The manufacturing route and resulting properties of adaptive composites are presented in the second part of this European project report. Manufacturing was performed using a specially designed frame to pre-strain the SMA wires, embed them into Kevlar-epoxy prepregs, and maintain them during the curing process in an autoclave. Composite compounds were then tested for strain response, recovery stress response in a clamped-clamped configuration, as well as vibrational response. Through the understanding of the transformational behavior of constrained SMA wires, interesting and unique functional properties of SMA composites could be measured, explained and modeled. Large recovery stresses and as a consequence, a change in vibrational response in a clamped- clamped condition, or a reversible shape change in a free standing condition, could be generated by the SMA composites in a controllable way. These properties were dependent on composite design aspects and exhibited a reproducible and stable behavior, provided that the properties of the matrix, of the wires and the processing route were carefully optimized. In conclusion, the achievements of this effort in areas such as thermomechanics, transformational and vibrational behavior and durability of SMA based composites provide a first step towards a reliable materials design, and potentially an industrial application.
Composites containing thin Shape Memory Alloy (SMA) wires show great potential as materials able to adapt their shape, thermal behavior or vibrational properties to external stimuli. The functional properties of SMA-composites are directly related to the constraining effect of the matrix on the reversible martensitic transformation of the embedded pre-strained SMA wires. The present work reports results of a concerted European effort towards a fundamental understanding of the manufacturing and design of SMA composites. This first part investigates the transformational behavior of constrained SMA wires and its translation into functional properties of SMA composites. Thermodynamic and thermomechanical experiments were performed on SMA wires. A model was developed to simulate the thermomechanical behavior of the wires. From the screening of potential wires it was concluded that NiTiCu, as well as R-phase NiTi appeared as best candidates. Requirements for the host composite materials were surveyed. A Kevlar-epoxy system was chosen. Finally, the quality of the SMA wire-resin interface was assessed by two different techniques. These indicated that a thin oxide layer seems to provide the best interfacial strength. A temperature window in which SMA composites can be safely used was also defined. The manufacturing and properties of the SMA composites will be discussed in Part II.
An important property of shape memory (SMA) wires is the generation of high stresses when the strain recovery is impeded during heating. These stresses are called recovery stresses and can reach stress levels up to 800 MPa. In a first step this paper compares and discusses the recovery stress generation and mechanism in different SMA-wires based on experimental results. All experiments were performed on a specially equipped thermomechanical testing apparatus. Complex stress-, strain-, and temperature profiles can be programmed to study the thermomechanical behavior of a SMA. The knowledge of these recovery stresses was applied for composite materials. Embedding pre-strained SMA-wires in a composite result in a material with adaptive properties that are related to the reversible martensitic transformation in the SMA-wires. The behavior of the SMA-composites was studied in three ways. Starting from the experimental results on SMA-wires and the knowledge of composite materials, the behavior of the SMA- composites was predicted. A computer simulation model has been used for the same purpose. Thirdly, thermomechanical experiments were performed on the SMA-composites. The theoretically calculated and the simulated results were validated by comparison with these experimental results. In conclusion, links were established between the recovery stress behavior of a SMA-wire and the thermomechanical behavior of SMA-composites. This knowledge can be used to accurately design SMA-composites based on material data of individual SMA-wires.
It is well known that composites, although strong and lightweight, can suffer badly when impacted. This can have catastrophic consequences to a structure. By embedding superelastic shape memory alloys into a composite structure, it is possible to reduce impact damage quite significantly. Superelastic shape memory alloy (SMA) wires absorb a lot of the energy during the impact due to their 'elastic' and hysteretic behavior. The mechanism behind superelasticity is the reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and recoverable strains of up to 8% can be achieved. This paper will report on the results, in which superelastic shape memory alloys were pre-strained to 1.5% and 3% and then embedded into glass fiber/epoxy composite plates. These plates were then impact tested. The effect of embedding wires at different depths of the specimen, different types of wires (martensitic NiTi and stainless steel) and also different volume fractions of wires was also investigated. The results of the impact tests were examined by ultrasonic C-scan to determine the size of the delamination area. The energy absorbed and the maximum impact force were also determined.
The peculiar thermomechanical and functional properties of adaptive composites with embedded shape memory (SMA) wires are directly related to the reversible martensitic transformation in the SMA-wires. The gradual transformation and the related strain recovery of the prestrained SMA-wires during heating is hampered by the rigid matrix. The constraining matrix thus influences the transformational behaviour of the embedded SMA-wires. The effects on the transformation heat and on the transformation temperatures of the forward and reverse transformation have been quantified and explained.
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