Holographic Interferometry in Crack Detection

Basic Concepts

Holographic interferometry can be used to produce a fringe pattern, which represents the field of surface displacement of an opaque object in response to some change in mechanical loading. The presence of a surface or near-surface crack in the object will cause some localized perturbation in this displacement field. If this perturbation is of a sufficient magnitude to cause a corresponding localized anomaly in the fringe pattern, holographic interferometry can be used to detect the presence of the crack-or even to observe its growth in real time.

This application of holographic interferometry is of potential usefulness in nondestructive inspection and related material testing procedures. It is particularly attractive for applications in which it is impossible or undesirable to polish or treat the surface to be inspected, or when the test must be conducted in a special environment which makes direct contact with the object impossible. As is the case with most holographic nondestructive testing techniques, the ability to make a full-field inspection, as opposed to point-by-point inspection, is a major advantage.

Clearly, it is the magnitude and nature of the surface displacement perturbation, induced by the presence of the crack, which determines the feasibility of this technique in a given application. This sets limitations on the usefulness of the technique and also affects the choice of loading configuration for a given inspection or test. The minimum perturbation of displacement which can be measured by holographic interferometry is λ/2, where λ is the wavelength of the laser light. This measurement can be realized only if the displacement deviation is in the appropriate direction (roughly normal to the object surface in most holographic setups); hence, it is desirable that the displacement deviation be of the order of at least a few wavelengths.

A second criterion for direct detection of cracks is that the effect of the cracking be fairly localized in the neighborhood of the crack, or else yield abrupt changes of fringe curvature at the crack location. If the second criterion is well satisfied, the first can be relaxed.

Holographic interferometer has been used to study deformation near cracks qualitatively, in order to detect the presence of cracks, and quantitatively, in order to study fracture mechanics. The procedure described here is concerned with detection techniques requiring only qualitative interpretation of interferometric fringe patterns.

Detection of Micro Cracks

Experiments have been reported by Friesem and Vest in which the feasibility of using double-exposure holographic interferometry to detect the presence of small cracks in structural members was assessed. The detection of such cracks in components is a problem of considerable practical importance since they may propagate during loading and lead to failure of the member. The experiments reported by Vest et al, were conducted under laboratory conditions; however, the test specimen used was similar to one which might be encountered in industrial application.

The test specimen was a channel, fabricated of a high-strength steel alloy. A number of bolt holes were drilled and reamed in the ribs of this channel. The objective of the experiment was to detect the presence of microcracks extending radially from these bolt holes.

The optical apparatus used to record holographic interferograms of the test specimen was a typical holographic system. The beam from a 20-mW HeNe laser was divided by a beam splitter into a reference beam and an object beam. Each of these beams was expanded and filtered by a microscope objective pinhole assembly. The reference beam was collimated to facilitate viewing and photography of the reconstructed real image. Mechanical stability and vibration isolation were achieved by mounting the test specimen and optical components on a large granite slab, which rested on air-filled inner tubes.

The specimen was carefully prepared in order to simulate fatigue cracks of various sizes, and to include several flaw-free holes for reference purposes.

Radial cracks of various lengths were induced at the periphery of several of the holes by a stress-etch technique. A 0.03-in. notch was cut into the periphery of the drilled hole. The area around this notch was then burned with the arc from a 500-V capacitance discharge.

Cracking was initiated by etching the area with hydrochloric acid. Crack length was controlled by adjusting the torque applied to the bolt and simultaneously observing the development of the crack with a microscope. After a crack of the desired length had been induced, the notched and burned material was removed by a tapered reamer, leaving a reamed hole of the desired nominal diameter. The specimen received no cleaning or surface preparation.

Several techniques for producing surface deflections by mechanical and thermal loading were tried. The technique which produced the most satisfactory differential displacement near the cracks was to draw a hardened bolt, having a tapered shank, into the hole. The hologram was reconstructed by illumination with a collimated beam.

