纤维增强复合材料界面疲劳裂纹扩展的模拟研究

脱粘是纤维增强复合材料界面裂纹扩展的主要表现形式.基于剪切筒模型和常用的实验加载方式,研究了纤维增强复合材料中纤维与基体界面在拉-拉循环荷载作用下的裂纹扩展.借助描述疲劳裂纹扩展的Paris公式,得到了疲劳裂纹扩展速率、扩展长度以及界面上摩擦系数与加载次数的关系.在分析中,考虑了疲劳加载引起的脱粘界面的损伤及损伤分布的不均匀性,同时还考虑了材料的泊松效应.     

SiCf/Al复合丝变形损伤过程的原位观察

采用扫描电镜原位观察方法研究了束丝SiC纤维增强铝复合丝在低频疲劳和静拉伸过程中的损伤过程。实验发现,经过较短时间的疲劳加载或在较低的载荷下就出现纤维裂纹,裂纹向基体方向扩展,没有明显的界面脱粘现象。损伤过程可分为三个阶段,包括以纤维裂纹萌生为主的损伤起始阶段、以纤维多次断裂和基体裂纹扩展为主的损伤累计阶段以及裂纹迅速扩展和主裂纹连接的失稳破坏阶段。根据剪滞模型计算的表观界面强度表明该复合丝为强结合界面。     

界面脱粘对正交铺设SiC/CAS复合材料基体开裂的影响

采用细观力学方法研究了正交铺设SiC/CAS复合材料在单轴拉伸载荷作用下界面脱粘对基体开裂的影响。采用断裂力学界面脱粘准则确定了0°铺层纤维/基体界面脱粘长度, 结合能量平衡法得到了主裂纹且纤维/基体界面发生脱粘(即模式3)和次裂纹且纤维/基体界面发生脱粘(即模式5)的临界开裂应力, 讨论了纤维/基体界面剪应力、 界面脱粘能对基体开裂应力的影响。结果表明, 模式3和模式5的基体开裂应力随纤维/基体界面剪应力、 界面脱粘能的增加而增加。将这一结果与Chiang考虑界面脱粘对单向纤维增强陶瓷基复合材料初始基体开裂影响的试验研究结果进行对比表明, 该变化趋势与单向SiC增强玻璃陶瓷基复合材料的试验研究结果一致。     

[±20°]钢丝/橡胶复合材料的疲劳损伤累积

对 钢丝帘线增强的橡胶复合材料在拉伸循环载荷下的疲劳损伤累积进行了研究。结果表明:在载荷控制的疲劳过程中,材料的周期最大应变发展曲线呈现明显的三阶段规律。帘线端头处基体裂纹的出现是宏观疲劳损伤的初始,损伤的累积表现为裂纹数量增加、帘线/基体脱粘和层间裂纹的扩展。以动蠕变为参量建立了线性疲劳损伤累积模型,该模型能够较好地预报两级加载条件下材料的第二级疲劳寿命。     

[±20°]钢丝/橡胶复合材料的疲劳损伤累积

对[±20°]钢丝帘线增强的橡胶复合材料在拉伸循环载荷下的疲劳损伤累积进行了研究.结果表明:在载荷控制的疲劳过程中,材料的周期最大应变发展曲线呈现明显的三阶段规律.帘线端头处基体裂纹的出现是宏观疲劳损伤的初始,损伤的累积表现为裂纹数量增加、帘线/基体脱粘和层间裂纹的扩展.以动蠕变为参量建立了线性疲劳损伤累积模型,该模型能够较好地预报两级加载条件下材料的第二级疲劳寿命.     

界面对纤维增强陶瓷基复合材料拉伸性能的影响

建立了桥联纤维细观力学模型, 研究了界面对纤维增强陶瓷基复合材料拉伸模量及强度的影响。分别引入纤维应力均匀系数和界面脱粘率作为界面完全脱粘和局部脱粘条件下界面性能的表征参数。研究表明, 应力均匀系数及界面脱粘率越大, 材料模量越低, 而断裂时纤维所承担的应力越高。基于混合率给出了拉伸强度表达式, 同时也分析了基体裂纹分布、界面脱粘和纤维拔出对强度的影响。计算结果表明, 本文强度模型给出的预测值与试验值吻合较好。      

加载载荷对ADB610钢疲劳裂纹扩展性能的影响研究

针对ADB610新型低碳贝氏体钢,采取紧凑拉伸试样、恒幅加载方式,在应力比为0.1时进行3种不同加载载荷下多试样的疲劳裂纹扩展试验,并测出裂纹扩展长度a和循环寿命N的a-N试验数据。然后采取七点递增多项式法对a-N试验数据拟合计算疲劳裂纹扩展速率。结果表明,加载载荷较小时,ADB610钢循环寿命较长;在疲劳裂纹扩展的初始阶段,加载载荷小的疲劳裂纹扩展速率相对较慢,但随着疲劳裂纹逐渐扩展,加载载荷小的疲劳裂纹扩展速率反而更大一些。     

