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PERMAS saves weight and improves endurance
2017-04-24 (Freeform Optimization)
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The freeform optimization is a method for shape optimization of Finite Element (FE) models, where the geometry of the model surface is modified while the element topology remains unchanged.

Freeform optimization is not a mathematical shape optimization method but it uses optimality criteria. They describe the relationship between a geometry change and its influence on a certain result quantity. Mainly, the relationship between a geometry change and equivalent stresses in the material is of great importance. This means that a high stress can be reduced by adding material (i.e. increase of part thickness) at the position of the high stress. The same holds for the inverse, i.e. a stress increases when part thickness is reduced.

Beside equivalent stresses all stress-like quantities like principal stresses, stress differences between different load cases, effective strains or safety factors can be used. Typical optimization solutions are stress homogenizations under weight conditions or a weight optimization under stress limits. The method is well suited for large models with many load cases and is mainly used to optimize the shape of models with free surface geometry like cast parts for housings of transmissions and engines. Applicable analysis methods are among others linear and non-linear static analysis including contact as well as frequency response analysis in dynamics.

The implementation of the method in PERMAS has now been extended by a combination with mathematical optimization methods in case of certain additional constraints, which are best represented by sensitivities. For example, this allows the limitation of displacements at bearings or an additional constraint on the compliance of the optimized structure. In dynamics, vibration amplitudes of selected nodes can also be restraint. Also, a third party software for endurance calculations can be invoked from within PERMAS in order to take its results as constraint or even objective function in a freeform optimization. By combining parametric and non-parametric optimization methods, many realistic requirements can be considered in one single optimization.

The set-up of an optimization model for freeform optimization is very easy and particularly tailored for PERMAS by its pre- and post-processor VisPER. A surface node set has to be selected only to sufficiently define the design space for freeform optimization. The related design elements are selected automatically. Stress and weight constraints as well as other constraints are very easily defined.

PERMAS freeform optimization targets to save weight and improve endurance in one single optimization. The time for development of structural parts can be significantly shortened while the structural behavior is improved by optimization.

The details of a freeform optimization is presented on the attached flyer. It is illustrated by an industrial example. More information on optimization can be found here.

PERMAS is making realistic simulations practical. PERMAS provides extremely fast and accurate solutions for realistic simulations of large models and complex situations in time. PERMAS supports better product designs through effective and rapid optimization of complex situations. PERMAS is an integrated FE analysis software. It combines thermo-mechanics, vibro-acoustics, and design optimization. For more information on PERMAS, a Short Description is available. More detailed information is available from the Product Description.

Topology Optimization in Dynamics
2017-03-29
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Comparison of frequency response of en engine bracket after an all static optimization (left)
and an optimization with both static and dynamic excitation (right).

Topology optimization is a standard method for shape finding under given boundary conditions and loads. Often, static load cases are used for topology optimization, because it is easy to apply and gives a good first impression of the best shape. For dynamically loaded structures, it is obvious that a topology optimization should include a dynamic analysis, too. To this end, the topology optimization method has to be extended for dynamic analysis. This extension has been developed in PERMAS to enable topology optimization to be used simultaneously for static and modal frequency response analysis.

The example of an engine bracket as shown in the picture to the left is used to show the effect of a concurrent use of static and dynamic loading in topology optimization. At the top of the picture, the design space and its boundary conditions are shown. So-called frozen regions are applied for the fixations of the part, which are not allowed to be modified during topology optimization. Release directions are also taken into account.

Below, the left column shows the optimized shape for a static load only, and the right column shows the optimized shape for a static load combined with a harmonic (sine or cosine) excitation. The weight of the optimized part is the same for both cases. The found designs are both fully converged results with a zero-one distribution of skipped (i.e. a filling ratio near zero) and remaining elements (i.e. a filling ratio near one).

The bottom of the picture compares the frequency response of the loading point for both cases. The shape found by static loading only shows a much higher amplitude when a harmonic excitation is applied compared to the other case, where the harmonic excitation has been taken into account during optimization. It can be concluded that a topology optimization under harmonic excitation urgently requires an optimizer, which can properly handle static and modal frequency response analysis simultaneously.

