Vincenzo Moramarco

An accurate validation of a computational model of a human lumbosacral segment

Clinical studies have recently documented that there is sufficient evidence to suggest that abnormal motion may be an indicator of abnormal mechanics of the spine and, therefore, may be associated with some types of low-back pain. However, designating a motion as abnormal requires knowledge of normal motions. This work hence aims to develop an accurate computational model to simulate the bio-mechanical response of the whole lumbosacral spinal unit (L1-S1) under physiological loadings and constraint conditions. In order to meet this objective, computed tomography (CT) scanning protocols, finite element (FE) analysis and accurate constitutive modelling have been integrated. Then the ranges of motion (ROM) under flexion, extension and lateral bending moment were measured and compared with experimental data, finding an excellent agreement. In particular, the ability of the model to reproduce the relative rotation between each couple of vertebrae was proved. Finally, the shear stresses for the most extreme load cases were reported in order to predict which are the most risky conditions and where the maximum damage would be located. The results indicate that the greater values of the stresses were located at L4-S1 levels just in the interfaces between disc and vertebrae across the posterior and posterolateral zone. This result can be clinically correlated with the existence of damage exactly where the stresses were maximal in the proposed finite element model.

Analysis and Comparison of Friction Stir Welding and Laser Assisted Friction Stir Welding of Aluminum Alloy

Friction Stir Welding (FSW) is a solid-state joining process; i.e., no melting occurs. The welding process is promoted by the rotation and translation of an axis-symmetric non-consumable tool along the weld centerline. Thus, the FSW process is performed at much lower temperatures than conventional fusion welding, nevertheless it has some disadvantages. Laser Assisted Friction Stir Welding (LAFSW) is a combination in which the FSW is the dominant welding process and the laser pre-heats the weld. In this work FSW and LAFSW tests were conducted on 6 mm thick 5754H111 aluminum alloy plates in butt joint configuration. LAFSW is studied firstly to demonstrate the weldability of aluminum alloy using that technique. Secondly, process parameters, such as laser power and temperature gradient are investigated in order to evaluate changes in microstructure, micro-hardness, residual stress, and tensile properties. Once the possibility to achieve sound weld using LAFSW is demonstrated, it will be possible to explore the benefits for tool wear, higher welding speeds, and lower clamping force.

A Constitutive Model for the Annulus of Human Intervertebral Disc: Implications for Developing a Degeneration Model and Its Influence on Lumbar Spine Functioning

The study of the mechanical properties of the annulus fibrosus of the intervertebral discs is significant to the study on the diseases of lumbar intervertebral discs in terms of both theoretical modelling and clinical application value. The annulus fibrosus tissue of the human intervertebral disc (IVD) has a very distinctive structure and behaviour. It consists of a solid porous matrix, saturated with water, which mainly contains proteoglycan and collagen fibres network. In this work a mathematical model for a fibred reinforced material including the osmotic pressure contribution was developed. This behaviour was implemented in a finite element (FE) model and numerical characterization and validation, based on experimental results, were carried out for the normal annulus tissue. The characterization of the model for a degenerated annulus was performed, and this was capable of reproducing the increase of stiffness and the reduction of its nonlinear material response and of its hydrophilic nature. Finally, this model was used to reproduce the degeneration of the L4L5 disc in a complete finite element lumbar spine model proving that a single level degeneration modifies the motion patterns and the loading of the segments above and below the degenerated disc.

Preliminary Study on Residual Stress in FDM Parts

The Fused Deposition Modelling (FDM) is nowadays one of the most widespread techniques for 3D object rapid prototyping. In recent years, the FDM evolved from rapid prototyping technique towards a rapid manufacturing method, changing the main purpose in producing finished components ready for use. However, as the parts are built as a layer-by-layer deposition of a feedstock wire, the FDM technique shows, during the building process, distortion and de-layering problems. This issue influences the shape and the final dimensions of the parts or it can prevent the finalization of the objects due to unsticking problems from the bed. Several techniques can be employed in order to obtain parts of correct shape and dimensions. Many of these, such as depositing glue on the bed, aim to constrain the object. As a consequence, the FDM parts could show residual strain and residual stress that could influence their mechanical behaviour. The aim of this work is to measure, by ESPI technique, the displacements around a hole drilled into the material. This can be considered as a preliminary indication of the level of residual stress inside the FDM parts.

Influence of Printing Constraints on Residual Stresses of FDM Parts

The Fused Deposition Modelling (FDM) is nowadays one of the most widespread and employed processes to build complex 3D prototypes directly from a STL model. In this technique, the part is built as a layer-by-layer deposition of a feedstock wire. This typology of deposition has many advantages but produces rapid heating and cooling cycles of the feedstock material that introduces residual stresses in the part during the build-up. Consequently, warping, de-layering and distortion of the part during the print process are common issues in FDM parts and are related to residual stresses. The common techniques employed to obtain parts of correct shape and dimensions, such as depositing glue on the bed, have the aim to constrain the object on the printing bed, though this increases the residual stresses in the parts. The aim of the present work is to measure the residual stresses in several points of printed parts, both on top and bottom, in order to verify if the constrain conditions used during the printing produce substantial variation from a point to another. The residual stresses have been measured in ABS parts employing the hole-drilling method. In order to avoid the local reinforcement of the strain gage, an optical technique, i.e. ESPI (electronic speckle pattern interferometry), is employed to measure the displacement of the surface due to the stress relaxation and, consequently, calculate the residual stresses.

