Brittle FractureDOE-HDBK-1017/2-93BRITTLE FRACTURE MECHANISMStress-TemperatureCurvesOne of the biggest concerns with brittle fracture is that it can occur at stresses well below theyield strength (stress corresponding to the transition from elastic to plastic behavior) of thematerial, provided certain conditions are present. These conditions are: a flaw such as a crack;a stress of sufficient intensity to develop a small deformation at the crack tip; and a temperaturelow enough to promote brittle fracture. The relationship between these conditions is bestdescribed using a generalized stress-temperature diagram for crack initiation and arrest as shownin Figure 2.Figure 2 illustrates that as the temperature goes down, the tensile strength (Curve A) and theFigure 2 Stress-Temperature Diagram for Crack Initiation and Arrestyield strength (Curve B) increase. The increase in tensile strength, sometimes known as theultimate strength (a maximum of increasing strain on the stress-strain curve), is less than theincrease in the yield point. At some low temperature, on the order of 10F for carbon steel, theyield strength and tensile strength coincide. At this temperature and below, there is no yieldingwhen a failure occurs. Hence, the failure is brittle. The temperature at which the yield and tensilestrength coincide is the NDT temperature.Rev. 0Page 3MS-04
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