On the tensile behaviors of 2D twill woven SiO2f/SiO2 composites at ambient and elevated temperatures : mesoscale analysis and in situ experimental investigation

SiO2f/SiO2 composites are among the most ideal high-temperature wave-transparent materials used in hypersonic vehicles. The purpose of the study is the thorough experimental investigation of the tensile behavior of a 2D twill woven SiO2f/SiO2 composite, and the establishment of an accurate and efficient simulation method for such composites. The digital image correlation (DIC) method was utilized to capture local deformation data during tensile tests. Meanwhile, a progressive failure analysis (PFA) model employing the exponential damage evolution law was subsequently developed with UMAT in ABAQUS. Simulations of the mechanical properties and strain distributions show good consistency with experimental results. The results at room temperature and 900 °C demonstrate that the strain distributions exhibit obvious periodic patterns related to the woven structure. In addition, band-shaped strain concentrations can be observed at the intersection zones between adjacent longitudinal and transverse fiber bundles. These zones are regarded as critical regions. This was validated by the damage evolution observed in the simulations. Owing to the grain coarsening of quartz fibers and the embrittlement of different constituents at 900 °C, notable degradation of the mechanical properties and brittle fracture characteristics were observed

The effect of off-axis angles on the mesoscale deformation response and failure behavior of an orthotropic textile carbon-epoxy composite

The deformation response and failure behavior of an orthotropic textile carbon-epoxy composite were investigated under off-axis tensile loading. Digital image correlation (DIC) was utilized to effectively capture the full-field and mesoscale strain distribution. The macroscale mechanical performance was strongly sensitive to the fiber bundles orientation relative to the loading direction. Based on the experimental data, a quantitative relation between the rotation angle and off-axis angle was established, and a negative correlation between the failure strength and the rotation angle was observed. The underlying failure mechanisms of the specimens with different off-axis orientations were analyzed using scanning electron microscopy (SEM) and DIC techniques. The load-bearing mechanisms were different between the on- and off-axis cases. High local shear strain eventually resulted in the brushy shear-type fracture in the off-axis case, and the local micro cracks developed during the loading caused the reduction and imbalance of local load-bearing capacity

The setup and application of the multi-scale in-situ test system for fatigue damage analysis

This essays aims at introducing the setup for the multi-scale in-situ test system which is applied for the fatigue crack initiation test. The setup of the experiment system is first introduced, including the image capturing system, optical path system, image acquisition and storage system and the three-axis mobile platform. Then the preparation of micro speckle and the corresponding technique for spatial adjustment are improved to realize the DIC measurement in micro scale. Finally three experiments from macro-meso scale to macro scale: fatigue initiation test, the observation of micro crack and the fatigue crack growth rate in macro scale were conducted to verify the application of the system. The test result can indicate the location of crack initiation, the crack growth rate and the evolution of displacement/strain field, etc

A Study on Establishing a Microstructure-Related Hardness Model with Precipitate Segmentation Using Deep Learning Method

This paper established a microstructure-related hardness model of a polycrystalline Ni-based superalloy GH4720Li, and the sizes and area fractions of γ’ precipitates were extracted from scanning electron microscope (SEM) images using a deep learning method. The common method used to obtain morphological parameters of γ’ precipitates is the thresholding method. However, this method is not suitable for distinguishing different generations of γ’ precipitates with similar gray values in SEM images, which needs many manual interventions. In this paper, we employ SEM with ATLAS (AuTomated Large Area Scanning) module to automatically and quickly detect a much wider range of microstructures. A deep learning method of U-Net is firstly applied to automatically and accurately segment different generations of γ’ precipitates and extract their parameters from the large-area SEM images. Then the obtained sizes and area fractions of γ’ precipitates are used to study the precipitate stability and microstructure-related hardness of GH4720Li alloy at long-term service temperatures. The experimental results show that primary and secondary γ’ precipitates show good stability under long-term service temperatures. Tertiary γ’ precipitates coarsen selectively, and their coarsening behavior can be predicted by the Lifshitz–Slyozov encounter modified (LSEM) model. The hardness decreases as a result of γ’ coarsening. A microstructure-related hardness model for correlating the hardness of the γ’/γ coherent structures and the microstructure is established, which can effectively predict the hardness of the alloy with different microstructures

The setup and application of the multi-scale in-situ test system for fatigue damage analysis

