·Table of Contents
Advances in Non-destructive Materials Characterization
ROBERT E. GREEN, JR.
Center for Nondestructive Evaluation
The Johns Hopkins University
Baltimore, MD 21218, USA
Although historically nondestructive techniques have been used almost exclusively for detection of macroscopic defects in structures after they have been in service for some time, it has become increasingly evident that it is both practical and cost effective to expand the role of nondestructive evaluation to include all phases of materials production. This paper will briefly describe the CNDE and describe several innovative nondestructive evaluation methods developed there. Among the topics to be discussed will be pulsed lasers, optical interferometers, electromagnetic acoustic transducers (EMATs), and air-coupled transducers for non-contact generation and detection of ultrasonic and acoustic emission waves; high resolution optical holographic interferometry for full-field vibration monitoring, leak testing of sealed packages and layup of composites; process control of composite tow placement, evaluation of thick composites; integrity of adhesive bonds; computer assisted tomography imaging of internal structure of ultralightweight metals; real-time viewing and recording of x-ray diffraction images showing the vibrational modes of quartz crystals, images of defects in gallium arsenide crystals and substructure in nickel alloy single crystal turbine blades.
Keywords: acoustic emission, adhesive bonds, composites, crystals, holography, lasers, process control, tomography, topography, ultrasound, x-rays
THE JOHNS HOPKINS CENTER FOR NONDESTRUCTIVE EVALUATION
The Johns Hopkins University Center for Nondestructive Evaluation (CNDE) is an interdisciplinary center for education and research in NDE, with participation within the University from the Engineering School, the Medical School and the Applied Physics Laboratory. The CNDE is dedicated to the development of more sophisticated and accurate methods for the nondestructive evaluation of materials and structures and to the education of talented students who will enter the NDE field. The CNDE has expertise in a wide range of NDE technology areas. Special emphasis is placed on development and optimization of those NDE techniques which lend themselves to intelligent manufacturing processes for electronic, composite, ceramic, and other high-tech materials; multiparameter imaging of inhomogeneities; non-contact evaluation of materials and structures in unique environments, such as high temperature, radiation, and space; and advanced sensors for industrial applications. The CNDE is a cooperative venture between the University, Industrial Affiliates, and Government Agencies.
ULTRASONIC ATTENUATION DETECTION OF FATIGUE DAMAGE
Ultrasonic attenuation measurements can detect extremely small microstructural alterations during fatigue testing of metals and give early warning of damage. Experiments were conducted using ultrasonic attenuation measurements to continuously monitor damage development in aircraft aluminum alloys and nuclear reactor pressure vessel steel during fatigue cycling. The ultrasonic attenuation change indicated microstructural alterations, probably microcrack formation, prior to detection of an additional ultrasonic pulse due to reflection from a crack as is used in conventional nondestructive ultrasonic testing. Similar ultrasonic attenuation measurements have been used to monitor fatigue damage development in thick graphite/epoxy composites.
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. 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, electromagnetic 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 generation/air-coupled detection system for process control of composite tape placement and a laser generation/laser detection system for inspection of railway rails.
COMPOSITE PROCESS CONTROL
Recently, non-contact laser and air-coupled ultrasonic systems have been developed to measure the elastic wave velocity and the attenuation of the wave propagating through composites during the cure process. The wave speed is a function of the elastic modulus and density of the composite while the attenuation is a monitor of the change in viscosity of the resin. In addition, a non-contact ultrasound system using a pulsed laser as the source and an air-coupled transducer as a detector has been successfully used to monitor the mechanical tape laying system for graphite/epoxy composites.
OPTICAL ACOUSTIC EMISSION DETECTION
The importance of acoustic emission monitoring is that proper detection and analysis of acoustic emission signals can permit remote identification of source mechanisms and the associated microstructural alteration of the material. Non-contact laser interferometric detectors have been used to record acoustic emissions from high strength steel and ceramic matrix composites. In several series of mechanical deformation tests, the dimensions of the gauge sections of tensile specimens were chosen to be very small so that any microstructural alteration would be visible on the gauge section surface and could be examined in detail using both optical and scanning electron microscopy. Because of the large flat frequency bandwidth (0 - 60 MHz) of the optical probe, acoustic emission signals were obtained in a frequency regime not normally detected with conventional transducers. Tests run on a series of stainless steel specimens revealed a large number of acoustic emission signals at 9 - 10 MHz prior to fracture. Scanning electron microscopic examination of the fracture surface resulted in a one-to-one correspondence between these high frequency signals and the fracture of intermetallic particles in the steel.
NONLINEAR ULTRASOUND ASSESSMENT OF ADHESIVE BONDS
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. Currently, 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 and NASA Langley Research Center, 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.
