Fracture Mechanism Of Specimens Research Articles

Dynamic fracture mechanism and behaviour of V-notch white marble specimens with strain gauge and digital image correlation in three-point bending tests

A three-point bending test was conducted on white marble specimens with V-shaped notches. The fracture mechanism of these specimens, including crack initiation toughness and propagation speed under different opening angles, was studied, and their special fracture behaviour was analysed. In this experiment, a falling weight loading device was used to impact the V-notch white marble specimens with different opening angles at the same impact speed. To measure the crack tip initiation toughness, a strain gauge theory is formulated in this study. The calculation results are in good agreement with the digital image correlation (DIC) calculation results. The error is approximately 10%, and the results obtained by the acute angle patching method are stable. The experimental results also show that when the opening angle is large, the specimens crack, and the energy dissipated by the crack initiation is considerable. Owing to the particularity of the white marble structure, the rock fracture behaviour during the dynamic fracture process is clearly observed in the experiment. All the specimens exhibited spreading crack branches at the crack tip under stress. The foregoing occurs because the original defects at the crack tip deform and grow after sustaining stress. Some of the cracks combine to form the main crack and produce branches. In this study, the microcrack areas on the crack tip and fracture path observed in the experiment are collectively referred to as the crack growth area. The expansion of the crack growth area precedes the expansion of the main crack. The expansion of the strain concentration area is calculated by DIC; it reflects the dynamic formation of cracks on the surface of the specimen. Scanning electron microscopy shows that microcracks mainly occur under the intergranular failure mode.

Mechanical behaviors and fracture mechanisms of CFRP sandwich composite structures with bio-inspired thin-walled corrugated cores

This paper investigates mechanical behaviors and fracture mechanisms of bio-inspired thin-walled corrugated-core sandwich composite structures made of carbon fiber-reinforced polymer (CFRP) under four-point bending and flatwise compression. Biomimetic thin-walled corrugated cores, inspired by nature's structures, were proposed and manufactured using the hot vacuum molding method. Quasi-static bending and compression tests were conducted to characterize the load capacity, compressive strengths, and failure progressions with the assist of the digital image correlation and thermography method. Measured full-field strains revealed critical bending/compression strains of the samples while a temperature increase was observed in full-field thermal maps, which was a sign of crack initiation. We concluded that the fracture mechanisms of specimens in bending tests depend on the fiber orientations of the core. The sandwich structure with a core angle of ±45 degrees exhibited a larger load capacity and bending stiffness than the structure with a core angle of 0/90 degrees. The cracks were mostly initiated at the core (0/90 degrees) and the facesheet (±45 degrees) with a core crushing and facesheet fracture, respectively, and propagated in the core with shear cracks due to fiber fractures. In addition, the failure of the thin-walled corrugated cores in flatwise compressions arose first by buckling and was followed by the subsequent collapse of the cores. The theoretical calculation provided a useful understanding of the different failure mechanisms of the sandwich structure. The proposed sandwich structures achieved prominent mechanical performances that were 26.5 MPa/gcm−3 in bending and 11.8 MPa/gcm−3 in compression and the experimental technique can depict the successive events that lead to the complex fracture of thin-walled corrugated core sandwich composites.

Assessment of cold resistance and fracture mechanisms of metals obtained by 3D-printing

The cold resistance and fracture mechanisms of specimens made of 08Г2С and 07Х25Н13 steels obtained by 3D printing by electric arc surfacing at low temperatures are considered. It is determined, that with a decrease in temperature, the impact toughness of steels decreases. The impact toughness of specimens cut along the deposition direction is higher than that of specimens cut in the transverse direction. It is shown, that the brittle component prevails in the fracture of 08Г2С steel at temperatures below –40 °C, and the viscous component is observed in the fracture of 07Х25Н13 steel over the entire temperature range. A relationship is established between the fractal dimension of the fracture surface and the amount of the viscous component. Keywords: 3D printing, cold resistance, fracture mechanisms, ductile-brittle transition, fractal dimension. uru.40@mail.ru

ВЛИЯНИЕ ПОСЛЕСВАРОЧНОГО ОТПУСКА НА МЕХАНИЧЕСКОЕ ПОВЕДЕНИЕ ФРИКЦИОННЫХ СВАРНЫХ СОЕДИНЕНИЙ СТАЛЕЙ 32Г2 И 40ХН В УСЛОВИЯХ МНОГОЦИКЛОВОЙ УСТАЛОСТИ