双金属层合板垂直界面裂纹疲劳的扩展过程

采用四点弯曲加载方式进行奥氏体不锈钢／低碳锅炉钢双金属层合板垂直界面裂纹的疲劳扩展实验,研究了组元强度配合、爆炸焊接影响的区域性能（晶粒大小、形变强化、界面脱粘、独立塑性区等）对裂纹扩展行为的影响,以及垂直界面裂纹疲劳扩展的不同过程及其所对应的扩展机制．结果表明：由于强度错配,裂纹起始于高强度材料一倜时其疲劳扩展速率提高,而起始于低强度材料一倜时其疲劳扩展速率降低;当裂纹尖端接近界面时,界面区域的存在对上述两种情况下疲劳裂纹的扩展均起到了一定的屏蔽减速作用．     

拉压加载下纤维增强铝合金层板疲劳裂纹扩展的压载荷效应与预测模型

基于增量塑性损伤理论与纤维增强金属层板疲劳裂纹扩展唯象方法, 推导出在拉-压循环加载下, 纤维增强金属层板疲劳裂纹扩展速率预测模型。并通过玻璃纤维增强铝合金层板在应力比R=-1,-2的疲劳裂纹扩展实验对预测模型进行验证。结果表明, 纤维增强铝合金层板疲劳裂纹扩展的压载荷效应分为两种情况: 在有效循环应力比R_C>0时, 表现为压载荷对铝合金层所承受残余拉应力的抵消作用; 当R_C<0时, 表现为压载荷抵消残余拉应力后, 对纤维增强铝合金层板金属层的塑性损伤, 对疲劳裂纹扩展存在促进作用。纤维铝合金层板疲劳裂纹扩展的压载荷效应不可忽略, 本文中得出的在拉-压循环加载下疲劳裂纹扩展速率预测模型与实验结果符合较好。     

SiC/Ti-6Al-4V复合材料在横向拉伸载荷下界面失效行为的数值模拟

采用有限元模拟了SiC/Ti-6Al-4V复合材料冷却过程和横向拉伸试验过程, 横向拉伸试样采用十字形试样。分别建立了平面应力和轴对称有限元模型, 采用平面应力有限元模型计算环绕纤维圆周的界面微区应力分布, 预测界面失效机制。采用轴对称有限元模型分析复合材料界面脱粘过程以及残余应力对界面径向应力分布的影响。结果表明: 对于SiC/Ti-6Al-4V复合材料十字形试样,在横向拉伸载荷下的界面失效由径向应力导致,界面失效模式为法向失效, 剪切失效模式未发生; 十字形试样在横向拉伸载荷下界面初始脱粘位置处于界面中间; 随横向拉伸应力增加, 十字形试样的界面脱粘对称向两边扩展; 界面径向应力随残余应力降低而升高。     

An Investigation of Interfacial Fatigue in Fiber Reinforced Composites

Based on the shear-lag model and the modified degradation formula for coefficient of friction, the interfacial fatigue and debonding for fiber reinforced composites under cyclic loading are studied. The loading condition is chosen as the kind that is the most frequently used in fiber-pull-out experiments. The stress components in the debonded and bonded regions are obtained according to the maximum and minimum applied loading. By the aid of theory of fracture mechanics and Paris formula, the governing equation is solved numerically and the interfacial debonding is simulated. The relationships between the parameters (such as the debond rate, debond length, debond force) and the number of cycles are obtained.     

Effects of the presence of compression in transverse cyclic loading on fibre–matrix debonding in unidirectional composite plies

Fatigue of composite materials is of great concern in load-carrying structures. The first type of damage to appear is generally transverse cracks in off-axis plies. These cracks form when fibre–matrix debonds coalesce. The underlying mechanism is hence fatigue growth of debonds at the fibre–matrix interfaces. In the present study, debond growth has been characterized under tensile and compressive cyclic loading of single glass fibres embedded in polymer matrix. The debond length was determined by in situ microscopy with transmitted polarized light showing the more damaging effect of tension–compression cyclic loading than tension–tension cyclic loading. A boundary element model has been developed and interfacial fracture mechanics concepts applied over the numerical results aiming to give an explanation of this experimental fact. These results may be used to formulate a fatigue growth law at a local microscopic level, at a stage prior to the formation of any visible damage, i.e. transverse cracks. Ideas of how to develop this methodology further are also discussed.     

