TWI Knowledge Summary

Mechanical testing of welded plastics joints

Introduction

Welded joints are potential sites of weakness in a plastic structure. Consequently, it is important to be able to show that a welded joint is fit for purpose. This can be achieved by appropriate mechanical testing techniques. There are several standard test methods for welded plastics joints. Many of these methods were developed by the plastic pipe industry as quality control techniques for welding processes. In addition, some standard plastic test methods can be applied to plastic welds. Some of the more common test methods are summarised below.

Tensile tests

Tensile tests are perhaps the most common methods used to characterise welds. Weld beads are usually removed in cases where they are removed in service. Typically, a tensile weld strength of 80-100% of parent material strength is acceptable, depending on the material and welding process. If failure occurs away from the weld, then a specimen with reduced cross-sectional area at the weld may be used to optimise welding parameters.

Bend tests

Butt welded joints in plastics may also be assessed using a bend test. Specimens are typically loaded in three point bending. The weld flash on the face in contact with the centre loading point is removed prior to testing. The angle at which either fracture occurs or a crack initiates is recorded. The bend angle and fracture appearance provide a guide to the ductility of the joint and the weld quality.

Impact tests

The resistance of a material to fracture under dynamic loading is often critical. A straightforward and cost-effective approach is to use the Charpy test, a semi-quantitative method that can be used to assess the energy absorbing characteristics of polymer welds. It can sometimes be more useful than tensile tests in distinguishing between different welding conditions. For example, a joint may show a tensile strength equivalent to its parent material, whereas the absorbed Charpy impact energy for the weld may be less than 50% of that of the parent material. However, when applying this test to welded plastics, accurate positioning of the notch at the weld line is essential. Also, any weld bead present should be removed.

Creep rupture tests

Creep rupture tests can be used to compare the long-term performance of plastic welds with those of the parent material. Normally, tests are carried out using tensile specimens under a constant load and elevated temperature and the time to failure is measured. Tests are normally carried out in water, but a surface-active medium can be used to accelerate failure. Although more expensive to perform than short-term static tests, creep tests provide more useful information when designing components which are under constant load.

Fracture mechanics tests

More rigorous fracture mechanics type test methods can be used to quantitatively characterise plastic welds, but these are not yet fully established and require extremely accurate notch tip positioning and test procedures. Testing can be conducted under three point bend loading using a single edge notch bend (SENB) specimen following standard methods for plastic and metallic parent materials. Some of the more brittle plastics can be characterised using linear elastic fracture mechanics (LEFM). However, many plastics show extensive crack tip plasticity requiring a more complex elastic-plastic fracture mechanics (EPFM) analysis.

Summary

There are many test methods that may be applied to welded and parent plastics. In order to select the most appropriate method, it is important to recognise the mode of loading in service and the limitations imposed by these methods. Also, it must be understood that the results of a short term test should not be interpreted to predict the long term performance of a joint in-service. Ideally, a plastics joint should be evaluated across a range of strain rates and modes of loading.

Further information

Other related content on mechanical tests in plastics welds includes:

Hot gas welding of plastics - a guide to best practice. Section 5. Fault identification, inspection & testing

Butt and socket fusion welding of plastic pipe - a guide to best practice. Section 6. Weld inspection and testing

Standards used for polyethylene pipe