ISO 12111-2011 pdf free download.Metallic materials – Fatigue testing – Strain-controlled thermomechanical fatigue testing method.
The tests shall be conducted on a tension-compression machine designed for smooth start-up with no backlash when passing through zero force. The machine shall be capable of controlling strain and measuring force when applying the recommended waveform. It may be hydraulic or electromechanical.
The load frame shall have high lateral stiffness when the crosshead is in the operating position and accurate alignment (both parallelism and concentricity) between the load train support references.
The complete load train (including force transducer, pullrodslgnps, and specimen) shall also have high lateral stiffness to minimize specimen bending.
5.1.2 Force measuring system
The force measuring system, comprising force transducer, conditioner and readout. This system shall meet the requirements of ISO 7500-1. Class 1 over the range of forces expected during the test series.
NOTE Class 1 requires that force indication errors should not exceed ± 1 % of reading over the verified range. The force transducer shall be suitable for the forces applied during the test.
The force transducer shall be temperature compensated and not have a zero drift or sensitivity variation greater than 0,002 % of full scale per one degree Celsius. During the test, it shall be maintained within its temperature compensated range.
5.1.3 Specimen gripping device
The gripping device shall transmit the cyclic forces to the specimen without backlash for the duration of the test. The geometric qualities of the device shall ensure correct alignment in order to meet the requirements specified in 5.1.4.
NOTE It is good design practice to reduce the number of mechanical interfaces to a minimum.
The gripping device shall ensure that the alignment is reproducible over successive specimens.
The gripping device materials shall be selected so as to ensure correct functioning across the range of test
5.1.4 Load train alignment
Load frame, including grips, shall be ahgned using a specimen, with a geometry as similar as possible to that of the test specimen, instrumented with strain gauges. The permitted maximum bending strain due to the machine shall be no more than 50 microstrain at zero force or 5 % of the applied axial mechanical strain, whichever is the greater. This shall be carried out at 12-month intervals and in the following events:
a) as part of the commissioning procedure of a newly acquired testing machine;
b) after an accidental buckling of a specimen, unless it can be demonstrated that the alignment has not changed; and
c) if any adjustment has been made to the load train.
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 pdnts 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.ISO 12111 pdf download.