F. Raymond Parker

Guided waves in anisotropic and quasi-isotropic aerospace composites: three-dimensional simulation and experiment.

Three-dimensional (3D) elastic wave simulations can be used to investigate and optimize nondestructive evaluation (NDE) and structural health monitoring (SHM) ultrasonic damage detection techniques for aerospace materials. 3D anisotropic elastodynamic finite integration technique (EFIT) has been implemented for ultrasonic waves in carbon fiber reinforced polymer (CFRP) composite laminates. This paper describes 3D EFIT simulations of guided wave propagation in undamaged and damaged anisotropic and quasi-isotropic composite plates. Comparisons are made between simulations of guided waves in undamaged anisotropic composite plates and both experimental laser Doppler vibrometer (LDV) wavefield data and dispersion curves. Time domain and wavenumber domain comparisons are described. Wave interaction with complex geometry delamination damage is then simulated to investigate how simulation tools incorporating realistic damage geometries can aid in the understanding of wave interaction with CFRP damage. In order to move beyond simplistic assumptions of damage geometry, volumetric delamination data acquired via X-ray microfocus computed tomography is directly incorporated into the simulation. Simulated guided wave interaction with the complex geometry delamination is compared to experimental LDV time domain data and 3D wave interaction with the volumetric damage is discussed.

Thermography inspection for early detection of composite damage in structures during fatigue loading

Advanced composite structures are commonly tested under controlled loading. Understanding the initiation and progression of composite damage under load is critical for validating design concepts and structural analysis tools. Thermal nondestructive evaluation (NDE) is used to detect and characterize damage in composite structures during fatigue loading. A difference image processing algorithm is demonstrated to enhance damage detection and characterization by removing thermal variations not associated with defects. In addition, a one-dimensional multilayered thermal model is used to characterize damage. Lastly, the thermography results are compared to other inspections such as non-immersion ultrasonic inspections and computed tomography X-ray.

Quantitative NDE Applied to Composites and Metals

This paper reviews recent advances at LaRC in quantitative measurement science applied to characterizing materials in a nondestructive environment. Recent demands on NDE have resulted in new thrusts to achieve measurements that represent material properties rather than indications or anomalies in a background measurement. Good physical models must be developed of the geometry, material properties, and the interaction of the probing energy with the material to interpret the results quantitatively. In this paper are presented NDE models that were used to develop measurement technologies for characterizing the curing of a polymer system for composite materials. The procedure uses the changes in ultrasonic properties of the material to determine the glass transition temperature, the degree of cure, and the cure rate. A practical application of this technology is a closed feedback system for controlling autoclave processing of composite materials. An additional example is in the area of thermal NDE. Thermal diffusion models combined with controlled thermal input/measurement have been used to determine the thermal diffusivity of materials. These measurements are remote, require no contact with the material under test and thus have interesting promise for NDE applications.

Thermal inspection of composite honeycomb structures

Composite honeycomb structures continue to be widely used in aerospace applications due to their low weight and high strength advantages. Developing nondestructive evaluation (NDE) inspection methods are essential for their safe performance. Pulsed thermography is a commonly used technique for composite honeycomb structure inspections due to its large area and rapid inspection capability. Pulsed thermography is shown to be sensitive for detection of face sheet impact damage and face sheet to core disbond. Data processing techniques, using principal component analysis to improve the defect contrast, are presented. In addition, limitations to the thermal detection of the core are investigated. Other NDE techniques, such as computed tomography X-ray and ultrasound, are used for comparison to the thermography results.

Simulation Based Investigation of Hidden Delamination Damage Detection in CFRP Composites

Guided wave (GW) based damage detection methods have shown promise in structural health monitoring (SHM) and hybrid SHM-nondestructive evaluation (NDE) techniques. Much previous GW work in the aerospace field has been primarily focused on metallic materials, with a growing focus on composite materials. The work presented in this paper demonstrates how realistic three-dimensional (3D) GW simulations can aid in the development of GW based damage characterization techniques for aerospace composites. 3D elastodynamic finite integration technique is implemented to model GW interaction with realistic delamination damage. A local wavenumber technique is applied to simulation data in order to investigate the detectability of hidden delamination damage to enable accurate characterization of damage extent.

Laser Ultrasonic and Photoacoustic NDE Using Front-Surface Piezoelectric Detection

Laser based NDE techniques offer a potential solution to many of the NDE problems found in the applications of high performance materials and aerospace structures. Typical of these structures and materials are the ones being developed for high speed civil transport as well as the currently used structures such as found in the commercial fleet. The response of the structure to a modulated laser beam can be used to probe both its thermal and mechanical properties which are often critical to its proper performance. Anther often mentioned advantage of laser based inspections is the laser beam can be remotely scanned over the surface of the object enabling the inspection of complex geometries.

Laser Ultrasonic and Photoacoustic Characterization of Subsurface Structures

There is a strong interest in applying laser ultrasonic and photoacoustic techniques to the NDE of some high performance structures, for example, the actively cooled panels of the National Aero-Space Plane. Both laser ultrasonic and photoacoustic techniques have been developed for years. Much significant work has been done on either the generation of waves, the mechanisms [1–3] or various techniques for the detection of these waves [4–6]. A few applications being pursued or conducted since the early stage of the development for these techniques [5–7]. However, there is little work concentrating on the interaction of these waves with structures and materials [8]. For successfully applying the laser ultrasonic and photoacoustic techniques to the NDE of a practical system, it is important to understand the interaction of laser generated ultrasounds and thermal waves with, for example, the subsurface structures or defects in the materials and their relationship to images obtained from the techniques.

Acoustic characterization of changes in viscoelastic properties of epoxy during cure

The transformation of an epoxy during cure from a viscoelastic liquid to a viscoelastic solid is characterized by the change in the shear and longitudinal ultrasonic velocities and attenuations. The measurement of these ultrasonic properties is reported for several different resins cured at room temperature. The velocities are used to calculate the bulk and shear moduli as a function of cure time. The results indicate a parallel development during cure of the shear and bulk moduli. Measurements of the longitudinal attenuation at several different frequencies indicate the viscoelastic response can be characterized by a single relaxation time which increases as the cure progresses.