Many investigations led to show that no crack begins to propagate to an increase of stress intensity factor. The life of the components of a structure containing premature cracks, can be governed by the degree of subcritical crack propagation. Thus, knowledge of crack propagation to determine the fatigue of the structure is necessary. One problem of steels of high resilience is their low toughness in the HAZ, when they are welded with a high heat input. In this work we have studied nine specimens that have been welded under a submerged arc welding process controlling the welding parameters and checking in the HAZ of such specimens, critical tensions at the ends of the cracks, the critical cracks lengths and stress intensity factors. It is intended to check that the parameters that indicate the values of fracture mechanics in the HAZ, after heat cycle to which the steel has undergone, under a process with a maximum heat input of 2.327kJ /mm, they are still valid, with the welding parameters applied. It is checked a correlation between the theoretical and experimental values.
TopBackground
Great deal investigations of crack propagation by fatigue led to show that no crack propagation begins until an increase of stress intensity factor, ΔKth, threshold as discussed depends on several known factors.
Initial investigations that were conducted by Frost (1960) showed a low degree of slowdown in growth due to fatigue at low stress, the data provided offered the presumption that below certain values, the crack does not spread. The existence of this threshold, was predicted by Clintock (1963) works under elastic–plastic analysis.
Later Lindner (1965) and Paris (1970) showed that the propagation of fatigue crack can be analyzed in the context of linear elastic fracture mechanics, and the threshold ΔKth, can be determined.
In the 60s, Elber (1971) demonstrated that fatigue cracks interfere each other via a locking mechanism. It was Schmidt (1973) who later it showed that the locking mechanism has a significant effect on the threshold behavior. The information provided led to indicate that there are four locking cracks.
Factors that may inGCHuence the threshold crack propagation by fatigue are: yield strength, grain size and other microstructural elements as residual stresses, CTOD mode, Young's modulus, and environmental temperature. The effects of most of these factors have on ΔKth, they can be explained by its relationship with the locking mechanisms of the crack.
Researchers as Kitagawa and Takahashi (1976), El Haddad, Dowling, Topper and Smith (1980), found a constant value of ΔKth for crack lengths greater than 0.5 mm.
For our steel EMZ 450, belonging to ferritic-pearlitic according to Barsom (1974):
Δ
Kth = 7(1 – 0.85R) MN/m
3/2 = 6.405MN/m
3/2=202.5 N/mm
3/2(1)Life of a structural element fatigue can be considered from three different states:
The life of the components of a structure containing premature cracks, can be governed by the degree of subcritical crack propagation. To this end, the many destructive and non-destructive testing can collaborate to establish any cracks before commissioning. Thus, knowledge of crack propagation through the concepts of fracture mechanics calculation to determine the fatigue of the structure is necessary.
Most testing the growth of fatigue cracks are performed on samples subjected to load fluctuations at constant amplitude. The lengths of the cracks to the cycles that are undergoing represented graphically as measured in Figure 1.
Figure 1. Effect of cycle-stress range on crack growth
Another observable effect on the specimens, is relating the initial size of the crack with life or number of cycles, noting that the higher the initial crack length, fewer life cycles Figure 2.
Figure 2. Effect of initial crack length on crack growth