This study presented a theoretical analysis and experimental investigation of the heat transfer mechanism in rigid insulation tile (RIT) materials for reusable aerospace vehicles. The structural characteristics of the materials with different orientations (in-plane (IP) and through-the-thickness (TTT)) and densities (similar to 400 kg/m(3) and similar to 200 kg/m(3)) were analyzed. A heat transfer test system was established to perform high-temperature experiments, from which the temperature distribution along the heat flow direction was obtained over time (0-1800 s). Experimental results showed that lower-density RIT materials in the TTT orientation exhibit better thermal insulation performance. An equivalent orthogonal unit cell model was proposed to characterize the structural features of RIT materials in different directions. A correction factor was introduced to modify the extinction coefficient, and a theoretical model was established to predict the thermal conductivity in various orientations. The effects of different structural parameters on heat conduction and radiation heat transfer were analyzed. At a hot-end temperature of 1273 K, the theoretical temperature distribution showed good agreement with experimental data, with temperature deviations ranging from -7.4% to 8.87% at various locations within the material. This work provides an effective tool for predicting the thermal insulation performance of RIT materials in different application environments.

This study utilized aramid pulp (AP) as an interlayer and employed the resin pre-coating (RPC) technique to enhance the compressive damage resistance and post-damage stability of carbon fiber reinforced polymers (CFRP) laminates in the anchor system. Compressive and tensile tests were used to simulate the mechanical behavior of CFRP laminates in prestressed unbonded reinforcement (PUR) systems. Test results demonstrate that the introduction of AP can improve the compressive damage resistance and post-damage stability of CFRP laminates, and the RPC solution seems to further enhance this effect. In particular, after the anchorage compression load of 250 kN, the CFRP(AP) specimens and CFRP(AP + RPC) specimens exhibited the most significant enhancement in tensile strength, with increases of 16.2 % and 26.6 %, respectively, compared to the original CFRP specimens (CFRP specimens without any interlayer strengthening treatment). Additionally, cyclic tensile tests were conducted on specimens subjected to an anchorage compression load of 250 kN, and their stress-strain characteristics were analyzed. Test results indicate that the incorporation of AP can significantly stabilize the damaged structure of CFRP laminates, thereby enhancing their compressive damage resistance and post-damage stability. The above findings suggested that the mechanical properties of CFRP laminates were significantly undermined by high anchor pressures, while the AP and RPC treatments between the interlayers could substantially enhance the compressive damage resistance and post-damage stability of CFRP laminates and enable them to withstand high compressive loads during the anchor process in the PUR system. Meanwhile, according to the stress-strain variation trend of CFRP specimens under cyclic tensile tests, monitoring the stress-strain behavior of CFRP laminates can serve as a valuable reference scheme for assessing whether CFRP laminates are damaged in practical engineering applications.

This work utilizes an enhanced elevated-temperature split Hopkinson pressure bar (SHPB) experimental system to perform three-point-bending (3-p-b) tests on rigid insulation tile (RIT) materials with a porosity of similar to 87 %, assessing fracture toughness across a range of extreme temperatures spanning from 78 K to 1423 K. Based on the regulation of temperature change on fiber spacing and the influence of inertia effect, the application of boundary effect model (BEM) in dynamic elevated-temperature environment is expanded. Moreover, the experimental results indicate that the fracture toughness of RIT materials significantly dependent on temperature and loading rate. For example, from 293 K to 78 K, the fracture toughness at various loading rate increased significantly, with a maximum increase of similar to 34.69 %. The temperature rises to the viscous-brittle transition temperature (similar to 973 K). Viscous flow and micro-crack self-healing lead to a significant increase in fracture toughness from 973 K to 1173 K at various loading rates, with an increase of more than 25.97 %. Near the raw material firing temperature (similar to 1473 K), the dominant fracture mode of fiber compaction and softening changed fundamentally, resulting in a significant decrease in fracture toughness at various loading rates, with a maximum decrease of 33.77 %. The fracture analysis results show that the significant difference in crack propagation mode and fiber fracture mechanism causes the loading rate sensitivity of fracture toughness. These findings will provide an important reference for evaluating the fracture properties of RIT materials under extreme temperature and high loading rate scenarios.

