Microcrack Initiation Research Articles

Material processing, microstructure, and composite properties of low carbon Engineered Cementitious Composites (ECC)

Traditional PVA fiber-reinforced Engineered Cementitious Composites (ECC) show high tensile ductility and superior durability with tight crack width, but the high cost and embodied carbon can hinder its wider application in infrastructures. The objective of this study is to develop a better understanding of the fresh and hardened properties of an ECC that employs a lower embodied-carbon binder, Limestone Calcined Clay Cement (LC3), and lower-cost PP fiber that is widely available. Specifically, the interrelations between material processing, microstructure, and composite properties were studied experimentally. The results showed that ECC with high tensile ductility up to 9% tensile strain and tight crack width with 50 μm at 2% tensile strain can be achieved. It was found that a matrix paste with higher viscosity generally enhanced fiber dispersion uniformity and robustness in tensile strain-hardening. The paste viscosity is increased when OPC is replaced by LC3 and can be tuned with superplasticizer content. Larger maximum flaw size leads to lower first crack strength, beneficial for microcrack initiation and multiple cracking. This study generates fundamental knowledge linking processing-microstructure-performance of PP-LC3-ECC. This class of low embodied carbon ECC with tight crack width is expected to contribute to reducing the carbon footprint of the built environment.

Identification of microstructure factors affecting hydrogen embrittlement of a 2205 duplex stainless steel

Indentation of key microstructure factors affecting hydrogen embrittlement (HE) of duplex stainless steel (DSS) is fundamental for regulating the HE resistance. This study investigated H-assisted cracking of 2205 DSS with different microstructure based on the initiation and evolution of microcracks during deformation. Load and H repartitioning during straining were simulated and experimentally revealed. H-assisted microcrack initiation was frequently found at the austenite grain boundary owing to H-enhanced planar slip promoting the stress and H accumulation; whereas the ferrite phase acted as preferential crack propagation path. The narrower ferrite block and lower inter-connectivity between the ferrite blocks provided better HE resistance.

Fracture behavior of polypropylene fiber reinforced concrete modified by rubber powder exposed to elevated temperatures

• Elevated temperature tests conducted on RPFC notched beams. • Effect of different temperature on the fracture behavior of RPFC was studied. • The damage evolution within the fracture process zone was monitored by AE and DIC. • Fracture energy and fracture toughness first increase and then reduce with increasing temperature. In this paper, the deterioration of fracture properties of rubber powder modified polypropylene fiber concrete (RPFC) after elevated temperatures is discussed. Three-point bending tests were conducted on 75 notched beams to discuss the effects of rubber powder content (0%, 5%, 10%, 15%), polypropylene fibers content (0%, 1%), and ambient temperature (25℃, 100℃, 200℃, 400℃, 600℃) on the fracture properties of RPFC. In addition, digital image correlation (DIC) techniques were used to measure crack development in RPFC notched beams. The fracture patterns, micro-crack initiation and extension and macro-crack development in the fracture process were identified using acoustic emission (AE) techniques. The results show that incorporating rubber powder into polypropylene fiber concrete can delay the crack propagation process, improve the deformation ability and make the fracture path more tortuous. With the rubber powder content increase from 5% to 15%, the initiation fracture toughness of RPFC notched beam decreased, the unstable fracture toughness and the fracture energy increased. When the ambient temperature rises from 25 ℃ to 600 ℃, the fracture energy and fracture toughness of RPFC increases first and then decreases, the crack mouth opening displacement increases, and ductility increases significantly. AE analysis further shows that the growth rate of accumulate counts decrease significantly with the rubber powder content in RPFC, indicating the rate of damage development has slowed down in some extent.

