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What is the drop-weight test (or 'Pellini' test)?

 

The drop-weight test (often referred to as 'Pellini' test) was developed at the Naval Research Laboratory, Washington DC, as a simple method to determine the nil-ductility transition temperature (NDTT). The NDTT was defined in the 1950s (Puzak et al, 1952) as the test temperature in explosion bulge tests at which the plate remained flat at fracture, that is crack propagation occurred in the presence of elastic strains only. The drop-weight test was developed to simplify the determination of the NDTT, it is standardised in ASTM E208.

The drop-weight test specimen is illustrated in Fig.1. It consists of a rectangular coupon with a brittle weld bead deposited on one face. The weld contains a notch, from which a crack is initiated by impact loading the specimen to a fixed amount of deformation under three-point bend. Tests are carried out at a variety of temperatures, the NDTT being defined as the maximum temperature at which the brittle crack spreads completely across one or both of the tension surfaces on either side of the brittle weld bead.

Fig.1a Drop-weight test specimens: Side view. Lower photo shows close-up of weld-bead and notch
Fig.1a Drop-weight test specimens: Side view. Lower photo shows close-up of weld-bead and notch
Fig.1b Drop-weight test specimens: Top view. Lower photo shows close-up of weld-bead with central notch just visible as thin black line
Fig.1b Drop-weight test specimens: Top view. Lower photo shows close-up of weld-bead with central notch just visible as thin black line

The importance of the NDTT as a reference temperature for the ductile to brittle transition temperature of ferritic steels was established from studies of service failures (Puzak et al, 1954 and 1958) and structurally representative crack arrest tests (Pellini and Puzak, 1963).

The correlation between the structural fracture transition behaviour and NDTT was originally established for 1940s ship steels; however, it was subsequently found that the NDTT could also be applied to reference the fracture transition of fully killed and alloy steels as well as QT quench and tempered steels. Given its importance as a fracture transition reference temperature, correlations between the NDTT and structurally representative test results continue to be sought (Wiesner and Hayes, 1995; Wiesner 1996, Wiesner et al, 1998).

References

  1. C S Wiesner and B Hayes: 'A review of crack arrest tests, models and applications', Proc Conf 'Crack Arrest Concepts for Failure Prevention and Life Extension', Paper 3. Abington, TWI, 27 September 1995.
  2. C S Wiesner: 'Predicting structural crack arrest behaviour using small-scale material characterisation tests', International Journal for Pressure Vessel and Piping, 1996. 69, (2), 185-196.
  3. C S Wiesner, S J Garwood and B Denham: 'The specification of crack arrest properties for storage tanks background and recommendations', ASTM STP 1337, Symposium on Effects of Product Quality and Design Criteria on Structural Integrity, St. Louis, Mo. 5 May 1997. West Conshohocken PA. ASTM, 1998.
  4. Puzak P P, Eschbacher E W and Pellini W S: 'Initiation and propagation of brittle fracture in structural steels'. Weld. J 1952, 31 (12) 561-s to 581-s.
  5. ASTM E208 'Standard test method for conducting drop-weight test to determine NDTT (Nil-Ductility Transition Temperature) of ferritic steels'. American Society for Testing and Materials.
  6. Puzak P P, Schuster M E and Pellini W S: 'Crack starter tests of ship fracture and project steels'. Weld. J. 1954. 33 (10) 481-s to 496-s.
  7. Puzak P P, Babecki A J and Pellini W S: 'Correlations of brittle fracture service failures with laboratory notch-ductility tests'. Weld. J. 1958. 37 (9) 391-s to 410-s.
  8. Pellini W S and Puzak P P: 'Fracture analysis diagram procedures for the fracture-safe engineering design of steel structures'. Welding Res. Counc. Bull. No.88, May 1963.

TWI is UKAS accredited to perform Pellini testing.

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