Abshnct-The specimen strength ratio (R,) determined from small specimen tests was correlated with plane strain fracture toughness (RI,) values for many beats of AS33B-1 steel. A variety of toading rate and specimen size results suggest that K,, can be predicted from the small specimen strength ratio up to values of R, near 2.0. Also, conservative estimates of cleavage-initiated, elastic-plastic fracture toughness can extend beyond R, values of 2.0. The ASTM E399 size criterion appears to be too restrictive for the class of steel studied, and a more appropriate requirement would reduce the ASTM criterion by a factor of four.
JNTRODUCTION
IMPROVED SA~ZI'Y and reliability of structural components has resulted from recent fraCWe-Safe design criteria based upon a fracture mechanics rne~~olo~.
The material property requirements for preventing fracture are specified in terms of fracture toughness parameters. Perhaps the most widely known methodology is linear elastic fracture mechanics (LEFM), for which the fracture toughness parameter is the critical plane strain stress intensity factor, I&. The laboratory detestation of Ic,, however, can be prohibi~vely expensive due to test specimen size requirements and accurate instrumentation necessary to measure "valid" K,, values[ I]. Therefore, there is a continuing effort to obtain mechanical properties from small, inexpensive specimens and relate these properties, directly or indirectly, to fracture mechanics parameters (in particular, I&=). These small specimen fracture tests can hopefully be used as quality control tests to ensure structural reliability.
There are generally two methods for using small specimens to estimate KI, values. The tist approach is to determine small specimen fracture toughness directly using elastic-plastic extensions of LEM. The second method is to correlate easily measured, small specimen mechanical properties with K,,. The National Materials Advisory Board (NW)
has recently published a review of rapid, inexpensive tests for determining fracture toughness [2]. Although specimen size requirements can be drastically reduced using direct mea&es of elastic-plastic toughness (.I, COD, etc.), these techniques are not considered rapid and inexpensive tests by the NMAB. Their review indicates that the specimen strength ratio parameter (R,) has promise as an inexpensive fracture parameter which seems to correlate well with KI; for certain specimen size ranges. The specimen strength ratio is the ratio of the net section nominal stress (UN) at maximum load to the yield strength (a,). Maximum load and the initial crack length are the only measurements required (or is usually known), thus making the R, parameter very attractive for quality control testing. The NMAB report presented R, data only for the precracked Charpy specimen, which has potential up to a~~~ately I&/o, = 0.75 in.'" (3.8 r&n>. The report suggests that l-in. (25.4 mm) thickness compact specimens should also be investigated for specimen strength ratio determination. The report also states that the correlation of R, with K,, will be improved if only specific alloy grades are investigated. No attempt at correlation of impact specimen strength ratio with dynamic fracture toughness (KM) was made in the report.
The purpose of this paper is to determine the correlation between fracture toughness (&, Kd and the specimen strength ratio (R,) measured with a variety of specimens and loading rates. The specimen strength ratio correlations for impact precracked Charpy, l-in. (254mm) thickness dynamic bend, and l-in. (25.4mm) thickness static and intermediate rate dynamic compact results are determined for many heats of A533B-1 steel. Additionally, the use of the /-integral initiation toughness measured directly from small specimens is investigated.
Up to fourteen heats of A533B-1 steel were examined in this study with HSST Plate 02 being a common heat used throughout the entire series of correlations. Not all heats were tested Ei% Vd. It, NO. ?-A