At modern mechanical facilities, the friction-welded joints are getting widespread as the most advanced production technique characterized by high efficiency, processability, cost-effectiveness, and safety. Moreover, it allows producing high-quality joints of a large number of different analogous and opposite metals and alloys. Despite all these advantages, one should consider that metal, in the process of welded joint formation, suffers a local thermo-deformational effect, which causes the gradient nature of the structure and residual strains of a welded joint. These factors directly influence the structure’s working ability and durability under fatigue loads, which are the most common cause for parts failure. The paper contains the assessment of the post-weld tempering influence on the cyclic life of welded joints of 32G2 and 40HN steels produced using the rotational friction welding technique. The authors tested laboratory specimens with welded joints under the high-cycle fatigue using the simulation machine with the two-point fastening of a revolving specimen under the action of even twisting moment. The study involved the statistical processing of the obtained results of cyclic life. Based on the metallographic analysis, the authors identified the weak points in welded points where the fatigue cracks initiation and progress occurred in the initial state and after tempering. The paper presents the fractographs illustrating the fracture mechanism of specimens under the study. The authors identified the influence of different tempering temperature modes on the cyclic life of the studied welded joints and the nature of their fracture. The study shows that tempering at the temperature over 400 °C promotes fracture acceleration under the effect of fatigue loads due to the development of return and polygonization processes in the vulnerable area of the thermomechanical action zone.

Microstructural tailoring of As-Selective Laser Melted Ti6Al4V alloy for high mechanical properties

As-Selective Laser Melted (as-SLMed) Ti6Al4V in horizontal direction possess poor ductility because of acicular α’ martensite and columnar β grains in microstructure. However, traditional ductility improvement methods including high-temperature preheating or heat treatment will decrease not only strength but also the efficiency of SLM fabrication to application. For a satisfactory mechanical property of as-SLMed Ti6Al4V alloy, some special temperature evolution conditions brought about from processing parameters such as layer thickness, hatch spacing, energy density and area ratios between support structure and part, were carried out in the paper. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the microstructure. The microstructure of as-SLMed specimens are found to be different extent decomposing of α’ martensite. Also, the formation mechanisms of such microstructural variables are proposed based on temperature evolution vs time during SLM process. Moreover, the tensile test presents high elongation (>8%) is achieved for horizontally as-SLMed Ti6Al4V specimens without lowering their strength (the ultimate tensile strength ∼ 1260 ± 30 MPa and yield strength ∼1160 ± 50 MPa), which markedly exceed ASTM standard for castings. The mechanical properties and fracture mechanisms of specimens are obviously affected by the width of α’/α martensite and the content of β phase. This work marks an important step forward in the understanding of microstructural tailoring in SLM process for a high-performance Ti6Al4V alloy.

Corrosion and Mechanical Degradation of Ni-Ti Superelastic Alloy in Neutral Fluoride Solution

The corrosion behavior and mechanical degradation of Ni-Ti superelastic alloy in neutral fluoride solution were investigated with a focus on the effect of temperature during immersion. Corrosion was accelerated by increasing the temperature, and mass loss increased with immersion time. Furthermore, Ti ions (Ni ions) preferentially dissolved in the early (later) stage of immersion. The tensile strength of the immersed specimen decreased when the mass loss was approximately 0.040-0.050 mg/mm3. The fracture mechanisms of specimens immersed in 2.0% NaF solution were active path corrosion at room temperature and presumably hydrogen embrittlement at 80 °C. For long-term (e.g., 180 days) immersions at 80 °C, hydrogen embrittlement did not occur, because the absorbed hydrogen diffused to the center of the immersed specimens, rendering the hydrogen distribution comparatively uniform. These findings suggest that the fracture mechanism of immersed specimens can change as a result of the temperature during immersion.

Internal Morphology of Cracking of Two 3-D Pre-Existing Cross-Embedded Flaws under Uniaxial Compression

Abstract This paper studies the internal morphology of cracking of two 3-D pre-existing cross-embedded flaws in poly(methyl methacrylate) specimens under uniaxial compression. In this study, observable cracking behaviors include the crack initiation, growth, wrapping and coalescence, and the ultimate fracture of the specimen. The experimental results show six modes of crack initiation and two types of crack wrapping. The initiation of silky cracks in 3-D cracking behaviors is reported first, as well as the closed-mode wrapping of petal cracks. Substantial wrapping of petal cracks, which plays an important role in crack coalescence, appears to be the major difference between the 2-D and 3-D cracking behaviors of pre-existing flaws. Moreover, three modes of crack coalescence, which include the coalescence of petal cracks, antiwing cracks, and oblique secondary cracks, are found. Crack coalescence is predominantly created by the linkage of petal cracks, which is further classified into six categories. Note that only limited vertical growth of petal cracks is observed, and the ultimate failure of specimens is induced by the continued growth of the propagated wing crack. It was found that the ultimate fracture face of the specimens consists of continuous wing cracks and fragmented silky cracks. This paper provides the internal morphology basis for the 3-D fracture mechanism of specimens containing two 3-D pre-existing cross-embedded flaws.

On Fracture Mechanism of Specimens in the "Brazilian Test" Scheme

The fracture of a specimen loaded according to the "Brazilian test" has been studied with the use of the strength criterion with respect to the maximum shear stresses. A comparison made between the simulation results and the experimental data showed that the criterion proposed adequately describes the process of fracture.