This research work is concerned with the determination of elastic-plastic deformation behaviour of structural members to combined tension-torsion loadings. Three aspects of the work were examined. In the first, an adaptive control system is developed for performing combined tension and torsion tests on solid and thin-walled tubular specimens by using systems supplied by National Instruments to control the axial and torsion loads on the specimen through data acquisition boards. The LabView software in conjunction with data acquisition board, servo-controllers and servomotors form the adaptive control system for the torque-tension machine. The second aspect was to carry out experimental investigations, where solid steel rods (structural steel) were subjected to non-proportional combined tension-torsion loading paths. In these loading paths, initial elastic tension followed by torsion, holding corresponding axial displacement constant and initial elastic torsion followed by tension, keeping the corresponding angle of twist constant, were examined The experimental programme also considered the non-proportional combined tension-torsion loading of thin walled steel tubes. It has been observed experimentally that, when the rod is initially subjected to an axial load keeping the corresponding axial displacement constant and then followed by subsequently application of the torque the rod behaves as if its axial load carrying capacity decreases without affecting its torque carrying ability. Similarly when the rod is initially subjected to a torque, keeping its corresponding angle of twist constant, and then followed by subsequently application of the axial load the rod behaves as if its torque carrying capacity is reduced without affecting its axial load carrying ability. The third aspect was devoted to the finite element analysis. The finite element analysis package ANSYS (version 5.7) was used for the analysis of the combined tension-torsion loadings of the steel rods. The geometry of the steel rod was supplied to the Ansys package and meshed .The preconditioning conjugate gradient (PCG) and the spare direct solver were used to solve all the combination of loads. Experimental results obtained were compared with computed values from the finite element analysis and are presented. Reasonably good agreement is obtained between the experimental and computed stresses and displacements. This work has direct bearing on the relaxation of tightening torques or axial loads as experienced by critical engineering components, such as couplings, bolted joints, rotating shafts, and steel structures that are subjected to similar types of combined loadings. It is also hoped that the information generated in this research work will guide the designer towards the use of more realistic materials and achieve better design.