Papers by Author: R.G. Rodríguez-Cañizo

Abstract: This work assesses the Crack Compliance Method (CCM), which has been extensively used for the experimental evaluation of residual stresses, by the Finite Element Method (FEM) to validate its experimental applicability through numerical evaluation. The CCM is a very powerful method that is based on Fracture Mechanics theory, but its experimental application and set up has not been totally scientifically validated. In this paper, a numerical evaluation is presented on the basic applications of the CCM. The assessment of the CCM is performed on bending beams with and without prior straining history. To determine the best position and orientation of the strain gages, as well as the optimum number of readings, a number of numerical simulations where also performed for the correct performance of the experimental evaluation of the CCM. The prior straining history condition, in the analyzed components, is induced by an axial pulling before the beam is bent. Three levels of preloading are considered: low, medium and high (which are related to the yield strain of the simulated material); Isotropic and Kinematic hardening rules are also considered. After the residual stress field is induced by bending, a slot cutting is simulated and the strain relaxation produced is captured, which is used later in the CCM program for the quantification of the original residual stress field. The results obtained in this work, provide a quantitative demonstration of the effect of hardening strain on the distribution of the residual stress in beams. In the same manner, the theoretical formulation of the CCM has been evaluated validating the application of this method for the determination of residual stress fields in mechanical components.

Abstract: This paper presents the modelling of the effects due to load conditions on the cervical section defined between C3 and C5 after a cervical plate implant is used to transfer the compression loads from C3 to C5 as C4 is considered to be damaged as a result of a medical condition. For this study, three different scenarios which describe the common motion condition of the head-neck system are modelled. The first one refers to the effect of the head weight over the considered section. In the second case the average patient weight is supported by C3 and C5 vertebrae. The last case simulates extreme loading conditions as vertebrae lesions occur when these are compressed beyond its failure limit; the ultimate stress to compression load failure value is applied to C3. The stability and mechanical behaviour of cervical plates under compression loading conditions is evaluated using the Finite Element Method (FEM). Cervical plates are useful to restore stability of the spine by improving the inter-vertebral fusion, particularly when the cervical body has been damaged. The results show that the stresses on the plate and fixation screws, for the three cases, are within the elastic range. Conversely, it has to be considered that cortical and trabecular bone densities vary from one patient to another due to a number of factors, which can influence the fixation conditions of the screws. In the case of this analysis, healthy bone conditions were considered and the obtained results show that the risk of the integrity of the screwimplant- vertebrae system is not compromised.

Abstract: The main results of a static analysis with a finite element model of the cervical section between C3 – C5 of a human spine are reported. In this case, it is assumed that the element C4 is completely damaged and has to be replaced. Therefore, a bone graft was installed between the anterior side of C3 and C5. Besides, a cervical plate of 55 mm. was fixed at the same side with 4 expansive screws. The resultant stresses caused by compression loads were analyzed and the displacements between the graft and adjacent vertebrae were calculated. Three loading conditions were applied: 80 N, 637.5 N and 6374.5 N. The first one corresponds to the head weight. In the second case, it is assumed that the average patient weight is supported by those vertebrae, while in the last one; the compression load failure is applied on the vertebrae. Results show that displacements were lower than 3 mm between the graft and the adjacent vertebrae. In accordance with the concept of spine stability after Müller [1], the arrangement is a stable one. Another advantage is that no wires are used in this surgical technique. Two more issues should be noticed. There is no risk that the plate may be broken and the geometry of the bone graft allows bone regeneration. These results are on line with those observed in preliminary experimental tests with porcine vertebrae.