ISO 12111:2011 pdf free download – Metallic materials一Fatigue testing一 Strain-controlled thermomechanical fatigue testing method.
7.6.2.2 Method 1
The thermal strain is compensated by recording the free expansion thermal strains (specimen at zero force) as a function of specimen temperature prior to test initiation. The temperature cycle used must be identical to that used for the subsequent thermomechanical fatigue test. These values can be fitted to an appropriate algebraic function or functions (typically, one for the heating portion of the cycle and one for the cooling) where temperature is the independent variable. The function(s) can then be used to calculate the instantaneous compensating strains for the thermomechanical fatigue test in real time.
This approach can be incrementally expressed by:
7.6.2.3 Method 2
The thermal strain is compensated by recording the free expansion thermal strains (specimen at zero force) as a function of cycle time prior to test initiation. The temperature cycle used must be identical to that used for the subsequent thermomechanical fatigue test. These recorded values can be recalled at the appropriate corresponding times within the cycle to provide the strain compensation values. This approach can be incrementally expressed by the following
7.6.2.4 Summary
Method 2 does not provide closed-loop control of mechanical strain and can result in a significant overstrain to the specimen if the desired specimen temperature is not consistently achieved.
It is generally not sufficiently accurate to take the free thermal strain range, divide it into equal time or temperature-based increments, and use this constant increment for subsequent compensation calculations. This approach does not account for the normal nonlinear thermal expansion (a) nor does it account for potential temperature lags experienced during reversals. Method 1 will minimize damage to the specimen if a temperature problem develops during the test.
7.6.3 Compensation accuracy
The accuracy of the thermal strain compensation routine shall be checked prior to the initiation of the thermomechanical fatigue test by subjecting the specimen to thermal cycling in mechanical strain control, at zero mechanical strain. Here, the thermal strain compensation method shall be used to actively compensate for the induced thermal strain of the specimen.
During this cycle, the maximum acceptable resulting stress shall be calculated from the peak mechanical strains from the specific test being considered, along with the corresponding moduli for the two temperatures at which the maximum and minimum mechanical strains occur:
The absolute values of the measured stresses during the thermal cycling at zero mechanical strain shall not exceed the allowable stress range as computed above, ensuring that the mechanical strain range controlled during the actual test is within 2 % of the mechanical strain range.
7.7 Temperaturelmechanical strain phasing
Temperature/mechanical strain phasing is typically not a problem if a common control clock is employed for both temperature and mechanical strain.
The temperature value used in assessing the temperature/mechanical strain phasing shall be the response (feedback) value measured on the specimen during thermal cycling, and not the command values.
However, if two independent clocks are used, the following apply.
— The mechanical strain value used in assessing the temperature/mechanical strain phasing shall be that calculated from the instantaneous total strain and the compensating thermal strain component in time within the cycle.