Transgranular Cracking Research Articles

Mitigation of hydrogen embrittlement in alloy custom age 625 PLUS® via grain boundary engineering

Hydrogen embrittlement is the deterioration of mechanical properties in a metal exposed to hydrogen, often characterized by brittle, intergranular fracture at low applied stresses. While grain boundary engineering has been applied to mitigate this issue, ambiguity in the mechanisms behind hydrogen embrittlement leads to ambiguity in the mechanism by which grain boundary engineering helps to mitigate this problem. In this study, grain boundary engineering was applied to improve resistance to hydrogen embrittlement in Custom Age 625 PLUS®, an alloy frequently used in corrosive environments where hydrogen embrittlement is of particular concern. Iterative low strain cold rolling followed by annealing at intermediate temperature successfully produced a grain boundary engineered microstructure with large twin-related domains and a high fraction of interconnected coincident site lattice (CSL) boundaries. Rising step load testing demonstrated that grain boundary engineering increased the stress intensity at which failure from hydrogen embrittlement occurred and caused a shift from intergranular to transgranular crack propagation. Evidence of localized plasticity on fracture surfaces suggest that hydrogen-enhanced localized plasticity (HELP) is the dominant mechanism of hydrogen embrittlement.

Damage progression and acoustic emission in brittle failure of granite and sandstone

This paper experimentally evaluates and quantitatively compares stress-induced damage progression and acoustic emission (AE) in granite and sandstone based on continuous acousto-optic-mechanical (AOM) observations. Experimental results reveal that the step-rise characteristics of the AE event rate preceding the entire macrofracturing process and the apparent white patching phenomena are the most significant acousto-optical evidences for process zone nucleation in granite but not in sandstone. During the unstable crack growth stage in granite, the robustly high AE event rate level has been mechanically correlated with the reactivation and intensification of fracture process zones (FPZs) by the stress build-up, and the relatively low AE event rate level has been revealed as a result of the progression of FPZs into macrocracks. It is the first time that the process zone nucleation in granite has been mechanistically related to the clustering of three types of grain-scale microcracks, i.e., grain boundary cracks, intragranular cracks and transgranular cracks, due to the grain interlocking effect. Comparatively, the clustering of microcracks is insignificant in the brittle failure of sandstone. Mechanistic correlations among the AOM characteristics in the two rock types are also investigated in detail.

In-situ SEM investigation and modeling of small crack growth behavior of additively manufactured titanium alloy

The fatigue crack growth of wire and arc additive manufactured Ti-6Al-4V alloys is investigated using in-situ SEM fatigue testing. The small crack initiation and propagation behaviors within grains and across grains are systematically studied, and the inter- and trans-granular crack growth rates are quantified. A synergy and competition among many factors such as persistent slip bands, grain boundaries, and crack branching governs the growth rate. A modified model based on the CTOD is proposed to characterize the growth rate considering the microstructure dissimilitude and local yield strength variations. Results show the proposed model improves the prediction accuracy.

Effect of microwave heating on fracture behavior of granite: An experimental investigation

Hard rock breaking is a great challenge for deep drilling and mining engineering, and the microwave-assisted rock breaking technique offers a promising solution. However, the lack of knowledge of the effects of microwave irradiation on hard rocks limits the further development of this technology. Laboratory tests have been conducted to investigate the variations of fracture properties of granite subjected to microwave heating. The semi-circular bend (SCB) specimens were prepared, and mode I fracture tests were conducted on the microwave treated granite specimens. The acoustic emission (AE) events was recorded and the strain field was calculated by the digital image correlation (DIC) technique. The test results demonstrated that the temperature of the sample surface increased linearly as the increasing heating time, and increasing microwave power contributes to enhancing the heating rate. According to the results of scanning electron microscope (SEM), intergranular cracks were the main form of cracks, and the transgranular cracks usually appeared in samples with high microwave power or longer heating time. The generation of micro-cracks can be attributed to the high temperature and the thermal shock. With the increasing heating time, the fracture toughness decreased exponentially, the AE active period prolonged, and the fractal dimension of the fracture surfaces (FSD) continuous increased. Compared to an untreated sample, the maximum principal strain and fracture process zone (FPZ) size of the microwave heated sample were larger at the same loading level, which indicated the enhancement of ductility. According to the experimental results, the relationships between FPZ size, fracture toughness and FSD were empirically regressed. Understanding the effects of microwave heating on the fracture properties is of significant to improve hard rock breaking efficiency.

