Engineering science and design methods are generally based on the assumption that engineering materials are homogeneous, or in the case of composites, have properties which vary with the orientation of the material in a known and uniform manner. A second useful and widely applied assumption is that engineering materials are sufficiently ductile that small local inhomogeneities can be ignored. However, in many real world structures, particularly those which are highly engineered to meet demanding design criteria or which employ advanced materials, local inhomogeneities, such as cracks and de-laminations, determine the large-scale structural performance.
Another type of highly-engineered structure in which it is necessary to determine local material properties is typified by turbine blades for use in the hot section of modem jet engines, which are among the most highly engineered of modem structures. Such parts must withstand high stresses and high temperatures in an abrasive and chemically hostile environment.
To meet these design demands, blades grown of single crystals of metal are given multiple coatings, for example, to resist heating, abrasion, and corrosion. The reliability of these blades is dependant on the ability to detect any flaw in a bonding between the various layers and the underlying metal of the blade. Many methods have been devised and put to use. X-ray examination has been found useful for detecting in-depth density variations; yet x-ray examination requires access to both sides of a specimen. Further, x-rays can involve the use of hazardous materials, and are not always successful in detecting cracks or de-laminations which do not produce density variations.
Dye-penetrant, ultrasound, and eddy-currents have been used to detect weld flaws, de-lamination in structural composites, and other undesirable structure attributes. These methods, however, can be labor intensive, require physical contact with the test object, and often require special preparation of the material's surface.
A belief that thermal imaging (thermography) would lead to more cost-effective material flaw detection has led to a number of techniques employing thermal radiation and the test specimen's reaction to the thermal radiation to detect material flaws.
One such technique employs a high-power, pulsed thermal wave, which propagates into the material as a planar wave of thermal energy. When the thermal wave reaches a defect within the depth of the object being tested, that portion of the wave which encounters the defect is reflected with the amount of the reflected energy dependant on the nature of the discontinuity. The reflected wave is detectable at the surface by thermal imaging. This method, while being useful in some circumstances, requires high-power flash lamps and does not detect flaws which are normal to the surface of the object being tested.
Another thermal imaging process for detecting material defects, particularly stress cracks, employs a laser which performs a raster scan of a test object while the test object is being thermally imaged. This method requires a laser of relatively high power and detects surface cracks by their greater absorption of energy. This method can be somewhat sensitive to the angle of incidence. Further, the decay of the transient thermal response of the crack must be carefully analyzed to distinguish surface blemishes from cracks having significant depth. This method also requires a relatively clean surface, free of surface film and paint.
Another known technique, sometimes referred to as the "mirage effect," employs a heating laser which generates an output which is intensity modulated to provide a periodic optical signal used to periodically heat a point on the surface of an object. The optical beam from a probe laser passes parallel to the surface of the object through the heated zone. This probe beam is deflected from a normal path due to density variations in the air above the surface of the sample caused by the heating of the surface by the probe laser. The deflection of the probe laser from its nominal path indicates the presence of surface or sub-surface cracks, flaws or voids in the object being tested. This type of apparatus requires a scan head located relatively close to the surface of the object being tested to recover the output of the probe beam.
One method for detecting flaws in welds involves heating the weld with an infrared source and observing the heated weld with an infrared camera to detect the thermal response of the weld, which may be indicative of weld flaws.
In principle, thermographic testing involves exposure of the analyzed surface to even heating. Variations in the thermoconductivity of the features below the surface then allow heat to flow away from the surface more rapidly in some places than other, establishing temperature gradients along the surface that provide an indication of the sub-surface features in an object.
One type of apparatus which employs the thermographic technique has a linear heat source which is passed in spaced relation over the surface in front of a row of detectors. The heat source establishes a temperature gradient on the surface being tested. The sensors monitor the temperature gradient for variations which indicate surface defects. This process suffers from requiting the heat source and the detector to be moved closely over the surface of the object to be tested.
One variation on this process of using lateral heat flow to detect sub-surface flaws involves using a linear heating source, such as a laser projected through a cylindrical lens and an infrared scanner which views a line of material spaced from, and off-set from, the linear heat source. Variations in the temperature gradient between the heat source and the infrared scanned line indicate the presence of cracks and material defects in the tested sample. This system is experimental in nature and requires a relatively intense heat source, with a change of temperature gradients in the material which can amount to several degrees or more.
What is needed is an apparatus which is able to detect sub-surface cracks and flaws and which can image entire test objects from a distance while utilizing low levels of thermal stimulation.