Computer Modelling in Safety Engineering

Engineering Applications
Rapid advances in computer technology over the past few years have opened up many possibilities for new applications of computer modelling in the engineering field. These include:

• Finite Element Analysis (FEA) - a numerical method using geometric models divided into meshes of many elements for modelling a wide range of physical phenomena in two and three dimensions.
• Boundary Element Modelling (BEM) - just the surfaces are meshed rather than the volume as in FEA.
• Dynamic Simulation - combining Computer Aided Drafting (CAD) and dynamic simulation programs for modelling the motion of individual parts, assemblies and the interaction of parts on a step time basis.

Typical applications include modelling mechanical failures and the behaviour of structures in three dimensions, analysing crack growth, and predicting stresses and strains of components and structures.

The Health and Safety Laboratory (HSL) has been keeping pace with these advances and has expertise in all these areas. Using state of the art computer hardware and software, we have successfully applied these techniques in the investigation of many safety-related problems.

HSL is well placed to apply theoretical models, either alone or with experimental validation using our comprehensive facilities, to meet a wide variety of customer needs.

We are committed to setting and maintaining the highest standards of service to our customers to meet their requirements on quality, value for money and timeliness. We operate a certified ISO 9001:2000 quality management system covering all of our scientific activities and have a proven track record of successful project management.

Finite Element Analysis
HSL's mechanical engineers and material scientists use Finite Element Analysis (FEA) to predict the deflection, strains and stresses of components and structures in both two and three dimensions. For example, we applied FEA in a project looking at the fracture toughness of different steels used to manufacture lifting chains. Experimental and theoretical results were combined to develop performance criteria for the chains. The results are being used to recommend fracture toughness values for inclusion in European and other Standards as well as in the investigation of industrial accidents.

FEA has also been used to investigate the integrity of gas cylinders which have had the manufacturers' identification markings ground offillicitly with an angle grinder. There was concern that the strength of the cylinders may be reduced. A range of FEA models were created for different types and degrees of grinding and the models validated against hydraulic pressure tests. It was found that certain types of grinding can introduce significant stress concentrations. These may initiate crack growth during cyclic loading of the cylinder as it is charged and emptied during normal use.

We also have a dynamic FEA capability which can be used for example to model impact events.

Boundary Element Modelling
Boundary element modelling (BEM), where just the surfaces are meshed rather than the volume as in FEA, can be used to obtain results quickly. The method is particularly suited to the analysis of cracked components as crack growth can be modelled automatically without time consuming re-meshing. Both the crack direction and the stress intensity at the crack tip are calculated automatically. HSL has used BEM in a number of incident investigations, for example a lorry loader crane failure and a reformer tube explosion. It has been also been used in a research project evaluating published stress intensity factor solutions for pressure vessels.

Dynamic Simulation
HSL's mechanical engineering specialists use computer-based techniques to model mechanical failures and the behaviour of structures in three dimensions. Using a Computer Aided Drafting (CAD) package coupled with a dynamic simulation program the motion of individual parts, assemblies and the interaction of parts are calculated on a time step basis. Physical quantities such as displacements, velocities, accelerations, forces, torques etc. can be analysed. The results are not merely computer animations but are based on calculations using Newton's laws of motion.

We have applied these methods in a number of incident investigations for HSE. Rendered 3D bitmap images, fly-round sequences and dynamic simulations have proved extremely useful in court cases to explain the engineering and technical aspects of incidents. For example, results from dynamic simulations were combined with on-site video footage to produce a video that helped explain the sequence of events and possible causes of a fatal incident at the Avonmouth Bridge. Four workmen fell 30 metres to their deaths when the maintenance platform they were working on became detached from the bridge. The HSL investigation team examined all the components and looked in detail at the construction and method of restraint of the platform. A three dimensional, computer simulation was used to model the movements of the platform under the prevailing weather conditions.

HSL's expertise in computer modelling covers a wide variety of engineering applications including:
• Evaluating the performance of an existing component or assembly, e.g. modelling crack growth.
• Assessing the remaining life of a component or assembly..
• Determining the causes of failure of a component or assembly.
• Determining the fracture toughness of steels.
• Static and dynamic analysis of components and assemblies, e.g. stability problems, fairground rides.
• Assessing driver visibility from vehicles.
• Validating computer models by traditional hand calculations and practical experimentation.
• Validating and assisting in the development of design guidance for industry.

Further Details
Contact our Business Development Group by telephone (+44) 01298 218218, fax (+44) 01298 218822 or email hslinfo@hsl.gov.uk.

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