Nuno Peixinho

Study of Viscoplasticity Models for the Impact Behavior of High-Strength Steels

This study reports on modeling the mechanical behavior of high-strength steels subjected to impact loading. The materials studied were steel grades of interest for crashworthiness applications: dual-phase and transformation induced plasticity (TRIP) steels. The challenges associated with the numerical simulation of impact events involving these materials include the modeling of extensive plastic deformation, particularly the change of material properties with strain rate. Tensile testing was performed at different strain rates on the materials studied. The test results were used to compare and validate constitutive equations that provide a mathematical description of strain-rate dependence of the material properties. The Cowper-Symonds equation and modified variants were examined. The crashworthiness performance of thin-walled sections made of dual-phase and TRIP steels was also investigated. Axial crushing tests were performed at different speeds on top-hat and hexagonal tubes. The experimental results were compared with numerical simulations obtained using an explicit finite element program (LS-DYNA) and the original and modified Cowper-Symonds equations.

Characterization of dynamic material properties of light alloys for crashworthiness applications

This paper presents results on the tensile testing of AZ31B-H24 magnesium alloy and 6111-T4 aluminium alloy at different strain rates. These materials are strong candidates for use in crashworthy automotive components and parts due to their well-balanced combination of strength, stiffness and density. To test their application in the auto industry an understanding of material behaviour at relevant strain rates is needed, as well as constitutive equations suitable for use in analytical and numerical calculations. Mechanical properties were determined from tensile tests using flat sheet samples, employing two different test techniques: a servo-hydraulic machine and a tensile-loading Hopkinson bar. The test results were used to compare different mechanical properties of the tested materials and to validate constitutive equations intended to provide a mathematical description of strain rate dependence. The Cowper-Symonds equation was examined.

Application of Dual-Phase and TRIP Steels on the Improvement of Crashworthy Structures

The improvements in vehicle crashworthiness observed in recent years have been closely linked to advanced high-strength steels that are currently being produced or in process of development. Amongst these, Dual-Phase and TRIP (Transformation Induced Plasticity) steels have presented excellent properties for use in crashworthy structures. For these steel grades an understanding of material behaviour at relevant strain rates is needed as well as constitutiv eequations suitable for use in analytic and numerical calculations. In this study the crashworthiness of thin-walled sections made of Dual-Phase and TRIP steels was investigated. Tensile tests were performed at different strain rates in a range of interest for crashworthiness problems. The results allowed the determination of parameters of Cowper-Symonds equation. Crush tests were performed at different speeds for top-hat and hexagonal tubes manufactured using laser welding. The experimental results were compared with numerical simulations obtained with LS-DYNA software. The influence of different material parameters on the accuracy of the simulations was examined.

A Novel Methodology to Assess the Relaxation Rate of the Intervertebral Disc by Increments on Intradiscal Pressure

The Intervertebral Disc (IVD) is subjected to several types of loading during daily routine events. However, the overloading on this structure induces higher Intradiscal Pressure (IDP), which could cause severe damage on its structure. This study describes a new approach to that allows monitorize and pressurize nuclear region of the IVD, with a cartilaginous endplate access, by the insertion of an external fluid, while a Motion Segment (MS-assembly composed by vertebra-disc-vertebra) is compressed at a physiological load. This methodology includes the use of a pneumatic structure that applies a certain pressure on the hydrostatic system, forcing a fluid to enter into the MS through a screw, with a drilled hollow along its entire length. Preliminary results indicated that this methodology presents high potential to efficiently pressurize the IVD, providing a useful tool to better understand the response of this structure under pressure.

Experimental study of impact energy absorption in aluminium square tubes with thermal triggers

This study presents an approach for the improvement of crashworthiness properties of aluminium tubular structures using initiators introduced through localized heating. The main objective of this approach is to improve the ability to absorb impact energy in a progressive and controlled manner by a local modification of material properties. Through a localized heating in areas previously chosen for initiation, associated with the softening of the aluminium alloy the deformation can be introduced precisely, forcing the tubular structure to deform in a mode of high energy absorption and reducing the maximum load in a controlled manner. This study presents the properties for an aluminium alloy 6061-T5 modified by thermal treatment by the use of a laser beam. Experimental results are presented of quasi-static and impact tests of tubular structures using the proposed approach. This concept appears as possible and effective in the experimental work presented.

Dynamic Material Properties of Stainless Steel and Multiphase High Strength Steels

This work presents results of tensile testing of H400 stainless steel, DP600 and TRIP600 at different strain rates. Mechanical properties were determined from tensile test using flat sheet specimens and recurring to different test techniques: servo-hydraulic machine and a tensile-loading Hopkinson bar. The test results were used to compare different mechanical properties of the tested steels and to validate constitutive equations intended to provide a mathematical description of strain rate dependence, namely the Cowper-Symonds equation. Following previous research work in dynamic material proprieties of multiphase and stainless steel grades, the energy absorption in quasi-static crushing of thin walled section made of the tested materials was subsequently investigated. Crush tests were performed in top-hat and hexagonal section tubes manufactured using laser welding. The experimental results were compared in order to assess the efficiency of the different steel grades for energy absorption.

