13th International Conference on Fracture June 16-21, 2013, Beijing, China -At the first 5s, the applied stress on the specimen was constant and equal to the mean stress σm. The mean stress being small compared to the material yield stress, creep and associated dissipated energy is negligible. Thus, during these 5s, the intrinsic dissipative sources d1 of both specimen and dummy were assimilated to zero. The measured thermal drifts are due to the convection (loaded specimen and dummy) or conduction (loaded specimen) and represent the initial conditions. -At the second 15s, the specimen was loaded with a stress such as σ=σm+σacos(2πft), where σa is the alternate stress amplitude and f is the loading frequency (f=14Hz). The temperature fields acquisition was fromt=5s and lasted 15s (210 cycles). The temperature field variations acquired due to the intrinsic thermo-mechanical sources. The alternative temperature variation are due to the thermo-elastic sources Sth (thermoelastic coupling effect) while the mean temperature increase is due to the dissipative sources d1. 0 5 10 15 −0.2 −0.1 0 0.1 0.2 0.3 0.4 Time(s) Temperature variation( oC ) Constant stress σ=σ m Loading start Figure 3: Temperature variation during a temperature field measurement under Rσ=0.2, σmax=380MPa, 316L specimen. The measured temperature fields were assumed to be representative of the material temperature through the specimen thickness, and the thermo-mechanical sources d1 and Sth were estimated using the local energy balances equations. By combining the balance energy equations before and after the start of the the loading on both loaded specimen and dummy, the following energy balance equation was obtained: ρC( ∂θ ∂t − [ ∂θ ∂t ]t=0−) −k∆2θ +ρC( θ τ2D th )=d1 +Sth (1) where ρ is the material density, C is the calorific capacity, k is the thermal conductivity and ∆2 is the Laplacian operator, θ is the local temperature variation of the specimen due to the thermo-mechanical sources. Eq.(1) underlines that these thermo-mechanical sources produce three major effects: a change in heat rate (ρC(∂θ ∂t −[ ∂θ ∂t ]t=0−)), energy exchanges by conduction (k∆2θ) and energy exchanges by convection and radiation (ρC( θ τ2D th )). τ2D th is here a time constant characterizing the convection and radiation losses. 4
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