预报三向碳碳材料在大偏轴角拉伸时的切口强度模型

三向碳碳材料在大偏轴角拉伸时,用纤维束的剪切强度来确定其切口强度,不会招致太大误差。先测定纤维束的剪切强度和基体碳的剪切强度,然后把3DC/C看成为正交异性均匀材料作平面力分析求得离切口较远处的位移场,以此为边界条件,求切口尖端附近的三维应力场。用Zweben的统计准则求出三向碳碳材料在大偏轴角拉伸下的切口强度,并用实验验证。      

三向碳碳材料在小偏轴角拉伸下的切口强度预报

在小偏轴角拉伸下,带切口的三向碳碳材料平面试件首先沿切口平面作自相似扩展。扩展到断裂的纤维束数与宽度方向的纤维束总数之比达到某一数值时,试件沿纤维束(或在另一些单层中垂直纤维束)方向剪断。断裂是灾难性的,所以这种切口强度预报与简单拉伸时的切口强度预报是相当的;区别仅在于在小偏轴角拉伸时,纤维束除受拉伸应力外,还作用有不可忽视的剪应力。本文对纤维束作了拉剪联合作用的强度测量,以得到其强度的概率分布的参数。据此预报了三向碳碳材料在小偏轴角拉伸时的切口强度,以及整个试件剪断时的断裂的纤维束数与纤维束总数之比值,理论预报与实验结果吻合良好。      

碳/碳复合材料疲劳损伤失效试验研究

对单向碳/碳复合材料纵向拉-拉疲劳特性及面内剪切拉-拉疲劳特性进行了试验研究; 对三维四向编织碳/碳复合材料的纵向拉-拉疲劳特性及纤维束-基体界面剩余强度进行了试验研究。使用最小二乘法拟合得到了单向碳/碳复合材料纵向及面内剪切拉-拉疲劳加载下的剩余刚度退化模型及剩余强度退化模型, 建立了纤维束-基体界面剩余强度模型。结果显示: 单向碳/碳复合材料在87.5%应力水平的疲劳载荷下刚度退化最大只有8.8%左右, 在70.0%应力水平的疲劳载荷下, 面内剪切刚度退化最大可达30%左右; 三维四向编织碳/碳复合材料疲劳加载后强度及刚度均得到了提高; 随着疲劳循环加载数的增加, 三维四向编织碳/碳复合材料中纤维束-基体界面强度逐渐减弱。      

三向碳碳材料的非线性双模量力学模型和强度准则

为了在平面十字形试件上实现拉拉、拉压、压压和拉剪的两向应力状态试验,设计并制造了双向加力装置和大剪切变形的测量仪,在试验基础上建立了三向碳碳材料的应力应变关系和强度准则.用纤维所受应力的正负号来决定两个方向里正应力异号时的模量(E_拉≠E_压)应该用哪一个.假定应变势存在,以保证弹性系数矩阵的对称性.非线性来自剪切变形.强度准则用二次型函数拟合实验数据,理论预报与实验吻合.      

碳/碳复合材料纤维束/基体界面强度的表征

为了获得准确表征纤维束界面强度的方法,对影响纤维束界面强度的因素进行了研究.通过顶出方法对纤维束界面强度进行表征,利用SEM和Micro-CT研究了纤维束界面层微结构对纤维束界面强度的影响,并通过有限元分析方法对纤维束界面的断裂行为进行分析.结果表明：在纤维束界面层含有不同程度的脱粘和裂纹缺陷结构,影响了纤维束的界面强...     

三维机织碳/碳复合材料双轴压缩载荷下的力学行为

基于三维机织碳/碳复合材料的细观结构特征, 设计平板十字形试样, 在材料双轴力学性能试验机上开展了复合材料单轴、 双轴加载压缩试验, 对比分析了三维机织碳/碳复合材料在双轴压缩载荷下的力学行为。研究表明: 三维机织碳/碳复合材料的压缩行为表现为非线性、 脆性断裂; 双轴载荷作用下非线性特征更为显著, 压缩模量随应力的增加而增大, 强度与模量相较于单轴有较大幅度增加, 双轴压缩载荷作用下材料的强化效应显著; 试样破坏位置并未出现在试样中心区, 而是发生在试样的加载端部或十字形试样的加载分枝根部, 主要表现为基体开裂、 纤维断裂和层间脱粘, 碳布及其层间界面剪切强度的强弱直接影响材料的压缩强度。     

