A. Germaneau

Refinement of digital image correlation technique to investigate the fracture behaviour of refractory materials

Refractory materials exhibit a heterogeneous microstructure consisting in coarse aggregates surrounded by fine grains that form an aggregate/matrix composite. This heterogeneous microstructure often leads to a complex mechanical behaviour during loading. This paper is devoted to the study, thanks to an optical method, Digital Image Correlation (DIC), of the fracture behaviour of two industrial refractory materials in relation with their microstructure resulting from both the chosen constituents and the sintering process. The aim is here, specifically, to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. DIC is indeed a helpful and effective tool, in the topic of experimental mechanics, for the measurement of deformation in a planar sample surface. This non-contact optical method directly provides full-field displacements by comparing the digital images of the sample surface obtained before and during loading. In the present study, DIC has been improved to take into account the occurrence of cracks and performed so as to better identify the early stage of the cracking behaviour. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex and a discontinuity of displacement should be taken into account. Then each subset which crosses a crack can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening).

In vitro production and biomechanical experimental analysis of thoracolumbar burst fractures

Spinal traumatisms constitute a current pathology in traumatology (10%) and thoracolumbar burst fractures are the most frequent and generally concern young patients. Burst fractures are characterised by compression loadings involved lesion of the vertebrae endplate between the anterior and the posterior walls (Magerl et al. 1994). Determinations of instability and treatment indications remain particular points of questioning. Various treatment solutions exist: immobilisation by an aircast, surgical treatment by posterior or anterior instrumentation and, recently, kyphoplasty with cement injection after vertebral reduction. Choice of the treatment can be delicate and few biomechanical studies have been performed to establish the mechanical response of each option. For that, a cadaveric model of the burst fracture is helpful. This study concerns the development of experiments to generate burst fractures.

Utility of cement injection to stabilize split-depression tibial plateau fracture by minimally invasive methods: A finite element analysis

Background: Treatment for fractures of the tibial plateau is in most cases carried out by stable fixation in order to allow early mobilization. Minimally invasive technologies such as tibioplasty or stabilization by locking plate, bone augmentation and cement filling (CF) have recently been used to treat this type of fracture. The aim of this paper was to determine the mechanical behavior of the tibial plateau by numerically modeling and by quantifying the mechanical effects on the tibia mechanical properties from injury healing. Methods: A personalized Finite Element (FE) model of the tibial plateau from a clinical case has been developed to analyze stress distribution in the tibial plateau stabilized by balloon osteoplasty and to determine the influence of the cement injected. Stress analysis was performed for different stages after surgery. Findings: Just after surgery, the maximum von Mises stresses obtained for the fractured tibia treated with and without CF were 134.9 MPa and 289.9 MPa respectively on the plate. Stress distribution showed an increase of values in the trabecular bone in the treated model with locking plate and CF and stress reduction in the cortical bone in the model treated with locking plate only. Interpretation: The computed results of stresses or displacements of the fractured models show that the cement filling of the tibial depression fracture may increase implant stability, and decrease the loss of depression reduction, while the presence of the cement in the healed model renders the load distribution uniform. Highlights:A patient‐dependent finite element model from a clinical case has been developed.A biomechanical analysis of cement contribution on a stabilized personalized model of tibiaStress distribution in bone structures was analyzed according to stabilization solutions.Stress distribution was studied a different stages of healing process.

Contribution of minimally invasive bone augmentation to primary stabilization of the osteosynthesis of Schatzker type II tibial plateau fractures: Balloon vs bone tamp

Background: Schatzker type II tibial plateau fractures necessitate the least invasive treatment possible. Arthroscopic reduction by bone tamp followed by osteosynthesis is the current gold standard for this type of tibial plateau fracture. The objective of this study was to compare this technique to anterior approach tuberoplasty with balloon reduction. The comparison criteria were residual articular step off, metaphyseal cavity volume formed during reduction, and mechanical strength to separation and to depression displacement. Methods: Fractures were created on 12 human cadaveric tibiae and reduced by a minimally invasive approach in six specimens by a balloon, and by bone tamp in the six others. Articular step off and metaphyseal‐epiphyseal cavity volume were measured by TDM. Mechanical tests were performed up to assembly failure to characterize structural strength. Secondary displacements, fracture depression displacement and separation were measured by optical methods. Findings: There was no significant difference in step off measurement after balloon reduction or bone tamp (0.29 cm vs 0.37 cm; p = 0.06). The cavity volume formed by balloon reduction was significantly smaller than the volume created by bone tamp reduction (0.45 cm3 vs 5.12 cm; p = 0.002). The compressive load required for assembly failure was significantly greater in the balloon group than in the bone tamp group (1210.17 N vs 624.50 N; p = 0.015). Interpretation: There exists a correlation between load to failure of the assembly frame and the metaphyseal volume required for bone fracture reduction. The minimally invasive balloon technique has fewer negative effects on the osseous stock, thereby enabling better primary structural strength of the fracture.

