ISO 12111 pdf download – Metallic materials – Fatigue testing 一Strain-controlled thermomechanicalfatigue 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 extensome(er
The extensorrieter shal be suitable for measuring dynamic strain over long penods during *$lcti there should be minimal drift, slippage or instrument hysteresis. II shall measure axial extension dwectly 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 net cause the extensometer contact points or knife edges to slip.
The contact pressure and operating force of the extensorneter 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 shal 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 (typicaly In the several hundred kHz range and lower). This wIN 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 CorTvnonty used methods of attachment are: resistance spot welding (outside the gauge length) and fixing by binc5ng or by pressing a sheathed thermocouple against the specimen sialace
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 specimens thermal emissivity over the duration of the test. Potential solutions may Include two-colour pyrometers and pre.oxidizlng the specimen surface.
5.4 Instrumentation for test monitoring
A oornputenzed 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
In general, the considerations discussed in 61.2 also apply to tests on tube specimens. In addition, tolerances should be maintamed such that wall thickness variation is maintained within 1 % of the nominal wall thickness around the circumference.
Tubular specimens have the advantage over solid specimens of minwnizing the radial temperature adient. The specimen wall should be sufficiently thick in order to be representative of the material microstructure.
As a general rule, the ratio of mean diameter to wall thickness should be in the range 5 to 30. in order to satisfy thin-wall specimen criteria. Buckling tendanaes at high axial strain ranges will tend to push specimen design to the lower end of this range.
6.1.4 Solid rectangular specimens
In general. the considerations discussed in 6-1.2 also apply to tests on rectangular specimens. However. these tests require specific geometries and fixtures in order to avoid problems of buckling
The gripping system may necessitate the use of fiat mechanical or hydraulic jaws. However, flat parallel jaw faces require additional measures to ensure alignment in the two unconstrained degrees of freedom.
In general, the width of the specimen is reduced in the gauge length to avoid failures in the grips. In some applications. It might be necessary to add end tabs to maaase the grip end thickness as well as to avoid failure in the grips,
6.2 Preparation of specimens
In any TMF test proamme designed to characterize the intflnsic properties of a material, it is important to observe the recommendations given in 6.3 to 6.7 in the preparation of specimens. A deviation from these recommendations is possible if the test programme aims to determine the influence of a specific factor (surface treatment, oxidation, etc.) that is incompatible with these recommendations, In all cases, this deviation shaM be noted in the test report.
ISO 12111 pdf download – Metallic materials – Fatigue testing 一Strain-controlled thermomechanicalfatigue testing method
