Microcrack Initiation Research Articles

Submerged deflecting abrasive waterjet peening for improving the surface integrity and solid particle erosion resistance of Ti-6Al-4V alloy

Waterjet peening is a promising new surface treatment technology, which is increasingly used to improve a metal's surface integrity and mechanical properties. In this study, the surface integrity and solid particle erosion (SPE) resistance of Ti-6Al-4V alloy treated with submerged deflecting abrasive waterjet peening (SDA-WJP) at different standoff distances (D = 3, 6, and 9 mm) were investigated. First, the surface integrity of specimens before and after SDA-WJP treatment was compared, including microstructure, surface roughness, micromorphology, microhardness, and compressive residual stress (CRS). The test results showed that the SDA-WJP-treated specimens formed refined grain deformation layers of 8–24 μm and minimum roughness of Ra = 0.28 μm and Sa = 0.55 μm. The thicknesses of the refined grain hardening layer and CRS field were 140–180 and 120–200 μm, respectively. Finally, the SPE behavior and mechanism of the samples before and after SDA-WJP treatment were studied in the SPE test. The highest increase in SPE resistance was 51 % under different SDA-WJP processes. The microscopic morphology of the eroded surface was observed by scanning electron microscopy. The shallow embedding depth of the particles on the eroded surface after SDA-WJP treatment led to difficult platelet formation for removal. The introduced refined grain hardening layer and CRS field were critical in preventing micro-crack initiation and propagation. This work optimizes the SDA-WJP process parameters for improving the surface integrity and SPE resistance of Ti-6Al-4V alloy, thus providing a reference for the application of SDA-WJP for the improvement of the SPE resistance of aero-engine blades.

Effect of stress amplitude on damage and fracture characteristics of austenitic heat‐resistant steel Sanicro 25 at 973 K

AbstractHigh‐temperature damage characteristics related to the deformation behavior of Sanicro 25 alloy were investigated under stress‐controlled cyclic loading. The results show that the strain curves exhibited creep‐like strain characteristics. The increase in mean stress increased the strain rate and weakened the life. Comparatively, the increase in stress amplitude only accelerated the tertiary‐regime strain rate, leading to a decrease in life and fracture strain. Microcrack initiation at grain boundaries and primary Z phases at different stress amplitudes can be evidenced by the observation of micro‐voids and high geometrically necessary dislocation densities around them. Subsequently, the increase in stress amplitude can significantly affect crack propagation. Dislocation accumulation at the transgranular crack tip caused by cyclic stress can assist the propagation of the transgranular crack at higher stress amplitude ( ). Conversely, transgranular cracks are easily suppressed and stop propagating at grain boundaries under constant loading ( ). The main reason was that the stress concentration of the transgranular crack tip at grain boundaries was dissipated through discontinuous dynamic recrystallization.

Effect of test temperature on the hydrogen embrittlement susceptibility of a duplex stainless steel

This study investigates the ductility loss of a hydrogen-charged 2205 duplex stainless steel at various test temperatures. The initiation and propagation of hydrogen-induced microcracks during tensile straining are statistically characterized to get a glimpse of the interactions between hydrogen and the dual-phase microstructure. The hydrogen contents and distribution are also analyzed. Thermodynamic models are utilized to determine the stacking fault energy of the austenite phase. The results show that the steel's hydrogen embrittlement (HE) susceptibility and the thickness of the hydrogen-induced brittle layer decreased with increasing temperature. Hydrogen desorption during straining was significantly accelerated when the test temperature was 42 °C. By comparing the observed deformation microstructure with the calculated stacking fault energies, we conclude that the stacking fault energy of the austenite strongly depends on hydrogen concentration rather than environmental temperature. The high HE susceptibility of the specimens tested at 22 and 32 °C resulted from the combined effects of hydrogen-enhanced planar slip and the early formation of hydrogen-induced microcracks.

