Inspectioneering: Brittle Fracture Detection

Brittle fracture is characterized as the sudden rapid fracture of a component subject to stress (either residual or applied), where the material exhibits little-to-no evidence of ductility or measurable plastic deformation [1,2]. A running fracture can occur at extremely high speeds (as high as 7,000 feet/second in some steels) and is usually distinguished by a flat cleavage fracture surface with little-to-no shear lips. Furthermore, rapid fracture can occur at average stress levels (well below the expected yield strength of the material) [3]. While historically, the overall number of documented cases of structures that have failed due to brittle fracture is relatively low compared to other failure modes such as fatigue, gross overload/yielding, or buckling, brittle fracture failures often have catastrophic consequences and pose a serious health and safety risk given their sudden nature. In the past, brittle fractures have occurred in all types of engineering structures, such as storage tanks, pressure vessels, ships, bridges, aircraft structures, and even buildings [3]. Even today, despite the widespread use and acceptance of modern fracture mechanics principles and more damage-tolerant design and fabrication practices, engineering structures and, in particular, pressure-retaining equipment may still be at risk for brittle fracture failure (or possibly ductile tearing). While it is neither practical nor feasible to detect a brittle fracture itself before it occurs, identifying crack like flaws, particularly at/near welds or other geometric discontinuities, represents the primary line of defense in quantifying potential failure risk since unstable fractures initiate at smaller defect locations. To this end, as highlighted herein, understanding the importance of reliably and accurately detecting and characterizing crack-like flaws in pressure equipment is paramount.