TWI Industrial Member Report Summary 686/1999
R J Wise and S D Smith
This report concentrates on the measurement of melt orientation of polymer chains and welding residual stress using birefringence. Tests are reported for the model polymer PMMA using four different welding techniques.
Welded joints in glassy thermoplastic polymers may be the weakest part of a structure made from such materials. This can be due to the geometry of the weld, residual stresses, or to polymer chain orientation because of melt flow. Each aspect of weld strength reduction due to welding is called a heterogeneity in this report. A study of welding procedures should therefore determine whether preferred joining techniques can be identified from a study of how much of each heterogeneity they create, and how much weakening is introduced. This must be done in two stages. The level of each weld heterogeneity must be measured for a wide range of welding procedures.
Further work would develop a predictive technique for equating the level of heterogeneity with the loss of strength at a welded joint.
- To use birefringence to measure polymer chain alignment introduced by welds and levels of welding residual stresses.