Table 2. Mechanical properties of pipe steel API 5L X65 at 20°C. Yield stress Re (MPa) Ultimate strength Rm(MPa) Elongation at failure A % Charpy Energy KCV (J) Fracture Toughness KJc (MP√am) Hardness HV 465.5 558.6 10.94 285.2 280 205 2.1. Transition temperature for elastic to elastoplastic behaviour in tension Tt Tensile tests at very low temperature exhibits brittle fracture and ductile failure at high temperature. At very low temperature, fracture always occurs at yield stress. This phenomenon was proven by compressive tests where no failure occurs, but yield stress is easily determined. When test temperature reaches transition temperature, failure occurs with plasticity at ultimate stress. In other words, transition temperature is defined when ultimate strength is equal to yield stress. Plasticity is a thermal activated process and yield stress decreases exponentially with temperature according to the following relationship: ) ( R R AExp BT e e − = + μ (3) where Re μ is a threshold, A and B are constants and T is temperature in Kelvin. Similarly the ultimate strength decreases to temperature according to: ) ( R R CExp DT m m − = + μ (4) where Rm μ is a threshold, C and D are constants. Tensile tests have been performed on standard specimens in a temperature range [120 - 293 K] with a strain rate of about 10-3s-1 [6]. Stress-strain diagrams have been recorded and the (static) yield stress and ultimate strength determined. Values of yield stress Re, and ultimate strength Rm are reported on Figure 1. Data are fitted with equation (3) and (4). Values of Re μ, Rm μ and constants A, B, C, D are reported in Table 3. Yield stress value at 0K is independent of loading rate and equal to 2320 MPa. This value is generally considered as equal to cleavage stress. Transition temperature is determined as the intersection of two curves at temperature 123 K. Figure 1. Evolution of static and dynamic yield stress and ultimate strength versus temperature for API 5L X65 pipeline steel.
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