A new test specimen for mixed-mode I+II fracture studies is proposed. An extended version of edge notch disc bend (ENDB) specimen, which was recently proposed to assess mixed-mode I+III fracture ...toughness, is designed and developed for introducing opening-in-plane-shear sliding mode. This new specimen that is called asymmetric ENDB specimen (A-ENDB), was numerically analysed to determine the stress intensity factors and T-stress along the crack front for a wide range of geometric and loading conditions. The finite element results demonstrated that (i) the contribution of mode III component is negligible under asymmetric loading of ENDB specimen and; (ii) the full modes I and II mixities including pure mode I, pure mode II and intermediate mode mixities can be achieved by changing loading support distances. The capability of the proposed asymmetric ENDB specimen was then experimentally assessed by conducting several mixed-mode I+II fracture toughness tests on a brittle model polymer (PMMA). The mode II fracture toughness value obtained from the A-ENDB specimen was noticeably greater than KIc value. Although the fracture trajectory of all A-ENDB specimens was nearly self-similar and along the plane of initial crack, different fracture surface morphologies were observed for mixed-mode I+II. The fracture surface consists of mirror area and ridge markings. While, towards to pure mode II, both density and length of the ridge marking were increased. It is shown that by using the same shape test specimen, loading type, and test configuration, both mixed-mode I+II and I+III can be studied.
Developing ultrahigh strength steels that are ductile, fracture resistant, and cost-effective would be attractive for a variety of structural applications. We show that improved fracture resistance ...in a steel with an ultrahigh yield strength of nearly 2GPa can be achieved by activating delamination toughening coupled with transformation induced plasticity. Delamination toughening associated with intensive but controlled cracking at Mn-enriched prior-austenite grain boundaries normal to the primary fracture surface dramatically improves the overall fracture resistance. As a result, fracture under plane-strain conditions is automatically transformed into a series of fracture processes in "parallel" plane-stress conditions through the thickness. The present "high-strength induced multi-delamination" strategy offers a different pathway to develop engineering materials with ultra-high strength and superior toughness at economical materials cost.
In the present paper, the microstructural evolution and tensile properties of additively manufactured stainless steel CX were investigated. Using scanning electron microscope (SEM), several powder ...particle morphologies were identified in the stainless s steel CX feedstock powder where the spherical morphology was found to be the dominant one. In addition, X-ray diffraction (XRD) technique detected austenite and martensite phases in both stainless steel CX powder and as-built sample, whereas no carbide peak appeared on the XRD patterns. Moreover, lath or needle-like martensite phase was observed in the microstructure of the as-built sample. The level of porosity was very low in the as-built sample, indicating the manufacturing of a nearly fully dense sample. Furthermore, a high ultimate tensile strength together with a good elongation to fracture was obtained for the horizontally-built stainless steel CX sample. Finally, examination of the fracture surfaces after tensile tests confirmed the ductile failure mode of the samples, in which the pull-out of the scan tracks and coalescence of the voids resulted in the tear and final rupture. This study demonstrates the successful additive manufacturing of stainless steel CX with outstanding tensile properties.
•The self-healable glass fibers-epoxy composites were fabricated.•The composites containing 14 wt% microcapsules had optimum results.•The maximum healing efficiency in tensile strength was 97.5%.•The ...maximum healing efficiency in interlaminar shear strength was 144.5%.•The crack pining and deflecting by microcapsules were observed.
