Finite element analysis (FEA) is a reliable method of predicting whether a proposed design will be able to meet set design specifications, prior to manufacture. FE analysis indicates whether a product will work, where and how it will wear out, and whether it is likely to perform to your expectations.
Having supplied precision elastomeric rubber components for defence and aerospace industries since the 1930’s, at Martin’s Rubber, we like to think that we know a thing or two about polymer applications. To enable us to grow with the demands and developments of materials technology, we have recently invested in Nonlinear Finite Element Analysis software, which combines our 150 years of ‘know how’ with the latest in polymer FEA methodologies.
Why use nonlinear finite element analysis?
Nonlinear FEA delivers cost saving through analysis prior to production, time saving through analysis without the need for in-situ testing and analysis of product failures to develop and improve designs.
Types of FEA analysis
There are generally two types of FE analysis that are used for FEA elastomer analysis: 2D modelling and 3D modelling. 2D modelling, such as axisymmetric analysis, is typically used in seal section analysis as increased mesh density can be afforded without the need to simulate the entire revolved section. However, 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.
What is linear and nonlinear FEA analysis?
Analysis can then be further subdivided into problems that require either linear or nonlinear methodologies to be employed. Linear FE analysis methodologies commonly included in 3D CAD packages are far less complex and are not sufficient for polymer FEA analysis as they naturally try to linearise the material behaviour. Nonlinear Finite Element Analysis methodologies are often carried out on polymers. This is because polymers are inherently nonlinear and require specialised material models to allow for their nonlinear stress-strain relationship and time/temperature dependent material properties. We have the expertise to cover both Static & Dynamic scenarios, geometric nonlinear scenarios for large deflections or strain, material nonlinear scenarios such as creep, plasticity, viscoelasticity, hyperelasticity etc. and contact nonlinear scenarios such as friction.
Simulating the behaviour of the unique material response of rubber is unlike standard linear FE analysis on materials such as steels, which have well-defined linear responses to strain. On the other hand, 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, such as polymer FEA, required to feed the analysis models with fully representative data. One typical material model is the Ogden strain energy function:
The Ogden model will allow for the nonlinear material response, leading to far more accurate rubber FEA and analysis results. As a word of caution, using material data from online sources is ill-advised as it is typically a best case scenario rather than a true representation of the material real world response to stress-strain. This is particularly true when temperature is applied; there really is no alternative to testing the polymer material under conditions that closely replicate the application conditions, and fully characterise its specific responses.
Use of nodes within Nonlinear Rubber FEA
Linear FE analysis and Nonlinear Finite Element Analysis use a system of points known as nodes, which make a grid called a mesh. 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.
Points of interest may consist of:
- Fracture point of previously tested material
- 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
Save costs with polymer FEA from Martin’s Rubber
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 maybe 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, at Martin’s Rubber, we are able to advise whether a proposed design will function to your design specifications prior to manufacture or construction 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 product failure to help determine what design modifications are necessary to ensure the product will function as required on future variants.
If you have a new design to verify, or an existing application with a problem, contact our technical team to see what we can do for you.