摘要
碳纤维增强复合材料(CFRP)内部损伤的不确定性及异质材料多模式损伤耦合的复杂特性,严重制约了构件的低冗余设计以及在苛刻环境下的长寿命可靠应用。随着试验和数值模拟手段的不断进步,有力推动了碳纤维增强复合材料力学性能退化及失效过程的研究,本文在总结国内外复合材料损伤行为研究的基础上,概述了静态、冲击、疲劳载荷以及湿热环境对CFRP损伤演化及失效行为的影响,对CFRP的多种损伤模型、试验方法及所取得的成果进行归纳,并展望了高精度仿真预测建模分析及各向异性材料复杂损伤模式的损伤机理分析等方面的研究发展趋势。
关键词
碳纤维增强复合材料(CFRP)由于具有密度小、比强度高、比模量大、耐腐蚀、抗疲劳、结构性能可设计性强、易于一体化成型等诸多优异特性,已被广泛应用于航空航天、交通、风电、海洋、建筑等众多领
CFRP的广泛应用使得对其力学行为和性能进行准确预测的要求日渐提
CFRP结构损伤的萌生与扩展直接影响其使役性能及结构可靠性。针对从初始缺陷到形成宏观裂纹,直至断裂失效的演化过程,复合材料多尺度力
前苏联学者KACHANO
DUGDAL
PAULINO

图1 双线弹性-软化内聚力模
Fig.1 Two-line elastic-softening cohesive zone mode
损伤力学从唯象学强度理论出发,建立损伤起始判据及损伤演化方程,考虑材料损伤本构关系,系统性地分析损伤对材料力学性能的影响。但其损伤模型存在很多物理意义不明确的参数,且不能反映材料破坏过程中细观结构的演化。
WAGNER
RYBICK
扩展有限元(XFEM)是基于有限元框架和断裂力学原理提出的研究材料失效点裂纹出现及扩展的最著名的方法,解决了裂纹扩展带来的网格重新划分问题,同时可以处理裂纹面和裂尖处不连续的问题,但是对于局部应力或应变变化较大、位移存在间隔的情况具有一定的局限性。
DUARTE
断裂力学以裂纹为边界条件,主要研究裂纹顶端的应力场、应变场和位移场,应用在三维断裂力学问题和疲劳裂纹扩展问题。但由于裂纹顶端应力场存在奇异性,导致直接采用常规数值方法分析断裂力学问题困难。
CFRP结构的冲击损伤是一个高度非线性问题,包括材料非线性、结构大变形引起的几何非线性和接触非线性等,在冲击载荷下,由于吸收冲击能量而出现基体开裂、分层、纤维断裂等形式的损伤,大多数情况下多种损伤行为共存。复合材料在冲击载荷作用下分层和厚度方向的剪切行为导致的层剪裂纹是主要的损伤方式,纤维、界面可以吸收大量能量从而减缓裂纹的扩展,预防材料突然失效。
KIM
LIM
在疲劳载荷作用下,复合材料轻微孔隙裂纹等结构内部的初始损伤进一步扩展,产生多种形式的损伤及相互耦合作用,CFRP的疲劳损伤演化是非线性的,如

图2 疲劳损伤演化曲
Fig.2 Fatigue damage evolution curv
最初,Hashi
基于刚强度退化的唯象学模型是从宏观力学尺度描述刚度或强度逐渐退化的演化规律,YANG

