significantly lower than the real application like machining and grinding. However, brittle materials like BK7 behave very differently under high-speed conditions due to the more promising temperature and stain rate effects Therefore, it is important to study its material behavior under high-speed condition. In this study, single scratch tests and consecutive scratch tests were performed on BK7 under scratch speeds of 1, 5 and 20 m/s, which were much higher than the traditional scratch tests. The surface morphology as well as the subsurface cracks of the scratch grooves was inspected under AFM and FIB-SEM. The thermal effect that caused the changes in ductile-brittle transition (DBT) and scratch morphology was simulated and explained by a thermal-stress coupled finite element analysis. Finally, the changes in material removal behavior as well as the crack initiation mechanism due to speed effect was revealed.

The mill products like sheet always have one or more severe textures inevitably, and its effect on mechanical properties is not a negligible issue. The orientation dependent tensile-creep behavior induced by rolling texture of Ti65 titanium alloy sheet has been systematically investigated at 650 ℃. There are some anisotropic characteristics between TD and RD of Ti65 sheet. The UTS and TYS of TD are higher than RD at 650 ℃. Besides, the creep endurance time of TD (172.6–174.5 h) is about three times longer than RD (55.6–65.1 h) at 650 ℃ and 240 MPa. Moreover, the grains are inclined to form Texture Ⅲ ( 1 ¯ 2 1 ¯ 6)[1 2 ¯ 11] and (01 1 ¯ 3)[1 2 ¯ 11] after creep along with TD, but to form Texture Ⅰ ( 1 ¯ 2 1 ¯ 0)[10 1 ¯ 0] after creep along with RD. Finally, the crack initiation site is different during creep in TD and RD. The reason for anisotropic properties of tensile and creep has been summarized in two aspects: (ⅰ) the change of the SFs (Schmid factors) value between TD and RD; (ⅱ) the difference of creep mechanism between TD (grain boundary sliding) and RD (dislocation slip). Anisotropy of Ti65 sheet should be fully considered to increase structural efficiency in the engineering design and application.

Scored tablets are produced to facilitate easy and uniform division of tablets. One of the most critical quality attributes of the scored tablet is its dividing uniformity. To clarify the tablet dividing mechanism, we proposed a novel algorithm on the basis of fracture and statistical mechanics. Powder formulations composed of microcrystalline cellulose, cornstarch, and lactose were prepared to realize tablets with different score line shapes. As a result, the probability of cracks in the tablet and the dividing uniformity were well estimated. The dividing uniformity was observed to be more sensitive to changes in the formulation factors rather than the shape of the score line. The proposed algorithm demonstrated that the variation in the crack initiation threshold originating from the formulation factors was more significant than that of the stress concentration on the basis of the shape of the score line.

Introduction Dehydration has been considered as a potential contributor to vertical root fractures (VRFs) after root canal treatment (RCT). A loss of water could cause embrittlement of dentin and detrimental shrinkage strains. Senior patients have the highest risk of VRF. In this study, we characterized the spatial distribution in shrinkage of tooth roots with respect to donor age and prior RCT. Methods Single-rooted human teeth with and without prior RCT were collected from young (age 60 years) adults. Transverse slices were sectioned from the apical, middle, and coronal thirds of the roots, and digital image correlation was used to evaluate shrinkage during free convection. Crack initiation and growth analysis was performed via optical microscopy, and bound water in dentin was characterized by Raman spectroscopy. Results The rate of shrinkage was significantly higher (p ≤ .05) in the apical third than in the middle and coronal thirds of all teeth regardless of donor age. The highest shrinkage strain occurred in the apical third of old donor teeth with prior RCT. In addition, the RCT-treated old teeth suffered the highest percentage of water loss with dehydration. Cracks initiated from the root surface and extended toward the canal with loss of water and shrinkage. Conclusions The apical third undergoes significantly larger shrinkage strains with dehydration than the remainder of the root. Prior RCT exacerbates the extent of shrinkage, particularly in the teeth of seniors and after clinical function, which could increase the propensity for VRF.

Large area, time resolved, in-situ optical scanning of constant displacement (four-point bend) tests has been used to monitor the environmentally assisted cracking (EAC) initiation and crack growth behaviour of AA7085 and AA7449 T7651 thick plate materials, exposed to warm-humid air (50 % RH & 70 °C). Subsequent fractography has revealed the dominant initiation sites originating at surface connected gas pore clusters, in combination with reactive Mg-containing constituent intermetallic compounds co-located on grain boundaries. Cracks were also shown to grow at stress intensities substantially below the KIEAC threshold measured via DCB testing, which should not therefore be taken as a limit below which EAC cannot occur.

