Development of DEM models for analysis of multi-type single lap joints : A microscale approach

Pang, Kai and Hou, Xiaonan and Ye, Jianqiao (2026) Development of DEM models for analysis of multi-type single lap joints : A microscale approach. PhD thesis, Lancaster University.

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Abstract

Currently, the concept of lightweighting is a prominent area of research to decrease the energy consumption and emissions. One of the most effective strategies for achieving lightweighting is to utilize advanced lightweight materials in place of traditional materials. Meanwhile, there are no fabrication processes capable of producing the entire structure as a monolithic unit without joints. The majority of structures are constructed by assembling different types of structural elements. Some traditional joining methods, such as bolting and riveting, can introduce high stress concentrations around the joint area and may cause cracks during the drilling process. In contrast, adhesive bonding offers notable advantages for joining multi-material structures, including reduced life-cycle maintenance costs and weight, more uniform stress distribution, and enhanced design flexibility. Microstructures such as microstructural surface roughness and internal defects of constituents are crucial factors in determining the performance and fracture mechanism of adhesive joints. However, the research dedicated to the examination of the failure mechanisms for adhesive joints influenced by microstructures at microscale is limited. This work conducts systematic experimental and numerical investigations into the effect of microstructural roughness and defects on the performance and fracture mechanism of multi-type adhesive SLJs. The adherend materials used in this study are Al and PPA, bonded with an epoxy adhesive (Loctite EA 9497). Firstly, the mechanical properties of the Al adherend, PPA adherend, epoxy adhesive, and multi-type SLJs (Al-Al SLJ, PPA-PPA SLJ, and hybrid SLJ) with three roughness grades are obtained through experimental studies. The mechanical properties of adherends and adhesive are used to determine the microparameters of the contact model of adherends and adhesive particles in the DEM model. The calibrated microparameters are validated by several experimental data, especially the interlaminar-like properties of the thin adhesive layer in joints. Then, the microstructural roughness and microstructural defect in SLJs are experimentally investigated through SEM and microCT scanning. The measured microstructural roughness and microstructural defect are realistically introduced into the DEM models for further calibration, including DEM Al-Al SLJ models, PPA-PPA SLJ models, and hybrid SLJ models. Compared to the experimental results, the developed DEM SLJ models can predict the performance and capture the microstructural fracture mechanisms of multi-type SLJs. Finally, the effects of microstructural roughness and microstructural defects on the performance of multi-type SLJs are investigated, including the failure load and stiffness. The effects of microstructural roughness and microstructural defects on the microscale fracture mechanisms of multi-type SLJs are also explored and discussed, including the crack initiation, coalescence, and propagation within the adhesive and interface.