高强度双头螺柱断裂失效分析

通过对双头螺柱化学成分、力学性能和金相组织进行分析,以及对其断口进行宏观形貌和微观形貌观察,最终确定双头螺柱断裂失效是由于表面螺纹沟槽处的大量氢鼓泡受到拉应力的作用在沟槽表面形成微裂纹造成的。     

A critical evaluation of indentation crack lengths in air

An extensive overview is presented of Vickers indentation crack lengths in ceramics in air. Measurement of such crack lengths is one of the most common and powerful assessments of the fracture properties of ceramics and the overview provides a critical evaluation of observed behavior as functions of material type and indentation load, and an extensive basis for comparison of results from new materials and analyses. The overview considers single crystals, polycrystals, transforming materials, glasses, and multiphase materials, including cermets, glass-ceramics, and tooth enamel. The coverage extends over structural and electronic ceramics, including oxides, carbides, nitrides, and titanates. The data are presented in a single format for ease of interpretation in terms of idealized indentation fracture and for inter-material comparisons; most data are unique to this work, but the results of selected studies from the published literature are included. The overview considers the precision and accuracy of crack length measurements and demonstrates a simple quantitative evaluation and ranking scheme for ceramic fracture based on load-adjusted crack length and cracking susceptibility. Indentation hardness and cracking threshold are also determined and related to the susceptibility. Material toughness is related to cracking susceptibility by fracture mechanics analyses: typical crack length measurements in air are shown to provide estimates of inert toughness with a relative uncertainty of ±50%.     

Effect of chitosan and Dysphania ambrosioides on the bone regeneration process: A randomized controlled trial in an animal model

The focus of this triple‐blind study was on evaluating the effect of chitosan combined with Dysphania ambrosioides (A) extract on the bone repair process in vivo. In total, 60 male Wistar rats (Rattus norvegicus albinus) weighing between 260 and 270 g were randomly selected for this study and distributed into four groups (n = 15). Group C (chitosan), Group CA5 (chitosan + 5% of D. ambrosioides), Group CA20 (chitosan + 20% of D. ambrosioides), and Group CO (Control‐Blood clot). In each animal, bone defects measuring 2 mm in diameter were performed in both tibias for placement of the substances. After 7, 15, and 30 days, the animals were sedated and sacrificed using the cervical dislocation technique and the tissues were analyzed under optical microscope relative to the following events: inflammatory infiltrate, necrosis, osteoclasts, osteoblasts, fibroblasts, periosteal, and endosteal bone formation. The data were evaluated to verify distribution using the Kolmogorov–Smirnov test, and variance, using the Levene test; as distribution was not normal, data were subjected to the Kruskal–Wallis and Dunn nonparametric tests (p < .05). A significant inflammatory infiltrate was observed in Group CA5 (p = .008) in the time interval of 7 days, and in Group C at 15 (p = .009) and 30 (p = .017) days. Osteoblastic activity was more significant in Group CA20 (p = .027) compared with CA5 in the time interval of 7 days. Group CA20 demonstrated a significantly higher endosteal and periosteal bone formation value in the time interval of 7 (p = .013), 15 (p = .004), and 30 days (p = .008) compared with the other groups. The null hypothesis was refuted, bone regeneration was faster in spheres with an association of chitosan and 20% extract, and complete bone repair occurred clinically at 15 days and histologically at 30 days. The spheres proved to be a promising method for the biostimulation of alveolar bone repair and bone fractures.     

