Marcílio Alves

Inertia effects in axisymmetrically deformed cylindrical shells under axial impact

Abstract The axisymmetric buckling of elastic–plastic cylindrical shells subjected to axial impact are studied using a finite element analysis. This study reveals that shells, subjected to axial impact, are both velocity and mass sensitive, so that larger energies can be absorbed by a shell for high-velocity impacts when decreasing the striking mass. It is shown that the inertia characteristics of the shell, together with the material properties, determine particular patterns of the axial stress wave propagation, thus, causing either dynamic plastic or dynamic progressive buckling to develop during the initial phase of the shell response. Domains of the load parameters, where the different buckling phenomena develop, are obtained for two particular shells. Strain rate effects are also considered when discussing the energy absorbing properties of the shells.

On the elastic modulus degradation in continuum damage mechanics

Abstract To measure accurately the elastic modulus of a metal, E, can be a difficult task when a specimen undergoes plastic strains. Moreover, some failure criteria, such as those associated with Continuum Damage Mechanics, require the change of elastic modulus with strain to define a measure of damage, D, in a material or structure. Thus, it is important to assess the possible geometrical influence of a specimen on the measurement of the elastic modulus at different deformation levels. It is shown in this article, with the aid of a numerical simulation, that any plastic strains induce important geometrical effects in the evaluation of E, which have a significant influence on the evaluation of the scalar damage parameter, D.

Impact failure of beams using damage mechanics: Part I-Analytical model

Abstract A simple theoretical method, which is based on ductile damage mechanics and which retains strain rate effects, is presented for predicting the failure of beams made from a perfectly plastic material and subjected to impact loads. For this class of materials, the strains can be estimated by defining a hinge length. The definition adopted here leads to reasonable predictions for the plastic strains and the strain rate, as shown by comparing the results with numerical calculations and experimental data. The equivalent strain and the strain rate can be used in the damage model to predict the failure of beams, as shown in a companion paper (Alves, Jones, Int J Impact Eng 2002;27(8):863–90).

MEASUREMENT OF DUCTILE MATERIAL DAMAGE

The damage related to the voids present in ductile materials is an important parameter in some failure criteria. It is possible to express this damage in terms of changes in the elastic modulus, hardness, and electrical potential. This article reports on some damage measurements in a mild steel specimen subjected to traction using different experimental techniques. The results indicate different values for the damage on the same specimen, according to the technique employed, suggesting that the damage parameter used in a theoretical model must be carefully related to the appropriate constitutive law. *Communicated by T. Santos.

Performance of polymeric reinforcements in vehicle structures submitted to frontal impact

Preformed structural reinforcements have shown good performance in crash tests, where the great advantage is their weight. These reinforcements are designed with the aim of increasing the rigidity of regions with large deformations, thus stabilising sections of the vehicle that work as load path during impact. The objective of this work is to show the application of structural reinforcements made of polymeric material PA66 in the field of vehicle safety, through finite element simulations. Simulations of frontal impact at 50 km/h and in ODB (offset deformable barrier) at 57 km/h configurations (standards such as ECE R-94 and ECE R-12) were performed in the software LS-DYNA® and MADYMO®. The simulations showed that the use of polymeric reinforcements leads to a 70% reduction in A-pillar intrusion, a 65% reduction in the displacement of the steering column and a 59% reduction in the deformation in the region of the occupant legs and feet. The level of occupant injuries was analysed by MADYMO® software, and a reduction of 23.5% in the chest compression and 80% in the tibia compression were verified. According to the standard, such conditions lead to an improvement in the occupant safety in a vehicle collision event.

Dynamic Characterization of a Fiber-Metal Laminate

The mechanical strength of a fiber-metal laminate is not so well explored at high strain rates, although its constituents are prone to exhibit such effects. In this paper, we describe an investigation of aluminium-fiber glass material using the Split Hopkinson bar device. We report on various experimental issues related to these tests, giving some emphasis to the use of high speed filming to obtain information on the specimen strain and strain rate.

Performance of metallic defences submitted to vehicle impact

Abstract The main function of a road guardrail is to redirect an out of control vehicle avoiding a frontal collision or a dangerous veering off the road trajectory. Second, the energy absorption imposed by barrier deformation or any other energy dissipation method is beneficial to the car occupant safety. This study evaluates the performance of a W-Beam guardrail system, in accordance to Brazilian Standards, in respect to the kinetic energy absorption and to the vehicle return angle to the road. This evaluation of the guardrails is performed in a virtual environment using the finite element method. For the correct representation of the model, the material was characterised through experimental tests. Simplifications in the model are taken from the literature and a validated model of a vehicle is provided by the US National Crash Analysis Center.

Failure Site Of Axisymmetric Notched Specimens As Predicted ByContinuum Damage Mechanics

When a circular notched specimen is subjected to an axial force along its main length, stresss and strain develop around its minimum cross-section. This stress/strain field promote failure of the specimen, which can occur at any position along the minimum area. This paper addresses the problem of how the failure site can be predicted using a Damage Mechanics model, valid for a ductile material.

Design Issues of a Kolsky Tension Bar

In general, materials exhibit an increase of strength when loaded at high strain rates, which should be taken into account when dealing with structural impact. Kolsky developed an equipment operating based on elastic wave propagation capable of submitting a material sample to high strain rates. This paper presents some design features of such a tensile wave machine, including mechanical and electronic design issues, which may be helpful in a design phase.

NanoComposites Behavior under High Strain Rate Tests

To differentiate the effect of high strain rate on nanocomposites behavior, the Split Hopkinson Pressure Bar (SHPB) was employed. In addition to the SHPB tests, low velocity impact tests were also performed. The fiberglass used was in a plain weave configuration with density of 6 oz/yd, while the epoxy system was supplied by Hunstman Inc. (RemLam M+HY956). The nanoparticles used were from Southern Clay (Cloisite 30B) and Nanocor (Nanomer I30E), while the nanographite with surface modification was supplied by Nacional Grafite. The fiber volume fraction was kept constant and equal to 65%, while the nanoclays employed were 5 wt% and 10 wt%. The optimum condition selected for the nanographite was 3 wt%. The dynamic stress-strain curves showed significant nonlinearity and strain-rate sensitivity. It seems that nanoclay/nanographite content and its nano-structures (intercalated) can also influence the composite behavior. Stiffness seems to be directly proportional to the nanoclay content. An increase on strain rate can lead to a correspondent increase on stiffness. The main failure mechanism changed from fiber breakage to delamination with the dispersion of nanoclay and nanographite. The 3% nanographite condition performed even better than the optimum nanoclay condition (5%). The damage tolerance (capacity of sustain damage with a catastrophic failure) increased approximately 9% with a 5 wt% nanoclay dispersion and close to 42% with the 3 wt% nanographite addition.