John J. Peters

Physical basis of tap test as a quantitative imaging tool for composite structures on aircraft

Tap test is a simple but effective way for finding flaws in composite and honeycomb sandwich structures; it has been practiced in aircraft inspection for decades. The mechanics of tap test was extensively researched by P. Cawley et al., and several versions of instrumented tap test have emerged in recent years. This paper describes a quantitative study of the impact duration as a function of the mass, radius, velocity, and material property of the impactor. The impact response is compared to the predictions of Hertzian-type contact theory and a simple spring model. The electronically measured impact duration, τ, is used for generating images of the tapped region. Using the spring model, the images are converted into images of a spring constant, k, which is a measure of the local contact stiffness. The images of k, largely independent of tapper mass and impact velocity, reveal the size, shape and severity (cf. Percent stiffness reduction) of defects and damages, as well as the presence of substructures and...

Non‐Contact Inspection of Composites Using Air‐Coupled Ultrasound

Conventional ultrasonic tests are conducted using water as a transmitting medium. Water coupled ultrasound cannot be applied to certain water‐sensitive or porous materials and is more difficult to use in the field. In contrast, air‐coupled ultrasound is non‐contact and has clear advantages over water‐coupled testing. The technology of air‐coupled ultrasound has gained maturity in recent years. Some systems have become commercially available and researchers are pursuing several different modalities of air‐coupled transduction. This paper reports our experience of applying air‐coupled ultrasound to the inspection of flaws, damage, and normal internal structures of composite parts. Through‐transmission C‐scans at 400 kHz using a focused receiver has resolution sufficient to image honeycomb cells in the sandwich core. With the transmitter and receiver on the same side of a laminate. Lamb waves were generated and used for the imaging of substructures. Air‐coupled scan results are presented for flaw detection a...

A prototype tap test imaging system: Initial field test results

This paper describes a simple, field-worthy tap test imaging system that gives quantitative information about the size, shape, and severity of defects and damages. The system consists of an accelerometer, electronic circuits for conditioning the signal and measuring the impact duration, a laptop PC and data acquisition and processing software. The images are generated manually by tapping on a grid printed on a plastic sheet laid over the part’s surface. A mechanized scanner is currently under development. The prototype has produced images for a variety of aircraft composite and metal honeycomb structures containing flaws, damages, and repairs. Images of the local contact stiffness, deduced from the impact duration using a spring model, revealed quantitatively the stiffness reduction due to flaws and damages, as well as the stiffness enhancement due to substructures. The system has been field tested on commercial and military aircraft as well as rotor blades and engine decks on helicopters. Field test resu...

Air‐Coupled Ultrasonic Measurements in Composites

Air‐coupled ultrasound is a non‐contact technique and has clear advantages over water‐coupled testing. Research of air‐coupled ultrasonics, especially using capacitance and micromachined transducers, has been extensively reported in the literature. This paper reports our experience of applying piezoceramic air‐coupled transducers for nondestructive evaluation of composites. The beam profiles of air‐coupled piezoceramic transducers, with and without apodization, were mapped out. The transmission of air‐coupled ultrasonic energy through composite plates of different thickness was measured experimentally; model calculation of the transmission coefficient, taking into account the frequency bandwidth of the transducer, agreed with the measurement results. The occurrence of diffraction phenomenon (“Poisson bright spot”) while imaging flaws in composite laminates was investigated. The resolution of scanned images obtained with air‐coupled transducers was investigated for different frequency, focusing, and apodiz...

Towards a Generic Manual Scanner for Nondestructive Inspection

Although NDE imaging modalities are gaining acceptance in airline maintenance hangars operations, the high cost/complexity of these units precludes their wide use. With low cost position encoders (for handwriting capture) available for PC’s and hand held inspection instruments available with data ports or alarm outputs, a low cost generic manual scanning instrument should be realizable. This report summarizes recent efforts at a generic manual scanning platform, which includes software and minimal electronics, that is aimed at facilitating the merging of available handheld NDE instruments and an encoder to produce C‐scans.

Development of a Fieldable Air‐Coupled Ultrasonic Inspection System

This paper describes the development of a non‐mechanically encoded, simple, field‐worthy air‐coupled ultrasonic scanning system that gives quantitative information about the size of damage and underlying structure in composite and aluminum aerospace structures. The system consists of the AIRSCAN® air‐coupled ultrasonic testing system, the Flock of Birds® real‐time motion tracking equipment, a lightweight composite yoke, and laptop PC with data acquisition and processing software. Through transmission C‐scan images are generated manually by moving transducers attached to a yoke across the part’s surface. The prototype has produced images for a variety of aircraft composite and metal honeycomb structures containing flaws, damages, and repairs. Field tests on commercial and military aircraft as well as rotor blades have begun. Initial test results are shown.

Comparison of local stiffness of composite honeycomb sandwich structures measured by tap test and mechanical test

This paper shows that the local spring stiffness of composite honeycomb sandwiches, such as those used in aircraft flight control structures, can be obtained with a tap test. A simple spring model is invoked for converting the time of contact measured in a tap test to the local stiffness. The validity of the model is verified using test results obtained on aircraft components. The stiffness obtained from the tap test is compared with that measured in a static loading test. Good agreements are obtained for a variety of composite sandwiches with and without defects.

Development of a magnetic CAM for the computer aided tap test system

A novel magnetic cam mechanism was developed for a computer aided tap test system. It makes use of the repulsion between opposing permanent magnets and provides a simple way for position-encoding the tap test data and producing “C-scan” like images of the local stiffness. This paper describes the design and operation of the magnetic cam and presents tap test images acquired with the magnetic cam on aircraft in various field tests.

Development of Nondestructive Inspection Methods for Composite Repair

This paper describes the development and implementation of two complementary nondestructive inspection methods for repairs made on aircraft composite honeycomb structures: computer aided tap testing (CATT) and air‐coupled ultrasonic testing (AC‐UT). The CATT, being a semi‐automated and quantitative technique, is exploited to map out the interior conditions of a repaired part. The same repair is also imaged with air‐coupled ultrasound and both compared with the results from destructive sectioning.

Imaging of flaws in composite honeycomb aircraft structures using instrumented tap test

This paper describes the development of an instrumented tap test and imaging system for the inspection of defects and damages in composite aircraft structures, especially honeycomb sandwich constructions. The resulting tap test images based on the impact duration and displayed in a C- scan format can readily reveal the shape and extent of honeycomb core damages. The images also reveal considerable detail of internal substructures such as core splices, ply build-up, and changes of foam core density. Based on a grounded-spring mechanical model, the tap scan images can be converted into images that show the quantitative changes of the local stiffness. A manual tap scan imaging system was assembled for laboratory experiments. Using actual composites parts for airplanes, impact damages and substructures were imaged. In addition, effects of the tapper mass, impactor radius, tap velocity and operator dependence were studied. A compact fieldable system is being built and an automated tapping and imaging system is also under development. The technique was compared to other similar devices currently available.