In fcc materials, and in bcc materials at temperatures above the transition temperature but at lower homologous temperatures(<0.3 TM), distortion(net section yielding) always accompanies overload fracture in sections that do not contain a severe stress raiser. At higher homologous temperature, an increase in temperature may cause a change from transgranular(TG)to intergranular (IG) fracture with a concurrent decrease in ductility. That is, there is a minimum ductility at elevated temperature(within the"creep"range)where the fracture mechanism changes from TG to IG fracture with a concurrent loss in ductility Creep, or time-dependent strain, is a relatively long-term phenomenon and can be distinguished from overload distortion by relating the length of time at temperature to the amount of distortion, as discussed in the article " Creep and Stress Rupture Failures"in this Volume. The specific mechanisms and associated kinetics of creep are temperature dependent While creep is sometimes considered to be limited to temperatures above one- half of the absolute melting point and is usually associated with an intergranular mechanism at those temperatures, it is important for the failure analyst to know that long-term creep deformation and even fracture can also occur at lower temperatures and via other mechanisms Changes in operating temperature can affect the properties of a structure in other ways. At temperatures higher than about 30 to 40% of TM depending on the alloy, microstructural changes may occur over time and degrade properties, allowing distortion and even fracture to occur. For example, if a martensitic steel is tempered at a given temperature and then encounters a higher temperature in service, yield strength and tensile strength will decrease because of overtempering Long-time exposure to moderately elevated temperatures may cause overaging in a precipitation-hardening alloy, with a corresponding loss in strength. It is well known to metallurgists that exposure to cryogenic temperatures may cause cracking in a martensitic steel due to the volume change accompany ing the transformation of retained austenite. What may not be as well appreciated is that even if cracking does not occur, the transformation may create a distortion failure due to dimensional growth or warpage in a close-tolerance assembly, such as a precision bearing When the temperature is changed, different coefficients of thermal expansion for different materials in a heterogeneous structure can cause interference between structural members(or can produce permanent distortion because of thermally induced stresses if the members are joined together). The failure analyst must understand the effect of temperature on properties of the specific materials involved when analyzing failures that have occurred at temperatures substantially above or below the design or fabrication temperature Effect of Stress Raisers and Complex Stress States. In sections that do contain stress raisers, net section yielding can still occur if the state of stress is plane stress; that is, one normal stress component is 0. If the stress raiser results in sufficient constraint to produce plane strain, then gross yielding and distortion will not be observed. However, localized distortion may accompany crack extension as the inherent ductility of the material manifests itself. At the microscale, the fracture may or may not show evidence of ductility. That is, the material may be microscale ductile or brittle. If it is microscale ductile, there may still be no evidence of ductility at the macroscale. In inherently brittle materials, where the fracture stress is equal to the flow stress, no gross or localized distortion accompanies fracture, as discussed further in the article Fractures Appearance and Mechanisms of Deformation and Fracture "in this Volume The yield stress is generally taken as the critical value at which plastic deformation initiates. In uniaxial tension, it is clear when the applied stress reaches the yield point. However, for more complex multiaxial stress states, the point at which yield is anticipated may not be as clear. Theories such as maximum shear stress, maximum distortion energy, and others are detailed in Ref 4 and may be applied by the failure analyst if there is uncertainty as to whether the stresses known to be applied were sufficient to cause the distortion observed References cited in this section J W. Jones, Limit Analysis, Mach. Des., Vol 45(No. 23 ), 20 Sept 1973, p 146-151 2. D. Goldner, Plastic Bending in Tubular Beams, Mach. Des., Vol 45(No. 24), 4 Oct 1973, p 152-155 3. D.J. Ulpi, Understanding How Components Fail, 2nd ed, ASM International, 1999, p 16-19 4. J.A. Collins, Failure of Materials in Mechanical Design, 2nd ed, wiley and Sons, 1993 Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Inappropriate Specifications Thefileisdownloadedfromwww.bzfxw.comIn fcc