ISO 12108:2018

ISO 12108:2018 pdf free.Metallic materials一Fatigue testing一Fatigue crack growth method.
5 Apparatus
5.1 Testing machine.
5.1.1 The testing machine shall have smooth start-up and a backlash-free force train if passing through zero force (tension — compression). Cycle to cycle variation of the peak force during precracking shall be less than ±5 % and shall be held to within ±2 % of the desired peak force during the test. ΔF shall also be maintained to within ±2 % of the desired range during test. A practical overview of test machines and instrumentation is available.
5.1.2 If a dynamic force calibration is appropriate or required (e.g. by the purchaser), it should be conducted according to Iso 4965-1. Dynamic force calibration is appropriate when inertial forces act on the force transducer or any dynamic errors occur in the electronics of the force indicating system, as described in ISO 4965-1. Test frequency and amplitude as well as grip mass can affect the inertial forces acting on the force transducer. Examples for which dynamic force calibration can be appropriate are configurations with the load cell on the moving piston or the part.
5.1.3 In terms of testing machine alignment, asymmetry of the crack front is an indication of misalignment. For tension-compression testing, the length of the force train should be as short and stiff as practical. Non-rotating joints should be used to minimize off-axis motion. It is important that adequate attention be given to alignment of the testing machine and during machining and installation of the grips in the testing machine. Regarding the relevance of alignment, a distinction shall be made between:
— Crack growth tests with rigid gripping and rigid load train which can also undergo compressive forces and stresses (e.g. corner crack test pieces): a sufficient alignment of the load train can be important for these test pieces to obtain correct and reproducible crack growth data, and
— Crack growth tests only with tensile load and fixed with bolts and using cardanic joints (e.g. CT- specimens): due to the use of cardanic or similar joints in the load train alignment checks are not necessary.
If an alignment check is appropriate (e.g. when using a rigid load train and grips) and required (e.g. by the purchaser), it should be conducted according to Iso 23788 and using an alignment class 5 according to ISO 23788. If alignment check is conducted, the results shall be reported.
5.1.4 Accuracy of the force measuring system shall be verified periodically in the testing machine. The calibration for the force transducer shall be traceable to a national organization of metrology. The force measuring system shall be designed for tension and compression fatigue testing and possess great axial and lateral rigidity. The indicated force, as recorded as the output from the computer in an automated system or from the final output recording device in a non-computer system, shall be within the permissible variation from the actual force. The force transducer’s capacity shall be sufficient to cover the range of force measured during a test. Errors greater than 1 % of the difference between minimum and maximum measured test force are not acceptable.
The force measuring system shall be temperature compensated, not have zero drift greater than 0,002 % of full scale, nor have a sensitivity variation greater than 0,002 % of full scale over a 1 °C change. During elevated and cryogenic temperature testing, suitable thermal shielding/compensation shall be provided to the force measuring system so it is maintained within its compensation range.
5.2 Cycle-counter.
An accurate digital device is required to count elapsed force cycles. A timer is to be used only as a verification check on the accuracy of the counter. It is preferred that individual force cycles be counted. However, when the crack velocity is below 105 mm/cycle, counting in increments of 10 cycles is acceptable.
5.3 Crack length measurement apparatus.
Accurate measurement of crack length during the test is very important. There are a number of visual and non-visual apparati that can be used to determine the crack length. A brief description of a variety of crack length measurement methods is included in Reference 114]. The required crack length measurements are the average of the through-thickness crack lengths, as covered in 9.1.ISO 12108 pdf download.

Leave a Reply

Your email address will not be published. Required fields are marked *