A Displacement Controlled Fatigue Test Method for Additively Manufactured Materials

Abstract

A novel adaptive displacement-controlled test setup was developed for fatigue testing on mini specimens. In property characterization of additive manufacturing materials, mini specimens are preferred due to the specimen preparation, and manufacturing cost but mini specimens demonstrate higher fatigue strength than standard specimens due to the lower probability of material defects resulting in fatigue. In this study, a dual gauge section Krouse type mini specimen was designed to conduct fatigue tests on additively manufactured materials. The large surface area of the specimen with a constant stress distribution and increased control volume as the gauge section may capture all different types of surface and microstructural defects of the material. A fully reversed bending (R = −1) fatigue test was performed on simply supported specimens. In the displacement-controlled mechanism, the variation in the control signal during the test due to the stiffness variation of the specimen provides a unique insight into identifying the nucleation and propagation phase. The fatigue performance of the wrought 304 and additively manufactured 304L stainless steel was compared applying a control signal monitoring (CSM) method. The test results and analyses validate the design of the specimen and the effective implementation of the test bench in fatigue testing of additively manufactured materials.

High Cycle Fatigue Performance of LPBF 304L Stainless Steel at Nominal and Optimized Parameters

“…All experiments were conducted at room temperature monitored with an infrared temperature sensor. The temperature variation reported here [51] remained within ±2 • C. The specimens failed at random locations within the gauges since defects are present randomly in the material. In Krouse type specimens, the stress concentration near the defects within the gauges was expected to be high, though the nominal stress distribution was assumed uniform.…”

“…As we can see, for both the nominal and optimized parameters, horizontal specimens demonstrate higher fatigue strength than the inclined and vertical specimens while inclined specimens have higher strength than vertical ones. The fatigue strength of the material in the horizontal direction is higher not only in the nucleation phase but also during propagation as we can see from the analysis of the control signal [51] shown in Figure 10. During the fatigue test, the crack initiates within the gauge and propagates across the cross-section.…”

“…A total of 7 specimens for each build orientation and process parameters were prepared without any additional surface preparation. Detailed FEA simulation results and sensitivity analysis on the designed specimen can be found here [51].…”

“…A displacement controlled test was performed in this study since the displacement controlled fatigue test tends to be a more stable approach than load control and the driving force decreases with the crack growth in displacement control [50]. Later implementing the control signal monitoring (CSM) method [51,52], the crack nucleation, and propagation phase were identified and compared for different build directions and process parameters.…”

Editorial on Special Issue “Fatigue and Fracture Behaviour of Additive Manufacturing Mechanical Components”

“…In the property characterization of additive manufacturing materials, mini specimens are preferred due to the specimen preparation and manufacturing cost, but mini specimens demonstrate higher fatigue strength than standard specimens due to the lower probability of material defects resulting in fatigue. In Reference [1], a novel adaptive displacement-controlled test set-up was developed for fatigue testing using mini specimens. In this study, a dual gauge section Krouse type mini specimen was designed to conduct fatigue tests on additively manufactured materials.…”