​​​Review 2023 - FEMFAT

The transformation of the mobility industry has an deep impact on development methodologies and the way how we will use data over the entire life cycle of an product. How can utilize this new enabler technologies to create a smart and sustainable mobility for our future?

The ever-increasing demands on modern lightweight structures constantly call for novel technologies. One promising possibility is the integration of sensing and also actuating devices to allow the loading and damaging conditions of a structure to be recorded, analyzed, localized and predicted in a way that non-destructive testing becomes an integral part of the structure. Such a structural health monitoring (SHM) system with its ability to continuously provide information during operation helps to address weight-saving demands by reducing uncertainties with respect to the structure’s integrity. This can be exploited for, e.g., a less conservative structural design, a change from scheduled to condition-based maintenance or to elongate the operational lifetime of the structural components. A pre-requisite to use SHM is a certain damage tolerance of the considered structure, i.e. that damages grow stable for a certain amount of time such that they can be detected early enough before they become critical. Here, the current talk presents lab experiments in which progressive damage under fatigue loading is reproduced for both, metallic and composite materials. It is demonstrated that various SHM methods, such as electro-impedance tomography or ultrasonic wave analysis, are able to locate and to measure growing cracks, debondings and delaminations. However, the development of novel SHM methods, of required equipment and its installation is not cheap. Compared to aviation industry, in automotive industry the demands to a SHM system regarding localization and quantification might be lower as a workshop check is cheaper and easy to organize. Challenges are, however, shorter development times and high cost pressure on the vehicle’s marked price. As a consequence, robust SHM methods with high technology readiness level and with low sensor and instrumentation costs are sought. Here, e.g., strain-based SHM methods might meet the demands. Such solutions will also be discussed in the current talk.

Continuous advancements in the electrification of the whole automotive industry rely heavily on the use of lightweight materials. Due to the overall requirements for mass reduction that such rapid development imposes, lightweight materials such as aluminum play a major role in the optimization and development of powertrain components and vehicles while at the same time ensuring the structural integrity of the system.

Different joining and welding processes and technologies might be found nowadays within various automotive battery packs or module designs. Therefore, from the design release perspective it is vital to understand failure modes and effectively predict possible failures during the lifetime period of the product. 
A fatigue testing program was designed for two types of welded joints, butt and one-sided T-joint, considering two loading scenarios, tension and 3-point bending. 

The test specimens were produced out of the standard 6060-T66 extruded aluminum with the rectangular profile, 3 mm thick. The profile was cut and welded with the MIG welding process with the AlSi5 used as a filler material. Welding was done before the EDM cutting of the final specimen shapes in order to minimize the effect of the residual stresses. 
The high cycle fatigue (HCF) tests were conducting with the R=0.1 ratio, with 12 test samples tested to generate single S-N curve. With two types of specimens and joint (butt and T-joint) and two loading cases (tension and 3-point bending), the total of 48 specimens was tested in order to generate 4 S-N curves. 

Beside the HCF tests, hardness measurements and detail microscopy scans were conducted for both, butt and T-joints. These measurements served as a basis for a detail FE model creation, used for the extraction of the weld notch factors and calibration of the model with respect to the HCF test results. 

Based on the notch factor extraction FE results and the HCF test results, customized joints were added to the Femfat Weld database (for both discussed joints and two loading scenarios) and compared against the experimental results. 

With our traditional FEMFAT news and more you get informed about new features of FEMFAT 2023 and maybe some hints what more you can expect in the future.

Due to their lightweight potential, short fiber reinforced polymers are getting more important in structural applications especially in the automotive sector. To use this potential, the knowledge about material behavior for the lifetime estimation is important. Based on measured reference data, influence factors like temperature, fiber orientation, mean stress, etc. are considered with parameters in the lifetime calculations. So, this is a linear approach without considering time effects. Depending on the polymer and the operating temperature, viscoelasticity as a polymer specific behavior is getting important during operation. Depending on the load case (strain rate dependency), the material behavior differs. In application fields where, short fiber reinforced polymers are used like, mobility, energy, etc. different types of loads including fatigue, creep and even impact are applied to the components. Especially in automotive applications, constant load or rather non-load are the major part in the whole load spectra. Since there are dominant rest times during a service live, creep or/and relaxation effects needed to be investigated in more detail.

