ANSYS Analysis Services

The following sections identify some of the areas of analysis expertise, the sections are specific in nature, but they are often intermixed, for instance transient and non-linear analysis. The following sections are intended to give an indication of available activities.

ANSYS also has an adaptive meshing capability for obtaining high quality meshes. The technique can be applied to most of it's element library, both structural and thermal. In addition to its structural and thermal capability, the program contains the Computational Fluid Dynamics program FLOTRAN, a magneto-statics capability and the highly non-linear analysis program DYNA-3D. These additional features are provided as an integral part of the overall program and are tightly bound to the thermal/structural program, either directly or through the use of the macro language.

The analysis can be carried out on beam, plate/shell, cable/struts, solids and contact elements.

In addition to structural analysis, thermal analysis falls into the "elastic" regime.

The results from the mode extraction, mode frequencies and mode shapes can then be used for further response analysis by the application of response spectra. Typically, force, displacement, velocity, acceleration and power spectral density spectra can be used. Mode frequency and spectral analysis is commonly used for seismic, wind and wave loading. The input spectra can also be the so called non-linear spectra to mimic non-linear response of the structure.

Frequency analysis will identify likely interactions between components and between attached vibration sources and the structure. Frequency analysis can therefore be used to tune or de-tune structures, especially when combined with design optimisation techniques.

Under most circumstances CREA would carry out a mode extraction analysis on models prior to their use in transient analysis. Knowledge of the response frequencies of a model helps to define the transient model, in particular, it will identify the required meshing densities.

A non-linear analysis can exploit varying integration time steps, which when used with predictive algorithms allows efficient analysis between non-linear events. With damping being an element property, variation in damping throughout a structure can be modelled, this allows modelling of vibration isolation features such as rubber shock absorbers.

As with mode frequency analysis there is a reduced analysis option which uses the Guyan Reduction technique to reduce the model to retained degrees of freedom and a constant integration time step. This is an efficient solution technique for structures which remain elastic, as is the case with most structures. Options in the solution module allow for gaps. As with the full transient scheme damping is an element property which can be varied at will. Using a reduced model, the solution times are relatively short.

Response spectra can be generated from all transient analyses allowing the modelling of attached structures and equipment by modal response techniques, in addition to transient analysis.

All of these non-linearities can be assessed under dynamic and static loading conditions as dictated by the product being analysed.

Bearing on deforming material

Contact is not always the best solution to the problem as it can be difficult to achieve convergence or the solution can become unnecessarily time consuming.

Solid modelling is used to model the effects of loads through the thickness of materials, where these effects are significant, and where the design rules allow. In many cases, to use through the thickness results effectively, it is necessary to work outside the limits of design codes, as these are more often than not based upon centre line analysis. It will be necessary to relate the solid model results to the code of practice by various means.

An example would be through the thickness shear stress. The design code may well give a limiting stress which is based upon the average shear through the material, i.e. Force/Area. The solid model will give a shear distribution through the material, therefore, it will be necessary to average the stresses for code checking AS WELL AS comparing the maximum Von Meses stress to yield allowables.

Solid modelling techniques will allow the investigation of surface effects, such as local yield in contact problems, indentation and local deformation. Solid modelling is also an ideal tool for casting design as it can model features such as: complex shapes, fillets, variation in thickness due to the requirement to release moulds and complex load paths. The results will show the stress distribution under all expected loading conditions, and will enable consideration of re- design to increase the usage of material. CREA have experience of casting design in the Offshore Oil and Gas industry.

Solid modelling also allows the analysis of non-isotropic materials, such as laminates (there are very effective laminate shell elements which should be considered first), soil, rock and concrete. With care, these different modelling techniques can be combined, and can also be combined with centre line models.

The ANSYS solid modeller is a fully featured Boolean modeller with the basic capability of many of the leading stand alone products.

In design optimisation, a target is set: this can be minimising cost or material, for cost optimisation, it is necessary to be able to define a relationship between the model and the cost of production. This relationship can be complex, as it will be calculated through the ANSYS parametric programming language. The function can include factors for manufacture complexity, material cost variations, for instance different grades of material, different testing, etc. The optimisation could be to minimise deflection, to avoid interactions, i.e. de-tune or tune a structure, minimise a certain stress, etc. A maximum as a target can be achieved by inverting the target.

The design is validated by design constraints, these are classified as design constraints, dimensions, materials, etc. and limiting states, stress, deflection, frequency, etc. Given the model and the constraints, the optimiser will then proceed to find "an optimum" design. The design is considered as "an optimum" since the program tends towards the first minimum point that it finds in the optimisation process. The optimisation region is defined by initial analyst defined design conditions and/or program randomly selected conditions. This is where analyst intervention can be essential if it is evident that there are better optimums than the one the program has found.

The design optimisation technique is also an ideal tool for "what if" analysis as it will automatically track given features of the model. A parametric model would be created which would then be run in the optimiser, totally under analyst control, but varying key design parameters. Analysis results can then be graphed against design parameters to study the influence of design change.

Design optimisation can be carried out using most of the analysis modules of ANSYS, either singly or in combination, however, notice has to be taken of the size of the model as design optimisation is necessarily iterative on the whole solution, including pre and post processing.

Sub Modelling

The sub modelling technique allows the refinement of an existing analysis model. For instance, in modelling a casting the results of a "coarse" model may well show a stress hot spot. Sub modelling allows a refined sub model of a part of the structure to be taken and re-analysed. Pre processors in the ANSYS suite will extract and interpolate displacement fields to generate the sub model. The program will also allow a solid model to be generated as a sub model of a shell model. The only constraint is that the solution should be elastic at the boundary, and that any non-linear response within the sub model does not influence the cut boundary.

Sub Structuring

The sub structuring technique involves the generation of so called "super elements", which are FE models of parts of the overall structure. This technology allows a model which is largely elastic, but which has a non-linear region to be solved more efficiently. The elastic regions would be generated as super elements, then attached to general elements for the non-linear region. The program then solves the elastic parts once, and only iterates on non-linear portion. Sub structuring can be used for all manner of mixed analysis, especially where run times are expected to be significant.