Thermal excitation involves the application of heat to the surface of a structure through the use of heat lamps. When heat is applied to an object, the temperature increases and surface starts to swell and expand. The Laser Shearography sensor measures the relative change in the out-of-plane bending strain due to this effect, whereby the visualized fringes are proportional to the change in thermal energy distributed on the surface.
Dantec Dynamics offers two types of thermal excitation systems, including; short-wave halogen heat lamps and medium-wave quartz emitter lamps. The selection of the appropriate thermal excitation system is dependent on the working distance of the sensor with respect to the test object and component structure. Both these attributes together determine the efficiency of the thermal excitation system for a given application. Halogen heat lamps, for example, are better suited for applications at working distances larger than 80 cm from the test object surface than quartz emitters, due to the higher achievable output power of the heat lamps. However quartz emitters are often quicker at inducing thermal loading due to the higher peak emission wavelength output, which are adsorbed more efficiently most materials applications.
Measurements using thermal excitation can be conducted using two approaches; during or after the application of heat. These methods are representative of when the reference and snapshot measurements are acquired. For most applications, measurements are acquired after the heat has been applied. This involves heating up a sample, when the heat lamps are switched on, and then waiting a specific lapse time between the time when the lamps are switched off and a reference image is acquired. Measurement snapshots are then acquired of the object swelling back to it‘s original shape after heat dissipates from the surface.
Thermal excitation is a comparatively cost-effective solution for loading (compared to the other excitation methods e.g. vacuum and vibration excitation) and it has a certain user „convenience” factor in that the loading does not need to be coupled to the sample. Additionally, a laser shearography sensor can be used at a distance, hence utilizing the technology’s higher inspection speed capabilities. It is usually an effective method for the inspection of; glass-fiber (GFRP), carbon-fiber (CFRP) and aramid-fiber (AFRP) laminates and also CFRP and AFRP-sandwich and honeycomb components.