It is vital to recognise the importance of getting the joint geometry right in a bonded assembly, as an adhesive joint is extremely vulnerable to peel stress. For example, at flange points in a composite box section, rapid crack growth can occur through the adhesive joint under impact conditions and the tensile stress component in particular. Crack growth through a continuous bead of adhesive in a composite joint structure can lead to catastrophic failure in fatigue and impact conditions.
Careful attention to the joint design can mitigate the effects of rapid crack progression. The crack develops and propagates through the adhesive and tends to stop at the boundary. For the crack to continue, a new crack front must be created which requires additional energy. This sequence of crack initiation, propagation and arrest leads to higher resistance to run-away crack growth in composite materials. Advanced analysis undertaken by Engenuity can determine the size and spacing of adhesive beads to resist crack growth.
Research is underway in many industries to determine and enhance the fracture toughness of adhesive bonds, working with a variety of substrates. The auto-industry is looking for improvement of composite joints in a future generation of plastic composites vehicles. The aerospace industry is also seeking to enhance the fracture toughness in repair patches for aircraft structures, in order to arrest development of fatigue cracks.
Engenuity is playing a vital role in analysing and determining the optimum solutions for bonded joints in a wide range of industry sectors. Finite element analysis is an important tool for determining the loading and performance properties in composite joints, in concert with other test programmes.