1-120 Section 1Multiaxial Fatigue Complex states of stress are common in engineering components, and in fatigue analysis they may cause serious difficulties.. The simplest situations tha
Trang 11-120 Section 1
Multiaxial Fatigue
Complex states of stress are common in engineering components, and in fatigue analysis they may cause serious difficulties There are many methods available, but none of them are adequate for all cases The simplest situations that might be handled reasonably well involve fully reversed loading by in-phase or
expensive, so it is often desired to use uniaxial test data for predicting the multiaxial behavior A typical
principal stresses σ1a, σ2a, σ3a With the concept of the octahedral shear yield criterion,
(1.6.18)
or general yielding, creep, and random frequencies of loading further complicate the problem and require more sophisticated methods than outlined here
Strain vs Life (ε-N) Curves
A strain-based approach is necessary in fatigue when measurable inelastic strains occur In general, total strain consists of elastic, plastic, and creep strains, with the latter two being in the category of inelastic strains,
(1.6.19)
Simplistically, HCF means a thin loop (a straight line at very long life) and LCF means a fat loop
Coffin-Manson equation) for fully reversed loading,
(1.6.20)
FIGURE 1.6.25 Hysteresis loop.
σe= 1 (σa−σ a) +(σ a−σa) +(σa−σa)
2
2 3
2
3 1 2
εt =εe+εp+εc
f b
c
= ( )2 + ( )2