Mode Crack Growth Research Articles

Effect of CrN coating on the hot salt corrosion fatigue behavior of titanium alloy

The dwell fatigue behavior of CrN coated titanium alloys were carried out in air and hot salt corrosive environment at 500 ℃. The results indicate that the decrease of the elastic modulus difference between the coating and the substrate alleviates the deformation mismatch issue at high temperature. Stress relaxation at the crack tip caused by Cr transition layer oxidation can improve the fatigue properties of the coated samples. Furthermore, the CrN coating enhances the corrosion fatigue properties, which is related to the barrier effect of the coating on the corrosive medium. The crack growth mode in the substrate transition from stress corrosion crack to fatigue damage crack.

Study on fatigue crack growth behavior of the TA15 titanium alloy repaired by laser deposition

Laser deposition repair was employed to repair the body and surface of the forged TA15 titanium alloy. Subsequently, the fatigue crack growth behavior of the repaired components after annealing was investigated. The results showed that the fatigue crack growth rate (FCGR) of the body-repaired specimen decreased from the initial cracks in the heat-affected zone (HAZ), interface, and repaired zone (RZ). The increase in the repair layer thickness drastically reduced the FCGR in the surface-repaired specimen. The main crack growth mode was crossing the primary α and transformed β in the substrate zone (SZ), primarily crossing the lamellae α and along lamellae α in the HAZ, and crossing the lamellae α in the RZ. The RZ developed a tighter bond with the substrate interface, making crack growth more difficult. Crossing the lamellae α in the basket-weave structure required higher amount of energy and exhibited significant increase in tortuosity. Hence, the FCGR of the initial crack within the RZ exhibited the lowest rate. An increase in the thickness of the repair layer increased the proportion of the basket-weave, improving the resistance against crack growth, and increased the tortuosity associated with the crack path, reducing the FCGR. It was confirmed that the proposed body and surface repair methodology improved the FCGR of investigated specimens.

On the influence of secondary branches on crack propagation in rolling contact fatigue

Gear and bearing failures are most often caused by rolling contact fatigue (RCF). Understanding the growth of a main surface-initiated crack into the depth of the piece, as well as the growth of subsurface-initiated branches towards the surface is necessary, in order to improve fatigue life prediction and reduce fatigue damage repair or component replacements before catastrophic failure. The purpose of this work is to analyze, based on 3D finite element simulations, the initiation and mode I growth of secondary branches in a case hardened gear, their influence on the main crack growth in mode II, and the competition between mode I and mode II crack growth. The residual stress field issued by the case hardening is taken into account and its role is analyzed. The reasons why branch cracks develop only from the upper main crack face and why only those initiated when the main crack is still shallow can reach the surface, inducing spalling are explained.

Crack growth behaviour of P91 steel under trapezoidal loading at high temperature

The increased incentives to utilize renewable energy sources so as to supplement the energy requirements of the present civilization pose a challenge to the power plants running on fossil fuels, as they are required to operate in a ‘flexible’ manner instead of operating at a constant load as intended during their design and inception. This flexible mode of operation at elevated temperature causes a combined creep and fatigue type of loads to be applied on the components. This allows for a comprehensive and effective understanding of synergistic creep-fatigue crack growth behaviour to assess long term failure of components and accessories in plant. The use of P91 steel has been widely prevalent in power plant components owing to its high creep resistance, superior mechanical properties and excellent thermal conductivity, cost-effectiveness, and minimal coefficient of thermal expansion when compared to austenitic steels and Ni-based alloys. The American Society for Testing and Materials (ASTM) has proposed a standard for experimentally evaluating creep-fatigue crack growth, E 2760-19, wherein compact specimens, C(T) are tested under load-controlled conditions. The creep-fatigue crack growth regime and mode of fracture undergone by P91 steel at 600 °C were studied using C(T) specimens at two different force ratio 0.1 and 0.5 and with two different hold time 10 and 600 sec. The Creep fatigue crack growth behaviour has been characterized using stress intensity factor range parameter ∆K and (Ct)avg parameter to study the effect of hold time and force ratio.