Turbine discs in gas-turbine engines always experience high-temperature and high-stress environment. Extensive investigations have been performed towards achieving high performance. Deformation behaviors and degradation factors above normal service temperature are important to be comprehensively understood for further development and optimization of turbine disc superalloys. In this study, the low cycle fatigue behaviors were investigated under total strain amplitude control in U720Li disc superalloys. The results revealed that with the increasing strain, the number of dislocation increased and dislocation configuration transferred from pile-up at the matrix/γ′ precipitates interfaces to penetrating into γ′ precipitates. The great extent of recrystallization was found above the normal service temperature through transmission-EBSD combined with TEM analysis. The low cycle fatigue property above the normal service temperature was weakened to a large extent after recrystallization of small-angle grains formed by the dislocation substructures and movements. Large extent of local recrystallization could also provide much more sites for the crack initiation and propagation.

The potential of hydratable magnesium carboxylate compounds (HMC) combined with MgO powder as a binder of castables was evaluated. The green mechanical and thermal shock resistance properties of HMC-bonded castables (HMCC) and calcium aluminate cement-bonded castables (CACC) were compared. The green mechanical strength of the HMCC reached the mechanical strength of the CACC. The residual strength ratio of the HMCC is higher than that of the CACC and increases first and then decreases with the MgO/HMC ratio. The residual strength ratio's highest value is 56.8% (MgO/HMC ratio is 2/3), which is 1.8 times that of CACC. Interestingly, the microcracks in the HMCC after thermal shock are radial, while those in CACC are linear. The micromechanical testing on the matrix of castables after thermal shock showed that the ratio of the energy for crack initiation to the work of fracture of the HMCC is lower than that of the CACC, but its fracture toughness is higher than that of the CACC, indicating that the microcracks in the HMCC initiate easily but propagate hardly.

Rail transport system experience complex loading conditions that alters the microstructure of wheel and rail material near the contact interface region. This paper presents microstructure and hardness study of virgin and used pearlitic rail steel. Samples are prepared from the railhead section taken from nearby Kazipet Railway Division, (South Central Railway). The microstructure and hardness variation along the depth from the surface is presented. In continuation to the author’s previous study [1] , along the transverse direction, this time samples are taken in a longitudinal direction. Microstructural observations show the existence of pro-eutectoid ferrite at the grain boundaries for the sample near to the rail surface. Presence of this softer material favours micro-crack initiation causing wear debris formation and rolling contact fatigue defects. Hardness test results reveal an increase in hardness values for material points close to the surface, while base material hardness remains almost the same. Higher hardness values at the surface level are indicating the formation of a brittle material which is more susceptible to crack initiation and propagation. This experimental investigation enriches knowledge domain in the formation of rolling contact fatigue defects. This will help railway engineers to schedule proper maintenance planning and material scientists to develop superior rail material.

The localized loss of material due to corrosion activities leads to the formation of pits. It produces undesired effect of stress riser which harms the structural integrity. To safeguards the structure with pre-existing pits, finite element analysis (FEA) can be very helpful. The present study investigates the influence of pit shape on crack initiation and propagation in a rectangular plate using the extended finite element method (XFEM). For this purpose, various pit shape such as semi-circular, semi-elliptical, rectangular and bullet-shape were designed. The damage behavior of the assigned material was modelled using cohesive segment approach. Moreover, the rectangular plate was strained by tension and bending loading to study their influence on crack initiation. It was observed that the presence of pit can greatly influence the crack initiation under both tension and bending loading conditions.

The paper presents fracture parameters for edge and internal cracks subjected to mechanical load by the finite element method. The influence of crack length and crack position on edge and internal crack in TBCs on the strain energy release rate (SERR) and stress intensity factor (SIF) are discussed in detail. The numerical results show that the edge crack near the top surface results in high SERR and SIF mode I. In edge crack, the decreasing trend in the SERR was observed with a decrease in crack length and crack position change towards the TC/BC interface. About 60.06% reduction in the SERR was observed for 100 µm edge crack positioned near the top surface when shifted towards TC/BC interface. Similarly, 60 µm and 80 µm edge crack show a 30% and 56% reduction in SERR. Whereas, 20 µm and 40 µm edge crack shows 1.76 and 1.48 times more value of SERR. The internal crack shows a small value of SERR and SIF mode I irrespective of crack length and position compared to edge crack. For internal crack, as the crack length was increased from 20 µm to 100 µm, the reduction in SERR value increases from 59.23% to 90.23%. Similarly, the edge crack shows high SIF mode I compared to internal cracks irrespective of crack length and position. The edge crack appears to be more unstable than the internal crack and propagates early to cause the failure. The crack length and position strongly influence the crack initiation and propagation, resulting in the failure mechanism.

