Additive Manufacturing (AM), especially laser powder bed fusion (L-PBF), has revolutionized how metallic parts are made, allowing complex designs and single-step production. However, understanding how these parts behave under long-term stress—particularly fatigue—is essential for ensuring their reliability in industries like aerospace, biomedical, and automotive. Fatigue performance becomes even more critical in applications where components face millions of cycles, such as turbine blades, implants, or engine parts.
While surface finish, defects, and residual stress are known influencers of fatigue behavior in 3D printed metals, the geometric scale—or in this case, the section diameter—has been relatively underexplored. This study dives deep into how varying the gauge diameter impacts the fatigue resistance of 316L stainless steel, a material widely used for its corrosion resistance and toughness.
Who’s Involved?
The study was conducted by Megan Trombley, Andrew Birnbaum, and John Allison, and published in Scientific Reports (2025), underlining the scientific rigour and peer-reviewed reliability of the results. Their research utilized 130 ultrasonic fatigue specimens, all made using the same L-PBF process parameters, to isolate the variable of gauge diameter. The researchers also employed advanced methods like X-ray diffraction (XRD) and low-stress surface grinding to understand the internal mechanics affecting fatigue life.
Size Matters: Diameter vs. Fatigue Strength
The research examined three different specimen diameters—1.5 mm, 2.5 mm, and 5.0 mm—under fully reversed loading conditions (R = −1), typical of high-cycle fatigue (HCF) testing. The smallest diameter (1.5 mm) exhibited the highest fatigue strength at 122.0 ± 32.8 MPa. The 2.5 mm samples followed with a strength of 98.7 ± 7.0 MPa, while the largest diameter (5.0 mm) performed the worst, with a strength of just 89.5 ± 5.6 MPa.
This inverse relationship aligns with Weibull's weakest link theory: larger volumes are more likely to contain crack-initiating flaws, reducing their durability. For L-PBF components, this insight is vital because AM often produces parts with variable cross-sections, especially in medical implants and aerospace brackets.
Surface Finish: The Rough Truth
The team observed that most fatigue failures initiated at surface crevices, underscoring the influence of surface condition. To test this, additional 5.0 mm specimens underwent low-stress surface grinding. Removing just 75 μm of the surface layer raised fatigue strength from 89.5 MPa to 170 MPa. When the entire surface contour was removed, strength further increased to 179 MPa—a 100%+ improvement.
This drastic enhancement emphasizes the importance of post-processing in AM parts, especially for structural applications. The as-built surface roughness acts like a stress riser, serving as the cradle for crack initiation, and its removal allows the intrinsic material strength to manifest.
Residual Stress & Heat Treatment: Hidden Influencers
Using X-ray diffraction (XRD), the researchers found that residual stress distributions were dependent on specimen diameter. The 1.5 mm samples exhibited lower axial residual stress compared to the 5.0 mm ones. This is crucial because tensile residual stress at the surface exacerbates crack growth under cyclic loading, while compressive stress is generally beneficial.
Stress-relief annealing, a common post-processing heat treatment, reduced residual stress by 51% in the 5.0 mm specimens. As a result, fatigue strength improved by 30%. This reveals that simple thermal treatment can significantly alter part performance, especially for thicker cross-sections that are prone to higher internal stresses.
Microstructural Observations: Fine Details Matter
Further microscopic evaluations showed that gauge diameter not only influenced mechanical performance but also subtly impacted internal microstructure. While all parts were printed using the same parameters, thermal gradients differed due to section thickness, affecting grain growth and defect distribution. For instance, smaller specimens had finer microstructures, which are typically associated with higher fatigue strength due to fewer stress concentrators.
The presence of unmelted powder particles and gas pores were observed, particularly near the surface. These defects are notorious for acting as crack starters. Grinding and stress-relief annealing not only eliminated surface imperfections but also reduced the impact of internal anomalies.
Ultrasonic Fatigue Testing: Speeding Up the Process
Traditional fatigue testing methods can take months, especially for high-cycle applications. The researchers used ultrasonic fatigue testing at 20 kHz, drastically reducing test time to just hours. This method, validated by previous studies for austenitic steels, allowed them to collect statistically significant data across multiple samples, making their findings robust and broadly applicable.
With over 130 specimens tested, the statistical analysis using a Random Fatigue Limit (RFL) model and Maximum Likelihood Estimation (MLE) ensured that the observed trends were not anomalies but indicative of genuine material behavior.
Key Takeaways:
• Smaller diameter specimens (1.5 mm) had the highest fatigue strength (122.0 MPa), while larger ones (5.0 mm) had the lowest (89.5 MPa).
• Surface roughness significantly reduced fatigue performance; removing 75 μm boosted strength to 170 MPa.
• Stress-relief annealing cut residual stress by 51% & improved fatigue strength by 30%.
• Surface-initiated cracks were the dominant failure mode in as-built parts.
• X-ray diffraction confirmed that axial residual stresses decrease with smaller gauge diameters.
• Ultrasonic fatigue testing at 20 kHz enabled rapid, high-cycle testing for robust data analysis.
