Definition of Brittle Fracture
Brittle fracture is a sudden, often catastrophic failure which is inherent to “brittle” materials (discussed below). It involves little or no deformation (yielding), and it has been experienced in steel structures such as pressure vessels, tanks, pipes, ships, bridges, beams, and other similar structures, often of welded construction. The most well known incidents were those occurring to over 200 ships constructed during World War II, 19 of which completely broke in two.
The material property “Brittleness” indicates that the material is prone to failure without deformation. Examples of brittle materials include chalk, brick, glass, and hardened steels. Brittle structures can, and literally have shattered like glass. Brittle materials are prone to fracture when they are stressed in the vicinity of a notch or stress concentration.
The opposite of brittleness is toughness, which for practical purposes can be defined as a material’s ability to resist brittle fracture. Toughness is discussed in more detail in Section 523. Toughness depends on material strength, thickness, and for steels, temperature. To resist brittle fracture, higher strength materials and thick materials require greater toughness than low strength and thin materials. Steels lose toughness as temperature decreases.
Brittle fracture can occur in ferritic steels, such as carbon, carbon-½ moly, chromemoly, and 400 series stainless steels, within the normal atmospheric temperature range. The regular 300 series stainless steels are not susceptible to brittle fracture until temperatures are below -300°F. However, after exposure above 1100°F, sigma embrittlement makes 300 series weld metals with large amounts of ferritic phase susceptible to fracture well above room temperature.
Brittle fractures are infrequent. Most occur during hydrotest rather than in operation. However, brittle fractures can be catastrophic due to fragmentation of the structure and fast release of energy. Due primarily to higher quality construction and maintenance standards, our Company has rarely experienced brittle fractures. However, to illustrate the importance of this design factor, two incidents of brittle fracture experienced by the Company are described below.
Brittle fracture is characterized by a flat fracture surface, and occurs at average stress levels below those of general yielding. Brittle fracture cracks grow at very fast speeds (up to 7000 ft/s), so brittle fracture happens quickly and unexpectedly.
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