In recent years, as the height and size of building layers have increased, the thicknesses of the components used have also increased, making brittle fracture more likely to occur. Fracture tests have often been performed on reduced scale steel. It is necessary to develop an approach for brittle fracture evaluation that is less sensitive to the specimen size. In this study, we focused on the relationship between brittle fracture properties and geometrical shape and conducted a three-point bending test for isometric specimens with a 4:2:1 geometry. The results of the three-point bending tests were organized by quasi-CTOD, and it was found that the results were affected by the specimen size and notch depth. The modified Weibull stress model based on the local approach was able to evaluate the critical conditions of brittle fracture in a unified manner. Fracture probabilities at the initiation point of brittle fracture were defined and applied to each Weibull stress model, and the results were validated. The plastic strain distribution and microcrack appearance suggest the involvement of triaxiality in crack initiation. The inclusion of triaxiality in the fracture probability makes the initiation of brittle fracture more likely, and it suggests the possibility of including multiaxiality in the local approach.

Mechanical properties of BTA deep drilled components made of AISI 4140+QT under quasi-static loading are analyzed in instrumented compression tests, inspired by the tube-flattening test according to DIN EN ISO 8492. Digital image correlation (DIC) analysis is used to observe spatial strain distribution on the front side of specimens during compression. It is found that microstructural changes resulting from drilling, particularly the formation of white etching layers (WEL), play a key role in the loading capability of the components under compression. Higher forces needed to be applied to compress specimens with WEL and crack initiation was observed at lower displacements.

Ultrasonic fatigue tests were carried out at room temperature and under zero mean stress (R=-1), on the aeronautical steel maraging 300. Hourglass shape testing specimen dimensions were obtained by modal numerical simulation, using the mechanical proprieties of this steel: density, Young Modulus and Poisson ratio. Ultrasonic fatigue results were achieved under three loading stress: 585, 486 and 389 MPa, corresponding to high, middle and low applied stress, respectively. Fracture surfaces were analyzed with scanning electronic microscope, in order to investigate the origin and causes of crack initiation and propagation under this testing conditions. Finally, general conclusions were addressed concerning the ultrasonic fatigue endurance of this aeronautical steel, together with the crack initiation and propagation behavior.

This work shows the after effects of a trial examination concerning the low-velocity impact response of Jute/Kevlar fiber epoxy hybrid composite laminates. Trial tests were executed according to ASTM principles utilizing an instrumented falling weight impact testing machine. Impact tests were directed to portray the type and extent of the damages observed in laminate subjected to various impacting heights: 50 to 250 mm. The reaction of these laminates to drop-weight low velocity impact at energy levels ranging from 0.4 to 2.3 J was examined. At impact height of 250 mm, there is a calamitous failure of laminates. It was discovered that the laminates displayed two kinds of failure modes viz., crack initiation and perforation of the laminate.

Four sets of alkali-activated aluminosilicate (AAAS) composites based on ceramic precursors were studied in terms of their characterization by mechanical fracture parameters as a basis for considerations of durability. AAAS composites made of brick powder as a precursor and alkaline activator with various silicate moduli (Ms = 0.8, 1.0, 1.2, 1.4, and 1.6) were investigated. The sets of AAAS composites differed in terms of the used filler: quartz sand or brick rubble. Two different precursor particle size ranges of 0–1 mm and 0–0.3 mm were used for both types of filler. The test specimens had nominal dimensions of 40 × 40 × 160 mm and were provided with a notch at midspan after 28 days of hardening. The notches were cut up to 1/3 of the height of the specimens. The specimens were subjected to three-point bending fracture tests during which force vs. deflection (F–d) and force vs. crack mouth opening displacement (F–CMOD) diagrams were recorded. Tensile strength ft,ID and specific fracture energy GF,ID values were identified using the inverse method based on a neural network ensemble. The obtained F–CMOD diagrams were subsequently evaluated using the double-K fracture model supported by the ft,ID and GF,ID values. The double-K model allows the quantification of two different levels of crack propagation: initiation, which corresponds to the beginning of stable crack growth, and unstable crack propagation.

High water pressure is one of the primary reasons for fracture water inrush through numerous cracks at the entry points of deep tunnels. Two types of tensile shear failure modes and one type of compression shear failure mode have been established for fracture water inrush under high water pressure condition. In this study, the condition for critical water pressure for two-dimensional crack initiation is proposed and the criterion for water inrush in fractured rock mass is revealed. Next, the model of crack cementation weakening and block collapse under continuous action of water pressure is established. Finally, the collapsing process of the block under the action of hydraulic pressure is simulated by immersed body–spectral element method (IB–SEM) system.