Experimental study on the effective stress law and permeability of damaged sandstone under true triaxial stress

The effective stress law and the permeability play a notable role in dealing with the gas-solid coupling problem of porous media . The key parameter to quantify the influence of pore pressure to effective stress is the effective stress coefficient α ij . However, current relevant researches are mainly focused on the pore elastic flow characteristics in the elastic stage or hydrostatic pressure under conventional triaxial stress state, which are difficult to reflect the actual formation stress condition. To this end, the present study was conducted to investigate the influence of induced damage on the effective stress coefficient and the permeability of sandstone under true triaxial stress. The results showed that the effective stress coefficient and the permeability were closely related to the damage evolution and fracture propagation . α ij exhibited obvious anisotropy, α 1 was larger than α 2 and α 3 , and α 3 > α 2 appeared in the horizontal direction. α ij increased with increasing the fracture density and opening, even greater than unity. Increasing the initial horizontal stress could reduce α ij under the damage state. The elastic modulus E ij also exerted anisotropy during loading, E 1 basically exhibited an increasing trend followed by a decrease as the axial strain increased, E 2 and E 3 were continue to degrade with increasing the axial strain. Loading the deviatoric stress could cause damage, which induced the initiation and propagation of microcracks , and in turn led to the nonlinear stress-strain relationship and volume expansion of the rock, so the permeability increased, and it had increased sharply before reaching the peak strength. Under the influence of σ 2 , α ij first increased and then decreased with increasing the intermediate principal stress coefficient b under the damage stage. The porosity effect also affected the relationship between α ij and the permeability related to the induced damage, and α ij revealed an increasing trend as the permeability increased. • Effective stress coefficient a ij increases with fracture growth, even beyond unity. • a ij and E ij exhibit anisotropy with loading. a 1 is the largest, and a 3 > a 2 . • Increasing the horizontal stress can reduce a ij related to the induced damage. • a ij first decreases and then increases with increasing b under the damage state. • a ij increases with the increase in permeability attributed to the induced damage.

Thermal effect on the fracture behavior of granite using acoustic emission and digital image correlation: An experimental investigation

The irreversible deterioration of bearing rocks via the thermal effect is a severe challenge faced by underground engineering. A comprehensive understanding of the fracture behavior of rocks subjected to high-temperature treatments can contribute significantly to engineering infrastructure design and safety. We conducted three-point bending tests on semi-circular bend granite samples exposed to high-temperature treatments. Complementary acoustic emission (AE) monitoring and digital image correlation analyses were used to examine the following: (i) AE timing response characteristics associated with microcracks initiation and propagation; (ii) acquired AE waveform frequency and shape parameters, which are closely related to the microcracking scale and mode; and (iii) fracture parameters, including the fracture process zone (FPZ) and crack opening displacement. The in-depth reasons for changes in the fracture behavior were then investigated. According to the results, the decreasing trend of multifractal index Δα indicates that high-temperature treatments reduce the distribution difference in the value of AE counts and make microcracks with relatively low energy more prone to be generated. Above heating temperature of 600 °C, microcracking activities tend to be low frequency and high amplitude and shear fracturing gradually becomes dominant. Observations show that the cohesive stress in granite is remarkably reduced with the elevated thermal damage, and the accumulated elastic energy becomes easier to dissipate. This promotes FPZ development and makes granite more prone to cracking and opening. Moreover, the evolution of crack-opening displacement of granites before the peak load is highly correlated with cumulative AE hits, with correlation coefficients ranging from 0.922 to 0.997. Finally, it is found that the equivalent crack length should be adopted in the evaluation of fracture toughness, and the fracture toughness of lower-heat-treated granites is more closely linked to the FPZ length.

Generation mechanism of fracture-induced electromagnetic radiation and directionality characterization in the near field

The electromagnetic radiation generated during the fracturing process is generally regarded as vector field that holds an imperative significance in rock engineering and fracture mechanics. With this motive, based on the theory of fracture mechanics and electromagnetics, the nonlinear field at the crack tip and the atomic bond breaking condition are analyzed in this study. Considering the electromagnetic field pattern generated by the oscillating dipole, the generation process of the electromagnetic radiation signal is divided into two stages namely, initiation of the forced oscillation of the dipole and the damped dipole oscillation. Followed by this, the experimental investigations on limestone and shale under the condition of Brazilian tests are performed. In the whole loading process, four sets of three-axis electromagnetic antennas are used for simultaneous measurement in different directions. The experimental and theoretical results exhibit close resemblance. The superposition effect of the electromagnetic field from microcrack initiation to macro-fracture is analyzed further. Based on the obtained results, three indicators are proposed to characterize the directionality of the electromagnetic radiation generated by the rock fracture. It is observed that the changing law of the direction angles is consistent, showing a strong correlation with the symmetrical position. The direction of the magnetic flux density vector generated by rock Brazilian tests tends to be parallel to the crack surface.

