GIGAcycle fatigue from internal DEFects – GIGADEF

1) Fabricate fatigue specimens containing controlled internal defects
2) Design and manufacture an in situ synchrotron ultrasonic fatigue machine
3) Develop techniques for detecting and monitoring an internal crack
4) Analyze tomographic images to understand the internal damage mechanisms

1) The first challenge was to set up a methodology to manufacture a fatigue test specimen containing a centred internal defect (cast Aluminium). These defects should initiate an internal fatigue crack. In addition, a procedure was put in place to fabricate an initial internal crack in a rolled titanium.
2) Promote internal initiation by testing in the field of gigacyclic fatigue: ultrasonic fatigue to have reasonable test times (13h = 1 billion cycles!).
3) 100 µm internal crack detection: laser vibrometer and infrared thermography, 3D synchrotron crack propagation monitoring (1 µm resolution).
4) The results show that the internal cracks propagate as under vacuum in Titanium and that the morphology of the cracks is very complex in Aluminium. These cracks are stopped during a very important part of the service life by the grain boundaries which form very strong structural barriers at this level of stress (~ 25% of the elastic limit).

The GIGADEF project made it possible to design, manufacture and develop a unique experiment in the world and to obtain the first experimental results. With this experience, we have a tool that will help us understand the role of internal defects on fatigue behavior and competition with surface defects. Now that the experimental means are operational, it is necessary to initiate experimental campaigns of greater scope on different materials to obtain experimental results capable of making progress on our understanding of the role of internal faults in fatigue. The stakes are high, indeed the questions posed by the extension of the life of structures beyond their initial end of life can lead to very long lifetimes (beyond a billion cycles). This «life extension« favors internal initiation which was not foreseen in the initial calculations.

Two articles in international journals among the top 5 results in the world on the subject. 6 international conferences including a prize for the best poster at the largest international conference on fatigue (FATIGUE 2018). Two French conferences, a dedicated website to present the project and an article for the general public in a CNRS review of the DR8 (MICROSCOOP). Two articles for international journals are at the end of the drafting.

The goal of the GIGADEF project is to study the impact of internal defects on the fatigue behaviour of casting materials. This topic is important for casting manufacturers in order to optimize the fatigue design and also to reduce component rejection during production. Many researchers have worked on the influence of surface defect on the fatigue behaviour but according to our knowledge, only two very recent papers in the literature deal with the propagation of internal crack under fatigue loading. The aim of the GIGADEF project is to design, build and use an ultrasonic fatigue device to perform in situ test on a synchrotron.

Two materials will be tested: a ferritic nodular cast iron (automotive application) and a cast Aluminium alloy A357 T6 (aeronautical application). These two materials are already available and fully characterised by the researchers of the GIGADEF project (fatigue, gigacycle fatigue, long and short crack, Kitagawa diagram, environmental effect, fatigue crack mechanisms). Furthermore, these two materials contain natural 3D markers for DVC (silicon particles and graphite nodules).

Le main goal of the GIGADEF project is to detect and follow the propagation of an internal crack initiated from an internal casting defect.

In order to reach this goal, it is necessary to perform tests in the Very High Cycle fatigue regime in order to promote internal initiation. The GIGADEF project is composed of four French research groups and a casting technical center (P’, MATEIS, I2M+PIMM and CTIF). The project is divided in four main topics:

1) Production of fatigue samples containing isolated internal casting defect. The nodular cast iron sample will be obtained by successive EDM machining and laboratory X tomography in order to isolate an internal shrinkage. Cast Aluminium specimens will contain an artificial reproducible internal centred defect.

2) Design, build and test an ultrasonic fatigue device dedicated to in situ synchrotron measurements. This test device must be able to open the crack during 3D image acquisition.

3) Establish the experimental protocol to detect and follow the internal crack during the test:

- Thermography analysis in order to detect the initial crack and to monitor its propagation by means of an inverse approach using a thermo-mechanical modelling.

- Drop of frequency method in order to detect the crack and to follow the propagation by calibration (link resonance frequency – defect size).

- Potential drop technique in order to detect the smallest initial crack and to follow the crack propagation by means of numerical Finite Element simulation of the potential drop.

4) 3D image analysis in order to understand fatigue crack mechanisms from internal defect and determine as precisely as possible the crack front in order to compute the stress intensity factor. 3D DVC in order to follow experimental deformation around porosity. The influence of environment on fatigue propagation mechanisms will be studied in air and different vacuum levels in order to understand the environment inside and surrounding the internal crack.

The GIGADEF project aims at designing a novel fatigue in situ experience that could be unique over the world. This project will bring at the international level the group of laboratories involved. This project is also very important for casting manufacturers that need quantitative data in order to be able to take into account the influence of internal defect on the fatigue behaviour of real components (casting components, additive manufacturing components).