Size Effect on the Critical Stress of Nitinol Wires

Nitinol has the best shape memory and superelasticity properties of all known polycrystalline shape memory alloys (SMAs) due to diffusionless Martensitic transformation. Due to these unique properties, Nitinol is increasingly used in different fields such as biomedical, structural and aerospace engineering. However, under certain stresses Nitinol exhibits unrecovered strain, or permanent set, that limits the applicability of Nitinol wire. This study showed that there exists a critical range of stress beyond which the permanent set is negligible. The goal of this paper is to determine range of critical stress using two different methods i.e. constant stress experiment and isothermal tensile test and to show variation of this range with changes in wire diameters.

Correlation between permanent deformation-related performance parameters of asphalt concrete mixes and binders

This paper examines methods to predict the performance of hot asphalt concrete mixes based on performance parameters of binders. Specifically, relationships between binder parameters determined from multiple stress creep and recovery tests were correlated to the creep parameters of hot asphalt concrete mixes obtained from cyclic load compression testing. For the determination of creep parameters, a modified expression of the creep curve is proposed to cover the entire spectrum of permanent deformation; including the tertiary creep phase. Non-recoverable compliance, unrecovered strain, and recoverable strain of binders show good correlation to creep parameters of hot asphalt concrete mixes such as creep rate and high temperature performance ratio. Additionally, unrecovered strain and non-recoverable compliance of binders correlates well with mean rut depth of asphalt concrete mixes. However, no correlation has been detected between the difference in non-recoverable compliance of binders and permanent deformation parameters of asphalt concrete mixes.

Improvement of Actuating Properties of a SMA Coil Spring by Change in Coil Orientation

The present authors have invented a new method of changing coil orientation of a coil spring. The major merit of coil orientation change for SMA are reduction of space and control of two way shape memory effect. For instance, when a coil is heated above Af temperature with a load, the coil contracts. There is unrecovered strain which is attributed to elastic deformation by the load. The elastic strain can be eliminated by changing coil orientation. By removing elastic strain, the necessary space can be reduced. Besides generation and control of two way shape memory effect is presented using the reverse of coil orientation.

A Comprehensive Treatise of the High Temperature Specification Parameter |G*|/(1-(1/tanδsinδ)) for Performance Grading of Asphalts

The term |G*|/(1-(1/tanδ sinδ)) has been suggested as one of the best candidates for the replacement of the Super-pave specification parameter |G*|/sinδ, which has been found to be inadequate in rating polymer-modified binders for high temperature performance grading. This refinement of the Superpave specification parameter evolved through a theoretical derivation based on fundamental concepts. It was shown to be more sensitive to the variations in the phase angle δ than the original Superpave specification parameter. It thus described the unrecovered strain in the asphalt binders more accurately, and hence related to actual field performance data. This article provides a comprehensive treatise of the parameter |G*|/(1-(1/tanδ sinδ)) giving details of its derivation, salient features that are attributed to its success, comparison with actual field performance data for validation and a one-on-one comparison with the existing parameter |G*|/sinδ. It is shown that for all available field data, the parameter |G*|/(1-(1/tanδ sinδ)) does a better job in correlating with the rutting behavior than the parameter |G*|/sinδ for unmodified as well as modified asphalts. Since it is obtained in the same manner as the parameter |G*|/sinδ through the determination of |G*| and δ from a stress-controlled or strain-controlled dynamic shear rheometer, it means that no retraining of technicians and staff is required and implementation for the use of this parameter is immediate, thereby saving enormous amount of time and money. This parameter has the further advantage of being in a form easily adaptable to modeling, and thereby directly applicable for pavement design purposes.