In the last decades, a new type of distress has been observed more and more frequently on asphalt pavements. This distress, ascribable to fatigue failure, has been named top-down cracking (TDC) because it consists in longitudinal cracks that initiate on the pavement surface and then propagate downwards. A series of surveys recently carried out on Italian motorways highlighted that TDC can affect up to 20%–30% of the slow traffic lane. Therefore, in order to achieve a better understanding of such distress, this paper reviews causes, models and experimental tools and highlights future challenges for TDC. The literature review indicates that TDC can evolve on the pavement surface in three stages (i.e., single crack, sister cracks, alligator cracking) and, below a certain depth, the cracks can form angles of 20°–40° with respect to the vertical plane. Even though multiple factors contribute to TDC development, thick pavements are more likely to fail due to TDC induced by tire-pavement contact stresses, especially in the presence of open-graded friction courses (OGFCs). Moreover, in literature there are several TDC models based on mechanics (e.g., fracture mechanics or continuum damage mechanics) which allow a rigorous study of crack initiation and propagation. Future challenges include the identification of a reliable and feasible test method, among those proposed in literature, to study the TDC performance of asphalt mixtures and the implementation of TDC in pavement management systems (PMSs) through the definition of criteria for TDC recognition in the field as well as for the rehabilitation depth evaluation. Finally, more research is needed for open-graded asphalt mixtures, which present critical drawbacks in terms of TDC.

In this paper, the S-N curve of Zn-Al alloy coated steel wire was obtained by setting 30 groups with different corrosion-fatigue tests. The results show that the corrosion-fatigue resistance of Zn-Al alloy coated steel wire is preferable to high-strength steel wire. The crack initiation time of Zn-Al alloy coated steel wire is longer than high-strength steel wire, and the propagation speed is far lower than high-strength steel wire. Finally, a life prediction model considering corrosion-fatigue coupling effect is proposed. The calculation results show that the model considering corrosion-fatigue coupling effect can reflect the corrosion-fatigue coupling effect to some extent. The correctness of the theory is checked by comparing the calculation results with the experimental results.

Steel fenders are frequently employed in bridge pier shields to engross collision energy due to vessel impact. However, vulnerability to corrosion is a major drawback in existing steel fenders. To end this, an innovative composite fender structure consisting of corrugated steel and Preplaced Aggregate Fibre Reinforced Concrete (PAFRC) is introduced. Corrugated plates of steel are intended to absorb energy and stiff guard outer panels made with PAFRC exhibit enhanced resistance to impact and ductility. Six steel-PAFRC fenders were prepared with the steel plates of two different thicknesses, where the four top panels of the PAFRC are reinforced with short and long hooked end steel fibres while bottom panels are made with non-fibrous preplaced aggregate concrete (PAC). All steel-PAFRC fenders were tested through a drop weight impact testing device to investigate their impact behaviour and failure mechanism. Collision energy at crack initiation and the ultimate crack of PAFRC panels, crushing deformation of corrugated plates and failure configuration were also examined. Additionally, a numerical model of composite fenders was developed and verified through experimental evidence. Research revealed the crashworthiness of PAFRC panel was higher when compared to the PAC panel. The relative resistance of corrugated plates of steel and the PAFRC panels had a dramatic effect on the composite fender’s impact response.

Metal matrix composites have significant applications in the engineering field. During service, the fracture is one of the most typical failure modes. In this study, the digital image-based technique (DIT) combined with a micro-scale damage cohesive finite element model (CFEM) was employed to reconstruct the microstructures of metal matrix composites with the arbitrary morphology of particles. The cohesive finite element in this model can predict the process of crack initiation and propagation spontaneously, and the effectiveness of the model was verified by the in-situ tensile tests of the compacted graphite iron (CGI) sample. We found that the morphology and distribution of the graphite particles play important roles in crack initiation and propagation under tensile and compressive loadings. The relationship between the graphite particle size and yield strength was established, and an optimal graphite particle size was found for the maximum yield strength, which is different from the previous results. The relationship between microstructures and tensile properties of CGI could contribute to the design and development of metal matrix composites with optimal mechanical properties.

In the present study, the mechanism of fatigue and dwell-fatigue crack initiation was investigated in Ti-6Al-4V with a bi-modal microstructure. While substantial dwell-fatigue life debit was evidenced, microstructural configurations associated with crack nucleation are similar and suggest a unique mechanism for fatigue and dwell-fatigue loadings. Pairs of α grains well oriented for basal slip and separated by a (0001) twist grain boundary were found to be critical configurations for crack initiation. Intense slip is localized at the boundary and precedes crack formation along this boundary. Data from a prior fatigue study was revisited and supports the occurrence of this mechanism.

