Detection of transverse cracks in laminated composites is important for the development and verification of damage-growth models. Historically, it has been assumed that the matrix cracks propagate all the way across the width of a test coupon. When this happens, the dye-penetrant technique works well for the detection of cracks. However, when a crack starts at one edge but does not run all the way across (a partial crack) or when the crack is totally internal, the dye-penetrant method does not give satisfactory results. Moreover, the dye-penetrant technique cannot be used for in-situ testing. The objective of this paper is to report the development of an ultrasonic method for in-situ detection of partial or internal across-the-width cracks.
In the transmission mode, a pair of 20-MHz focused transducers was used in the pitch-catch arrangement. The angle of incidence was chosen to exceed the critical angle for the excitation of a longitudinal wave. This results in the excitation of a shear wave which is more sensitive for the detection of transverse cracks.
Sometimes access is limited to only one side of the plate. To cover these situations, a reflection technique was also developed. Here, higher-order Lamb-wave modes were used (Modes 1 and 3; frequency range 4 MHz 11 MHz). The advantage here is that in the absence of a crack, there is no reflection, i.e., the wave sees a crack against a dark background. Moreover, the inspection can be done at a lower frequency where wave propagation is less sensitive to plate surface roughness. A disadvantage is that the spatial resolution of the reflection mode is not as good as that of the transmission mode.
The technique was used to examine a number of graphite/epoxy [0,90,0] coupons, 0.5mm1mm thick, which had been subjected to monotonic tensile loading prior to the ultrasonic test. A large number of transverse cracks were detected. As expected, a majority of the cracks ran all the way across the width of the test coupon. Some of the cracks started at one edge but were arrested in the middle. Occasionally a crack that started and stopped in the interior was detected. The cracks were also recorded by an examination of the edges using an optical microscope; an excellent correlation between the optical and the ultrasonic images was observed.
It is well known that a closed crack cannot be detected by the use of ultrasound. It is considered possible that what is ultrasonically perceived as a partial crack may, in fact, be an across-the-width crack with a part of its length closed (due, for example, to residual stresses). In order to guard against this possibility the following experiments were conducted. A plate was subjected to a significant bending moment, guaranteeing that at least one half of the crack (the half that occupies the tensile half of the plate) is open. No difference in the ultrasonic images with or without bending was observed. It is concluded that the technique is suitable for the detection of partial and internal cracks.