Rigid insulation tile (RIT) material is a kind of thermal insulation material with excellent performance, which is widely used in reusable aerospace vehicles. An improved split Hopkinson pressure bar (SHPB) device has been developed to study the dynamic fracture response of RIT specimens due to the random overlapping of internal fibers and high porosity (similar to 89 %). In addition, by proposing a 5 % secant method to determine the dynamic fracture load, the inertial effect in the dynamic loading process is considered, and the application field of the boundary effect model (BEM) is extended to high-rate loading. Macro/meso-structure fracture analyses were performed using an industrial camera and a scanning electron microscope (SEM), respectively. The test results show that the load-displacement curve, the fracture energy dissipation energy density and the fracture toughness predicted by the enhanced BEM of the RIT material have significant loading rate sensitivity. The source of the loading rate sensitivity is determined by the fracture analysis results. It is mainly due to the fact that the internal meso-structure of the material responds differently to different loading rates, resulting in significant differences in crack propagation modes and fiber fracture mechanisms. Finally, based on the fracture energy analysis results, the finite element analysis (FEA) methods of RIT materials under static/dynamic loading were established by using the extended finite element method (XFEM) and the cohesive zone model (CZM). The FEA results are in good agreement with the experiment and can simulate the failure process of the specimen.

The mechanical properties of rigid insulation tile (RIT) materials at elevated temperatures (700 1000 ℃) were studied through compression tests and the digital image correlation (DIC) method. To reduce measurement error in a thermal environment, an image gradient zero-mean normalized cross-correlation algorithm (ZNCCGI) was added to the DIC algorithm. The DIC algorithm was verified via RIT material mechanical tests at room temperature. Furthermore, the compressive stress–strain curves and Young's modulus of RIT materials at elevated temperatures were obtained. The experimental results show that the Young's modulus of RIT materials significantly increased at 800 ℃. Moreover, the compressive yield strength was significantly improved at 800 ℃, which resulted in a random distribution of ceramic fibers and viscous flow deformation at elevated temperatures. Scanning electron microscope analysis demonstrated that the compressive damage occurs due to the breaking of ceramic fibers.

To investigate the effect of volume fraction (VF) on the fracture properties of silicon carbide particle-reinforced 6061 Al matrix composites (SiCp/6061Al), we propose a model for calculating the grain size G and establish a boundary effect model (BEM) that incorporates the VF of SiCp. To validate the rationality of the proposed BEM, three-point-bending tests are conducted at three different temperatures on SiCp/6061Al with three distinct VFs of SiCp. The comparisons show that the calculation results of the proposed BEM are highly consistent with those obtained in accordance with American Society for Testing Materials (ASTM) standards. Our findings reveal that both the tensile strength and fracture toughness of SiCp/6061Al decrease with an increase in VF of SiCp, whereas these properties increase with rising temperature. Fracture analysis indicates a mixed fracture mode, encompassing reinforcement fracture, matrix fracture, and interface debonding. This study provides theoretical support and data references for understanding the influence mechanism of VF of SiCp on the fracture properties of SiCp/ 6061Al, thereby facilitating the design and optimization of these advanced materials.

In this study, experiments and finite element modeling (FEM) are performed to study the tensile mechanical properties of SiC particle-reinforced 6061 Al-matrix composites (SiCp/6061Al) at various volume fractions (VFs) of SiCp. The Young's modulus, yield strength, and tensile strength of SiCp/6061Al display an overall upward trend with the increment of the VF of SiCp. The fracture analysis results demonstrate that the fracture of SiCp/6061Al is a hybrid of matrix fracture, reinforcement fracture, and interface debonding. An algorithm is developed to construct the mesostructure of particle-reinforced composites, based on the random sequential absorption algorithm. The VFs of the particles of the representative volume element (RVE) model constructed using the novel algorithm are as high as 42%. The tensile mechanical properties of SiCp/6061Al are predicted by replacing the irregular particle reinforcements in SiCp/6061Al with spherical particles in the RVE model. Comparisons reveal that the stress-strain curves obtained via experiment and FEM are highly consistent in the elastic-plastic stage, which verifies the effectiveness and rationality of the novel algorithm. In this study, tensile experiments and finite element simulations are conducted to evaluate the tensile mechanical properties and fracture modes of SiC particle-reinforced 6061 Al-matrix composites with various volume fractions of SiCp.image (c) 2024 WILEY-VCH GmbH

