A pilot study into microstructuralaspects of fatigue in AA6082-T6

and AA2024-T3

L.G.J. GoorenMT05.10

Supervisor:

Dr.Ir. R.H.J. Peerlings

Contents

1 Introduction 3

2 Fatigue 4

3 Experimental Techniques 7

3.1 Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2 Material as received . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3 Uniaxial fatigue testing . . . . . . . . . . . . . . . . . . . . . . . 8

3.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Microscopic analysis of the fractured samples 12

4.1 Microscopy methods . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2 Surface condition . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.3 The fracture surface . . . . . . . . . . . . . . . . . . . . . . . . . 15

5 Conclusion and recommendations 19

A ESEM images 22

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Acknowledgements

I would like to take this opportunity to thank the following people for helpingme on the way and for helping me during this project. I would like to thank Marcvan Maris for helping me with the ESEM, Cees Meesters and Ron Peerlings forgiving me useful advice during the project.

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Chapter 1

Introduction

Nowadays airplanes are largely made of aluminium alloys. Due to cyclic load-ing, components made of these materials may develop fatigue cracks, which canultimately cause failure. By understanding the mechanics of fatigue, it is pos-sible to predict numerically when a component fails. To validate such models,experimental test are needed to compare with numerical predictions.

The main objective of this report is to examine the possibilities to accom-plish fatigue tests with the equipment available in the MaTe laboratories andsubsequently visualize microstructural changes which may result in fatigue. Mi-croscopic methods which are used include optical microscopy and the environ-mental scanning electron microscopy. First a short introduction in fatigue isgiven. This will be used for the experimental setup. Next, the fatigue effects arevisualized.

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Chapter 2

Fatigue

Fatigue is a form of failure that occurs in structures subjected to dynamic andfluctuating stresses. It is possible for failure to occur when stresses are lowerthan the yield stress [1] [2].

Several parameters are used to characterize the fluctuating stresses which resultin fatigue damage. If identical cycles of a harmonic form are assumed, we canwrite

σ = σm + σa sin(ωt) (2.1)

Where σm is mean stress and the stress amplitude is:

σa =σr

2=

σmax − σmin

2(2.2)

where σr is ∆σ and σmax and σmin are the maximal and minimal stress. Thestress ratio R is the ratio of minimum and maximum stress amplitudes.

R =σmin

σmax(2.3)

For a reversed stress cycle, the value of R = −1 results. For this value thestress alternates from a maximum tensile stress to a minimal compressive stresswhich are symmetrical relative to the zero stress level, σm = 0. Another typeis repeated stress cycle, which uses −1 < R < 1. The maxima en minimaare asymmetrical to the zero stress level. The fatigue life depends on stressamplitude and mean stress. Increasing the mean stress will lead to a decreasingfatigue life.

The fatigue damage process occurs by the initiation and propagation of cracks.This happens mostly in Mode I cyclic tensile loading. At a certain crack length

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Figure 2.1: Illustration of the transition from flat fatigue crack propagation ona slant plane [5]

the material cannot sustain the load anymore and will fracture under 45◦ in ashear mode (figure 2.1). Fatigue crack growth is characterized by three stages,which are affected by the slip characteristics of the material, characteristic mi-crostructural dimensions, applied stresses and the extent of near-tip plasticity.

Stage I: fatigue crack initiation

At points of high stress concentration micro-cracks will form. Crack nucleationsites include surface scratches, sharp fillets, threads, dents. In addtition, cyclicloading can produce microscopic surface discontinuities resulting from disloca-tion slip steps that may also act as stress raisers, and therefore as crack initiationsites.

Stage II: crack growth and fatigue striations

At a certain stage during the loading, one or a few micro-cracks become dom-inant and continue to grow faster than the remaining micro-cracks. This stageof the fatigue damage process is termed (macroscopic) crack growth and showsa certain amount of stable crack growth in each loading cycle.

In this stage striations are formed. Most alloys shows striations. Fatigue stria-tions are microscopic marks on the fracture surface which are left behind everyloading cycle. Each striation is thought to represent the advance distance of thecrack front during a single load cycle.

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Stage III: final fracture

When the crack reaches a critical size, final fracture will occur very rapidly,usually within a single cycle. Striations will not appear in the region of finalfracture. The character of the final fracture may be ductile or brittle, dependingon the material and loading conditions.

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Chapter 3

Experimental Techniques

This chapter describes the experimental details of the fatigue tests which werecarried out in the course of the project.

3.1 Specimens

Two different types of specimens have been machined from sheet metal (seefigure 3.1). To have a constant influence of anisotropy, the specimens were allmade in the rolling direction. No surface finishing has been done.

Figure 3.1: Two specimen types (dimensions in mm, dashed arrow indicates therolling direction)

The specimens follow the ASTM standard E466, ”Standard Practice for Con-ducting Constant Amplitude Axial Fatigue Tests of Metallic Materials”. A spec-imen with a continuous radius between the ends and a specimen with tangen-tially blending fillets between the uniform test section and the ends have been

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considered. The latter was used for static tensile testing to determine relevantmechanical properties of the material. This is described in the next section.