Experimental investigation on damage evolution and failure mechanism of pre-corroded AA7075-T6 under multiaxial fatigue loading

To investigate the tension–torsion fatigue failure of pre-corroded AA7075-T6, fatigue damage accumulation, macro-crack formation and propagation are quantitatively characterized through 3D-DIC based strain maps and damage severity curves, micro-cracking characteristics are further examined by SEM, TEM, and EBSD observation. Experimental results show that corrosion pit induces fatigue macro-cracks formation, and the identified nucleation time, amount, and location are diverse for different loading conditions. Four dominated fatigue micro-crack initiation forms and corresponding mechanisms were analyzed, namely, pit bottom, sidewall dis-continuous surface, corrosion micro-feature (micro-pits/jut-ins), and pit subsurface, which depends on multi-factors involving corrosion macro-morphology, corrosion micro-morphology, loading condition and underlying microstructure.

The influence of phosphorus on the temper embrittlement and hydrogen embrittlement of some dual-phase steels

The temper embrittlement (TE) and hydrogen embrittlement (HE) of some dual-phase (DP) steels were investigated, focusing on the influence of the phosphorus (P) content. An increased P content of 0.02 wt% caused little TE and enhancement of HE susceptibility in DP steels. Hydrogen had little influence on strength, but caused a significant decrease in ductility. For the DP steels with the different microstructures, the hydrogen-induced cracks initiated at the specimen surface, and hydrogen-induced micro-crack initiation was mainly martensite related. The increased martensite related cracking and the tendency of interface cracking was attributed by the enhanced element segregation (by both increased tendency of P segregation and promoted by other element segregation like Mn). The increased P content can enhance HE susceptibility through both interface weakening related HE mechanisms (HESIV, HEDE, or defactant) and ferrite ductility loss related HELP mechanism.

Welding heat input for synergistic improvement in toughness and stress corrosion resistance of X65 pipeline steel with pre-strain

The microstructure, hydrogen embrittlement sensitivity, fracture toughness, and influence of pre-strain on the sulfide stress corrosion cracking (SSCC) sensitivity of the coarse-grained heat-affected zone of X65 pipeline steel with large deformation were studied to determine the optimal welding heat input in an acidic H2S environment. After pre-straining, the microstructures of the specimens changed insignificantly, but their ductility decreased. The acicular ferrite microstructure obtained under a low heat input demonstrated the synergistic improvement in the SSCC resistance and fracture toughness under pre-strain, especially at t8/5 = 20 s. The M/A constituents interface potentially causes microcrack initiation and interconnection under pre-strain.

Damage evolution of extruded magnesium alloy from deformation twinning and dislocation slipping in uniaxial stress-controlled low cycle fatigue

The damage evolution of an extruded AZ31 magnesium alloy in uniaxial stress-controlled low cycle fatigue is investigated using a quasi-in-situ method. In the three studied samples, that is, slipping dominated (SD), twinning/detwinning/basal slipping (TDBS), and twinning/detwinning/slipping (TDS), the fractions of slip bands and/or twins keep increasing till a critical cyclic number, and are essentially unchanged in the subsequent cycles. Inter-granular micro-crack initiation is found to be favorable in the TDS and SD samples. While both the inter-granular and intra-granular micro-cracks are detected in the TDBS sample. Moreover, the non-Schmid behavior in the crack trans-granular propagation path is investigated.

Microstructure distribution and bending fracture mechanism of 65Mn steel in the laser surface treatment

Laser surface treatment of 65Mn steel was conducted using a high-power diode laser. Based on the numerical simulation of the temperature field, the distribution of the microstructure and the peak temperature at various depths were analyzed. An empirical model was proposed to predict the hardened depth. The bending property and fracture mechanism were studied. The results indicated that the martensitic microstructure was coarser near the outer surface of the hardened layer owing to the higher peak temperature. The peak temperature decreased exponentially with increasing depth and increased linearly with laser power and the reciprocal of the square root of the scanning rate. The maximum hardness of the laser-hardened layer was approximately 950 HV, and the self-tempering effect decreased the maximum hardness. Because of the higher brittleness of the hardened layer and the concentration of deformation near the hardened layer, the bending properties deteriorated after laser surface treatment. However, the hardened layer increased the resistance to early bending deformation. Tempering at 150 °C slightly increased the bending strength. The bending fracture consists of three stages: first, tensile load induces the initiation of microcrack, and the crack propagates in an intergranular manner near the crack source; second, the crack propagates in a transgranular manner, and the fracture morphology exhibits quasi-cleavage; and finally, the fracture morphology changes to a ductile fracture in the substrate region.