As a critical component of a welded structure, the integrity of the welded joint significantly affects the safety and reliability of the structure in service. Therefore, it is essential to investigate the integrity of welded joints. However, given the harshness of standard measurements renders them unsuitable for application to small volume zones, welded structures or in-service structures. In this study, the fracture properties of a Q235 steel butt-welded joint was evaluated by boundary effect modelling (BEM) method and the ball indentation (BI) method. The results demonstrate that the fracture toughness distribution obtained by the two methods are consistent, with the highest fracture toughness in the weld metal, followed by the heat-affected zone and the base metal, and the lowest fracture toughness in the fusion zone, which is in agreement with the results of the metallographic testing. Furthermore, the relative errors of the zones obtained by the ball indentation method and the boundary effect modelling method ranging from 10.16 to 19.05%, which indicates safety margin ought to be considered for employing the BI method to determine fracture properties of in-service structures.

This work investigates the tensile mechanical properties of rigid insulation tile (RIT) materials at room temperature (26 C) and elevated temperatures (700-1000 C) through experimental and digital image correlation (DIC) methods. Experimental results indicate that the tensile strength of RIT materials changes nonlinearly with increasing temperature and that the primary fracture mode under tension load is fiber root fracture. In terms of the DIC algorithm, this work builds an elevated-temperature DIC test system. A zero-mean Gaussian filtering algorithm and an image gradient difference square sum algorithm (SSDGI) are proposed to improve the precision of the DIC algorithm. The derived tensile stress-strain curve of RIT materials reveals that the elastic modulus changes nonlinearly with increasing temperature. The reliability and precision of the DIC algorithm are confirmed through the tensile test of silicon carbide particle-reinforced aluminum matrix composites. Hence, this work provides guidance for evaluating mechanical properties of RIT materials under elevated temperature environments.

详细介绍了首次火星探测70m天线结构保型设计的实现方法和关键要素:用伞形支撑结构实现反射体等刚度支撑,达到结构变形均匀化,提高主反射面的吻合精度,为保型设计奠定坚实基础;用三自由度调整机构实现副反射面随动控制,补偿主反射面最佳吻合以及副面支撑结构变形带来的电性能下降,为保型设计提供有力保障.工程中用到的等刚度设计理念为国内鲜有的设计理念,对我国后期大口径天线设计具有参考意义.三自由度调整机构能够有效补偿副面支撑结构变形以及主反射面最佳吻合后焦点位置调整带来的电性能下降.

This study aimed to evaluate the fracture toughness of three-dimensional random fibrous (3D RF) materials with 87% porosity and shallow surface cracks at room temperature (20 & DEG;C) and cryogenic temperature (- 100 & DEG;C). The ratios of crack length to specimen width of three-point-bending (3-p-b) specimens were 0.2 and 0.5, respectively. The experimental results revealed that the average fracture toughness KIC at - 100 & DEG;C was higher than that at 20 & DEG;C. In addition, the experimental data were analysed using the boundary effect model (BEM), with almost all experimental data included in the upper and lower boundaries with 96% reliability, as specified by a normal distribution. Furthermore, based on the fracture toughness KQ computed using the formula of American Society for Testing Materials (ASTM), the expression of the fibre gap size G is derived. When the confidence level was 95%, no significant difference was observed in the fracture toughness model when the fibre gap size G was replaced with the average fibre spacing size D2 in the fracture toughness model. Thus, this study is expected to provide a reference for the safety assessment of the insulation layer of space shuttles at cryogenic temperatures.

