Tap test is an age-old technique for inspecting adhesively bonded composite parts such as honeycomb panels for flaws and damage. With this technique one takes a hand-held mass, such as a coin or a machined piece of metal, and tap on the surface. A good region without defects or damages will produce a crisp and solid sound whereas a bad or damaged region gives a dull sound. The method is simple and cheap, but is also subjective, inaccurate, and highly affected by the inspector's hearing and background noise. Imagine trying to perform a tap test in an aircraft hanger with all the background noise completing for the ear's attention.
Over the years, a number of instruments have been developed to move the tap test method from a subjective method to an objective method. These efforts resulted in several instrumented tap test devices that have proven useful. While some devices measure the acoustic information produced in response to a physical impact, most instruments use force data produced by an impact. However, prior the the Computer-Aided Tap Testing system, all instruments stopped short of bringing in another powerful tool to the tap testing... a visual image. When a visual image of the surface stiffness is produced by systematically tapping the area and recording the force response, the probability of detecting defects or damage increases. The size, shape and location of the damage can be assessed more accurately. An image also provides an electronic record for archiving and later reference.
Contact Time Measurement
The physical quantity measured by the CATT is the contact time between the tapping instrument and the part surface. A tapping device instrumented with a force sensor is use to measure the contact time. Areas with skin delaminations, disbonds or damaged core, leave the surface less supported than undamaged areas. When the tap instrument impacts a damaged area the contact time is usually much longer than it is in an undamaged area. To get a mental picture of this, imagine dropping a bowling ball on a hardwood floor and then dropping the ball on a mattress. The bowling ball will be in contact with the mattress longer than it will be in contact with the hard floor.
In tap testing, the contact time is dependant on the local stiffness of the surface and the mass of the tap instrument. The contact time is not highly affected by the force of the impact so variation in the tap strength does not appreciably affect the results. The images below show the force-time history on an oscilloscope of an undamaged (left) and a damaged area. The contact time was approximately 500 microseconds for the undamaged are and approximately 1600 microseconds for the damaged area. Three taps of significantly different force were made at each of the two locations. The force of the taps results in a change in the voltage of the signal (vertical axis) but has little effect on the contact time (horizontal axis). Laboratory testing has demonstrated high consistency (less than 5 percent variability) between different operators despite differences in grasping force, strike angle and tapping force.
Force-time histories for an undamaged area and a damaged area. The contact time was around 500 microseconds for the undamaged area and 1600 microseconds for the damaged area. Impact force affects the amplitude of the signal but not the contact time.
Contact Time and Localized Stiffness Relationship
The relationship between the contact time and the surface stiffness is given by the following equation:
From this equation, it can be seen that if the mass (m) of the tap instrument is known and the contact time (t) is measured, it is possible to determine the surface stiffness (k). As shown in the graph below, the stiffness determined from tap test measurements with the CATT compare well with the stiffness at the same locations measured in static load tests.