SiCf/Ti6242复合材料热机械疲劳损伤原位研究

采用声发射技术和高分辨X射线成像技术对SiC_f/Ti6242复合材料在热机械疲劳过程中的损伤进行了原位监测。实验中采用同相位梯形波加载来模拟钛基复合材料整体叶环服役时的实际载荷工况,应力变化范围130～1300 MPa,温度变化范围100～500℃,500℃时保持最大载荷2min。结果表明,声发射信号主要出现在加载阶段和保载阶段,声发射系统检测到纤维断裂、基体裂纹信号,而在卸载阶段声发射信号非常稀少。利用高分辨X射线成像技术对声发射确认已损伤的样品进行检查,发现试样内部存在连接多个纤维的基体裂纹;在样品裂纹区域,部分断裂纤维的钨芯出现缩颈。声发射技术和高分辨X射线成像技术均表明SiC_f/Ti6242复合材料热机械疲劳损伤机制由纤维断裂和基体裂纹共同控制。     

泡沫铝准静态压缩变形的声发射特征研究

为了研究泡沫铝在压缩过程中的屈服平台和密实阶段的力学性能,使用万能试验机、声发射仪和高速相机对泡沫铝试件在不同加载速率下进行了单轴压缩试验,得到了泡沫铝材料在塑性变形-损伤过程中的声发射特征参量和变形场图像。通过分析声发射信号的振铃计数和能量计数,研究了屈服平台和密实阶段的力学性能。结果表明:弹性和屈服平台阶段的声发射振铃计数曲线与应力-应变曲线有较高的吻合度;密实应变与声发射能量计数有较好的对应关系;不同的加载速率下,声发射能量信号表现出不同的增长特征,在屈服平台阶段能量的吸收与耗散更明显,且声发射能量-应力曲线与泡沫铝的吸能效率的变化趋势相同。     

T700／环氧复合材料拉伸损伤机理声发射实验研究

以声发射技术为手段,对T700／环氧复合材料试样进行了拉伸损伤实验,依据其损伤过程和声发射特性,发现在加载初期主要发生的是基体、界面的损伤;加载的后期微损伤发生的数量不多,但是损伤性质严重,其主要是碳纤维损伤;同时拉伸过程声发射信号存在峰值和”平静期”,且重复性很强。     

飞机复合材料拉伸过程损伤的声发射特性分析

利用声发射技术开展对拉伸载荷下飞机复合材料T型结构件的损伤发生过程研究,对复合材料试件拉伸载荷下损伤产生过程的机理进行了初步探讨。采用参数分析方法对拉伸试验采集到的声发射信号进行分析,得到该T型结构件拉伸过程中基体开裂、分层、纤维断裂至完全丧失承载能力时的声发射信号特征及其对应的载荷。结果表明,声发射技术能够准确预测复合材料T型结构件拉伸载荷下的损伤发生过程,可为该类结构件的检测提供参考。     

3Cr13不锈钢气阀试样断裂过程的声发射特性

为了探讨往复式压缩机气阀的无损故障诊断技术,对3Cr13不锈钢气阀试样进行连续加载的拉伸试验,利用声发射技术对其拉伸过程中的损伤线定位声发射特性、断裂声发射特性进行分析,并采用S3400型扫描电镜进行断口形貌分析。试验结果为:气阀试样在距离3号佳感器30mm处出现较强声发射峰,在拉伸时间为175.5s时,试样发生断裂。气阀试样在承受83kN拉力时,试样表面呈现规则的层状形貌。当3Cr13不锈钢气阀试样的挠度为0.48和0.74时,气阀试样的振铃计数分别为3.7×103和27.1×103个。在3Cr13不锈钢气阀试样断口上有大量微裂纹,是断裂过程中产生幅度为43~80dB信号的主要原因。在3Cr13不锈钢气阀试样断口表面,其主要成分为Cr元素,其重量百分比含量为13.8%。     

