The capability of Shape Memory Alloys (SMAs) to modify the reference configuration of an SMA-composite
through martensitic transformation is explored. It is intended that through careful selection of a thermomechanical
loading path the composite can be "processed" such that the constituent phases are in a preferential reference
configuration. Specifically, for materials which have preferred loading conditions (i.e., compression versus tension),
such processing results in a residual stress state which takes advantage of the improved properties. The
composite under investigation is assumed to be composed of an SMA phase and an elasto-plastic second phase.
For analysis of such a composite, a Finite Element (FE) mesh based on a realistic microstructure is constructed
by using the results of X-ray tomography. The resultant microstructure is analyzed using FE techniques. It is
shown that through an isobaric loading path, transformation generates plastic strains in the elasto-plastic phase
which modify the composite reference configuration. The effect of different applied loads is considered.
The experimental characterization of fatigue crack initiation and growth of structural materials can be very expensive and time consuming. Fatigue specimens are typically controlled by a single dominant defect and several specimens are needed to examine the fatigue response for each loading condition of interest. Time and expense add up as millions of load cycles are sometimes required to initiate a crack, and replicate tests are necessary to characterize the inherent statistical nature of fatigue. In order to improve the efficiency of experimentation, we are developing laser-based techniques to produce fatigue test samples with arrays of defects. Controlled arrays of oval shaped micro-defects are laser-micromachined in titanium alloy (Ti-6Al-4V). Crack initiation from the individual defects in the arrays is monitored using a DC potential drop technique. Results indicate the utility of this approach in multiplying the amount of fatigue data generated per specimen-test. The new fatigue test approach is applicable to a wide range of material systems and initial defect structures.
A generalization for a two-tier approach to damage identification based on structural performance and levels or magnitude of damage is presented. The two tiers are defined as health or damage monitoring and situation assessment. Damage monitoring involves the inspection of a structure for continual degradation caused by accumulated damage. Situation assessment results from a known incident with a high probably of damage. Initial work on damage monitoring of structural components examines the response of a flat plate as the first step in a series of analyses that will address more complex structures. Damage is included in the computational study in the form of damage to joints such as weld lines. Trends in local and global responses have been evaluated in order to develop an understanding of the implications of varying amounts of damage in the joints on structural response. Numerical based visualization techniques are used to isolate regions of mode shape variation with increasing damage. Implications and use of the developed techniques for monitoring and sensor placement requirements are noted.