INFLUENCE OF MATERIAL CUTTING ON THE CYCLIC FATIGUE OF TRIP STEEL

Formed parts of the car-body represent cyclic loaded products. For the proper design of these parts is very important not only knowledge of deformation and stress material behaviour at production by forming technologies but important is also knowledge of other factors which influence subsequent product utility properties at the car operation. It's obvious that truly important parameter for the proper car-body part proposal is the fatigue strength of the given material. Mainly for cyclic exposed stampings of chassis assembly is knowledge of used materials fatigue strength one of the basic presumptions for the safe car operation. Measured dependences of stress size on the number of cycles (Wohler's curves) are thus very important input parameter at design for the given car-body part. However from the practical point-of-view they don't reflect influence of individual production technologies on the fatigue strength of processed materials. Regarding fact that formed parts from steel sheets are during production cut by several different technologies is submitted paper focuses on the determination of material cutting influence on the fatigue strength. For experiment there was chosen ultra-high strength TRIP steel and the most commonly used material cutting technologies (laser cutting, shearing and machining). For samples prepared as mentioned was on the six stress levels observed material cutting influence on the achieve number of cycles up to fracture. Results of experiments were compared with the reference samples which were machined. Results from measurements are presented mainly graphically as stress versus number of cycles and fatigue fracture surfaces images from the electron microscope.