Experimental study on the effect of anchors on the creep mechanical properties and acoustic emission characteristics of fractured sandstone under uniaxial compression

This study conducts laboratory tests to investigate the creep and acoustic emission (AE) of unanchored and anchored fractured sandstone samples under uniaxial compression by taking the Triassic fine sandstone collected from the Xiaolangdi Reservoir area as the research object and ensuring the same testing conditions using an RLJW-2000 rock rheometer and a PCI-Ⅱ AE instrument. Moreover, this study identified the effects of anchors on the creep mechanics and AE characteristics of fractured sandstone samples by analyzing the strain, long-term strength, AE event count, AE rate, AE energy rate, and rising angle – average frequency (RA-AF) characteristics of each sample before and after anchoring. According to the study results: (1) Under the same stress level, the anchored sample's instantaneous strain, creep strain, and total strain are smaller than those of the unanchored sample. Anchors exerted the largest inhibitory effect on creep strain, followed by the total strain, while anchors inhibited instantaneous strain to the smallest extent. (2) Compared to the unanchored sample, the anchored sample experienced a significant increase (as high as 50%) in long-term strength. Anchor reinforcement significantly improves the long-term strength of samples. (3) Under the same stress level, the AE event count and cumulative AE event count of the anchored sample are smaller than those of the unanchored sample. Anchors significantly inhibited microcracks initiation, propagation, overlapping, and penetration in samples. (4) The maximum AE rate of the anchored sample is only 69.30% of that of the unanchored sample, whereas the peak AE energy rate of the anchored sample is only 23.21% of that of the unanchored sample. Anchors reduce the AE event count generated and the AE energy released by samples per unit of time, exerting a significant crack-arrest effect on them. (5) The presence of anchors changes the type and number of microcracks in the samples' creep process. Anchors greatly inhibit the generation of high-RA and low-AF AE signals and generate low-RA and high-AF ones. As a result, more tensile microcracks occur in samples under stress levels close to the failure stress level (especially the last stress level), which inhibit the formation of shear microcracks in samples and causes samples to be more prone to macroscopic tensile-shear failure. (6) The microcrack characteristics of samples characterized by RA-AF values are consistent with the macrocrack characteristics of samples. Therefore, RA-AF values can satisfactorily characterize the microcracks' variation characteristics in the sandstone samples' creep process before and after anchoring.

Fatigue damage evolution and failure of pre‐corroded aluminum alloy 7075‐T651 under air and corrosion environments

AbstractThis paper presents characterization for fatigue failure analysis of precorroded AA7075‐T651 under air and exfoliation corrosion (EXCO) environments, with respect to damage accumulation, macrocrack and microcrack behavior via 3D digital image correlation (DIC), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). It is shown that that corrosion pits induce fatigue cracking with different nucleation times, and locations. It is furthermore shown that while the corrosive environment accelerates crack formation, it significantly hinders fatigue crack propagation at low stress amplitudes due to crack closure and tip passivation. Five dominant fatigue microcrack initiation types (pit bottom, corrosion tunnel, corrosion micropit, corrosion jut‐in, and chapped feature) and relevant mechanisms were identified, related to corrosion pit macro–micro morphology, material microdefect, local material grain structure, and loading condition.

Impacts of nano-clay particles and heat-treating on out-of-phase thermo-mechanical fatigue characteristics in piston aluminum-silicon alloys

Abstract. In this article, the effect of nano-clay particles and heat-treating on thermo-mechanical fatigue (TMF) behaviors and failures of piston aluminum-silicon (AlSi) alloys was investigated. For this purpose, thermo-mechanical fatigue tests were conducted under out-of-phase (OP) loading conditions. Two loading conditions were checked based on different maximum temperatures (250, 300, and 350 °C) and various thermo-mechanical loading factors (100, 125, and 150%). The minimum temperature was constant in all tests at 50 °C under a heating/cooling rate of 10 °C/s and a dwell time of 5 s. Results showed that the nano-composites had a longer fatigue lifetime, at least 2 times higher, compared to the Al alloy, when the maximum temperature was 250 °C and the thermo-mechanical loading factor was 100%. However, no effective change was seen for the stress value and the plastic strain. At higher maximum temperatures, the change in the material behavior was lower. The fracture analysis by scanning electron microscopy (SEM) demonstrated that both materials had a brittle behavior due to cleavage and quasi-cleavage marks. The damage mechanism was also due to the Si-rich phase and intermetallics, respectively for the crack propagation and the micro-crack initiation.

