A variety of non-destructive examination (NDE) techniques are available for inspecting plastic welds. Many of these techniques are similar to the ones used for inspecting metal welds. Traditional techniques include visual testing, radiography, and various ultrasonic techniques. Advanced ultrasonic techniques such as time of flight diffraction (TOFD) and phased-array ultrasonics (PAUT) are being increasingly studied and used for inspecting plastic pipeline welds. Research in the use of optical coherence tomography (OCT) and microwave reflectrometry has also been conducted.
The main purpose of NDE is to detect defects in the weld and the joint fit-up. Examples include joint mismatch, cracks, porosity, voids, inclusions, lack of penetration and lack of fusion (cold joints). However, the lack of defects does not necessarily mean the weld is adequate and thus NDE does not provide a good indication of weld strength, long term performance, or its ability to handle cyclic loading. Although destructive testing gives a better indication of actual joint strength, it's impractical to use on production welds since the sample is destroyed. Hence, the need to use and develop NDE techniques to ensure sound welds are being produced during production welding.
Visual Testing (VT) Visual testing is one of the most basic and inexpensive methods. It consists of a technician visually observing the joint and comparing it to a set of acceptance criteria based on code of construction. Its use is mainly limited to joint fit-up and detecting surface breaking flaws. It can be done as an in-process inspection, while the welding is occurring, or on the completed weld. In-process inspections can show problems with the weld such as discoloration, indicating thermal degradation, bubbles, scratches, and weld bead problems. Visual inspection of the completed weld can detect joint fit-up and tool problems such as mismatch, angular misalignment, notches, and insufficient weld size as well as surface breaking weld defects like porosity, cracks, and excessive flash. As with most NDE techniques, technical training, experience, and skill is important for a successful inspection.
Leak Testing
Leak testing is commonly used for piping and pressure vessels due to the safety, reliability, or environment consequences of a leak once in service. The part to be tested is filled with water, air, or other type of fluid. Depending on the application, testing can be done at atmospheric pressure, under a positive pressure, or under vacuum. Evidence of leak can either be visual or by observing a change in pressure. When positive pressure is used, testing with non-compressible fluids is safer than using a gas. Gasses under pressure contain a significantly higher amount of stored energy than liquids and a rupture can pose a significant safety risk.
Radiography (RT) Radiography uses an X-ray tube or radioactive source to give an image of the weld. Its use is similar to metal welds and provides a volumetric inspection which can show defects in the weld not visible from the surface. For plastics, a lower radiation intensity is typically required than that for metal welds, due to the lower material density. Because of the higher cost, it's typically only used in critical applications like pipelines and not for commercial products.
Ultrasonic Inspection
Various forms of ultrasonic inspection are used for plastic welds. Plastics have higher attenuation and a lower sound velocity compared to metals which can make these techniques more challenging. In 2010, a joint European project, TestPEP, was started to develop and validate ultrasonic NDE techniques on plastic welded pipe. The use of plastic pipe is limited in safety critical applications due to the lack of accepted NDE techniques and acceptance criteria. For butt welded plastic pipe, TestPEP proposed the use three different ultrasonic techniques. TOFD with PAUT probes to inspect the majority of the weld, traditional PAUT to look for flaws in the middle of the fusion zone, and a creeping wave to inspect the top of the weld.