Introduction to Crack Growth
Understanding Crack Initiation and Progression in Engineered Parts
May 13, 2009
Cracks can occur naturally in engineered components due to the combination of environmental effects and material and geometric properties. Crack growth can be difficult to predict and measure because it is difficult to predict or measure the local stresses at the crack tip.
However, there is a great body of data that can be used to assist with crack growth prediction, and technology does exist to measure crack lengths with a reasonable degree of accuracy.
Crack Initiation
The most common reasons for crack initiation in a component include:
- Notches, corners, or other geometric inconsistencies in the component
- Material inclusions, impurities, defects, or material loss due to wear or corrosion
- Mechanical or thermal fatigue
Once a crack has been initiated, repeated loadings can cause the crack to lengthen. Depending on the stress level and the number of load cycles applied, crack growth can be stable, with a predictable rate; unstable, with imminent failure; or occasionally a decrease or cessation of growth altogether.
Crack growth can be encouraged by factors other than applied load. Environmental effects, such as corrosion due to exposure to water or other solvents, as well as component wear can result in an overall loss of material in the component cross section, increasing the stress carried by the remaining material. This increased stress can accelerate crack growth and ultimate failure.
Material Type and Crack Growth Prediction
Homogeneous materials such as steel or aluminum have relatively predictable crack growth rates that have been verified through extensive material coupon tests. Based on the current crack size and the stress per load cycle, the remaining life of the part can be estimated using reference graphs generated through coupon testing.
However, crack growth in composite materials, such as fiberglass, are far more difficult to predict. Crack growth can depend on the materials in the composite, orientation of the materials in the matrix, orientation of multiple layers in the matrix, etc. Crack growth in composite materials can also be influenced by other damage mechanisms, such as delamination, that generally do not occur in homogeneous materials.
Detecting and Measuring Cracks
Cracks are difficult to defect when they initiate because cracks begin at the microscopic level. There are a variety of non-destructive inspection (NDI) methods that can be used to detect and measure cracks. The most successful NDI method for a specific application will depend on the material to be examined, crack dimensions, orientation, and location in the part; and overall part geometry. Some of the NDI methods used for crack detection and measurement include dye penetrants, x-ray, acoustic methods, magnetic-based systems, or microwaves.
Cracks can cause catastrophic damage to engineered systems if not detected and monitored before they grow to a critical length. By understanding the properties of the material involved, engineers can predict crack initiation and repair cracked components before failures occur.
Sources
Beer, F., Johnston, E.R., Mechanics of Materials, Second Edition, McGraw-Hill, 1992.
Krieth, F. (ed.), The CRC Handbook of Mechanical Engineering, CRC Press, 1998.
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