Numerical study of mechanical properties and microcrack evolution of double-layer composite rock specimens with fissures under uniaxial compression

Layered and heterogenous fissured rock masses are ubiquitous in nature. Fissures exert significant influence as release surfaces and nucleation sites for failure in layered rock masses. However, key mechanisms controlling microcrack initiation and extension in layered and heterogenous rock masses remain unclear. We complete a suite of numerical (granular mechanics) and complementary physical compression experiments on layered rocks containing fissures in different modalities where the overlying layer is both stronger and stiffer. We complete this to understand the impact of local specimen and system stiffness on ultimate strength and modes of failure (stability). We systematically examine the influence of fissure angles, lengths and positions on the mechanical response, including observations of microcrack evolution and tendency for instability (i.e. rock bursting), constrained by acoustic emission (AE). Peak strength and overall elastic moduli vary in a predictable and systematic manner. The strengths and stiffnesses of the double-layer composite specimens fall between those of the homogeneous weak and strong single layer specimens. The AE counts, micro crack growth and failure characteristics of the double-layer composite specimens are closely related to the fissure angles, lengths and positions. Whether the weak rock or the strong rock contains fissures, the rock burst tendency of the double-layer composite specimens decreases in varying degrees–defining the potential of pre-blasting/fracturing to soften the mass. Fissure angles and lengths do not have to be too large. Additionally, failure in the double-layer composite specimens results not only from crack propagation in the weak layer, but also failure of the entire specimen when the length of the fissure in the strong layer is sufficiently large. These observations provide a reference for stability evaluation and disaster mitigation of layered rock masses in engineering.

The development of modern diagnostic methods and means of controlling the initiation of microcracks in metal structures that are in a stress-deformed state

The possibility of detecting the depth of a micro defect in metals has been proved at the moment of its formation under the action of a load based on the results of simultaneous monitoring of temperature changes on the opposite surfaces of the object of control. Taking into account the fact that in the case of the formation of a defect at a depth other than ½ the sample thickness along its central axis, there will be a time delay in the observation of the maximum temperature on the opposite surfaces of the object of study. The research results are the basis for improving the existing method and system for controlling the moment of microcrack initiation.

Study on Acoustic Emission Characteristics of Fatigue Damage of A7N01 Aluminum Alloy for High-Speed Trains.

Online monitoring of the fatigue damage process of A7N01 aluminum alloy base metal and weld seam was conducted based on acoustic emission (AE) and digital microscopic imaging technology. The AE signals were recorded during the fatigue tests and analyzed using the AE characteristic parameter method. Fatigue fracture was observed using scanning electron microscopy (SEM) to analyze the source mechanism of AE. The AE results show that the AE count and rise time can effectively predict the initiation of fatigue microcracks in A7N01 aluminum alloy. The digital image monitoring results of a notch tip verified the prediction of fatigue microcracks using the AE characteristic parameters. In addition, the AE characteristics of the A7N01 aluminum alloy under different fatigue parameters were studied, and the relationships between the AE characteristic values of the base metal and weld seam and the crack propagation rate were calculated using the seven-point recurrence polynomial method. These provide a basis for predicting the remaining fatigue damage in the A7N01 aluminum alloy. The present work indicates that AE technology can be used to monitor the fatigue damage evolution of welded aluminum alloy structures.

Effect of processing route on microstructure and tensile properties of hot isostatic pressed 30CrMnSiNi2A steel: Comparison of prealloy and master alloy routes

The prior particle boundaries (PPBs) are non-negligible powder metallurgy defect, which seriously weaken the mechanical properties of hot isostatic pressed (HIPed) steels. In present study, the master alloy route is proposed to improve the PPBs in HIPed 30CrMnSiNi2A steel, and the Fe-4.40Cr-4.80Mn-4.60Si-6.80Ni-1.00Mo-0.40V-1.12C master alloy system with low solidus temperature and wide solid-liquid temperature interval is designed. The microstructure, tensile properties and fracture behavior of HIPed compacts prepared by prealloy and master alloy routes are systematically compared. The results show that the structure in prealloyed sample is granular bainite (GB); and substantial Si oxides distributed along particle boundaries constitute serious PPBs. The yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE) of the sample are 723 ± 10 MPa, 1238 ± 16 MPa and 12.3 ± 0.4%, respectively. In master alloy route, the formation of local liquid disrupts the distribution of Si oxides along particle boundaries and promotes the coarsening of oxides, which results in dispersed distribution of oxides and significant reduction in oxide density. In addition, since the structural uniformity depends on element diffusion, a relatively uniform structure is obtained through the optimization of powder size. The structure of optimized sample mainly consists of GB and martensite; and good tensile properties with YS of 798 ± 7 MPa, UTS of 1310 ± 8 MPa and TE of 15.3 ± 0.6% are obtained. The higher tensile properties are mainly due to the improvement of PPBs, which avoids the initiation of microcracks along PPBs and further avoids premature failure of material.

