Explicit FE Code Research Articles

Numerical modelling of composite structures under impact loads

This paper describes recent progress on the materials modelling and numerical simulation of the impact response of fibre reinforced composite structures. A continuum damage mechanics (CDM) model for fabric reinforced composites is developed as a framework within which both in-ply and delamination failure may be modelled during impact loads. Damage evolution equations are derived and appropriate materials parameters determined from experiment. The CDM model for in-plane failure has been implemented in a commercial explicit FE code, and new techniques are used to model the laminate as a stack of shell elements tied via contact interface conditions. This approach allows the interlaminar layers and strength reduction due to delamination to be correctly modelled. The code is applied to predict the response of a composite shell impacted at low velocity by a steel impactor, and comparison between predicted structural response and failure modes with observed test results is given.

Robustness optimization for vehicular crash simulations

Over the past 10 years (1990-2000), the computer analysis of vehicle crashworthiness has become a powerful and effective tool, reducing the cost and time to market of new vehicles that meet corporate and government crash safety requirements. Crash simulation is fundamentally computation-intensive, and it requires fast and powerful supercomputers to ensure reasonable turnaround time for the analyses. Because the explicit FE method is computationally efficient, researchers generally prefer it for crash simulation. Explicit FE codes are available from several third-party mechanical computer aided engineering (CAE) software vendors, and the major automotive manufacturers, including Ford Motor Company, use them for crash simulation. The article focuses on how this technology has been extended to perform design optimization and robustness assessment.

A constitutive relation and failure criterion for Ti6Al4V alloy at impact rates of strain

Shear stress-strain curves have been obtained for thin-walled tubular specimens of Ti6Al4V alloy at strain rates from ∼7×10 −4/s to ∼1000/s. A specially developed infrared radiometer has been used to measure the rise in specimen surface temperature during the high strain rate tests where the deformation process is essentially adiabatic. The data obtained were used to obtain material constants for a Zerilli–Armstrong type constitutive relation giving the best fit with the observed mechanical response. This relation was incorporated in the ABAQUS/ explicit FE code and shown to predict the experimentally measured flow stress in the impact tests with reasonable accuracy. A failure criterion, based on a critical value of effective plastic strain, is also incorporated, the critical condition being chosen to give agreement between the predicted and the measured overall failure strains. The validity of the constitutive relation and associated failure criterion is then assessed in terms of their ability to predict the observed mechanical response in tensile impact tests on specimens of both cylindrical and rectangular cross-section where the early onset of localised plastic flow and significant temperature rises in this region provide a more rigorous test of the proposed model. High-speed photography was used to monitor the changing specimen cross-section and the infrared radiometer was used to determine the specimen surface temperature in the region of localised deformation. Reasonable agreement was obtained between the experimental results and the numerical predictions using the ABAQUS/ explicit FE code.

Integral hydro-bulge forming of pressure vessel heads

A new process has been proposed to form pressure vessel heads by hydro-bulging circular flat plates as an alternative to traditional processes. This new process is especially intended for the manufacture of pressure vessel heads of very large diameter that may not be possible to form by traditional processes, and also for products of various sizes or very small production batches. A pressure vessel head usually includes two parts: a central elliptical plate and a circular skirt. The skirt may be formed by traditional processes (press forming), whilst the central elliptical plate may be formed by hydro-bulge forming, the vessel head being manufactured by welding these two parts together. Experiments have been carried out to investigate this process, and it has proven feasible to hydroform the central elliptical plates in pairs with a fixed boundary. The explicit non-linear FE code LS-DYNA3D is used to simulate two different process schemes: free hydro-bulge forming and hydro-bulge forming with a fixed boundary. The wrinkling occurring during free hydro-bulge forming is predicted and analyzed. The simulation of the model with a fixed boundary gives results in good agreement with experimental results, thus this process is proven feasible. The simulation also shows that a thickness-enhanced ring boundary can improve product quality.

The inside-outside contact search algorithm for finite element analysis

A new contact search algorithm (Inside–Outside Algorithm) for the sheet forming simulation has been developed and implemented in the dynamic explicit FE code: ‘DYNAMIC’. The inside–outside algorithm is derived based on the feature of the inside–outside status of a nodal ‘mesh normal vector’ in respect to a surface segment for the judgment of the contact of FE nodes with the tool surface. This new algorithm includes local search, local track and penetration calculation processes. Almost no additional CPU time is required for the local search process, because the calculations for both global and local search are combined. Moreover, the problems of conventional contact searching algorithms, such as iterations for local search and the deadzone problem, are resolved. Therefore, the quick, robust contact searching and accurate evaluation of penetration have been achieved. The numerical results show that the new contact searching algorithm is more cost effective and robust than conventional ones. © 1997 John Wiley & Sons, Ltd.

Simulation of the precision forging of bevel gears using implicit and explicit FE techniques

The implicit FE code MARC has been used to simulate the precision forging of a bevel gear. The true coupled thermomechanical forming conditions have been adopted to describe the metal flow during the process and the results obtained have been validated by extensive experimental work. An attempt has been made also to use a time-consuming explicit FE code, DYNA 3D; however, limitations were apparent in simulating the total procedure due to structural restrictions of this code. A CAD/CAM/CAE system able to successfully analyse such a precision forging operation is on hand.

Finite-element simulation of axisymmetric preforms in precision forging at elevated temperatures

The explicit FE code DYNA 3D was used to simulate precision forging at elevated temperatures (warm- and hot-working conditions) of two axisymmetric preforms, namely a conical shell and bevel gear preform. For simple and quick calculations the coupled thermomechanical process was simulated as an isothermal mechanical process for a particular temperature distribution of the steel preform. The metal flow, preform profiles, forging loads, stress and strain distributions in the workpiece and the tooling are well predicted by this technique. The upper-bound elemental technique (UBET), a kinematic admissible upper-bound approach, was also employed for quick engineering-approximation prediction of the forging loads and metal flow for the axisymmetric preform. The results obtained from the two simulating techniques are in good agreement for the forging loads, up to a particular degree of deformation, whilst the material flow is well predicted by the FEM but is only approximated when UBET is employed.