Successive double scratch tests under the critical load of median crack initiation were conducted to investigate the crack behaviors under the influence of interaction between the arranged grits during grit-arranged grinding BK7 glass. After the inspection of the subsurface crack morphology by the advanced focused ion beam, it is found that with the separation distance increasing, the crack behaviors under the 2nd scratch experience three stages: a new type of crack—premature lateral crack initiation, no crack initiation and median crack reappearing. The crack behaviors that occurred in the former two stages can alleviate the subsurface damage. The initiation mechanisms of the unique crack behaviors were analyzed by a stress field analytical model. Also, the benefit of these unique crack behaviors on the grinding process was discussed.

We describe here the distinct high cycle fatigue properties of advanced bainitic steels to elucidate the significant role of retained austenite with different morphologies on sub-surface fatigue crack initiation process. The inter-plate film-like retained austenite at the straight front of small crack tip transformed to martensite, leading to the arrest of small crack growth and altered the active slip systems. However, the blocky retained austenite or martensite/austenite islands located at prior austenite grain boundaries induced intergranular fatigue cracking because of ease of activation of slip in retained austenite.

In this study, a combination of acoustic emission (AE) method (AEM) and wave transmission method (WTM) is used to investigate the behaviors of AE and ultrasonic properties corresponding to initial fracturing in granitic rocks. The relationships of AE characteristics, frequency spectra, and spatial locations with crack initiation (CI) are studied. The anisotropic ultrasonic characteristics, velocity distributions in different ray paths, wave amplitudes, and spectral characters of transmitted waves are investigated. To identify CI stress, damage initiations characterized by strain-based method (SBM), AEM and WTM are compared. For granite samples, it shows that the ratio of CI stress to peak strength estimated by SBM ranges from 0.4 to 0.55, and 0.49–0.6 by WTM, which are higher than that of AEM (0.38–0.46). The CI stress identified by AEM indicates the onset of microcracking, and the combination of AEM and WTM provides an insight into the detection of rock damage initiation and anisotropy.

In the low cycle fatigue (LCF) regime, fatigue failure of metallic materials with high strength and less impurities generally dominates by multiple surface crack propagation and coalescence, in which its final failure shows a stochastic nature on crack initiation, propagation and coalescence under cyclic loadings. According to this, the competing failure modes of multiple surface cracks and interior cracks are studied through coupling numerical simulations with fracture mechanics methods. In particular, a probabilistic procedure for modeling multiple surface crack propagation and coalescence is established by incorporating Monte Carlo simulation with experimental evidences, including surface crack density and crack length distributions measured from LCF replica tests of 30NiCrMoV12 steel. In addition, it calculates the probability of coalescence of neighboring cracks with allowance for their interactions and local plastic deformation at the crack tips. Finally, it estimates the remaining usage lives of specimens from initial state to critical cracks by propagation and coalescence of dispersed cracks.

The fatigue failure is predominate in the aircraft fuselage. The fuselage skin consists of the shell and stingers. Fatigue plays a significant role in the crack growth of the skin structures. The proper maintenance and scheduled test intervals may avoid sudden skin failure and crack path (CP). With exiting the cracks, the propagation rate increases dramatically. Therefore, shortening the regular inspection intervals is recommended. Nevertheless, the young machines may have also unexpected skin rupture. The cracks are emanating from the notches such as rivets and the holes under the cyclic loading. The stresses concentrated around these notches. The stress concentration provides the unique conditions for crack initiation. Therefore, the cracks tend to connect with each other. Hence, the number of cycles to failure is decreased dramatically. Although the last decades, the fracture toughness, design, and new alloying element are enhanced. According to the traditional fuselage failure, the crack inspections are recommended even after a few thousand cycles.

Geared components can be manufactured from sheet metals by sheet-bulk metal forming. One relevant load case in service are overload events, which might induce elevated strain rates. To determine the characteristic hardening and fracture behavior, specimens manufactured from the deep-drawing steel DC04 were tested with strain rates ranging from 0.0001 to 5 s−1. The gear wheels manufactured by sheet-bulk metal forming are tested at crosshead velocities of 0.08 mm/s and 175 mm/s. The tests are analyzed by measuring deformed geometry and hardness. While the tensile tests results show obvious strain-rate dependency, the hardness measurements show no strain-rate depended effect. The analyses are complemented by finite-element-simulations, which assess the homogeneity of deformation and point out the mechanisms of failure. Both coupled and uncoupled ductile damage models are able to predict the critical areas for crack initiation. The coupled damage model has slight advantages regarding deformed shape prediction.