Fatigue study on AISI/SAE 1015 steel with shot peening under corrosive environments

This work evaluates alternative four-point bending fatigue stress in 2 < pH < 4 corrosive media of AISI/SAE 1015 steel subjected to shot peening surface treatment. A tri factorial experimental design with replications was used. The intervening factors were: i) load levels, 0.96, 0.85, 0.73, 0.65; ii) corrosive media; pH2, pH4, ambient; iii) shot-peening surface treatment; without any shot peening, S230 shot peening and CW-41 shot peening, for which a total of 108 specimens were tested with a reliability level of 95% and a probability of failure of 3%. Tests were performed on a universal machine in a three-point bending assembly with an electro-pneumatic system. The cycle counting and control were performed by a programmable logic controller. The results of the stress-life method in each one of the scenarios show that the best condition was shown in the probes with shot peening S230 and CW-41 treatment under atmospheric conditions, where the material tends to increase its fatigue behavior; the number of cycles reached 36990 and 37773 respectively. The corrosive effect of pH2 and pH4 reduced on probes without shot peening treatment with a maximum of 9862 and 8670 cycles respectively; however, for S230 and CW-41 shots, an increase in cycle number in pH2 and pH4 was evidenced, with the best result for S230 shot at pH4 with a maximum range of 21568. Likewise, the fractography analysis shows mixed fractures with fatigue failure mechanism; the characteristics found were beach marks, secondary cracks, localized oxidation (filiform, plates, pitting, stress, and compression corrosion), nucleation, microvoids, cleavage facets, intergranular and transgranular cracks.

Mechanical characteristics and pore evolution of red sandstone under ultrasonic high-frequency vibration excitation

Ultrasonic vibration rock breaking is a new type of rock breaking technology. By studying the mechanical properties of red sandstone under ultrasonic vibration, the mechanical behavior and damage mechanism of rock under the impact of high-frequency vibration can be revealed more comprehensively from macro- and microscopic standpoints. In this paper, the cylindrical red sandstone specimen is used as the study object subjected to vibration excitation via the ultrasonic vibration device. The change in the mechanical parameters of red sandstone specimens is analyzed via a single-axial compression test. The red sandstone specimens are vibrated to study the effects of high-frequency vibration on their natural frequency. The latter’s natural frequency is measured using the knocking method, while the micro-disruption characteristics of the red sandstone are observed via electron microscopy. The T2 spectrum, aperture distribution, porosity, and nuclear magnetic resonance image (MRI) evolution characteristics of red sandstone specimens are obtained via nuclear magnetic resonance technology. The results show that ultrasonic vibration deteriorates the red sandstone compressive strength and elastic modulus by 55.3% and 26.9%, respectively, after 120 s of excitation. Under ultrasonic vibration excitation, the rock specimen’s natural frequency is reduced by 2.4% due to its mass and elastic modulus variation. Many transgranular cracks are generated in the sandstone, splitting the crystal nucleus into smaller blocks. The generation of new micropores is observed in the T2 spectrum, and the maximum increase in the dimensions of micropores and mesopores at the two peaks is 58.7% and 4.67%, respectively. The variation trend of rock specimen porosity is completely consistent with the variation in micropores’ content. MRI images indicate that the microcrack aggregation occurs in the edge area of the contact surface between the exciter and rock.

