TWI Industrial Member Report Summary 603/1997
G E Hale
As the load carrying capacity of plastics continues to grow in importance, a high level of structural integrity becomes an essential part of the long-term viability of the structure.
For plastics a comprehensive knowledge of accepted failure modes, eg brittle fracture, creep fatigue etc, is required. At the same time, the deformation rate and temperature sensitivity of plastics must also be taken into account. Application of fracture mechanics principles to plastics provides a mechanism for understanding how cracks develop and grow in structures fabricated from plastic materials. Standardised test methods are needed to yield meaningful fracture mechanic parameters which can be used for assessment purposes and to compare materials.
The ESIS (European Structural Integrity Society) Technical Committee on polymers and composites is made up of representatives from industrial and academic organisations who work together to develop test methods for determining the fracture mechanics parameters of polymeric-based materials. This group has been instrumental in developing a draft international standard for LEFM (Linear Elastic Fracture Mechanics) testing of plastics. However, many of the more common engineering thermoplastics only exhibit linear elastic behaviour at low temperatures or high loading rates.
By applying the concepts of elastic plastic fracture mechanics (EPFM), it has been possible to develop a testing protocol which evaluates the J-crack growth resistance curves of tougher thermoplastics. The report summarises work undertaken by laboratories across Europe to assess the effectiveness of this testing procedure, which thermoplastics it applies to, and to propose changes which will make the test method acceptable.
- To evaluate and analyse J-crack growth resistance curve data determined in accordance with an ESIS testing protocol.
- To identify those factors which have the greatest influence on accurate determination of J R-curves for plastics.
- To refine the testing procedure so that it can form the basis of an international standard.