Advances in Nondestructive materials characterization
Robert E. Green, Jr.
Center for Nondestructive Evaluation
The Johns Hopkins University
Baltimore, MD 21218, USA
- International Symposium on NDT Contribution to|
the Infrastructure Safety Systems, 1999 NOV 22-26 Torres,
published by UFSM, Santa Maria, RS, Brazil
The Johns Hopkins University Center for Nondestructive Evaluation (CNDE) is an interdisciplinary cooperative center between the university, government, and industrial organizations dedicated to the development of more accurate and innovative methods for the nondestructive evaluation of materials and to the education of talented students who will enter the nondestructive testing field. Innovative nondestructive materials characterization methods developed in the CNDE include the following. Lasers, optical interferometers, electro-magnetic transducers (EMATs), and air(gas) coupled systems are used for non-contact generation and detection of ultrasonic waves to inspect art paintings, wood, metals and composites. Non-linear ultrasonic generation of harmonics is used to determine the quality of adhesive bonds. High resolution optical holographic interferometry is used for leak testing and imaging surface acoustic waves. Computer assisted x-ray tomography permits imaging of the internal structure of ultralightweight metals. X-ray diffraction images defects in gallium arsenide wafers, defects and vibrational modes of quartz crystals, and microstructure of nickel alloy single crystal turbine blades.
Keywords: lasers, ultrasound, holography, x-rays, tomography
Although piezoelectric transducers are commonly used for nondestructive testing, several problems are associated with the requirement that they be bonded to the test material with an acoustical impedance matching coupling medium. For velocity measurements, which are necessary for material thickness measurements and to locate the depth of defects, the coupling medium can cause transit time errors on the order of one percent. Due to partial transmission and partial reflection of the ultrasonic energy in the couplant layer, there may be a change of shape of the waveform which can further affect velocity measurement accuracy. This can also lead to serious errors in absolute attenuation measurements, which is the reason that so few reliable absolute measurements of attenuation are reported in the scientific literature. It is also important to note that the character of the piezoelectric transducer itself exerts a major influence on the components of the ultrasonic signal, since conventional transducers do not respond as a simple vibrating piston and have their own frequency, amplitude, and directional response. Therefore, a method of non-contact generation and detection of ultrasound is of great practical importance. Several such techniques are presently available in various stages of development, namely capacitive pick-ups, electro-magnetic acoustic transducers (EMATs), laser beam optical generators and detectors, and more recently air(gas)-coupled ultrasonic systems. However, as the name implies, capacitive pick-ups cannot be used as ultrasonic generators and, even when used as detectors, the air gap required between the pick-up and test structure surface is extremely small, which in essence causes the device to be very nearly a contact one. One major problem with EMATs is that the efficiency of ultrasound generation and detection rapidly decreases with lift-off distance between the EMATs face and the surface of the test object. They can obviously only be used for examination of electrically conducting materials. Because of the physical processes involved they are much better detectors than generators of ultrasound. Laser beam ultrasound generation and detection overcomes all of these problems and affords the opportunity to make truly non-contact ultrasonic measurements in both electrically conducting and non-conducting materials, in materials at elevated temperatures, in corrosive and other hostile environments, in geometrically difficult to reach locations, and do all of this at relatively large distances, i.e. meters, from the test object surface. Furthermore, lasers are able to produce simultaneously both shear and longitudinal bulk wave modes as well as Rayleigh and plate modes. Air(gas)-coupled ultrasonic systems have been under development for some time and research is underway to optimize them for practical non-contact ultrasonic applications. These systems have been used to inspect art paintings, lumber, composite pre-pregs, and composite panels. Most recently researchers in the Johns Hopkins CNDE have developed a non-contact laser genration/air-coupled detection system for process control of composite tape placement and a laser generation/laser detection system for inspection of railway rails.
Nonlinear ultrasonic waves differ from linear ultrasonic waves, which are commonly used for nondestructive evaluation, in several important aspects. An initially sinusoidal longitudinal wave of a given frequency distorts as it propagates, and energy is transferred from the fundamental to the harmonics that appear. Monitoring the amount of harmonic generation developed in materials subjected to mechanical or thermal loading or other environmental alterations can be used to access the state of resulting damage. Recently, adhesive bonds are being used to replace rivets on aircraft and aerospace structures. Therefore, a nondestructive evaluation technique is required to determine the quality of these adhesive bonds. In cooperation with Boeing Aircraft, CNDE researchers have proven that nonlinear ultrasound measurements of the harmonic content of ultrasound focused at the adhesive bond line can distinguish good bonds from bad ones.
