Fatigue - Femto Engineering - Femto Engineering

Fatigue

As Femto Engineering we are keen to keep up with the newest and most up to date calculation methods and strive for the most accurate analyses. Nowadays more and more projects involve fatigue problems for new design checks, residual life analyses, or crack investigations.

Investing in fatigue

Fatigue is the weakening of a material caused by cyclic loading that results localized structural damage and the growth of cracks. Once a fatigue crack has initiated, it will grow a small amount with each loading cycle. The crack will continue to grow until it reaches a critical size. A lot of structures and equipment are subjected to repetitive loading and likely to develop cracks or failures. Hence we want to calculated the fatigue damage as accurate as possible. At the moment specific interest is the simulation and calculation of fatigue lifetime estimations for welded steel constructions. Our calculations are mostly performed with industry wide accepted standards like DNV-GL or FKM.

The subject of fatigue is not always intuitive or transparent. Users from academia and from engineering sectors, standards and commercial fatigue solvers have many different available calculation methods. And, we do not always have the material data at hand for each specific case. Let alone large benchmarks and/or the resources to acquire the best known calculation method for the case. Therefore, we are sponsoring and involved in for example several PHD projects at the TU Delft like the 4D-Fatigue project and it’s follow-up SCReen.

4D Fatique

The 4DFatigue JIP project is started to find out how multiaxial damage accumulation works. Specific cases show that a construction may have a lifetime reduction up to 15 times if it is loaded by multiaxial and non-proportional loading instead of simple constant uniaxial loading. A new testing equipment is developed: the hexapod.


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SCReen JIP

The SCReen JIP objective is to increase the fatigue life time estimate accuracy using the total life fatigue damage criterion (phase 1) and to achieve fatigue resistance similarity between small-scale and full-scale specimen test data (phase 2). This project focusses specific towards steel catenary risers.


 

 

We believe these are excellent opportunities to further develop our physical background and calculations tools. We believe that through our direct cooperation with the universities, knowledge transfer and the implementation of the program results in our tools are guaranteed. Actually, we expect that new criterions will be validated on short-term which may lead to increasing the confidence level and improvement of the fatigue lifetime estimate accuracy in our analyses. And if so, these calculation methods will be implemented within the SDC Verifier software so that we can all benefit from this.

 

 

SDC Verifier benefits in fatigue

Normally the fatigue calculation of your construction is extensive and complicated. You need information about the working conditions, loading history and material data. This is to be prepared on top of your FE model with loads, boundary conditions, load combinations, load factors and safety factors. Next up is the recognition and classification of all the details within the model. Recognition defines sections of base material, welds or free edges. And then on each of these, normally based on a specific standard, the accompanied fatigue resistance classification is to be defined. If everything is in order you can start the actual verification of the fatigue damage by translating stress results and perform calculations according to the standards and rules.

SDC Verifier can help you with all these steps.

  • The basis of the calculation.
    SDC Verifier offers the engineer a user-friendly interface for combining the loads into a matrix of load sets and groups, taking into account all the safety factors and partial load factors. Not only does this simplify the processing of a large amount of load history data, but it also shortens the solution time.
  • The recognition and classification.
    An excellent and efficient weld finder is used to easily recognize the areas of interest. After the recognition is complete, the classification must be determined for all welds in the model. Note, you still need to specify which classification to accompany which weld section in the model. Hence, as an Engineer you will need some know how of the rules in the standards. But the process within SDC is straightforward and automates most of the steps for you. SDC’s selector helps creating components for all welds, weld intersections, weld tips, etc. The process automatically reorients the stresses in the welding directions so different classifications can be made along the weld, perpendicular and parallel to the weld and for shear.
  • The verification of the construction.
    After all of the preparations above, SDC Verifier performs the fatigue check. This is according to a standard of your choice from the Offshore, Heavy lifting, Heavy machinery, Maritime, Shipbuilding and Civil industries. The following engineering standards are currently available:

    • Eurocode 3 (EN1993-1-9, 2005)
    • EN 13001 (2018)
    • FEM 1.001 (3rd, 1998)
    • DNV-RP-C203 (2016)
    • DIN 15018 (1984)
    • FKM Guidelines

Femto’s own validation test

Recently we are also involved in doing validation tests ourselves. Real-time fatigue tests at non-scaled welded specimen are performed. Hence our engineers keep a sense for practice and get their knowledge up to date. If you are planning on doing similar tests, watch out because problems with testing equipment and unexpected results are inbound 😉.

Specimen

The specimens have a full penetration V-weld. We performed fatigue tests on normal and annealed specimens. And several material properties are checked with tensile tests, charpy impact tests and Vickers hardness tests.


Physical testing

This specific case studies a cruciform joint with a 3 point bending load such that the loading is easily scaled up to higher levels. Hereby is made use of different load ratios, constant amplitude loadings and random loadings.


Result

Our case showed very consistent higher life expectancy for the weld compared to the calculations based on the standard. Around a factor 4 higher lifetime.


 

 

Also, just a headsup for everyone, many of our specimens failed at compression side of the joint. Fatigue failure due to pure loading in compression is very much possible and even can be very similar to pure tension! This is something that many engineers seem to wipe of the table instantly, for example by only calculating with maximum principle stresses in the FE model, so watch out.

From our own tests it is has become even more clear that fatigue calculations are not straight forward. General thoughts do not always match results from practice. And also widely accepted and used standards and fatigue calculation software can easily miss an accurate fatigue life estimation of a construction.

Do you have questions about the right software, are you looking for training courses, would you like to get in touch with our support department or do you have another question? We are ready to help you!
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January 14, 2021
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At Femto Engineering we help companies achieve their innovation ambitions with engineering consulting, software, deployment, training, support and R&D. We are an authorized distributor for Simcenter Femap, Simcenter Nastran, Simcenter 3D, Simcenter Star-CCM+ and SDC verifier. Get in touch and let us make FEA and CFD work for you.

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