An animation of both optimizations over all iterations is shown here.

A paper is available on the engine bracket optimization, too.

Why you should use PERMAS for Topology Optimization in Dynamics? Although the need for a topology optimization with harmonic excitation is obvious, such a feature is not sufficiently supported in many other optimizers. The reason lies in the very nonlinear relation between small changes of structural stiffness and mass and the resulting change of frequency responses. This nonlinearity becomes very critical in case of plastics, because parts of this material show more eigenfrequencies in a typical frequency range than metal parts. Moreover, it is not sufficient to monitor the response just for one frequency, but a monitoring over the complete frequency range is required. PERMAS handles all these cases properly and provides the right means for topology optimization under harmonic excitations. PERMAS delivers a fully converged shape of the optimized parts, which is very close to the final design or can be directly used for production. By using PERMAS, dynamically loaded parts can be successfully optimized and high costs for testing and re-design of parts can be significantly reduced.

PERMAS is making realistic simulations practical. PERMAS provides extremely fast and accurate solutions for realistic simulations of large models and complex situations in time. PERMAS supports better product designs through effective and rapid optimization of complex situations. PERMAS is an integrated FE analysis software. It combines thermo-mechanics, vibro-acoustics, and design optimization. For more information on PERMAS, a Short Description is available here. More detailed information is available from the Product Description here.

New software PCGen to model fluid tanks for PERMAS
2017-02-01
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Tank modelling with PCGen

Fluid tanks are applied in a wide range from space flight in launchers and satellites to storage tanks in refineries and power plants. Such tanks are usually almost rotationally symmetric, i.e. they have a few deviations like longitudinal ribs or bolted flanges. It is also characteristic for these models that the structure mainly consists of shell elements. In addition, the fluid is also modeled to calculate the dynamic coupling between structure and fluid. For a highly efficient modeling of fluid tanks, PCGen (PERMAS Component Generator) has been developed as extension to VisPER (Visual PERMAS), which combines the modeling of geometry, structure, fluid, and meshing in one single tool.

The new apporach of PCGen can be described as follows:

  • A library of parametric geometrical models is available for all typical parts of a tank.
  • These parts are piled up from the bottom until the tank is completed. Subsequently the parameters are updated to the required values.
  • Thicknesses of the shell elements and possibly the filling level of the fluid are added.
  • Material properties of structure and fluid are defined.
  • Finally, the model is compatibly meshed after some settings for the mesh size, and it is stored for subsequent computations with PERMAS.
  • Completed!

In order to take the vibrations of the fluid surface into account, PCGen generates wave elements on the fluid surface. The frequencies of the surface waves are computed and shown by PCGen under the simplifying assumption of a rigid structure. After a coupled vibration analysis with PERMAS, these surface eigenfrequencies can be compared with the FEA results.

In the adjacent picture, some of the modeling phases are shown for a perfectly rotationally symmetric storage tank. The total time to model the fluid tank is about 6 minutes.

A film about the complete modeling of the tank is shown here. An additional flyer shows the most important details about PCGen.

A paper is available, which also shows details about bolted joints of fluid tanks.

PERMAS is making realistic simulations practical. PERMAS provides extremely fast and accurate solutions for realistic simulations of large models and complex situations in time. PERMAS supports better product designs through effective and rapid optimization of complex situations. PERMAS is an integrated FE analysis software. It combines thermo-mechanics, vibro-acoustics, and design optimization. For more information on PERMAS, a Short Description is available here. More detailed information is available from the Product Description here.

One-stop FSI analysis for liquid sloshing in an earthquake
2016-12-22
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Fluid-structure coupled modal time-history analysis for simulation
of storage tanks in an earthquake

Liquid storage tanks with critical content (like oil) in an earthquake have to withstand the structural loads and, in addition, the maximum filling level has to be restricted to avoid spill-over of the liquid. Many national standards provide corresponding requirements, e.g. in ASCE 4-98 or GB50191-93 for the nuclear field.

Because the large liquid mass implies high loading of the tank structure due to rapid movements in an earthquake, the simulation is best done using fluid-structure interaction (FSI) analysis, or more precisely, fluid-structure coupled (FSC) analysis. In this way, the stresses in the structure and the wave height at the free liquid surface can be determined in one single simulation.