This essays aims at introducing the setup for the multi-scale in-situ test system which is applied for the fatigue crack initiation test. The setup of the experiment system is first introduced, including the image capturing system, optical path system, image acquisition and storage system and the three-axis mobile platform. Then the preparation of micro speckle and the corresponding technique for spatial adjustment are improved to realize the DIC measurement in micro scale. Finally three experiments from macro-meso scale to macro scale: fatigue initiation test, the observation of micro crack and the fatigue crack growth rate in macro scale were conducted to verify the application of the system. The test result can indicate the location of crack initiation, the crack growth rate and the evolution of displacement/strain field, etc

The study of the relationship between life limiting factor and stress level for FGH96

FGH96 is a Chinese made powder metallurgy nickel based superalloy. This reserach aims to investigate the effect of stress on the microstructure of the worst fatigue life at 600°C. The specimens were first polished by abrasive paper and then electropolished to elimate the effect of surface residual stress. The fatigue tests were conducted at 600°C, and the loading ratio was 0.05 at the frequency of 5Hz. Tests were conducted at 900, 1000, 1100, 1200MPa respectively. The fatigue life under the same condition were compared and the fractography of specimens were examined under SEM to further identify the life-limiting factors of the material. The initiation mode transferred from facet initiated to inclusion initiated from lower stress level to higher stress level. Meanwhile, the initiation position transferred from internal to surface with the increase of stress level. This means at higher stress level, fatigue life is mainly limited by inclusion at surface while at lower stress level the limit is controlled by internal facet

Effect of surface treatment and synergistic effect of O and S elements on hot corrosion behavior of GH738 superalloy

The hot corrosion behavior of GH738 superalloy after solution heat treated (SHT), room temperature machining (RTM) and low temperature burnishing (LTB) coated mixed salt (25% NaCl + 75% Na2SO4) on its surface was investigated at 700 °C. The results showed that the surface grain refinement of the LTB specimens and produced numerous multiple nano-twins, which was conducive to the diffusion of Cr and Al elements from the inside to the outside of the matrix. Dense and continuous Cr2O3 and Al2O3 layers were formed on the surface of the corrosion product layer and between the external corrosion layer and the surface of the matrix, respectively, which improved the hot corrosion resistance. The mechanism of the synergistic effect between O and S elements during corrosion process was found. The sulfide (SO3) reacted with the alloy elements in the matrix to form metal oxides and S, and this process promoted the diffusion of O element into the matrix. The O2 entering the internal corrosion layer and the O2 generated by the decomposition of SO3 together cause the partial pressure of oxygen in the matrix to rise, promoting the reoxidation of the metal sulfide to release S element. The oxygen in the matrix is consumed, resulting in the decrease of oxygen partial pressure. This leads to the reaction between S element and metallic elements with strong affinity to generate metal sulfide (such as TiS). In this process, O element promotes the diffusion of S element to the matrix

The study of the relationship between life limiting factor and stress level for FGH96

FGH96 is a Chinese made powder metallurgy nickel based superalloy. This reserach aims to investigate the effect of stress on the microstructure of the worst fatigue life at 600°C. The specimens were first polished by abrasive paper and then electropolished to elimate the effect of surface residual stress. The fatigue tests were conducted at 600°C, and the loading ratio was 0.05 at the frequency of 5Hz. Tests were conducted at 900, 1000, 1100, 1200MPa respectively. The fatigue life under the same condition were compared and the fractography of specimens were examined under SEM to further identify the life-limiting factors of the material. The initiation mode transferred from facet initiated to inclusion initiated from lower stress level to higher stress level. Meanwhile, the initiation position transferred from internal to surface with the increase of stress level. This means at higher stress level, fatigue life is mainly limited by inclusion at surface while at lower stress level the limit is controlled by internal facet

Influence of orientation and temperature on the fatigue crack growth of directionally solidification superalloys DZ125

The influence of orientation and temperature on the fatigue crack growth of a directionally solidification superalloys named DZ125 were investigated in this study. The specimens were designed based on ASTM E647 standard and the compliance method was employed to monitor the crack growth. Tests were conducted at room temperature, 760°C and 850°C respectively. The longitudinal and transverse orientation were both studied in this tests. Results show that the orientation dependence is weak at elevated temperature compared with that at room temperature. However, the temperature influences the fatigue crack growth rate significantly and the largest difference can be an order of magnitude on the da/dN-ΔK graph. But the reduction in higher ΔK area of elevated temperature can still be observed. Potential reasons and hypothesis were explained in this paper. In addition, the crack growth mode appears to transfer from type I with the increase of temperature which also reflects the influence of temperature