​​​Review 2021 - FEMFAT

In the development of automotive parts, it is costly and time-consuming to manufacture prototypes and conduct tests, and it takes a great deal of time to evaluate fatigue durability in particular. Therefore, there is a growing need to predict the fatigue life as a preliminary study, and design using fatigue analysis tools is considered important.
In this case study, a multi-objective optimization of the stiffness and mass of an automobile lower arm was performed by coupling the fatigue analysis software FEMFAT with modeFRONTIER, and the results were discussed.

Chery has generated an internal program based on the 2nd development of Altair HyperMesh Interface and FEMFAT solver. Using it an entire process of car body quasi-static fatigue analysis can be achieved, including modal analysis, integration of unit stress calculations, weld definition and model processing, integration of spot definition tool, generation of max files and ffj files, submission of calculations and the generation of reports. The whole process can make the fatigue analysis more clear, standardized, to solve the problems of artificial irregularities and error-prone in use, and ensure the consistency of analysis results.

The present study deals with the life correlation of axle housings of Heavy Commercial Vehicles for different loading patterns like vehicle level road loads and in-door bench testing loads with experimental results. Investigation has been carried out to understand the underlying cause for failure before acceptance in bench testing and improvements in life by enhancing the surface finish of housing plates has been proposed.
Design of the axle housing depends upon the vehicle usage pattern and its payload. Therefore, to have the realistic life estimates, the road roughness was captured by measuring the micro strain of the axle housing at vehicle level. Life calculation of housings for transient forces is carried out using FEMFAT. Similar life estimates for the equivalent damage using In-door bench testing loadings has been carried out. Both the design life estimates doesn’t reveal any failure before acceptance, which is contrary to the experimental testing’s results for bench loadings as housing developed crack before acceptance. Failure investigation reveals surface roughness levels increase from design intended 10 μm (max) to 22 μm at press forming bending radius zones created micro notches leading to initiation of cracks. In addition, the tensile strength of the housing plate also lowered from 610 to 500 MPa due to hot working. Life re-estimates carried out by modifying the SN curve to account for the losses reveals a close correlation between different loading patterns captured at vehicle level and bench loadings with experimental results. Life calculation iterations has been carried out by varying the surface finish factor ‘fsr’ but maintaining the tensile losses due to hot working in the component to be constant. It is observed that surface finish enhancement up to 3 μm will make the axle housing plate to meet acceptance life even though the tensile values of plates has been lowered to 500MPa from 610MPa by hot forming process. Enhanced surface finish can be achieved by using abrasive or pressurized sand blasting of housing plates at the required zones. Alternatively, if plate tensile strength is maintained up to 620 MPa after forming, then the surface finish can be lowered as low as 78 μm and still life meets acceptance.
The case study helps the manufacturer to achieve a trade-off between surface finish and the tensile strength as higher surface finish requirement increases the cost of manufacturing. The correlation between different loadings patterns and life enhancement by surface finish helps the designer to improve the fatigue life without undergoing for major design modifications, which adds cost and weight penalty to the component.

Durability of passenger cars mainly depend on the integration of BIW structure with its sub-components against the road load excitation. To ensure this integration, dynamic response of sub-components need to be accounted during design/validation. In general, the road load excitations generated from Proving Grounds are random cyclic loads in time domain because of different road profiles which the passenger car undergoes. To simulate the dynamic response of sub-components in time domain is a biggest challenge in CAE as it requires a lot of solving time and memory usage. FEMFAT Harmonic is one of the alternative approaches to capture dynamic responses in Frequency domain with good accuracy, less solving time and memory usage. In this study, Frequency response analysis is carried out to determine the natural modes of each sub-component and FEMFAT Harmonic is used to convert the time based road loads into modal based response loads. These modal response loads are then mapped with modal stresses and the durability cycle of each sub-component mounting is calculated using FEMFAT Channelmax. As an Output, for a full vehicle CAE simulation accuracy improved drastically with ~80% reduction in solving time and memory usage using FEMFAT Harmonic.

In the automotive industry the regulatory and customer requirements are increasingly becoming stringent for all design parameters like emissions, crash regulations, customer comfort, weight and cost. It has becoming imperative to be low on vehicle weight and first time right. This has led to numerous changes in the way simulations use to be executed.