Dynamic fracture effects such as fracture delay and dependence of the material strength on the loading rate are known from multiple experiments on crack initiation and spallation. Classic linear oscillator (mass-spring system) is considered and critical spring stretch fracture condition is added. Due to relative simplicity of the model and availability of analytical solutions, the dynamic fracture effects can be studied analytically for the case of dynamic loading. Moreover, the mass-spring model can be calibrated to simulate some known experimental results on dynamic crack initiation due to pulse loading.

The direct current potential drop method (DCPD) is commonly used to measure the crack length in fatigue tests. Due to a propagating crack the cross section of the specimen is reduced leading to a densification of potential field lines, whereby the measured potential difference increases. First experiments on notched round bars have shown that the location of a crack can be determined with a multiple potential drop measurement (Hartweg and Bar, 2019). In this study single-edge notched specimens were equipped with three potential probes – on the frontside (Uf), on the backside (Ub) and on the narrow side (Un) of the specimen. During the fatigue test the three potentials were measured simultaneously using amplifiers of the control electronics. To enable a direct comparison between the potential drop measurement and the real crack geometry, the crack front was marked on the fracture surface by introducing overloads in defined intervals. The measured potential values were normalized on the starting potential and quotients of the potentials were formed (Pf/Pn and Pb/Pn). By plotting the potential quotients over the cycle number, the time as well as the location of the crack initiation site can be identified. In addition, it was possible to identify subsequent cracks and to determine the shape of the crack front of long fatigue cracks based on the measured potential data.

The extensive application of various types of point-supported glass facades may bring potential thermal breakage risk and impacts on indoor human beings safety. In this work, point-supported glass facades with five various types were tested under thermal loads. The present results showed that installation forms influenced significantly the first breaking time, the location of crack initiation and the final falling out area. It demonstrated that the one-point-supported glass facades had the longest time for the first crack occurrence whereas the glass eventually fell completely out of the frame. However, the six-point-supported glass facades had the shortest first breaking time, but ultimately no glass pieces fell out of the frame. To calculate the temperature variation and stress distribution of glass panel, a thermal-mechanical model was developed. In addition, an optimization simulation was further conducted using the bound optimization by quadratic approximation method to obtain a better thermal resistance performance of glass facade. This work provides significant insights on the effects of various installations upon the thermal response of glass facades and helps to understand the failure mechanism and build safer facades by the structural optimization method.

In this article, a new algorithm is developed for predicting crack initiation and growth direction in 3D solid concrete. The algorithm combines the anisotropic damage-plasticity model used for simulation of concrete material degradation (crack) and irreversible plastic deformation and the eXtended Finite Element Method (X-FEM). The Voyiadjis local anisotropic damage-plasticity model is expressed in a non-local form to prevent strain and damage localization in damaged points. Also, a new crack propagation direction criterion based on second order damage tensor of anisotropic damage, containing different material degradation components in different directions is proposed for 3D problems. Computational algorithms with an elastic predictor and plastic and damage corrector returning map are also developed. Finally, three numerical examples are solved and results of crack propagation paths predicted by the proposed algorithm are compared with available experimental data and those of other numerical simulations. It is shown that the results from the proposed method are in better agreement with experimental data compared to other reported numerical methods.

Asphalt is known to have self-healing characteristics. This study was aimed to investigate the healing behavior of asphalt binders under short- and long-term oxidative aging conditions, and to evaluate the relationship between healing potential and chemical properties. Two asphalt binder types were evaluated which included one neat (unmodified) asphalt and one styrene-butadienestyrene (SBS) modified binder. Healing characterization was based on the recently developed linear amplitude sweep based healing (LASH) test. Chemical characterization consisted of the saturates, aromatics, resins, and asphaltenes (SARA) fractionation, and gel permeation chromatography (GPC) tests. Based on the obtained healing index from the LASH test, oxidative aging generally had negative effects on healing, and that the SBS modifier slightly reduced the healing potential within the crack initiation phase. Presence of more light fractions, small molecules, and molecules that are longer with less branches would promote the molecular self-diffusion across crack surfaces, thereby enhancing the healing process. The negative effect of oxidative aging on healing may be attributed to molecular agglomeration which results in a less dispersed structure with reduced capability of the asphalt to diffuse across crack interfaces. This study provided a preliminary validation of the LASH test and the healing index.