Microstructural effects on short fatigue crack propagation in cast Al-Si alloys

The role of microstructure on the short fatigue crack behaviour of cast Al-Si alloy under high cycle loading conditions is investigated by tracking the progress of cracks through fine, medium and coarse microstructured samples. Generally, in the early stage of crack growth, short cracks mainly grow transgranularly, with cracks growth close to low index crystallographic planes (mainly {100}) in some grains and frequently inhibited at grain boundaries. In the late stage, the crack growth mode gradually shifts from crystallographic stage-I to non-crystallographic stage-II. The short cracks grow faster and fluctuate significantly according to local microstructure. Our results show that finer microstructures offer greater crack nucleation and propagation resistance.

Crack nucleation and growth induced by thermal phase transition for polymer bonded energetic composite materials

Understanding thermophysical and thermomechanical damage behaviors of polymer-bonded energetic composite materials (PBECMs) is fundamental to the design of advanced safe munition structures. Herein, the effects of thermal expansion and phase transition in energetic crystal, thermal softening of binders, binder-crystal interface debonding, and heating rate on cracking damage in PBECMs are quantitated from the viewpoint of mechanics. An original thermomechanical crystal model integrating with anisotropic thermoelasticity, thermal expansion and phase transition is developed for energetic crystal. Damage and debonding at the interface and intracrystalline fracture are modelled by a bilinear traction-separation cohesive law. The results show that the occurrence of phase transition induces a prominent increase of volume deformation and contributes to mutual compression between crystals and mismatch shear deformation at the interface, thereby multiplying crack density. Inhomogeneous thermal stress field and crack distributions are related to anisotropic thermal expansion and phase transition behaviors of crystals. The predicted thermally cracking behaviors in PBECMs, including critical temperature to crack formation, crack nucleation position and growth mode, are consistent with experimental results.

Microstructure, mechanical properties and fatigue crack growth behavior of an Al-Zn-Mg-Cu-Si-Zr-Er alloy fabricated by laser powder bed fusion

An Al-Zn-Mg-Cu alloy modified by Si-Zr-Er was prepared by laser powder bed fusion (LPBF), the tensile strength of as-printed alloy reached 445 MPa. Precipitates observed in the alloy included Al2CuMg, MgZn2, Mg2Si and Al3(Zr,Er). The accumulated dislocations near the crack tip could combined with the second phases to hinder the propagation of fatigue crack. Transgranular and intergranular modes of fatigue crack growth were both observed in the alloy, and deflection was prone to occur when the crack encountered grains with high misorientation. The fatigue life (N = 1.02 × 106 cycles) reached highest under the condition of r = 0.5 and Pmax = 0.3kN.

Enhanced Fatigue Resistance of Nanocrystalline Ni50.8Ti49.2 Wires by Mechanical Training

In this paper, the fatigue resistance of superelastic NiTi shape memory alloy (SMA) wires was improved by combining mechanical training and nanocrystallization. Fatigue tests were performed after mechanical training with a peak stress of 600 MPa for 60 cycles of nanocrystalline (NC) NiTi wires, and the associated microscopic mechanism was investigated by using transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD). The results showed that stress-controlled training effectively improved the functional stability (the accumulated residual strain decreased by 83.8% in the first 5000 cycles) of NC NiTi SMA wires, as well as increased the average structural fatigue life by 187.4% (from 4538 cycles to 13,040 cycles). TEM observations and TKD results revealed that training-induced dislocations resulted in lattice rotation and preferential grain orientation. The finite element method (FEM) simulation results indicated that the training-induced preferential grain orientation tended to decrease the local stress concentration and strain energy density. Combined with fractography analysis, the uniform deformation caused by mechanical training changed the crack growth mode from multi-regional propagation to single-regional propagation, improving the structural fatigue life.

Fatigue assessment and crack propagation of floorbeam cutout in orthotropic bridge decks

Based on the fatigue cracking of floorbeam cutout in a bridge, the field testing under random traffic flows was firstly conducted to investigate the fatigue crack initiation, and then a multi-scale numerical model, based on extended finite element method (XFEM), was established to reveal the propagation mechanism of crack growth. It was found that floorbeam cutout exhibits a complex three-dimensional deformation state under wheel loads, leading to an I-II-III mixed crack growth mode. The crack propagation direction is normal to the free edge of floorbeam cutout, which is the crack propagation direction at initiation. Increasing floorbeam thickness can facilitate preventing the crack propagation of floorbeam cutout.

