Combination and interaction of ageing parameters on single lap shear adhesive joints

Abstract

For adhesives used in structural adhesive bonds, the service lifetime and environmental ageing are fundamental elements when considering safe design and ensuring the integrity of a structure over its desired lifetime. Ideally, the environmental conditions in which a bonded structure must operate can be quantified, and their effect on structural degradation estimated. It is known that, on the long term, the mechanical performance (stiffness and strength) of an adhesively bonded structure strongly depends on the environmental temperature and the amount of moisture. Both of these environmental factors adversely affect the structural integrity of an adhesive joint, and thus its durability and lifetime. This work analyses aluminium-epoxy single lap shear samples after ageing. An experimental approach is applied to improve the understanding of the mechanical behaviour of adhesive bonds when exposed to multiple environmental factors acting simultaneously. This paper extensively discusses and compares the individual and combined influence of four different environmental factors, static and dynamic loading, UV radiation, temperature and humidity, on the residual strength of a single lap shear joint (ASTM D 1002-05). A design of experiment approach is adopted to define the interaction between the different ageing parameters mentioned above. Moreover, an ANOVA procedure, carried out on various test batch results, analyses the sensitivity of the various environmental factors. The main observed mechanical characteristics are the stiffness and (residual) strength of the single lap samples. After failure, an analysis of the fracture surface yields further information on the sample condition after ageing. The results of the study, for the specific investigated test conditions, indicate that the residual strength of the single lap joint depends strongly on temperature, our observations even show an increase by a factor of 3. On the other hand, other effects may cause a reduction of mechanical properties.

Introduction

The long-term performance of structural adhesive joints heavily relies on the resistance to ageing of the adhesive used. Ageing of adhesives is defined by DIN 50035-1 as the irreversible physical and chemical change of a material over time. According to J. Comyn [1] and A. Herzig et al. [2], this is mainly caused by the simultaneous effects of mechanical loadings and environmental factors, such as temperature, humidity and UV radiation. Moisture uptake is recognised as one of the major factors affecting the structural integrity of adhesive joints over time [3]. In the case of epoxy based adhesives, Jurf and Vinson [4] noted that the uptake of moisture reduces the modulus in the transition region, while these adhesives are relatively insensitive to moisture in the glassy and rubbery regions. They suggest that the effect of moisture uptake is irreversible, because of moisture induced effects such as micro-cracking and chemical decomposition. Apicella et al. [5] and Hand et al. [6] recognised this theory and stated that moisture diffusion caused irreversible damage by producing microcavities that are responsible for the loss in cohesive strength.

Several authors [7,8] report that the induced failure mechanism varies from cohesive failure in the adhesive to apparent interfacial failure with increasing moisture content. The partial recovery of strength after drying has been attributed to the reversible part of the moisture absorption in the (adhesive) bulk material. The remaining strength reduction is associated with the irreversible interfacial damage, leading to interfacial failure. Mubashar et al. [9] studied the moisture absorption-desorption effects of single lap epoxy adhesive joints. The samples consisting of Al2024 T3 showed a recovery of up to 100% of the original strength in joints conditioned up to 182 days. It was noted that the apparent interfacial failure left a thin layer of primer on the adherend due to mechanical interlocking caused by the pre-treatment.

In addition, the moisture absorbed causes plasticisation and swelling of the adhesive bulk material. This phenomenon can be reversed by drying the adhesive joint at a higher temperature. However, moisture absorption depends on the temperature of the environment. Many studies [[10], [11], [12], [13]] that investigate the influence of moisture absorption on the glass transition temperature (T_g) of a polymer show a decrease in the T_g. Odegard [14] states that even sub-T_g temperatures cause physical changes in the molecular structure of the epoxy polymers. Physical changes in the molecular structure reduced the free volume, which affect the structural reliability of the epoxy. Bellini et al. [15] shows with an experimental study a decrease in strength of structural epoxy joints as a result of hydrothermal ageing and improved performances when aged with increased air temperatures. Bai et al. [16] report that changing the temperature affects the fatigue life. More specifically, there is a 50% reduction in joint strength and about 20–30% reduction in residual stress compared to room temperature measurements. In contrary to the effect of moisture, the tensile strength of an epoxy adhesive decreases by 13.9% after an exposure to UV of 744h, while the modulus increased significantly by 105%, according to Nguyen et al. [17].

In order to make reliable predictions about the life span of an adhesive bond, it is necessary to carry out tests in truthful conditions. Possart and Brede [18] describe the complexity and influences of (artificial) ageing on failure behaviour of adhesive joints. According to Costa et al. [19], no consistent trends are reported in studies on the influence of temperature and humidity on the failure behaviour of adhesive bonds. More studies would certainly provide clearer insights on the inherent correlations between the various environmental factors, especially with cyclic loading.

The influence on stiffness and strength of environmental factors cannot be underestimated. Although extensive studies of mechanical fracture and ageing of adhesive bonds have already been conducted and published, the simultaneous effects and the interaction of all these factors are still a blind spot. Ageing tests seldom are conducted with the test specimen also under dynamic loading. This is particularly the case when the specimen is exposed to a combination of degradation factors such as UV radiation, moisture and temperature. This experimental research contributes to the state of art by investigating the interaction of environmental ageing factors on dynamically loaded samples and their influence on the mechanical properties of the adhesive bond.