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Literature Review on Crack Length Measurement Techniques on Environmental Fracture Toughness Tests

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Literature Review on Crack Length Measurement Techniques on Environmental Fracture Toughness Tests

By Fokion Oikonomidis and Mehran Shahriarifar

Background

There is an industry requirement to generate accurate fracture toughness data in aggressive environments for engineering critical assessments. The use of clip gauges in aggressive environments limits the quality of the test data and the range of tests performed by TWI’s world-leading environmental testing laboratory. On many occasions the severity of the test environment does not allow the use of a clip gauge. Therefore, to maintain a competitive advantage in the field, TWI needs to develop the direct current potential drop approach further and perform a detailed comparison study to properly understand if the load line displacement indirect estimations are within the level of accuracy suggested by the air testing standards. This report presents the findings of a literature review on the topic of crack length measurement techniques for fracture toughness testing in adverse environments.

Key Findings

  • The applicability of the direct current potential drop (DCPD) technique for crack length measurement during fracture toughness testing has been studied by numerous researchers. However, there are very limited published resources on the use of DCPD in aggressive environments such as seawater under cathodic protection (CP).
  • There is high degree of uncertainty in the published literature on the accuracy of determining the fracture toughness at the initiation point of stable crack growth when the specimens are in aggressive environments. This uncertainty exists regardless DCPD or clip gauges for UC are used.
  • Some published work shows that resistance (R-)curve results from all three techniques (DCPD, unloading compliance (UC), and normalisation) are comparable.
  • The main issue associated with DCPD is the offset due to plastic blunting and material deformation. To match the results from DCPD to those from UC and normalisation, curve adjustment methods can be used.
  • Applying the normalisation method recommended in Annex A15 of ASTM E1820 for J-R curve evaluation requires careful consideration of the displacement limit for plasticity functions.

Impact

This project originated from the industry requirement to generate accurate fracture toughness data in aggressive environments. The use of clip gauges is limited in those environments. For that reason the use of DCPD as an alternative method has been considered. The project focuses on the effect of DCPD or UC on R-curves generated in 3.5% NaCl deaerated water solution under CP. Compact tension (CT) and single edge notched bend (SENB) specimens have been extracted from the parent material of a forging F22-75k Low Alloy Steel. The effect of K-rate on fracture toughness results generated in aggressive environments using the DCPD technique is also under investigation. The literature review has been a significant part of this project in order to provide the latest research trends and developments in the topic.

The literature review has highlighted the importance of the normalisation method and the investigation of the accuracy of the DCPD and UC methods near the initiation of crack growth. These two topics have been added as subsequent items in the scope of work of the current project.

 

 

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