On the one hand the knowledge of this behavior is necessary to describe the stiffness and/or strength after a certain period of time at constant load, and on the other to find a static load limit (creep rapture). 
The investigated materials here are basically polypropylene (PP) and polyetherethercetone (PEEK) with different fiber contents. 

In these studies, a load sequence has been developed which delivers the impact of creep on the fatigue behavior. For this, cyclic loads were combined with static loads, block by block. So, starting with fatigue load for a defined number of cycles, constant load is attached followed by another fatigue block after a given time. This sequence is repeated until fracture occurs. To get a clearer picture of the creep influence, the static load level as well as share related to the cyclic load part was varied. 

Cyclic tests at a constant R-ratio and room temperature serve as a reference. 

The results show, that there is an impact on fatigue life of more than a decade caused by creep loads during fatigue. Depending on the constant stress level related to the cyclic load level, the constant load leads to further damage or even relaxation. So static loads can even increase the bearable number of cycles at a certain load level. This effect increases with a larger constant load share. Since during an average lifetime of an automobile constant loads are dominant and due to the insights of these studies, they needed to be considered in the lifetime estimation. 

The FEMFAT Rotating Principle Stresses Influence is a functionality which aims to take multiaxial effects into consideration. The influence factor is described in the FEMFAT documentation and papers such as [1] and [2]. In this study, the definitions in [2] have been used to derive analytical solutions to some special load cases. The analytical solutions have been used to analyze the influence of loading parameters such as phase, mean stress and wave form and how they interact. The background for this study has been to better understand the results of rotating principle stresses influence in FEMFAT. Any hypotheses for explaining the results are not included in the study, only the results are presented. The first example studied is based on an example from [2] and deals with two perpendicular normal loads. One of them is static and the other one is cyclic, with zero mean stress. The analytical solution can be used to calculate the static loading which results in the highest rotating principle stress influence. The second example is an extension of the first example and handles two cyclic loads, with phase shift and mean stress. It can be shown that the most critical phase may vary from 0° to 180°, depending on the mean stress and wave form. The third example is basically a reformulation of the second example, with one axial and one torsional load. In the solution it can be seen that all mean stress terms are squared, which results in an independence of mean stress sign. This may also be seen directly in the formulation in [2]. In other words, a bar subjected to a cyclic torsional load and a static axial mean stress, will result in the same rotating principle stress influence independent if the axial mean stress is in tension or compression. The analytical solution of the third example has been validated using FEMFAT. This has been made by applying cyclic loads to FE-models and evaluating them in FEMFAT. A brief comparison with literature [3] is also presented, but does not take aspects such as specimen geometry into consideration. Finally, the differentiability of the analytical solution is studied and some new alternative formulations of the influence factor are presented. To summarize, this study aims to increase the understanding of the effects of the rotating principle stresses influence factor. The equations for the analytical solutions will be published in the presentation at the FEMFAT user meeting 2023. MAN Energy Solutions in Copenhagen develops two-stroke engines for marine applications.
References:
[1] Fatigue Analysis of Multiaxially Loaded Components with the FE-Postprocessor FEMFAT-MAX, Christian Gaier and Helmut Dannbauer
[2] Investigations on a statistical measure of the non-proportionality of stresses, C. Gaier, A. Lukacs, K. Hofwimmer
[3] A Comparative Study of Multiaxial High-Cycle Fatigue Criteria for Metals, Papadopoulos et al. Int. J. Fatigue Vol. 19, No. 3, pp. 219-235, 1997

The paper presents a comparison of methods for the fatigue life evaluation of engine housings with respect to the internal engine loads. The motivation for the comparison is that, according to current calculation methodology, low safety factors occur around the connections of the engine to the frame. This means that the connection points are not designed to be operationally stable according to numerical calculation. The safety factors are based on a quasi-static calculation methodology and an analysis with FEMFAT TransMax, which uses individual stress results from an FE analysis. The loads result from an external multibody simulation. The experimental strength tests and the evaluation of endurance vehicles show that the connection areas can be regarded as operationally stable. To verify whether the deviations are due to the quasi-static calculation approach, a calculation methodology with a dynamic approach is developed. To this end, the model was expanded to include key components of the entire vehicle to represent the dynamic behavior of the engine under internal engine excitation.

