Advanced Nondestructive Evaluation Methods Research Articles

On the Local Constitutive Behavior of Friction Stir Welded AISI 304 Stainless Steel Joints

The uniaxial tensile test is often used to determine the mechanical properties of a material like its yield strength and elastic limit. However, some of the recent advances in imaging Non Destructive Evaluation (NDE) modalities offer experimental tools which, apart from determining the conventional properties, also make it possible to visualise and map the dynamic strain evolution during monotonic loading and correlate it with the micro mechanisms of deformation. Infrared thermal imaging (IRT) and digital image correlation (DIC) are two such advanced NDE methods which are being widely used in experimental mechanics. Infrared thermal imaging maps the thermal gradient including the dynamic thermal transients that may occur during the tensile testing and is based on the detection of infrared radiation. Digital image correlation, a non-contact optical method based on grey value correlation before and after deformation, maps the magnitude of deformation on the surface of the object under load. In this investigation, the global and local properties of a friction stir welded joint of 304 Austenitic Stainless steel through the simultaneous application of thermal imaging and digital image correlation. By characterising the various stages of tensile deformation, this study enabled correlation of the thermal and strain evolutions and to provide deeper insights into the micro-mechanisms of the associated deformation phenomenon.

Swept-frequency ultrasonic phase evaluation of adhesive bonding in tri-layer structures.

As modern aerospace and automotive designs continually strive for higher performance, and thus rely on advanced composite structures where adhesive bonding is a preferred method of joining, the need for a robust quantitative nondestructive bond strength measurement method has increased. As such, advanced nondestructive evaluation methods have been researched for increased sensitivity to weak interfacial bonding and ultimately to detect "kissing" bonds. In this work, a phase-based method for interrogating bonded joints and detecting weak adhesion is developed by using swept-frequency phase measurements of ultrasonic waves reflected from an adhesive joint and modeling adhesive interfaces as a distributed spring system. The method's sensitivity to bond strength is explored by ultrasonic phase evaluation of tri-layer joints with bond quality varied by controlling ultraviolet light exposure and extracting interfacial stiffness constants of the bonds. Mechanical tensile tests found each joint failed adhesively, allowing a linear correlation to be drawn between interfacial stiffness and tensile strength, consistent with previous theoretical research. The ultrasonic phase measurement method identifies intermediate bond strengths, rather than simply detecting good or bad bonds. This technique has the potential for the verification of bond quality in lightweight aerospace and automotive designs utilizing advanced composite structures with adhesive attachments.

Microstructure-sensitive computational modeling of fatigue crack formation

Recent trends towards simulation of the cyclic slip behavior of polycrystalline and polyphase microstructures of advanced engineering alloys subjected to cyclic loading are facilitating understanding of the relative roles of intrinsic and extrinsic attributes of microstructure in fatigue crack formation, comprised of nucleation and growth of cracks at the scale of individual grains or phases. Modeling of processes of early stages of fatigue crack nucleation and growth at these microstructure scales is an important emerging frontier in several respects. First, it facilitates analysis of the influence of local microstructure attributes on the distribution of driving forces for fatigue crack formation as a function of the applied stress state. This can support microstructure-sensitive estimates of minimum life, as well as characterization of competing failure modes. Second, it can inform modification of process route and its manifestations (e.g., residual stress, texture) to alter microstructure in ways that promote enhanced resistance to formation of fatigue cracks. Third, microstructure-sensitive modeling, even conducted at the mesocopic scale of individual grains/phases, can facilitate parametric design exploration in searching for microstructure morphologies and/or compositions that modify fatigue resistance. Fourth, such technologies offer promise for integration with advanced nondestructive evaluation methods for prognosis and structural health monitoring. Finally, as a longer term prospect in view of uncertainties in modeling mechanisms of cyclic slip, crack nucleation and growth, such modeling can serve to support more quantitative predictions of fatigue lifetime as a function of microstructure. We first discuss computationally based microstructure-sensitive fatigue modeling in the context of recent initiatives in accelerated insertion of materials and integration of computational mechanics, materials science, and systems engineering in design of materials and structures. We then highlight recent application of such strategies to Ni-base superalloys, gear steels, and α–β Ti alloys, with focus on the individual grain scale as the minimum length scale of heterogeneity. Finally, we close by outlining opportunities to advance microstructure-sensitive fatigue modeling in the next decade.

X-ray tomography and nuclear magnetic resonance imaging applications for advanced ceramic materials: 37715 Ellingson, W.A.; Sawicka, B.D.; Kriz, R.J.; Gronemeyer, S.; Ackerman, J.L. Argonne National Laboratory, Argonne, Illinois (United States), CONF-870422-8, DE87-011534, 33 pp. (Feb. 1987)

Advanced nondestructive evaluation methods are being developed to characterize ceramic materials and allow improvement of process technology. If one can determine porosity, map organic binder/plasticizer distributions, through-volume density, and detect inclusions, machine operations can then be modified to enhance the reliability of ceramics. Two modes of X-ray tomographic imaging - advanced film tomography and computed tomography - are being developed to provide flaw detection and density profile mapping capability. Nuclear magnetic resonance imaging is being developed to determine porosity and map organic binder/plasticizer distribution. Results obtained with these methods are presented.