42CrMo锚栓断裂分析

风电用42CrMo锚栓在安装过程中突然发生断裂，对断裂试样进行化学成分分析，采用金相检验与扫描电镜观察相结合的方法，分析研究断裂原因。结果表明，断口为沿晶断裂，锰元素含量超标和磷、硅、铜合金元素晶界偏聚是造成沿晶断裂的主要原因；另外，热处理时淬火温度过高造成奥氏体晶粒粗大，以及回火温度接近高温回火脆性区造成杂质元素在奥氏体晶界偏聚，这些因素的共同作用也会导致材料发生脆性断裂。     

Fabrication of high-fracture-strength and gas-tightness PDC films via PIP process for pressure sensor application

High-fracture-strength and high-gas-tightness thin-film gas-pressure sensors were fabricated using a silicon oxycarbonitride polymer-derived-ceramic (SiCNO-PDC) material via a multiple polymer-infiltration-pyrolysis (PIP) process. To obtain dense SiCNO ceramic films, two types of liquid polyvinylsilazane (PVSZ) precursors were chosen; a high-ceramic-yield precursor with high viscosity (PVSZ-1) was designed to construct the skeleton of ceramic film, whereas a relatively high-ceramic-yield precursor with low viscosity (PVSZ-2) was designed to infiltrate in ceramic defects. The results confirmed that the PVSZ-2 can effectively fill both intergranular and intragranular defects of ceramic films pyrolyzed by the PVSZ-1, and produce the sponge-like structures with nanosized pores. Although the density of the ceramic films only increased by 2.2%-5.2% after PIP process, the gas tightness was fundamentally improved, and all ceramic films after PIP process could keep gas-tight condition without loss of pressure after 72 hours. Similarly, the fracture strengths of the ceramic films after PIP cycles have also been improved, and the value could reach 108 MPa after only three PIP cycles. In addition, because a linear relationship among the load, resonant frequency, and deflection was detected in our ceramic films, the wireless passive gas-pressure sensors with high-sensitivity and high-temperature resistance have been fabricated. It strongly indicates that the ceramic films obtained by our PIP process have real potential to be used as thin-film gas-pressure sensing elements in high-temperature and high-pressure fields.     

焊接热输入对2219薄板铝合金焊接接头性能的影响

针对2219薄板铝合金,采用不同焊接热输入的TIG焊开展了对比试验,分析了焊接接头的力学性能、组织形貌和断裂特征。研究结果表明,单道焊比双道焊的接头抗拉强度的平均值提高了19.6%,断后伸长率基本相同;双道焊的接头在熔合线附近,焊缝区形成柱状枝晶,组织大小不一、形状不规则,热影响区晶粒粗化,几何形态变化,组织形态不均匀,接头在焊接温度升高时,晶粒粗化,强度降低,断口呈现了沿晶界撕裂扩展的痕迹,局部呈“河流花样”特征;单道焊的接头在熔合线附近,焊缝区等轴晶分布较多,组织均匀一致,晶粒细小,接头在经历了加热、快速冷却、常温放置后,强化相溶于固溶体,Cu原子扩散、聚集,产生点阵畸变,晶粒得到细化,强度升高,断口呈现了典型的塑形断裂痕迹,韧窝形态明显。     

Effect of α phase morphology on fatigue crack growth behavior of Ti−5Al−5Mo−5V−1Cr−1Fe alloy

Taking a Ti−5Al−5Mo−5V−1Cr−1Fe alloy as exemplary case, the fatigue crack growth sensitivity and fracture features with various tailored α phase morphologies were thoroughly investigated using fatigue crack growth rate (FCGR) test, optical microscopy (OM) and scanning electron microscopy (SEM). The tailored microstructures by heat treatments include the fine and coarse secondary α phase, as well as the widmanstatten and basket weave features. The sample with coarse secondary α phase exhibits better comprehensive properties of good crack propagation resistance (with long Paris regime ranging from 15 to 60 MPa·m^1/2), high yield strength (1113 MPa) and ultimate strength (1150 MPa), and good elongation (11.6%). The good crack propagation resistance can be attributed to crack deflection, long secondary crack, and tortuous crack path induced by coarse secondary α phase.     

