ISO 12111:2011 pdf download – Metallic materials – Fatigue testing 一Strain-controlled thermomechanical fatigue testing method.
Alternatively, an analog system capable of measuring the same data may be used and would ndude
— an X-Y-Y recorder used to record force, extension, and temperature hysteresis loops:
— a strip-chart recorder for several time-dependent parameters: force, extension and temperature;
— a peak detector per signal:
— a cycle counter.
The recorders may be replaced with storage devices capable of reproducing the recorded signals either in photographic or analog form. These devices are necessary when the rate of recorded signals is greater than the maEimLwn slew-rate of the recorder. They allow permanent records to be reproduced subsequently at a lower rale
5.5 Checking and verification of the apparatus
The testing machine and its control and measurement systems shoidd be checked regularly.
Specifically, each transducer end assuciated eledrunics shall always be checked as a unit. The force measuring syslern(s) shall be verified according to ISO 7500-1
— The strain measuring system(s) shall be verified according to ISO 9513.
— The temperature measuring system(s) shall be traceable to the relevant national standard.
It is good practice before each series of tests to check the base length of the extensometer, the force cell and extensometer calibrations using a shunt resistor or other suitable method and the thermocouple or pyrometer caibrations.
6 SpecImens
6.1 Geometry
6.1.1 General
The total specimen bending is comprised of bending Iron, load frame misalignment and specimen bending from test specimen asymmetry. To minimize the bending contribution due to the test specimen, It is important to carefully control deviations from the intended test specimen geometry.
6.1.2 SolId round specimens The gauge portion of the specimen in a TMF test represents a volume element of the material under study, which implies that the geometry of the specimen shall not affect the use of the results.
This geometry should fulfil the followrng conditions:
— provide a uniform cylindrical gauge portion;
minimize the risk of buckling in compression to avoid failure initiation at the transition radius:
— provide a uniform strain distribution over Its whole gauge portion:
— allow the exterisometer to measure the strain without interference or slippage.
6,1.3 Round tubular specimens
In general, the considerations discussed m 6.1.2 also apply to tests on tube specimens. In addition, tolerances should be maintained such that wall thickness vanabon is maintained within 1 % of the nominal wall thickness around the cWcumference.
Tubular specimens have the advantage over solid specimens of minlmzing the radial temperature gradient. The specimen wall should be sufficiently thick in order to be representative of the matenal 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 thinwalI specimen criteria, Buckling tendencies 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 flat 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 increase the grip end thickness as well as to avoid failure in the grçs.
6.2 Preparation of specimens
In any TMF test programme designed to characterize the intrinsic properties of a material, it is important to observe the recommendations olven 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 shell be noted in the test report.