S. Pattofatto

The Conformance Test for Robotic/Prosthetic Fingertip Skins

Although the requirements for robotic and prosthetic skins may considerably differ, conformance is a critical property when a tactile sensory system is embedded on the skin. The skins inability to deflect or adapt to the surface of an object will prevent the spatial features to be transmitted to the embedded tactile sensors. By characterizing viscoelastic materials like Technogelreg, silicone and polyurethane, and applying their material coefficients to a finite element model, comparisons were made possible with the experimental surface deflection of a human fingertip that was indented by a wedge. Simulations showed that the silicone as the inner layer, and polyurethane or Technogelreg as the outer layer offered the optimal combination that approximates the human fingertips conformance

Links between mechanical behavior of cancellous bone and its microstructural properties under dynamic loading

Previous studies show that in vivo assessment of fracture risk can be achieved by identifying the relationships between microarchitecture description from clinical imaging and mechanical properties. This study demonstrates that results obtained at low strain rates can be extrapolated to loadings with an order of magnitude similar to trauma such as car crashes. Cancellous bovine bone specimens were compressed under dynamic loadings (with and without confinement) and the mechanical response properties were identified, such as Young׳s modulus, ultimate stress, ultimate strain, and ultimate strain energy. Specimens were previously scanned with pQCT, and architectural and structural microstructure properties were identified, such as parameters of geometry, topology, connectivity and anisotropy. The usefulness of micro-architecture description studied was in agreement with statistics laws. Finally, the differences between dynamic confined and non-confined tests were assessed by the bone marrow influence and the cancellous bone response to different boundary conditions. Results indicate that architectural parameters, such as the bone volume fraction (BV/TV), are as strong determinants of mechanical response parameters as ultimate stress at high strain rates (p-value<0.001). This study reveals that cancellous bone response at high strain rates, under different boundary conditions, can be predicted from the architectural parameters, and that these relations with mechanical properties can be used to make fracture risk prediction at a determined magnitude.

Influence of the Impact Velocity on the Perforation Resistance of a Glass-Reinforced Polypropylene Material

The perforation resistance of a thermoplastic woven composite material (Twintex-based) was experimentally investigated. First, the efficiency of the inversed perforation experiment conducted on a Hopkinson bar was used to measure accurately the strain-rate sensitivity at two perforation velocities (0.01 and 45 m/s). The results showed an increase in perforation maximum load and perforation energy. Then, direct tension experiments were performed to obtain the mechanical response and fracture force of the yarns at two different tension velocities 0.05 and 2000 mm/s. Low strain-rate sensitivity was measured and no modification in the fracture process was observed. On the contrary, the use of imaging techniques, from low to highest impact velocities reachable with the perforation experiments, showed that the fracture process was clearly modified from macrocracking at 0.01 mm/s to early fragmentation at 45 m/s, which may contribute to the strain-rate sensitivity of the strength of the material.

On the piercing force enhancement of aluminum foam sandwich plates under impact loading

This article tends to explain the observed piercing force enhancement under impact loading of the sandwich panels, which is made of rate-insensitive aluminum skin sheet and Cymat foam core. Using an elastic–plastic damageable model for the skin and an isotropic foam constitutive model for the core, a numerical model is proposed and validated by the comparison with experimental results. Virtual tests allow revealing that the damage evolution in the skin sheet is strain rate dependent because of the wave propagation effects. Under impact loading, the damage area is larger and it leads to a larger deflection of the skin plate at its breaking. Therefore, the foam cores are more compressed under impact loading and induce the piercing force enchancement of the sandwich.

Modeling of a Polishing Tool to Simulate Material Removal

In plastic injection mould and prosthesis industries, `mirror-effect` polished surfaces are required for obtaining transparent parts or surfaces without scratches. Traditionally done manually, we have proposed to automate polishing on 5-axis machining centre using a passive elastomeric carrier. One of the main advantages of automatic polishing is the repeatability of the machine movements in order to achieve restricted form deviations. However, the material removal rate (MRR) during polishing depends on parameters such as contact pressure, relative velocity and tool wear. We have thus developed a model dedicated to our process to compute the effective MRR along the polishing tool path regarding the contact area and the contact pressure between the tool and the part

