Design of Additively Manufactured Test Article for Multiaxial Fatigue Fracture Characterization

The Aerospace Corporation

Mentors:  Wei Cai (Stanford), James Tuck-Lee (Aerospace Corporation) and Jacob Rome (Aerospace Corporation) 

Goal:

To design a mechanism or procedure to test the biaxial fatigure strength of 3D-printed Inconel 718 using a uniaxial Instron testing machine. The design should permit the development of independent controls of the stress state on the crack, enable fully reversible loading, and minimize production costs and time by minimizing sample size, support structures, and post-print modifications.

Problem:

The simplest way to obtain a biaxial stress state is simply to control two perpendicular loads, as shown in the figure just above this paragraph. This type of test setup allows for independent, fully reversible control of the two stress components and very simple testing samples; however, it is much less common and more expensive than traditional uniaxial testing machines. For this reason, a common approach for biaxial testing involves the use of a linkage system to translate the uniaxial load into perpendicular components, see the top figure. Most commonly the angles of the linkages are varied to control the ratio of the two stress components. This type of system allows for fully reversible loading, but the linkage-sample system has to be setup differently for each desired loading scenario, and during testing, the ratio between the stress components cannot be changed. Taking advantage of the freedom to design afforded by 3D printing, the problem is to design a class of test articles that can be mounted on a uniaxial testing machine and that, through changes in the geometry in the test article, can be used to tailor the biaxial loading history of a fatigue crack in the sample.

What did the team do?

The team stepped over a sequence of progressively more advanced designs as they learned from the  analysis of each generation, as shown in the figure above. The final design, also shown above,  translates uniaxial compression along the Y direction into biaxial tension in a perpendicular plane and the ratio of stresses in the X and Z directions is controlled by the thicknesses of the legs along those axes. The stresses are fully reversible, as compressive stresses can be induced by reversing the applied load. The desired strain of 1% is achievable using the maximum load of 10 kN, and the stress state in the test region away from the crack is acceptably uniform: <10% difference for crack lengths up to 25% the test region diameter. During printing, only three of the regions will require specific print supports, and those supports are located on the same side and are entirely external, which minimizes the machine time required for removal.