Phase transformation induced nonlinear elastic alloys: toward the metallic spider web and flaw-tolerant structures
Abstract
Conventional man-made structural systems have limited capability for damage tolerance, as failure in a small subset of the structure often leads to a progressive failure in the entire structure. For example, the failure of a single truss member significant increases the load on all other members due to addition of moment from loss of symmetry. In other words, deviations from the designed configuration in such structures beget more deviation, and the entire system is situated at an unstable equilibrium. There is a need for structural systems that are inherently more damage tolerant, or structurally resilient, where progressive and catastrophic failures modes are arrested. One sample of such resilient system is found in the web of ordinary garden spiders. These structures show remarkable tolerance to damage and defects, and the removal of up to 10% of the threads not only does not cause failure, but rather increases the load capacity of the web. The commonly reported high strength-to-weight ratio of the spider silk was not the primary reason for the resilient properties of the spider web. Rather, it is the silkÕs nonlinear stress-strain response that is most important as it was shown that the strain-stiffening response of the silk creates localized failures instead of global ones under intense loading. This behavior makes failures in spider webs isolated and repairable instead progressive and catastrophic. Unfortunately, the application of this concept is difficult in practice because most high-strength engineering materials show a linear elastic response only or an elastic-plastic response as in the case of metallic alloys. The strain-stiffening response, where the tangential modulus increases with increasing strain, is only broadly observed in elastomers and other polymers which do not possess adequate strength or do so only at small dimensions. As such, we face a strong need to develop higher strength and higher toughness materials with the strain-stiffening nonlinear elastic response. We have shown one such solution in a metallic alloy based on the simultaneous activation of elastic strain and stress-induced phase transformation in shape memory alloys (SMAs). With this method, it is possible to control the stress level and curvature of the strain-stiffening response in a large number of alloy systems. However, we do not currently understand the origin of the nonlinear elastic response and how it can be precisely controlled. In addressing this technical challenge, we hypothesize that the variant selection, size, and morphology of residual martensite are sufficient to predict the degree of cycle softening observed in an SMA system regardless of microstructure. We believe this to be the case since retained martensite, not any other microstructure feature, is the most direct cause for the reduction in transformation triggering stress during cycling. To test this hypothesis, in-situ synchrotron XRD will be applied to study the characteristics (variant, size, morphology) of a) retained martensite and b) the first martensite plates forming during loading, and these characteristics evolve with superelastic cycles for the TiNb alloy different microstructures. The effects of microstructure (precipitates, dislocation density, grain size) and alloy system will be categorized based on their effects on martensite characteristics and compared with the cycle softening response. The hypothesis is rejected if alloys with significantly different martensite characteristics result in similar nonlinear elastic stress-strain responses. Through fundamental research, the goal of the proposed work is to understand how a controlled processing method for creating non-linear elastic metallic alloys with tunable strain-stiffening properties can be developed for multiple alloy systems, which can then be used to create spider-web-inspired resilient engineering structures.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 14, 2022
- Source ID
- W911NF2210121
Entities
People
- Ji Ma
Organizations
- Army Contracting Command
- United States Army
- University of Virginia