C/SiC复合材料拉伸过程的声发射研究

利用声发射（AE）技术对C／SiC复合材料试样拉伸试验过程进行动态监测。通过声发射多参数分析法对拉伸过程中的声发射累计能量和平均持续时间随载荷或时间的变化进行了综合分析；同时对拉伸过程中典型AE信号的频率特征进行了分析，揭示了C／SiC复合材料拉伸损伤的演化过程及规律，给出了材料拉伸损伤发展的不同阶段以及各阶段损伤类型。通过声发射累计能量随载荷变化的斜率突变定义了材料临界损伤强度。     

不同拉伸速度下的碳布/环氧树脂复合材料声发射评价

采用电子万能实验机控制,分别以1、2、5 mm/min的加载速度对碳布/环氧树脂复合材料进行拉伸,在拉伸过程中用声发射检测设备采集拉伸过程中产生的声发射信号,建立采集的声发射信号特征与时间、载荷的相关图,通过对相关图的分析,判断碳布/环氧树脂材料在拉伸过程中的损伤情况,并结合相关图分析不同拉伸速度对碳布/环氧树脂复合材料的影响,判定碳布/环氧树脂复合材料的临界失效载荷。结果表明:声发射检测可用于评价复合材料加载过程中的损伤情况,可将最大承载载荷的70%～80%作为碳布/环氧复合材料的失效参考载荷。     

基于SVM的碳纤维/环氧复合材料声发射源识别

对单丝碳纤维/环氧树脂复合材料的试样以及碳布/环氧树脂试样拉伸,对其拉伸过程中产生的声发射源信号利用声发射波形参数作为不同神经网络的输入对其进行模式识别,发现SVM神经网络比BP神经网络在对声发射源信号进行模式识别时具有更高的预测准确率。在增加SVM网络训练集时,SVM网络的预测准确率有较大的提高。利用声发射波形参数与SVM对碳/环氧树脂复合材料进行模式识别得到较好的效果。     

蜂窝夹层复合材料的压缩损伤声发射特征

应用声发射技术对蜂窝夹层复合材料压缩损伤过程进行了试验研究。分析载荷与声发射信号关联图,依据其损伤过程和声发射特征,发现随着加载条件下载荷的增加,复合材料的损伤逐步增大。在加载初始阶段,仅有少量声发射信号,各种表征信号量小幅度增加;在加载中期,声发射信号增多,各种表征信号量不断增大;在加载后期,声发射信号有明显突增,各种表征信号量急剧增加。复合材料压缩损伤破坏与声发射的幅值、能量、撞击、上升时间、持续时间和计数等参量特征相关。根据各阶段特征参量滤波后所得信号分布与实际断裂位置相吻合。     

单轴压缩作用下不同高宽比硬岩试样的破坏模式与板裂破坏机理（英文）

为了研究板裂破坏与试件高宽比(H/W)之间的关系以及板裂破坏的发生条件、破坏特征和力学机理,对6组花岗岩试样进行单轴压缩试验。利用应变监测仪和高速摄影仪记录试样的破坏全过程,通过分析应力、应变以及声发射的监测结果确定裂纹的发生和扩展路径。结果表明,H/W的变化可以改变试样的宏观破坏模式。当H/W降低至0.5时,试样破坏模式以板裂破坏为主;板裂破坏发生时试样承载力降低,裂纹拓展方向近似平行于加载方向。此外,发生板裂破坏的试样的裂纹扩展过程和声发射信号均呈典型拉伸破坏特征,表明板裂破坏本质上是一种特殊的拉伸破坏。     

Fatigue-life prediction of SiC aluminum composite using a Weibull model

The feasibility of the acoustic emission technique in predicting the residual fatigue life of 6061-T6 aluminum matrix composite reinforced with 15 vol.% SiC particulates (SiC_p) is presented. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were induced at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. The number of cumulative AE events increased exponentially with the increase in strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion. Cumulative events during post fatigue tensile tests reduced with a decrease in the residual fatigue life. Based on the high cycle fatigue damage accumulation model, a Weibull probability distribution model is developed to explain the post fatigue AE activity of specimens during tensile tests. Using the model, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change significantly with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.     

