A Test Method for Accelerated Humidity Conditioning and Estimation of Adhesive Bond Durability

The ability to determine the durability of adhesive bonds remains an elusive task, especially when the service environment involves exposure to diluents such as water. Moisture continues to be of major concern for many adhesive bond systems for a number of reasons including:

1) many adhesives are hydrophilic, picking up significant amounts of moisture over time;

2) most adhesives and some adherends allow moisture permeation, eventually reaching the adhesive/adherend interface;

3) the high surface energies of metallic and certain other substrates result in moisture migrating to the adherend surfaces and displacing the adhesive from the substrates, and possibly oxidizing the adherend, etc., and

4) absorbed moisture induces swelling stresses which can reduce the bond strength.

Recognition of this susceptibility to moisture has led to extensive studies aimed at evaluating the effects of moisture, developing an understanding of the responsible mechanisms, and predicting the performance of adhesive bonds subjected to humid environments. While some studies have focused on the effect of humidity on neat adhesive samples, most studies have recognized the significance of the adhesive/adherend interactions, and have evaluated strength of actual bonded joints. Unfortunately, the time required for typical bonded geometries to reach moisture equilibrium can be quite long. Single lap joints (SLJ) and double cantilever beam (DCB) specimens with a width of 25mm may take several years to equilibrate, depending on the temperature and adhesive. Such lengthy conditioning times hamper the development of improved adhesives, and may delay the acceptance of these adhesives because of the time required to certify them. Methods to accelerate the conditioning of test specimens would be of significant benefit to adhesive formulators and users.     

考虑脱粘的复合材料加筋板屈曲后屈曲及承载能力数值分析

建立了复合材料层合加筋壁板的屈曲后屈曲有限元分析模型。该模型采用界面单元以有效模拟筋条和壁板之间的连接界面, 连接界面和复合材料层板分别采用Quads和Hashin失效准则作为失效判据, 引入材料刚度退化模型, 采用非线性有限元方法, 研究了复合材料加筋壁板在压缩载荷下的前后屈曲平衡路径及破坏过程。数值分析结果与实验结果吻合良好, 证明了该方法的合理有效性。详细探讨了筋条尺寸及界面单元强度等参数对加筋壁板屈曲后屈曲行为及承载能力的影响规律, 研究表明增加筋条截面惯性矩及筋条密度在一定程度上能有效提高加筋板的屈曲载荷与极限强度, 筋条密度增加到一定程度会引起结构破坏形式由失稳破坏?湮顾跗苹? 界面强度与铺层方式对极限强度有重要影响, 界面脱粘是引起加筋板最终破坏的重要因素。      

泡沫夹层板脱粘损伤的小波分析检测技术研究

研究了一种通过对结构的应变响应进行复高斯二维小波分析来有效识别泡沫夹层板脱粘损伤的检测方法.数值模拟了无损板和多种脱粘损伤情况下的泡沫夹层板在悬臂条件、正弦激励下的应变响应,对损伤前、后的应变响应用复高斯二维小波进行小波变换,得出有损情况与无损情况下相应位置小波变换模极大值的差值,并用三维图表示出来.结果表明,相应位置小波变换模极大值的变化与脱粘损伤有很好的对应关系.小波变换模极大值变化量三维图中的峰值可有效表征脱粘损伤的位置和损伤程度.     

单纤维VS复合物纤维应力传递

纤维复合材料的力学特性很大程度上依赖于纤维和基体的应力传递。本文建立了单纤维和复合物纤维平面拉伸轴对称有限元模型，它考察了纤维拉伸时伴随基体破裂、界面分离以及基体破裂与界面分离同时发生时的应力传递特性，计算结果给出具有参考价值的有益结论。     

Face/core interface fracture characterization of mixed mode bending sandwich specimens

Debonding of the core from the face sheets is a critical failure mode in sandwich structures. This paper presents an experimental study on face/core debond fracture of foam core sandwich specimens under a wide range of mixed mode loading conditions. Sandwich beams with E‐glass fibre face sheets and PVC H45, H100 and H250 foam core materials were evaluated. A methodology to perform precracking on fracture specimens in order to achieve a sharp and representative crack front is outlined. The mixed mode loading was controlled in the mixed mode bending (MMB) test rig by changing the loading application point (lever arm distance). Finite element analysis was performed to determine the mode‐mixity at the crack tip. The results showed that the face/core interface fracture toughness increased with increased mode II loading. Post failure analysis of the fractured specimens revealed that the crack path depends on the mode‐mixity at the crack tip, face sheet properties and core density.     

Indentation debonding test for polymer coatings adhered to a substrate

Indentation debonding tests have been carried out on seven laminated glass/epoxy circuit boards coated with an epoxy film in an apparatus which allowed quantitative evaluation of competing substrate/film configurations by measurement of the indentation radius and debonding radius. The resulting adhesion performance was the debond strain. The effect of indenter shape is discussed in terms of a mechanical model; the experimental results are shown to demonstrate equivalence between conical and spherical indenters.     

扩展有限元法在SRM脱粘分析中的应用研究

采用扩展有限元法来研究固体火箭发动机中壳体/绝热层脱粘问题.简述了扩展有限元在双材料界面裂纹中的实施,重点介绍了其核心算法--位移模式的选取和应变矩阵的求解,通过相互作用积分求出应力强度因子和能量释放率.把问题简化成一个经典的算例,由计算结果可以看出扩展有限元算法的独特优越性和在解决脱粘问题时的有效性.     

压缩载荷下复合材料整体加筋板渐进损伤非线性数值分析

建立了考虑脱粘的复合材料整体加筋板渐进损伤有限元分析模型。该模型采用界面单元模拟筋条与壁板之间的连接界面, 连接界面和复合材料层板分别采用Quads准则和Hashin准则作为失效判据, 基于ABAQUS软件, 建立了含连续损伤状态变量的材料刚度退化方案。基于该模型, 采用非线性有限元方法研究了压缩载荷下复合材料整体加筋壁板在考虑初始几何缺陷时的破坏过程, 分析了结构相应失效模式的细观损伤机制; 详细讨论了轴向刚度比对结构承载能力及破坏模式的影响。结果表明: 考虑脱粘损伤的有限元模型能有效模拟加筋板的破坏过程; 在加筋板铺层设计合理的情况下, 增加筋条与壁板刚度比能有效提高加筋板截面单位面积的承载能力。      

复合材料加筋壁板长桁终止端混合连接设计分析

对缩短胶接面长度以提高起裂载荷的复合材料加筋壁板长桁终止端新型混合连接设计进行了研究。采用经试验验证的有限元方法,对长桁终止端混合连接的传载与失效机制进行分析,并研究了胶接面长度对结构起裂载荷的影响规律。研究结果表明：新型混合连接设计中紧固件提供的法向约束以及剪切载荷传递路径可显著提升终止端结构的起裂载荷,并增强终止端处界面的抑制损伤扩展能力;结构的起裂载荷随着胶接面的缩短而提高;当胶接面前缘退至连接区末排紧固件之后,可避免结构在最终破坏前发生界面失效,结构强度由蒙皮的机械连接强度决定。