含紧固穿透分层复合材料层板的疲劳扩展

对含预置穿透分层复合材料层板开展压缩强度和压缩疲劳试验,获得结构的极限载荷,并研究层板的分层扩展特性。基于ABAQUS软件建立含穿透分层复合材料层板有限元模型,通过VCCT计算能量释放率,采用B-K准则来模拟层间分层的扩展。引入VUMAT子程序,采用改进的Hashin准则判断单元损伤;基于累积损伤理论和剩余强度理论,弱化材料性能,对含穿透分层和含紧固穿透分层复合材料层板的疲劳力学行为进行分析,讨论了紧固件等参数对层板分层扩展的影响。     

复合材料层板疲劳损伤的有效能耗分析法

通过对纤维增强复合材料层板疲劳过程中能量耗散的不可逆过程的分析,并考虑不同加载次序,建立了一种以有效能耗为基础的非线性疲劳损伤累积法则.并从理论和实验证明了该法则能正确的反映载荷次序效应等疲劳累积损伤规律.     

玻璃纤维-Al 混杂复合层板的疲劳裂缝扩展行为

本文研究了一种新型结构复合材料玻璃纤维-Al 混杂复合层板(Glass Aluminum Laminates,GLALL)在不同外载荷作用下的疲劳裂缝扩展行为,以及疲劳破坏规律。试验发现,由于未断高强度纤维对裂缝的桥接作用降低了裂缝尖端的有效应力场强度因子,GLALL 的疲劳裂缝扩展速率远低于单一Al 合金。伴随 Al 合金层内疲劳裂缝的扩展,GLALL 疲劳裂缝附近区域会产生脱层破坏,脱层区宽度随外加载荷增大而增大。脱层区宽度越小,纤维对疲劳裂缝的桥接作用越强。     

电阻法预报CFRP层板疲劳损伤演变过程

通过类比方式,以电阻变化作为复合材料的损伤参数,建立了用电阻损伤参数预报CFRP层板疲劳损伤的数学模型,通过拉-拉疲劳试验,对CFRP层板的疲劳损伤与疲劳寿命预报进行研究,建立了累积损伤中电阻与疲劳损伤状态之间的关系.研究表明,在疲劳损伤过程中,电阻损伤参数的变化同材料的疲劳损伤过程密切相关,电阻损伤规律可分为下降阶段、平稳阶段和破坏阶段;碳纤维材料的电阻变化能够反映纤维的损伤程度和破坏形式,可以作为表征复合材料纤维损伤的有效参数.     

带切口碳纤维复合材料层板损伤的实验研究

本文采用渗透剂增强的X射线照相法分析了含切口复合材料层板的损伤扩展与材料的破坏机理。试验结果表明这种方法是可行的,可以清楚地观察到材料的内部损伤的发生与扩展。材料的破坏是以切口前缘附近损伤区的扩展为特征的,这个损伤区以基体开裂、分层和少量纤维断裂的形式出现。     

残余应力与胶粘剂对ARALL层板疲劳裂纹扩展特性的影响

研究了粘剂性质、含量及残余应力状态对纤维－铝合金胶接层板（ＡＲＡＬＬ）疲劳裂纹扩展特性的影响，分析了裂纹扩展过程中的分层状态的变化，结果表明，ＡＲＡＬＬ层板内富胶层的剪切形变和伴随裂纹扩展的分层区越大，即这两方面耗散能量越多，则疲劳裂纺扩展速率越低；胶粘剂含量的影响不明显，给层板施加预应力极大降低了层板的疲劳裂纹扩展速率，其本质在于裂尖在同样的疲劳载荷下实际所受到的有效应力降低。     

T300/648复合材料迭层板接头挤压疲劳累积损伤问题的实验研究

本篇论文根据“剩余强度,剩余刚度”理论,讨论了碳纤维增强复合材料迭层板承载孔的挤压疲劳损伤,应用孔的永久变形描述损伤的产生和扩展,提出了“损伤函数”的定义和测定方法,并提出了新的“疲劳损伤点”的概念。实验还研究了温度对连接孔静挤压强度和疲劳寿命的影响。实验结果表明,温度的影响尤为重要。对损伤失效试件,采用“揭层方法”、“X射线”及“SEM方法”进行检测和损伤机理的研究。     

基于连续损伤力学的UHTCC疲劳损伤扩展模型研究

为研究超高韧性水泥基复合材料的损伤扩展规律,该文对该材料的预制单边切口试件进行了三点弯曲疲劳试验。基于连续损伤力学,建立两种疲劳损伤扩展模型：一是以J积分作为自变量的双对数线性模型,二是以疲劳应力水平作为自变量的单对数线性模型。结果表明,两种模型计算结果与试验结果吻合良好。但J积分取值很大程度上依赖于试件的几何特征,导致模型1受试件形状影响较大;实际应用中,由于疲劳应力水平的获得相对直观方便,故模型2更加实用。     

FRP补强疲劳损伤钢结构裂纹扩展研究

钢结构在服役过程中,因超载、疲劳等原因会引起损伤累积,极大地影响了结构的安全使用.对纤维增强复合材料(FRP)补强钢结构张开型裂纹疲劳扩展进行了实验研究.比较了不同的钢结构表面处理方式对FRP与钢结构粘接接头剪切强度的影响,根据剪切强度选择最佳表面处理工艺.对FRP补强后钢结构试样进行疲劳实验,与未补强试样的结果加以对比.结果表明,用FRP增强后钢结构FRP增强区疲劳裂纹扩展速率得到抑制、疲劳循环次数得到了提高.     