In-situ investigation on the anisotropic behavior of the additively manufactured dual-phase Ti-6Al-4V alloy

In this work, in-situ SEM tensile investigations were performed to study the fracture mechanism, anisotropic behavior, and the defect effects of selective laser melted (SLM) and then heat-treated Ti6Al4V with a dual-phase α + β lamellar structure in three loading directions (90°/45°/0° to the substrate). The results showed that the surface steps formed due to the deformation mismatch during the plastic stage. The initiation of microcracks was mainly due to the separation between the adjacent lamellae or the lamellae fracture. Near-surface defects could induce the strain concentration and then premature failure, whereas the defects deep inside the specimens presented little effect on ductility deterioration. Based on the grain orientation data, the calculation of most likely active slip systems illustrated the significant difference in the slip traces between the two adjacent α laths accounted for the deformation mismatch and the arising of surface steps. The difference in the overall Schmid Factors revealed the inhomogeneous deformation between the adjacent columnar prior β grains. Finally, the theoretical analysis was conducted and well verified by the experimental results helping a further understanding of the deformation mechanism.

Microstructure refinement, strengthening and ductilization mechanisms in Al–Mg–Mn–Er–Zr alloy with high Mn content by friction stir processing

In the present work, the friction stir processing (FSP) has been conducted on the extruded Al–6.0Mg–1.1Mn–0.19Er–0.1Zr alloy and subsequently the microstructure evolution and mechanical property were systematically investigated after FSP. The results showed that FSP is an effective approach to achieve remarkable refinement and homogeneity of microstructure. The coarse band shape grains are gradually replaced by the fine equiaxial grains with a mean diameter of 2.54 μm after fully dynamic recrystallization, which can be ascribed to severe plastic deformation and considerable heat input from the stir effect. Additionally, both the primary intermetallic compounds and the short rod-like secondary dispersoids are refined and evenly distributed in the matrix during FSP. The tensile tests demonstrated that the elongation of the FSPed alloy can be improved by 105% without sacrificing any strength compared with that of the base material (BM), proving that FSP is a good method for improving alloy with a desirable combination of strength and ductility after FSP. Compared with the BM, the grain boundary strengthening and the dispersion strengthening mainly contribute to the compensation for the decrease of dislocation strengthening in the FSPed alloy. Based on the observation of the fracture surfaces and the side view near the fracture, the ductilization can be attributed to the refinement of the primary intermetallic compounds and the rod-like secondary dispersoids by suppressing the initiation and propagation of microcracks during the tensile deformation.

Modelling of the intergranular fracture of TWIP steels working at high temperature by using CZM–CPFE method

Grain boundaries (GBs) of metallic materials will become the weakest and most vulnerable place when the materials deform at high temperature. This could directly affect deformation flow, plastic strengthening, the initiation and propagation of microcracks, and finally the fracture failure of materials. In this research, to reveal the high temperature deformation mechanisms of twinning–induced plasticity (TWIP) steels, the crystal plasticity finite element method (CPFEM) was employed to model the mechanical response of grain interiors. The cohesive zone model (CZM) of the bi–linear traction separation law (TSL) was developed to describe the damage and failure of GBs. By combining CZM and CPFEM, an analysis method was proposed to investigate the effect of the microstructure morphology, orientation and size of grains, and temperature. Furthermore, the damage initiation, accumulation and fracture at GBs were represented by the representative volume element model based on the CZM–CPFE method to explore the influences of GB angle, grain size and local microvoids (including the location and size) on the GB cracking. The stress damage cloud maps, the distribution of quadratic stress damage initiation criterion (QUADSCRT) and the scalar stiffness degradation (SDEG) describing the damage and fracture along the GBs were used to articulate the relationships of GB angle, size effect, local microvoids and the expected failure strain. The results show that low angle grain boundaries (LAGBs), the small average grain size (<21.2 μm), microvoids at the horizontal GBs and microvoids with a diameter below 1.5 μm can delay the initiation of microcracks and fracture failure along the GBs. The predicted damage and failure of GBs and the intergranular cracking characteristics in plastic deformation of TWIP steels at high temperature were corroborated by microscopic in–situ SEM experiments, in which the deformation characteristics inside the grains, and microcrack initiation and propagation near GBs were observed at 500 and 750°C. This research enhances the understanding of intergranular fracture, mechanical response and microstructure evolution of TWIP steels worked at different temperatures, and also provides a new way and strategy for studying the deformation behaviours of TWIP steels considering GB properties.

