The application of laser-generated ultrasound to the study of aluminium-epoxy bonded systems

Abstract

The poor performance of acoustic wave techniques in predicting adhesively-bonded joint failure under destructive loading is a long-standing problem, known to derive from unreliable adhesive defect detection. This thesis examines the feasibility of applying a relatively new technique, generating ultrasound with pulsed Nd:YAG lasers, to the study of aluminium alloy adherends joined by epoxy layer bonds. Laser generation is a non-contacting method which produces highly repeatable ultrasonic sources in metals, without damping motion at the sample surface. Pulses created in this fashion have bandwidths around 20 MHz and radiate both along the sample surface and into the material bulk. Displacements at the sample surface recorded by a broad bandwidth non-contact detector, such as a 532 nm wavelength laser Michelson interferometer, are therefore able to resolve details in time-varying traces which are not visible when narrowband transducers are used. In particular, individual reverberations between the interfaces of epoxy layers less than 100 μm thick are detected in transmission through adhesively bonded joints, on time domain traces. An epoxy layer sandwiched between two thick aluminium adherends presents a three layer case which is seldom discussed in the literature. I have therefore adapted theory developed for surface waves in thin layers overlying deep substrates, and for waves transmitted through multilayer structures, into an explicit formulation for an elastic layer embedded in adherend half-spaces that can be used for both through-transmission and interface-parallel waves. The case of travelling waves in a viscoelastic layer has not yet been examined as the current formulation requires unfeasibly long computation times. A numerical solution assuming elastic behaviour, gives strong indications that embedded epoxy layers support travelling waves directed along the interfaces, despite the fact that a single interface between epoxy and aluminium will not support non-dispersive Stoneley interface waves. Experimental work presented in Chapters 4 to 7 is preceded by a review of laser generation and non-contact detection methods, which introduces techniques that I have employed. As well as using laser interferometers, 1 have also built my own electromagnetic acoustic transducers (EMATs), to provide a cheaper alternative detection scheme. Chapter 4 concentrates upon on-epicentre detection of direct through transmission pulse arrivals, using analysis both of the entire reverberation wavetrain following the main arrival and of consecutive pulses within it, in order to extract information on the bonds' cohesive and adhesive properties. Chapter Five examines variations in surface-travelling waveforms on unbonded, free-surface aluminium plates with thicknesses varying from 63 mm down 28 μm, in a search for non- dispersive waves that would be suitable for probing adhesive bonds. Rayleigh arrivals on samples over 10 mm thick and the symmetric zero-order Lamb mode on plates under 200 μm thick both propagate from the NdtYAG laser source as sharp pulses, but intermediate plate thicknesses only allow waves with highly dispersive characteristics, which tend to mask any dispersion due to bonds. The plate wave experiments allow a full intercomparison between interferometer and EMATs, both out-of-plane motion sensitive and in-plane motion sensitive. Chapter 6 uses Rayleigh-like surface waves travelling along 25 mm thick adherends to initiate interface-parallel travelling waves in an adhesive layer bonding a second adherend to the surface, which are subsequently detected on emerging at the free surface beyond the bond. These surface-interface-surface travelling (SIST) waves penetrate under increasingly longer bonds as the wave frequency decreases, a fact confirmed by the behaviour of pulses given a narrowband frequency modulation when generating laser beams interfere to produce a spatially modulated source. The interference source optical arrangement, described in Chapter 5, can be altered to give Rayleigh arrival modulation frequencies from 20 MHz to below 1 MHz. Finally, Chapter 7 examines alternative pathways for surface waves incident upon the edge of a bonded joint region, and demonstrates that SIST waves are an efficient mechanism for transferring ultrasound between the two adhesive-adherend interfaces, given the observed emergence of clearly discernible SIST waves on the second adherend of a lapped bond joint. I conclude that through transmission pulse analyses arc capable of extracting quantitative information about bond properties and should be developed as the basis for laser generated ultrasonic bond testing. SIST waves, however, require further research before they can be employed in a practical manner