Hydrothermal aging effects on high strain rate behaviour of adhesively bonded composite joints

Composite materials have attracted significant attention owing to their excellent properties and potential industrial applications. Most structural composites are characterized based on their ability to withstand loads for longer time duration under critical environmental conditions. Some applications may require the joining of composite structures at various points. This has been successfully achieved through adhesively bonded joints which exhibit good damping, resistance and vibration properties. Therefore, it is important to investigate the behavior of these materials under both mechanical loads and environmental effects.

To this note, researchers at ENSTA Bretagne in France: Mourad Nachtane (PhD), Professor Mostapha Tarfaoui, Dr. Sassi Sonia, Dr. Ahmed el moumen together with Professor Dennoun Saifaoui from the Laboratory for Renewable Energy and Dynamics Systems in Morocco recently studied the influence of hydrothermal aging effects on the mechanical behaviors of adhesive bonded composite joints at high strain rates. The experiment was based on the Split Hopkinson Pressure Bar Machine, that was utilized to analyze the progression of the microstructural damages and the dynamic compressive properties of the composite joints. Their work is published in the journal, Composites Part B: Engineering.

First, the samples of the adhesively bonded composites were subjected to hydrothermal aging conditions, preferably 50°C of temperature and 80% of relative humidity at different time intervals. The hygrothermal aging experiment was used to determine the moisture absorption behaviors and the weight gained by the specimen as a function of time. Secondly, in-plane dynamic compression tests was conducted at different impact pressures to quantify the effect of hygrothermal aging on the dynamic response and kinetics of damage of the composite / adhesive assembly. . Finally, the strain-stress characteristics of the composite specimens were evaluated under severe hygrothermal conditions at higher strain rates in the 445-1240 s^-1 range.

Environmental effects exhibited significant changes in the dynamic properties and damages behavior of the composite specimen. For instance, optical microscopy and scanning electron microscopy observations revealed failure modes for each specimen. For adhesively bonded composite joints, the moisture absorption amount was reported to about 0.28% of the weight at saturation level. In all the cases, however, an increase in the strain rates and moisture absorption resulted in a corresponding decrease in the failure stress, failures strain, and initial modulus. Moreover, the samples exhibited a decrease in the fracture strength with moisture absorption at higher strain rates and especially when the sample was loaded in the in-plane direction. This was attributed to the significant role of the matrix properties which were more prone to stain rate as compared to fiber dominant properties.

The strain rates and aging time highly influenced the dynamic compressed behaviors of the composite assemblies. The samples became more prone to damages with the increase in the strain rate. For example, laminated splitting failure was dominant at low strain rates while delamination dominated at higher strains. Furthermore, the main physical and chemical degradation of polymer structures due to water absorption was observed to be as a result of the leaching, hydrolysis, swelling and plasticization effects.

In summary, the scientists noted that critical understanding of the moisture absorption and desorption behavior is a crucial consideration in the evaluation of the perceived material and structural performance. Based on the study findings, Mourad Nachtane, first author mentioned that their study provides essential results that will pave way for advancing further investigation in hydrothermal aging effects in other materials.