A Method of Predicting the Stresses in Adhesive Joints after Cyclic Moisture Conditioning

Abstract

The durability of adhesive joints is of special concern in structural applications and moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying environmental conditions throughout their life. This paper presents a methodology to predict the stresses in adhesive joints under cyclic moisture conditioning. The single lap joints were manufactured from aluminium alloy 2024 T3 and the FM73-BR127 adhesive-primer system. Experimental determination of the mechanical properties of the adhesive was carried out to measure the effect of moisture uptake on the strength of the adhesive. The experimental results revealed that the tensile strength of the adhesive decreased with increasing moisture content. The failure strength of the single lap joints also progressively degraded with time when conditioned at 50°C, immersed in water, however, most of the joint strength recovered after drying the joints. A novel finite element based methodology, which incorporated moisture history effects, was adopted to determine the stresses in the single lap joints after curing, conditioning and tensile testing. A significant amount of thermal residual stress was present in the adhesive layer after curing the joints, however, hygroscopic expansion after the absorption of moisture provided some relief from the curing stresses. The finite element * Corresponding author. Tel: +44 1509 227535; fax: +44 1509 223934 Email address: i.a.ashcroft@lboro.ac.uk (I.A.Ashcroft) 2 model used moisture history dependent mechanical properties to predict the stresses after application of tensile load on the joints. The maximum stresses were observed in the fillet areas in both the conditioned and the dried joints. Study of the stresses revealed that degradation in the strength of the adhesive was the major contributor in the strength loss of the adhesive joints and adhesive strength recovery also resulted in recovered joint strength.The presented methodology is generic in nature and may be used for various joint configurations as well as for other polymers and polymer matrix composites.

Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: State of the art

“…Lin and Chen [55] observed that the reduction in the elastic modulus of epoxy samples for the saturated, dried and re-saturated groups is 29%, 9% and 42% respectively, compared with the unaged group samples. Very similar results were also reported by Mubashar et al [56]. Roy et al [57] found that the 20% drop in the elastic modulus of epoxy adhesive samples aged in water at 40°C would be fully restored upon drying.…”

Characterization of Pre-bond Contamination and Aging Effects for CFRP Bonded Joints Using Reference Laboratory Methods, Mechanical Tests, and Numerical Simulation

“…The drastic reduction of the G IIC values ascertained when comparing the findings for sets of aged and unaged CFRP joints coincides with the fact that the onset of crack propagation was observed at the side of the specimen where the moisture concentration and thermal effect are higher [34]. Additionally, the crack propagation occurred near the adhesive region, and diffusion through the adhesive is regarded as the primary access route for moisture to enter a joint [35].…”

A review of the relationship between design factors and environmental agents regarding adhesive bonded joints

“…The appearance of microcracks caused by swelling becomes more pronounced when cycles of water absorption and desorption occur, since the microcracks act as routes for the diffusion of water, intensifying the moisture absorption of the material and consequently increasing the degradation. Under cyclic exposure to moisture and drying, the ductility obtained by absorbing water can be lost [4,143]. This is yet another reason for needing to determine the behavior of adhesives and resins based on absorption and desorption history.…”