FATIGUE DAMAGE AND SHAKEDOWN IN METAL MATRIX COMPOSITE LAMINATES

SUMMARY

Fatigue failure of metal matrix composite laminates is often preceded by a substantial loss of stiffness associated with cyclic plastic straining and subsequent low-cycle fatigue crack growth in the matrix. Experimental observations suggest that two principal crack patterns are involved; these are related here to the deformation modes predicted by the bimodal plasticity theory of fibrous composites. The relation is utilized in modelling the damage process such that matrix crack growth is regarded as a shakedown mechanism leading to a saturation damage state. For a given program of variable cyclic loading, evaluation of the saturation state is formulated as a non-linear optimization problem, where the total damage in a laminate is minimized subject to non-linear constraints derived from the ply yield criterion, hardening rule, and physically motivated bounds on the damage parameters. Effective elastic stiffness reduction and local stress redistribution predicted by the optimization procedure are compared with experimental measurements on several B/AI laminates. Stress transfer to and overloading of the fibres in certain plies appears to cause final fatigue failure of the laminate.     

A RANDOMIZED ENDURANCE LIMIT IN FATIGUE DAMAGE ACCUMULATION MODELS

The use of random parameters for describing S-N curves requires special caution. A Peculiar Fatigue Curves (PFC) method based on a linear damage accumulation rule is proposed for modelling the fatigue damage accumulation process. The fatigue properties of a component are presented in the PFC form by randomizing its endurance limit which facilitates a simple technique to predict fatigue life and residual life distributions. Possible usages of the models and their restrictions are illustrated for some common load histories of a component. Particular attention is paid to the stochastic relations between random variables having in some cases improper distributions. The problem of experimental testing of the models is also discussed.     

带中心孔的钛合金扩散连接层合板裂纹扩展特性

为了研究含孔TC4钛合金扩散连接层合板的裂纹扩展行为,进行了含ϕ6中心孔的8mm厚单层板材、三层(3+2+3)扩散连接层合板和三层(3+2+3)含ϕ12止焊区扩散连接层合板的疲劳对比试验,试验中施加标识载荷,通过断口判读裂纹形态和尺寸,得到裂纹扩展(a,N)数据,建立了裂纹扩展da/dN-a曲线,对单层板、层合板和止焊层合板的裂纹扩展行为和规律进行了对比。结果表明：单层板出现规则的半椭圆形孔壁裂纹;层合板由于存在层合界面的影响,出现不规则的半椭圆孔壁裂纹;而止焊层合板以角裂纹为主,裂纹扩展过程分为三阶段;三类试件的da/dN-a曲线可用双对数线性关系描述;三层板的裂纹扩展特性不弱于单层板;止焊层合板的裂纹扩展性能有明显改善,提高了含孔层板的损伤容限特性。     

FATIGUE DAMAGE–DIFFUSE LIGHT TRANSMISSION DEPENDENCE OF GRP LAMINATES

Abstract— The influence of cyclic loading on diffuse light transmission of GRP laminates was investigated. Light transmission decreased due to the presence of matrix microcracks and interface debonding. The dependence of defects caused by cyclic loading on the variation of light transmission was established by methods of scattering medium optics. This dependence can be applied both in investigations of the kinetics of GRP laminate failure and in the estimation of cumulative fatigue damage.     

FATIGUE DAMAGE IN 1045 STEEL UNDER CONSTANT AMPLITUDE BIAXIAL LOADING

The progressive nature of fatigue damage under multiaxial stress states has been investigated. Experiments were performed on thin-wall tubular specimens of 1045 steel in tension, torsion and combined tension-torsion loading. Two equivalent strain amplitudes, one in the high cycle fatigue (HCF) region and one in low cycle fatigue (LCF) region were employed in this study. Four recently proposed damage theories were evaluated. Crack depth was used as a damage parameter in comparing damage curves under different loading modes.
Different types of crack systems were observed in the HCF and LCF regions. The damage curve obtained in tension loading can be used to evaluate the damage behavior under combined tension—torsion loading. The results of torsion loading show that torsional damage behavior is different from the above two loading modes.     

FATIGUE DAMAGE IN 1045 STEEL UNDER VARIABLE AMPLITUDE BIAXIAL LOADING

Abstract— During constant amplitude loading, two different types of crack systems have been reported In the high cycle fatigue (HCF) region, cracks nucleate on a small number of maxium shear strain amplitude planes One of these cracks becomes a dominant crack and leads to failure of the specimen In the low cycle fatigue (LCF) region, equally developed microcracks are observed over the entire gage section and grow during the majority of the life. The failure is due to a linking in which the microcracks join up during the last few cycles of the fatigue life.
To investigate the interaction of these two types of crack systems in biaxial fatigue, experiments were performed on thin-wall tubular specimens in tension, torsion and combined tension-torsion loading The test program included step loading and block loading in which two equivalent strain amplitudes were employed. One of the equivalent strain amplitudes is in the HCF region and the other was in the LCF region
Fatigue lives were predicted from constant amplitude damage curves when a single crack system dominated the fatigue process Two competitive crack systems were sometimes developed on the maximum shear strain amplitude planes in a single specimen under block loading This resulted in a conservative prediction of the fatigue life.     

CUMULATIVE FATIGUE DAMAGE UNDER RANDOM LOADS

Abstract This paper presents a simple algorithm of hysteresis loop counting to make on-line data processing much easier. It involves a modification of the range pair counting method and the rain flow method and yields practically the same result as these methods. In addition, the applicability of the present method to cumulative damage estimation is examined in experiments on fatigue crack growth rates under stationary random loads and low cycle fatigue lives under stationary random loads together with non-stationary random loads such as seismically excited loads.