Interfacial damage in fibre‐reinforced composites subjected to tension fatigue loading

ABSTRACT The interfacial behaviour of fibre‐reinforced composites subjected to tension fatigue loading is studied based on the shear‐lag model. The governing equations of this problem are obtained and solved. In order to describe the interfacial debonding, the Paris fatigue crack growth formula as well as a modified degradation model for the coefficient of friction is adopted. Finally some important values related to interfacial debonding are obtained. In the present investigation, Poisson's contraction is considered.     

GMT-PP复合材料的界面粘结状况与动态疲劳关系研究

采用动态疲劳试验研究了不同界面粘结状况的GMT-PP复合材料的疲劳行为。结果表明:GMT-PP界面粘结状况对其拉伸疲劳性能有明显的影响,界面粘结的改善有利于抗疲劳性的提高。进而通过扫描电镜的观察发现具有良好界面粘结的GMT-PP在静态拉伸时破坏形式为基体破坏,而动态拉伸疲劳破坏则以界面脱粘为主。由于界面粘结强度越高,界面脱粘过程越慢,因而,材料的抗疲劳性越好。      

Interface debond crack growth in tension–tension cyclic loading of single fiber polymer composites

Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension–tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.     

3D in situ observations of glass fibre/matrix interfacial debonding

X-ray microtomography was used for 3D in situ observations of the evolution of fibre/matrix interfacial debonding. A specimen with a single fibre oriented perpendicular to the tensile direction was tested at a synchrotron facility using a special loading rig which allowed for applying a load transverse to the fibre. Three distinguishable damage stages were observed: (i) interfacial debond initiation at the free surface, (ii) debond propagation from the surface into the specimen and (iii) unstable debonding along the full length of the scanned volume. The high resolution microtomography provides both qualitative and quantitative 3D data of the debonding initiation and propagation. Thus, microtomography is demonstrated as a promising technique which can assist micromechanical model development.     

复合材料的界面损伤过程与弹塑性本构关系

探讨了颗粒增强复合材料在三向拉伸荷载作用下界面逐渐开裂的过程及材料弹塑性本构关系的改变。应用W eibull 统计学理论描述界面由局部到全部的开裂过程。对于进入弹塑性阶段的材料本构关系采用割线模量理论。数值分析结果表明, 界面开裂过程取决于荷载模式、界面强度以及颗粒形状等几方面因素。      

短纤维增强橡胶密封复合材料界面疲劳损伤行为

依据广义自洽方法,建立了包含芳纶纤维、界面相、橡胶基体和等效介质的代表性体积单元(RVE)模型。采用自定义材料子程序对内聚力疲劳累积损伤模型进行编译,分别在基体/界面相的界面和纤维/界面相的界面设置内聚力单元,研究界面相性能参数对纤维增强橡胶密封复合材料(SFRC)界面疲劳损伤行为的影响。探讨了界面相厚度和模量的确定方法,获得了不同界面相厚度和模量下SFRC界面脱粘起始位置以及脱粘起始疲劳次数。结果表明,较低的界面相模量能够抑制界面脱粘的产生;随着界面相厚度的增加,界面脱粘的起始疲劳次数增加,SFRC抗疲劳损伤能力得到提高。     

Investigation of sub-critical fatigue crack growth in FRP/concrete cohesive interface using digital image analysis

The cohesive stress transfer during the sub-critical crack growth associated with the debonding of FRP from concrete under fatigue loading is experimentally investigated using the direct shear test set-up. The study focused on high-amplitude/low-cycle fatigue. The fatigue sub-critical crack growth occurs at a load that is smaller than the static bond capacity of the interface, obtained from monotonic quasi-static loading, and is also associated with a smaller value of the interfacial fracture energy. The strain distribution during debonding is obtained using digital image correlation. The results indicate that the strain distribution along the FRP during fatigue is similar to the strain distribution during debonding under monotonic quasi-static loading. The cohesive crack model and the shape of the strain distribution adopted for quasi-static monotonic loading is indirectly proven to be adequate to describe the stress transfer during fatigue loading. The length of the stress transfer zone during fatigue is observed to be smaller than the cohesive zone of the interfacial crack under quasi-static monotonic loading. The strain distribution across the width of the FRP sheet is not altered during and by fatigue loading. A new formulation to predict the debonding crack growth during fatigue is proposed.     

Acoustic emission based tensile characteristics of sandwich composites

Sandwich composite static and fatigue testing results indicated the predominant failure to be the core damage followed by interfacial debonding, resin cracking and fiber rupture. Under static testing, crack was observed to initiate in the core and ensue planar propagation near the interface with the facesheets; whereas, onset of crack initiation in the facesheets served as a precursor to the catastrophic failure. Multiple failure initiation and propagation sites in the core and intermittent interfacial debonding were consistently observed under fatigue. An acoustic emission based stiffness reduction model is presented that seems to accurately identify the extent of damage in sandwich composites subjected to fatigue loading conditions.