HOLOGRAPHIC FULL FIELD DEFORMATION IMAGING
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. In order to solve this problem a double-pulse holographic interferometric technique was developed. The package to be tested was 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 was pressurized causing the package to deflect elastically and the valve to the container was closed maintaining the pressure on the package. The holographic image of the original undeflected package was 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. Heterodyne holographic interferometry also permits full-field imaging of surface displacements on solid materials due to wave propagation. A full-field laser pulse was 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, was 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 was 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 themselves 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 ultralightweight metals made in the Ukraine using the Gasar process, from hollow spheres prepared at Georgia Institute of Technology, and 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 possessed a marked sub-structure.
PULSED X-RAY DIFFRACTION
Flash, or pulsed, x-ray generators, normally used for radiographic applications such as ballistic studies and baggage inspection, have also been used for x-ray diffraction applications. This type of generator produces x-ray tube currents of the order of thousands of amperes as opposed to the tens of milliamperes in conventional tubes. However, the burst of emitted x-radiation only lasts a few tens to perhaps a hundred nanoseconds. The result is extremely high x-ray photon flux over a very short span of time. Although these generators cannot be used for continuous observation of material alterations, they can be used in a fairly rapid sequential fashion, since the x-ray flux delivered per pulse is comparable in intensity to a synchrotron source in the wavelength regime useful for diffraction. CNDE investigators are among a very few researchers who have used such devices, appropriately modified, for generators in x-ray applications of materials testing. Although flash x-ray generators, by their very nature, cannot be used for continuous observation of material alterations, they can be used in a fairly rapid sequential fashion. Many materials processes, such as crystal growth, are relatively slow processes, and therefore, flash x-ray generators can be optimally used to study these processes. These pulsed x-ray systems can also be used to image alterations in materials which occur very rapidly. This type of generator produces x-ray tube currents of the order of thousands of amperes as opposed to the tens of milliamperes produced by conventional tubes. Since, the burst of emitted x-radiation only lasts a few tens to perhaps a hundred nanoseconds, the result is extremely high x-ray photon flux over a very short span of time. Pulsed x-ray diffraction systems have been used to record x-ray diffraction patterns during rapid solidification of metals during the melt spinning process, exploding metal foils, shock-wave compression of pyrolytic boron nitride, phase transformations in ferroelectric crystals caused by polarity switching and electrically initiated temperature jumps, and to characterize shaped-charge metal jets.
The present paper has given several examples of where nondestructive characterization of materials can not only be used for the historical purpose of locating and sizing defects, but can do much more. Examples have been given where nondestructive characterization has been capable of detecting alterations in structures prior to macroscopic defect detection. Moreover, it has been shown that nondestructive characterization affords a most promising opportunity for process control during production of materials, devices, and structures.
- Robert E. Green, Jr., "Nondestructive evaluation of thick-composite fatigue damage", SPIE, Vol. 2459, pp.30-41, Society of Photo-Optical Instrumentation Engineers, Bellingham, Washington (1995).
- Allison Murray, Marion F. Mecklenburg, C.M. Fortunko, and Robert E. Green, Jr., "Air- Coupled Ultrasonic System: A New Technology for Detecting Flaws in Paintings on Wooden Panels", Journal of the American Institute for Conservation 35. pp. 145-162, (1996).
- Robert E. Green, Jr., "Physical Techniques for Nondestructive Materials Characterization", in New Perspectives 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 of Progress in Quantitative NDE, D.O. Thompson & D.E. Chimenti (Eds.)Vol. 18, pp. 2079-2086, Kluwer Academic/Plenum Press, New York (1999).
- Robert E. Green, Jr., John M. Winter, Jr., and Kirsten G. Lipetzky, "Materials Characterization by X-ray Topographic and Tomographic Imaging", in Nondestructive Evaluation and Material Properties IV, P.K. Liaw et al. (Eds.), The Minerals, Metals &
Materials Society, Warrendale, PA, pp. 21-28 (1999).
- Robert E. Green, "Noncontact Acoustic Emission System for Remote Fault Detection in Machinery and Structures", in Failure Analysis: A Foundation for Diagnostics and Prognostics Development, Henry C. Pusey et al. (Eds.) Society for Machinery Failure Prevention Technology, Haymarket, VA, pp. 71-80 (1999).
- J.M. Winter, Jr., R.E. Green, Jr., A.M. Waters, W.H. Green, "X-ray Computed Tomography of Ultralightweight Metals", Research in Nondestructive Evaluation, Vol. 11, pp. 199-211 (1999).
- Francesco Lanza di Scalea, Tobias P. Berndt, Robert E. Green, Jr. and B. Boro Djordjevic, "Advances in Optical Methods for Non-contact Nondestructive Evaluation", Nondestructive Characterization of Materials IX, R. Green, Jr. (Ed.), pp. 149-155, American Institute of Physics, Melville, NY (1999).