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This study focuses on the prediction of the fracture mechanics behaviour of a highly flexible adhesive (with a tensile elongation of 90%), since this type of adhesive is becoming widely used in automotive structures due to their high elongation at break and damping capacity. Despite their extensive applications, the understanding of their fracture mechanics behaviour under varying loading rates and temperatures remains limited in the literature. In addition, current prediction models are also unable to accurately predict their behaviour due to the complex failure mechanism that such bonded joints have. This study aims to determine whether a simple triangular cohesive zone model (CZM), which predefines the crack path, can reproduce the load–displacement curves of adhesives under various temperatures and strain rates. To achieve this, a calibrated CZM is used, adapting the model for reference joints and then validating it with independent test results conducted in a wide range of loading and environmental conditions. The tests were performed at speeds between 0.2 and 6000 mm/min and at three different temperatures ranging from −30 °C to 60 °C. Mode I fracture toughness was measured using the DCB (double cantilever beam) specimens. Using a simple triangular CZM may not be optimal for predicting the mechanical response of highly flexible adhesives with complex failure mechanisms and multiple crack paths. However, by correctly adjusting the cohesive zone properties for a limited set of reference conditions, it is possible to accurately predict the mechanical response of these joints across various test speeds and temperatures, significantly reducing costs and effort. [ABSTRACT FROM AUTHOR]

In this study, the effect of temperature (below and above Tg) on the rate of crack growth and on the fatigue threshold energy release rate of a polyurethane (PU) adhesive is analysed. The effect on the results of the Paris law relation used is also investigated. Tests were performed under a displacement-control strategy and at different temperatures. Out of the experimental data, the threshold energy release rate was determined as a function of temperature, something which up until now has been much more common for adhesive systems other than polyurethane. Results showed that the threshold energy release rate diminishes with increasing temperature, while the crack growth rate increases for a given strain energy release rate. The ratio between the threshold energy release rate and critical strain energy release rate remained constant for tests above the Tg, indicating that fatigue tests need only be performed at one temperature when above the Tg at least for the range of temperatures considered in this study and for the adhesive tested. A Paris law relation taking into account the effect of R-ratio during the displacement-control fatigue tests proved effective in unifying results at different temperatures. [ABSTRACT FROM AUTHOR]

The investigation of the behaviour of adhesive joints under high strain rates is an active area of research, primarily due to the widespread use of adhesives in various industries, including automotive manufacturing. Understanding how adhesives perform when subjected to high strain rates is crucial for designing vehicle structures. Additionally, it is particularly important to comprehend the behaviour of adhesive joints when exposed to elevated temperatures. Therefore, this study aims to analyse the impact of strain rate and temperature on the mixed-mode fracture characteristics of a polyurethane adhesive. To achieve this, mixed-mode bending tests were conducted on test specimens. These specimens were subjected to three different strain rates (0.2 mm/min, 200 mm/min, and 6000 mm/min) and tested at temperatures ranging from −30 °C to 60 °C. The crack size was measured using a compliance-based method during the tests. For temperatures above Tg, the maximum load supported by the specimen increased with an increasing loading rate. GI increased by a factor of 35 for an intermediate strain rate and 38 for a high strain rate from low temperature (−30 °C) to room temperature (23 °C). GII also increased for the same conditions by a factor of 25 and 95 times, respectively. [ABSTRACT FROM AUTHOR]