多向细编碳/碳复合材料界面力学性能测试与表征

本文用自制装置研究了多向细编C/C复合材料纤维束性能,分析了工艺过程的影响。同时用界面微脱粘实验技术研究了C/C复合材料界面性能,给出了相应的理论模型和界面应力分布,提出了由界面脱粘力,纤维、基体和复合材料性能表征界面剪切强度的方法,为C/C复合材料优化设计提供了定量参数。结果表明:织物结构、织物编织工艺以及织物/基体复合对纤维的强度影响很大,降为原始纤维的20%左右,对模量影响小。不同界面层次,纤维/基体的界面结合情况和界面剪切强度不同,Z向纤维束中纤维/基体结合好,具有最高的结合强度,SEM观察证实有大量基体碳在纤维上枝联。     

多向细编碳／碳复合材料界面力学性能测试与表征

本文用自制装置研究了多向细编Ｃ／Ｃ复合材料纤维束性能，分析了工艺过程的影响。同时用界面微脱粘实验技术研究了Ｃ／Ｃ复合材料界面性能，给出了相应的理论模型和界面应力分布，提出了由界面脱粘力，纤维、基体和复合材料性能表征界面剪切强度的方法，为Ｃ／Ｃ复合材料优化设计提供了定量参数。结果表明：织物结构、织物编织工艺以及织物／基体复合对纤维的强度影响很大，降为原始纤维的２０％左右，对模量影响小。不同界面层次，纤维／基体的界面结合情况和界面剪切强度不同，Ｚ向纤维束中纤维／基体结合好，具有最高的结合强度，ＳＥＭ观察证实有大量基体碳在纤维上枝联。     

纤维束波动效应对平纹编织复合材料损伤行为的影响

平纹编织复合材料中纤维束波动效应会引起随动材料主方向变化及面外剪切应力集中,为了研究其对平纹编织复合材料力学性能及损伤行为的影响,提出改进的像素法细观有限元单胞模型。模型根据纤维束波动曲线定义了材料主方向的变化,采用Hashin准则模拟纤维束的损伤起始,并引入剪切修正因子考虑面外剪切应力对面内拉伸损伤的影响。模型可以预测平纹编织复合材料的面内拉伸强度和损伤演化过程,结果表明:纤维束材料主方向波动会引起平纹编织复合材料面内拉伸强度下降;面外剪切应力集中是导致复合材料最终失效的主要原因,且随着剪切修正因子增大,复合材料面内拉伸强度显著降低;纤维束材料主方向波动和面外剪切应力集中均对平纹编织复合材料的损伤行为和破坏机理产生了影响,需要在数值分析中对其进行准确描述。      

纤维束增强复合材料的双层次随机扩大临界核模型

针对纤维束增强复合材料, 提出了双层次随机扩大临界核模型。以复合材料制造工艺为基础建立了纤维间距和纤维束中纤维根数的计算模型, 将复合材料中的纤维分为2 个层次: 纤维束和纤维束群, 并提出父核和子核的概念对临界核的构成进行了区分, 用Beyerlein 公式计算纤维束群中纤维束相继失效引起的纤维束的平均应力集中因子, 用Sivasambu 公式来计算纤维束中纤维相继断裂造成的纤维的应力集中因子。然后, 以纤维断裂蔓延的主要模式为基础, 将逐渐增大的无效长度引入纤维束内部, 根据统计学理论推导相应的复合材料破坏概率计算公式。编制了相关程序, 通过该程序分别预测了S 玻纤、E 玻纤、玄武岩纤维无捻单向纤维布增强复合材料试件的拉伸强度。对3 种复合材料板进行了拉伸强度及基体的拉伸和剪切实验, 并对比了预测结果与实验结果。研究结果显示, 直接将实验对象的材料、几何参数代入就能得到与实验结果吻合的预测结果。      

Size effect in fracture of unidirectional composite plates

Fracture of notched, unidirectionally reinforced composite plates with well-bonded ductile matrices is typically preceded by the formation of long, discrete plastic shear zones aligned in the fiber direction. Onset of fracture is associated with a critical tension stress in a certain small process zone ahead of the notch tip; the critical stress is often equal to the tensile strength of the unnotched composite plate. Length of the shear zones can be estimated by plastic limit-analysis, and the local tension stress ahead of the notch found by superposition of the stress caused by remotely applied loads with the stress induced by the shear tractions in the plastic zone, which is shown to be dominated by a logarithmic singularity. In as-fabricated boron-aluminum composites, this fracture mechanism was analyzed and confirmed by numerous experiments (G.J. Dvorak, J. Zarzour and Y. Benveniste, Engineering Fracture Mechanics 42, 501–517, 1992). Since the notch tip field is not described by a stress intensity factor, experimental notched strength data cannot be interpreted in terms of a single material property, such as toughness. An alternative scaling procedure is outlined for prediction of notched strength of wide plates on the basis of data obtained from small size specimens. This revised version was published online in July 2006 with corrections to the Cover Date.     

