FATIGUE LIMIT TESTING METHOD FOR SPECIMENS

Abstract

Fatigue limit testing method for specimens comprising subjecting a specimen (10) to be tested to successive test blocks (1, 2, 3, 4, 5, 6, 7), each test block (1, 2, 3, 4, 5, 6, 7) comprising applying to the specimen successive cyclic loads according to load parameters with an amplitude bigger than the load parameters of cyclic loads of the preceding test block; subjecting said specimen to successive deformation tests (a, b, c, d, e, f), each deformation test being performed between two successive test blocks and comprising the application of a isolated specific load to the specimen and performing deformation measurements from said element while being subjected to said specific load; and characterizing a fatigue behavior of the specimen considering at least a variation occurring on the successive deformation measurements and considering the load parameters of cyclic loads preceding each deformation measurement.

Claims

1. Fatigue limit testing method for specimens comprising: subjecting a specimen to be tested to successive test blocks, each test block comprising applying a vibration to the specimen, the vibration being defined by reciprocating successive cyclic loads without rest time in between according to load parameters, the load parameters of the cyclic loads of each test block having an amplitude higher than the load parameters of cyclic loads of the preceding test block; pausing cyclic loads between successive test blocks; subjecting the specimen to a deformation test during a pause between successive test blocks, each deformation test comprising the application of an isolated specific load not contiguous to other loads to the specimen starting from and finishing in an unloaded state of the specimen, and performing deformation measurements from the specimen while being subjected to the isolated specific load; and characterizing a fatigue behavior of the specimen considering at least a variation occurring on successive deformation measurements and considering the load parameters of cyclic loads preceding each deformation measurement.

2. The fatigue limit testing method according to claim 1 wherein during the application of the cyclic loads, no deformation of the specimen is measured.

3. The fatigue limit testing method according to claim 1 wherein the isolated specific load: is lower than a known yield strength of the specimen when the specimen has not suffered fatigue; or is comprised between 15% and 60% of a known yield strength of the specimen when the specimen has not suffered fatigue damage.

4. The fatigue limit testing method according to claim 3 wherein the known yield strength is obtained from tensile testing performed on an additional specimen which has not suffered fatigue damage, and which has identical shape, dimensions and material than the specimen subjected to cyclic loads during the test blocks.

5. The fatigue limit testing method according to claim 1 wherein the load parameters of the cyclic loads comprised on each test block comprises: cyclic loads having a frequency comprised between 1 Hz and 80 Hz; and/or between 4000 and 8000 cycles; and/or cyclic loads all with the same frequency and amplitude; and/or cyclic loads all with the same frequency, the frequency being equal to the frequency of the cyclic loads contained in the other test blocks; and/or the same number of cycles than the other test blocks.

6. The fatigue limit testing method according to claim 1 wherein the increase in the amplitude of the load parameters of the cyclic loads between successive test blocks follows a predefined increase pattern.

7. The fatigue limit testing method according to claim 6 wherein the predefined increase pattern is either linear, is a logarithmic increase pattern or is a hybrid pattern combining an initial phase following a linear pattern and a final phase following a logarithmic pattern.

8. The fatigue limit testing method according to claim 1 wherein the test blocks and the deformation tests are alternated until the breakage of the specimen occurs.

9. The fatigue limit testing method according to claim 1 wherein the deformation measurements are obtained from an extensometer and/or from an optic sensor facing a surface of the specimen comprising a stochastic pattern.

10. The fatigue limit testing method according to claim 1 wherein the method further comprises calculating an undamaged fatigue limit of the specimen considering the results of successive deformation tests each set in relation to the amplitude of the previous test block.

11. The fatigue limit testing method according to claim 10 wherein: the results of successive deformation tests are included in a bidimensional chart with an axis representing the deformation suffered by the specimen on each deformation test and the other axis representing the maximum amplitude of the loads of the test block performed previous to each deformation test, and the undamaged fatigue limit of the specimen is obtained from an intersection between an asymptote line defined by the results of the successive deformation tests and an axis corresponding to zero deformation of the specimen.

12. The fatigue limit testing method according to claim 1 wherein the specimen on which the fatigue limit testing is applied to a flat and rectangular specimen that is elongated in a longitudinal axis direction and the cyclic loads are applied thereon parallel to the longitudinal axis direction, so that the obtained fatigue behavior corresponds with a fatigue behavior characterization of the material from which the specimen is made.

