15 th International Conference on Experimental Mechanics Track_H: Biomechanical Applications 441 PAPER REF: 2757 PSEUDO-DYNAMIC ANALYSIS OF THE BIOMECHANICS OF THE HIP REPLACEMENT USING A FORCE METHOD BASED ON THE NEWMARK ALGORITHM Pedro Talaia 1(*) , António Ramos 2 , Francisco J. Queirós de Melo 3 , and José A. O. Simões 4 1, 2, 3, 4 Biomechanics Research Group, Dep. of Mechanical Eng., Universidade de Aveiro, Aveiro, Portugal 2 Institute of Mechanical Engineering (IDMEC – pólo feup), Universidade do Porto, Porto, Portugal (*) Email: ptalaia@ua.pt ABSTRACT A pseudo-dynamic analysis is applied to predict the stress distribution in an in-vitro Charnley cemented hip replacement under dynamic forces resulting from walking and jumping. Consequences of the stress distribution in the implant region from cyclic fatigue load are discussed. Application of this method in biomechanics appears to be an useful tool for the characterization of mean and peak stresses, making possible the prediction of future damage and a programmed clinical examination in patients using hip prosthesis. INTRODUCTION A pseudo-dynamic technique is a sequential procedure where a current structure configuration obtained by a time integration algorithm is prescribed to a scaled or real sized model in a test rig. This combines a Newmark algorithm version operating with the internal force and compliance of the test-specimen. With the structure discretized in a set of degrees of freedom, a force vector, (internal restoring force vector), is measured at the level of the degrees of freedom using a set of load cells. The internal restoring force vector is then feedback to the time integration Newmark algorithm [1] where new displacement, velocity and acceleration vectors are calculated; the so-updated displacement vector is then prescribed again to the test rig continuing the cyclic sequence of the method. This technique has been mainly applied in piping structures; the authors have not found any report about the eventual use in biological structures, particularly in the simulation of the effect of dynamic loads on a bone structure. The aim of present work is the application to non-conventional pseudo-dynamic analyses to determine the dynamic response of in vitro cemented hip replacement. RESULTS AND CONCLUSIONS The results of the displacements at the loaded end of the implanted femur are depicted in Fig. 1. This result does not correspond to the expected behaviour for an SDoF spring-mass model; in fact this dynamic system is equivalent to the femoral model in test, once a single degree of freedom dynamic model is here analyzed. The reason for this apparent anomaly can be assigned to the machine sliding parts, where a misalignment between of the load axis and the cylinder axis arising from the load eccentricity could generate a bending moment at the crosshead ram and additional friction in the sideway bushes. A numerical analysis [2] for a similar dynamic model was repeated with a Coulomb type friction included. Results for displacement are depicted in Fig. 1.b. The equivalent friction factor  was close to 0.2 , presenting a good agreement in the displacement with the