Biomechanical analysis of the thoracolumbar spine under physiological loadings: Experimental motion data corridors for validation of finite element models

Biomechanical studies that involve normal, injured or stabilized human spines are sometimes difficult to perform on large samples due to limited access to cadaveric human spines and biological variability. Finite element models alleviate these limitations due to the possibility of reusing the same model, whereas cadaveric spines can be damaged during testing, or have their mechanicals behaviour modified by fatigue, permanent deformation or structural failure. Finite element models need to be validated with experimental data to make sure that they represent the complex mechanical and physiological behaviour of normal, injured and stabilized spinal segments. The purpose of this study is to characterize the mechanical response of thoracolumbar spine segments with an analytical approach drawn from experimental measurements. A total of 24 normal and fresh cadaveric thoracolumbar spine segments (T11–L3), aged between 53 and 91 years, were tested in pure flexion/extension, lateral bending and axial torsion using a specific experimental setup. Measurements of global and intervertebral angle variations were performed using three-dimensional mark tracking methods. Load/angle curves for each loading were fitted by a logarithmic approach with two coefficients. The coefficients for the functions describing the response of the spinal segments are given and constitute predictive models from experimental data. This work provides data corridors of human thoracolumbar spine motion segments subjected to pure bending in the three physiological planes. These data could be very useful to validate finite element models of the human spine.

Finite element analysis of mechanical behavior of stabilization techniques for tibial plateau fractures

adepartment of mechanical engineering, Faculty of technology sciences, university of mentouri BrothersConstantine, Constantine, algeria; binstitut pprime upr 3346, Cnrs – université de poitiers – isae-ensma, France; caBs lab, université de poitiers, France; ddepartment of neurosurgery, prismatics lab, Chu, poitiers, France; edepartment of orthopaedic surgery and traumatology, Chu, poitiers, France

Biomechanical analysis of different cross-link configurations spinal instrumentation systems: a preliminary study

Percutaneous techniques were initially developed in the management of thoraco-lumbar fractures. Initially transmucular systems have been developed, which generally provide less muscle trauma, blood loss and surgical time. They also benefit from a reduction in the duration of hospitalization, a lower rate of infection, and a reduction in postoperative pain. Nevertheless, there is no system of crosslink binding in these minimally invasive approaches, and those, which are vectors of pain and discomfort from a clinical point of view (Skovrlj 2015). Crosslinks are often used clinically as part of spinal instrumentation, but relatively little basic biomechanical research has been done in terms of their contribution. Dick et al. (1997) used a synthetic spine of polyurethane and reported that stiffness with the concomitant use of transverse systems increased by 60% in axial rotation, but that no increase in flexion, extension or lateral inclination were Observed. Conversely, Lynn et al. (1997) performed cadaveric biomechanics showing that concurrent use of crosslinks increased stiffness not only in rotation but also in lateral flexion. Our study aims to determine the contribution of one or two transverse devices in a short assembly, for which the results are very variable in the literature and if it is necessary to develop percutaneous.

Development of an experimental model of burst fracture with damage characterization of the vertebral bodies under dynamic conditions

Background: Burst fractures represent a significant proportion of fractures of the thoracolumbar junction. The recent advent of minimally invasive techniques has revolutionized the surgical treatment of this type of fracture. However mechanical behaviour and primary stability offered by these solutions have to be proved from experimental validation tests on cadaveric specimens. Therefore, the aim of this study was to develop an original and reproducible model of burst fracture under dynamic impact. Methods: Experimental tests were performed on 24 cadaveric spine segments (T11‐L3). A system of dynamic loading was developed using a modified Charpy pendulum. The mechanical response of the segments (strain measurement on vertebrae and discs) was obtained during the impact by using an optical method with a high‐speed camera. The production of burst fracture was validated by an analysis of the segments by X‐ray tomography. Findings: Burst fracture was systematically produced on L1 for each specimen. Strain analysis during impact highlighted the large deformation of L1 due to the fracture and small strains in adjacent vertebrae. The mean reduction of the vertebral body of L1 assessed for all the specimens was around 15%. No damage was observed in adjacent discs or vertebrae. Interpretation: With this new, reliable and replicable procedure for production and biomechanical analysis of burst fractures, comparison of different types of stabilization systems can be envisaged. The loading system was designed so as to be able to produce loads leading to other types of fractures and to provide data to validate finite element modelling. HIGHLIGHTSA specific experimental dynamic setup for spinal loading has been developed.A biomechanical analysis was performed during impact on spines from a dynamic optical method.Mechanical effects of a dynamic impact on spine were characterized by X‐ray imaging and optical analysis.A reproducible model of burst fracture on human cadaveric specimens was developed.

Identification of Thermal Parameters by Treating the Inverse Problem

aim of this work is to identify the tensor of thermal conductivity and volumetric heat of an anisotropic material with conductivity tensor diagonal, the heat conduction follows the linear Fourier law. The temperature field in the plate is obtained by solving the analytical heat equation. The solution of the direct problem is simulated by applying the Finite Element Method 2D. The inverse problem is solved by returning the intermediate step of the MEF. An optimization method based on conjugate gradient algorithm has enabled us to develop an identification of thermophysical parameters procedure.

Identification of Elastic Parameters by Treating the Inverse Problem

The aim of this work is to lay the groundwork for identifying digital mechanical parameters of materials with elastic. Most of the tests do not allow identifying these parameters automatically. The use of the finite elements of calculations for sizing works is thus limited by a poor understanding of the mechanical properties. In this context, it raises the issue of inverse analysis [1] [2]. From this information about the parameters of the laws of material behavior, is it possible to obtain the displacement field from in situ measurements and how does digital technology obtain a determination of these parameters accurately and systematically? In this work we present a new approach by providing a formulation is easily used by treating the inverse problem. It is based on the finite element method, which, in a direct problem, gives the displacement field knowing the mechanical properties and an inverse problem gives the mechanical knowledge of the field trips. The resolution of the direct problem has yielded results. The latter is in agreement with the simulation code of commercial calculation. This allowed us to address the inverse problem with no understanding by offering an alternative identification using a database previously determined [3].