Document b 2 - 2 0 1 4 - 0 1
Application of IVlarker Load Systems
for Fractography of Fatigue Cracks
J. Schijve
1 March 2014
T
U
D e l f t
Introduction
The present investigation is a continuation of research carried out in 2013 and reported in a paper presented at the International Fatigue Congress in Melboume in March 2014 [1]. It presents reach on the application of moderate OverLoads (OL) as marker loads to obtain marker load striations on the fatigue fracture surface. It should enable a reconstruction of successive crack front profiles. It was expected that this experimental technique should be more atfractive and less time consuming than the application o f existing techniques such a the marker load spectrum proposed by Piasick et al.[2]. A n example of a marker load spectrum with OL's is shown in Figure 1. It was shown that possible delay effects of the relatively small OL's was negligible. It was also indicated that crack front shapes of initially small part through-cracks could indeed be reconstructed. This question is significant for practical problems because a large part of the fatigue life of notched structural elements is covered by the initial growth of small invisible cracks.
At the same time the investigation also learned that the fracture surface of the fatigue cracks encompasses various parts with a chaotic morphology without striations. This phenomenon was observed in specimens of various aerospace alloys of the 2000 an d 7000 series, including so-called clean alloys. The chaotic character is mainly caused by grain boundaries, and to a minor extent by inclusions and probably other not fully recognized causes.
Recently small sheet specimens were found which probably were left from an old investigation. The specimens were labelled to be 7075-T6 specimens, partly with a fine grained structure, and for another part with a coarse grained structure. The structure was checked with optical microscopy, see Figure 2 which confirms the two different grain sizes. It was then decided to carry out crack growth tests with the OL marker loads in order to see whether they both should reveal the same chaotic fracture surface morphology. A specimen with the larger grain size might be less affected with the grain boundary effect. In view of exploring this issue, it is not necessary to look specifically to small part-through cracks. A simple specimen with a central hole and two saw cuts was adopted. The specimen dimensions are: width 100 mm, thickness 2.4 mm. The tip to tip distance of the two saw cuts is 10 mm which corresponds to an virtual crack length o f ao = 5 mm. The specimens were loaded with the OL-marker load spectrum in Figure 1.
Fatigne crack propagation tests on fine grained and coarse grained 7075-T6 specimens
Test results
Typical SEM pictures are shown in Figure 3 for both coarse and fine grained material. It includes pictures with easily and less easily detectable OL-striations. The pictures were made at approximately midthickness positions of the fatigue crack. The chaotic areas on the fatigue crack surface are similar for both grain sizes, and also similar to pictures obtained in the preceding test series of Ref [1]. However, relatively large areas with abundant baseline striations were observed on the fatigue crack surface of the coarse grained specimen, see Figure 4. This was not the case for the fine grained specimen.
Because the SEM also provides the x/y-coordinates of the location on the fatigue crack it is possible to reconstitute the crack growth curves. It requires that no OL-striations o f a crack front are overlooked. The locations of the SEM pictures are presented in Figure 5. The reconstructed crack growth curves are shown in Figure 6. The crack growth rate in the specimen with the fine grained structure is approximately 1.4 times slower than in the specimen with the coarse grained structure.
In conclusion
The present tests clearly confirm that the grain boundaries are responsible for the chaotic parts of the morphology o f fatigue cracks in the present 7075-T6 specimens. This was already concluded in the preceding investigation for several aerospace Al-alloys reported in Ref [1]. It was also reported recently in the literature [3]. It is desirable that papers on fi-actographic investigations of fatigue crack growth include a fiill representative survey o f all details of the fatigue crack.
Ackowledgement: The SEM pictures were made by Frans Oostrum as the operator of the electron microscope. His assistance was essential for the fi-actographic observations.
References
[1] Schijve J., Application of marker loads for fi-actography of fatigue cracks. Paper presented at the 11th Intemational Fatigue Congress, 2-7 March, Melboume.
[2] Piascik. R.S. and Willard, S.A., The characteristics of fatigue damage in the fiiselage riveted lap splice joint. NASA / TP-97-206257,1997.
[3] Chen, Y,Q., Pan, S.P., Zhou, M.Z., Y i , D.Q., Xu, D.Z. and Xu, Y.F., Effects of inclusions, grain boundaries and grain orientations on the fatigue crack initiation and propagation behariour of 2524-T3 A l alloy. Materials Science & Engineering A, Vol. 580, 2013, 150-158.
Block of 10 marker load
cycles
Smax =65 MPa
3000 cycles
Smin = 5 MPa
50 Intermediate cycles
TT
One period of the load history repeated until failure.
Number of cycles in one period: 3130
Fatigue crack propagation tests on fine grained and coarse grained 7075-T6 specimens
Figure 2a: Fine grained 7075-T6. Section perpandicular to the rolling direction. Optical microscopy. Characteris grain size dimension: Length 55 mu, width 11 ^m.
Figure 2b: coarse grained 7075-T6. Section perpandicular to the rolling direction. Optical microscopy. Characteris grain size dimension: Length 150 mu, width 30 i^m.
Figure 3b: Fine grained specimen.
Fatigue crack propagation tests on fine grained and coarse gmined 7075-T6 specimens
Figure 3d: Coarse grained specimen.
Fatigue crack propagation tests on fine grained and coarse grained 7075-T6 specimens Fine grained • • • mis sing data poir t 0 2 4 6 8 10 12 14 16 18 20 a (mm) 9 . Coarse grained 1.8 1.6 1.4 ^ 1-2 E, 1 ^ 0.8 0.6 0.4 0.2 n 1.8 1.6 1.4 ^ 1-2 E, 1 ^ 0.8 0.6 0.4 0.2 n 1.8 1.6 1.4 ^ 1-2 E, 1 ^ 0.8 0.6 0.4 0.2 n 1.8 1.6 1.4 ^ 1-2 E, 1 ^ 0.8 0.6 0.4 0.2 n •—