배열회수보일러 복수예열기 부식 파손 분석

In this work, we have performed a corrosion failure analysis of a leaking tube connected to an upper header of a condensate pre-heater in a heat recovery steam generator. It was revealed that the leakage position in the tube was the location where the materials were easily vulnerable due to tensile residual stresses induced by the material manufacturing process and welding process. In addition to an imbalance in the module induced by temperature difference during operation of the pre-heater, the weight of the modules and thermal fatigue provoked a type of stress of tensile-tensile fatigue on the tube. Thus, the leakage position of the pre-heater was exposed to the tensile stress on the inner surface of the tube facing the gas, which rendered the unstable oxide layer susceptible to corrosion and the formation of pits on the water side. The cracks propagated along with the degraded microstructure in a transgranular cracking mode under fatigue loading and finally resulted in water leakage.

Nanoscale Characterization of Stress Corrosion Cracking in a Failed Alloy C-276 Component

A failure analysis was performed on an Alloy C-276 pull rod which underwent unexpected brittle, intergranular fracture after exposure to 280°C to 300°C aqueous solutions designed to replicate secondary side environments in nuclear energy systems: Pb-containing alkaline (pH300°C 8.5 to 9.5), and sulfate-containing acidic solutions (pH280°C 3 to 5). The component was characterized using advanced electron microscopy methods to demonstrate the benefits of these techniques for determining the nanoscale chemical, mechanical, and material factors contributing to failure, and to provide insight into the mechanisms of stress corrosion cracking (SCC) responsible for failure. Site-specific transmission electron microscopy specimens containing crack tips were prepared using focused ion beam. Nanoscale chemical characterization methods revealed that Pb was present in some oxidized regions of cracks, suggesting that the element may be inhibiting or impairing the passivity of the Cr-rich oxide. Complementary nanoscale microstructural analysis was performed. At an intergranular to transgranular cracking mode transition, it was observed that the transgranular crack (and corrosion process) propagated along the (110) crystallographic plane. Also, the cracking mode was highly dependent on the tensile stress direction relative to grain boundary orientation, the crystallographic orientation of grains, and geometrically necessary dislocation structures. A comparison of results with proposed mechanisms for SCC of Ni alloys in similar environments are discussed; the highly directional nature of cracking is consistent with a slot-tunnel corrosion mechanism.

Effects of GNP on the mechanical properties and sliding wear of WC-10wt%Co cemented carbide

WC-10Co cemented carbides reinforced with 0, 0.5, 1, and 2 wt% graphene nanoplatelet (GNP) were fabricated by ball milling and spark plasma sintering (SPS). The microstructure, structural and phase analysis, hardness, and fracture toughness of WC-10Co/GNP composites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and Vickers indenter. Tribological behaviors of the fabricated composites against an alumina counterface were studied using a pin on disk configuration. It was found that GNP refined the microstructure, increased the fracture toughness, and postponed the stable-to-unstable friction transition. While transgranular fracture and crack deflection were observed in the base composite, crack bridging, micro-crack formation, and crack deflection were the major toughening mechanisms in GNP-reinforced cemented carbides. The addition of 1 wt% GNP resulted in the highest hardness and wear resistance. However, at higher GNP contents, both hardness and wear resistance decreased due to the agglomeration of GNPs. Widespread abrasive grooving and Co binder extrusion were characterized as the main controlling mechanisms of wear in GNP-free cemented carbides. The wear of GNP-reinforced cemented carbides was dominated by the formation of a lubricating surface layer and its cracking or fragmentation. Plastic flow is much less likely to occur in the presence of GNPs.

Experimental and Computational Approach to Fatigue Behavior of Polycrystalline Tantalum

This work provides an experimental and computational analysis of low cycle fatigue of a tantalum polycrystalline aggregate. The experimental results include strain field and lattice rotation field measurements at the free surface of a tension–compression test sample after 100, 1000, 2000, and 3000 cycles at ±0.2% overall strain. They reveal the development of strong heterogeneites of strain, plastic slip activity, and surface roughness during cycling. Intergranular and transgranular cracks are observed after 5000 cycles. The Crystal Plasticity Finite Element simulation recording more than 1000 cycles confirms the large strain dispersion at the free surface and shows evidence of strong local ratcheting phenomena occurring in particular at some grain boundaries. The amount of ratcheting plastic strain at each cycle is used as the main ingredient of a new local fatigue crack initiation criterion.

