Evaluation of different crack driving forces for describing the fatigue crack growth behaviour of PET-G

To predict the fatigue crack growth in full-scale components made of polymeric materials, a methodology to transfer laboratory crack-growth data to structures is needed. This requires the similitude hypothesis to be applicable using a valid crack driving force, which allows the description of the R-ratio effect on the crack growth behavior. Whereas in metals it is common to use the range in the applied stress intensity factor, Delta K, as the crack driving force, in polymeric materials and composites the use of the energy release rate G has been considered and Delta root G has been proposed as the crack driving force to ensure the similitude principle. Therefore, the aim of this paper is to assess the validity of Delta root G as a proper crack driving force and to explore the way of practical application in an amorphous thermoplastic copolyester Polyethylene terephthalate glycol (PET-G).