materials, and in bcc materials at temperatures above the transition temperature but at lower homologous temperatures (<0.3 TM), distortion (net section yielding) always accompanies overload fracture in sections that do not contain a severe stress raiser. At higher homologous temperature, an increase in temperature may cause a change from transgranular (TG) to intergranular (IG) fracture with a concurrent decrease in ductility. That is, there is a minimum ductility at elevated temperature (within the “creep” range) where the fracture mechanism changes from TG to IG fracture with a concurrent loss in ductility. Creep, or time-dependent strain, is a relatively long-term phenomenon and can be distinguished from overload distortion by relating the length of time at temperature to the amount of distortion, as discussed in the article “Creep and Stress Rupture Failures” in this Volume. The specific mechanisms and associated kinetics of creep are temperature dependent. While creep is sometimes considered to be limited to temperatures above one-half of the absolute melting point and is usually associated with an intergranular mechanism at those temperatures, it is important for the failure analyst to know that long-term creep deformation and even fracture can also occur at lower temperatures and via other mechanisms. Changes in operating temperature can affect the properties of a structure in other ways. At temperatures higher than about 30 to 40% of TM depending on the alloy, microstructural changes may occur over time and degrade properties, allowing distortion and even fracture to occur. For example, if a martensitic steel is tempered at a given temperature and then encounters a higher temperature in service, yield strength and tensile strength will decrease because of overtempering. Long-time exposure to moderately elevated temperatures may cause overaging in a precipitation-hardening alloy, with a corresponding loss in strength. It is well known to metallurgists that exposure to cryogenic temperatures may cause cracking in a martensitic steel due to the volume change accompanying the transformation of retained austenite. What may not be as well appreciated is that even if cracking does not occur, the transformation may create a distortion failure due to dimensional growth or warpage in a close-tolerance assembly, such as a precision bearing. When the temperature is changed, different coefficients of thermal expansion for different materials in a heterogeneous structure can cause interference between structural members (or can produce permanent distortion because of thermally induced stresses if the members are joined together). The failure analyst must understand the effect of temperature on properties of the specific materials involved when analyzing failures that have occurred at temperatures substantially above or below the design or fabrication temperature. Effect of Stress Raisers and Complex Stress States. In sections that do contain stress raisers, net section yielding can still occur if the state of stress is plane stress; that is, one normal stress component is 0. If the stress raiser results in sufficient constraint to produce plane strain, then gross yielding and distortion will not be observed. However, localized distortion may accompany crack extension as the inherent ductility of the material manifests itself. At the microscale, the fracture may or may not show evidence of ductility. That is, the material may be microscale ductile or brittle. If it is microscale ductile, there may still be no evidence of ductility at the macroscale. In inherently brittle materials, where the fracture stress is equal to the flow stress, no gross or localized distortion accompanies fracture, as discussed further in the article “Fractures Appearance and Mechanisms of Deformation and Fracture” in this Volume. The yield stress is generally taken as the critical value at which plastic deformation initiates. In uniaxial tension, it is clear when the applied stress reaches the yield point. However, for more complex multiaxial stress states, the point at which yield is anticipated may not be as clear. Theories such as maximum shear stress, maximum distortion energy, and others are detailed in Ref 4 and may be applied by the failure analyst if there is uncertainty as to whether the stresses known to be applied were sufficient to cause the distortion observed. References cited in this section 1. J.W. Jones, Limit Analysis, Mach. Des., Vol 45 (No. 23), 20 Sept 1973, p 146–151 2. D. Goldner, Plastic Bending in Tubular Beams, Mach. Des., Vol 45 (No. 24), 4 Oct 1973, p 152–155 3. D.J. Wulpi, Understanding How Components Fail, 2nd ed., ASM International, 1999, p 16–19 4. J.A. Collins, Failure of Materials in Mechanical Design, 2nd ed., Wiley and Sons, 1993 Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Inappropriate Specifications The file is downloaded from www.bzfxw.com