Low Cycle Fatigue
Low cycle fatigue (LCF) is failure caused by repeated high-amplitude cyclic strain that pushes material into plastic deformation each cycle, leading to crack initiation and failure after a relatively small number of cycles, typically below 10^5. It is evaluated with strain-controlled testing because stress-based elastic assumptions do not capture accumulated plastic damage.
On a CNC or millwork shop floor, LCF dictates how machining decisions affect a part’s survival under repeated start-stop cycles, thermal expansion, or heavy load reversals. Machining alters surface integrity—roughness, residual stress, work hardening, grain refinement—and those changes can dominate fatigue life. NASA broaching studies observed up to a two-order-of-magnitude spread in LCF life from process parameter differences. For Inconel 718, turning with a 4 mm nose radius produced 1.3–1.4× longer high-temperature LCF life than a 1.2 mm radius, correlating with residual stress and work-hardening state. In a CNC cell, tool geometry, feed, speed, coolant strategy, and finish quality are therefore not merely dimensional concerns; they determine whether the surface survives plastic cycling in service. Shop teams use LCF data to qualify process windows, compare tool wear states, and validate machined surfaces against cyclic loading rather than only checking print dimensions.
How is LCF different from HCF on the shop floor?
HCF is mostly elastic and typically involves higher cycle counts, while LCF involves plastic strain each cycle and usually fails after far fewer cycles. This distinction determines whether stress-based or strain-based process controls matter most.
Why do machinists care about residual stress in LCF parts?
Machining-induced residual stress changes the mean stress state at the surface. Compressive residual stress can delay crack initiation, while tensile residual stress can accelerate it, and NASA studies show machining-induced residual stress strongly influences LCF life.
Why is strain-controlled testing used for LCF?
LCF damage is driven by cyclic plastic strain, not applied load alone. Strain-controlled testing tracks strain amplitude and plastic response, which is necessary because components are cycled into the plastic region rather than remaining fully elastic.