Test conditions in stress wave factor measurements for fibre- reinforced composites and laminates M. Bhatt and RJ. Hogg An acousto-ultrasonic technique using Vary's definition of the stress wave factor is evaluated (SWF = grn). AET's 206 AU model is used for the study. Instrument variables are sequentially eliminated and reduced to the instrument gain and the background noise. Regions of validity are then defined in terms of the background noise levels, and of the instrument gain, enabling stress wave factor measurements to be made on various unreinforced and glass-reinforced polyester resins, with and without introduced defects. The stress wave factor is found to be most reproducible when independent of background noise and at high gains. The equivalence of readings taken using through-transmission and those taken by placing transducers on the same side is investigated. Readings taken using through- and same surface- placement are found to be equivalent only at high gain, with doubtful results for low gain. It is found that the technique needs to be re-appraised. The study emphasizes that before the stress wave factor technique can be used reliably, the regions under which the stress wave factor is reproducible must be carefully defined. Keywords: stress wave factor, acousto-ultrasonic technique, composite materials, polyester resins, introduced defects, instrument gain, background noise A novel technique that has recently become available in the UK is the commercial equipment that allows stress wave factor measurements, as marketed by AET (USA), the model 206 AU. In essence, the technique consists of exciting a broadband piezoelectric transducer by using an electrical pulse, allowing the transducer response to propagate through the system under test and analysing the signal using what are termed acoustic emission techniques. This latter consists of counting the number of oscillations that exist above a particular noise threshold within the transducer pulse as modified by the system. The number of oscilla- tions is characteristic of the energy spectrum that exists due to interaction with the system. Ifa low-energy pulse is generated at a pulse rate g, then the number of oscillations (or 'ringdown counts'), n, may be counted within a time window r. The stress wave factor (SWF) is then defined as SWF =gm and is considered to be a material parameter, given that other conditions are constant, characteristic of the system under test. In work by the several groups that have used the technique so farh-~31, the stress wave factor has been shown to be sensitive to deviations from perfection within a speci- 0308-9126/88/010003-08 $3.00 © 1988 Butterworth ~ Co (Publishers) Ltd NDTInternationalVolume21 Number 1 February1988 men and has been considered to be capable of distinguish- ing inferior material correctly. In addition, the stress wave factor is variously claimed to provide information about flaw population, regions of residual stress, local microcracldng and other failure phenomena. However, it should be emphasized that the stress wave factor is only a comparative number as cited within the literature, and it is uncertain that one specimen may be compared and ranked correctly with another made of the same material. In fact, a quick survey of the sources cited above indicates that, apart from being a comparative technique rather than an abso- lute one, there appears to remain the problem of consistent reproducibility from one reading on a single site to the next reading and of the conditions under which the readings are taken. Initial work[lZ,q indicates that the stress wave factor and its interpretation are open to considerable ambiguity. This viewwas reinforced by work done in the UKprior to the pre- sent study, which indicated that a reappraisal of the tech- nique was in order. Experimental In order to evaluate the technique, it is necessary to identify all the contributions to the cause of the ambiguity in the results. The factors are many and varied, some central to the test technique and some the result ofmaterialvariation. 3