Yuval Freed

Thermomechanically Coupled Micromechanical Analysis of Shape Memory Alloy Composites Undergoing Transformation Induced Plasticity

In this investigation, fully thermomechanically coupled constitutive and energy equations for shape memory alloys (SMAs) that include the effect of transformation induced plasticity are presented. This is followed by a micromechanical analysis for the establishment of the fully coupled thermomechanical constitutive equations that model the overall behavior of SMA composites undergoing transformation induced plasticity. The effects of the thermomechanical coupling and permanent inelasticity which arise by the phase transformation were examined. It was found that the response of the monolithic SMA depends upon the thermomechanical coupling. The permanent inelasticity and the resulting induced temperature become significant especially in the case of several repeating cycles. In addition, the induced average temperature caused by the thermomechanical coupling as well as the stress—strain behavior of SMA/epoxy and SMA/aluminum composite materials were determined. A significant thermomechanical coupling, which corresponds to an appreciable temperature increase, was observed for the SMA/aluminum metal matrix composite material. For the case of SMA/epoxy polymeric matrix composite material the thermomechanical coupling was found to be negligible. This results from the absence of inelastic strains in the polymeric matrix in spite of the existence of rate of dilatation effect in the epoxy phase.

Micromechanical investigation of plasticity–damage coupling of concrete reinforced by shape memory alloy fibers

Although concrete is an extremely popular material in the building industry, it is very weak in tension, as compared to its strength in compression. Smart prestressing of concrete with shape memory alloy fibers is a promising solution to this limitation. To this end, already stretched shape memory alloy fibers are embedded in a concrete matrix at a relatively low temperature which corresponds to that of the shape memory effect. Upon heating (activating) the resulting composite, the shape memory alloy fibers regain their original shape, and compressive stresses are transmitted to the concrete. In the present study, a robust micromechanical framework is proposed to analyze the concrete that has been prestressed by shape memory alloy fibers. The offered methodology accounts for the evolution of plastic strains in the concrete phase, in addition to the coupled evolving damage. Initial yield surfaces of the SMA/concrete composite are obtained for several loading cases. These surfaces are further generalized to incorporate the effect of the activation temperature change. The relation between the residual macroscopic plastic strain and the activation temperature change, as well as the macroscopic stress–strain response of the activated composite are presented.

Investigation of shape memory alloy honeycombs by means of a micromechanical analysis

Shape memory alloy (SMA) honeycombs are promising new smart materials which may be used for light-weight structures, biomedical implants, actuators and active structures. In this study, the behavior of several SMA honeycomb structures is investigated by means of a continuum-based thermomechanically coupled micromechanical analysis. To this end, macroscopic inelastic stress?strain responses of several topologies are investigated, both for pseudoelasticity and for shape memory effect. It was found that the triangular topology exhibits the best performance. In addition, the initial transformation surfaces are presented for all possible combinations of applied in-plane stresses. A special two-phase microstructure that is capable of producing an overall negative coefficient of thermal expansion is suggested and studied. In this configuration, in which one of the phases is a SMA, residual strains are being generated upon recovery. Here, the negative coefficient of thermal expansion appears to be associated with a larger amount of residual strain upon recovery. Furthermore, a two-dimensional SMA re-entrant topology that generates a negative in-plane Poissons ratio is analyzed, and the effect of the full thermomechanical coupling is examined. Finally, the response of a particular three-dimensional microstructure is studied.

Thermomechanically micromechanical modeling of prestressed concrete reinforced with shape memory alloy fibers

Concrete is a very popular material in civil engineering, although it exhibits some limitations. The most crucial limitation is its low tensile strength, compared to its compressive strength, which results from the propagation of micro-cracks. This may be prevented by using prestrained shape memory alloy wires that are embedded in the concrete matrix. Upon activation, these wires regain their original shape, and consequently initial compressive stresses are transmitted to the concrete matrix. In this study, a thermomechanically micromechanical model of prestressed concrete reinforced by shape memory alloy fibers is presented and examined for different reinforcement aspects. It was found that there is a strong relation between the activation temperature deviation and the behavior of the prestressed concrete. The relation between the fibers volume fraction and the composite response and the effect of the shape of the reinforcing fibers and residual strain orientations is examined in detail.

Toughness of a ±450 Interface

Experiments are carried out to determine the delamination toughness for a crack along the interface between two transversely isotropic materials. The material chosen for study consists of carbon fibers embedded within an epoxy matrix. A crack is introduced between two layers of this material, with fibers in the upper layer along the +450-direction and those in the lower layer along the −450-direction both with respect to the crack plane. The Brazilian disk specimen is employed in the testing.