Rate dependency mechanism of crystalline rocks induced by impacts: Insights from grain-scale fracturing and micro heterogeneity

Rocks in nature contain a large number of defects and exhibit strong heterogeneity on grain scale. In this study, a novel three-dimensional multiscale method is proposed to investigate the dynamic behaviors and microfracturing in granitic rocks. In this numerical method, the heterogeneity in mineral components is reproduced by a series of a space filling Voronoi tessellations and particle filling in subgrains as computational nodal points to allow for transgranular fracturing. The rupture strength, temporal deformation fields, and failure patterns are compared with the experimental results to verify the reasonability and accuracy of the proposed method. Then, the underlying mechanism of grain-scale fracturing leading to fractal fracture surfaces, pervasively grain pulverization and deflection/penetration cracking model are discussed. It's shown that the fracture surface roughness is fractal dominated by two competitive mechanisms. The crack initiation time of intergranular tensile crack, transgranular tensile crack and shear cracks is gradually increased. The ratio of the number of tensile cracks exceeds 90% and that value of intergranular crack decreases from 56% to 33% as the strain rate increases. The appearance of multicracks activation and the transition from intergranular fracturing to transgranular fracturing is the underlying mechanism of rate dependency for granitic rocks.

Effect of grain orientation on the compressive response of highly oriented MAX phase Ti3SiC2

The MAX phases comprise of a group of layered ternary carbides that exhibit unique mechanical properties which bridge the gap between their metal and ceramic constituents. To study the effects of the global grain orientation, Ti, Si and TiC powders were hot pressed to synthesize highly oriented bulk Ti3SiC2. X-ray diffraction (XRD) was used to verify the grain orientation and a Lotgering factor of 0.87 with respect to the c-axis was obtained. Prepared Ti3SiC2 samples have been compressed in two orientations, loading along the c-axis (∥c-axis) and perpendicular to the c-axis (⊥c-axis) at 10−3 s−1 using a standard load frame and at 102 s−1 using a Kolsky (split-Hopkinson) bar. The average compressive strength along the ⊥c-axis orientation was 761 MPa under quasi-static conditions and 987 MPa under dynamic loading, exhibiting a 30% increase on average. The ∥c-axis orientation exhibited no rate dependence in compressive strength; however both orientations exhibited an increase of strain at failure under dynamic conditions by over 0.5%, on average. The orientation-dependent failure behavior at different strain rates were examined using high-speed imaging and 2D digital image correlation (DIC) during loading and via scanning electron microscopy (SEM) post-mortem. Results indicate that the ⊥c-axis fracture surface exhibited a mixture of transgranular and intergranular cracks, kink bands and delaminations, whereas ∥c-axis was limited to a combination of intergranular and transgranular cracks. Such fracture distinctions due to the availability (or lack thereof) for kink band formation appear to be responsible for the anisotropic compressive behavior.

Hydrogen enhanced cracking via dynamic formation of grain boundary inside aluminium crystal

By in situ bending tests inside an environmental transmission electron microscope, we found that from the pre-crack tip of single crystalline aluminium cantilevers, dislocations are emitted and self-organized to build a low angle grain boundary (LAGB), along which the crack propagates more easily in hydrogen atmosphere than in vacuum. Atomistic modelling suggests that the LAGB formed can strongly trap hydrogen atoms, and its cohesive strength can be substantially reduced. These results imply that the hydrogen-induced transgranular crack in a polycrystalline metal can proceed by repeating a process of dynamically forming a hydrogenated LAGB and its subsequent separation in grain.

