Development of advanced methods to characterise the fracture behaviour of flexible bonded joints
- DIRECTORS: Javier Zurbitu, Albert Turon and Jordi Renart
- UNIVERSITY: Universitat de Girona
Joining technologies play an important role in one of the most important technical challenges of today: climate change. The main objectives are to reduce the carbon footprint and to increase the environmental sustainability and for this purpose, industry has to develop more efficient and durable systems that may favour the reduction of CO2 emissions and renewable energy generation. Transport or energy sectors support innovation in lightweight materials (e.g. composites) and structures to reduce energy consumption, and multi-material structures are used to provide their integrity. While conventional joining technologies show limitations when mechanical requests are high, advanced joining technologies such as adhesive joints take advantage. The lack of knowledge about the mechanical behaviour of bonded joints, however, produce oversized structures and duplicated joints by adding extra mechanical joints for precaution.
Most studies are based on rigid bonded joints using epoxy adhesives although the catalogue of structural adhesives is extensive. Thus, experimental studies are lacking to analyse the structural behaviour of flexible adhesives, which may provide improved properties against dynamic loads or harsh environments. The most widely used test standards are the DCB test (ISO-25217) and the Wedge Test (ASTM D3762) to evaluate the quality and the durability of bonded joints, respectively. Their principal limitation, however, is that crack length measuring is needed and that results show a strong dependence on joint dimensions. Moreover, there is not yet consensus regarding the use of the appropriate method specially when using flexible adhesives or in durability studies.
The present Thesis gives a step forward and proposes different solutions for the characterisation of the fracture behaviour of flexible bonded joints, contributing to the use of bonded joints in industry. In this way, the influence of the width-to-thickness ratio (i.e. stress state) on the mode I fracture toughness of flexible bonded joints has been evaluated through an experimental study. An analytical model has been developed to predict the fracture toughness of flexible bonded joints and it is proved that the fracture behaviour of flexible bonded joints is dominated by the elastic behaviour of the adhesive. Acoustic emissions technique is proposed to reduce subjectivity in crack length measurement and mode I crack growth is monitored in bonded joints, providing an estimation of the size of the fracture process zone (FPZ). The method is valid for rigid and flexible bonded joints and evidences in numerical and experimental results show the nature of acoustic events.
Bonded joints applied in industrial environments, however, must also withstand environmental conditions (e.g. temperature and humidity), but the selection of the appropriate method to characterise their durability is not evident. A durability study of flexible bonded joints has been conducted to evaluate the suitability of different experimental methods available in the literature. For this purpose, bonded joints have been subjected to load and environmental conditions and the fracture toughness has been evaluated at different exposure times. It is concluded that neither the Wedge Test (ASTM D3762) nor the experimental methods that require crack length measurement are appropriate to evaluate the durability of flexible bonded joints exposed to the combined effect of loads, temperature and humidity. To compensate the lack of experimental methods to address durability studies, a new data reduction method named Wedge Driven Test Plus (WDT+) has been developed. It is valid for rigid and flexible bonded joints and avoids crack length measurement to evaluate the fracture toughness. The influence of test speed and the wedge diameter on the test procedure have been analysed by comparing the WDT+ and the DCB test. It is proved that the proposed method is robust, less sensitive to test speed than the DCB test (ISO-25217) and applicable in bonded joints with any bondline thickness. Finally, a new experimental method has been developed to evaluate the durability of bonded joints exposed to load and environmental conditions. The test method applies the WDT+ to provide the evolution of the energy release rate (ERR) during the test. For this purpose, a new test rig has been developed and, despite its low TRL level, the proposed method shows a great potential to be used in future projects with an industrial application.