Rapid, Local Characterization of the Fatigue Crack Growth Behavior

ABSTRACT: SA Photonics will leverage its experience using high-frequency surface currents to interrogate the physical parameters of very small fatigue cracks growing on titanium alloys as they propagate through regions of varying microtexture. In some regions, clusters of grains with similar crystallographic orientation persist over millimeter length scales, and affect fatigue crack growth in complex ways. During Phase I, we will evaluate a technique based on these principles: (1) high-frequency stimulation to achieve much smaller skin depth than the micro-crack dimensions, (2) highly repeatable current injection points and parasitic electromagnetic loads, (3) multi-position interrogator instrumentation that is retracted from the test piece during fatigue cycling and (4) mixed analytic/numerical crack shape determination from interrogator data. Thus, the measurements we will carry out in Phase II will allow us to use standard fatigue cycling methods and cost-effectively track the crack tip propagation near and below the surface without intervention. The reduced cost of gathering data will enable much more complete characterization of fracture dynamics in materials with varying microtexture.; BENEFIT: The proposed instrumentation and technology will enable detection of cracks and measurement of crack shape parameters during fatigue cycling. This will substantially reduce the cost of acquiring extensive data on crack propagation and fracture mechanics, which will be critical to determining the performance limits of specialized materials used in very high-stress, high-temperature applications such as jet engines. The great expansion of data will give engineers the confidence to produce new designs that push closer to the materials' physical limits and achieve heretofore unrealized performance.