Experimental and numerical analysis of dynamic splitting mechanical properties and crack propagation for frozen sandstone

To reveal the dynamic tensile properties and crack growth law of frozen sandstone under impact loading, dynamic splitting tests (−30–20 °C) with different strain rates were carried out using a split Hopkinson pressure bar (SHPB), and the crack growth process of the specimens was captured using a high-speed camera. The study focused on the analysis of dynamic tensile mechanical properties, energy dissipation, crack growth, and failure modes of frozen sandstone. Numerical simulation analyses were also performed using LS-DYNA. The results indicated that: (1) The dynamic tensile strength has an obvious strain rate enhancement effect, and the strain rate sensitivity coefficient increased with a decrease in temperature. The Logistic function can characterize the relationship between the dynamic strain rate enhancement factor and the strain rate of frozen sandstone. (2) In the dynamic Brazilian disk (BD) test, the energy absorption rate showed a transition between the strain rates of 40 s−1 and 50 s−1, and there were two main failure modes of the specimen. The Y-direction secondary crack in mode II was caused by tensile stress in the Z-direction. (3) In the dynamic BD test, the central main crack (CMC) did not occur at the central position first, and its Z-direction propagation rate was linear with the strain rate. In the dynamic flattened Brazilian disk (FBD) test, the condition of CMC initiation was that the platform loading angle (2α) was greater than 10°. The dynamic tensile strength increased with an increase in 2α, and the correction coefficient was introduced in the calculation. This study can serve as a theoretical reference and basis for the study of the dynamic tensile properties of rock-like materials.

Fatigue crack growth rate and fracture toughness evaluation of 15-5 precipitation hardening stainless steel fabricated by laser powder bed fusion process

The present article reports the fatigue crack growth (FCG) rate and fracture toughness of 15-5 Precipitation Hardening (PH) Stainless Steel (SS) specimens fabricated by Laser powder bed fusion (L-PBF) additive manufacturing (AM) technique. The effect of notch orientation (parallel and orthogonal in reference to build direction) on FCG and fracture toughness is investigated. The FCG and fracture toughness data of L-PBF specimens in the solution annealed (SA) and SA + aged condition were compared to that of wrought 15-5 PH SS. It was found that dependence of FCG on the build orientation was insignificant up to the stress intensity factor range (ΔK) of 30 MPa√m, and beyond that, a higher FCG rate is recorded for L-PBF specimens. The SA specimens were found to have a slightly lower FCG rate at Paris region as compared to SA + aged specimens. The fractography showed predominantly transgranular mode of crack growth, with presence of fine pores and signature of plastic deformation at higher stress intensity. The crack branching and zig-zag path was observed for the L-PBF specimens with notch orientation orthogonal to build direction (horizontal notch). The fracture toughness (K1C) of horizontally notched L-PBF fabricated 15-5 PH SS specimens in aged condition showed higher values (51–62 MPa√m) as compared to vertical (parallel) notched specimens (40–46 MPa√m). The observed fracture toughness values of L-PBF 15-5 PH SS specimens appear to be significantly lower than the reported fracture toughness values of wrought 15-5 PH SS material.

Numerical modeling of rolling contact fatigue cracks in the railhead

Rolling contact fatigue (RCF) crack is one of dominating damages on the rail surface due to repeated wheel and rail contact. RCF defects, including squats and head checks, can potentially cause rail fracture failure if not treated in time. It is critical to accurately predict the pertinent damage for different operational conditions. A two-stage simulation strategy was proposed to investigate the mechanisms of RCF-induced cracks on the rail, consisting of a dynamic FEM for wheel and rail interaction and a coupled static FEM/BEM for crack growth. It was found that there are two critical wheel positions for crack propagation during wheel approaching to and departing from the pre-existed crack. The different cracking mechanisms at these two critical wheel positions are correlated with the local physical phenomena, i.e., the traction at the wheel/rail interface, the relative distance between the contact patch and crack, and the sliding at crack faces. The twisting of the cracks from the transverse plane to the longitudinal plane at the railhead significantly reduces crack growth in mode I, enhances crack propagation in modes II and III, and shifts the dominant crack growth from the crack front center to the crack corners. The increased incline of the crack in the rolling direction promotes the crack to grow downward in opening mode I.

