TESTING 1-2-3
DYNAMIC FRACTURE TOUGHNESS OF HIGH STRENGTH CAST STEELS
The dynamic fracture toughness of quenched and tempered chromium and molybdenum steels was evaluated with regard to deoxidation practice and compared to a lightweight Fe-Mn-Al-C steel.
T brittle fracture. What are those factors? 1
2 3
Procedure—Four cast steels were tested and compared to study the effect of material composition and different deoxidation practices.
Results and Conclusions—The yield and tensile strength of chromium and molybdenum steels were relatively uniform, whereas deoxidation practice had a
signifi cant infl uence on ductility. AN MCDP STAFF REPORT
he fracture toughness of high strength steel can be characterized using either stress intensity or a J-integral approach to fracture mechanics. For steels with low toughness where cleavage fracture is dominant, little plastic deformation occurs around the crack tip and a linear elastic fracture mechanics (LEFM) approach to toughness (KIc) is applicable. However, for high energy absorbing materials that have a great deal of ductil- ity, LEFM is no longer valid and failure is governed by
plastic fl ow around the crack tip. Such materials exhibit crack tip blunting, and as such, toughness is characterized by an elastic-plastic fracture mechanics (EPFM) approach by calculating toughness using the critical value of the J-integral near the
ADDING IT ALL UP
Breaking down the industry’s latest research papers is as easy as 1-2-3: “Dynamic Fracture Toughness of High Strength Cast Steels” L. Bartlett, A. Dash, D. Van Aken, V. Richards and K. Peaslee, Missouri Univ. of Science and Technologhy, Rolla, Mo. Background—The dynamic fractue toughness of a material under high loading rates is often diffi cult to determine. Many different metallurgical factors give cast steels the ability to resist
onset of stable crack propagation (JIc). Fracture toughness often depends
on loading rates. Loading rates of less than 2 MPa√m/s are typical for evalu- ating static toughness using KIc and JIc. However, to qualify high strength steels for high energy absorbing ap- plications, knowledge of the dynamic fracture toughness (JId), is required. Determination of dynamic fracture toughness under high loading rates is often a challenge because crack exten- sion during impact loading is diffi cult to measure. Instrumented Charpy impact tests on fatigue pre-cracked specimens provide a reproducible way of measuring the time dependency of force and crack displacement at elevated loading rates and thus, pro- vides a method for measuring dynamic fracture toughness. Researchers from the Missouri
Univ. of Science and Technology in Rolla, Mo., are currently developing and characterizing high strength cast steels for high energy absorbing ap- plications. In a recent study, dynamic fracture toughness was determined for four cast steels with regard to microstructure, cleanliness and heat treatment. Team members included Laura Bartlett, Abhilash Dash, David Van Aken, Von Richards and Kent Peaslee.
Jul/Aug 2012 | METAL CASTING DESIGN & PURCHASING | 45
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