1 Scope
This document specifies the conditions for controlled fatigue testing of metallic material specimens (without the introduction of stress concentrations) at room temperature with axial equal force. When this document is used, the purpose of the test is to provide fatigue information such as the relationship between the applied stress and the number of cycles to failure for different stress ratios and given material conditions (e.g. hardness and microstructure).
This document applies to axial force controlled fatigue testing of specimens with circular and rectangular cross-sections, but not to the testing of product components and other specially shaped specimens.
Note 1: This document does not cover fatigue testing of notched specimens as the shape and dimensions of notched specimens are not standardised. However, the fatigue test procedure described in this document can be applied to the fatigue testing of notched specimens.
Note 2: Engineering stress is used throughout this document and is defined as the ratio of the axial force to the original cross-sectional area of the specimen at the test temperature.
2 Normative references
The contents of the following documents constitute essential provisions of this document through normative references in the text. Among them, note the date of the cited documents, only the date of the corresponding version applicable to this document; do not note the date of the cited documents, the latest version (including all the revision of the list) applicable to this document.
3 Terminology and definitions
The following terms and definitions apply to this document.
4 Test plan
4.1 General provisions
Recent research has shown that metals do not normally exhibit a "fatigue limit" of "infinite cycle cycles" sustained at one stress. Usually the "plateau" of stress life is the "fatigue limit" in the conventional sense, but failures below this stress level do occur.
4.2 Presentation of fatigue results
4.2.1 Overview
The design of the study and the use of the results dictate the most appropriate method for selecting the results from the many available, graphical and other means. The results of fatigue tests are usually represented graphically. When reporting fatigue data, it is advisable to specify the test conditions. In addition to graphical representations, tabular numerical data are also desirable where the presentation format permits.
4.2.2 Wöhler or S-N curves
The most common method of graphing the results is to plot the number of cycles at failure as the horizontal coordinate and the stress amplitude or other stress values dependent on the stress cycle as the vertical coordinate. The smooth curve drawn across the approximate centre line of the test data points is known as the Wöhler or S-N curve. The cycle cycle is in logarithmic coordinates and the stress axes are in linear or logarithmic coordinates. One curve is plotted for each set of test results for each stress ratio. The test results are usually plotted in the same graph. An example of a graphical report is given in Figure 3, where the stress axes are in linear coordinates.
4.2.3 Average stress graph
The fatigue strength derived from the Wöhler curve or the S-N curve is plotted as a constant life line on a fatigue strength graph. The results can be reported directly graphically, for a given fatigue life the stress amplitude versus average stress graph is given in Figure 4; or the maximum and minimum stresses versus average stresses are plotted as in Figure 5; or the maximum stresses versus minimum stresses are plotted in Figure 6. The test results can be plotted in the same graph.
5 Specimens
5.1 Specimen shape
A specimen type with a fully machined, smooth cylindrical scale as shown in Figure 7 is usually used.
Specimens commonly referred to as "funnel type" specimens may be used with caution. In these specimens there is a continuous arc between the clamping end and the minimum diameter of the circular specimen or the minimum width of the plate specimen. Unlike smooth equal-diameter or equal-width specimens where the material is equally stressed in the spaced section, funnel-shaped specimens are stressed in the thin flat unit of the smallest cross-section. As a result, the fatigue results produced may not be representative of the response of the bulk material, particularly in long-life fatigue states where inclusions control the behaviour of high hardness metals and where there is double crack initiation from surface to subsurface [9]. In fact, such results may be non-conservative, especially in the case of long life, where the largest microscopic discontinuities may not be in the plane cross-section of the maximum stress.
It is important to note that for specimens with rectangular cross-sections, it may be necessary to require a simultaneous reduction in the test section in both width and thickness. If this is the case, a transition arc is required in both the width and thickness directions. Similarly, when rectangular cross-section specimens are required to take into account the surface conditions of the material in practice, it is desirable that at least one side of the test area of the specimen remains unfinished. As a rule, fatigue tests using rectangular cross-section specimens are generally not comparable to those using circular specimens due to the difficulty of obtaining small roughness or the early onset of fatigue cracking at the corners of the rectangular cross-section.
