复合材料气瓶的多轴疲劳寿命预测研究

采用等效应变法计算气瓶疲劳寿命的ASME压力容器规范,不能考虑多轴非比例加载效应,具有明显的不足。针对复合材料气瓶,采用修正的Brown-Miller算法研究其多轴低周疲劳寿命,并分析了自紧压力、金属内衬厚度和单层缠绕层厚度对复合材料气瓶疲劳寿命的影响,给出了提高复合材料气瓶疲劳寿命的方法。     

薄壁金属内衬复合气瓶自紧压力的有限元分析及性能研究

本文研究自紧压力对薄壁金属内衬复合材料气瓶性能的影响规律。采用有限元分析方法对气瓶的自紧过程进行数值模拟,缠绕成型复合材料气瓶,进行水压疲劳及爆破试验验证,并在试验过程中引入声发射监测。试验及分析结果表明,自紧压力对薄壁金属内衬复合材料气瓶的疲劳及爆破性能影响较大,自紧压力过大会使得气瓶复材层出现树脂开裂、纤维断裂等损伤,导致复合气瓶爆破强度值下降。疲劳试验结果表明,自紧压力过大容易导致薄壁内衬提前进入屈服状态,降低疲劳寿命。针对薄壁金属内衬复合材料气瓶,其自紧压力的选取必须充分考虑金属内衬在自紧及工作过程中的应力值,使其在合理区间。     

全复合材料车用天然气气瓶使用寿命的计算与分析

本文在系统地研究了一些使用因素对某型车用天然气气瓶强度影响的基础上,利用损伤理论进行了全复合材料车用天然气气瓶使用寿命的计算与分析.首先系统地阐述了基于疲劳损伤理论的气瓶寿命的计算方法;再根据车用全复合材料天然气气瓶在使用过程中的一些因素对气瓶强度的影响进行了探讨;最后在某型车用气瓶强度分析的基础上依据损伤理论以及计算方法给出了该型气瓶的使用寿命情况,同时也系统地分析了一些使用因素对气瓶寿命的影响.该计算方法与分析过程可以为车用天然气气瓶的安全使用以及检测提供理论基础和依据.     

含凹坑缺陷的薄壁圆筒形压力容器疲劳寿命数值模拟研究

利用有限元分析软件MSC.Patran和疲劳分析软件MSC.Fatigue对含凹坑缺陷的薄壁圆筒压力容器进行静力分析和疲劳寿命分析,得出满足G0>0.1的不同深度和长度的凹坑的最大主应力、应力集中系数和疲劳寿命的情况.通过对数据的分析和处理得到:随着凹坑深度的增加,最大主应力、应力集中系数是线性增加的,而疲劳寿命则是线性减小的.并且随着凹坑长度的增加,最大主应力和应力集中系数是不断减小的,而疲劳寿命也有上升趋势,但变化不明显,由此可知疲劳寿命的主要影响因素是凹坑的深度.     

金属内衬纤维缠绕复合气瓶自紧压力优化

为了达到提高承载能力和疲劳寿命的目的,往往在气瓶固化之后水压试验之前,对气瓶进行自紧处理。本文以市场上常见的钢制内胆E玻璃纤维复合气瓶为研究对象,采用Ansys有限元分析软件,建立各向同性金属材料和各向异性复合材料层的有限元分析模型,分析了自紧压力变化对气瓶应力和疲劳寿命的影响,最后确定气瓶最佳自紧压力。     

反应器盘管撞击凹坑的残余应力模拟及其疲劳分析

在无损检测未发现反应器盘管壁厚减薄及微裂纹等缺陷的前提下,重点研究材料硬化、塑性变形和残余应力对其正常工作和疲劳寿命的影响。借助ANSYS的APDL编程,对撞击后盘管凹坑的残余应力、应变场和后续运行工况进行数值模拟,本构关系为双线性等向强化弹塑性模型。通过模拟获得的残余变形与实际吻合,在此基础上,对操作工况凹坑最大应力点进行疲劳分析,预测盘管的疲劳寿命。     

复合材料气瓶高低温条件下的疲劳及爆破性能研究

本文缠绕成型130 L复合材料气瓶,进行了充放气疲劳及水压爆破试验,并且采用声发射的检测方法对充放气疲劳试验过程进行了监控。试验结果表明:充放气疲劳过程中复合材料气瓶除了承受内压载荷作用,同时还承受由于充放气导致的温度变化产生的热应力,内压以及热应力对气瓶的损伤较大,会导致气瓶疲劳后的爆破压强下降。     

基于VCCT的复合材料低周疲劳性能研究

基于VCCT建立复合材料低周疲劳模型,对层合板结构分层损伤进行疲劳寿命预测。采用ABAQUS软件通过直接循环法计算复合材料低周疲劳分层扩展情况,在模拟中指定分层扩展所沿的界面,基于VCCT可以计算界面单元裂纹尖端的断裂能量释放率,通过Paris准则来判断疲劳裂纹的产生和扩展。     

复合材料天然气气瓶预紧压力的研究

本文针对铝内衬全缠绕复合材料天然气气瓶,应用ALGOR FEAS有限元分析系统进行了气瓶材料的弹塑性历程分析,设计了气瓶的预紧压力。采用轴对称的应力－应变关系对气瓶金属内衬、复合材料进行了应力分析,确定了气瓶的应力分布状态。研究表明,通过预紧压力设计,降低了铝内衬工作状态下的最大拉应力,实现了提高复合材料气瓶疲劳寿命的目的。     

