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This program is developing improved design and analytical techniques for predicting the integrity of structural components containing flaws under nonlinear fracture mechanics (NLEFM) conditions.

Researchers are conducting fundamental studies of fracture processes and behavior of materials ranging from brittle geomedia and ceramics to very ductile metals containing weldments. Metals bonded to ceramics through functionally graded joints are another area of exploratory research. These studies also include determination of the effects of multi-axial loading conditions, mismatched materials, and residual stresses. A principal goal of these research efforts is development of verified procedures to predict the integrity of full-size structural components using experimental data obtained from small test specimens. Tests of intermediate-sized specimens and full-scale structural components are used to validate predictions. These research and testing efforts have led to the development of several new experimental capabilities. Additionally, new instruments and technology are being developed to monitor the health of structural components, such as pressure vessels during normal operating conditions.

Major advances have been made in three areas of experimental measurement and analysis. Phase-shifted moiré interferometry (PSMI) provides high resolution surface displacement measurements and related analyses. The microtopography technique and data analysis methodology (MicTop) allows in-depth interrogation of three-dimensional ductile fracture processes. An active monitoring system (3-D Crack Growth Monitor) allows detection of crack growth initiation and monitoring of subsequent growth of part-through (surface) cracks in both test specimens and actual structures.

INL’s PSMI system is used to experimentally measure surface displacements (in the x and y directions) in a spatially-continuous fashion with very high resolution. The whole-field nature of the resultant displacement maps make the technique ideal for studying deformation in areas where large gradients or highly irregular displacement fields exist. Examples of such measurements where this technology have been applied include geo-materials, ceramic composites, weldments, and graded materials. This unique capability incorporates optical phase shifting to provide accurate displacement values at very small physical spacings. The system couples phase-shifting control and image acquisition with data processing algorithms to obtain almost real time (only a few seconds to acquire and process) displacement and strain maps.

Microtopographic analysis begins by mapping the height contours of fracture surfaces created by crack growth in ductile materials. These fracture surfaces may be those from laboratory test specimens or sections from a failed structural component, or some other source. The deformation created in the fracture process zone at the tip of a growing crack is "recorded" in the fracture surface that is created. The height data collected from matching fracture surfaces are analyzed and parameters that define the fracture process can be determined. Crack-tip-opening displacement (CTOD) for initiation of crack growth, crack-tip-opening angle (CTOA) as a function of crack growth, and other aspects of the fracture process are determined by this method. These results can be used to characterize the fracture behavior of the material, or as input to fracture prediction models. This technology is very effective as a research tool, and has unique capabilities for failure analysis as well.

The 3-D Crack Growth Monitor (CGM) system is an evolution of the electric potential change method of crack growth assessment. The basic technology makes a periodic point-to-point voltage measurement across a crack mouth which can be correlated to an incremental change in the crack’s length. INL’s 3-D CGM expands this approach to a multi-point, three-dimensional field-type measurement that allows, for example, local depth changes in a part-through surface crack to be detected and quantified. This technology has proven very useful in determining crack initiation states in tests of surface cracked plate specimens. It has also been successfully employed to monitor various cracks and defects in full-scale pressure vessel tests.

The third technology is the ability to detect the formation of cracks, wall thinning due to corrosion and/or erosion and to follow the development of the degradation. This capability has been demonstrated, in part, using specimens containing surface cracks (the crack configuration often observed in structural components).