The current study focuses on development of an innovative and robust approach for simulations called as integrated durability simulations. In this approach shift from manual traditional static loadcases to measured wheel forces is demonstrated along with improvement in robustness to ensure low weight and first-time right simulations by leveraging measured wheel forces on customer roads, proving grounds and automation scripts in FEMFAT.

Using this approach, the complete process from load synthesis, fatigue analysis, design improvement deck creation to report generation is automated. Good correlation for dynamic strains and fatigue failure locations is demonstrated with this new real world and robust approach.  Huge benefit in turnaround time is demonstrated using integrated durability simulation approach which will be highlighted.

Customer and legislation in the transportation sector require the development of dis-ruptive commercial vehicle concepts. Especially E-Mobility drivetrains demand new layouts to include the batteries and the electric drivetrain. On top of that, a high number of variants are needed to fulfill the specific customer demands, making the development even more complex. Simulations at an early stage in the development process help to cope with the increasing complexity.
A virtual full vehicle test on the digitized test tracks represents the final prototype test-ing on the proving ground for the durability approval in the simulation environment. Therefore, a multibody simulation model of the vehicle, used for driving dynamics, with models of the front and rear axle including the steering and powertrain is equipped with an adequate tire model and a flexible frame structure. The flexible frame structure is derived from the full finite element model used for structural strength analysis. Since the full finite element model has several million degrees of freedom, a reduced order modal representation of the frame structure is used to approximate the full model be-havior. In the frequency range of interest a reduced order model with only a couple of hundreds degrees of freedom was found to be sufficient. This reduced order model is then integrated in the multibody simulation to allow the flexible deformation of the frame structure. The process of the virtual full vehicle test is shown in Fig. 1.
The flexible frame structure deforms due to the loads when riding over the digital test tracks. The local stress and strain histories can be derived for the flexible body structure when the modal stress and strain tensors are multiplied with their respective modal par-ticipation factors. In a subsequent durability analysis, the multiaxial stress history, to-gether with the local material and load properties, are evaluated to calculate the lifetime of the structure. In this project, FEFMAT is used for the durability analysis of the bus frame structure.
The weld seams of the frame structure are thereby of special interest due to the lower dynamic loading capacity. Since the frame structure has several thousand weld seams, an efficient process for the detection and the definition of the weld seams is necessary. The detection and definition (e.g. square, tee) of the weld seams is based on the geom-etry and topology information of the finite element model. In a next step the weld seam definitions are checked using the graphical user interface.
The frame structure of the bus is predominantly modeled with shell elements. For the evaluation of the durability of the weld joints the results from a structural stress and a notch stress based approach are implemented. A comparison of the approaches with Wöhler experiments of specimens and a comparison of virtual and real world proving ground tests of the full vehicle are shown.
In this talk, the simulation process for virtual rough road testing at MAN Truck & Bus is presented with special emphasis on durability analysis of the weld junctions. Further-more, improvements of the process and the resulting savings in computation time and disk space are outlined.

Automotive structures are becoming lighter in view of the stringent emission control regulations. As thinner sheet metal panels are used in order to reduce weight of body structures, number of spot welded joints and their durability has become important than ever before. Hence, it has now become imperative to validate strength and optimize number of spot welds during early stages of product development through simulation.
Though number of simulation techniques, for evaluation of fatigue life of spot welded joints, are already available, each technique has its own advantages and limitations. The techniques available can be broadly categorized into; 1. Force based assessment method and 2. Stress based assessment method. Force based assessment method does not require special modelling to represent spot weld joint and hence can be used for quick identification of critical joints from more than 5000 spot welded joints in an automotive Body in White (BIW). The accuracy of the predicted fatigue life however depends on the quality of the elements representing spot welded joint. Stress based assessment technique requires special kind of modelling for spot welded joints but is more robust and demonstrates correlation with test results.
“SPOT STRESS” method for assessment of fatigue life of spot welded joints in FEMFAT has been in use ever since its inception. From FEMFAT 5.4 version onwards, additional technique called “SPOT STRESS EXTENDED” is introduced. Comparison of newly introduced technique with its predecessor is discussed in this paper in terms of the correlation achieved with test results. Correlation with test results is studied using ‘coach- peel” and ”tensile shear” specimen in order to capture mixed loading patterns.

