Effect of microstructure on fracture toughness on industrial alloys

• Main Author: Wee, Benjamin William Wei Wen.
• Other Authors: Seow Hong Pheow
• Format: Final Year Project
• Language: English
• Published: 2011
• Subjects: DRNTU::Engineering::Materials::Composite materials
• Online Access: http://hdl.handle.net/10356/43938
• Institution: Nanyang Technological University

This report will cover the steps taken, the tests conducted and the equipments used to investigate the effect of microstructure on fracture toughness on API-5L. This is a widely used pipeline material for the transportation of combustible fluids. In ensuring that the minimal mechanical requirements are met, the properties of the material are evaluated through various mechanical tests. Existing theories have been developed to estimate the fracture toughness of the material through indirect mechanical tests, such as Vickers Hardness test and Charpy Impact test, instead of performing Crack-Tip Opening Displacement or other forms of fracture toughness tests. Due to the complex nature of fracture phenomena, there has yet to be unification among the different existing theories. API-5L tests specimens were heat treated for 30 minutes before being cooled at various rates. The microstructure of the material was evaluated using the scanning electron microscope before proceeding onto further mechanical tests which included Vicker’s Hardness Test and Charpy Impact Test. The effect of microstructure on fracture toughness on API-5L was further evaluated through Crack-Tip Opening Displacement Test. Results indicated that the different microstructures obtained through different cooling rates had varying influence over the material’s hardness, toughness and fracture toughness. At temperatures below 925°C, the microstructure of the material is stable. Existing theories of “Lawn, Evan and Marshall” and “Barson and Rofle” were also re-evaluated with the test values obtained from this project. High hardness does not essentially result with low toughness and high toughness does not essentially result with high fracture toughness in the material. To substantiate, estimations of fracture toughness through indirect mechanical tests do not essentially provide accurate results on the material’s true properties.