Structural Health Monitoring of Fatigue Cracks for Steel Bridges with Wireless Large-Area Strain Sensors.

This paper presents a field implementation of the structural health monitoring (SHM) of fatigue cracks for steel bridge structures. Steel bridges experience fatigue cracks under repetitive traffic loading, which pose great threats to their structural integrity and can lead to catastrophic failures. Currently, accurate and reliable fatigue crack monitoring for the safety assessment of bridges is still a difficult task. On the other hand, wireless smart sensors have achieved great success in global SHM by enabling long-term modal identifications of civil structures. However, long-term field monitoring of localized damage such as fatigue cracks has been limited due to the lack of effective sensors and the associated algorithms specifically designed for fatigue crack monitoring. To fill this gap, this paper proposes a wireless large-area strain sensor (WLASS) to measure large-area strain fatigue cracks and develops an effective algorithm to process the measured large-area strain data into actionable information. The proposed WLASS consists of a soft elastomeric capacitor (SEC) used to measure large-area structural surface strain, a capacitive sensor board to convert the signal from SEC to a measurable change in voltage, and a commercial wireless smart sensor platform for triggered-based wireless data acquisition, remote data retrieval, and cloud storage. Meanwhile, the developed algorithm for fatigue crack monitoring processes the data obtained from the WLASS under traffic loading through three automated steps, including (1) traffic event detection, (2) time-frequency analysis using a generalized Morse wavelet (GM-CWT) and peak identification, and (3) a modified crack growth index (CGI) that tracks potential fatigue crack growth. The developed WLASS and the algorithm present a complete system for long-term fatigue crack monitoring in the field. The effectiveness of the proposed time-frequency analysis algorithm based on GM-CWT to reliably extract the impulsive traffic events is validated using a numerical investigation. Subsequently, the developed WLASS and algorithm are validated through a field deployment on a steel highway bridge in Kansas City, KS, USA.

Mode II fracture of wood: Comparison between End-Notched Flexure and Compact Shear testing

End-Notched Flexure (ENF) and Compact Shear (CS) tests were performed on Australian Radiata Pine to quantify Mode II fracture properties in wood. Capabilities and limitations of each fracture test are compared with respect to ease of testing, underlying reduction schemes to determine Mode II fracture toughness (or strain-energy release rate), stable crack growth and mixed mode behaviour during crack growth. In particular, different reduction schemes are presented with and without the need for cumbersome crack length measurements. In contrast to CS tests, it is found that ENF tests yield stable and reliable Mode II fracture with negligible mixed-mode behaviour. Considering resistance curves, crack length measurements are only needed to determine initial fracture energy values associated with crack length values up to 20 mm while compliance-based beam theory can be used to calculate steady-state fracture energies by means of an equivalent crack length that only requires global load–displacement test data.

Peridynamic simulationons of RCF crack growth in laser quenched rail material

Laser quenching (LQ) is a promising technology which could be applied on rails to improve their anti-wear properties. However, severe rolling contact fatigue (RCF) damages tend to occur on the LQ treated rail materials. The peridynamics (PD) simulation is a novel method, which is good at addressing issues on discontinuous materials. Thus, the present study aims at exploring the growth mechanism of RCF cracks in the LQ treated rail material using the PD simulation. Meanwhile, rolling-sliding contact experiments were conducted to verify the simulation results. The results indicated that the RCF crack initiated closely to the front edge of the quenching region. During one wheel/rail contact cycle, the crack growth mode was dominated by the sliding mode when the wheel was close to the crack, and by the opening mode when the wheel was above the crack. The crack growth process in the LQ treated rail could be divided into four stages: crack initiation in the quenching region; crack propagation in depth; branch cracks generation at the bottom of quenching region and crack growth along the boundary between the quenching and substrate regions; crack growth into the substrate or back to the surface. The crack growth rate was increased significantly with the increase in crack length in the quenching region, then decreased gradually in the boundary and substrate regions.

