Drop Weight Tear Testing
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 60,000 ft-lbs (80 kJ).
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: Mininum 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. Typically, a fractured surface must exhibit a minimum of 85% shear area to be considered a passing test.
DWTT For Transition Temperature of Carbon Steel
ASTM E436 is used to determine the temperature range where carbon and low alloy steels morph from brittle to ductile. This method can be used for 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 Guardrail Box Tubing
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.
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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