为研究侧面涂层和应变隔离垫厚度及两者刚度对刚性隔热瓦及组件力学性能的影响,构建一种对称结构的刚性隔热瓦分析模型,进行了拉伸试验和有限元分析.结果表明,在拉伸载荷作用下刚性隔热瓦的应力分布不均匀,具有明显应力集中现象,其拉伸强度水平与最大应力密切相关,随涂层厚度或刚度增加,刚性隔热瓦最大应力上升,拉伸强度降低;当刚性隔热瓦粘结应变隔离垫时,应变隔离垫厚度增加或刚度减小,刚性隔热瓦最大应力下降,拉伸强度增大;当刚性隔热瓦含涂层时,增加应变隔离垫可减缓涂层对刚性隔热瓦的影响;并对有限元分析结果进行了试验验证,计算结果与试验结果吻合良好,表明建立的刚性隔热瓦分析模型合理,揭示了涂层和应变隔离垫参数与刚性隔热瓦失效的关联关系.

Yielding loads (Py) from small three-point-bending (3-p-b) specimens with notch tips in the heataffected zone, fusion zone, weld metal, and base metal of Q235 (common carbon steel in China) were used to estimate the corresponding fracture toughnesses. To model the elastic and plastic fracture around the Q235 welded joint, the boundary effect model initially developed for quasibrittle fracture of heterogeneous solids with a large crack-tip fracture process zone was adopted in this study, bypassing the stringent ASTM standard requirements on the specimen size, initial crack length, and un-notched ligament. It was found that the weld metal had the highest fracture toughness value around 130 MPa root m, followed by the heat-affected zone of 109 MPa root m, the base metal of 83 MPa root m, and the fusion zone of 77 MPa root m. Microstructures and notch-tip plastic zones in each section of the welded joint were measured and used to explain the fracture toughness measurements.

This study examines the effect of temperature on the dynamic compressive performance of random fibrous (RF) composites at temperatures up to 1273 K in the through-the-thickness (TTT) and the in-plane (IP) directions, using an improved high-temperature split Hopkinson pressure bar (SHPB) system. The results revealed that in the IP direction, the RF composite presented a shear fracture mode below 1073 K and initiated multiple major cracks in the specimens at 1273 K. However, the composite showed a layered fracture mode in the TTT direction from 288 to 1273 K. The dynamic strength in both directions showed a consistent trend when observed under static loading below the critical temperature. The change in the strain-rate sensitivity (SRS) of the dynamic strength was insignificant for temperatures below the transition temperature of viscous-flow and brittle deformation of the RF composite. However, above the transition temperature, the SRS of the dynamic strength became significant.

We studied the dynamic mechanical behaviour of three-dimensional (3D) random fibrous (RF) materials at various strain rates using a high-temperature split Hopkinson pressure bar (SHPB) system from 288 to 1423 K. The failure mode of 3D RF materials in the in-plane (IP) direction exhibits a shear fracture mode with an angle of similar to 30 degrees at a strain rate of 300/s, and the bonding between fibres remains intact at 288 K despite the fracture in fibres. The variation of dynamic strength at a strain rate of 500/s (from 288 to 773 K, the strength increases from 4.72 +/- 0.61 to 5.27 +/- 1.03 MPa; from 773 to 1423 K, the strength decreases from 5.27 +/- 1.03 to 3.36 +/- 0.93 MPa) aligned well with observations of the static experiment with increasing temperature. We determined the strain rate sensitivity factor by fitting strength versus strain rate at various temperatures and observed a significant effect of the strain rate sensitivity on strength at 288, 773, and 1273 K. However, the effect of strain rate on the strength is low at the transition temperature of viscous flow and brittle deformation of 3D RF materials. These results can be used as the theoretical basis for the preparation and performance evaluation of 3D RF materials.

