Available online 26 January 2007

Keywords: Fatigue; Fractography; Fatigue life; Fatigue testing

found that there is no simple correlation between striations and stress cycles [5]. In particular such correlationfails at low amplitude of stress intensity factor [4] Where striations are found it is generally true that each

* Corresponding author. Tel.: +972 57 8179956; fax: +972 8 8685298.E-mail address: e_hershko@hotmail.com (E. Hershko).

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Engineering Failure Analysis 15 (2008) 20271350-6307/$ - see front matter 2007 Elsevier Ltd. All rights reserved.1. Introduction

Apparently the most common mode of failure in metallic parts is fatigue. It is particularly important inaircraft parts.

Determination of a maintenance policy after the occurrence of a fatigue failure in service is based on ndingthe number of loading cycles which caused the failure. Then, this information need to be related to a measur-able parameter, e.g. ight hours, landings, rotor revolutions, etc.

The counting of striations was suggested many years ago as a method to estimate the duration of crackpropagation. Several authors claim a one-to-one correspondence between striation and stress cycle [13].The striation width was also suggested for estimation of the stress amplitude [4]. However, other researchersAbstract

A well-known method for determining the number of fatigue load cycles prior to failure is to perform a striation count-ing on the fractured surface.

The primary objective of this study is to evaluate the accuracy of a striation counting performed using a high-resolutionscanning electron microscope. Fatigue experiments were conducted on two aluminum alloy AA-2024-T3 specimens and twolow alloy steel AISI-4130-O specimens. We then performed a fractographical analysis of the fractured specimens and com-pared the results to the experimental data. The second objective of this study is to determine several guidelines regarding themethod of striation counting process which will raise its accuracy. This study shows that high accuracy can be achieved bycounting fatigue striation using a scanning electron microscope, but one must be aware of several problems and dicultieswhich can occur during the counting process. Several guidelines that will raise the accuracy were determined. 2007 Elsevier Ltd. All rights reserved.Assessment of fatigue striation counting accuracy usinghigh resolution scanning electron microscope

Emmanuel Hershko *, Nir Mandelker, George Gheorghiu, Haim Sheinkopf,Izack Cohen, Ofer Levy

Materials Division, Aircraft Maintenance Unit, Israeli Air Force, P.O. Box 02538, Israeli Defence Forces, Israel

Received 2 January 2007; accepted 8 January 2007doi:10.1016/j.engfailanal.2007.01.005

striation was produced by one load cycle, on the other hand, it is not generally true that every load cycle pro-duces a striation. The strong correlation of striation spacing and crack growth rate/cycle is only valid at med-ium crack growth rates [4]. Special techniques are needed to obtain a good accuracy [1]. It has been recentlyclaimed that very ne striations are sometimes observed that are ignored in ordinary failure analysis [6].

The primary objective of this study was to evaluate the accuracy of a striation counting by a high-resolutionScanning Electron Microscope to determine the number of load cycles during crack propagation.

Fatigue experiments were conducted on two aluminum alloy AA-2024-T3 specimens and two low-alloysteel AISI-4130-O specimens. In each experiment, the number of applied load cycles was recorded and frac-tographic analysis of the fractured specimens was done and compared to the directly measured cycles.

It is worth noting that the information revealed by studying the fracture surfaces (particularly the numberof load cycles prior to failure) relates only to the propagation stage of the fatigue crack life. Therefore, it is notpossible to determine the number of load cycles which occurred during the initiation stage.

Thorder

E. Hershko et al. / Engineering Failure Analysis 15 (2008) 2027 212. Experimental

2.1. Measurement of crack growth rate (da/dN)

The rst stage of the experiment was to physically measure the crack growth rate in a specimen. Standardspecimens were prepared in accordance with ASTM-E647 (standard test method for measurement of fatiguecrack growth rates) from the commonly used in the aircraft industry aluminum alloy AA-2024-T3 and lowalloy steel AISI-4130 annealed (two specimens from each material).

In order to measure the rate of crack growth, a COD (Crack Opening Displacement) device was attached tothe edge of the specimens (Fig. 1). This device measures the displacement at the specimens edge during eachload cycle. Then the information is used to calculate the length of the crack, based on the specimen geometryand the materials elasticity.

The specimens were cyclically loaded on MTS tension compression machine (Fig. 2) at constant ampli-tude until fracture occurred. The ratio of the minimum load to the maximum load (R) was 0.1.

During the experiment, we conducted a continuous monitoring of the crack length (a) as a function of thenumber of load cycles (N). It allowed the calculation of the crack growth rate (da/dN).

