The aspects of this invention deal with nondestructive materials characterization, particularly as it applies to the rapid and quantitative model-based characterization of hidden features. Examples of materials characterization include assessment of material loss from corrosion, characterization of hidden geometries such as the size, depth, and presence of defects around cooling holes or sealant grooves, and the detection and assessment of size and depth for buried inclusions. A common technique suitable for these inspections involves eddy current sensing.
Conventional eddy-current sensing involves an excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
In many inspection applications, large surface areas of a material need to be tested, such as a lap joint of an aircraft. This inspection can be accomplished with a single sensor and a two-dimensional scanner over the material surface. However, the use of a single sensor has disadvantages in that the scanning can take an excessively long time and care must be taken when registering the measured values together to form a map or image of the properties. These shortcomings can be overcome by using an array of sensors, but each sensor must be driven sequentially in order to prevent cross-talk or cross-contamination between the sensors. Alternatively, multiple sense elements can be used with a single drive winding. With known positions between each array element, the material can be scanned in a shorter period of time and the measured responses from each array element are spatially correlated.
Furthermore, detection of damage is often insufficient by itself and more quantitative or detailed assessments are required to determine the appropriate course of action. For example, prediction of corrosion fatigue life is still difficult, but limited information about the shape and nature of corrosion damage can provide useful information for prioritization of dealing with detected corrosion damage. Decision support for maintenance and repair for individual aircraft, as well as for depot and fleetwide initiatives, requires such information.
In another application of materials characterization, the structural integrity of titanium castings used to achieve significant cost savings during the manufacture of complex aircraft structural components depends largely on the capability of non-destructive inspection (NDI) methods to detect detrimental flaws. The primary defects found in titanium castings are voids or local porosity, cracks and inclusions. Inclusions can originate from contamination during manufacturing processes or from the shell material from investment casting molds. A specific type of deleterious inclusion of particular importance for titanium alloy component integrity is hard alpha inclusions in titanium castings. Hard alpha inclusions are particularly harmful when they reside in the near-surface region, where they are more likely to serve as initiation sites for fatigue cracks in cyclically loaded structures.
Considerable effort has been invested in NDI for titanium castings. Porosity, cracks and high-density inclusions (i.e., tungsten) in castings are not usually considered a problem because they are controlled by specifications and standard NDI sensitivity. The detection of shell inclusions and some types of alpha-stabilized nuggets presents a more difficult detection problem. X-ray sensitivity to these features is poor, to the point of non-detectability at material thickness of 0.75 in. (19 mm) or greater in many cases. Phased array ultrasonic testing (UT) has become the method of choice for detection of inclusions, but suffers from what is considered a dead zone (poor sensitivity) in the first 0.06 in. (1.5 mm) of the surface. In the areas where immersion scanning cannot be performed, the near-surface dead zone is roughly 0.15 in. (3.8 mm) using contact phased array inspection. Scanning from the opposite side of the part, if possible, is currently the only way to cover such dead zones. Electromagnetic inspection of the near-surface region of titanium typically looks for variations in material conductivity, where the hard alpha and other inclusions possess a different conductivity relative to the surrounding titanium matrix.