The research presented in this thesis was carried out using optical methods to generate and detect ultrasound, using a scanning configuration that takes ad- vantage of the high spatial resolution and non-contact nature of laser ultrasonics. The subject of laser ultrasonics has been divided into the theory of generation and detection of ultrasound, with more details on the specific experimental setups used in this work provided in chapter 3.
Generation of ultrasound by laser irradiation has many advantages over other techniques due to the remote nature of the method[166]. Laser ultrasound is a non- contact method with both generation and detection lasers able to be positioned at distances on the scale of centimetres to metres away from the test material. This is a particular advantage when access to the test material is restricted or if the material under inspection is at an elevated temperature or in a hazardous environment[10].
eration, both for the material under test and for the operator themselves. Safe operation of laser inspection equipment in a laboratory environment requires the use of protective filtered glasses of a rating appropriate to the laser wavelength and power being used, and care has to be taken to ensure that no sensitive material comes into contact with the laser beam. In an industrial setting steps must be taken to shield the general environment from the laser radiation as it may be im- practical to supply eye-protection for all workers. Consideration must also be given to how the incident laser beam will interact with the material under test, as an in- appropriate choice of incident power or wavelength will lead to undesirable surface damage, the effects of which can include material ablation, plastic deformation and formation of cracks[27]. The cost of laser ultrasonics is considerably higher than for other non-contact methods, such as EMATs and air coupled transducers, however, the benefits make it suitable for consideration as a scanable non-contact system.
For a low laser power the interaction with the material surface occurs in the thermoelastic regime, in which the laser impact causes rapid localised heating of the material, without the material being melted. At high laser powers the irradiated material is melted to form a plasma, damaging the surface, and this is known as the ablative energy regime. The thermoelastic regime is the power regime required for NDT[166]. The following discussion uses a model in which localised heating by the incident laser radiation gives rise to the production of thermoelastic stresses and strains within the material, producing an ultrasonic wave[27,85,166–168].
In this work laser generation of ultrasound is performed using a pulsed Nd:YAG laser, of infrared wavelength 1064 nm, with an energy per pulse of 144 mJ, a pulse width of 10 ns, and a pulse rise time of 10 ns that was fired at a rate of 2.7 Hz[95,169]. A beam of electromagnetic laser radiation illuminating a material will penetrate into the material and the photons in the incident beam will interact with the electrons in the material surface, leading to the incident energy being ab- sorbed[27,170]. For a beam incident on a conductor some of the energy of the beam will also be reflected via scattering interactions between the photons and the elec- trons in the conduction band of the material, and this reduces the amount of energy that is absorbed in the material. The fraction of the incident energy absorbed is dependent upon the wavelength of the radiation and the optical reflectivity of the material surface[85]. In general more energy is absorbed by rough surfaces than
shiny reflective surfaces, which direct a lot of the incident radiation away through scattering[85]. The electromagnetic absorption and the resulting temperature change is assumed not to change the thermal, elastic or electromagnetic properties of the material and the resulting mechanical deformation is assumed not to change the
thermal profile of the material[167].
The absorption of the incident photons by the surface electrons in a con- ductor acts to screen the interior of the metal from the radiation, such that the absorption only occurs in a thin layer at the surface, the depth of which is deter- mined by the material skin depth, δ[27]. At the skin depth the amplitude of the incident radiation falls to 1/eof its initial value, and the depth at which this occurs is dependent upon the frequency of the incident radiation, f, the conductivity of the metalσ and the relative permeability of the metal,µr, via
δ = (πσµrµ0f)−1/2 , (2.67)
where µ0 is the permeability of free space. The choice of laser used for ultrasonic
generation should take into consideration this skin depth to ensure that it is not too small, so as to ensure efficient generation of ultrasound by improving the amount of electromagnetic energy absorbed. For aluminium samples illuminated by a 1064 nm wavelength laser the skin depth is approximately 5 nm[27].
The size of the skin depth dictates the region of the material that will absorb the incident radiation, and therefore dictates the size of the region which will be heated by this absorption process[27,85,166,167]. For a laser incident on a conductor, the absorption of the photons generates heating within the skin depth, causing a localised material expansion outwards from the absorption region, as shown in figure 2.6. The expanding heated material is restrained by the surrounding cooler material, and the resulting heat gradient produces stress and strain fields within the material[27,166,167].
For incident radiation in the thermoelastic energy regime a 3D model of the thermal expansion can be produced by considering the heated absorption volume,
V, which is given by the product of the area of the spatial extent of the laser spot incident on the surface and the skin depth of the absorption. The thermal gradient that is developed produces an expansion of the material, which is equivalent to the insertion of a small volume, δV, in the absorption region, which will produce localised material strains and stresses[27,167].
At the top surface of the material only the compressional stresses and strains, e.g. σxx,σyy andσzz, are present, however, within the skin depth shear stresses and strains, e.g. σxy, σxz and σyz, also exist. The thermal gradient δ T present in the material with thermal expansion coefficientαT produces localised strains of,
xx=yy =zz = 1 3
δV
Figure 2.6: Schematic diagram illustrating the absorption of the incident laser energy within the skin depth of a sample that leads to the formation of a thermal gradient, which in turn produces ultrasonic stresses and strains.
from which the volume expansion gives localised surface stresses of,
σxx =σyy =σzz = (λ+ 2 3µ)
δV
V , (2.69)
whereλandµare the Lam´econstants[27]. The presence of these stresses and strains produces ultrasonic waves by the process described in section 2.1.2. A contribution to the generation of the ultrasonic stresses in the material also arises from the ra- diation pressure exerted on the illuminated surface by the incident electromagnetic radiation, however, this is several orders of magnitude smaller than that arising from the thermoelastic mechanism and so is negligible in this case[27,167].
To achieve a high degree of compressive stress in the heated region a Q- switched pulsed laser is used for generation. The pulsed laser allows the heat energy to be deposited into the material over a very short period of time, producing a very steep thermal gradient[27,166,167]. This model of a thermal expansion in the absorb- ing region is valid in the limit of no thermal diffusion[167].
As the heating is not instantaneous, with a finite laser pulse rise time, a broadband range of ultrasonic frequencies are generated simultaneously within the sample, as seen in the experimental waveform shown in figure 2.3 for Lamb wave generation. The heating profile within the material will mimic the Gaussian beam profile produced by the Nd:YAG generation laser, and variations in the pulse time and Gaussian width have been shown to affect the character of the thermal heat- ing, and therefore the generated ultrasound, within the material[27]. The effect on the character of the ultrasound produced by variations in the spatial profile of the generation area is discussed in section 2.3.1.
2.1.3, the surface acoustic waves described in section 2.1.4, and other wave modes depending on the sample geometry[171]. Laser thermoelastic generation has been shown to produce Rayleigh waves with a simple bipolar (spot source) or monopolar (line source) wave shape, making them ideal for the studies carried out in chapter 4[27,171], and in thin plate-like materials the preferentially produced surface wave becomes a Lamb wave[172].
In the higher energy ablative generation regime the energy densities involved are sufficient to vaporise the sample surface (or sometimes a sacrificial coating can be applied to the test sample if this generation mechanism is necessary), produc- ing a plasma, and the net force of this process is downwards into the material[27].
From the momentum transfer into the material the ultrasound generated by this mechanism can be modelled as arising from a normal impulsive force applied to the surface. The ablative regime may be used if higher amplitude waves are required and the preservation of the material surface is not important[173].