Crack initiation,propagation, and arrest in sintering powder aggregates

Cracking during sintering is a common problem in powder processing and is usually caused by constraint that prevents the sintering material from shrinking in one or more directions. Different factors influence sintering-induced cracking, including temperature schedule, packing density, and specimen geometry. Here we use the discrete element method to directly observe the stress distribution and sinter-cracking behavior in edge notched panels sintered under a uniaxial restraint. This geometry allows an easy comparison with traditional fracture mechanics parameters, facilitating analysis of sinter-cracking behavior. We find that cracking caused by self-stress during sintering resembles the growth of creep cracks in fully dense materials. By deriving the constrained densification rate from the appropriate constitutive equations, we discover that linear shrinkage transverse to the loading axis is accelerated by a contribution from the effective Poisson's ratio of a sintering solid. Simulation of different notch geometries and initial relative densities reveals conditions that favor densification and minimize crack growth, alluding to design methods for avoiding cracking in actual sintering processes. We combine the far-field stress and crack length to compute the net section stress, finding that it characterizes the stress profile between the notches and correlates with the sinter-crack growth rate, demonstrating its potential to quantitatively describe sinter-cracking.     

Tough,strong, hard,and chemically durable enstatite-zirconia glass-ceramic

Strong glass-ceramics (GCs) have been envisaged and widely researched for various applications, including large architectural panels, ballistic impact protection, bioactive medical implants, and odontological prostheses. Here, we report on the development and characterization of a novel hard, strong and tough enstatite-zirconia (MgSiO₃-ZrO₂) glass-ceramic derived from a 51SiO₂–35MgO–6Na₂O–4ZrO₂–4TiO₂ (mol%) glass. The best GC was developed by treating glass samples for nucleation at 700°C for 12 hours, followed by crystal growth at 1090°C for 3 minutes. It was characterized by X-ray fluorescence (XRF), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM), and contained plate-like enstatite, zirconia, and Ti-containing crystals. We investigated the nucleating ability of ZrO₂ and TiO₂ in inducing internal nucleation. In the early stage of crystallization, enstatite spherulites were observed, which were precipitated by heterogeneous nucleation on previously nucleated ZrO₂ nano-crystals. At more advanced stages, at high temperatures, they transformed into plate-like crystals. The ball-on-three-balls strength, elastic modulus, and Vickers micro-hardness of the GC are 323 ± 26 MPa, 146 ± 13 GPa, and 6.9 ± 0.1 GPa (load = 5N), respectively. The indentation (K_C), single-edge notched beam bending (K_IC), and crack tip (K_tip) fracture toughness are 2.8 ± 0.6 MP.m^0.5, 2.2 ± 0.3 MP.m^0.5, 1.9 ± 0.3 MP.m^0.5, respectively. The crack propagation profile after a controlled Vickers indentation was quite intricate. The enstatite and zirconia crystals enhanced crack deflection, bridging and branching, hindering crack propagation. According to the ISO 6872 for dental materials, the chemical solubility of our GC is 80 ± 5 μg/cm². Due to this positive combination of high strength, toughness, hardness, and chemical durability, this new glass-ceramic is envisioned as a candidate for several applications and could be further developed for memory disc substrates, architectural cladding and tiles, ceramic glazes, and dental materials.     

铸态Ti-20Zr-20Al钛合金动态压缩力学性能与断裂机制研究

采用X射线衍射仪、金相显微镜、分离式霍普金森杆和扫描电镜等手段研究了Ti-20Zr-20Al钛合金在动态压缩条件下的微观结构、力学性能和断裂机制。结果表明,Ti-20Zr-20Al铸态合金由Zr基体相和TiAl/Ti₃Al针状相组成; 随着应变率增加,合金的抗压强度增加,失效应变显著增加; Ti-20Zr-20Al合金的断裂机制为解理断裂; 随着应变率增加,试验压缩断裂后碎片总数增大,平均粒径减小。采用DID模型模拟的材料碎片尺度与实验结果比较吻合,Grady模型与实验结果的偏差较大。