Evaluation of cement stresses in finite element analyses of cemented orthopaedic implants

Stress analysis of the cement fixation of orthopaedic implants to bone is frequently? carried out using finite element analysis. However, the stress distribution in the cement laver is usually intricate, and it is difficult to report it in a way that facilitates comparison of implants for pre-clinical testing. To study this problem, and make recommendations for stress reporting, a finite element analysis of a hip prosthesis implanted into a synthetic composite femur is developed. Three cases are analyzed: a fully bonded implant, a debonded implant, and a debonded implant where the cement is removed distal to the stein tip. In addition to peak stresses, and contour and vector plots, a stressed volume and probability-of-failure analysis is reported. It is predicted that the peak stress is highest for the debonded stem, and that removal of the distal cement more than halves this peak stress. This would suggest that omission of the distal cement is good for polished prostheses (as practiced for the Exeter design). However; if the percentage of cement stressed above a certain threshold (say 3 MPa) is considered, then the removal of distal cement is shown to be disadvantageous because a higher volume of cement is stressed to above the threshold. Vector plots clearly demonstrate the different load transfer for bonded and debonded prostheses: A bonded stein generates maximum tensile stresses in the longitudinal direction, whereas a debonded stem generates most tensile stresses in the hoop direction, except near the tip where tensile longitudinal stresses occur due to subsidence of the stein. Removal of the cement distal to the tip allows greater subsidence but alleviates these large stresses at the tip, albeit at the expense of increased hoop stresses throughout the mantle. It is concluded that a thorough analysis of cemented implants should not report peak stress, which can be misleading, but rather stressed volume, and that vector plots should be reported if a precise analysis of the load transfer mechanism is required.

Autoclave Cure Simulation of Composite Structures Applying Implicit and Explicit FE

Simulation of the autoclave manufacturing technique of composites can yield a preliminary estimation of induced residual thermal stresses and deformations that affect component fatigue life, and required tolerances for assembly. In this paper, an approach is proposed to simulate the autoclave manufacturing technique for unidirectional composites. The proposed approach consists of three modules. The first module is a Thermo-chemical model to estimate the temperature and the degree of cure distributions in the composite part during the cure cycle. The second and third modules are a sequential stress analysis using FE-Implicit and FE-Explicit respectively. User-material subroutine is used to model the Viscoelastic properties of the material based on theory of micromechanics.

Virtual Manufacturing of Composite Structures Applying Implicit and Explicit FE Techniques

Virtual manufacturing of composites can yield an initial early estimation of the induced residual thermal stresses that affect component fatigue life, and deformations that affect required tolerances for assembly. Based on these estimation, the designer can make early decisions, which can help in reducing cost, regarding changes in part design or material properties. In this paper, an approach is proposed to simulate the autoclave manufacturing technique for unidirectional composites. The proposed approach consists of three modules. The first module is a Thermochemical model to estimate temperature and the degree of cure distributions in the composite part during the cure cycle. The second and third modules are stress analysis using FE-Implicit and FE-Explicit respectively. User-material subroutine will be used to model the Viscoelastic properties of the material based on micromechanical theory. Estimated deformation of the composite part can be corrected during the autoclave process by modifying the process-tool design. The deformed composite surface is sent to CATIA for design modification of the process-tool.