Dent Resistance of Aluminium and Magnesium Alloys

Abstract This study presents results concerning the stiffness and denting resistance of 6111-T4 aluminium alloy and AZ31B-H24 magnesium alloy. Experimental results of dynamic denting are compared with numerical simulations performed using LS-DYNA software. The experimental tests were performed on 0.9 and 1.09 mm thick plates clamped in a circular area with a diameter of 80 mm. Dynamic denting was accomplished by dropping different indenters from heights ranging from 0.36 to 1.7 m. The obtained experimental results enabled the properties of the two light alloys tested to be compared. The results of the numerical simulations display a good correlation with experiments if dynamic effects are introduced into the constitutive equation of the material through the Cowper-Symonds coefficients.

Numerical Simulation of Quasi-Static Compression Behavior of the Toe Cap Component for Safety Footwear

Geometrical changes can improve stiffness substantially. The project S3 - Safety Slim Shoe presents the potential to reduce the weight in safety toe cap components combining a new geometric redesign deeply associated to local stiffeners to realize the full potential of AHSS - Advanced High Strength Steels. The investigation aimed to examine the potential energy absorption capacity for a substantial thickness reduction of slim toe cap models. In this paper the normative quasi-static compression test in the context of the experimental validation of the two last and approved prototype models were focused. A non-linear FEA - Finite Element Analysis of the elasto-plastic deformation mode was performed, and several numerical parameters such as: hardening effects of extrapolated True-Stress-Strain material curves and simulation convergence conditions were carried out. Experimental results of the toe cap deformation behavior confront a weight saving range of over 40%, compared with the original steel toe cap. The safety footwear, in the context of the global evolution of footwear, has adopted creative and different orientations. Particularly, as an active element in the prevention of accidents fitted in PPE - Personal Protective Equipment, it is appropriate that its optimization solutions amplify the market targets, mainly combining ergonomic aspects, biological and mechanical features since its conception. The toe cap is the main component for its weight contribution, approx. 35% of total weight for each safety footwear model, and due normative context demands with high deformation resistance and impact loading (1). Currently the toe cap protection takes a subdivided position on design selection and materials, with a clear definition of two distinctive conception trends: metallic and non-metallic models. If on one hand the metallic models, especially high carbon steels with Heat Treatment processes, emphasize the security concept by the implicit mechanical strength performance, on the other hand, the weight of these solutions is a disadvantage compared to the metal-free solutions, and relegates the first one for outdated and heavier footwear concept. The non-metallic solutions with higher trend effects in the global market call on low density materials such as: reinforced Polyester composites with glass fiber, HDPE - High Strength Polyethylene, and several advanced developments in the optimization of energy

Process Development for Manufacturing of Cellular Structures with Controlled Geometry and Properties

This study presents experimental results on the behaviour of aluminium alloy metal structures and foams manufactured by lost-wax casting and using 3D printed components for internal structure definition. Results for tensile tests, metallurgical properties, surface quality and geometry tolerances were obtained and discussed. The analysis focused on development geometries, used for adjusting manufacturing parameters and prototype geometries intended for geometrical and mechanical validation. The results are indicative of the viability of the method for producing foam structures suitable for mechanical loading.

Study and characterization of the crest module design: A 3D finite element analysis

STATEMENT OF PROBLEM The importance of the crest module design in the cortical bone region triggered a need to understand its geometry and its influence on stress management and bone stimulation. PURPOSE The purpose of this study was to determine the effect of different crest module designs in the cortical bone region in terms of critical stress distribution and bone stimulation. MATERIAL AND METHODS Several 3-dimensional finite element analyses were conducted of a mandibular cross section with osseointegrated dental implants. For the numerical models, different crest module designs (cylindrical, divergent, convergent, and cup shaped) were analyzed. An average value of a maximum occlusal load of 250 N was applied to each dental implant design, 30 degrees from the top surface. The concentration and distribution patterns of principal and maximum shear stresses and strains were obtained and analyzed. RESULTS According to the comparative finite element analyses, the most pathologic stress and strain peaks around the implant collar in the cortical bone region were found in divergent crest modules with angles 14 degrees or larger. Nevertheless, the highest physiologic peaks of passive bone stimulation through compression, and the lowest tensile and shear stresses and strains in the cortical bone region were promoted by extended divergent crest module designs. CONCLUSIONS A slightly divergent and smooth crest module design extended to the cancellous bone increases the surface area available. This results in the dissipation of critical stresses expressed around the collar of the cortical bone region, not only promoting a higher bone-implant contact area and a physiologic bone stimulation but also boosting a healthy and strong bone-implant interface.