Numerous structural components made of high strength low-alloyed steels are often exposed to low-cycle fatigue loading during their exploitation life. In order to study the fatigue behavior of one grade of structural steels produced in Yugoslavia during the eighties of the last century, a push-pull constant amplitude strain-controlled fatigue tests were conducted at room temperatures. This paper analyses behavior of two different materials, parent steel (PM) and its coarse-grained (CG) simulated heat-affected zone (SHAZ) exposed to the action of low-cycle fatigue (LCF) loading performed in the range from 1 to 104 cycles. LCF tests were conducted at strain ratio R = -1 at strain amplitude range from 0.3% to 0.8%. The materials' Coffin-Manson parameters appearing in the strain-life and cyclic stress-strain curves were calculated. Fracture surface analysis of both materials was done. The results showed that plasticity under dynamic load conditions is satisfactory for both materials which give them good resistance to crack initiation and propagation.

In this study, simulated heat-affected zone (HAZ) of Nb-free and Nb-bearing steel were obtained, and SEM, TEM, and slow strain rate tensile (SSRT) tests were performed to investigate the effect of Nb on the stress corrosion cracking (SCC) behavior of high-strength low-alloy (HLSA) steel in simulated seawater with or without hydrogen charging. The addition of Nb significantly refined the grains and uniformed the microstructure of HLSA. Nb hardly affected the SCC susceptibility of BM and HAZ without hydrogen-charging. However, after charging with 10 mA cm−2, the SCC resistance of Nb-bearing steel, especially the coarse grain HAZ (CGHAZ) improved drastically, and the process of crack initiation and propagation was inhibited owing to the hydrogen trap function of NbC precipitates.

The microarchitecture of bone tissue presents many features that could act as stress concentrators for the initiation of bone microdamage. This was first identified by John Currey in a seminal paper in 1962 in which he presented the mechanical and biological evidence for stress concentrations at the bone surface, within the bone through the action of stiffness differentials between architectural features including between lamellae, and at the level of the lacunar and canalicular walls. Those early observations set the stage to consider how microscopic damage to bone tissue might affect the properties of bone at a time when most in the scientific community dismissed microcracks in bone as artifact. Evidence collected in the nearly 60 years since those important initial observations suggest that some of these architectural features in bone tissue are more effective as crack arrestors than as crack initiators. Sites of higher mineralization in the bone matrix, particularly interstitial sites in both cortical and trabecular bone, may serve preferentially as locations for crack initiation, whereas those boundaries identified by Currey as both stress concentrators and stress arrestors are more effective at stopping cracks than at initiating them.

Surface mechanical attrition treatment (SMAT) is considered as an effective technology to influence fatigue and corrosion resistance as these properties are largely related to surface structure and property. Despite the increasing number of studies that report the effects of SMAT on separate fatigue or corrosion behaviors, the impact of SMAT on fatigue property under corrosive environment is left unexplored. In this paper, the corrosion fatigue properties of AZ31B magnesium alloy before and after SMAT process are investigated. Hydrogen evolution tests and potentiodynamic polarization tests are carried out to obtain the corrosion resistance, and it is found that SMAT promotes corrosion rate. The surface roughness of SMATed specimen also increases, which accelerates corrosion rate. In terms of fatigue properties, the fatigue life of SMATed specimen is found to be higher, in both air and 3% NaCl solution. And the larger the size of stainless steel balls used during processing SMAT specimens, the longer the fatigue life. Under 3% NaCl corrosive environment, the beneficial effects induced by SMAT process gradually emerge with increasing stress amplitude during fatigue tests. Reduced fatigue life of SMATed specimen under corrosive environment is attributed to roughened interface between deformed surface layer and bulk material. Nevertheless, the crack initiation sites for SMAT specimens are the same in both air and NaCl solution, all located at the subsurface area.

Fatigue crack initiation and propagation behavior were investigated on a tubular specimen of Al 7075 alloy subjected to multiaxial mixed mode bending-torsional loadings. Tests under pure bending and pure torsional loadings were also conducted to segregate the effect of multiaxiality. A single crack nucleated on the plane of maximum shear stress in all the tests, except for pure torsion. For pure bending, crack propagated on the plane of maximum shear stress in a mixed mode condition tracing an inverted S-shaped trajectory; whereas, crack trajectory under combined bending-torsional load consisted of mode I dominant region, transition region, and pure mode II region. Furthermore, analyses of fracture surfaces were conducted to determine the micro-mechanisms governing crack initiation and propagation behavior.

To solve the fatigue crack initiation problem, it is necessary to calculate the fatigue crack initiation size. According to the aspect of macroscopic crack development, the calculation method of crack initiation size is determined by the critical dimension of fatigue crack initiation. Simultaneously, the elastic–plastic fracture mechanics (EPFM) method is used to study short crack fatigue life by considering the cumulative damage rule of crack initiation life, and the calculation model of fatigue crack initiation size caused by symmetric cyclic torsional alternating stress is established under low strain. The crack initiation dimension of component is estimated, which lays a foundation for the study of fatigue life. Then the feasibility of this method is verified by comparing with the crack initiation size, which is determined by predicting the non-propagating crack size (NPCS).