2.2. Experiment parameters determination

The magnitude of the load P applied on the specimen was calculated based on its geometry and materialelasticity in order to induce a reasonable crack growth rate, as shown in Fig. 3 and Eq. (1).

DP DK b w

pf a0=w 1e second objective of this study was to develop guidelines regarding the method of striation counting into improve its accuracy.Fig. 1. A COD device attached to a specimen.

22 E. Hershko et al. / Engineering Failure Analysis 15 (2008) 20272.3. Critical crack length

Based on the load chosen to be used in the experiment, the critical crack length of the specimen was cal-culated by comparing Eq. (2) with Eq. (3):

Fig. 2. The specimen and device loaded on the tension compression machine.

Fig. 3. A schematic illustration of the experiment specimen.

f acr=w DK1c b w

pDP

2

f acr=w 2 acrw

0:886 4:64 acrw 13:32 acrw 2 14:72 acrw

3 5:6 acrw 4

1 acrw 3=2 3

2 3 ) acr=w ) acr 4The parameters used and the calculation results are shown in Table 1.

2.4. Fractographic analysis of fracture surfaces

a0[in]W [in]b [in]K1C psif(a0/w)DK ps

DP [lbacr

E. Hershko et al. / Engineering Failure Analysis 15 (2008) 2027 230.375 1.21.89 1.960.316 0.472

in

p 32,000 2,70,0004.268 14.34

iin

p 12,000 32,900

] 1100 15163.2. Fractographical analysis of fracture surfaces

As stated above, 3 lines leading from the origin to the nal rupture zone were chosen on each fracture sur-face (denoted top, middle and bottom) and the striation density was measured at 10 dierent locations alongthe lines. Based on this data, graphs were generated showing the crack density (dN/da) as a function of thedistance from the origin (L) a separate graph was made for each line. The total number of striations (i.e.,load cycles) in each specimen was then calculated by approximating the area under the corresponding graph.

Fig. 4 shows a typical area in the fracture surface of one of the aluminum specimens. Fig. 5 shows a typicalfracture surface of one of the steel specimens. Figs. 6 and 7 show the graphs representing the middle line in thealuminum 1 and the steel 1 specimen, respectively.

Table 3 shows the total number of striations calculated for each line in each specimen.

Table 1The parameters and calculation results

Aluminum specimen Steel specimenThe fractured specimens were rst examined using a stereomicroscope in order to determine the length ofthe fatigue crack (i.e., the critical crack length), the location of the fracture origin/origins and the crack prop-agation path.

Next, the fracture surfaces were examined by JEOL JSM-7000F Field Emission Scanning Electron Micro-scope (SEM). Three separate paths connecting the main origin to the nal rupture zone were chosen and 10areas equally spaced along each path were photographed. The number of fatigue striations (N) in a unit oflength (a) was counted in each area and graphs were generated showing the relationship between the striationdensities (dN/da) in each area as a function of the distance of that area from the origin (L).

Note: A separate graph was generated for each of the three paths chosen on each specimen.These graphs were then compared with the direct mechanical measurements.

3. Results

3.1. Preparation of the specimens and measurement of crack growth rate

The results of the measurements performed using the COD are summarized in Table 2.It is important to note that the total number of load cycles applied to the specimens was measured from the

start of measurable propagation occurred at the indicated distance from the crack origin.1.096 1.76

Table 2The results obtained from the fatigue experiment

Specimen Total number of load cycles Distance from origin (mm) Critical crack length (mm)

Aluminum 1 50,250 3.8 32Aluminum 2 35,875 0 27Steel 1 59,381 0.67 29Steel 2 97,752 1.63 33

Fig. 4. Typical fracture surface of an aluminum sample and measurement lines.

Fig. 5. Typical fracture surface of a steel sample. One measurement line is shown.

24 E. Hershko et al. / Engineering Failure Analysis 15 (2008) 2027

Fig. 6. Experimental and fractographic results aluminum sample.

Fig. 7. Experimental and fractographic results steel sample.

Table 3The total number of striations calculated for each line, in each specimen

Specimen Line Total number of striations Average total number of striations

Aluminum 1 Top 54,784 57,696Middle 59,098Bottom 59,207

Aluminum 2 Top 36,563 33,162Middle 31,590Bottom 31,334

Steel 1 Top 58,204 59,446Middle 58,795Bottom 61,340

Steel 2 Top 50,837 57,397Middle 60,941Bottom 60,412

E. Hershko et al. / Engineering Failure Analysis 15 (2008) 2027 25

26 E. Hershko et al. / Engineering Failure Analysis 15 (2008) 20274. Discussion

The relative errors (in percentage) of the average total counted striations with respect to the number of loadcycles applied to the specimen are shown in Table 4. The table also shows the relative error with regard to theline where the total number of striations counted was closest to the total number of the applied load cycles.

