Investigation of the mechanical properties and behaviour of hybrid polymer composites embedded with shape memory alloys

Abstract

The increasing requirement for light weight constructions and the unsatisfactory performances of traditional metals and conventional engineering materials, especially in their failure to positively respond to environmental stimuli, in a demanding environment have made the search for the development of alternative materials inevitable. Such alternative materials being sought, which are the so-called adaptive, multifunctional, smart or intelligent composites would facilitate the realization of some engineering applications that are simply difficult to achieve with the existing conventional materials. Composite materials have found increasing applications in construction, aerospace and automotive industries due to their good characteristics of light weight, improved strength, corrosion resistance, controlled anisotropic properties, and reduced manufacturing and maintenance costs. However, there is a growing demand to improve on composite materials to have “smart" capabilities so as to be able to sense, actuate and respond to the surrounding environment. Shape memory alloys (SMAs) are metallic alloys that can undergo martensitic phase transformations as a result of applied thermomechanical loads and are capable of recovering permanent strains when heated above a certain transformation temperature. SMAs possess sensing and actuating functions and have the potential to control the mechanical properties and responses of their hosts due to their inherent unique characteristics: shape memory effect (SME) and pseudoelasticity. When integrated into structural components, they perform sensing, diagnosing, actuating and repair or healing functions, thereby enhancing improved performance characteristics of their hosts. Amongst the commercially available SMAs, NiTi (Nickel-Titanium) alloys in forms of wires, ribbons, bars, particles and porous bulks are the most widely used because of their excellent mechanical properties and superior material characteristics. Embedding SMAs into composite materials can create smart or intelligent hybridized composites. This thesis details an investigation of the mechanical properties and behaviour of the hybridized composites formed by embedding NiTi SMA wires into 60D polyurethane. The composites were produced by the vacuum process of manufacturing. The properties of the implanted SMA wires were enhanced by ageing and pre-straining. Uniaxial tensile and four point bending tests were conducted to ascertain the significance of embedding SMA wires into the polyurethane host

matrix. It was found that the embedded SMA results in an increasing in elastic modulus, tensile strength and bending stiffness. It was found that these improvements in the properties can not be sustained at high temperature owing to degradation of interfacial strength between the SMA and polyurethane as a result of the high recovery stress generated by the SMA upon activation. Some measures that can ameliorate the interfacial breakdown were suggested.