Resistance to Plane-Stress Fracture (R-Curve Behavior) of A572 Structural Steel

The R-curve behavior of A572 Grade 50 steel was established over the temperature range-40 to +72°F by using state-of-the-art procedures. Both linear-elastic-fracture-mechanics (LEFM) and crack-opening-stretch (COS) analytical techniques were used in assessing experimental results obtained under load-control and displacement-control testing conditions. This study represents a pioneer effort in that it is the first known attempt to evaluate the R-curve behavior of a low-strength structural steel in some depth.

Results showed a steep K_c transition behavior for 1.5-in.-thick (38 mm) plate, with minimum K_c values of 57, 155, and 318 ksi √in (63,171, and 350 MNm^-3/2) obtained at-40, +40, and +72°F (-40, +4.5, and + 22°C), respectively. A similar behavior was observed for 0.5-in.-thick (12.7 mm) plate, with minimum K_c values of 150, 273, and >380 ksi √in. (165, 300, and >418 MNm^-3/2) obtained at the corresponding test temperatures. The results are discussed in relation to the influence of material and testing method, as well as in relation to earlier K_Ic results obtained at cryogenic temperatures.

The minimum K_c values measured demonstrate extensive crack tolerance for A572 Grade 50 steel under all combinations of the test conditions studied. With one exception, these minimum behaviors can be translated into total critical flaw lengths that are at least seven times the plate thickness (2a_cr ≧ 7B) for cracks embedded in large planar structures and subjected to tensile-stress levels equal to three fourths the yield strength. The applicability of a_cr calculations obtained from R-curve measurements generally, and on the A572 Grade 50 steel specifically, is discussed in relation to typical structural members such as H-beams.