Combination and interaction of ageing parameters on single lap shear adhesive joints
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 (Tg) of a polymer show a decrease in the Tg. Odegard [14] states that even sub-Tg 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.
Section snippets
Materials and sample configuration
Adhesive bonded joints are commonly achieved by bonding thin substrates together in a lap geometry. Therefore single lap joint samples based on ASTM D 1002-05 [20] as shown in Fig. 1 are used. The aluminium plates are, as defined in the standard, Aluminium alloy 2024 T3. Holes were punched with a diameter of 5 mm at 7.5 mm from the edge in the middle of the plate to ensure fastening in the test set-up.
The pre-treatment consists of immersion in boiling water followed by soaking in a 1% aqueous
Results
The residual strengths of the aged samples are reported in the whisker-boxplots of Fig. 6, Fig. 7. The first digit defines the load, the second the presence or absence of UV radiation, the third a constant or fluctuating temperature as described in subsection 2.4 and the fourth the level of humidity.
Samples that are aged at an increasing temperature, which is the first part of the temperature cycle show a clear increase in residual strength. This is because post-curing takes place at higher
Discussion
Fig. 12 and Fig. 13 plot the lap shear strength –displacement as a function of ageing parameters. They present that the strength of joints conditioned with a temperature cycle is significantly higher than all other ageing parameters. Even moisture doesn't affect the strength significantly as might be expected based on the literature [16]. After visually analysing the failure surface (see Fig. 14), it can be stated that the samples conditioned in this specific investigation with static load
Overall conclusions
The ageing of adhesive single lap joints, with a unique test set-up, was successfully performed. Furthermore, the residual strength of these single lap-shear samples was defined with a universal testing machine. After an extensive statistical analysis, taking into account the chosen parameters and conditions under which these tests took place. The following conclusions can be drawn:
- •
In this research, the residual strength generally increases with a factor 3 in test cases where temperature
Acknowledgements
This work is supported by the M-group, part of the KU Leuven Bruges Campus. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
References (28)
- et al.
Effect of thermal history on water sorption, elastic properties and the glass transition of epoxy resins
Polymer
(1979) - et al.
Effects of environmental exposure on adhesively bonded joints
Int J Adhesion Adhes
(1991) The durability of adhesive joints in the presence of water
Int J Adhesion Adhes
(1996)- et al.
Moisture absorption-desorption effects in adhesive joints
Int J Adhesion Adhes
(2009) - et al.
Segmental relaxation of water-aged ambient cured epoxy
Polym Degrad Stabil
(2007) - et al.
The glass transition temperatures of highly crosslinked networks: cured epoxy resins
Polymer
(1982) - et al.
Absorption and glass transition temperature of adhesives exposed to water and toluene
Int J Adhesion Adhes
(2014) - et al.
Effect of operating temperature on aged single lap bonded joints
Defence Technology
(2020) - et al.
Effects of ultraviolet radiation and associated elevated temperature on mechanical performance of steel/cfrp double strap joints
Compos Struct
(2012) - et al.
Effects of ultraviolet radiation and associated elevated temperature on mechanical performance of steel/CFRP double strap joints
Compos Struct
(2012)
Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: state of the art
Compos B Eng
Dynamic strength of adhesive single lap joints at high temperature
Int J Adhesion Adhes
Environmental (durability) effects in Adhesive bonding, Science, technology and applications
Ageing Phenomena in polymers: a short Survey in adhesive joints, ageing and durability of Epoxies and Polyurethanes
Cited by (3)
Adhesively bonded joints – A review on design, manufacturing, experiments, modeling and challenges
2024, Composites Part B: EngineeringImproving the wettability of oxide layers to enhance the bonding strength of shot-blasting steel substrates by using simple resin pre-coating method
2024, International Journal of Adhesion and AdhesivesEffect of Thermal Aging on Dynamic Mechanical Performance of a Novel Structural Adhesive
2022, International Journal of Heat and Technology