What is the drop-weight tear test?

The drop weight tear test (DWTT), specified in API RP 5L3 or ASTM E436, was developed in the early 1960s at the Battelle Memorial Institute, USA, to overcome some limitations of the 'Pellini' drop-weight test. Drop weight tear testing is a material characterisation test aimed at avoiding brittle fracture and ensuring crack arrest in pipelines (seamless or welded).

The DWTT specimen is illustrated in Fig.1. The specimen is of the full material thickness (but up to 19mm only), contains a shallow pressed notch and is loaded in three point bending. A series of specimens is broken under impact loading at a variety of temperatures and the proportions of ductile fracture (shear) and cleavage on the fracture surfaces are measured. In this way, a transition curve of percentage shear versus temperature is constructed for the material, see Fig.2.

From correlations with full-scale pipe burst tests, a transition temperature corresponding to between 70 and 80 percent shear is normally stipulated in application standards at or below the minimum operating temperature together with a supplementary minimum Charpy energy requirement to avoid low tearing energy materials. Successful service of pipelines has validated this approach for the avoidance of brittle fracture in pipelines (Eiber, 1965).

The successful service record of pipelines qualified using the test and the less than perfect correlation of Charpy properties with crack propagation and arrest behaviour supports its continued use as a qualification test for pipeline material.

References

1. Eiber R J: 'Correlation of full-scale tests with laboratory tests'. Proc. AGA Symposium on line-pipe research. American Gas Association, Dallas, 1965, pp83-118.
2. API RP 5L3: 'Recommended practice for conducting drop weight tear tests on line pipe', American Petroleum Institute.
3. ASTM E436 'Standard test method for drop weight tear tests of ferritic steels', West Conshohocken, PA. American Society for Testing and Materials.
4. Willoughby A A: 'Crack Arrest Concepts: Part 1 and 2'. TWI Research Bulletin, Vol.28, (1987), pp.235-240 and 275-277.