Influence of the clamps configuration on residual stresses field in friction stir welding process

Friction stir welding is a joining process developed in 1991 by The Welding Institute. This welding technique is a solid-state joining process leading to joints with good mechanical performance and low residual stresses. In all welding techniques, the clamping systems have an important role in determining the quality of the welds and mechanical characteristics. Even more in friction stir welding, the position of the clamps plays a critical role because it is mainly a mechanical process with high forces involved. In this article, the correlation between the residual stress field and configurations of clamps has been established numerically. For this purpose an uncoupled thermo-mechanical finite element analysis has been carried out. The mechanical loads due to the tool have been also implemented into the model. The thermal and mechanical models have been validated on temperature field recorded by an infrared camera and residual stress field measured by X-ray diffraction analysis. The friction stir welding test was conducted on 6-mm-thick 5754 H111 aluminium alloy plates.

Temperature Field in FSW Process: Experimental Measurement and Numerical Simulation

Friction Stir Welding (FSW) is a relatively new welding process, which was developed at The Welding Institute (TWI), United Kingdom, in 1991. FSW is a solid-state joining process, i.e. no melting occurs. The welding process is promoted by the rotation and translation of an axis-symmetric non-consumable tool along the weld centreline. Thus the FSW process is performed at much lower temperatures than the conventional fusion welding. Nevertheless the control of the temperature field is fundamental to guarantee a high quality joint. In the present work the temperature field during the welding process was measured using an infrared camera. The test was conducted on 6 mm thick 5754 H111 aluminium alloy plates, in bead on plate configuration, with constant tool rotation rate and feed rate. Furthermore a finite element model was implemented and validated on experimental measurement data to evaluate the temperature field also into the plate.

Numerical Prediction of Temperature and Residual Stress Fields in LFSW

In recent years important innovations have been introduced in FSW technology such as, for example, the Laser assisted friction stir welding (LFSW). In this technique the FSW process is assisted, during the weld, by the pre-heating of a de-focalized laser that allows an easier advancement of the tool. This technique should lead higher welding speeds, lower clamping forces and lower residual stress. Moreover, the prediction of residual stress is a relevant, complex and still open issue for welding process design. In this work an uncoupled thermo-mechanical finite element model has been developed to predict the temperature and residual stress fields in LFSW. Both the thermal and mechanical parts of the model have been validated on the base of experimental data. The temperature field has been recorded by an infrared camera, while residual stress field has been measured by X-ray diffraction analysis. The LFSW test was conducted on 6 mm thick 5754 H111 aluminium alloy plates.

Discussion on X-Ray and HDM Residual Stress Measurements

The stress field remaining in some materials without application of external sources of stress is known as residual stress. These residual stresses are produced in almost all manufacturing processes or may occur during the life of structures. They have a fundamental role in welded joints because they affect the way to design structures (e.g. the safety coefficients), their fatigue life and their corrosion resistance. Quantify, as well as possible, the residual stress field is one of main issues for mechanical engineers. To this purpose, in the last decades, several techniques have been developed. Hole Drilling Method (HDM) and X-Ray Diffractometry (XRD) are two of the most diffused and the only standardized techniques to measure the stress field in depth on welded structures. Although both methods declare to make an accurate measurement of the stresses, few comparisons of these techniques applied on the same structure are described in scientific literature.

Influence of End-Plates on Biomechanical Response of the Human Lumbosacral Segment

Diseases of lumbar spine and associated diseases of the intervertebral disc are a major focus of contemporary spinal care. Low back pain, in fact, is becoming in the recent years one of the most diffuse chronic pathologies and represents one of the highest direct and indirect costs for national welfare. It can affect for years the patient with obvious disabling consequences and many times without a complete explanation of the causes (Devor & Tal, 2009). Some clinical studies (Latorraca & Forni Niccolai Gamba, 2004) showed that the greatest proportion (about 90%) of spinal diseases, like lumbar hernias, is located in the lumbar spine segment. Recent investigations based on clinical-radiological observations pointed out those rachidian affections such as lumbar and sciatic pains that in the worst cases last for years with very invalidating effects on the patient mainly depending on disk-vertebral insufficiency caused by degenerative phenomena. At the same time, in absence of pathological disease, daily activities, lifting stationary work postures, heavy physical work and vibrations are factors that contribute to low back disorders (Natarajan et al., 2004). Usually, orthopaedic therapy is essentially based on the experience of the surgeon who predicts the best solution for each patient. Using mathematical models and computer simulations could potentially be an important tool to support clinical decisions in order to predict the appearance and evolution of spine pathologies, for preoperative planning and implant design. The present work is focused on the analysis of the lumbar spine with the aim of studying the influence and the roles that the different components of the spine play on its biomechanical response. In particular the presence of the cartilagineous endplates separating the soft tissues of the intervertebral disc from the strong bone of the vertebrae will be studied. The endplates, in fact, are demonstrated to perform a double function in the global response of the spine respectively mechanically “protecting” the discs from a direct contact with the vertebrae avoiding the disc to bulge axially under compression and, at the same time, acting as the favourite nutrients transport route for the same discs. The endplates also absorb the