Accelerated random fatigue testing by means of a tri-axial electrodynamic shaker: solutions for combing the multiple test specifications

Industrial products are subjected to extreme conditions of the surrounding environment throughout their entire life. Severe environment conditions may seriously deteriorate the components durability. Therefore, the environmental effects must be taken into account from the early stages of development. The capability to withstand such adverse conditions is assessed during the product design process by means of laboratory testing. Typically, the laboratory tests force the specimen to work in high severity environmental conditions that are accelerated in order to speed up the testing process. Accelerated random fatigue tests represent a powerful testing procedures to evaluate the fatigue life characteristics of the specimens, overcoming the time cost limitations. Within this framework, the combination of the vibration control technique and the shaker system become critical for replicating in the laboratory the in-service dynamic response of the specimen. The methodologies for performing vibration control tests are constantly evolving thank to the enormous advances of the available technologies. MechVib, the research group in mechanics and vibration of the Engineering Department of the University of Ferrara, has recently purchased a 3-DoF shaker table (Dongling 3ES-10-HF-500). The Dongling patented technology adopts a hydraulic orthogonal decoupling bearings unit for connecting three independent electrodynamic shakers of 10 kN rated force. This avant-garde test facility has the capability to address simultaneous multi-axis vibration control testing. This work presents a test campaign where the 3-DoF shaker table is used in combination with Multi-Input Multi- Output (MIMO) control techniques to carry out accelerated random fatigue tests on specially designed specimens. The paper investigates the effects caused by simultaneous multiple random excitations on the fatigue-life of the specimens. In particular, the paper reviews the currently available solutions for combining the test specifications. The pros and the cons of each method are theoretically and experimentally compared, providing a critical analysis of the issue. The test results show that different combinations of the multiple test specifications can significantly affect the fatigue behaviour of the specimen, resulting in altered failure modes and differing test durations.