Experimental Study of Charpy Impact Characteristics of High-Strength Spiral Welded Gas Pipeline

Charpy upper shelf energy is widely used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. The measurement of this fracture criterion particularly for modern steels and its apportion into different components (i.e. fracture and non-related fracture energy) are of great importance for pipeline engineers in order to transfer laboratory data from Charpy experiment to real structure. As the conventional Charpy impact test has only one output (i.e. the overall fracture energy) the instrumented test has been used to achieve full failure information from impact test samples. In this paper the results of instrumented Charpy impact experiments on high-strength spiral welded pipeline steel of grade API X70 are presented. First, the instrumentation technique including the design and implementation of a strain gauge load-cell and the details of the data-recording scheme are reviewed. Next, the experimental data obtained from the Charpy impact machine so instrumented are given. These include test data obtained at room temperature from different sets of standard full size Charpy V-notched specimens taken from the pipe material, seam weld and heat affected zone (HAZ). The instrumented Charpy machine was able to capture the load history in full during the fracture process of the test specimens resulting in a smooth load-time response. This eliminated the need for filtering used in similar test techniques. From the recorded test data the hammer displacement, impact velocity and fracture energy were numerically calculated. The numerical results showed good agreement between the instrumentation data and those read from dial indicator. From fracture energy plots it was found that the maximum and minimum fracture energy were associated with the pipe material and seam weld (in average), respectively. In all test samples a significant amount of energy was consumed in non-related fracture processes including crack initiation, bending and gross deformation of test specimen, and indentation at the support anvils and at the impact point. This non-related fracture energy should be accounted for if the current failure models are going to be used for toughness assessment of high-strength low-alloy gas pipeline steels.Copyright © 2006 by ASME