图3 剩余刚度、强度衰减曲
Fig.3 Residual stiffness strength and reduction curve
MATZENMILLER
(1) |
该模型引入损伤变量,并且研究了损伤与损伤本构的关系,以及应力应变对损伤的影响。XIAO
剩余刚度模型需要通过大量试验获取拟合参数且参数形式复杂,也考虑复杂的加载模式和预测多轴应力场下复合材料的疲劳寿
HALPIN
SAHU
(2) |
ALUKO
(3) |
剩余强度的退化对于损伤累积不敏感,开始时变化非常缓慢,临近失效时快速下降,出现“突然失效”现
HARRIS
GERENDT
渐进损伤模型使用一个或多个可观察损伤机理的损伤变量表示损伤扩展和疲劳寿命,是目前最受关注的疲劳损伤分析方
疲劳寿命模型根据S-N曲线确定失效准则,是三类模型中最简单的一种,需对特定案例校准,并且没有考虑损伤机理;唯象学模型可以表征疲劳损伤累积的特征,多用于工程领域;渐进损伤模型具有分析损伤机理的独有优势,考虑损伤的物理机制,模型的成本高、复杂性高,多用于科研领域。
在湿热环境中,高温会引起CFRP树脂后固化,提高其交联程度;同时树脂基体吸湿后会发生溶胀、塑化,使复合材料产生孔隙和裂纹,玻璃态转化温度降低,热膨胀系数提高,最终影响材料的力学性能。另一方面,界面处和基体内部会产生内应力、水分子通过界面进入纤维,引起纤维的破坏。
Fick模型认为经过材料一定时间吸水后达到动态平衡,此后质量不再增加。Two-Stage模型认为材料达到动态平衡后,吸水溶胀,界面处产生孔隙继续吸水,并且不会达到平衡状态。Langmuir模型认为材料吸收的水分子分为游离水和化合水,游离水填充分子间隙,化合水与材料内部分子成键,Langmuir模型第一阶段和Fick吸湿模型吻合,但第二阶段出现分散。吸湿曲线都经历两个阶段,即快速吸湿阶段和平衡吸湿阶段,如

图4 吸湿模
Fig.4 Moisture absorption mode
GRACE
ZHANG
NANDAGOPAL
湿热环境会对CFRP的碳纤维、树脂基体及界面产生不同程度的损伤,其中对碳纤维的影响很小,对基体及界面影响显著,湿热老化使复合材料产生结构微裂纹,加速水分子扩散,引起基体开裂、界面脱粘、层间分层等损伤,导致复合材料强度和刚度降低,基体和界面相关的力学性能明显下降。
从理论分析、试验、仿真等方面概述了在不同载荷及湿热环境条件下的复合材料力学性能退化及失效过程的研究进展,根据以上的分析和讨论,提出今后研究应关注的以下几个问题,需要进一步地发展与完善。
(1)损伤力学缺乏基于微观尺度相互作用的参数(包含原子层级的复杂相互作用,很难直接测量,只能作为唯象模型使用,需结合微纳米力学对模型进行参数标定),断裂力学存在裂纹尖端奇异性问题。近场动力学适用于损伤、断裂、冲击等失稳问题及热扩散、水扩散等渗流问题,研究领域包含多种复合材料和层合板结构,兼有分子动力学方法和无网格方法的优点,避免了传统宏观方法在面对不连续问题时的局部奇异性,真正地实现了微观到宏观的跨尺度分析,但是因为其形式复杂且缺乏成熟的商业软件,亟待开展更多的研究。
(2)CFRP的应变率效应显著,复合材料的动态力学性能以及损伤状态等均与受载时的应变率大小有关,近年对损伤本构关系的影响还处于初级阶段,尤其是对具有复杂微结构及纤维随机分布等复杂特性复合材料的研究很少,需要深入研究。
(3)CFRP在多轴交变载荷或疲劳载荷加载下,材料失效的预测需要大量的计算,且模型通用性较低,损伤机理尚未明确,因此引入多种修正系数和非线性项,发展基于多尺度力学分析方法的高效率、高适用性疲劳寿命仿真模型,可以促进其在大规模工程结构的应用与发展。
(4)CFRP在海洋等水环境中应用时不可避免地受到湿热和冲击、疲劳等外部载荷的耦合影响,需要考虑恶劣的工作环境和复杂应力状态造成的复合材料性能退化及失效。目前,在这方面的研究还比较少,强度理论和分析模型也亟待开发,针对该方面的研究也是一个重要的发展方向。
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