Micro-fracture behaviors of needled short-chopped fiber reinforced phenolic aerogel composites based on in-situ X-ray micro-CT

Needled short-chopped fiber reinforced phenolic aerogel composites (PAC) are the most promising thermal protection materials for space applications. However, the complex microstructures deriving from the irregular needling yarns severely limits the revealing of their fracture mechanisms. Herein, for the first time, the micro-fracture behaviors of PAC are innovatively revealed using in-situ X-ray micro-CT under tensile loading. Furthermore, the high-precision finite element analysis of stress evolution is realized by establishing PAC model based on the micro-CT slices. The results indicate that the needling yarns will result in the enrichment of resin aerogel, leading to the stress concentration and micro-cracks initiation during loading. And the needling yarns can effectively hinder the separation and orientation of short-chopped fibers and retard the stress transmission between fibers and matrix, which are the main strengthening mechanisms of PAC. The present results will give references for revealing the fracture mechanisms of composites enhanced by needled fibers.

Interior crystallographic plane induced cracking behavior of Ni-based superalloy in high-temperature and vacuum environment

Tension-tension fatigue tests at 750 °C were performed to state on fatigue structure-properties of Ni-based superalloy in long-life regime under high temperature by means of ultra-depth-of-field overlay three-dimensional, scanning electron microscopy, energy dispersive spectrometer, electron back scattered diffraction, focused ion beam and transmission electron microscope. Results show that interior crystallographic plane-fisheye failures are divided into the single plane & multiple planes cracking. The formation of plane with micro-crack initiation and growth is mainly under Mode Ⅱ cracking from the Copper grain with large size and the highest Schmid factor or the Goss grain with small size and high Schmid factor. Moreover, owing to high thermal activation energy, grain-related interior cracking is mainly induced by climbing and bypassing mechanisms with bowing of dislocations and zig-zag dislocations, and assisted by shearing mechanism with tangled dislocations. Finally, the interior plane-fisheye cracking behavior with microstructure in high-temperature and vacuum environment is elucidated.

Multiaxial fatigue life prediction model based on an improved strain energy density criterion

Based on the critical plane approach and the shear damage model, a new damage parameter based on the strain energy density criterion is proposed. This parameter takes into account the interaction between shear stress and hydrostatic stress during microcrack initiation and the contribution of hydrostatic strain energy density to fatigue damage. On this basis, a new multiaxial fatigue life prediction model is established by combining the Basquin-Manson-Coffin equation. The analysis results of five metal materials show that the predicted life correlated well with the experimental observations. The merits of the proposed model are further highlighted by the comparison between the results of the proposed method and those of two fatigue damage prediction models.

Study of material recrystallized regions on deformation and cracking behaviours in cast steel brake discs

Localized regions of recrystallized material have been observed on steel brake discs used in high-speed rail systems. Because the effects of recrystallized regions on steel brake disc cracking behaviour are not well known, further study is warranted. In the present study, the characteristics of localized regions of recrystallized material were characterized by transmission electron microscope, and the coupled effect of severe braking thermal cycles and frictional shear force was found to be the root cause of material recrystallization. In situ tensile testing was used to compare the deformation and cracking behaviours in samples taken from a in-service railway brake disc and a virgin brake disc. The results showed that the localized recrystallized material degrades the mechanical properties of the in-service brake disc due to strain localization. The localized recrystallization of the material also changed the sample fracture from a ductile mode in the virgin steel brake disc to a mixed mode. Electron back-scattered diffraction (EBSD) analysis showed that the nonuniform distribution of low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs) in the localized recrystallized material reflected strain localization. HAGBs promoted local strengthening of the small-size grain region, resulting in grain-scale incompatibility deformation and enhanced strain localization. The difference in the degree of deformation between the large-size and small-size grain regions accordingly became irreconcilable, which led to stress concentrations and microcrack initiation. Based on these results, it was recommended that the microstructural condition of steel brake discs be monitored during maintenance inspections and that the sliding surfaces be turned to remove the recrystallized material if necessary.

