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How do you apply fracture energy criteria to assess the fatigue and fracture behavior of structures?

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Fatigue and fracture are two common modes of failure for structures subjected to cyclic or dynamic loading. To prevent or mitigate these failures, engineers need to understand the material properties and the stress intensity factors that affect the crack initiation and propagation. One of the methods to assess the fatigue and fracture behavior of structures is to apply fracture energy criteria, which relate the energy dissipated by the crack growth to the applied load and the geometry of the structure. In this article, you will learn how to use fracture energy criteria to evaluate the resistance and toughness of materials and structures under different loading conditions.

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Syed Fadzil Syed Mohamed

Fellow of The Institution of Engineers, Malaysia (FIEM)

1 Fracture energy concept

Fracture energy is the amount of work per unit area required to create a new crack surface in a material. It is also known as the fracture toughness or the critical strain energy release rate. Fracture energy can be expressed as a function of the stress intensity factor, which is a measure of the stress distribution near the crack tip. The stress intensity factor depends on the applied load, the crack size and shape, and the geometry of the structure. The higher the fracture energy, the more resistant the material is to crack growth.

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Syed Fadzil Syed Mohamed

Fellow of The Institution of Engineers, Malaysia (FIEM)
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The fracture energy, calculated from the area under the load-displacement curve, is an important measure in assessing a structure's resistance to crack propagation. Engineers can quantify the material's fracture toughness and forecast when catastrophic failure will occur by using the proper fracture energy criterion. This approach enables preventive maintenance and ensures the structural integrity of crucial components, hence improving safety and dependability in diverse mechanical systems.

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2 Fracture energy criteria

Fracture energy criteria are based on the principle of energy balance, which states that the energy supplied by the external load must equal the energy dissipated by the crack growth and the elastic deformation of the structure. There are different ways to formulate fracture energy criteria, but one of the most widely used is the Griffith criterion, which assumes that the crack grows when the energy release rate equals the fracture energy. Another common criterion is the Irwin criterion, which modifies the Griffith criterion by introducing a plastic zone correction factor to account for the plastic deformation near the crack tip.

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3 Fracture energy testing

Fracture energy testing is a method to measure the fracture energy of a material or a structure under a specified loading condition. There are various types of fracture energy tests, such as single-edge notched bending (SENB), compact tension (CT), double cantilever beam (DCB), and three-point bending (TPB). These tests involve applying a load to a specimen with a pre-existing crack and recording the load-displacement curve. The fracture energy can be calculated from the area under the load-displacement curve using the appropriate fracture energy criterion.

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4 Fracture energy applications

Fracture energy applications are the practical uses of fracture energy criteria to assess the fatigue and fracture behavior of structures. Fracture energy criteria can help engineers to design structures that can withstand cyclic or dynamic loading without failure, or to predict the remaining life of structures that have developed cracks. Fracture energy criteria can also help engineers to select suitable materials for specific applications, or to improve the material properties by introducing reinforcements, coatings, or treatments.

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5 Fracture energy limitations

Fracture energy limitations are the challenges and drawbacks of applying fracture energy criteria to assess the fatigue and fracture behavior of structures. Fracture energy criteria are based on some assumptions and simplifications that may not be valid for all materials and loading conditions. For example, fracture energy criteria may not account for the effects of temperature, environment, loading rate, or crack closure on the crack growth. Fracture energy criteria may also not capture the complex interactions between multiple cracks or between cracks and other defects. Therefore, fracture energy criteria should be used with caution and complemented by other methods and tools.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you apply fracture energy criteria to assess the fatigue and fracture behavior of structures?

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1 Fracture energy concept

2 Fracture energy criteria

3 Fracture energy testing

4 Fracture energy applications

5 Fracture energy limitations

6 Here’s what else to consider

Mechanical Testing

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