Local tensile stress as a fracture criterion for brittle crack propagation in steels

Significance 

Structural components are susceptible to various failures. Among the numerous causes of material failures, the formation of cracks has recently attracted significant attention of researchers. To this note, several methods have been developed to investigate fracture mechanics in materials and especially in steel. It helps in the evaluation of the propagation of cracks which is a key consideration in structural design and optimization. Presently, fracture mechanics emphasizes mostly on cleavage crack propagation and arrest behaviors in steel materials.

Unfortunately, the relationship between cleavage crack propagation and arrest and material resistance have not been fully explored. This is of great significance because the critical conditions in arrest behavior and crack propagation are mandatory requirements in the development of steel fracture with enhanced crack arrest toughness.

Several methods such as energy balance approaches have been devised to explain the cleavage crack propagation and arrest behavior in materials. Unfortunately, they are not suitable for steels. Therefore, researchers have been looking for alternatives and effective methods favorable for steels and have identified local fracture stress criterion as a promising solution. It takes into consideration the correlation between material resistance and the continuous crack propagation. Unfortunately, this approach consists of various limitations that result in result discrepancies.

For instance, it takes a cleavage crack as a two-dimension problem whereas for steels, crack propagation is highly influenced by three-dimensional effects. In addition, there is a need to accurately evaluate the local tensile stresses since the three-dimensional effects are capable of changing stresses states rapidly. However, it is difficult to experimentally observe the three-dimensional effects during the crack propagation in steels.

The University of Tokyo Researchers Dr. Fuminori Yanagimoto, Dr. Kazuki Shibanuma, Dr. Shuji Aihara, Dr. atsuyuki Suzuki and Dr. Toshiyuki Matsumoto investigated the local tensile stress in steels through a combination of experiments and finite element analysis. They used side-grooved specimen to eliminate the three-dimensional effects thus reducing the crack propagation to a two-dimensional problem. Local tensile stresses were determined from the measured crack velocities. Furthermore, dynamic stresses intensity factors were also evaluated from the experimental results. The work is published in the research journal, Materials and Design.

From the experimental results, the authors observed that the local tensile stresses were almost constant for all the steel specimens. On the other hand, the dynamic stress intensity factors varied significantly with changes in temperatures, crack length and crack velocities. Therefore, the authors postulated that the dynamic stress factors were negligible for cleavage crack propagation and arrest behavior in steels. The experimental results were consistent with those available in the literature thus indicating the feasibility of the study.

The study is the first to evaluate the local tensile stress in steel by a combination of experimental and numerical analysis where the local stresses represent the material resistance to the crack propagation. As a significant contribution of the study, the local fracture stress criterion is regarded as a key consideration in describing the cleavage crack propagation and arrest behavior in steels. Therefore, it will advance fracture mechanics that will, in turn, lead to better performance materials for different applications.

Local tensile stress as a fracture criterion for brittle crack propagation in steels - Advances in Engineering

About the author

Kazuki Shibanuma is an Associate Professor at the School of Engineering, The University of Tokyo. He obtained his BSc, MSc and PhD at Kyoto University, in 2006, 2007 and 2010 respectively. He is also a Visiting Academic at Imperial College London since 2018.

His research covers a wide range of fracture phenomena of solids and structures such as fatigue, brittle fracture, ductile fracture and creep. He is an author of over 50 peer-reviewed journal publications at the moment.

His latest achievements can be found in ResearchGate

Reference

Yanagimoto, F., Shibanuma, K., Suzuki, K., Matsumoto, T., & Aihara, S. (2018). Local stress in the vicinity of the propagating cleavage crack tip in ferritic steel. Materials & Design, 144, 361-373.

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