To this end, beside the structural model PERMAS provides fluid elements and their coupling to the structure as well as special wave elements for the free liquid surface to determine wave heights. A time signal of horizontal excitation of the tank baseplate due to an earthquake is a possible load case. The simulation is then performed by a modal time-history analysis, where the coupled vibration modes are calculated first and used afterwards to compute the coupled response behavior of the tank.

The adjoining example shows at the top the tank model with the first vibration mode, where the displacements show the structural mode and pressure distribution shows the coupled fluid mode. Below, the acceleration signal of the earthquake is shown over a period of 20 seconds. The next row contains the surface wave at certain point in time on the left and two stress distributions of the tank walls at different points in time on the right. At the bottom, a comparison is given between the requirements of the mentioned standards and the simulation results. The requirements following the American and Chinese Standard for Seismic Analysis of Safety-Related Nuclear Structures are cited from Zhang Liqiang, Tang Qionghui, Fluid Structure Interaction Analysis of Liquid Sloshing Phenomenon in Storage Tank, 18th National Conference on Structural Mechanics of Reactors, October 2014, Chengdu.

For this tank, the fundamental eigenfrequency of the surface wave is met very well. For wave height and overturning moment, the simulation results are between the values of the two standards.

Now, which run time has to be spend to solve the coupled analysis task? Less than 10 minutes!

Animations over time for surface waves and structural stresses are available here.

PERMAS is making realistic simulations practical. PERMAS provides extremely fast and accurate solutions for realistic simulations of large models and complex situations in time. PERMAS supports better product designs through effective and rapid optimization of complex situations. PERMAS is an integrated FE analysis software. It combines thermo-mechanics, vibro-acoustics, and design optimization. For more information on PERMAS, a Short Description is available here. More detailed information is available from the Product Description here.

PERMAS uses production processes in optimizing sheet metal parts
2016-12-01
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Rolling, blanking, and beading to achieve the stiffest sheet metal
under given weight.

In the development of sheet metal parts various production processes like rolling (to get thickness variations), blanking (to cut out the final sheet), and beading (to generate beads for higher stiffness) are very important. In order to optimize sheet metal parts for weight and stiffness, these production processes have to be represented by suitable model modifications. The integration of this optimization in Finite Element (FE) analysis facilitates the direct identification of the necessary production steps.

To this end, the rolling is implemented by the optimization method called free sizing to achieve variable sheet thicknesses. This method works like topology optimization and modifies the thickness of shell elements in a wide range to get the desired part properties.

The blanking process is implemented by topology optimization, where an unambiguous selection of required elements is generated.

For the stiffening of the sheet metal part, bead generation by shape optimization is available, where the nodes of the shell mesh are moved normal to the sheet to get the stiffening effect.

By applying the Multi-Modal Optimization (MMO) approach in PERMAS, all these previously mentioned methods can be combined in one single optimization.

The adjoining example of a simple shell model under torsional loading shows the effect of the three combined optimization methods. The stiffest structure under a given weight and symmetry conditions is the corresponding objective. The results of the different production steps and the final result of the optimization are shown.

An animation of the shape changes over all iterations of the optimization is available here.

PERMAS is making realistic simulations practical. PERMAS provides extremely fast and accurate solutions for realistic simulations of large models and complex situations in time. PERMAS supports better product designs through effective and rapid optimization of complex situations. PERMAS is an integrated FE analysis software. It combines thermo-mechanics, vibro-acoustics, and design optimization. For more information on PERMAS, a Short Description is available here. More detailed information is available from the Product Description here.

PERMAS to explore buckling load factors and mode shapes
2016-10-27
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Three samples of rib height with different buckling mode shapes.

For many structures, buckling is an important design constraint and has to be checked by buckling analysis beside other types of analysis. Mainly beam- and shell-like structures are typically subject to buckling. To check the buckling performance a linear buckling analysis to calculate buckling load factors and buckling mode shapes is used.

Frequently, high load factors are desired and ribs are a successful means to increase load factors for shell structures. Besides, buckling mode shapes are also of importance, because mode shapes change with rib design. In order to make a rib design with highest load factor and a desired mode pattern, a design exploration is very useful.