Passenger car closures experiences extreme slam loads repeatedly depending upon customer usage pattern. It is important to ensure the closure strength and durability against severe slam loads. Closure Slam test predicts the durability of automotive closures subject to a repetitive slamming load. In BIW and closure, Latch & Stricker mount, door inner panel cutouts and check arm mounting regions are the critical locations in durability point of view.
Excessive loads are generated at check arm mounting region during abuse conditions when opening and closing of door. Repeating the abuse conditions will lead to durability concerns in these mounting regions. To provide an accurate estimation of fatigue life of automotive door check arm mounting location, it is crucial to understand the loading conditions associated with opening and closing effort of door. A duty cycle is derived by applying loads at door handle location which represents the actual loading pattern applied on check arm mounts. Duty cycle contains the door closing and opening effort in a specific pattern which represents the door opening and closing condition.
In this paper, durability analysis of closure check arm mounting regions using FEMFAT ChannelMax is presented. It is observed that, in slam test and CAE analysis, pass-pass and fail-fail correlation is achieved by using duty cycle approach for closing and opening efforts at door handle location. New method is deployed for durability evaluation of the check arm mounting regions as it is predicting the physical behavior accurately.

The powertrain of an electric vehicle comprises of many auxiliary units which needs to be accommodated in the vehicle. Most of the OEM’s want to keep the platform of their vehicle same and keep both ICE and electric vehicle options. Hence, the EV powertrain package space is completely defined by the body developed by the ICE. Apart from the electric motor and gearbox, there are other heavy parts such as DC-DC converter, on board charger, power distribution unit, etc. that are a part of the EV powertrain need to be rigidly supported inside the available space. A fabricated structure (cradle) needs to be developed within the available space to accommodate the EV powertrain along with its auxiliary parts. This entire system with lumped masses on the cradle will have a complex dynamic behaviour. The modelling of this behaviour and dynamic assessment of the same is very crucial.
To develop this entire system digitally, a CAE model calibration activity was done to establish the co-relation. A 4 poster test was done to capture signals of the proving grounds as per our standard duty cycles on the mule vehicle. Since this is a dynamically important problem, modal transient analysis followed by FEMFAT assessment for welds and parent material is used.
The fatigue results from FEMFAT co-related well with the 4 poster vehicle. With the help of this assessment, improvements were done and a robust cradle structure was developed. Post that, the prototypes were built and physical trails showed no failures. The methodology used and the co-relation accuracy of FEMFAT with 4poster vehicle results is show cased in this paper.

In order to improve the endurance performance of the pickup truck, the fatigue life analysis of the truck frame was carried out by the virtual iteration technology. The load iteration and output were completed by FEMFAT_LAB software. The fatigue life analysis was completed by the nominal stress method in FEMFAT software, and the simulation results were benchmarked with the vehicle road durability test results to form a closed cycle of simulation analysis.

The fatigue life is one of the important criterion in assessing design reliability under the influence of cyclic loads. The majority of catastrophic failures in fabricated structure manufactured using welding as a joining process, are due to variability and uncertainties in the fatigue life assessment. Welding is one of the most convenient & extensively used manufacturing process across every industry. During welding process, residual stresses are generated due to non-volumetric changes in heating & cooling cycle. These residual stresses have a significant impact on fatigue life of component leading to poor quality joints. To alleviate the effects, designers and process engineers rely upon their experience and thumb rules but has its own limitations. This approach often leads to conservative designs & advances the pre-mature failures. Hence there is a need to consider the detrimental effects of welding during fatigue life assessment for robust product development. Recent advances in computational simulation techniques provide us opportunity to explore this complex phenomenon & generate deep insights towards fatigue life assessment.

The study proposes a numerical framework to assess fatigue life of welded joints by integrating influence of thermo-mechanical stresses with field stresses. The framework is divided in two stages namely Welding simulation and Fatigue life estimation using FEMFAT.  At first, the numerical assessment of residual stresses was accomplished and the results were validated with experimental data from X-ray diffraction testing.  Later on, these residual stresses were super-imposed over non-identical structural model using mesh mapping technique. In next stage, the road load response from vehicle was measured using Road Load Data Acquisition (RLDA) method to characterize the field environment. For evaluation of fatigue life, the ChannelMAX module was used, where integration of multi-axial loading behaviour from RLDA along with thermo-structural loads from welding was done. A comparative study on fatigue life of component, both with & without residual stresses, was performed. The damage factor computed in fatigue analysis noticeably manifested that tensile residual stresses are more susceptible to failures & needs to be addressed during product development process.