Simulation of micro-crack initiation and propagation under repeated load in asphalt concrete using zero-thickness cohesive elements

The fracture resistance of asphalt materials under repeated traffic loads significantly influences the service life of asphalt pavements. The numerical simulation of asphalt materials based on a form of fracture mechanics is considered as an efficient way to model fracture damage. Unlike traditional fracture mechanics approaches assuming the existence of infinitely sharp crack tips leading to stress singularities preceding the crack tips, an alternative approach, called Cohesive Zone Modelling (CZM), is presented to model crack initiation and propagation. To make the CZM more consistent with reality and thus yield more reliable results, a heterogeneous model was developed to explicitly model the different material phases (aggregate and asphalt mastic) captured by X-ray Computed Tomography (CT) (with a resolution of 0.014 mm3). A Two-Dimensional (2D) Finite Element (FE) model was reconstructed; the model embedded zero-thickness cohesive elements both in asphalt mastic and at interfaces between mastic and aggregates, in conjunction with viscoelastic material properties. Crack paths in asphalt mixture samples at different deformation levels in the Indirect Tensile Fatigue Test (ITFT) were compared to simulation results. The 2D model was then applied to predict crack initiation and propagation at 25 ℃. It is found that high resolution X-ray CT applied on small-size samples is a suitable method to visualize detailed crack geometry of asphalt concrete and reconstruct realistic aggregate morphology for simulations. The 2D FEM developed can approximately model the whole cracking process. Based on the model, it is found that factors such as aggregate size and distance between aggregates can affect crack initiation. At 5 ℃ microcracks tend to initiate predominantly at aggregate-binder interfaces. At 25 ℃ microcracks tend to initiate and propagate within the mastic; as the microcracks propagate. At both temperatures, microcracks were observed develop continuously throughout the modelling fatigue process.

Effect of natural defects on the fracture behaviors and failure mechanism of basalt through mesotesting and FDEM modeling

As an important geological medium for engineering construction in Southwest China, basalt typically contains natural defects, such as hidden joints and amygdales, which significantly affect its mechanical properties. In this work, the fracture behaviors of basalt with different defects were studied in terms of its strength, deformation and failure characteristics, and crack evolution laws using a comprehensive mesotesting scheme and finite-discrete element method (FDEM) modeling, revealing the failure mechanism of basalt. The results showed that: (1) Intact specimens exhibiting high strength and smooth stress–strain curves burst instantly when loaded to the peak strength, and the corresponding acoustic emission events are abnormally concentrated, indicating a fragmentation failure mode. (2) The strength of fractured samples decreases significantly, the stress–strain curve is bimodal or multimodal, and the acoustic emission events are very active during the entire failure process, which is closely related to the local damage generated during each significant stress drop, with tension dominant, shear dominant, or tension–shear composite failure modes. (3) For amygdaloidal basalt samples with a serrated stress–strain curve, each small-scale stress drop is accompanied by the initiation, propagation, and coalescence of microcracks, resulting in relatively scattered acoustic emission events, indicating a splitting failure mode. (4) Because of the small particle size, dense arrangement, good uniformity, and high strength at the grain scale, intact basalt exhibits a relatively uniform stress field easily induced under loading, leading to an extremely high strength, fragmentation failure mode, and significant brittleness. However, a heterogeneous stress field near the original defects, such as hidden joints or amygdales, is prominent, and it significantly affects the fracture paths, which is the main reason for the strong randomness of the mechanical properties, evident strength reduction, and significant change in the failure modes. The research results can provide reference for revealing the occurrence mechanism, guiding warning systems, and formulating control methods for high-stress disasters in hard brittle rocks.