On effect of pre-bending process on low cycle fatigue behaviour of high strength steel using lock-in thermography

The application of High Strength Steel (HSS) on different structural components is becoming more attractive. They have a fine grain structure, low carbon content for improved weldability, and controlled internal purity. These steel grades are frequently used for structural applications. There are a lot of steel components of the considered applications that are subjected to bending and fatigue loading conditions, respectively. It is well known that under critical loading conditions, cyclic stress, which exceeds the material yield stress, can occur at some critical locations such as the inner side of bent components. Combining these two aspects, i.e. the bending process followed by fatigue loading, causes multiple micro-crack initiation inside the inner surface of the bending area, which is followed by propagation of the cracks up to final rupture of material. The main objective of the present study is to investigate the effect of pre-bending process of HSS subjected to low cycle fatigue loading conditions, since so far only very limited amount of research has been focused in this direction. For this purpose, in the first step, a new test set-up was designed to take into account the effect of pre-bending process when the fatigue load has been applied. Lock-in thermography technique was used to monitor the incremental temperature variation during fatigue cycling at the bending root. Using the temperature evolution, the crack initiation and propagation lifetimes were separated from total lifetime. Fractography and Scanning Electron Microscopy (SEM) analyses were performed to study the fracture surface of specimens after bending and fatigue testing. Furthermore, numerical technique approach was used to model the bending and spring back processes along with the fatigue loading in order to understand effect of bending process on fatigue behaviour of tested material. The developed finite element model provides more information about the multiaxial strain and stress states at and near the bending root after bending process and applied axial fatigue load.

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