Figure 1 (left): The Rotatamatron 3000: the world’s largest rotating bending fatigue tester; Figure 2 (right): View of the 2.75” reduced section sample in the Rotatamatron 3000
To provide full scale testing of the fatigue performance of our products, Apollo designed, built and commissioned the world’s first large scale rotating bending fatigue tester. Nicknamed the Rotatamatron 3000, the machine is capable of applying a constant stress across a 2.75 in (70 mm) diameter reduced section with a maximum load of 50,000 lbs. This unique testing ability has allowed Apollo to lead the scientific community globally on understanding the impacts of laser surface treatments on fatigue performance of production sized components undergoing rotating bending fatigue.
The need for this capability arose from limitations of scale down mock ups in published literature, which where the surface treatment is nearly the same size scale as the test sample cross section. Many high-performance, widely used materials on this reduced scale experience either through-hardening from the laser surface treatment or a critical stressed region (residual stresses in-excess of the yield strength of the material) that extends significantly or completely through the cross section. This scenario is not truly representative of the “surface” nature of laser cladding or heat-treating processes on real world components. In the absence of any existing testing capabilities, Apollo sponsored a project to develop the world’s first machine with the capability to test large scale samples for rotating bending fatigue performance. This one of a kind machine is available for fee-for-service testing.
The machine is displacement controlled, and a hydraulic jack is used to apply the necessary force through the bearings adjacent to the sample to set the displacement. The reaction load is balanced by the outer bearings and the machine frame securely anchored to the floor. The design creates fully-reversed four-point bending with a constant stress through the sample reduced section. With the load applied the sample is rotated at speeds of up to 300rpm. As the test progresses the number of cycles are recorded, and when a setpoint reduction in load, typically 80% of the initial load, is reached the test is considered finished. A significant reduction in load to maintain a set displacement is indicative of the presence of a fatigue crack and effective reduced load carrying cross section, and the stoppage of the test is necessary to avoid damage to the fatigued surfaces and damage to the machine itself. Liquid nitrogen is used to break open the fatigued samples to complete the failure analysis and identify the metallurgical initiation and propagation characteristics of the fatigue failure.
For more information, read Kurtis Bell’s thesis on Full-Scale Testing of the Fatigue Life of Laser Additive Manufacturing Repaired Alloy Steel Components >
Figure 3 (left): Rotatamatron 3000 sample at the end of a fatigue test; Figure 4 (right): Higher magnification view of the resulting test fatigue crack
Residual Stress Testing
Apollo-Clad has the ability to measure near surface residual stresses according to the Hole-Drilling Strain Gauge Method. Following established techniques, a strain gauge rosette is applied to the surface of the test material. A strain indicator is soldered to the strain gauge, and a high-speed drill is used to slowly remove material in a targeted area on the gauge. As material is removed, the region in the vicinity of the hole relaxes according to the stresses that are present. These effects are mapped to a maximum depth of approximately 1 mm (0.040 in) in approximately 25 μm to 50 μm steps (0.001 in to 0.002 in).
The results of this testing allow Apollo to generate residual stress maps like the ones shown below in Figure 3 that can directly measure the effects of surface treatments or treatment conditions/parameters on a given material and geometry. Residual stress testing is often combined with rotating bending fatigue testing to provide maximum metallurgical insight into the performance capabilities of laser clad and laser heat treated components.