The following presentation covers the fatigue life calculation of a swing arm based on a multibody simulation. Strain gages are being used to measure the occurring strains during field studies. Via virtual iteration statically applied forces are being calculated to reconstruct the measured strains. The current calculation process uses those time dependent forces in combination with a Femfat-Max calculation to compute the damage values. Due to the static calculation dynamic effects are being neglected. Therefore, a dynamic approach is needed to take those effects into account. The main goal of this thesis is to compare the existing calculation with a dynamic approach. If possible, the new approach should be used for other components as well. The modal damage analysis is based on X-form files calculated with Adams, the corresponding modal stresses, and the number of repetitions for each file. The results showed that the best solution according to a compromise between computation time and output quality is accomplished by using 80 Hz filtered measurements in combination with the Craig-Chang reduction method and other filter methods to speed up the calculation. Compared to the static analysis no disadvantages were brought forward.

Standard fatigue estimations processes for long load-time histories are based on superposition of unit loadcases, which can either be a superposition of static loadcases or of dynamic loadcases using modal coordinates. In both cases, linearity of the model is a key constraint that does not allow to apply this approach to cases where nonlinearities are relevant. However, especially for the assessment and optimization of local body attachment point designs, realistic results do require the consideration of contact nonlinearities.

In the presentation, we show & compare two different approaches to tackle this problem being applied to a trailer towing test example for which also physical test results allow correlation.

The first approach (FEMFAT Elastoloads) uses stress response approximations from a multi-dimensional load space exploration, whereas the second approach (Ford inhouse method) develops damage equivalent block loads from the full system first followed by a regular fatigue assessment considering all nonlinearities for the developed block loads.

The presentation will conclude with a review of pro’s and con’s for both methods.

Carbon fiber reinforced polymers (CFRP) are increasingly used in the aerospace and automotive industry due to their high lightweight potential. New automated manufacturing processes were developed and lots of knowledge on the new technologies was built up.
However, new kinds of material variability arise due to new processes and are classified as defects in a first attempt. Such defects might initiate complex damage modes during service life due to the interaction between carbon fibers and epoxy resin. Safe operation has to be guaranteed. Therefore, a profound understanding of defects and damage progression and their detection is necessary. Defects and damages have to be considered also in the design stage, otherwise parts with defects have to be classified as rejects. Especially the current fatigue design of CFRP is conservative.
A profound fatigue life prognosis in early design states is necessary to achieve an optimized fatigue design. In the case of composites the different interacting damage mechanisms on microscopic scale have to be taken into account, which makes fatigue life prognosis much more complicated than for metallic components. The effects of manufacturing defects are usually not considered in fatigue life prognosis tools and should be incorporated as the initial material state. Therefore, their influence on the macroscopic scale has to be quantified taking into account the damage processes on the microscopic scale that initiate damages.
The approach of this thesis is to investigate common manufacturing defects of high-pressure resin transfer molded non-crimp fabrics (NCF), to understand the damage mechanisms initiated by each defect and to quantify the detrimental effect on static and fatigue strength. Therefore, coupons with artificially introduced manufacturing defects are tested under different static and fatigue load cases, different R-values and material orientations and validated with numerical simulations. Non-destructive testing and monitoring of the stiffness evolution allow an in-depth understanding of the damage processes. The investigated defects are an out-of-plane fiber waviness, a fold in fiber direction and a locally compacted region. In-plane tension and compression tests and out-of-plane delamination tests are performed using in-house designed test rigs.
The results of these investigations were published in several journal publications and conference proceedings.
For example, it was found that the fiber waviness has the most detrimental effect on the compressive load cases in fiber direction. Static strength and fatigue life are accordingly decreased depending on the defect angle. It was observed, that different fracture patterns result from different defect angles including mainly kink bands and delamination. These damage mechanisms were simulated with non-linear finite element analysis, whereby it was possible to simulate the kink banding as well as the delamination processes.
Beside the effects of defects, the potential of a Structural Health Monitoring method for damage localization in CFRP structures is experimentally and numerically evaluated. The direct resistance measurement of CFRP was proposed to detect and localize damages. First, the resistance change of the same NCF material was measured and drilled holes were successfully detected. Second, an anisotropic CFRP plate was equipped with electrodes and the potential for damage localization could successfully be demonstrated.