In the last few decades, great effort has been spent on advanced material testing and the development of damage models intended to estimate the ductility and fracture of ductile metals. While most studies focused on static testing are applied at room temperatures only, in this paper, multiaxial tests have been executed to investigate the effects of dynamic action and temperature on the mechanical and fracture behavior of an API X65 steel. To this end, a Split Hopkinson Bar (SHB) facility for dynamic tests, and a uniaxial testing machine equipped with a high-temperature furnace, were used. Numerical simulations of the experiments were setup for calibration and validation purposes. Based on the experimental results, the Johnson–Cook and Zerilli–Armstrong plasticity models were first tuned, resulting in a good experimental–numerical match. Secondly, the triaxiality and Lode angle dependent damage models proposed by Bai–Wierzbicki and Coppola–Cortese were also calibrated. The comparison of the fracture surfaces predicted by the damage models under different loading conditions showed, as expected, an overall significant increase in ductility with temperature; an appreciable increase in ductility was also observed with the increase in strain rate, in the range of low and moderate triaxialities.

An experimental campaign based on multiaxial tests is carried out to characterize the ductile behavior of 17-4PH steel and a Ti6Al4V titanium alloy, and to calibrate numerical ductile damage models, accordingly. This study aimed to identify a minimal set of four specimen types to ensure the robust tuning of the damage models, using only a conventional uniaxial machine for testing. Two different shear–tension candidate geometries are identified, modified, and used together with cylindrical and notched bar specimens to evaluate material plastic strain at fracture under several stress states, characterized by different triaxialities and Lode angles. Finite element analysis and digital image correlation techniques are used to identify local data not directly measured from the tests. Three recent ductile damage models are calibrated using the experimental data. The accuracy of the proposed approach is validated and presented for the two alloys, by comparing the results with calibrations performed on the same materials using more conventional multiaxial tests. It is shown that the new methodology is effective, and how either one of the two shear–tension geometries in addition to tensile tests could replace, with the same level of accuracy, typical more complex calibration procedures involving tests that require dedicated facilities.

A detailed solid model of an entire lumbar segment is reconstructed through a reverse engineering approach based on a real patient Computed Tomography DICOM scan. The main structurally and functionally relevant elements have been considered in the model, including the L1-S1 vertebrae, intervertebral discs, ligaments, and the simplified structural and bracing effects of the principal lumbar muscles. Using the model, a structural Finite Element (FE) analysis is setup to provide an insight of stresses and range of motion of the "spinal mechanical system" under typical everyday as well as accidental loads. Organic bone, discs, ligaments physical and structural material data have been retrieved, and both static and fatigue load cases have been taken into account, collecting real world data from a review of literature works and of information available from accessible databases. A validation of the FE model is provided based on the so called Yamamoto protocol first, and then using experimental measurements performed on real patients, again taken from literature works. The numerical model proved to be accurate in the description of the stress states of the different parts of the spine and in reproducing the overall range of motion of the lumbar segment. A tentative design modification of rigid interbody cages and vertebral fixation systems used in present spinal fusion neurosurgery is then proposed, aimed at providing a certain level of compliance to the implant. This should restore a limited range of motion of the two adjacent vertebrae hosting the implant itself, reducing the possibility of subsequent adjacent segments diseases. The new cage and fixation system have been integrated in the FE model to replace a prospective "damaged" L4-L5 disc. The results of the numerical simulation demonstrated that the implant can withstand static and fatigue everyday life loads, as well as accidental overloads without failing. Additionally, the analysis showed that a limited mobility is maintained between the fused vertebrae.