The simple adjoining example shows the influence of the rib height not only on the buckling load factor but also on the mode shape. To explore this behavior, an automatic sampling process is performed, which scans a range of rib heights to get the related load factors and the heights when mode shapes change.

This sampling process is performed by PERMAS in an integrated manner, i.e. sampling is part of one solver run only and no other software is needed. The required pre-processing steps to define the sampled variable and its values is supported by VisPER, the graphical model editor of PERMAS.

A movie showing the history of changing rib heights is available here. An extended abstract is available from here.

PERMAS is making realistic simulations practical. PERMAS provides extremely fast and accurate solutions for realistic simulations of large models and complex situations in time. PERMAS supports better product designs through effective and rapid optimization of complex situations. PERMAS is an integrated FE analysis software. It combines thermo-mechanics, vibro-acoustics, and design optimization. For more information on PERMAS, a Short Description is available here. More detailed information is available from the Product Description here.

INTES released Multi-Modal Optimization in PERMAS
2016-09-09
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A multi-modal optimization of a cylindrical shell
with simultaneous topology and shape optimization.

For many years, topology optimization and shape optimization were used independently and successively to optimize structural parts or assemblies using Finite Element (FE) models. Now, INTES released PERMAS Version 16, where both types of optimization were unified to allow an simultaneous optimization of shape and topology.

This combination of different optimization methods is named 'Multi-Modal Optimization'.

On the attached image the multi-modal optimization of a cylindrical shell under a concentrated load is shown, where the shape of the shell and the element topology of required and neglectible elements is optimized by both a simultaneous use of topology and shape optimization. The stiffness of the structure is maximized under a given weight. The topology shows a converged result, where the great majority of the elements show a clear assignment to the required or to the neglectible element sets. The shape is modified due to a few shape basis vectors. The idea for this example is taken from the paper DOI 10.1007/s00158-013-0894-9.

The Multi-Modal Optimization opens a new class of optimization problems, which could not been solved so far.

Pre- and post-processing of optimization models is fully supported by VisPER, the PERMAS Graphical User Interface. The definition of design space, shape basis vectors, and all other optimization parameters is supported by using wizards as guided model description features, which make the optimization of topology and shape an easy task for the analyst.

A movie showing the iteration history during optimization is available here.

PERMAS is making realistic simulations practical. PERMAS provides extremely fast and accurate solutions for realistic simulations of large models and complex situations in time. PERMAS supports better product designs through effective and rapid optimization of complex situations. PERMAS is an integrated FE analysis software. It combines thermo-mechanics, vibro-acoustics, and design optimization. For more information on PERMAS, a Short Description is available here. More detailed information is available from the Product Description here.

PERMAS to calculate bolt loosening
2016-08-16
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Model set-up (top), contact pressure under bolt head and
in the thread (middle), and the bolt head rotation
due to bolt loosening (bottom)

In mechanical engineering, bolt connections are used very frequently for part connections. Beside other design conditions, the most frequently asked question is about the self-loosening of such bolt connections under operational loads. Can this self-loosening be calculated using FE analysis?

From an analysis point of view, it requires a nonlinear static analysis including frictional contact and a cyclic loading based on realistic operation of the structure. Friction needs to be applied in the thread and at the bolt head. Moreover, large rotations and an update of the contact geometry during rotation is also required. PERMAS integrates all these functions to facilitate this kind of analysis.

The paper “Experimental and numerical studies of bolted joints subjected to axial excitation” by J. Liu, H. Ouyang, J. Peng, C. Zhang, P. Zhou, L. Ma, M. Zhu, published 2016 in Wear, describes a experimental set-up to detect the pretension loss in case of axial loading of a bolt, which is called relaxation. In cooperation with the authors, we got their FEA model of the set-up to use it for calculation with PERMAS. Beside relaxation, the model could also be used to demonstrate self-loosening of the bolt due to shear forces on the bolt connection.

The appendant picture shows the model set-up at the top, the contact pressure in the thread and under the bolt head in the middle, and the rotation of the bolt head due to self-loosening at the bottom. Movies to these pictures are also available from here. An extended abstract in German is available from here.