Innovative Applications
The Numerical Modelling and Optimisation Section at TWI has carried out many interesting and diverse projects for a range of industries, including medical, aerospace, power generation and microelectronics sectors.
Case studies:
Fatigue life analysis of a stent
Stents are inserted into the arteries around the heart to improve blood flow if the arteries have become damaged. Biocompatibles manufacture a stent from a solid tube of stainless steel (Fig.1).
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Fig.1. Photograph of stent (prior to expansion) |
The tube walls of the stent are cut into a lattice structure using a laser and the surfaces are then etched. Once inserted in the patient, the lattice is expanded by an internal balloon which is later removed. It had been suggested that the fatigue loading applied by the heart beating could result in the failure of a stent. Therefore, Biocompatibles requested that TWI carry out a finite element analysis of the stent to enable the expected fatigue life to be calculated.
The stent was modelled using beam elements, as shown in Fig.2. The analysis involved simulating a heart beat by applying radial pressure loading to all the elements in the model. The stent is under compressive loading in-vivo because it pushes the artery open. The blood pressure changes the compressive loading approximately 60 times a minute. Therefore the stent experiences an in-service loading of 1 Hz.
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Fig.2. Finite element model of stent |
The results of the analysis showed that the current design gave a predicted mean endurance of approximately an order of magnitude greater than the specified minimum target life. It was therefore concluded that the current design of stent was acceptable.
Electronic component reliability
Ball grid array (BGA) components are widely used in electronic assemblies. They enable a compact interconnection to be made using solder balls. Fig.3 shows a BGA component attached to a main circuit board. TWI has carried out finite element analyses to investigate the mechanical integrity of the connecting balls because of concerns over their durability. Fig.4 provides a close up view of the solder connecting ball, and also indicates the separate components modelled.
| Fig.3. BGA component | Fig.4. Model of BGA assembly and solder connecting ball |
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Case Study : Development of FSW tools
Friction stir welding was invented and patented by TWI in 1991. A tool with central probe is rotated between the components to be welded and traversed along the joint line forming a solid phase weld (Fig.5). Early in the development it was realised that the detailed form of the welding tool was critical for sound welds with good mechanical properties. Finite element analysis has been used to optimise tool design. Fig.6 shows a mesh of a TrifluteTM tool.
| Fig.5. Friction stir welding with a rotating tool | Fig.6. Photograph and mesh of Triflute TM tool |
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To find out more about how TWI could benefit your company, please contact us at nmo@twi.co.uk
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