Evaluation of the surface fatigue behavior of amorphous carbon coatings through cyclic nanoindentation

Diamond-like carbon (DLC) coatings, frequently used to reduce wear and friction in machine components as well as on forming tools, are often subjected to cyclic loading. Doping of DLC coatings with metals or metal carbides as well as the usage of multilayer architectures represent promising approaches to enhance toughness, which is beneficial for the coatings' behavior under cyclic loading. In this study, we utilized cyclic nanoindentation to characterize the tribologically induced surface fatigue behavior of single-layer tungsten-doped (a-C:H:W) and multilayer silicon oxide containing (a-C:H:Si:O/a-C:H)25 amorphous carbon coatings under cyclic loading. Columnar growth was observed for both coatings by focused ion beam microscopy and scanning electron microscopy, while the multilayer architecture of the (a-C:H:Si:O/a-C:H)25 coating was verified by the silicon content using glow-discharge optical emission spectroscopy. In cyclic nanoindentation of the (a-C:H:Si:O/a-C:H)25 multilayer coating, stepwise small changes in indentation depth were observed over several indentation cycles. The surface fatigue process of the single-layer a-C:H:W covered a smaller number of indentation cycles and was characterized by an early steep increase of the static displacement signal. Microscopical analyses hint at grain deformation, sliding at columnar boundaries, and grain detachment as underlying fatigue mechanisms of the a-C:H:W coating, while the (a-C:H:Si:O/a-C:H)25 multilayer coating showed transgranular crack propagation and gradual fracturing. In case of the (a-C:H:Si:O/a-C:H)25 multilayer coating, superior indentation hardness (HIT) and indentation modulus (EIT) as well as a higher HIT3/EIT2 ratio suggest a higher resistance to plastic deformation. A high HIT3/EIT2 ratio, being an indicator for hindered crack initiation, combined with the capability of stress relaxation in soft layers contributed to the favorable surface fatigue behavior of the (a-C:H:Si:O/a-C:H)25 multilayer coating observed in this cyclic nanoindentation studies.

Dynamic deformation and fracturing properties of concrete under biaxial confinements

The study of deformation and fracturing properties of concrete is essential to understand failure mechanisms of concrete material, especially under extreme and complex loading conditions, e.g. high strain rates and multiple confinements. In this study, dynamic deformation and fracturing behaviour of ordinary concrete under biaxial confinements are investigated by using a triaxial Hopkinson bar system, three-dimensional digital image correlation, and synchrotron X-ray computed tomography techniques. Results show that compressive strain localisation areas appear around aggregates due to the elastic difference between aggregate and matrix, where accompanied with initiation of interfacial cracks and propagation of main cracks. Transgranular cracks can also be observed in the central of specimen in CT slices due to the effect of strain concentration. In addition, CT slices with distinct properties in various directions indicate the anisotropic fracturing behaviour of concrete due to the effect of biaxial confinements.

Multi-Dimensional Study of the Effect of Early Slip Activity on Fatigue Crack Initiation in a Near-α Titanium Alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-α titanium alloy using cyclic four-point bending at up to 90% of the proof stress, with the finding from surface characterization that plasticity at such low stress levels was dominated by the basal slip and two types of cracking were seen parallel to basal slip traces. Detailed 3D analysis of both crack types highlighted out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The transgranular crack initiation was accompanied by the formation of crystallographic facet which was identified to be 6° away from the basal plane due to additional prismatic slip activation during multi-step crack formation. The intergranular crack facet along the boundary between primary α grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes, was formed by an easy cleavage in one step along the basal plane. Statistical evaluation demonstrated that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being orientated strongly out-of-surface plane favoured crack initiation. Based on those observations a new parameter involving these three geometrical factors was developed to predict surface crack initiation sites.