Evaluation of fatigue damage in reinforced concrete slab by acoustic emission

The applicability of acoustic emission (AE) technique for evaluation of fatigue damage in reinforced concrete (RC) slabs under cyclic loadings in both laboratory and as a structure in service is studied. The fundamental test performed in laboratory shows that the cracking process can be practically monitored by the measurement of AE signals. Analysis of the relationship between loading phase and AE activity indicates that the final stages of the fracture process can be evaluated by detecting AE signals generated near the minimum loading phase. Comparison of the results from the structure and that from the laboratory specimen demonstrates that AE energy can be an effective parameter for the evaluation of fatigue damage in RC slabs in service.     

Magnetic Barkhausen emission technique for detecting the overstressing during bending fatigue in case-carburised En36 steel

The effect of bending fatigue at different maximum stress levels on the magnetic Barkhausen emission (MBE) has been studied in case-carburised En36 steel specimens. The low frequency MBE profile has been measured after unloading the specimen at different number of fatigue cycles. It has been found that, beyond 1000 MPa, the MBE peak height decreases just after few thousand cycles and the percentage reduction in the MBE peak increases with maximum bending stress level. The reduction in MBE peak at lower stresses (<1400 MPa) is attributed mainly to the effect of residual stresses becoming more compressive below the surface due to the application prior tensile stress. At higher stresses (1500 MPa), the variation in the MBE peak also indicates the effect of cyclic hardening and softening with progressive fatigue cycles. The MBE profiles measured after monotonic loading and unloading with different maximum stress levels also show similar reduction in the MBE peak with increase in pre-stress level. However, at higher stresses (>1400 MPa), the cyclic loading shows larger reduction in the MBE peak than the monotonic loading. This is attributed to the effect of cyclic microplasticity induced enhancement of dislocation density in addition to the residual stress modification. This study clearly shows the MBE technique can be used to detect the maximum stress level seen by the specimen beyond 1000 MPa. Any overstressing of this case-carburised steel beyond the fatigue limit of 1150 MPa can be easily detected from the percentage reduction in the MBE peak. Since the crack propagation stage is insignificant in these hard steels, the detection of any overstressing using the MBE technique would be very useful in assessing and preventing the impending catastrophic failure.     

Post-impact damage characterisation on natural fibre reinforced composites using acoustic emission

The present study aims to characterise damage due to low velocity impact on jute fibre reinforced polyester composites. To attain this goal, a number of post-impact mechanical tests have been carried out, including tensile tests, three-point bending and indentation, using either a staircase or a continuous loading programme. On all these tests acoustic emission activity (AE) was monitored. The results, compared with damage observed under an optical microscope, show that AE is able to perform a reliable measurement of the level of damage also, on a natural fibre reinforced laminate. The main limitations of this study are owing to the rather low ultimate stress of the material and to the need to apply a loading to evaluate the damage produced by the impact event.     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.     

复合材料拉伸过程的声发射特性研究

为了研究16MnR／0Cr18Ni9Ti复合材料断裂过程的声发射特性,可以利用声发射技术对16MnR／OCr18Ni9Ti复合材料试件的拉伸过程进行全程监测。研究表明,材料拉伸断裂过程中,声发射信号丰富明显,可测性良好,并且不同破坏阶段的声发射信号具有不同的特征。通过对不同拉伸阶段声发射信号的参数分析,可以了解材料不同变形阶段的声发射特性,并据此来分析材料损伤的发生、发展及演变过程。与传统的力学试验方法相比,声发射技术在研究复合材料断裂过程方面具有明显的优势。     

Fatigue behaviour of bolted joints

This paper analyzes the tensile fatigue behaviour of bolted joints constituted by commercial steel bolts. They were tested under both monotonic and fatigue tensile loading, with different R-ratio. Results show that under increasing monotonic tensile loading the bolted joint is not the failure zone of the bolt, whereas such a bolted joint is the failure region under cyclic loading. The fatigue life decreases with the increase of the stress range and with the maximum stress, and pre-loading enlarges the fatigue life. Fatigue fracture surface shows a geometry of crescent moon in the case of short cracks and such a shape evolves towards a quasi-straight crack front in the case of long cracks. Fatigue fracture usually happens at the root of the first notch inside the bolted joint, although fracture initiation may happen in several consecutive notch roots, increasing the initiation angle of the fatigue crack as the applied stress diminishes.