The results of the new methodology, which are calculated with FEMFAT ChannelMax from modal input variables, show for the problem that more realistic safety factors are calculated with the same load signals. This can be attributed to the inertias of the masses, which lead to a changed force flow in the area of the connection points in the dynamic calculation. Based on the results of the dynamic calculation, it was investigated to consider the dynamic influence in the static non-linear analysis, which minimizes the duration of the calculation and allows the influence of non-linearities to be investigated.

Modern vehicles contain numerous subsystems for comfort, safety, digitalization, electrification and autonomous driving functions that are mounted to the body-in-white structure. These subsystems are submitted to stochastic vibrational loads induced by the road unevenness. The vibration fatigue behaviour of the subsystems and their connection to the body-in-white structure depend on a great number of influence parameters and associated uncertain scatter bands that are usually unknown and difficult to consider in the design process.

Even minor uncertainties in the characteristic properties of the mounted components can lead to significant changes in the oscillation behaviour. An inappropriate modification of the oscillation behaviour can result in a shift of the eigenmodes and natural frequencies towards a peak of the excitation and lead to resonance of the system. Consequently, the fatigue lifetime of the component or the connection to the body-in-white structure is massively reduced. In order to ensure the structural durability of the mounted components and body-in-white structure a numerical concept for consideration of uncertainties in the input parameters is created.

The methodology proposes an analyzation of the relevant natural frequency range of the system and critical hot spots with respect to the calculated damage. For the identification of the most significant influence parameters in relation to the target variables, sensitivity analyses are applied. The scattering of damage and fatigue lifetime is investigated by using a Monte-Carlo-Simulation. Finally, the reliability and robustness of the system is evaluated.

The proposed concept is illustrated by a current automotive subsystem mounted to thin plates which replicate the body-in-white structure. The implementation of the multiparameter variation is realised by the commercial optimization tool Optimus. Within the program a simulation tool chain is build up consisting of MCS Nastran for the conduction of a modal frequency response analysis and FemFat Spectral for the calculation of the damage and fatigue lifetime of the subsystem.

It was shown for the studied system that the damping, the mass and thickness of the mounted component, as well as the thicknesses of the plates have a major influence on the vibration behaviour of the system. Furthermore, a correlation between natural frequencies and calculated damage was derived. With decreasing natural frequency, the predicted damage to the system is increased. As a conclusion, it is recommended to consider a target value and limits for the natural frequencies in addition to the simulated fatigue life for mounted components submitted to vibrational loads.

A further aspect of this study is the application of super elements as a model order reduction technique. In order to save computing time and capacities the mounted components can be realised as super elements with a reduced number of relevant input parameters (mass, stiffness, damping) for implementation of the method in full-scale car simulations.

The simulation-based fatigue assessment of car body structures is an essential part of modern vehicle development processes and facilitates reaching the respective durability, lightweight and cost targets. This talk provides an overview of three selected simulation processes and how they complement each other.

The first one relies on inertia relief analyses and delivers the baseline fatigue results. The second one relies on a linear dynamic simulation and enables the identification of potentially critical areas due to vibrations. However, despite detailed and fine meshed FE models one observes that both approaches lead to overestimated damage results at certain areas, mainly at spot-welded connections. Previous investigations suggest this overestimation is due to the neglected contact stresses as both methods rely on a linear model. Therefore, the third simulation process relies on a mode based dynamic contact analysis to circumvent this limitation of the other two approaches.

Throughout this talk selected results from the fatigue assessment of a car body structure are presented to point out the respective characteristics of each simulation process.

In the “digital age”, leadership—in the meaning of the skill for “working on a system/business model”—is of paramount importance. Rapid technological change coming with digitalization enters all aspects of life, so businesses are forced to undertake extensive transformation processes. A strategic and pervasive positioning of information technology (IT) as business enabler and fundament of business models is of essence. Cultural business conflicts with the positioning of other operational units are unavoidable. Traditional methods of management are certainly being tested. The “digital society” needs executives with new competencies and businesses based upon fundamentally changed business cultures.

Address: Am Pöstlingberg 14, 4040 Linz, Austria

Different platforms were developed to meet market demands. As a result, different vehicle architectures emerged. This led to several design variations of vehicle sub-systems. One of them is exhaust systems. Among the exhaust system types, we will discuss vertical exhaust systems.

The vertical exhaust assembly consists of an Exhaust Gas Processor (EGP), which contributes 70% of the assembly's weight. This unit is placed above the frame on two vertical parallel rods. All-around welds are used to attach these rods to the EGP mounting bracket. The bracket is bolted to the long member. Welds on the EGP mounting bracket and vertical rods are stressed because of the cantilever of the whole structure.

