materials and is important in analysis of manufacturing operations involving deformation(see, for example, Forming and Forging, Volume 14, ASM Handbook). The amount of distortion that can occur at high rates of loading is difficult to analyze or predict precisely because The variation, or scatter, among replicate tests of mechanical properties is greater when strain rates are high than it is when strain rates are low Impulse or impact loading involves the propagation of high-velocity stress waves through the structure, which may not be well quantified, with the most extreme example being ballistic impact with strain rates on the order of 10/min Impulse loading creates an adiabatic condition that causes a local increase in temperature Effect of Temperature. Distortion failures caused by overload can occur at any temperature at which the flow strength of the material is less than the fracture strength. In this discussion, flow strength is defined as the average true stress required to produce detectable plastic deformation caused by a relatively slow, continuously increasing application of load fracture strength is the average true stress at fracture caused by a relatively slow, continuously increasing application of load. The flow strength and fracture strength of a material are temperature dependent, as is the elastic modulus(Youngs modulus, bulk modulus, or shear modulus) Figure 5 illustrates this temperature dependence schematically for polycrystalline materials that do not undergo a solid- state transformation. Two flow strengths are shown: one for a material that does not have a ductile-to-brittle transition in fracture behavior, such as metal with a face-centered cubic(fcc)crystal structure, and one for a body-centered cubic(bcc) material that exhibits a ductile-to-brittle transition modulus Fracture strength Ow a strength(bcc materials) F strength(fcc materials) Homologous temperature Fig 5 Diagram of the temperature dependence of elastic plastic, and fracture behavior of polycrystalline materials that do not exhibit a solid-state transformation. bcc, body- centered cubic; fcc, face-centered cubic; T, instantaneous absolute temperature; TM, absolute melting temperature of the material As shown in Fig. 5, the flow strength, fracture strength, and elastic modulus of a material generally decrease as temperature increases. If a structure can carry a certain load at 20C(70F), it can carry the same load without deforming at lower temperatures. Stressed members made of materials having a ductile-to-brittle fracture transition will sometimes fracture spontaneously if the temperature is lowered to a value below the transition temperature(e.g, see the article Overload Failures"in this volume) If the temperature is increased so that the flow strength becomes lower than the applied stress, a structure may deform pontaneously with no increase in load. a change in temperature may also cause an elastic-distortion failure because of a change in modulus, as might occur in a control device where accuracy depends on a predictable elastic deflection of a ontrol element or a sensing element. For most structural materials, the curve defining the temperature dependence of elastic and plastic properties is relatively flat at temperatures near 20C(70F). For steels, the modulus is slightly decreasing until temperatures of approximately 320 to 370C(600 to 700F), are reached, at which point modulus starts to decrease more rapidlymaterials and is important in analysis of manufacturing operations involving deformation (see, for example, Forming and Forging, Volume 14, ASM Handbook). The amount of distortion that can occur at high rates of loading is difficult to analyze or predict precisely because: · The variation, or scatter, among replicate tests of mechanical properties is greater when strain rates are high than it is when strain rates are low. · Impulse or impact loading involves the propagation of high-velocity stress waves through the structure, which may not be well quantified, with the most extreme example being ballistic impact with strain rates on the order of 104 /min. · Impulse loading creates an adiabatic condition that causes a local increase in temperature. Effect of Temperature. Distortion failures caused by overload can occur at any temperature at which the flow strength of the material is less than the fracture strength. In this discussion, flow strength is defined as the average true stress required to produce detectable plastic deformation caused by a relatively slow, continuously increasing application of load; fracture strength is the average true stress at fracture caused by a relatively slow, continuously increasing application of load. The flow strength and fracture strength of a material are temperature dependent, as is the elastic modulus (Young's modulus, bulk modulus, or shear modulus). Figure 5 illustrates this temperature dependence schematically for polycrystalline materials that do not undergo a solid￾state transformation. Two flow strengths are shown: one for a material that does not have a ductile-to-brittle transition in fracture behavior, such as metal with a face-centered cubic (fcc) crystal structure, and one for a body-centered cubic (bcc) material that exhibits a ductile-to-brittle transition. Fig. 5 Diagram of the temperature dependence of elastic, plastic, and fracture behavior of polycrystalline materials that do not exhibit a solid-state transformation. bcc, body￾centered cubic; fcc, face-centered cubic; T, instantaneous absolute temperature; TM, absolute melting temperature of the material As shown in Fig. 5, the flow strength, fracture strength, and elastic modulus of a material generally decrease as temperature increases. If a structure can carry a certain load at 20 °C (70 °F), it can carry the same load without deforming at lower temperatures. Stressed members made of materials having a ductile-to-brittle fracture transition will sometimes fracture spontaneously if the temperature is lowered to a value below the transition temperature (e.g., see the article “Overload Failures” in this Volume). If the temperature is increased so that the flow strength becomes lower than the applied stress, a structure may deform spontaneously with no increase in load. A change in temperature may also cause an elastic-distortion failure because of a change in modulus, as might occur in a control device where accuracy depends on a predictable elastic deflection of a control element or a sensing element. For most structural materials, the curve defining the temperature dependence of elastic and plastic properties is relatively flat at temperatures near 20 °C (70 °F). For steels, the modulus is slightly decreasing until temperatures of approximately 320 to 370 °C (600 to 700 °F), are reached, at which point modulus starts to decrease more rapidly