Sustainable shotcrete production with waste glass aggregates

This study employs statistical and experimental procedures to assess the applicability of crushed waste glass to replace natural sand in shotcrete production for use in tunneling, mining and excavation industries as a support system. Mechanical strength under different uniaxial and biaxial load combinations, fracture properties, and ultra-high-speed photography of the crack growth mode in the newly developed shotcrete mixes were studied and compared against the control mix at 0% waste glass inclusion. Results suggest that shotcrete mixes containing synthetic glass aggregates exhibit similar or higher strength properties (compared with conventional shotcrete with natural aggregates) at early and late ages in both fresh and hardened shotcrete samples. Under biaxial stress conditions, in particular, the new glass shotcrete designs demonstrate a higher load-bearing capacity of up to 35% increase. Compared to conventional shotcrete, replacing sand with recycled glass aggregates further exhibited the need for less water and binder consumption owing to the glass water-reducing effect. This in return could leave more water available for improved workability in the mixture hence producing a more cost-effective and eco-friendly shotcrete product. From the results, no impairment of performance was recorded by substituting sand with crushed waste glass even at a high substitution percentage of up to 100%; suggesting high improvement potential of crushed waste glass re-use in the shotcrete industry.

Low cycle fatigue behavior of Inconel 706 at 650 °C

Low cycle fatigue (LCF) behavior of 2- and 3-step-aged Inconel 706 (IN706) specimens that were prepared from the center and the periphery of forged disc was examined at 650 °C and an R ratio of −1. The resistance to LCF of IN706 alloy was greater in approximately half order at 650 °C than that of IN718 alloy for the same aging condition. Elevated temperature Coffin–Manson relationship between 2- and 3-step-aged IN706 specimens was similar with each other, despite a considerable difference in microstructure. The fractographic analysis suggested that cluster of carbides provided the sites for crack initiation for both 2- and 3-step-aged specimens. The mode of fatigue crack growth for each specimen was intergranular and transgranular, but with different magnitude for each mode. Depending on the location of specimen preparation form the forged block of IN706 alloy, no notable difference in LCF resistance was found. The role of grain boundaries, clusters of carbides and bands of acicular η platelets on the initiation and propagation mechanism of IN706 alloy was discussed based on detailed fractographic and micrographic analyses.

Mechanical Behaviour and Failure Mode of High Interstitially Alloyed Austenite under Combined Compression and Cyclic Torsion

Multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis.

Effect of SRB and Applied Potential on Stress Corrosion Behavior of X80 Steel in High-pH Soil Simulated Solution.

The effect of SRB and applied potential on the stress corrosion sensitivity of X80 pipeline steel was analyzed in high-pH soil simulated solution under different conditions using a slow strain rate tensile test, electrochemical test, and electronic microanalysis. The experimental results showed that X80 pipeline steel has a certain degree of SCC sensitivity in high-pH simulated solution, and the crack growth mode was trans-granular stress corrosion cracking. In a sterile environment, the SCC mechanism of X80 steel was a mixture mechanism of anode dissolution and hydrogen embrittlement at −850 mV potential, while X80 steel had the lowest SCC sensitivity due to the weak effect of AD and HE; after Sulfate Reducing Bacteria (SRB) were inoculated, the SCC mechanism of X80 steel was an AD–membrane rupture mechanism at −850 mV potential. The synergistic effect of Cl− and SRB formed an oxygen concentration cell and an acidification microenvironment in the pitting corrosion pit, and this promoted the formation of pitting corrosion which induced crack nucleation, thus significantly improving the SCC sensitivity of X80 steel. The strong cathodic polarization promoted the local corrosion caused by SRB metabolism in the presence of bacteria, whereby the SCC sensitivity in the presence of bacteria was higher than that in sterile conditions under strong cathodic potential.

Microscopic and macroscopic analyses of the interaction mechanism between defect growth and dislocation emission in single‐crystal aluminum

AbstractThe study on the interaction between defects and dislocation emission is of significance to predict the behaviors of fracture and plastic deformation in crystalline metals; however, the multiscale interaction mechanism has not been well understood. In this work, based on molecular dynamics simulation and distributed dislocation technique, the interaction mechanism between dislocation emission and a crack or a void is studied at the atomic and continuum scales. The results show that dislocation emission can change the mode of crack growth, i.e., from brittle to ductile, and the emitted dislocations cause crack‐tip blunt and decrease stress concentration near the defects. The increment of void growth is completely contributed by dislocation emission, and it can be predicted by the characteristics of dislocation emission. This work reveals the influence mechanism of dislocation emission on failure behaviors and deepens the understanding of the interaction between dislocation emission and defects.