Mean Stress Effects on Fatigue Crack Growth and Failure in a Rail Steel

MEAN STRESS EFFECTS ON FATIGUE CRACK GROWTH AND FAILURE IN A RAIL STEEL 1. Increasing R (ratio of minimum to maximum stress) was found to both accelerate cracking and reduce the critical crack size at instability (alternating or pulsating loads? Or nearing fracture?) 2. The equation of Forman et al. relating crack growth rate to ∆K and R gave the best fit. It is used to predict life in the finite range of S-N curve. 3. Pulsating load = zero-to-tension loading 4. Fatigue crack growth rate = dldN , where l=half length crack, N = number of cycles 5. Paris and Erdogan proposed the rate of fatigue crack growth dldN = A (∆K)m , where A and m are constants for a given material. 6. The limitation to Paris and Erdogan formula is that it does not take into consideration the effect of mean stress. Forman et al modified the formula to take account of a. Mean stress through the load ratio R b. The rapid increase in growth rate as Kmax approaches the fracture toughness of the material Kc 7. Forman et al equation for crack growth is: dldN= B (∆K)m1-RKc-∆K , where B and n are constants for a given material 8. Roberts and Erdogan also examined the problem of mean stress on cracking under cyclic loading and amended the equation (Paris and Erdogan) : dldN= C(Kmax ∆K2 ) p
Based on aluminium alloy, where C and p are constants. Note: p=2 for high-strength steel 9. SEM revealed common fracture characteristics under all testing conditions: a. localised areas of cleavage (brittle) are more prevalent as instability was approaching. b. Fracture of pearlite colonies appeared to be more common at higher R values c. Final fast fracture occurred solely by cleavage fracture 10. y7i6

Observations from this table: a. As R (ratio of min-max stress) increases, less number of cycles (cyclic loading) is required to initiate a crack and fracture the specimen. Also critical crack length is reduced. b. Given the same R