Marble specimens were used to investigate the effects of freezing and thermal cyclic loading on crack initiation and propagation and fracture modes. To this end, four loading types of freezing-cooling (F-C), heating-cooling (H-C), freezing-cooling- heating (F-C-H) and heating-cooling-saturating-freezing (H-C-F (sat.)) were considered. Forty-four toughness tests were carried out, and changes in the fracture mechanism in the marble were analyzed at −30 °C and 160 °C. The fracture toughness parameters, load-displacement curves, crack initiation angle for mode II and the mixed mode (I-II) were measured and then predicted using the conventional criteria. Crack initiation angles for the mixed mode (I-II) and mode II fracture toughness in different loading conditions were calculated to be about 58° and 78°, respectively. The F-C-H loading condition seemed to reduce mode I fracture toughness in the tested samples. The experimental data on fracture toughness for H-C, F-C-H and H-C-F (sat.) loading conditions were well predicted by the modified maximum tangential stress (MMTS), maximum tangential stress (MTS) and G-criterion. The results indicated that very good predictions are obtained by the extended maximum tangential strain (EMTSN) for the fracture toughness of marble envelopes in the entire range of mode mixities from pure mode I to pure mode II. Load-displacement curves for different loading conditions were monitored, indicating that the peak load significantly decreased for F-C-H and H-C-F (sat.) cycles.

In snubbing service, the ram blowout preventer (BOP) is switched frequently, and the stress condition of the packer is very complicated. Cracks, rubber drop, and other fatigue failures often occur. The load characteristic was firstly analyzed in this paper based on the stress condition of packer in snubbing service. The mechanical property of rubber commonly used for packer was tested, and the performance parameters were obtained. Based on the crack initiation approach, taking the rubber matrix as the research object, the effective stress, the maximum Green–Lagrange strain and strain energy density were selected as damage parameters. The distribution contour of three damage parameters of the rubber matrix of the packer under different stress conditions and speeds of lifting or lowering pipe were obtained using finite element method (FEM). The effect of different stress conditions and speeds of lifting or lowering pipe on the fatigue behavior for the rubber matrix was analyzed. The result shows that the value of three damage parameters is relatively high on the upper area of the rubber matrix, causing the first occurrence of fatigue in the upper part, which agrees well with the oilfield application. The speed of the pipe does not change the location of fatigue point and the value of damage parameters, so that it has little effect on the fatigue behavior of the packer's rubber matrix. Finally, based on Miner linear damage rule, a calculation method for the fatigue behavior of the packer is presented.

A discrete element model (DEM) of SiC ceramics containing two parallel flaws was used to study crack coalescence under uniaxial compression with different flaw inclination angles, ligament lengths, and ligament angles. A relationship is proposed between coalescence stress state, flaw inclination angle, and horizontal component of the ligament length during the failure process. The effects of coplanar flaws on various characteristics of the specimen, viz. compressive strength, Young's modulus, crack initiation stress, and Poisson's ratio, were studied. Coalescence modes between two parallel flaws observed in the DEM model were in good agreement with nine crack coalescence categories summarized in experimental studies. Meanwhile, the corresponding stress state at the moment of coalescence can be classified into three types – pre-peak, mid-peak, and post-peak. The results also showed that the horizontal component d of the ligament length of parallel flaws significantly influences the coalescence stress state. When parallel flaws overlap in the loading direction (d

cited By 0; Non-proportional axial-torsional loading fatigue tests on nodular cast iron revealed a non-symmetrical behavior of the fatigue life relatively to the phase shift angle δ = 90° between axial and torsion loads. This behavior was in agreement with fatigue crack orientation investigated using optical and electron microscopy. Elasto-plastic finite element model (FEM) were used to assess the crack orientation behavior and it could explain this non-symmetrical behavior by the non-symmetrical values of both σ I and σ I / σ II ratio relatively to δ = 90°. FEM revealed that for the δ < 90° loads, principal stress was applied essentially at critical planes of low angle ϕ n , thus occurrence of tensile cracks mode was higher for δ < 90° than for δ > 90° load, which reduces the fatigue life. σ I / σ II ratio strongly influenced the dominant crack mode. When the applied loads were torsion σ I /σ II = 1, cracks were observed to occur, equally, in modes I and II. Presence of the non-propagating failure (mode II) significantly increased the fatigue life. Conversely, for high phase shift, where σ I / σ II ≫ 1, crack mode I dominated and crack driving force remained high during the whole fatigue cycle, inducing a lower fatigue limit. © 2019