Since the architecture of exhaust system is new, the design was analyzed using standard generic load cases as previous RLD data was not available. During proto-testing of this vertical exhaust failure was observed. In order to investigate this failure RLD data was captured. Using this data analysis was done and failure co-relation was established. In order to front load RLD based fatigue load case generic platform based PSD was developed using RLD data of various vehicles of same platform. FEMFAT Lab was used to convert the RLD data into PSD format.

FEMFAT Spectral is used to evaluate the exhaust system for PSD based analysis. The PSD analysis has resulted in a 70% reduction in analysis time compared to RLD based fatigue analysis. A Generic PSD provides an edge for digital evaluation against random vibrations for the same platform of vehicles during the pre-proto stage. Introduction of Weld capability in Spectral module gives advantage of evaluation of welded joints for random load vibrations.

The energy sector is a major contributor to CO2 emissions. Regulation requirements and the global pursuit to become independent from fossil fuels call for the use of so-called future fuels. Future fuels such as green hydrogen and its derivatives including synthetic natural gas (SNG), green ammonia, and green methanol are an important ingredient to achieve climate targets.

In this context, much public and research attention is focussed on automotive engines. The field of large bore engines, which are mainly used in energy and shipping applications, is less prominent but of equal, if not greater importance, as these applications are harder to decarbonise while contributing significantly to global emissions. Large internal combustion engines face some unique challenges compared to automotive engines. These include a much longer life cycle, higher reliability requirements, and a greater demand for fuel flexibility.

As each of the afore mentioned future fuels might have a different impact on material behaviour, combustion temperature, and transient pressure build-up, changes to many different parts and components of the engine might seem necessary at first glance. For an economically viable engine development process, all potentially affected parts and their respective materials need to be screened in terms of sensitivity to each fuel type.

After establishing which parts are affected, they are analysed using Finite Element Method (FEM) and Multibody Simulation (MBS). The subsequent assessment of structural durability is of great importance for a robust, reliable, and efficient design of future fuel engines. This contribution illustrates how we at MAN Energy Solutions SE achieved these goals regarding the assessment of structural durability. It is shown how we automatized and integrated the commercial software FEMFAT in our in-house software simWorkflow.

The program simWorkflow mainly generates a simplified traceable input/output interface to several programs. In addition, we give an outlook on how to integrate this program in a Simulation Data Management (SDM) environment in the near future. Lastly, some ideas on how to deal with material degrading effects such as hydrogen embrittlement within FEMFAT are presented.

RICARDO analysis methodology has been developed with the target to optimize  stiffness (very important parameter for the motorcycle dynamic behavior), stress distribution and fatigue resistance, minimizing the weight but reaching the expected life.

FEA analyses and fatigue analyses met respectively the road tests and fatigue tests. Both analyses were fundamental to introduce the appropriate improvements to the structure and therefore to achieve a safe and reliable development of the final product already in the calculation phase.

FEMFAT play an important role in the RICARDO’s process. This was proved in the last two important projects to predict the fatigue life of frames and other motorcycle components, this has been confirmed by the validation tests carried out.

An advanced multi-layer material model has been developed to simulate the complex behavior in case carburized gears where hardness dependent strength and elastic-plastic behavior is characterized.  Also, an advanced fatigue model has been calibrated to material fatigue tests and implemented in FEMFAT software for fatigue life prediction in differential gears.  An FEA model of a differential is setup to simulate the rolling contact and transient stresses occurring within the differential gears.  Fatigue life is predicted using the calculated stresses and the FEMFAT fatigue model.  A rig test is set up and used to measure the fatigue life of the differential over a range of load conditions.  Fatigue life predictions are shown to correlate very well with the measured fatigue life in the rig test.  Also fatigue life predictions are shown to correlate well with validation tests carried out on a full-scale axle.

Situation
Festo has been developing an in-house SPDM (simulation data and process management) system for fatigue analyses since 2015. The system is called SAFEM (SAFE + FEM) and is based on the Python web framework django and the Java Script front-end VUE.js.
So far, SAFEM uses Abaqus and Ansys software for FE calculations and FEMFAT for the fatigue analyses.
To save licensing costs, Festo has started an investigation to implement the open-source FE solver Code_Aster. In addition, the use of Code_Aster opens the possibility of calculating many parallel jobs, limited only by hardware resources.

