Cooling tensile yield stress

During machining, the material subsurface is heated temporarily to high temperatures. By this the surface-near layers expand as a result of thermal compressive stress. The surface-near layers deform plastically in a state of reduced yield strength. After cooling down to room temperature, tensile residual stresses result. As in machining both mechanical and thermal influences act concurrently, they interfere with each other strongly nonlinear. A simple superposition is not... 

Effect of 2 h annealing (or stress relieving) temperature on room-temperature tensile properties of full hard (50% odd worked) tubing. With a room-temperature full hand strength of 999 MPa (145 ksl) UTS and 896 MPa (130 ksl) tensile yield strength. 15.8 mm (0.625 in.) OD X 0.96 mm (0.038 in.) wall 50% CW+anneal, 2 h in vacuum, vacuum cool to approximate 425 °C (800 °F), AC. 

The thennal stresses exhibit an maximal value on the particle-matrix boundary, acted by the radial stress, pb (see Eq. (7)). Accordingly, the presented calculation is considerable in the temperature range T (7, 7 ), where Tc is the critical final temperature of a cooling process at which the stress, pb = yield stress in compression or tension related to the compressive or tensile stress, pb > 0 or pb < 0,... 

Copolymer test specimens were prepared by pressing slabs for 0.5-3 min at temperatures approximately 20°C above the copolymer melting point, followed by cooling under pressure to room temperature in 5-10 min. Test samples were conditioned at 24°C and 50% relative humidity for at least 48 hr before testing. Stress-strain and tear-strength measurements were made on 0.6 to 0.8-mm-thick specimens. Thicker specimens yield lower values for tensile strength and elongation at break. The test methods used were ... 

Upon re-heating, regions of tensile stress may develop in the TGO. With an increase in the number of thermal cycles, the shear stresses induced in the bond coat upon cooling exceed the bond coat yield strength, so that it flows plastically from valleys to peaks at the undulation sites, allowing the undulation amplitude to increase (Fig. 11). As a result, the normal tensile stresses increase and ultimately lead to separation of the TGO [69]. 

Special attention must be paid to the shape and surface of specimens because polimiers are very brittle at low temperatures. In tensile tests, circular specimens yield the best results (see Fig. 10). To avoid internal thermal stresses during cooling, specimens should not be too large. 

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