Optimization of bend test set up to quantify effect of surface roughness on fatigue properties - A guideline to four-point bending and how to optimize specimen geometry.

Sammanfattning

Mechanical testing is a crucial part of engineering; it reassures safety in products, which is of utmost importance in the Aerospace industry. Additive manufacturing, AM, is being used more widely in this sector, and a common testing method for components with AM surfaces is four-point bending. This method is in theory great for investigating and determining the influence surface roughness has on fatigue life properties, but there are many uncertainties which compromise the validity of 4PB. This thesis project explores some of these uncertainties and attempts to increase the validity of four-point bending. The specimen geometry is one of the major points of research in this project. It explores how the design of the specimen can be optimized to minimize an inherent error of 4PB: wedging stress. This thesis also explores how sensitive four-point bending is to setup errors, specifically eccentric- and non-perpendicular-loading. The project is based on FEA simulations performed in Ansys Workbench.The wedging error was proven to be less severe than it was thought to be at the beginning of the project. The baseline specimen, which was based on a common specimen used at GKN Aerospace, showed localized stress peaks that were 1.37% larger than nominal stress. Which is within the total error limit of ±5%. Furthermore, this could be further minimized by increasing the difference between the inner and outer rollers. It was also found that if the inner roller distance is smaller than three tenths of the outer roller distance, the test transitioned towards pseudo-3PB behavior. Consequently, this method is not suitable for thin and wide specimens, since if the breadth of the specimen became bigger than double the height the error started increasing rapidly.Four-point bending proved to be more sensitive to eccentric loading and was barely influenced by non-perpendicular loading, at least when it comes to peak stress. Additionally, it was discovered that 4PB is less sensitive to eccentric loading when the inner rollers are applying the load.