Challenge
For the SAFEM workflow, FEMFAT must be able to read the Code_Aster results, and the FEMFAT results must be provided in the vtk file format (VTK is a file format for the open-source Paraview postprocessor).
There were two key challenges that must be met.
The first was to find a file format that Code_Aster can write and FEMFAT can read without main effort.
The other was to prepare the FEMFAT results so that they can be written to a vtk file along with the structural mesh information.

Solution
To solve the first challenge the SAFEM-Team tested several ASCII-based formats.
For reading the structure data a suitable format could be found which Code_Aster can write directly and FEMFAT can read directly. Unfortunately, the situation is different for the stress data. Here no format could be found which meets the requirements without additional processing. However, with slight modifications to the stress file written by Code_Aster it is possible to make it readable for FEMFAT. With this the basic feasibility to process Code_Aster results with FEMAT could be shown.
Because FEMFAT cannot write its results directly in VTK format, another way had to be found for the second challenge. The solution is to write the structural data as vtk file from the postprocessor and append the FEMFAT results to this file. For this the FEMFAT results must be converted to format that fits to vtk.
So far it is now possible to use Code_Aster with FEMFAT for some specific mechanical problems.

Next steps
The next steps are the automation of the workflow and the integration into SAFEM.

The approaches used to calculate the fatigue life of components must inevitably consider multiaxial stresses. Compared to proportional loading, the calculation of nonproportional loading is particularly challenging, especially since different materials exhibit the effects of nonproportional hardening and shifts in fatigue life.

Although there are calculation approaches that show good accuracy in predicting the fatigue lifetime, the expensive acquisition of necessary input data often prevents the application in broad engineering practice.

Therefore, the “classical” strength hypotheses

• maximum principal stress criterion according to Rankine
• maximum shear stress criterion according to Tresca
• distortion strain energy or octahedral shear stress criterion, respectively, according to von Mises

are used in such applications, even if the component to be verified is not proportionally loaded. It is known, however, that the service life estimated in this way has a low accuracy and, in many cases, results in unsafe predictions.

A method that are suitable for technical applications should ideally meet the following requirements:

• The method should be compatible with the widely used approaches for uniaxial and multiaxial proportional stresses
• The method should provide physically based sequences of equivalent stress, i.e. it should not produce jumps in the resulting stress sequence
• The method should be applicable on the basis of simple uniaxial properties.

One approach fulfilling these conditions is the concept of scaled normal stresses, suggested by Gaier and Dannbauer. The damaging influence of shear stresses is considered by scaling the normal stresses depending on the overall stress state and the ductility of the material. The ductility is represented using the ratio fτ/σ between the fatigue strength for shear stresses τW and for normal stresses σW.

In this contribution, the critical plane approach of scaled normal stresses, proposed by Gaier and Dannbauer, is utilized. A modification is proposed so it is possible to insert this critical plane approach into the algorithm of the well known FKM-Guideline. Correction factors accounting for nonproportional loading are investigated. Through appropriate parameterization of one of the studied corrections, proportional and nonproportional test results were observed to fall within one common scatter band.

The results of a comparative calculation with the algorithm of the FKM guideline and FEMFAT MAX applied on test results of fatigue experiments with multiaxial proportional and multiaxial non-proportional loading are presented.

In summary, this contribution presents a possibility of how an analytical fatigue life estimation for components subjected to multiaxial loading can be carried out in engineering practice, with only few required input data.

A live session with FEMFAT including tips and tricks for the usage and optimising the workflow

Address: Landstraße 31, 4020 Linz, Austria

Agenda:

09:00 - 09:45 | FEMFAT visualizer: model check - WELD definition - result verification (Dominik Hofmann)
 

09:45 - 10:00 | Coffee break
 

10:00 - 10:45 | Material generation and fatigue analysis of fiber reinforced plastics (Klaus Hofwimmer)
 

10:45 - 11:00 | Coffee break
 

11:00 - 11:45 | Strength assessment of non-welded and welded components according to the FKM guideline (Manuel Frank)
 

11:45 - 12:00 | Coffee break
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12:00 - 12:45 | Unlocking the Potential of Metal Plasticity in FEMFAT with new incremental Methods (Gerhard Spindelberger)

Address: Steyrer Straße 32, 4300 St. Valentin, Austria