为研究三维随机纤维材料服役环境下的力学性能,提高热防护系统安全服役性能和结构寿命.利用有限元建模软件,建立三维随机纤维材料微结构模型,研究其面内和厚度两个方向的三维随机纤维材料宏观力学性能与几何参数(纤维方向、纤维长度和纤维直径)的关系,获得了三维随机纤维材料厚度和面内宏观力学性能演化规律.结果 表明:三维随机纤维材料内部偏移纤维比重越大,厚度方向压缩强度就越高,面内方向压缩强度就越低;压缩强度随纤维长度增加呈上升趋势,在0.9 mm左右时达到临界值;压缩强度随纤维直径增加呈下降趋势.该研究结果可为三维随机纤维材料制备提供理论基础.

In this study, we perform three-point-bending tests for 3-dimensional random fibrous (3D RF) material specimens of two porosities (87% and 83%) with a crack. The crack with two different crack-length-to-specimen-width ratios (0.2 and 0.5) is considered. Using the tensile strength obtained from experiments and a characteristic average fiber spacing parameter from an equivalent orthotropic fiber network model, a simple theoretical model is developed to evaluate the fracture toughness of 3D RF material. This model gives the fracture toughness of 3D RF material, which is in good agreement with that from compact tension test, and better than the results calculated via the American Society for Testing Materials (ASTM) standard for three-point-bending test. This fact not only rationalizes the use of average fiber spacing size in the developed model, but also demonstrates the model efficiency in evaluating the fracture toughness of 3D RF material. Further, considering the temperature effect on the fracture strength of material, this theoretical model can be successfully used to predict fracture toughness of 3D RF materials at elevated temperatures (299 K similar to 1073 K).

In this study, we prepare the specimens of three-dimensional random fibrous (3D RF) material along its through-the-thickness (TTT) and in-plane (IP) directions. The experimental tests of tensile and compressive properties as well as fracture toughness of 3D RF material are performed at elevated temperatures. Then, the porosity (83%, 87% and 89%) and temperature dependence of the tensile and compressive strength, elastic modulus, fracture toughness and fracture surface energy of the 3D RF materials for both the TTT and IP directions are analyzed. From the results of the tensile strength and elastic modulus versus material porosities at various temperatures, we find that tensile strength and elastic modulus for the TTT direction are more sensitive to the porosity, but not for the IP direction. Fracture toughness increases firstly and then decreases at a certain critical temperature. Such critical temperature is found to be the lowest for the porosity of 83%. On the other hand, at below 1073 K, the temperature-dependent fracture surface energies with three porosities for the TTT direction show similar variation trends.

By means of finite element modeling (FEM) and fatigue experiments, we study the fatigue performance of the rounded welding region between the diaphragm plate and closed rib of orthotropic steel bridge deck in this work. A local sub-model of the rounded welding region from the orthotropic steel bridge deck was developed to analyze the stress distributions. Based on the analysis results we designed the fatigue specimen for the fatigue test of this detailed structure. The fatigue experimental results revealed that the crack initiates from the weld toe of the rounded welding region and the stress concentration at the rounded welding region is the main mechanism of fatigue crack initiation. In addition, we propose three improvements to reduce the stress concentration of the rounded welding region, and the local structure optimization scheme of the diaphragm–rib weld can effectively improve the fatigue resistance of the detailed weld structure.

The temperature dependence of the fracture toughness J(C) of a three-dimensional (3D) random fibrous (RF) material, with a porosity of 87% along the through-the-thickness (TTT) direction, was investigated using experiments and the finite element method (FEM) in this study. The temperature considered ranges from 299 to 1273 K. The experimental observations revealed the fracture toughness J(C) with crack length-to-width ratios of 0.4 and 0.5, which increased from 47.32 to 328.28 J/m(2) and from 44.92 to 280.09 J/m(2), respectively, as the temperature increased. Then, a 3D FE model, considering the meso-morphology characteristics of the 3D RF material, was developed to simulate a size-scaled compact tension (CT) specimen with a single edge crack. Using the elastic modulus and the fracture strength of the silica fibers at room temperature, we verified the effectiveness of the FE model, then predicted the fracture strength of the silica fibers and the bonding between the fibers at elevated temperatures. In addition, our developed FE model proved to successfully simulate the fracture toughness J(C) from 299 to 1273 K and reveal the deformation mechanism of the 3D RF material at different temperatures.