Flaws are inevitable in real parts, therefore as part of damage tolerant design, parts need to be engineered so that flaws do not necessarily result in catastrophic failure. In structural engineering, critical flaw size refers to the maximum size of a flaw that can exist under a given loading before immediate catastrophic failure may occur. In some NDE literature, the term "critical flaw size" may be used instead to refer to the reliably detectable flaw size. While the goal is to have no flaws in materials, during manufacturing there will be naturally occurring flaws of some size introduced into the materials, and these flaws will be able to grow. These flaws will be very small, so NDE can help determine a bound on the size of flaws likely in the material. Using crack growth estimates, NDE inspection intervals and procedures need to be designed to prevent flaws from ever reaching the critical flaw size.
Critical flaw size is determined based on the fracture toughness, geometry, and expected loading of a material. The critical flaw size is determined from experimental testing and fracture mechanics principals. Once the critical flaw size is determined, the next goal is to determine the amount of use a system can safely experience before any flaws reach critical flaw size. Fracture mechanics analysis can estimate the crack growth rate of flaws in current state of the material and therefore approximate the number of loading cycles required to grow these flaws to the critical size. The number of loading cycles until potential failure and the rate of load cycling for the system can be a basis for the amount of time until the next inspection is necessary. Due to the importance of catching flaws before they can reach their critical size, at least one NDE inspection is required before the estimated time that critical flaw size will be reached. Flaws in the current state of the material may be based on an assumed, statistically defined flaw size if none have been observed yet.
References and Resources
- Anderson, T.L. (2017). Fracture Mechanics: Fundamentals and Applications (4th ed.). CRC Press.
- NASA-STD-5009B, Nondestructive Evaluation Requirements for Fracture-Critical Metallic Components, 05/08/2019, https://standards.nasa.gov/standard/nasa/nasa-std-5009
