ISO 12111-2011 pdf free download.Metallic materials — Fatigue testing — Strain-controlled thermomechanical fatigue testing method.
5.2 Strain measuring system
The strain measuring system (optical, mechanical, etc.) including the extensometer and its associated electronics shall conform to ISO 9513, Class 1.
For gauge lengths less than 15 mm, a Class 0.5 system is recommended.
The strain shall be measured on the specimen using an axial extensometer. The extensometer shall be suitable for measuring dynamic strain over long periods during which there should be minimal drift, slippage or instrument hysteresis.
It shall measure axial extension directly over the gauge length of the specimen.
The transducer section of the extensometer should be protected from thermal fluctuations that give rise to drift. Given the transient nature of the temperature in a TMF test, it is recommended that the extensometer is actively cooled, so that the transducer section of the extensometer remains isothermal during the course of the test. The kinematic design of contacting extensometry should be such that lateral or angular motions of the specimen contact zone do not cause the extensometer contact points or knife edges to slip. The contact pressure and operating force of the extensometer should be low enough to avoid damaging the specimen surface and giving rise to crack initiation at the extensometer contact points or knife edges.
5.3 Heating system The heating system shall be capable of applying the maximum heating and cooling rates required by the TMF test series.
To minimize radial temperature gradients with a direct induction heating system, it is advisable to select a generator with a sufficiently low frequency (typically in the several hundred kHz range and lower). This will help to minimize “skin effects” during heating. During a test, the specimen temperature shall be measured using thermocouples, pyrometers, RTDs, or other such temperature-measurement devices. For thermocouples, direct contact between the thermocouple and the specimen shall be achieved without causing incipient failure at the point of contact. NOTE Commonly used methods of attachment are: resistance spot welding (outside the gauge length) and fixing by binding or by pressing a sheathed thermocouple against the specimen surface.
If the temperature within the gauge section is measured with an optical pyrometer, steps shall be taken during calibration to address possible variation in the specimen’s thermal emissivity over the duration of the test. Potential solutions may include two-colour pyrometers and pre-oxidizing the specimen surface. 5.4 Instrumentation for test monitoring A computerized system capable of carrying out the task of collecting and processing force, extension, temperature, and cycle count data digitally is recommended. Sampling frequency of data points shall be sufficient to ensure correct definition of the hysteresis loop especially in the regions of reversals. Different data collection strategies will affect the number of data points per loop needed, however, typically 200 points per loop are required.ISO 12111 pdf download.