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FINITE ELEMENT ANALYSIS (FEA)

Helps us to understand whether a product will work to everyone’s expectations.

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Types of FEA

If the cost of prototyping a part through manufacture is prohibitive, or you are on a tight timescale but looking at a lengthy test period, FEA will deliver time saving and the reduction of test cycles and therefore cost. This can be done through simulated analysis of product failure. FEA enables us to help you to develop and improve designs prior to cutting a tool. It can also be used to retrospectively analyse potential causes of a product failure.

Types of FEA

  • 2D modelling: this is typically used in seal section analysis as increased mesh density can be afforded without the need to simulate the entire revolved section.
  • 3D modelling: if the loading conditions are likely to be non-symmetrical, for example if shaft eccentricity is an issue, then 3D modelling must be employed to ensure the validity of the results.

Linear vs. non-linear FEA

  • Linear: this type of analysis method is commonly included in 3D CAD packages. It is generally used for materials such as steel, which has a well-defined linear response to strain and therefore requires a far less complex analysis. This type of analysis is not sufficient for polymers as it will naturally try to linearise the material’s behaviour. This usually means that a failure analysis of polymers will be needed.
  • Non-linear: polymers are inherently nonlinear and require specialised material models to allow for their nonlinear stress-strain relationship and time/temperature-dependent material properties. This methodology allows us to cover both static and dynamic scenarios, geometric non-linear scenarios for large deflections or strain, material non-linear scenarios such as creep, plasticity, viscoelasticity, hyperelasticity etc., and non-linear contact scenarios such as friction.
Rubber product analysis

How does FEA work?

Using a linear material model on polymer data will typically over or under-predict the response of the material.

Therefore, highly specialised material models are needed to allow for a nonlinear stress-strain relationship and temperature-dependent material properties, with specialist material testing required to feed the analysis models with fully representative data. One typical material model is the Ogden strain energy function:

Ogden’s hyperelastic material model for Nonlinear Fea

The Ogden model will allow for the non-linear material response, leading to far more accurate analysis results. Our investigations of rubber and polymer failures will hopefully eventually lead to total elimination of failure.

This means that in order to work with a true representation of the material real-world response to stress-strain, particularly when temperature is applied, we need to test the polymer material under conditions that closely replicate the application conditions, and fully characterise its specific responses, before using those outputs within the modelling software.

Use of nodes within Non-linear Rubber FEA

The analysis software uses a system of points known as nodes, which make a grid called a mesh.

We also implement a rubber moulding prediction system. This mesh contains the material and structural properties which define how the structure will react to certain loading conditions and must be borne out of accurate and representative material testing. Nodes are assigned at a certain mesh density throughout the material depending on the anticipated stress levels of a particular area. Regions which are estimated to receive large amounts of stress usually have a higher node density than those which experience little or no stress. FE analysis packages can automatically mesh entire 3D bodies. However, they are not clever enough (yet) to make a judgement as to where increased node density is required; it is not simply a case of uploading a model and applying the material and loading conditions, which is where the skill and knowledge of the FE analyst is required.

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Points of interest may consist of:

  • Fracture point of previously tested material
  • Fillets
  • Corners
  • Complex detail
  • High stress areas.

A wide range of functions are available such as:

  • Mass, volume, temperature
  • Strain energy, stress strain
  • Force, displacement, velocity, acceleration.

Multiple loading conditions which may be applied to a system to allow accurate replication of intended application such as:

  • Point, pressure, thermal, gravity, and centrifugal static loads
  • Thermal loads from solution of heat transfer analysis
  • Enforced displacements.
Finite Element Analysis (FEA)

Interpretation of results: investigation of rubber and polymer product failures

Results interpretation in polymer FEA is where the skill of the analyst and knowledge of polymers really comes into play; it is not simply a case of reviewing the analysis results against the tested material properties, because whilst the results will provide the calculated stress, deformation etc., it cannot account for the specific requirements of polymer design and manufacture. For example, whilst the predicted stress result may be within the material’s ultimate tensile strength (UTS), it might be that repetitive loading to a near UTS value will damage the polymer’s material structure, leading to degradation of the material and ultimate failure of the part.

Dynamic simulation of the anticipated results then gives a real insight into the likely articulation of a product. Using our polymer FEA software, we are able to advise whether a proposed design will function to your design specifications prior to manufacture of a mould tool; this has potential for significant cost saving on new development projects. We can also use the rubber FEA software in the case of previous product failures to help determine what design modifications are necessary to ensure the product will function as required on future variants.

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