5.2 Specimen dimensions
5.3 Specimen preparation
5.3.1 General requirements
In any fatigue test procedure designed to characterise the inherent properties of a material, it is important that the following recommendations are observed in the preparation of the specimens. Some deviations are permitted if the purpose of the test is to determine the effect of specified factors (e.g. surface treatment, oxidation, etc.) in relation to these non-conformities. In any case, these deviations should be noted in the test report.
5.3.2 Specimen processing procedures
The processing procedure selected may produce residual stresses on the surface of the specimen which may affect the test results. These residual stresses may be caused by thermal gradients during the machining stage or by deformations or microstructural changes in the material. During high temperature testing, the residual stresses generated may be partially or fully released and therefore the effect of residual stresses is minimal. However, the selection of a suitable machining process (especially before the final polishing stage) will reduce the residual stresses. For harder materials, grinding is preferred to turning or milling.
5.3.3 Sampling and specimen identification
5.3.4 Surface condition of the specimen
The following recommendations will minimise the above effects.
6 Test set-up
6.1 Test machine
6.2 Coaxiality check
6.3 Force transducer
6.4 Clamping of the specimen
7 Test monitoring equipment
7.1 Recording systems
7.2 Cycle counter
The cycle counter is the device necessary to record the number of cycles.
7.3 Specimen temperature measurement
Tests are normally carried out at room temperature (10°C to 35°C). In high and low temperature tests, the temperature of the specimen should be recorded in full. The temperature of the specimen may be measured using a thermocouple in contact with the surface of the specimen or other temperature measuring device with a maximum permissible tolerance of ±2°C. If the temperature range is exceeded during the test, this shall be indicated in the test report.
8 Checking and calibration
The test machine and the control and measurement systems used should be checked or calibrated at regular intervals. In particular, each sensor and the electronic equipment connected to it should always be calibrated as a whole.
The temperature measuring system should be calibrated according to the relevant standards.
9 Mounting of specimens
The specimen should be carefully mounted, positioned between the upper and lower jaws to ensure that the axial force is applied and that a predetermined stress pattern can be applied. For specimens of rectangular cross-section, ensure that the force is uniformly distributed across the cross-section of the specimen. The fixture is designed so that the torsional stresses caused by the tightening of the lock nut are not applied to the specimen when the round specimen is threaded at both ends. In some cases where threaded specimens are used, a portion of the force on the mating plane and coaxial surface is distributed along the threads to reduce the clamping torque.
10 Test frequency
The frequency of force cycles depends on the type of testing machine used and in many cases on the stiffness of the specimen.
The choice of frequency should depend on the material, the specimen and the test machine combination. If the frequency depends on the dynamic characteristics of the specimen and the test machine combination, it is necessary to measure the stiffness of the specimen prior to the test.
11 Application of force
The force application procedure should be consistent for each specimen in a set. The average force and the range of force values should be kept within ±1% of the force range.
12 Recording of temperature and humidity
The maximum and minimum air temperature and humidity should be recorded for each day during the test.
13 Failure criteria and test termination
13.1 Judgement of failure
Unless otherwise agreed, the criterion for failure shall be specimen fracture.
NOTE: In some special applications, other criteria such as the appearance of visible fatigue cracks, plastic deformation of the specimen, change in the rate or frequency of crack propagation, may be used.
13.2 Test termination
The test is terminated when the specimen fails or reaches a predetermined number of cycles.
14 Test report
Appendix A (Informative) Cross-reference between the chapter numbers of this document and ISO 1099: 2017
Bibliography
1 Scope
2 Normative references
3 Terminology and definitions
4 Test plan
5 Specimens
6 Test set-up
7 Test monitoring equipment
8 Checking and calibration
9 Mounting of specimens
10 Test frequency
11 Application of force
12 Recording of temperature and humidity
13 Failure criteria and test termination
14 Test report
Appendix A (Informative) Cross-reference between the chapter numbers of this document and ISO 1099: 2017
Bibliography
