Further enriching the event the "Academic Fatigue Research Award " was presented during the FEMFAT User Meeting for the third time. This award honors scientific works focused on fatigue life and durability, as well as applications and future developments in fatigue life prediction. We proudly presented the "Academic Fatigue Research Award " to Dr. Serkan Cavdar (Henkel) for his exceptional doctoral research on hyperelastic bonded joints.

To complement the wealth of knowledge shared, the conference offered immersive workshops, engaging evening events, and the chance to explore the breathtaking Stahlwelt and historic city of Linz. 

Once more, the ECS Simulation Conference exceeded expectations as an outstanding platform for knowledge exchange, captivating discussions, and invaluable networking opportunities in the dynamic realm of fatigue simulation and beyond. 

Continuation of the article recently published in FEMFAT Newsletter #30 on FKM-compliant fatigue strength assessment of the base material (1/3), this time focusing on the consideration of design factors. 

Since FEMFAT 2023 it is possible to assess the fatigue strength of non-welded and shell-meshed welded components according to the 7th edition of the FKM guideline 2020 'Analytical Strength Assessment for Components in Mechanical Engineering' (abbr. FKM7). Particularly in view of the complexity of the guideline, the aim was to provide a defined simulation process for the software-based assessment. This has been standardized by a template that can be loaded directly from the GUI following project-specific inputs. At this point the accompanying manual (FEMFAT_Template_FKM) should be highlighted, which explains all the details including the material generation. 

• n_σ           K_t - K_f ratio                  (= f_GR,af)
• K_f,σ      estimated fatigue notch factor (= K_f)
• K_R,σ     roughness factor                    (= f_1,af)
• K_V        surface treatment factor        (= f_2,af)
• K_S        coating factor                         (= f_GS,af / K_NL,E,σ ∙ j_S ∙ j_F,b ∙ j_G,b)
• K_NL,E,σ  factor for GJL under bending (= f_GS,af / K_S ∙ j_S ∙ j_F,b ∙ j_G,b)

The FKM guideline determines the design parameter for normal and shear stresses. Differences for shear stresses arise in the determination of the K_t - K_f ratio (n_τ), the estimated fatigue notch factor (K_f,τ) and the roughness factor (K_R,τ). The constant K_NL,E,σ is only relevant for normal stresses. In contrast, in FEMFAT a total influence factor f_tot,af is applied equally to normal and shear stresses. 

FEMFAT Material Data 

The equations of the design factors are hard coded in FEMFAT, i.e. additionally to the material generation acc. to FKM (Material Generator 2023) no material data need to be defined for the implemented 'FKM Guideline' methods described in the following sections. 

FEMFAT Influence Factors 

Relative stress gradient 

The constant K_f can be assigned as node characteristic from FEMFAT 5.4.3 onwards and is considered in the same way for normal and shear stresses. As an approximation, for not too mild notches (stress concentration factor K_t ≥ estimated fatigue notch factor K_f) can be taken from FKM7 tab. 4.3.1. Alternatively, eq. 4.3.2 which depends on the stress concentration factor K_t and the K_t - K_f ratio n) is also available for estimation. 

In FEMFAT with the 'FKM Guideline' method, the K_t - K_f ratio n(X’_max) = f_GR,af is determined acc. to SIEBEL & STIELER (FKM7, sec. 4.3.1.3.1). The relative stress gradient X’ is determined based on the v.Mises stress. In this, FEMFAT differs from the FKM guideline, which here treats the normal and shear stress separately from each other (FKM7, sec. 4.3.1.3.3). Properties from surface hardening can be considered in materials assigned to the boundary layer of the component. 

The FKM guideline edition 7 describes, as an alternative to the K_t - K_f ratio acc. to SIEBEL & STIELER, the determination of a material-mechanical K_t - K_f ratio (FKM7, sec. 4.3.1.3.2). The concept is currently not available in FEMFAT, but from version 2022, externally determined stress factors and/or gradients can be assigned as node characteristics. 

Surface roughness 

The roughness factor K_R of the FKM considers the influence of the surface roughness (in µm, acc. to DIN 4768) on the fatigue strength. In FEMFAT, the equation for normal stresses (FKM7, eq. 4.3.22) is available with the method 'FKM Guideline'. Furthermore, the component roughness factor K_R,σ,C is related to the specimen roughness factor K_R,σ,M 

This formula considers the FKM safety factors for load j_S, material j_F,b and casting j_G,b. The assignment of the externally determined factor f_GS,af is done by node characteristic. The factor K_NL,E,σ may only be considered for components made of GJL, whereby, the 'Normal Stress in Critical Plane' should be considered. 

We are excited to announce a new FEMFAT plast method that takes accuracy to the next level, all while considering the precise sequence influence. This breakthrough is made possible through the integration of a kinematic hardening model, adding an extra layer of sophistication to our simulation capabilities.

FEMFAT plast is available since many years and offers an efficient and reliable implementation of the Neuber method.

This method involves extracting stress data from a linear elastic finite element analysis and then approximating the non-linear elasto-plastic stresses. In simpler terms, it is employed to evaluate local plastic deformation, particularly in regions like notches. By accounting for localized plastic deformation, it provides a more accurate representation of stress distribution, leading to improved predictions of fatigue life.

The method offers significant advantages since a finite element simulation with elasto-plastic material behavior can be avoided in many applications.

So, what exactly does the new PLAST method bring to the table?

1. Enhanced Accuracy: The new method is designed to offer unparalleled accuracy in predicting metal plasticity behavior. By incorporating the kinematic hardening model, FEMFAT plast can capture the intricate details of how materials respond to variable amplitude loading, resulting in more precise simulations and predictions.

2. Sequence Influence: One of the key advantages of our new approach is its ability to account for the exact sequence influence of transient stress histories. There are two options available, stress rearrangement according Neuber or ESED (equivalent strain energy density). With our new method, you can gain insights into how materials evolve over time, enhancing the realism of your simulations.

3. Improved Consistency: The new PLAST method uses the selected equivalent stress method (e.g. Scaled Normal Stress). Additionally, now the rainflow counting comes after the stress rearrangement. In other words, the Neuber correction is directly applied in the cutting plane.

4. Additional Output: FEMFAT offers new output in terms of diagrams and text files. Now you can analyze strain histories as well as stress-strain diagrams.

For illustration of the new PLAST method a simple example of a notched specimen under pulsating tensile load (stress ratio R=0 ) is presented here. The fatigue analysis is carried out with both FEMFAT 2023 (new PLAST method) and FEMFAT 2022 (old PLAST method).

Examining the results, we can see significant differences in the equivalent stress histories:

• The old PLAST method delivers a pulsating stress response with constant amplitude like the original input signal (cp. figure 1).
• In contrast, the new method initially delivers decreasing amplitudes until they stabilize and remain constant due to material hardening (cp. top diagram in figure 2).

Additionally, the new PLAST method comes with diagrams for strain histories and stress/strain hysteresis (cp. figure 3).