利用分子動力學模擬探討黏著接合強度及其破壞模式

Abstract

本研究主要目的為探討接合面粗糙度及黏著劑厚度，對黏著接合強度及破壞模式的影響。本文選用聚乙烯做為黏著劑(adhesive)，金屬板為黏著物(adherend)當例子，透過分子動力學(molecular dynamics, MD)模擬，分別施加拉伸及剪力負載於黏著接合上，而其中剪力負載又可依粗糙表面的方向分為縱向及橫向兩種。結果發現當黏著接合承受拉伸負載時，由於粗糙接合面會導致介面應力集中，使得黏著接合拉伸強度降低，並且發生接合面破壞。而提升黏著接合的接著強度，破壞模式會轉變為黏著劑破壞，此時若接合面為平滑表面或低粗糙度表面，其拉伸強度並無太大的差異，但是當粗糙度較高時，則會影響黏著劑的內部分子密度排列，使其黏著接合拉伸強度降低。基本上黏著劑厚度對以接合面破壞為主的黏著接合強度沒有影響；而當破壞模式為黏著劑破壞時，因為不同厚度的黏著劑，其分子密度分佈有明顯的不同，導致當黏著劑較厚時，黏著接合拉伸強度有下降的趨勢。 當黏著接合承受橫向剪力負載時，由於黏著物的粗糙表面與黏著劑之間的互鎖效應(interlocking effect)，粗糙接合面可大幅提升黏著接合橫向剪力強度。當接合面為平滑表面或低粗糙度表面時，其破壞模式均為接合面破壞，因此黏著劑厚度對其橫向剪力強度無明顯的影響；但是當接合面粗糙度較高時，其破壞模式轉變為黏著劑破壞，且受到接合面粗糙度的影響，其黏著劑分子密度分佈較為均勻，並未隨厚度的影響而有所改變，使得不同厚度的橫向剪力強度幾乎一致。當黏著接合承受縱向剪力負載時，增加接合面粗糙度會提升介面的接觸面積，使得縱向剪力強度亦會隨之上升；此外，由於破壞模式為接合面破壞，因此黏著劑厚度與縱向剪力強度之間無明顯的趨勢。

This research aims to investigate the effect of interfacial roughness and adhesive thickness on adhesive joint strength as well as failure mechanism by using molecular dynamics simulation. The polyethylene (PE) is adhesive and metal plates are adherend. The strength of adhesive joints is determined by applying tensile and shear loading on adhesive joint. In addition, the shear loading includes transverse and longitudinal according to the rough interface direction. When joints are loaded in tension and adhesive failure, because rough interface would cause the stress concentration on the interface, the adhesive joint strength would decrease accordingly. When the failure mode is cohesive failure, the tensile strength has no significant difference between smooth interface and low rough interface joint. The high rough interfacial surface would affect the density distribution of PE polymer resulting in the reduction of adhesive joint strength. In addition, the adhesive thickness basically does not influence the strength when the failure mode is adhesive failure. For cohesive failure, because the adhesive density distributions are influenced by the adhesive thickness, the thicker adhesive exhibits the lower adhesive joint strength. When joints are loaded in transverse shear, the rough interface increases the transverse shear strength because of the interlocking effect between adherend and adhesive. When the interface are smooth and low roughness, the adhesive thickness does not effect on transverse shear strength since the failure mode are adhesive failure. High roughness lead to the adhesive density distribution more uniform so that the failure mode becomes cohesive failure and the transverse shear strength are almost same although the adhesive thicknesses are different. When joints are loaded in longitudinal shear, the contact area can be increased by raising the interfacial roughness so that the longitudinal shear strength increases. In addition, the adhesive thickness does not affect the longitudinal shear strength because all the cases are adhesive failure.