13. The fatigue limit testing method according to claim 1 wherein the specimen on which the fatigue limit testing is applied is a non-rectangular and/or a non-flat test piece on which the cyclic loads are applied from two separated subjection points of the test piece so that the obtained fatigue behavior corresponds to a fatigue behavior characterization of the non-rectangular and/or non-flat test piece.

14. The fatigue limit testing method according to claim 1 wherein each load of the cyclic loads produces a tensile stress on the specimen and/or every isolated specific load produces a tensile stress on the specimen.

15. The fatigue limit testing method according to claim 1 wherein the specimen is actively or passively cooled to a room temperature between successive test blocks.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] The foregoing and other advantages and features will be more fully understood from the following detailed description of an embodiment with reference to the accompanying drawings, to be taken in an illustrative and non-limitative manner, in which:

[0060] FIG. 1 shows a bidimensional chart with an axis representing time and the other axis representing the amplitude of each load applied to the specimen, including the cyclic loads performed during the test blocks, and the isolated specific loads (σ.sub.c) applied during the deformation tests;

[0061] FIG. 2 shows a bidimensional chart with an axis representing the deformation suffered by the specimen on each deformation test, and the other axis representing the isolated specific load (σ.sub.c) applied on each of said deformation test;

[0062] FIG. 3 shows a bidimensional chart with an axis representing the deformation suffered by the specimen on each deformation test and the other axis representing the maximum amplitude of the loads of the test block performed previous to each deformation test, and wherein the undamaged fatigue limit of the specimen is obtained, on said bidimensional chart, from the intersection between an asymptote line defined by the results of the successive deformation tests and an axis corresponding to zero deformation of the specimen;

[0063] FIG. 4 shows a schematic view of a fatigue test machine.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0064] The foregoing and other advantages and features will be more fully understood from the following detailed description of an embodiment with reference to the accompanying drawings, to be taken in an illustrative and not limitative, in which:

[0065] According to an example of the present invention, a metal specimen 10 is retained between two clamps 20, and an extensometer 30 is associated to said specimen 10 to measure its deformation, as shown on FIG. 4.

[0066] Then, specimen 10 is subjected to successive test blocks 1, 2, 3, 4, 5, 6 and 7 of cyclic loads, each including the same number of cyclic loads, all of the same frequency, produced by a relative movement between the clamps 20.

[0067] The cyclic loads of each test block are all of the same amplitude, but the amplitude of the cyclic loads of each test block are higher than the amplitude of the preceding cyclic blocks 1, 2, 3, 4, 5, 6 and 7, as shown on FIG. 1.

[0068] In this example, the increase of the amplitude between successive test blocks is linear, producing a regular increase between successive test blocks.

[0069] After each test block a deformation test a, b, c, d, e and f is performed by subjecting the specimen 10 to an isolated specific load of an amplitude equal or lower than the amplitude of each cyclic load of the initial test block, produced by a relative movement between the clamps 20. All the isolated specific loads of all the deformation tests are identical.

[0070] Each isolated specific load produces a deformation of the specimen, generally an elastic deformation, which is measured by the extensometer 30.

[0071] Because the cyclic loads of the successive test blocks 1, 2, 3, 4, 5, 6 and 7, of an increasing amplitude, produce fatigue damage on the specimen 10 and a stiffness reduction thereof, the specimen 10 suffers an increasing deformation on each successive deformation test a, b, c, d, e and f, which is measured by the extensometer.

[0072] FIG. 2 shows said increasing deformation on each successive deformation test.

[0073] After a certain number of test blocks 1, 2, 3, 4, 5, 6 and 7, the fatigue damage produced on the specimen is considerable and the deformation suffered by the specimen 10 on each deformation test a, b, c, d, e and f increases in an accelerated manner.

[0074] Using the deformation measurements obtained by the extensometer 30 on each deformation test a, b, c, d, e and f, when related with the maximum amplitude of the cyclic loads of the test block 1, 2, 3, 4, 5, 6 and 7 performed before each deformation test a, b, c, d, e and f, allow the calculation of the undamaged fatigue limit of the specimen 10.

[0075] In this example, said calculation is performed by including the deformation measurements in a bidimensional chart where the ordinate axis represents the deformation suffered by the specimen 10 on each deformation test a, b, c, d, e and f and the abscissa axis represents the maximum amplitude of the cyclic loads of the test block performed previous to each deformation test a, b, c, d, e and f.

[0076] Said deformation measurements included in said chart, shown on FIG. 3, determine a curve. Said curve define an asymptote which intersects with the abscissa axis. Said intersection point is the undamaged fatigue limit of the specimen 10.