In three out of the four specimens, a good conformance between the calculated number of striations and thereal number of cycles applied during crack propagation was found.

It is however notable that the conformance of the average error column with the real number of load cyclesis lower.

These errors are the result of the following problems and diculties that one must be aware of when count-ing fatigue striations by a SEM:

1. One must always make sure that the area of the striations counting is planar and normal to the electronbeam. Any inclination of the inspected area will result in an inaccurate density measurement, which directlydistorts the total number of calculated striations.

2. It was found that counting 10 equally spaced areas along three dierent lines from the origin to the nalrupture zone was a redundant procedure. No distinguishing characteristics were found inherent to anyone line on a specimen and no signicant variation of the striation density was observed. Hence, it is rec-ommended to concentrate on one line, along which to examine 23 dierent areas at each location, in orderto minimize the errors.

3. It is imperative to make sure that the line one follows from the origin to the nal rupture zone exactly fol-lows the path of the crack propagation. This can be achieved by carefully examining the shape and direc-tion of the fatigue striations and fan lines on the fracture surface. Counting striations in locations from twoor more dierent paths will aect the results of the calculation.

4. While examining the fracture surfaces, it became evident that there were several degrees of striations. Forexample, in steel specimens, large fatigue tears are visible in magnications of 25006000, ne fatigue tearsare visible in magnications of 10,00020,000 and even ner fatigue striations are visible in magnicationsof 20,00045,000 (the same is true for aluminum specimens, but there the smallest striations are alreadyvisible at 20,000 magnication). Although occasionally dicult to nd, only the nest striations truly rep-resent a single load cycle. Counting the larger striations (at lower magnications) will inevitably yieldresults far smaller than the actual number of load cycles applied to the specimen.

The inaccuracies found in the project were, therefore, a result of one or more of the above diculties. If allthe above conditions are met, counting striations using an electron microscope should yield the number ofload cycles with high accuracy.

Table 4The relative errors (in percentage) of the counted striations with respect to the number of load cycles applied to the specimen

Specimen Average error percentage (%) Best line error percentage (%)

Aluminum 1 14.8 9Aluminum 2 7.6 +1.9Steel 1 +0.1 0.98Steel 2 41.3 37.655. Conclusions

This study shows that a scanning electron microscope may be a very eective tool to determine, accurately,the number of fatigue cycles that propagated a crack. We used striation counting to evaluate the number offatigue cycles on aluminum alloy AA-2024-T3 and low alloy steel AISI-4130 annealed specimens.

We analyzed the results and identied several factors that may distort the results (inclination of theinspected area, straight line from origin to the nal rupture zone and degree of striation counted). Properaccount for these issues will increase the reliability of the results.

Our experiments were performed with a xed load value and frequency, in a controlled environment.Therefore it is unclear how accurate this procedure will be in an actual failure investigation, where these valuesmay vary constantly. This should be the subject of a further research.

References

[1] Connors WC. Fatigue striation spacing analysis. Mater Charact 1994;33(3):24553.[2] Odrico J, LeGall Ph. Striation counting on fatigue fractured light alloys. Rev Phys Appl 1974;9(July):67381.[3] Khan Z, Rauf A, Yonas M. Prediction of fatigue crack propagation life in notched members under variable amplitude loading. J Mater

Eng Perform (USA) 1997;6(3):36573.[4] Forth J, Schutz W, Crack Propagation Under Constant and Variable Stress Amplitudes: A Comparison of Calculations Based on the

Striation Spacing and Tests, AGARD (NATO), vol. CP-376, no. CP-376, November 1984. p. 17.117.9.[5] Goswami T. Flange bolt failure analysis. J Mech Behav Mater (UK) 1999;10(4):20514.[6] Gerberich WW, Nelson JC, Jungk JM. Fatigue life predictions in small volume components. In: Fatigue: A David L. Davidson

symposium as held at the 2002 TMS annual meeting, Seattle, WA, USA, 1721 February 2002. p. 12134.

E. Hershko et al. / Engineering Failure Analysis 15 (2008) 2027 27

Assessment of fatigue striation counting accuracy using high resolution scanning electron microscopeIntroductionExperimentalMeasurement of crack growth rate (da/dN)Experiment parameters determinationCritical crack lengthFractographic analysis of fracture surfaces

ResultsPreparation of the specimens and measurement of crack growth rateFractographical analysis of fracture surfaces

DiscussionConclusionsReferences