After a DWTT sample is broken it is evaluated for percent shear.
Drop Weight Tear Testing - DWTT is used to determine the fracture toughness material properties of seamless or welded line pipe. QTI can test heavy wall material with an energy capacity up to 30,000 ft-lbs (40 kJ). Additional weights could be added to achieve up to 37,000 ft-lbs (50 kJ) of energy.
There are several specifications such as API 5L3, and ASTM E436 that govern the specimen dimensions and how the drop weight test is performed depending on the material properties that which you are trying to determine.
DWTT for Pipeline
In 2011 the DWTT, as part of the testing regime to maintain quality throughout the manufacturing process, made its way into Title 49 of the Code of Federal Regulations which governs the Transportation Of Natural And Other Gas By Pipeline: Minimum Federal Safety Standards, under Part 192, Section 112 (49 CFR 192.112).
API 5L3 is a fracture toughness test used to determine the fracture ductility of a specimen. A set of two specimens are tested per heat number. Upon evaluation, a minimum of 85% shear average must be met to be considered a passing test.
ASTM E436 is The Standard Test Method for Drop Weight Tear Tests of Ferritic Steels, and is used research and development, to determine material suitability for a specific application, or a method of quality control for a manufacturing process.
Sample dimensions in both specifications are essentially identical at 3" W x 12" L with a +/- tolerance. A pressed notch is most commonly used and is the preferred notch. A chevron notch is sometimes employed on high energy material to help reduce the possibility of an inverse fracture, thus an invalid result.
DWTT For Nil-Ductility Transition Temperature of Ferritic Steels
ASTM E208 is the standard test method is used to determine the temperature at which the fracture mode of steel morphs from ductile to brittle when subjected to an impact load. Above the NDT, the steel will have elastic characteristics and will stretch or bend upon impact. Below the NDT, the steel will have become brittle, and therefore crack upon impact. A common application for this test is for steel plates that will used on the hulls of ships designed to be subjected to arctic conditions.
A crack-starter weld bead is placed on the surface that is then notched to provide brittle material which initiates a crack in the base material once the load is applied. The impact load is provided by a guided, free-falling weight with an energy of 250 to 1200 ft-lbf (340 to 1630 J) depending on the yield strength of the steel to be tested. The specimens are prevented by a stop from deflecting more than a few tenths of an inch.
Once the specimens are prepared according to spec, and the samples must be conditioned to a temperature estimated near the NDT temperature. Upon completion of the first test, the remaining specimens are conducted in subsequent temperature intervals of 10°F (5°C) for break and no-break performance.
A501 box tubing was commonly used as highway barrier guardrails. When it came into short supply, A500 tubing was used as an alternative. However, it was discovered that the material became brittle in prolonged cold temperatures due to the galvanizing process used to protect the steel from corrosion in the field. Therefore, a modified version of Drop Weight Tear Test (ASTM E436) was first proposed as a "highly recommended" practice in "A Guide To Standardized Highway Barrier Rail Hardware" written in 1979 by the AASHTO-AGC-ARTBA joint committee, to determine if each lot exhibits acceptable fracture toughness.
Since then, Drop Weight Tear Testing of galvanized guardrail tubing made from ASTM A500, Cold-Formed Structural Tubing for certain highway barrier and bridge rail designs has been adopted as a requirement by many state Departments of Transportation across the country. Due to the size constraints of the tube, such that a standard DWTT specimen cannot be achieved, a 2" W x 9" L specimen supported on a 7 inch span is the specified requirements to perform this test in the modified condition.
NYDOT spec 710-23
Calibrated Video Measurement System
Upon completion of the test, analysis of the broken sample is generally performed using a visual inspection method. However, QTI employs a high definition video measurement system that accurately measures the fracture surface. The data is then input into a spreadsheet to calculate the percent shear. This reduces the likelihood of an error being made by "eye-balling" the broken sample.
Typically the charpy impact test is also performed and paired with drop weight tear test data so that a comprehensive picture of the correlation between shear fracture appearance and temperature is provided.
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