The test specimen used in this holographic investigation was also examined by Magnaflux, eddy current, and x-ray techniques in order to provide additional evaluation of the technique. The holographic and Magnaflux inspections were comparable in ability to detect the presence of cracks. The Magnaflux technique, however, required careful cleaning of the specimen surface and detailed visual inspection with the aid of a low-power microscope.

The eddy current inspection was conducted by rotating a longitudinally split cylindrical probe in the hole. In this particular application the highly sensitive probe tended to give false indication of cracks, so meaningful comparison with this technique was not possible. The x-ray inspection was capable of identifying only the two largest cracks in the test specimen, and was therefore inferior to the holographic technique.

This experiment showed that it is feasible to detect the presence of small cracks in structural members using holographic interferometry. In the case of the steel test specimen utilized in this experiment, the holographic technique appeared to be comparable in accuracy with Magnaflux inspection and superior to eddy current inspection and x-ray examination.

Obviously these experimental results do not form the basis of any general conclusions regarding the applicability of holographic crack detection techniques. Each proposed application must be considered individually to discern if differential loading will produce the type of surface deflection necessary to make holographic inspection possible.

Real-time holographic interferometry can be used to monitor the growth of cracks in materials by observing the temporal change of surface deformation of the specimen. This was first demonstrated by Magill and Wilson, who observed the propagation of a small crack from a scratch in a stressed silicon disk. In their experiment, a holographic interference microscope utilizing near-image-plane holography was used, and the propagating crack was indicated by a discontinuity in a fringe pattern which resulted from application of an increment of force following the first exposure.

Some metals are susceptible to the formation and propagation of cracks when they are placed in certain environments, even though they may be stressed below their yield point. Although various physical mechanisms may be involved, this phenomenon is generally referred to as stress-corrosion cracking (SCC). Early detection of such cracking processes by holographic interferometry is possible if the cracking causes sufficient surface displacement. The concept involved is the same as that used for detection of microcracks. In this case, however, the observed surface displacement is due to stress release upon formation of a crack; hence, no externally applied differential load is required.

Real-time holographic interferometry has three particularly attractive features for this application:

• It is sufficiently sensitive to allow detection of cracks before they have grown to a readily observable macroscopic size.
• Cracking can be continuously monitored without removing the specimen from the corrosive environment for inspection.
• The entire specimen surface can be examined without the need for point-by-point inspection.

These results indicate that holographic interferometry can be used to monitor delayed cracking phenomena, such as stress-corrosion cracking, in real time. Due to the sensitivity of the technique, rather early indications of stress-corrosion cracking appear to be given. The method, therefore, may be useful in determining the susceptibility of materials to delayed cracking, since it would considerably shorten the time required to perform such a test.

There are, however, certain limitations which should be noted. First, the mechanical stability requirements inherent in real-time holographic interferometry must be satisfied. Second, the environment of the specimen must be transparent to laser light, so that the object may be observed, and it must remain at constant temperature to avoid extraneous fringes. Finally, gross corrosion of the surface cannot occur. If it does, the optical correlation between the hologram and the surface will be lost, and meaningful interferometric fringes will not be produced.

Conclusion

Holographic interferometry can be used to observe surface deformation caused by the presence of cracks. This can form the basis of an inspection scheme for locating small cracks in structural members, or other objects. When used on a real-time basis it also makes possible the monitoring of delayed-cracking phenomena such as stress-corrosion cracking. When the configuration allows quantitative fringe interpretation, holographic interferometry can also be used as a tool in experimental fracture mechanics.

In order for holographic interferometry to be useful as a nondestructive inspection technique for locating small cracks, the cracks must cause a displacement perturbation of sufficient magnitude and form to clearly indicate the presence of the crack. Each proposed application of holographic interferometry of this nature must be individually investigated to see if a viable test procedure can be developed. If this technique is applicable, it offers the advantages of a whole-field, visual inspection, and can be used even when the object must be placed in a special environment; it does not, in general, require cleaning or preparation of the object surface.