Session: 02-05-01: Session #1: 7th Symposium on Fastening and Joining Research and Advanced Technology

Paper Number: 95201

95201 - 2-D Analytical Model of Heat and Moisture Diffusion in Bonded Single Lap Joints 

In this study, a two-dimensional elastic analytical model for bonded single lap joints subjected to heat and moisture diffusion is presented. The distributions of peel and shear stress in the bond area are calculated as a function of the applied tensile-shear load on the joint. Local moisture concentration and bondline temperature determine the properties of the adhesive layer, which vary in the space and time coordinates. Adhesive diffusivity coefficient and absolute saturated concentration are affected by the material temperature, and scenarios of individual and combined heat and moisture diffusion are analyzed. The governing partial differential equation for the shear stress in the adhesive layer is obtained neglecting the effect of the bending moment at the edges of the overlap, following an approach similar to the model by Olaf Volkersen. Conversely, the peel stress distribution in the bond layer is derived by accounting for the bending deflection of the joint, as in Goland and Reissner’s study. A simplified shear stress formulation is then introduced for low-modulus adhesives, and is compared to the full model.

Different diffusion patterns generate two-dimensional gradients in the adhesive properties and consequently in the peel and shear stress distributions. As illustrated by the previous one-dimensional models found in the literature, softening of the adhesive layer occurring in the direction of the loading axis of the joint can contribute to a positive stress redistribution along the joint overlap, partially mitigating the effects of the adhesive strength degradation. However, the results of this proposed two-dimensional approach suggest that diffusive patterns in the direction transverse to the loading axis may severely impact the joint performance, especially in the initial stages of environmental exposure: significant increases in peak shear stress are observed in the innermost portions of the bond area, compensating for the adhesive softening near the lateral edges of the joint.

Diffusion across the joint width affects the distribution as well as the magnitude of the shear stress in the adhesive layer. Less significant gradients are observed for the peel stress distribution, under the same conditions. Moreover, an inverse relationship between stress gradients across the width of the joint and material diffusivity is found.

A 3-D Finite Elements Analysis of a single lap joint subjected to 2-D moisture diffusion is used to compute adhesive peel and shear stresses. The results are in reasonable agreement with the proposed analytical model. Minor differences are observed at the edges of the overlap, due to static limitations acknowledged in the literature.

Presenting Author: Marco Gerini-Romagnoli Oakland University

Presenting Author Biography: Marco Gerini-Romagnoli is a research associate at Oakland University, at the Fastening and Joining Research Institute led by Prof. Sayed Nassar. He also serves as OU site manager in the newly formed NSF IUCRC for Composite and Hybrid Materials Interfacing.<br/>His research focuses on lightweight joining, material characterization, and threaded fastening. He earned his Ph.D. in Mechanical Engineering from Oakland University, and he holds a dual MS/MASc degree in Mechanical and Automotive Engineering from Politecnico di Torino (Italy) and the University of Windsor (ON, Canada).

Authors:

Marco Gerini-Romagnoli Oakland University
Sayed Nassar Oakland University