The CNC turning program used a 0.010" radius tool instead of the specified 0.040". Additionally, a grinding step left transverse scratches across the radius.
The company revised their CAM software to force a 0.050" radius, added a roller-burnishing step, and implemented 100% DPI on all bolts. The SAS4 radius crack failure rate dropped by 97%. Frequently Asked Questions (FAQ) Q1: Can I weld an SAS4 radius crack? A: Almost never. Welding introduces high residual stress and heat-affected zone (HAZ) brittleness. The crack will likely re-initiate adjacent to the weld. Q2: How long does it take for an SAS4 radius crack to grow to failure? A: It depends on load cycles. In a high-cycle fatigue scenario (e.g., rotating machinery at 3000 RPM), a detectable crack may reach critical size in less than 100 hours . In low-cycle, high-force applications (e.g., hydraulic press), it could be 10–20 cycles . Q3: Is there a difference between SAS4 and other steel grades regarding radius cracking? A: Yes. SAS4 grades are often heat-treated to high hardness (40–50 HRC), which makes them more sensitive to radius-induced fatigue than lower-strength steels like 4140 or mild steel. Q4: What is the best NDT method for an in-service SAS4 component with a painted radius? A: Eddy current array (ECA). It penetrates coatings up to 0.020" thick and detects cracks as small as 0.005" deep. Conclusion The SAS4 radius crack is not a mysterious or rare phenomenon—it is a predictable consequence of high stress, inadequate geometry, and cyclic loading. By understanding the critical role of the radius as a stress concentrator, engineers and technicians can take decisive action: designing larger radii, specifying superior surface finishes, performing regular NDT, and resisting the temptation to patch rather than replace. sas4 radius crack
A properly designed radius distributes that stress over a larger area. However, if the radius is too small, incorrectly machined, or damaged, the stress concentration can initiate a crack. The CNC turning program used a 0