BS 7448 vs ASTM E1820 CTOD fracture test

Both BSI and ASTM have published standards for ‘combined’ fracture toughness test methods, allowing measurement of critical values of stress intensity (K), J and CTOD. The standards cover both ‘single-point’ values of fracture toughness and tearing resistance curves.

BS 7448: Fracture mechanics toughness tests, is published in four parts:

  • Part 1: ‘Method for determination of KIc, critical CTOD and critical J values of metallic materials’.
  • Part 2: ‘Method for determination of KIc, critical CTOD and critical J values of welds in metallic materials’ – superseded by BS EN ISO 15653.
  • Part 3: ‘Method for determination of fracture toughness of metallic materials at rates of increase in stress intensity factor greater than 3MPa √ms-1‘.
  • Part 4: ‘Method for determination of fracture resistance curves and initiation values for stable crack extension in metallic materials’.

BS 7448 supersedes previous test standards, which were restricted to the measurement of KIc (BS 5447) or critical CTOD (BS 5762) only.

ASTM E1820: Standard test method for measurement of fracture toughness, coexists with the older test method for determination of KIc (E399). It does not explicitly address the testing of weldments.

The British and American standards are very similar in principle, differing mainly in small details of the test and in the terminology used. Current research, however, suggests that there can be big differences in values of CTOD calculated to BS and to ASTM for certain materials. The largest difference between standards to note is in the equations used to calculate CTOD. BS 7448 assumes the specimen rotates about a fixed point in the un-cracked ligament, whilst ASTM E1820 derives CTOD from the J-integral.

Other differences to mention between the three standards are as follows:

  • ASTM E1820 covers more specimen geometries than does BS; a disk-shaped specimen can be used as well as the single edge-notched bend (SENB) and compact tension (CT) geometries.
  • ASTM E1820 includes the use of a so-called ‘normalization’ technique to derive a J-R curve directly from a load-displacement record, along with details of initial and final crack lengths. This technique is particularly useful in cases where unloading compliance techniques cannot be used, for example when high loading rates are applied.
  • ASTM E1820 allows J to be derived from either crack mouth opening displacement (CMOD) or load line displacement, whilst BS 7448 requires J to be derived from load line displacement.
  • ASTM E1820 does not allow the reporting of δm – the value of CTOD at the first attainment of maximum force plateau for fully plastic behavior, as allowed in BS 7448.
  • ASTM E1820 includes requirements for size-independent determinations of CTOD and J; the BS 7448 approach, by contrast, is to determine J/CTOD on full-size specimens that represent the structure being analyzed.

Note: ASTM E1290 (the standard test method for CTOD) was withdrawn in 2013.

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