Fracture behavior of a high pressure die casting AlSi10MnMg alloy with varied porosity levels

The fracture behavior of a high pressure die casting AlSi10MnMg alloy with varied porosity levels is investigated by synchrotron radiation X-ray computed tomography. Attention is focused on origin of crack and the evolution of porosity in the process of plastic deformation. Different from in-situ 3D tensile test, load-off specimen after plastic deformation is applied for this experiment and it is closer to the real scene. Results show that multiple defects are proposed in high pressure die casting AlSi10MnMg alloy including shrinkage, gas pore, externally solidified primary iron-rich phase (ESC-IMCI) and heterogeneous layers. In a lower-porosity sample, the surface defects like skin layer are a main crack source and a radial river pattern exhibits in crack surface. No new pores generated in the process of plastic deformation. In a higher-porosity sample, no obvious crack source and radial crack propagation path can be identified while the large crack surface fluctuation and tortuous morphology are obtained. The collapse of pre-existing small-sized pores exert a vital role in connecting large-scale pores. The micro-crack initiation and propagation originate from the expansion, consolidation and collapse of pores, especially the irregular pores.

Microcrack initiation and propagation in 3 C-SiC ceramic based on molecular dynamics nano-drilling

The evolutionary mechanism of microcracking has been the focus of research in materials processing. The experimental calculation and analysis of microcracking are inevitably difficult due to the limitations of the controllability of its emergence and expansion and the existing observation techniques. In this paper, an experimental model of nano-drilling of 3 C-SiC ceramic parts is constructed using molecular dynamics simulations to achieve real-time observation of the drilling process at the atomic scale, and the microcrack evolution behavior at different depths during the drilling process is discussed, and the microcrack initiation and propagation laws of 3 C-SiC ceramic drilling processing are proposed. The results show that when drilling reaches a certain depth, the compressive stress generated by drilling leads to a prominent region in the plastic deformation zone below the drill bit, where the atomic spacing becomes larger and the workpiece material splits, forming a microcrack eruption zone. The expansion direction of the microcrack is related to the dislocation slip direction, and it expands continuously with the continuous emission of dislocations at the crack tip. The influence of the special evolutionary behavior of the dislocation ring on the microcrack expansion during drilling is focused on, and it is found that the trend of microcrack expansion changed due to the creation of the dislocation ring, and the limit of microcrack expansion is significantly correlated with the dislocation ring evolutionary behavior. With the drilling simulation, the new microcrack expansion showed a periodic occurrence pattern with the special evolutionary behavior of dislocation rings.

Acoustic Emission Characteristics of the Water Weakening Effect on Cretaceous Weakly Cemented Sandstone

Rock mass stability is often affected by water–rock interaction in underground engineering construction. Cretaceous sandstones often have weak cementation, low strength and strong water-holding capacity, and their rock mass strength is easily weakened by these activities. In this paper, the uniaxial compressive strength (UCS) and tensile strength (TS) of weakly cemented Cretaceous sandstones from different sedimentary facies under natural and saturated conditions were tested, and the loading process was monitored by the acoustic emission (AE) technique. The results show that the existence of water obviously weakened the mechanical properties of weakly cemented sandstone. The UCS and TS of saturated braided river facies sandstone decreased to 41.24% and 35.95% of their natural states, respectively, while those of desert facies sandstone decreased to 32.90% and 26.98% of their natural states, respectively. The AE characteristics of sandstone from different sedimentary facies were similar during loading due to weakening by water, including a decrease in cumulative AE energy, b-value fluctuation and reduction in the peak frequency distribution range. Fracture in the Brazilian splitting test was mainly due to the rapid initiation and coalescence of microcracks near the peak point. However, in the uniaxial compression test, the macro fractures were caused by many microcracks that occurred continuously during loading and finally connected. The high quartz and low feldspar contents strengthened the mechanical properties of braided fluvial facies sandstone compared to those of desert facies sandstone and lessened the effect of water weakening.