Links between microstructural properties of cancellous bone and its mechanical response to different strain rates

Automobile accidents and sporting injuries may lead to osseous fractures. To reduce the number of road accidents and their societal costs, governments have partnered with car manufacturers to develop an overall road safety. To achieve this, researchers are working on improving the design of automotive structures. However, researchers must first quantify the risk of injury incurred during an impact. Indeed, in France, osteoporosis is responsible of approximately 150,000 fractures per year. A better understanding of this fracture mechanism will aid in the design of protective features that will guard against fracture under these loading conditions. Bone is generally divided into two micro-structural types: cortical bone and cancellous bone. Cortical bone is a compact bone, denser than cancellous bone, and accounts for 80% of the skeletal mass in the human body. Cancellous bone, also called trabecular or spongy bone, has a porous structure that protects the bone marrow, acts as a core material to support the shape of thin layers of cortical bone and assists in transferring joint forces to the thick load bearing cortical bone layers. Several studies have been able to make great progress on characterising and modelling the behaviour of cortical bone. Regarding the cancellous bone, studies are mainly focused on quasi-static loading cases (Follet 2002). In order to analyse and understand the mechanism of cancellous bone for speed ranges above the quasi-static regime, experimental work using Split Hopkinson Bar Technique (SHPB) has been undertaken. However, no modelling, including the different parameters of cancellous bone, has yet been developed to analyse and understand the mechanism of rupture of the cancellous bone. This issue raises a keen interest in the scientific community, and this comes through in the work presented here. Indeed, the aim of this study was to characterise the mechanical properties of cancellous bovine bone for compression loading under different strain rates and to identify links with the microstructural description.

Influence of Property Gradient on the Behavior of Cellular Materials Subjected to Impact Loading

Recent manufacturing advances offer the possibility of introducing controlled porosity gradients in metallic‐based cellular materials (foam, hollow spheres), which are promising structural materials used in applications involving lightweight structures, impact energy absorption, acoustical wave attenuation, etc. However, the influence of such porosity gradients on the overall mechanical properties of these cellular materials still lacks understanding. This paper presents a study of such porosity gradient influence using numerical simulation. Basic material behavior of a given sphere size is experimentally characterized using a recently developed testing device—a 60 mm diameter nylon Hopkinson bar system, which provides an interesting solution for both the impedance match and reasonable specimen size. Numerical results using realistic material constants permit the determination of optimized porosity gradient influence on various properties such as impact resistance, thermal diffusion, etc.

Mechanical response and failure processes of anterior cruciate ligament from static to dynamic loading rates

In daily life activities, knee joint ligaments and in particular the anterior cruciate ligament (ACL) are commonly injured and the knowledge of their mechanical properties is therefore necessary to design artificial materials. In most cases, rupture occurs at the bone–ligament interface, under dynamic loading (Subit et al. 2008), but the failure process depends on the elongation rate. This paper presents experimental results obtained using rabbit femur–ACL–tibia complexes (FATCs) submitted to a wide range of loading rates and analysed using digital image correlation (DIC).

Shock Enhancement due to Shock Front Propagation in Cellular Materials

In this study, crushing experiments are performed on four kinds of cellular materials using a large diameter (60 mm) nylon Hopkinson bar. The impact velocities are chosen around the critical velocity corresponding to the occurrence of a shock front predicted by the classical RPPL model (Reid and Peng, 1997). The experimental setup allows to measure the stress enhancement due to the shock front propagation. In particular, for one type of material, a high speed camera is used to capture about ten images at 20,000 fps and then the strain field during testing is obtained by a special image correlation program (CorreliLMT). This strain field allows to measure directly the shock front velocity. Moreover, an improved model, including the hardening curve, is proposed to predict this shock enhancement. Finally, numerical analyses using Ls-Dyna explicit code show that for all experiments a macroscopic homogeneous phenomenological material law can reproduce essential features of stress enhancement due to shock front propagation.

Experimental Analysis of the Shock Enhancement of a Cellular Structure Under Impact Loading

This study is part of a research program on the analysis of the impact behavior of metallic cellular materials (honeycomb, foam, hollow sphere agglomerate) used in the energy absorption design of aeronautical applications (Zhao [1]).