Frequently Asked Questions about Laser Bond Inspection (LBI)
Laser Bond Inspection can be applied to any bonded material, including composites of all forms, metals, polymers, and ceramics.
Laser Bond Inspection typically requires a back surface reflection to interrogate the bondline. The LBI method is most easily performed on flat-backed surfaces, but can be applied to additional geometries with some calibration. The maximum thickness that can be inspected is dependent on materials and bond strength. Fiber reinforced polymer composites up to one inch thick have been successfully tested.
Laser Bond Inspection was initially developed for inspecting bonded joints, but it can have other applications when material response to a tailored stress wave provides valuable information about structural integrity. For example, LBI can be used to inspect welded joints for cold (unfused) welds or for detecting subsurface cracks.
As currently configured, each inspection point requires about 30 seconds to complete. This is a configurable parameter that can be adjusted to meet specific customer requirements.
No appreciable heating of the structure occurs. The laser energy is delivered for such a short duration (100-300 nanoseconds) that no significant thermal load is transferred to the work piece. In addition, the target surface is protected by an overlay of EMAT sensors and protective tape. The short duration of heating during the laser shock process is insufficient to penetrate these protective layers.
Laser Bond Inspection is a nondestructive inspection method (NDI). The laser energy is calibrated for each application to deliver a shockwave that stresses the bond at a selected strength magnitude. This value is typically some percentage of the structure's safe operating threshold, meaning that if the bond fails in response to the applied stress, it was not manufactured to an acceptable standard. Laser Bond Inspection does no harm to acceptable bonds or structural materials, even when multiple inspections are performed at the same location.
The Laser Bond Inspection method measures how a structure responds to a controlled stress wave. LBI applies a tensile stress to an internal bondline, and provides an snapshot of the bond before and after the applied stress.
The LBI system is integrated with an Electromagnetic Acoustic Transducer (EMAT) for capturing bondline signatures before and after Laser Bond Inspection. By comparing a known pre-inspection trace to a post-inspection trace, the impacts of the inspection stresswave on the bondline can be determined.
EMAT stands for Electromagnetic Acoustic Transducer. It consists of a coiled wire oriented whithin a magnetic field. The coil oscillates in response to the vibrations produced by the stress wave, and provides a signature that can be compared to a pre-inspection baseline.
Laser Bond Inspection is designed to perform inspections at critical points to verify bond strength. LSPT has worked to enhance the capabilities of LBI and increase the percentage of a bonded structure that can be inspected. Our engineers have developed innovations in beam delivery infrastructure and inspection head configuration to make the LBI system versatile across a variety of applications.
The bondline is interrogated by a tensile stress wave of known magnitude. If no changes are detected in the bondline after this interrogation, the bond can be determined to have a minimum strength value equivalent to magnitude of the tensile wave. If this value exceeds the minimun required threshold for the structure, then the bond is sufficient for safe operation.
Laser bond inspection has been proven to provide good results on pi joint inspection capability. There are limitations on thickness of structures that can be inspected based on the attenuation of the shockwave.
Currently there are no standards in place for the application of laser bond inspection. LSP Technologies is working with government and industry to begin the development of standards.
LSP Technologies is constantly working to improve on laser technology and miniaturization of our equipment is a high priority on the list. Substantial R&D activities and funding are required by both LSPT and its suppliers in order to achieve this goal.
The cost per inspection is relatively low requiring only 30 seconds of operator time and an EMAT inspection circuit. Current EMAT inspection circuits are priced around $1.
Inspection of honeycomb and foam has typically been found to be unreliable. Irregular surface bonding of the honeycomb structure prevents adequate transmission and reflection of the shockwave.
Most of the LBI work has been in the inspection of fiber reinforced polymer (FRP) composites. This is the largest market sector of composites and has the greatest demand for the technology at this time. The technology has also been proven on other structures, such as glass adhesively bonded to metal, rubber bonded to metal, and several other combinations.
The sensitivity of the process is a scalable parameter. We have demonstrated the process to be detectable of defects that are less than half a millimeter in size. The custom tailoring of the laser beam, and hence the shockwave, allows the inspection to be almost infinitely customizable.
While a kissing bond may go undetected by conventional ultrasound inspection, the response of a kissing bond to a shockwave will show a different EMAT trace than a good bond. This difference is detectable because of a decoupling event between the kissing bond and mating surface.
The inspection process may or may not leave a void and both results have been observed in practice. There are many contributing parameters related to the material and the inspection process that affect the final outcome.
A pulse laser system is the only reliable means of generating a predictable shockwave with sufficient size, strength and duration to perform complex shockwave processing of components. Shockwave generation requires very high pressures that are difficult to predict, control and generate via any other means.