Research on the hard rock cutting characteristics of disc cutter under front-mounted water jet precutting kerf conditions

To reveal the rock-breaking mechanism of the disc cutter under the front-mounted water jet precutting kerf conditions and study the effect of water jet kerf depth (H) and cutter spacing (S) on rock-breaking performance. The rock-cutting test and numerical simulations by disc cutter under the water jet precutting kerf conditions are conducted. The effects of H and S on the rock-crushing state, rock-cutting force, and rock-breaking efficiency are investigated, and the failure mechanism of internal rock cracks is revealed and discussed. The results show that with the increase of H, the disc cutter's vertical force and rolling force can be reduced by 48.2% and 32.9%, respectively, at maximum, but fail to improve the internal crack expansion capacity of the rock. The optimal combination of S of 80 mm and H of 9 mm can promote the expansion of the main crack and improve the rock-cutting efficiency. Meanwhile, the numerical simulations result further show that the microcrack initiation and propagation can describe the rock damage process. The rock internal crack initiation stage is accompanied by tensile and shear cracks, while the crack propagation stage is dominated by tensile cracks. Microscopic observation of the scanning electron microscope (SEM) reveals that the fracture mechanism for the major crack is tensile type.

Effect of loading-induced damage on chloride ingress behavior of recycled aggregate concrete: A comprehensive review

The recycling technology of construction and demolition wastes is an inevitable choice towards the sustainable development of the construction industry, aiming at addressing the problems of raw materials shortage and waste disposal. The utilization of recycled coarse aggregate (RCA) in new concrete production has been identified as an effective way to reasonable application of natural resources replacement and environmental preservation. In service, external mechanical loading can easily cause the initiation and interconnectivity of microcracks inside concrete, specifically the complex multiple interface structures of recycled aggregate concrete (RAC) due to the old adhered mortar at the surface of RCAs, which can remarkably accelerate the process of water penetration and aggressive ions penetration into concrete. Thus, knowledge of mass transport properties in the loading-induced damaged RAC contributes to the durability evaluation of RAC and its extensive application in the practical engineering. This paper reviews the previous findings on the mechanical loading effects on the chloride transport properties of RAC. The effects of loading-induced damage on microstructure evolution of RAC as well as the damage characteristics were discussed. The research progress on the chloride ingress behavior of RAC subjected to mechanical loading was highlighted from the viewpoint of testing method, loading devices and numerical calculation, and the modified effects on the transport properties were further analyzed. On this basis, the future research on the durability-related properties of RAC would be finally proposed, which is helpful to further perfect theoretical development and technological innovation of RAC durability in the aggressive environment.

Experimental and numerical study on the mechanical behaviours of two rocks with circular openings

Abstract The presence of discontinuities in rocks can significantly compromise the performance of the rocks in terms of strength and stiffness, thereby threatening the reliability and safety of engineering structures built in or on the rocks, for example, tunnels, and slopes. In this paper, a comprehensive set of experimental tests, including Brazilian disc tests and uniaxial compression tests, were performed for two distinct rocks including granite and sandstone with seven patterns of opening holes, in addition to intact rock specimens. To investigate the micro-cracks initiation and propagation, a hybrid continuum-discrete element method incorporating a cohesive fracture model was adopted to simulate the laboratory testing. The stress–strain relations, micro-cracks initiation and propagation, Young’s modulus, compressive strength, tensile strength and micro-cracks rate and orientation were discussed. The results showed that the presence of the holes could significantly influence the tensile strength and compressive strength. However, its influence on the Young’s modulus was found to be relatively insignificant.

