University of Arkansas: Low Cycle Fatigue in 3D Printed 17-4PH Stainless Steel

Kaley Collins recently presented a thesis, ‘Extremely Low Cycle Fatigue Behavior of Additively Manufactured 17-4PH Stainless Steel,’ to the Department of Civil Engineering at the University of Arkansas. Researching the behavior of stainless steel and low cycle fatigue regimes, Collins explains that the goal of the thesis project was to develop strain-life curves to eliminate failure in metal structures.

In this study, the focus is on energy dissipating devices that allow structures to handle seismic load, standing up to catastrophic stress like earthquakes. With 3D printing and additive manufacturing processes at work in so many industrial applications today, users are benefiting from the many advantages of 3D printing, from greater affordability, speed, and efficiency, to the ability to create complex geometries not previously possible.

Centering around 17-4PH stainless steel, Collins compares seismic dissipation devices made through conventional methods as well as 3D printing. While the pros involved in 3D printing are often extolled, there are still many challenges to overcome as users take on new and innovative projects, as well as delving further into the functionality of complex parts rather than just rapid prototypes.

Internal defects continue to be a source of study for researchers around the world, hoping to eliminate issues with new tools, materials, monitoring systems, and more. In metal printing, voids can be the catalysts for failure, as well as ‘layered heat-affected’ areas that cause prints to weaken.

“A better understanding of how AM materials perform in the ELCF [extremely low-cycle fatigue] regime will provide an understanding of material performance during earthquake-type loadings and allow construction of optimized free-form geometries for earthquake dissipating devices,” explains Collins.

Fatigue loading for all experiments in the study was strain controlled, with samples stressed, running through cycles until the point of failure. Five samples (XS) were made by industry partners, explained the author, while the other eight (NS) were created at the National Institute of Standards and Technology. Before the research team removed them from the build plate, all NS specimens were heat treated—while XS specimens were not.

Specimen geometry from ASTM E606

Controlled strain evaluations were performed with a servo-hydraulic fatigue testing machine, with the use of a knifeblade extensometer directly afterward to assess whether suitable strain ranges were applied.

Applied strain ranges for each specimen

Micro-hardness test with diamond shaped indenter [4].

For all samples, typical behavior resulted in necking and fracture after several inelastic cycles; however, upon the use of heat treatment, there were ‘no observable effects’ on ELCF behavior.

ELCF fracture of wrought steel specimen reversed strain cycles.

Strain-life curves for AM and rolled 17-4PH stainless steel.

Overall, AM 17-4PH stainless steel demonstrated a lower fatigue life versus the wrought 17- 4PH stainless steel as inelastic cyclic strains were performed.

“Voids and defects within the AM steel due to unmelted particles contributed to the reduction in fatigue life,” concluded Collins.

“AM 17-4PH stainless steel exhibits higher post-yield strain hardening that wrought 17-4PH stainless steel. Micro-hardness measurements within the grip and gauge sections of the specimens showed that the AM steel post-yield hardening differs from that of traditionally fabricated steels.”

Fractographic image taken from AM steel failure surface.

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[Source / Images: ‘Extremely Low Cycle Fatigue Behavior of Additively Manufactured 17-4PH Stainless Steel’]

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