PSI - Issue 33

Dario Santonocito et al. / Procedia Structural Integrity 33 (2021) 724–733 Santonocito et al./ Structural Integrity Procedia 00 (2019) 000–000

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T σ

scatter index of the SN curve (= σ 0,PS10% /σ 0,PS90% ) thermal diffusivity of the material [m 2 /s]

α

ΔT s ΔT st ΔT 1 ΔT 2

absolute surface temperature variation during a static tensile test [K] stabilization temperature reached during fatigue test [K] estimated value of temperature for the first set of temperature data [K] estimated value of temperature for the second set of temperature data [K]

Φ

Energy Parameter [Cycles K] density of the material [kg/m 3 ]

ρ σ

stress level [MPa]

σ lim fatigue limit estimated with the Static Thermographic Method [MPa] σ 0 , σ 0 TM fatigue limit, fatigue limit assessed by Thermographic Method [MPa] σ 1 uniaxial stress [MPa]

2. Theoretical background 2.1. Thermographic Method

As observed by La Rosa and Risitano (La Rosa and Risitano, 2000), during a fatigue test, performed at a stress level above the fatigue limit σ 0 of the material and at a given stress ratio R and test frequency f, the temperature evolution exhibits three phases (Fig. 1a). In the first phase (Phase I), there is an increment until the temperature stabilize at a value equal to ΔT st (Phase II). As the material approaches to fail, temperature experiences a very rapid increment (Phase III), compared to the previous one. Fatigue limit can be identified in a rapid way as the first stress level at which the stabilization temperature is noticeably higher compared to the previous value. For each constant amplitude (CA) fatigue test, it is possible to evaluate the energy parameter Φ as the subtended area of the temperature versus number of cycle curve (ΔT-N). Generally, the higher the applied stress, the higher the stabilization temperature, but the energy parameter could be assumed as material property, at a given stress ratio and test frequency. It is also possible to perform a stepwise fatigue test (Fig. 1b), increasing the applied stress level and registering the relative stabilization temperature. As the specimen fail, it is possible to evaluate the energy parameter Φ and assess the number of cycles to failure for each stress level, as the specimen would be stressed at that stress level with CA tests, simply dividing the energy parameter for the different stabilization temperatures and neglecting Phase I and III, usually smaller compared to Phase II. In this way, knowing the N-σ values, it is possible to obtain the complete SN curve of the material with a very limited number of tests.

(a)

(b)

Fig. 1. Fatigue assessment by Thermographic Method: a) temperature trend during a fatigue test; b) rapid stepwise fatigue test.