In-situ SEM study on fatigue crack behavior of a nickel-based single crystal alloy at 950 °C and 1050 °C

As the blade material of engine, nickel-base single crystal superalloy has been subjected to complex service environment such as high temperature and alternating load for a long time. In this work, the mechanism of fatigue microcrack initiation and propagation in a nickel-based single crystal at 950 °C and 1050 °C was studied by using the in-situ scanning electron microscope high temperature fatigue test system. At 950 °C, the alloy deformation was controlled by slip and the crack shear γ/γ’ phase cracking, showing Stage I mode. At 1050 °C, the alloy crack tearing occurred at the γ phase parallel to the loading direction and γ/γ' interface perpendicular to the loading direction and presents a Stage II mode perpendicular to the loading axis, then changes into Stage I in the accelerating stage. The deformation mechanism of fatigue cracking at two temperatures was compared. Besides, the influence of the microstructure defects on the fatigue crack behavior was discussed. The results show that the propagation of the microcrack below 100 μm shows evident fluctuations, which is closely related to the microstructure and the development stage of the fatigue crack.

Influence of notch root radius on high cycle fatigue properties and fatigue crack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure

Notch high cycle fatigue (HCF) properties and microcrack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure under various notch radii were systematically investigated. Results indicate that the reduction of notch root radius significantly promotes fatigue microcrack initiation, and then dramatically reduces the HCF life and strength of the alloy. Cyclic deformation of the alloy is mainly controlled by the slipping and deformation twinning in α plates. The primary fatigue crack initiation micro-mechanism is α/β interface cracking induced by slipping and twinning at all notch HCF specimens. Moreover, the volume fraction of twinning would increase with decreasing of the notch root radius. Interestingly, when the notch root radius is the smallest (R = 0.14 mm) and the stress concentration factor is the largest (Kt = 4), in addition to slipping and twinning, basal stacking faults promoting the cracking of α/β interface could be another crucial HCF microcrack initiation mechanism of the alloy. Furthermore, with decreasing of notch root radius and increasing of stress concentration factor, the size of cycle plastic deformation zone at notch root gradually reduces to the size of α colony and even the α plate. Therefore, the order of influential degree of three different levels microstructure on the crack initiation mechanism of notched HCF can be arranged as α plate > α colony > β GB.

Soil particle size-dependent constitutive modeling of frozen soil under impact loading

In this study, the soil particle size dependence of the impact dynamic mechanical properties of frozen soil was investigated using a split Hopkinson pressure bar, and the meso-deformation mechanism of frozen soil was analyzed via electron microscopy. Under impact loading, the dynamic stress-strain curves of frozen soil showed strain hardening, strain softening, strain rate-temperature sensitivity, and soil particle size dependence, and the soil particle size dependence of frozen soil was considered to be significantly influenced by non-uniform meso-deformation, which primarily appeared in the form of microcracks surrounding the coarse particles. A constitutive model comprising a multiplicative hardening law was proposed, and soil particle size dependence was introduced through the equivalent strain gradient of the soil cell element. The impact compression process of frozen soil was simulated in ABAQUS/Explicit using the proposed model, and the mechanism of frozen soil deformation was summarized by the finite element analysis: the stress concentration around coarse particles induces the initiation of microcracks, the propagation of microcracks forms a crack network, eventually leading to the softening behavior of frozen soil.

Rubber particle bridging effect on crack width control of low carbon Engineered Cementitious Composites (ECC)

Engineered Cementitious Composites (ECC) exhibit high tensile ductility and multiple cracking behaviors. The weak bonding between hydrophobic fiber and matrix tends to enlarge the crack width of ECC. This paper investigated recycled crumb rubber (CR) to tailor the crack width control of ECC as an artificial flaw. The experimental results show that CR can enhance the multiple cracking and reduce the crack width of ECC, which can be attributed to the CR weakened matrix and particle bridging effect. Micro-CT scanning revealed how CR influences the initiation and propagation of microcracks qualitatively. A constitutive model of CR particle bridging was developed to evaluate the quantitative contribution of CR particle bridging to the crack width reduction of ECC. Although the effect of bridging on crack width reduction is not as strong as that of the weakened matrix, the combined effect is consistent with the experimental magnitude of crack width reduction. Further parametric analysis indicates that small particle size, higher content, and narrower size distribution range of CR are preferred for better crack width control of ECC. The insights gained provide guidance on ECC composite design for a tight crack width with better durability properties.