The process of Laser Bond Inspection is based upon stresswave interactions in bonded materials. Specifically, LBI looks at how the reflective stresswave interacts with the adhesive bond interface. The stresswave generated at the surface of the test area propagates into the material to the backside surface as a compressive stress. Once reflected, the wave propagates back toward the initiation point as a tensile stress. The tensile stresswave interrogates the bondline at a selected strength value. Figure 1 shows a time-lapse simulation of the stresswave propagating into a material.
How is the Stresswave Magnitude for an Inspection Determined?
In current practice, the LBI process is calibrated to individual structure configuations. A series of evaluations determine the value at which the bond line can be dissociated based on structural geometry and material composition. The magnitude of the stresswave is specifically tailored by controlling the duration and energy of the laser pulse. Once the laser fluence threshold has been determined, the stresswave is selected to provide approximately 80% of the threshold value required for bond dissociation. This value is the test fluence that can be customized depending upon the desired minimum strength threshold for a safe structure. The minimum level is determined by the design limit load of the structure, and the test level necessary to demonstrate an acceptable bond.
LSPT has stresswave magnitude predictive modeling capabilities for determining threshold values on different structures.
What Makes LBI Structures Safe to Use?
Laser Bond Inspection provides validation that structures meet their strength ratings and the intended design requirements. The process does not induce any deficiencies in the bond or the structural material if the structure has been assembled to acceptable specifications. If the bond passes inspection, there is no damage and the structure is safe to use. Even after multiple inspections, LBI does not degrade acceptable bonds. (See Bossi, et.al "Bond Strength NDT Using Laser Induced Stress Waves").