基于子模型技术的含凹坑缺陷变压吸附器疲劳寿命预测

基于子模型技术,采用有限元分析软件对含凹坑缺陷变压吸附器进行强度分析并对其疲劳寿命进行预测。结果表明:在一定尺寸范围内,凹坑缺陷对变压吸附器剩余寿命影响较小,故可不对凹坑部位进行补焊处理。     

Fatigue Crack Growth in Ferroelectric Ceramics Driven by Alternating Electric Fields

Electric-field-induced fatigue crack growth in ferroelectric ceramic PZT-5 with precracks was investigated. The experimental results showed that there were two distinct characteristics in the crack growth under electric loading. Under low electric loads, microcracks located ahead of the main crack emerged and grew and, as a result, impeded the growth of the main crack. On the other hand, under high electric loads, microcracks were absent, and the main crack was the only mode of fatigue cracking. The main crack grew macroscopically along the original path perpendicular to the electric field. Microscopically, the crack grew along the grain boundaries and grain breakaway was observed. The crack growth rate was nonlinearly related to the cyclic electric load. Similar to mechanical fatigue, there existed a crack growth threshold in the applied electric-field amplitude below which the crack ceased to grow. A steady crack growth occurred when the applied electric field exceeded this threshold. An empirical model for crack growth was obtained. Domain-switching effect and fracture-mechanics concepts were used to explain the observed crack closure and crack growth under electric loads.     

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP-blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF-PP with different amounts of maleic anhydride grafted PP (PP-g-MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF-blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF-PPs with varying PP-g-MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.     

High-Temperature Fatigue Behavior of Polycrystalline Alumina

The strength and fatigue behavior of a 99.5% polycrystalline alumina were measured as a function of temperature. Both the strength and fatigue behavior remained essentially constant up to 500°C; from 800° to 1100°C the strength and fatigue resistance decreased markedly and at >1100°C macroscopic creep was observed. It is believed that the decrease in strength and fatigue resistance is caused by a grain-boundary glassy phase enhancing subcritical crack growth. Proof-testing at room temperature was effective in improving the strength distributions at both room temperature and 1000°C; however, at 1000°C it was not effective, due to crack growth during the proof test. The good agreement between proof-test results and fracture-mechanics theory indicates that the same flaws control the strength at room temperature and at high temperatures.     

循环泵轴断裂原因分析

通过硬度测试 ,金相检验 ,断口分析等方法 ,对循环泵轴断裂原因进行了分析。结果表明 ,循环泵轴的失效方式为多裂纹源疲劳断裂 ,轴的表面有一圆弧过渡区 ,表面加工刀痕较明显 ,造成尖锐的缺口 ,使应力集中增大 ,易于裂纹产生 ,成为裂纹源 ,内螺纹应力集中处也是萌生的裂纹源的场所。轴的组织为网状铁素体加索氏体组织 ,疲劳抗力低 ,易于疲劳裂纹的扩展。疲劳裂纹扩展到临界尺寸时 ,轴的断面已不能承受给定载荷时 ,轴瞬间断裂     

Comparison of Slow Crack Growth Behavior in Alumina and SiC-Whisker-Reinforced Alumina

Results of dynamic fatigue experiments in water at room temperature on an indented Al₂O₃/25 wt% SiC whisker composite material have shown that this composite has a high resistance to slow crack growth. Aging tests in water revealed that the residual stress due to the indentation does not play an important role, and interrupted fatigue tests showed that the crack starts to grow at very low stress intensities, but the velocity is very low. Detailed fractography indicated that the slow crack growth path is intergranular in the whisker composite. The slow crack growth behavior in the whisker composite is discussed in association with the indentation residual stress, the change of the crack shape during the bending test. These results are compared with a bio-grade Al₂O₃ with lower resistance to slow crack growth, and important differences are pointed out.     

Crack-tip Degradation Processes Observed during in situ Cyclic Fatigue of Partially Stabilized Zirconia

It is proposed that reduced transformation zone widths in Mg-PSZ in cyclically versus critically propagated cracks are due to reductions in the crack-tip toughness, consistent with an intrinsic cyclic fatigue mechanism. Cyclic fatigue crack growth in Mg-PSZ was observed in situ in a SEM. Following cyclic fatigue, the samples were critically broken and the fracture surfaces observed. Extensive crack bridging by the precipitate phase was observed near the crack tip, and it is proposed that this crack bridging significantly affects the material's intrinsic toughness. Frictional degradation of the precipitate bridges occurs during cyclic loading and hence reduces the critical crack-tip stress intensity factor for crack propagation. Reductions in the critical crack-tip stress intensity factor also lead to reductions in the transformation zone widths during cyclic loading and hence the level of crack-tip shielding caused by phase transformation. This appears to be the mechanism of cyclic fatigue. A degree of uncracked ligament bridging was also observed and is linked with the frequency of random large precipitates. However, analysis shows that its effect upon crack growth rates under cyclic load is limited.     

CRACK GROWTH IN ADHESIVELY BONDED JOINTS SUBJECTED TO VARIABLE FREQUENCY FATIGUE LOADING

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Crack growth in adhesively bonded joints subjected to variable frequency fatigue loading

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

变幅载荷谱中加载顺序对疲劳裂纹扩展速率的影响

根据某高温高压换热器实际工况载荷波动所编制的载荷谱,用材料为2 1/4Cr-1Mo钢的CT试件,不同的载荷谱加载顺序,进行变幅载荷谱下的疲劳裂纹扩展速率试验研究。根据试验数据和超载迟滞效应理论,讨论了不同载荷谱加载顺序对疲劳裂纹扩展速率的影响。