Fatigue Behavior Of Adhesively-bonded Joints Research Articles

The Fatigue and Durability Behaviour of Automotive Adhesives. Part III: Predicting the Service Life

In Part I [1] a fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release-rate, G max, applied in the fatigue cycle, using logarithmic axes. In Part II [2] the mechanisms of failure were considered, particularly the mechanisms of environmental attack. The present paper, Part III, discusses the use of the relationship between da/dN and G max, which can be obtained in a relatively short timescale, to predict the fatigue lifetime of (uncracked) single-overlap joints cyclically loaded in tension. An analytical and a finite-element model have been derived to predict the number of cycles of failure, Nf , for lap joints and, particularly when the latter model was used to deduce the value of the strain-energy release-rate, G, in the lap joints, the agreement between the theoretical predictions and the experimental results is found to be very good.

The Fatigue and Durability Behaviour of Automotive Adhesives. Part II: Failure Mechanisms

In part I [1] a fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The cyclic fatigue tests were conducted in a relatively dry environment, of 23°C and 55% RH, and in a “wet” environment, namely immersion in distilled water at 28°C. The “wet” fatigue tests clearly revealed the significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The present paper, Part II, discusses the modes and mechanisms of failure for the two adhesive systems in both the “dry” and “wet” environments. The failure surfaces of the joints tested in Part I have been examined using a variety of analytical techniques and the surface chemistry and morphology compared with that of the “as prepared” (i.e. non-bonded) metal surfaces and cured adhesive. In the present investigation use has been made of an elemental mapping form of X-ray photoelectron spectroscopy (EM-XPS) along with conventional XPS. The surface topography has been examined using scanning electron microscopy and atomic force microscopy. Also, cross-sections of the joints have been studied using the transmission electron microscope. The results reveal that for both the aluminium alloy and EG steel joints that the failure path is complex, and is associated with electrochemical activity (i.e. corrosion) in the case of the latter joints when tested in the “wet” environment. In part III [2], the results presented in the earlier papers will be used to predict the lifetime of single-overlap joints subjected to cyclic fatigue loading.

The Fatigue and Durability Behaviour of Automotive Adhesives. Part I: Fracture Mechanics Tests

A fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release rate, Gmax , applied in the fatigue cycle, using logarithmic axes. Of particular interest was the presence of a threshold value of the strain-energy release rate, Gth , applied in the fatigue cycle, below which fatigue crack growth was not observed to occur. The cyclic fatigue tests conducted in a relatively dry environment of 23°C, and 55%; RH were shown to cause crack propagation at far lower values of Gmax compared with the value of the adhesive fracture energies, Gc , which were determined from monotonically-loaded fracture tests. Cyclic fatigue tests were also conducted in a “wet” environment, namely immersion in distilled water at 28 C. The “wet” fatigue tests clearly revealed the further significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The development and standardisation of “wet” fatigue tests may provide the basis for a very effective accelerated-ageing test.

Predicting the Fatigue Life of Adhesively-Bonded Joints

Abstract The fatigue behaviour of adhesively-bonded joints, which consisted of an epoxy-film adhesive bonding fibre-composite substrates, has been studied. Using a double-cantilever beam specimen, the rate of crack growth per cycle has been measured as a function of the maximum strain-energy release rate, Gmax. These data have then been modelled, and used to predict the fatigue lifetime of bonded single-lap joints. The agreement between the theoretical predictions and experimental results for the fatigue behaviour of the single-lap joints was found to be excellent.

Experimental Investigation of Epoxy Bonded Polymethylmethacrylate Joints

Abstract This paper presents an experimental investigation into various aspects of epoxy-bonded polymethylmethacrylate (PMMA) and PMMA-to-aluminium joints. The effects of adhesive thickness, overlap area, surface roughness, and environmental exposure on the joint strength were studied. Results indicated that the joint strength was not directly proportional to the overlap area, while sanding had a positive effect on the joint strength. A negative effect was observed when adhesive thickness was increased. The fatigue behaviour of adhesively-bonded joints under dynamic loading was found to be independent of frequency, for the range of values tested; however, it was dependent on the test temperature with greater reduction in fatigue life observed in PMMA-to-aluminium joints at higher temperature. Empirical equations from which the fatigue life of joints can be predicted were obtained by regression analysis. Intermittent fatigue testing of the joints was also performed. The epoxy adhesive tested proved to be a satisfactory choice for outdoor exposure. The rate of degradation of the adhesive was slow with the adherend itself degrading at a faster rate than the adhesive or the bondline.