Investigation on Internal Crack Defects in Medium Carbon Steel by Soft Reduction

This paper investigated the effect of soft reduction amount increasing from 1-4mm with a cooling time of 120s and 180s to reduce the internal crack defects by analyzing the microstructure, crack morphology, extent and nature of grain boundary ferrite, centre macrosegregation, and fracture surfaces in medium carbon steel. It was found that the large extent of grain boundary ferrite with coarse morphology weakens the bonding force of the PAGB and makes it brittle with the initiation of micro-cracks. The sever centre carbon macrosegregation of 1.3 takes place at cooling time of 180s with different reduction amounts, which becomes homogeneous at cooling time of 120s. The results show that the length of inter-granular and trans-granular cracks reaches 20mm with the carbon macrosegregation index of 1.3. The carbon macrosegregation index of 1.3 with a small fraction of 15% is enough to deteriorate the centre macrostructure of the ingot. The cleavage facet size of fracture surfaces is remarkably decreased below 100um at soft reduction parameters of 2mm/120s, 3mm/120s, and 4mm/120s with micro-cracks less than 100um, which reveals that the microstructure of specimens with these soft reduction parameters is more resistant to the cracks propagation.

Multi-Dimensional Study of the Early Slip Activity on Crack Initiation in a Near α Titanium Alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-a titanium alloy using cyclic four-point bending at up to 90% of the proof stress. Detailed surface characterization demonstrates that plasticity at such low stress levels is dominated by basal slip. Two types of crack mode were seen among four short cracks. Transgranular cracking parallel to basal slip traces was observed within three primary a grains and one case of intergranular cracking was seen along the boundary between primary a grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes. Detailed 3D analysis highlights out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The plane of the short crack was identified to be a basal facet and lattice rotation around the c-axis close to the crack plane suggests additional prismatic slip activation during multi-step crack formation. Statistical evaluation highlighted that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being orientated strongly out-of-surface plane favour crack initiation. Based on those observations a new parameter involving these three geometrical factors has been developed that predicts surface crack initiation sites.

In situ He+ irradiation of the double solid solution (Ti0.5,Zr0.5)2(Al0.5,Sn0.5)C MAX phase: Defect evolution in the 350–800 °C temperature range

Thin foils of the double solid solution (Zr0.5,Ti0.5)2(Al0.5,Sn0.5)C MAX phase were in situ irradiated in a transmission electron microscope (TEM) up to a fluence of 1.3 × 1017 ions⋅cm-2 (∼7.5 dpa), using 6 keV He+ ions. Irradiations were performed in the 350–800 °C temperature range. In situ and post-irradiation examination (PIE) by TEM was used to study the evolution of irradiation-induced defects as function of dose and temperature. Spherical He bubbles and string-like arrangements thereof, He platelets, and dislocation loops were observed. Dislocation loop segments were found to lie in non-basal-planes. At irradiation temperatures ≥ 450 °C, grain boundary tearing was observed locally due to He bubble segregation. However, the tears did not result in transgranular crack propagation. The intensity of specific spots in the selected area electron diffraction patterns weakened upon irradiation at 450 and 500 °C, indicating an increased crystal symmetry. Above 700 °C this was not observed, indicating damage recovery at the high end of the investigated temperature range. High-resolution scanning TEM imaging performed during the PIE of foils previously irradiated at 700 °C showed that the chemical ordering and nanolamination of the MAX phase were preserved after 7.5 dpa He+ irradiation. The size distributions of the He platelets and spherical bubbles were evaluated as function of temperature and dose.

Analysis of Fatigue Indicator Parameters for Ti-6Al-4V microstructures using extreme value statistics in the HCF regime

Fatigue Indicator Parameters (FIPs) have been introduced to serve as surrogate measures of driving force to form and grow fatigue cracks. With Ti-6Al-4V, we consider that transgranular cracks may form along slip bands due to irreversible cyclic plasticity or disruption of phase boundaries impinged by dislocation pile-up under mean stress. FIPs that capture these mechanisms are examined over several microstructure and loading conditions in the High Cycle Fatigue (HCF) regime. The peaks-over-threshold approach determines extreme FIP values and rank-orders fatigue behavior of microstructures. It is shown that fatigue crack formation is dominated by alpha-phase grains in the HCF regime.