In-process Inspection of Pultruded Tubular

Pultruded products offer many excellent properties, particularly high strength, low weight and excellent corrosion resistance. However their acceptance into many critical applications has been adversely affected by the inability to guarantee ‘defect-free’ quality. Common problems such as voids and delaminations will greatly reduce the strength of the finished product, but until now there has been no effective means of verifying their absence during manufacture. The water coupling required for conventional ultrasonic inspection is unacceptable during the pultrusion process. Manual inspection of finished products is very time-consuming, and runs the risk that an unrecognized production problem could result in large quantities of scrap product being produced. Air coupled ultrasonic Inspection has been proven over a number of years in critical applications such as inspection of honeycomb composite panels for space applications. Developments of the technique have been applied to in line inspection of pultruded products, using both scanning and surface wave approaches. For tubular products a surface wave technique can be used, applied from the outside of the tube only, immediately near the mandrel, while the tube is still at a temperature around 80 oC. Transducers can be positioned up to a metre apart, giving very rapid inspection. Experience has been gained in the USA developing a multi channel technique on pultruded tubes intended for support pole applications, It has proved possible to find critical defects which might otherwise have compromised the safety of the finished product. The system also provides immediate feedback for process control, and allows a central data connection facility. This will allow the US based head office to monitor production at plants all over the world, allowing ‘discrepancies’ between produced and shipped product quantities to be identified. Introduction In this paper I will attempt to describe some work carried out by QMI Inc. in California during the last year. This has resulted in the development of a technique, currently being implemented, which gives cost-effective 100% in-line inspection of pultruded materials. The Pultrusion Process The pultrusion process for continuous manufacturing of composite material profiles has been in use for many years but is still finding new applications. As the name suggests, pultrusion is a combination of ‘pulling’ and ‘extrusion’. Extrusion relies on pressure to force a heated material through a die, with pultrusion the material is pulled through the process. Reinforcing fibres, typically carbon or glass, are saturated in a resin bath then, formed to shape and passed through a heated die. The heat cures the resin and produces a finished material at the desired dimension. The process is typically capable of making continuous lengths of material in a wide range of sizes. Because of the need to ensure proper wetting of the fibres and curing in the die the process is relatively slow, typical line speeds are of the order of 50cm per minute.