# Interface Characterization within a Nuclear Fuel Plate
## 2 Overview of the Laser Shockwave Technique (LST)  
The characterization methods, equipment and techniques, advantages, limitations, etc., have been described in detail previously [1; 2; 3; 4; 5; 6; 7]. Prior to the development of the laser shock system, the program tried many mechanical testing techniques. Most \"pull or peel\" techniques required bonding an attachment to the plate. In most cases, the attachment mechanism failed first and since the fuel plates are thin there were concerns about altering the stress distribution in the plates. In general, the laser shock technique involves application of two complementary experimental techniques, laser-shockwave testing and laser-ultrasonic imaging, collectively referred to as the laser shockwave technique (LST), which allows the integrity, physical properties and interfacial bond strength in fuel plates to be evaluated. Characterization results include measurement of layer thicknesses, elastic properties of the constituents, and the location and nature of generated debonds. In laser shock testing, a high-power pulsed laser is used to generate shock waves within the specimen. The shock wave travels as a compression wave through the material to the free (unconfined) back surface, and reflects back through the material as a rarefaction (tensile) wave. This tensile wave is the physical mechanism that produces internal failure (i.e., interfacial delamination) within the specimen. To elevate the efficacy of the optical-to-mechanical energy transfer [2; 3; 4; 5; 6], the surface of the specimen is covered with an optically absorbing black tape and then covered with a transparent constraining medium as shown in Figure 1. To keep the plasma contained and enhance energy transfer, the INL system uses a strong high-temperature transparent tape. The generated shock wave, which is produced under the confinement cover, causes high-amplitude molecular displacements on the surface of the aluminum cladding. The shock wave source size (roughly the laser spot size) needs to be about two times the sample thickness (about 1.5 mm) to approximate one-dimensional (1-D)wave propagation. Under the 1-D approximation, shear stresses are neglected, and the shock wave is exclusively compressive when first generated. During testing of a fuel plate specimen, bond strength is determined by increasing the laser pulse energy incrementally, which increases the amplitude of the interrogating shockwave. The threshold stress value is imposed at the interface when debonding is imminent. The stress is indirectly calculated from the measured back-surface velocity, \(u\), (see Figure 1) as recorded by an optical velocimeter based on a solid Fabry-Perot etalon [8]. Assuming 1-D, elastic wave propagation, and ignoring multiple reflections, the simplified relation between the back-surface velocity and interior stress is given by Equation (1) [2; 3; 4; 5; 6; 7]. \[\sigma(t)=\rho cu(t)/2 \tag{1}\]  
where,  
\(c\) = Speed of sound in Al (6.4 mm/\(\upmu\)s; 6400 m/s; measured)  
\(\rho\) = Al 6061 density (2.70 g/cm\({}^{3}\); 2,700 kg/m\({}^{3}\))  
\(u(t)\) = Measured surface velocity (m/s). Equation (1) simply quantifies the maximum interior stress that makes it through the specimen to the back surface when the internal interfaces are intact. When an interface is debonded or has deteriorated, energy is dispersed or absorbed. While the shock wave energy reaching the back surface must have made it across at least partially intact internal interfaces, the actual stress field at the internal interface is more complicated, and may be higher or lower, due to specimen geometry effects and interaction of multiple reflections [2; 3; 4] from all interfaces. To be able to identify debonding of the interfaces during a test, the LST system also contains a fully functional and independent laser-based ultrasonic (LUT) imaging subsystem. This subsystem is used to identify and image debonding caused during the laser shock test.