Metal packages containing electronics which control heart pacemakers are routinely placed beneath the skin of patients. In order to prevent leakage of body fluids into these packages usually resulting in loss of life of the patient, the packages are hermetically sealed with polymer coatings. However, occasionally these polymer coatings possess small holes causing leaks when implanted. Since the polymer coating absorbs helium gas, it is not possible to use standard helium leak testing methods to check the packages for proper sealing. In order to solve this problem a double-pulse holographic interferometric technique was developed. The package to be tested is placed into a metal container with a glass faceplate permitting a holographic image of the package to be made with an Argon-ion laser. Then the container is pressurized causing the package to deflect elastically and the valve to the container is closed maintaining the pressure on the package. The holographic image of the original undeflected package is then superimposed on the real image of the deflected package resulting in an interference fringe pattern. If the package does not leak the fringe pattern remains constant in time. However, if the package leaks, the pressure will eventually equalize inside and outside the package and the fringes will disappear. By monitoring the movement of the fringes, an extremely accurate measure of the leak rate over a very large dynamic range can be obtained. This same technique has been used to determine the quality of the seal on many other packages including drug vials and food cans. An inverted version of this system has been constructed to monitor any leakage in the habitat modules to be used on the NASA space station. Heterodyne holographic interferometry also permits full-field imaging of surface displacements on solid materials due to ultrasonic wave propagation. A full-field laser pulse is used to record a holographic image of the entire front surface of a graphite/epoxy composite plate. Subsequently, a second point source laser pulse, possessing sufficient energy to cause ultrasonic wave generation in the plate by thermoelastic heating, is directed to the center of the back surface of the plate. After sufficient time for the resulting thermoelastically generated ultrasonic wave to travel to the front surface and progress away from the center, a third full-field laser pulse is used to re-expose the holographic plate. The resulting interference pattern shows the ultrasonic wavefront traveling outward from the source with the influence of the anisotropic character of the plate clearly evident. Defects such as delaminations between layers of the composite plate exhibit themself as alterations in the holographic image.
X-Ray Tomography of Ultralightweight Metals
Computed x-ray tomography (CT) permits retrieval of three dimensional information inside an object. To make a CT measurement, several radiographic images (or projections) of an object are acquired at different angles, and the information collected by the detector is processed in a computer. The final three-dimensional image, generated by mathematically combining the radiographic images, provides the exact locations and dimensions of the internal features of the object. Considerable effort is currently underway to produce innovative new metal configurations that possess marked reductions in weight while retaining a maximum amount of mechanical strength. X-ray CT is the only nondestructive technique which can reveal the internal structure of these metals. CT images have been recorded from ultralightmetals made in the Ukraine using the Gasar process, from hollow spheres prepared at Georgia Institute of Technology, from ultralightweight plates produced at Cymat, Alphoras, and Fraunhofer.
X-Ray Diffraction Imaging
Electro-optical systems optimized for rapid x-ray diffraction imaging have been used to orient single crystals, to study crystal lattice rotation accompanying plastic deformation, to measure the rate of grain boundary migration during recrystallization annealing of cold-worked metals, to determine the physical state of exploding metals, to monitor the amorphous to crystalline phase transformation of rapidly solidified metals, to rapidly measure residual stress (strain), to study the dynamics of structural phase transitions in ferroelectric crystals, and to record topographic images of lattice defects in quartz, nickel, and gallium arsenide crystals. In the case of quartz crystals used as frequency control resonators, x-ray diffraction topography not only reveals the defects in the quartz plates, but also reveals the vibration pattern of the plate in resonance. Application of synchrotron x-radiation permits defects in gallium arsenide wafers, used for microwave applications, to be imaged without contamination by removing them from their plastic carriers. The conventional technique for inspection of nickel alloy single crystal turbine blades is to use the Laue back-reflection x-ray diffraction technique to determine the orientation of one small localized point on the turbine blade at one time requiring an inordinately long time. An asymmetric crystal topographic (ACT) x-ray diffraction technique permits imaging of a large portion of the single crystal blade at one time, while incorporation of an x-ray sensitive electro-optical detector permits this to be done in real-time. Such an x-ray topographic image of a section of a single crystal turbine blade showed that the blade was not a single crystal and that it had marked sub-structure due to non-optimum thickness of the ceramic mold covering the flange area of the blade.
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- Winter, J.M., Jr., Green, R.E., Jr., and Green, K.A., "Application of Synchrotron and Flash X-ray Topography to Improved Processing of Electronic Materials", in Advances in X-ray Analysis, C.S. Barrett et al. (Eds.) Vol.35, pp.239-245 (1992) Plenum Press, NY (1992).
- Green, R.E., Jr., "Practical Applications of Nondestructive Materials Characterization", J.
Minerals, Metals & Materials, Vol. 44, 12-16 (1992).
- Murray, A., Boltz, E.S., Renken, M., Fortunko, C.M., Mecklenburg, M.F., and Green, R.E., Jr."Air-Coupled Ultrasonic System for Characterizing the Structural Stability of Wooden Panel Paintings", Nondestructive Characterization of Materials VI, pp. 103-110, Plenum Press, New York (1994).
- Robert E. Green, Jr., "Physical Techniques for Nondestructive Materials Characterization", in New Perpectives on Problems in Classical and Quantum Physics, P.P. Delsanto and A.W. Saenz (Eds.), Gordon and Breach, The Netherlands, pp. 139-156 (1998).
- Tobias P. Berndt and Robert E. Green, Jr.,"Feasibility Study of a Nonlinear Ultrasonic Technique to Evaluate Adhesive Bonds", in Nondestructive Characterization of Materials VIII, R.E. Green, Jr.(Ed.), Plenum Press, NY, pp. 125-131 (1998).
- Kirsten G. Lipetzky, Robert E. Green, Jr., and P.J.Zombo, "Development of X-ray Diffraction Methods to Examine Single Crystal Turbine Blades", in Nondestructive Characterization of Materials VIII, R.E. Green, Jr. (Ed.), Plenum Press, NY, pp. 423-430 (1998).
- K.G. Lipetzky, R.E. Green, Jr., R.W. Armstrong, and W.T. Beard, Jr., "The Evaluation of Quartz Resonators Via X-ray Diffraction Topography", in Review ofProgress in Quantitative NDE, D.O. Thompson & D.E.Chimenti (Eds.)Vol. 18, pp. 2079-2086, Kluwer Academic/Plenum Press, New York (1999).