Frequently Asked Questions
The ASME reference curve is a curve showing a lower bound relationship between the fracture toughness and temperature of ferritic steels, based on Charpy and drop weight testing (these are carried out in order to determine the reference nil-ductility temperature, RTNDT).
When fracture toughness data are not available, this indexing procedure, based on the reference temperature RTNDT , can provide a conservative low-bound estimate of fracture toughness for ferritic steels and their weldments.
The ASME reference critical stress intensity factor curve (see Figure 1), which adopted the above indexing methodology, represents a lower bound to a large number of static initiation fracture mechanics (KIC) tests. It was derived on the basis of a large number of tests on parent metal, weld metal and HAZ, with yield strength at room temperature up to 350N/mm2, in particular nuclear grade pressure vessel steels.
The equation for the K IC curve is given by (ASME 2007 Section XI Appendix G or ASME Section XI Appendix A):
|KIC = 36.5 + 22.783 exp[0.036( T - RTNDT )] (MPa √m, °C)
|KIC = 33.2 + 20.734 exp[0.02( T - RTNDT )] (ksi √in, °F)
According to ASME 2007 Case code N-610, for the evaluation of non-ductile failure under certain conditions, an alternative reference stress intensity factor curve (KIR curve) may be used in lieu of the KIC curve. The K IR curve includes the results of dynamic initiation (KID), arrest tests (KIa) and static initiation (KIC). Since dynamic (K ID) and crack arrest toughness (KIa) values are generally lower than static initiation (K IC) values (see Figure 2), KIR curve tend to have lower values of toughness for a given temperature compared with KIC curve (see Figure 3). The equation of the KIR curve (US units, as per original source) is given by:
KIR = 26.81 + 13.79 exp[0.0152( T - RTNDT )] (ksi √in, °F) Equation 2
Figures 1, 2 and 3 reprinted from ASME ASME 2007 BPVC, Section XI and ASME 2007 BPVC, Code Cases - Nuclear, by permission of The American Society of Mechanical Engineers. All rights reserved.
In theory, the reference temperature RTNDT for both the KIR and KIc curves is defined as the higher of the following cases:
- The drop weight NDTT (See TWI FAQ: What is the drop-weight test (or 'Pellini' test)).
- 33°C (60°F) below the minimum temperature at which the lowest of three Charpy results is at least 68 J (50 ft-lb).
In practice, a Charpy transition temperature alone is sometimes used as a reasonable indicator of RTNDT , since drop-weight testing is rarely carried out on materials for non-nuclear use. This is the basis of the fracture toughness estimation scheme given in API 579-1/ASME FFS-1.
According to the exponential curve fit for KIC and KIR, the toughness increases without bound and these curves do not predict an upper shelf. Determination of KIC on the upper shelf would be impracticable as very large specimens would be needed - a cut-off at KIC = 220 MPa √m has therefore been imposed for ferritic steels (API 579-1/ASME FFS-1).
Studies suggested that the reference temperature T0 defined in the fracture toughness master curve (See TWI FAQ: What is the fracture toughness master curve?) is the superior indexing parameter for fracture toughness of ferritic steels compared to the RT NDT approach (K Yoon etc.). Hence RTTo , which is calculated from T0, as an alternative to RTNDT in ASME KIC -reference or KIa -reference curves is described in ASME Case Code N-629 in the applications for nuclear pressure vessels:
RTTo = T0 + 35 Equation 3
The reference temperature T0 is derived from analysis of fracture toughness test data, as given by ASTM E1921-05.
ASME A-4200: 'Material Properties', ASME 2007 Section XI Division 1 Appendix A, pp 332.
ASME G-2110: 'Reference Critical Stress Intensity Factor', ASME 2007 Section XI Division 1 Appendix G, pp. 391-392.
ASME Case N-610: 'Alternative Reference Stress Intensity Factor (KIR) Curve for Class Components', ASME 2007 Cases of ASME Boiler and Pressure Vessel Code, Case N-610.
API 579-1/ASME FFS-1: 'Fitness-for-service, second edition', 2007
ASME Case Code N-629: 'Use of Fracture Toughness Test Data to Establish Reference Temperature for Pressure Retaining Materials Section XI, Division 1', ASME 2007.
K K Yoon: 'Fracture toughness data analysis using the master curve method'. WRC bulletin No. 486 Part 3.
W He: 'TWI Frequently asked questions: What is the fracture toughness master curve?'
ASTM E 1921-05: 'Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range.' Philadelphia, PA, 2006.