NOTCHED FATIGUE BEHAVIOUR OF TWO COLD ROLLED STEELS

Abstract— A SAE1010 plain carbon steel and a SAE945X HSLA steel were cold rolled to various thickness reductions. Centre notched specimens were tested under stress control at a stress ratio of—1. The effect of loading direction on the fatigue strength was examined. The notched specimen fatigue strength was only slightly increased by cold rolling, since two opposing factors: the smooth specimen fatigue strength and the notch sensitivity, were increased by cold rolling. The notched specimen fatigue strength in the transverse direction was approximately the same as that in the longitudinal direction. An empirical equation and equations derived from fracture mechanics and Neuber's rule were applied to predict the fatigue notch factor for the sharp and blunt notch geometries examined. A reasonable agreement between the predictions and the experimental results was observed for the sharp notches. For the blunt notches, the predicted fatigue notch factors were conservative.     

Plasticity spread in mild steel pre-cracked charpy bars

During dynamic service loading, small fatigue cracks are normally seen to emanate from the root of sharp machined stress concentration region. In a recent authors' paper at ICF4, the fracture strength of a charpy type notched beam has been studied in three different engineering materials, when a small fatigue crack emanates from the notch root. Fracture tests on these medium and high strength materials demonstrate the presence of a large size plastic zone near the crack tip [1–6]. To understand the mechanism of fracture for such complex geometry. it is important to know the size and shape of these plastically yielded regions. The present paper is mainly on the experimental measurement of plasticity spread as well as the stress intensity factor (S.I.F.) for such short cracks. Firstly, the S.I.F. is approximately measured by plain transmission photoelasticity on model castolite specimens. Secondly, plastic zones around crack tip are measured for a wide range of notch root radii and crack-length, by using photo-stress PS-3B coating on mild steel pre-cracked charpy type notched specimens. It is observed that for small scale yielding at the crack tip, the plasticity spread is around 60–65° angle to the line of crack-extension. On the contrary, as the gross applied stress approaches the yield strength of the material, the maximum plasticity spreads around 45° angle. Finally, it is noticed that at high stress level, the plastic zone boundary (for short crack) touches the free machined notch surface. These experimental observations explain the nature and degree of non-linearity in a load—C.O.D. diagram during the fracture test of a short cracked-notched specimen. These data are also useful to predict the crack-extension load for an elastic-plastic material.     

碳纤维复合材料破坏的超声波检测

本文对碳纤维增强复合材料O°、±45°和0°/90°铺层的无缺口、有直边缺口、有中心圆孔和有中心缺口层板,作了静态拉伸试验研究。采用超声波扫描成象检测方法,检测了碳/环氧复合材料在不同加载区域内的损伤分布及损伤程度,得到了载荷—损伤程度曲线和材料内部缺陷分布的分层图象。结果表明:带有垂直于载荷方向直边缺口的试件在受静态拉伸时,裂缝不是沿缺口长度方向作自相似扩展,而是沿纤维方向的界面扩展,缺口基本上不影响试件无缺口部分的承载能力;对于带中心缺口的试件,首先出现缺口尖端的界面分离;带中心圆孔的试件,损伤从孔周开始,逐渐沿纤维方向扩展。本文还分析了试件的损伤和破坏机理。     

预测复合材料缺口强度的场强法

本文以复合材料缺口附近的应力应变场为参量,提出了预测复合材料缺口强度的场强法,给出了一个估计复合材料缺口强度的普遍表达式,并由此导出了一个易于使用的工程估算方法。通过对两种材料四组实验结果的分析表明:场强法的预测结果是比较满意的。     

Off-axis notched strength of fiber–metal laminates and a formula for predicting anisotropic size effect

Experimental and theoretical studies of the effects of notch size and fiber orientation on the off-axis notched strength of the fiber–metal laminate GLARE-3 have been conducted. The notched strength decreased with increasing notch size, regardless of the fiber orientation. The notch sensitivity in GLARE-3 was highest in the fiber direction, and it decreased with increasing fiber orientation angle to the lowest in the 45° direction. A new multiaxial criterion for the tensile failure of notched orthotropic fiber composites was developed from a phenomenological point of view. From the proposed multiaxial failure criterion, an anisotropic size effect law for predicting the off-axis notched strength of orthotropic composite laminates was derived. By comparison with the experimental results on GLARE-3, it was demonstrated that the anisotropic size effect law can accurately and efficiently predict the off-axis notched strength of GLARE-3, regardless of the notch size and fiber orientation.