In the present study, the self-ability based on the microcapsule healing system on the glass fibers-epoxy smart composites was investigated. The microcapsules’ shell and healing agent comprised urea-formaldehyde and diluted epoxy, respectively. The synthesized microcapsules by one-stage in-situ polymerization method were dispersed with the different percentage of 7, 14 and 21 wt% into the epoxy resin. Then, the epoxy-glass fibers composites with the self-healing ability were fabricated by hand lay-up method. The self-healing composites damaged by various damage forces through a quasi-static penetration method. Therefore, the damage forces of 800, 1050 and 1300 N for tensile samples and 2500, 2900 and 3300 N for interlaminar shear strength (ILSS) samples were selected. The obtained results showed that adding 14 wt% microcapsules into the composite increased the tensile strength, due to the crack pining and deflection mechanisms. Whereas, by increasing the percentage of microcapsules up to 21 wt%, the shear strength of composite was reduced, compared with the pristine composite. The maximum obtained healing efficiency for both tensile and ILSS tests were 97.5 and 144.5%, respectively, which belonged to 21 wt% microcapsules-loaded composite. The microscopically investigations of interactions between microcapsules and micro-cracks at the fracture surface illustrated the crack pinning, deflecting, and bowing mechanisms. Also, rupturing the microcapsules, wetting the damaged area by healing agent and forming the polymeric shell in the damaged area were seen.
•The effect of high temperature on the changes in the mineral composition of granite and green sandstone was investigated.•The variation laws of crack propagation paths, fracture toughness and micro ...fracture surfaces of granite and green sandstone specimens under different temperature were obtained.•The relationship between the fractal dimension and the fracture toughness of granite and green sandstone specimens after thermal treatment was built.
As rock materials suffered from high temperatures, their mechanical properties, and physical parameters will be changed greatly, thus temperature has a great influence on the stability of geological engineering. In this study, a cracked straight-through Brazilian disc (CSTBD) specimen was applied to investigate the fracture behavior of two types of rock materials (hard rock and soft rock) under high temperature, which was made of granite and green sandstone and heating temperature from 25 °C (room temperature) to 700 °C. The changes in the mineral component of the granite and green sandstone materials with the temperature were obtained. The electro-hydraulic servo press was used for uniaxial compression tests, scanning electron microscope (SEM) was also used to observe fracture morphology and fractal dimension was calculated after high-temperature treatment, the relationship between the macro and microdamage mechanism of high-temperature treatment on rock materials was built. The experimental results can conclude that: (1) High temperature has a great effect on the changes in rock mineral components. After thermal treatments, the biotite in the granite is reduced, and the nimite in the green sandstone is reduced. (2) It can be found that the static fracture toughness of granite falls far more than that of green sandstone. When the temperature is over 600 °C, granite deformation becomes an obvious plastic failure feature and when the temperature is over 500 °C, green sandstone deformation becomes an obvious plastic failure feature. (3) The fractal dimension of the fracture surfaces of the green sandstone and granite is inversely proportional to the static fracture toughness. The fractal dimension of the green sandstone is larger than that of the granite.
In this study, the effects of micro-shot peening and artificial defect on the fatigue properties of EA4T railway axle steel were evaluated. Un-peened (UP) and micro-shot peened (MSP) EA4T axle steel ...specimens were drilled to introduce artificial defects on the surface. Then the fatigue tests were conducted on a rotary bending fatigue machine (R = −1). The test results showed that the fatigue limit of the UP specimens was improved by 25% after MSP; when the equivalent defect sizes were less than 60 and 70 μm respectively, they had no negative effect on the fatigue strength of the UP and MSP drilled specimens, otherwise the fatigue strength of both specimens decreased with the increase of the defect size. The fracture surfaces were analyzed by scanning electron microscopy (SEM) observations, which indicated that crack initiation sites for the as-prepared UP and MSP specimens were on the surface, while for the drilled specimens, they were on the vicinity of hole defects. The effect of artificial defect size on the fatigue strength of the UP and MSP specimens and the critical defect size were analyzed with fracture mechanics approach, the results of which showed that the relationship between defect size and fatigue strength of the UP and MSP specimens can be well evaluated by the modified Haddad model, and the critical defect size of the UP specimens was 65 μm while that of the MSP specimens was increased to 76 μm due to the introduction of compressive residual stress.
