Analytical and Experimental Study of Fracture in Bend Specimens Subjected to Local Compression

Abstract

This study presents finite element simulations and fracture predictions for experimental conditions in a recent ASTM round-robin program on weld fracture toughness. The simulations include the effects of weld residual stress and residual stress alteration due to local compression of notched SE(B) specimens. Simulations are performed for a variety of material models and local compression details. Fracture is predicted using crack-tip stress and strain as inputs to both a local and a global prediction scheme. The local prediction is based on the RKR micromechanical model. The global prediction is based on obtaining a critical value of the J-integral, computed using a modified domain integral. Results predict that fracture toughness in the round robin was markedly reduced by local compression of the specimens. The amount of the reduction depends on the position of the compression platen relative to the notch-tip, but is largely independent of the amount of local compression applied. Available data from the round robin do not completely confirm the prediction results, but some of these data may have been influenced by warm pre-stressing during fatigue pre-cracking.

Guide for Recommended Practices to Perform Crack Tip Opening Displacement Tests in High Strength Low Alloy Steels

“…In addition, transverse, equally spaced t marks (lines) should be made to identify the size of the crack during precracking. It is important to point out that the minimum fatigue precrack extent is 0.05 B or 1.3 mm [6]. There are three important crack-size stages: 1) machined notch size (a m ), 2) initial crack size (a 0 ), created most of the time by using fatigue, and 3) the final crack size after the toughness test (a p ).…”

“…If for any reason the crack initial condition deviates from the intended, the result could be unrepresentative of the region of interest, therefore it could lead to an erroneous interpretation of the results. According to the standards, the precracking should be done in conditions similar to the studied conditions, e.g., it is advisable to avoid aggressive environments, high or low temperatures, high load values and fatigue frequency less than 100 Hz [6]. Excessive applied load during the precracking accelerates the crack growth; nevertheless, it creates a bigger plastic region in front of the crack tip.…”

“…Excessive applied load during the precracking accelerates the crack growth; nevertheless, it creates a bigger plastic region in front of the crack tip. Using standard load equations to determine a proper load value [6] is recommended, in addition to increasing the frequency as much the machine allows it.…”

“…The conventional procedure recommends using two steps to grow the crack, the first part is performed by using high loads until the crack gets to 50% of its total depth, and then load is reduced (~50%) and maintained steady until the crack reaches the final size. More information about force and intensity factor (K) limits and, crack size in each stage could be access in [6,49]. An alternative method uses a gradually decremented load, which could be better than the former one because it may require less participation from the user.…”

“…That issue has been sorted by using different standardized specimens, the available technical standards offers different geometries and load modes, such as bending or tensile configurations. In addition, it is usual to find that those specimens offer a high constraint state and consider a crack growth under the plane-strain condition [3][4][5][6]. Although cracks in real structures do not always fulfill the requesting standard conditions, for example, residual stresses induced by welding procedures could change the constraint state and cause uneven fatigue cracks fronts [7,8], very shallow cracks are not considered standard, and some constraint conditions are far away from the options presented by the standards [9].…”