Stress Analysis
Engineers use stress analysis to design structures to withstand their intended in-service loads. Loads can be static (e.g. dead weight), dynamic (e.g. from vibrating machinery), or thermal (e.g. from thermal cycles in a chemical plant). Finite element analysis may be applied to assess the effect of any these types of loading on the stress in a structure.
Case studies:
Design of a battery crate and box
Concept designs of a battery box and crate for use in railcars were jointly developed by TWI and Lordgate Engineering Ltd (the crate is held inside the box). The battery crate was designed in stainless steel sheet that contained welded and bolted joints. The box was designed in mild steel sheet of rolled sections and with bolted construction. Both box and crate were assessed for their structural integrity under specific loading conditions using finite element analysis. Assessments were carried out in accordance with recognised standards. Figs 1 and 2 show part of the battery box and corresponding finite element mesh.
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Fig.1. Part of battery box |
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Fig.2. Corresponding finite element mesh |
The finite element analyses predicted stress distributions in the proposed designs under all critical loading conditions, and allowed estimates of fatigue lives to be calculated. Initial models of the box and crate were modified and re-run until acceptable designs were generated. The finite element results indicate the overall stress distribution in the components, and also allow detailed examination to be made of more highly stressed regions ( Figs 3 & 4).
Fig.3. Overall maximum principal
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Fig.4 Detail of highly stressed
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FE for ECA input
TWI is frequently involved in carrying out Engineering Critical Assessments (ECA) in order to demonstrate that a construction is 'fit-for-purpose'. Such assessments are usually carried out according to BS 7910: 1999. Part of the essential data required for this type of analysis is the stresses acting on the structure arising from pressure, thermal, residual or any other type of loading. These stresses may be determined using finite element analysis.
When assessing the fitness for purpose of a flange in a pipeline, finite element analysis may be used to determine the stresses for input into the ECA. In the case shown in Fig.5, stresses in the flange arose from bolt loading, thermal loading, internal pressure and other mechanical loads acting on the component. Using the worst case stresses generated from the finite element model, it was possible to calculate the maximum tolerable flaw size in several locations in the flange.
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Fig.5. Stresses in flange component for use in ECA |
Calculating SCFs
Stress Concentration Factors (SCFs) may be calculated using finite element methods when trying to minimise the severity of stresses at a particular feature, e.g. when optimising a weld profile by burr grinding ( Fig.6). FEA may also be used when designing test specimens where a minimum stress is required e.g. for stress corrosion cracking, and a deliberate SCF is to be introduced.
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Fig.6. FEA being used to determine SCFs |
To find out more about how TWI could benefit your company, please contact us at fea@twi.co.uk
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