Due to the difficulties of applying torsional loads to introduce mode III effects, there are few studies for investigating mixed mode I/III fracture in brittle materials. In this paper using three ...novel single or double edge‐cracked diametral compression disc shape specimens, complete ranges of mixed mode I/III are introduced. Fracture factors of the compressed disc specimens with different pre‐notch shapes are determined numerically for a wide range of notch depth and notch inclination angles. The ability of specimens was studied using mixed mode I/III fracture experiments on gypsum. Different fracture envelopes were obtained for the gypsum demonstrating the influence of disc geometry and notch type on mixed mode I/III behavior. For all samples, mode III fracture toughness (KIIIc) was approximately 1.2 to 1.8 times the corresponding value of KIc. Depending on the crack front type, ligament shape and mode mixity, different fracture surfaces were observed. While the mode I fracture surface was smooth and flat in all samples, for mixed mode I/III and mode III, rotation and partially segmentation saw tooth shape wrinkles were observed.
Structural topology optimization in the context of material degradation and fracture has been gaining considerable interest. In this work we explore the use of the phase field method for fracture in ...order to increase the fracture resistance, or strength, of a structure prior to failure by directly constraining the phase field approximation of the fracture surface energy. A density-based topology optimization framework is used and the total weight of the structure is minimized. The analytical sensitivities are derived and an efficiency gain based on the Schur-complement is presented for the computation of the sensitivities. The effectiveness of the proposed method is then demonstrated for two benchmark examples.
The generalized model and parameter equation for the internal fracture surface of tensile type anchor cable anchorage section are constructed, and the calculation model of ultimate bearing capacity ...is derived based on the principle of limit equilibrium. The shape of the internal fracture surface in the anchorage section and the expression of its parameter equation are experimentally studied, and the variation law of the parameter equation and fracture range with the diameter of the anchor cable and confining pressure is analyzed. The parameter equation is substituted into the calculation model to realize the calculation and verification of the model. Through theoretical derivation, a calculation model of ultimate bearing capacity of anchor cable is obtained, which comprehensively considers the average shear strength of anchorage section, the shape of fracture surface, fracture range and confining pressure. The tests show that according to the magnitude of the confining pressure on the anchorage section, the internal fracture surface takes on two shapes: cylindrical and trumpet‐shaped, the parameter equation of trumpet‐shaped fracture surface obeys logarithmic distribution approximately. The coefficient a decreases with the increase of confining pressure but increases with the increase of anchor cable diameter, the constant b increases with the increase of confining pressure but decreases with the increase of anchor cable diameter. The ranges of horizontal and vertical fracture decrease with the increase of confining pressure, and increase with the increase of anchor cable diameter. The deviations between the theoretical calculation and the experimental results are less than 6.5%.
Pelletization of biomass reduces its handling costs, and results in a fuel with a greater structural homogeneity. The aim of the present work was to study the strength and integrity of pellets and ...relate them to the quality and mechanisms of inter-particular adhesion bonding. The raw materials used were: beech, spruce and straw, representing the most common biomass types used for fuel pellet production, i.e. hardwoods, softwoods and grasses, respectively. The results showed that the compression strengths of the pellets were in general higher for pellets produced at higher temperatures, and much higher for wood pellets than for straw pellets. Scanning electron microscopy of the beech pellets fracture surfaces, pressed at higher temperatures, showed areas of cohesive failure, indicating high energy failure mechanisms, likely due to lignin flow and inter-diffusion between adjacent wood particles. These were absent in both spruce and straw pellets. Infrared spectroscopy of the fracture surfaces of the straw pellets indicated high concentrations of hydrophobic extractives, that were most likely responsible for their low compression strength, due to presence of a chemical weak boundary layer, limiting the adhesion mechanism to van der Waals forces. Electron micrographs indicating interfacial failure mechanisms support these findings. Infrared spectra of the fracture surface of wood pellets, pressed at elevated temperatures, showed no signs of hydrophobic extractives. It has been shown that both temperature and chemical composition, i.e. the presence of hydrophobic extractives, have a significant influence on the bonding quality between biomass particles during the pelletizing process.
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