A combined strain energy density and entire fracture surface approach for life prediction of short carbon fibre reinforced PEEK

Purpose – The correlation between fatigue response and surface topography of a PEEK composite, in which different preferred orientations of short carbon fibres were present as a result of extrusion process, was investigated. Fatigue tested specimens, all obtained from the same extrusion plate, were grouped based on their elastic modulus, evaluated before the tests. The detailed study of fractography combined with the fatigue failure process study would greatly address the manufacturability and performance of the materials. Design/methodology/approach – Fatigue life of a PEEK short fibre composite with different fibre orientations from extrusion process was investigated. Next, fracture behavior was analysed using the entire fracture surface method. Fatigue behavior was evaluated with a combined approach based on strain energy density (SED) and fracture surface factors. Findings – Relevant surface topography parameters were assessed on fracture surfaces from fatigue testing, and SEM analyses were carried out to assess the nature of failure mechanisms. A combined approach, based on the definition of a parameter depending on SED and fracture surface factors, is then presented for fatigue life estimation. While fatigue strength differed strongly depending on initial stiffness, the proposed parameter provided a general framework to capture all test results, including dependency on fibre orientation. Research limitations/implications – With its broad range of applications, the entire fracture surface approach can be used to analyse a variety of engineering problems involving various tested materials, geometric configurations and loading histories. Using the “entire” area of the fracture surface in relation to huge dimensions or irregular forms requires interpretation, which is a disadvantage of this method. Originality/value – The combination of fatigue fracture surface topography assessment with SED and fatigue life data seems a viable way for the development of methods aiming at calculating failure parameters and damage factors from post-mortem analyses.