The specimen with a continuous radius was used for fatigue testing. This sampleusually fails at the smallest width and so it is not difficult to locate possibledamage at the surface using a microscope when the sample did not fracture yet.

3.2 Material as received

Two aluminium alloys have been used to perform fatigue tests, AA6082-T6(Al Si1MgMn, Solution heat treated and artificially aged) and AA2024-T3 (AlCu4Mg1 Solution heat treated, cold worked and naturally aged). Mechanicaland chemical properties of the two materials are given in table 3.1. AA6082-T6is used in heavy duty structures, like truck frames, ships and bridges. AA2024-T3 is often used for high strength components in the aircraft industry.

Table 3.1: Chemical composition

alloy Si Fe Cu Mn Mg Cr Zn Ti NiAA6082-T6 0.7-1.3 ≤0.50 ≤0.10 0.40-1.0 0.6-1.2 ≤0.25 ≤0.20 ≤0.10 -AA2024-T3 0.50 0.50 3.8-4.9 0.30-0.94 1.2-1.8 0.10 0.25 0.15 0.05

A tensile test has been performed on both alloys to determine the yield strength,σy. For AA6082-T6 σy = 290MPa and AA2024-T3 σy = 350MPA. Accordingfigure 3.2 shows the tensile curves obtained in these tests the material specifi-cations of both alloys given by the manufacturer the minimal yield stress is 290MPa.

3.3 Uniaxial fatigue testing

In order to test in a controlled manner, all high cycle fatigue tests were per-formed in force control mode and at room temperature. The fatigue experimentswere carried out with a MTS 810 Elastomer Test System with a calibrated load-cell of 25 kN. To be able to compare both alloys the same σm and σa were used.

Experimental data found in the literature [4] indicates 108 cycles to failure ata fully reversed stress amplitude of 280 MPa. To prevent buckling and shortenthe experimental time, repeated stress cycles with a positive mean stress (σm =150 MPa) and higher stress amplitude (σmax = 280 MPa) were used in ourexperiments. The specimens are carefully oriented in the loading direction andfixed by mechanical clamps.

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0 0.05 0.1 0.15 0.20

50

100

150

200

250

300

350

400

450

500tensile test

ε [−]

σ [M

Pa]

AA2024−T3AA6082−T6

Figure 3.2: tensile test of material as received

The loading program is schematically described in figure 3.3:

1. Gradually increasing the force to σm ·A in 10 seconds.

2. The next 10 seconds the stress remains constant at the mean stress.

3. A tapered sine is added with range σr · A. The full amplitude is reachedin approximately three cycles.

4. This continues until the sample fractures (machine will shut down) or theprogram ends.

5. The force is relieved to zero.

The time needed for an experiment depends greatly on the frequency of the ap-plied loading. Applying a small and positive stress ratio limits the frequency ofthe current setup to about 0.5 Hz. Because of the stiffness of the specimens smalldisplacements must be generated the test system. Applying a higher frequencyis not possible, because the valves of the hydraulic system are not accurateenough to cope with small and rapid displacements. Tuning the system is im-portant to get a stable response and allows a slightly higher frequency. Smallerand more accurate valves would make a higher frequency possible.

A higher positive R allows a higher frequency, because σr is lower and thehydraulic pressure fluctuation is lower. This however stretches the experimental

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Figure 3.3: Schematic view of the fatigue program

Table 3.2: Indication of the time needed for an experiment depending on fre-quency and presuming 100000 cycles till failure

frequency [Hz] time [h]0.5 55.51 27.85 5.510 2.8

time. Therefore a frequency of 0.5 Hz has been used. To indicate the time(depending on frequency) an experiment takes table 3.2 is included.

3.4 Results

There is a large scatter in the measured fatigue life of the specimens, as shownin table 3.3. AA2024-T3 has a relative scatter of ±50%.

Note that this scatter is related to just three experiments. To obtain a betterindication of the scatter, more experiments are necessary. A scatter of 50% isrelatively large for the applied stresses. Normally, relatively large scatter occursfor low stress amplitudes and fully reversed stress cycles.

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Table 3.3: Fatigue life (N-cycles)

alloy 1 2 3AA6082-T6 25074 33271 -AA2024-T3 125019 79562 50601

Fatigue life is sensitive to test and material parameters. Several reasons can bementioned:

1. Surface effects. Minor scratches and dents affect the fatigue life. This isprobably the most important cause of scatter in our experiments. Polish-ing the surface of the specimens before testing will reduce these effects.Besides that, polishing also has the advantage that changes in the surface’sappearance are more easily detected.

2. Test system variations. During long experiments the response differs morefrom the input command. At the end of the experiments there is no propersine left. This is probably because small particles stick at the valves. Aftercleaning the test system this problem did not occur.

3. Due to small differences in geometry, the stresses in the specimen differfrom each other. The specimens are carefully milled. However, an inaccu-racy of ± 0.1 mm is still possible. There is also some variation in thick-ness of the material as received. The influence on the fatigue life is yetnot known. More experiments are needed to understand the importanceof this.

4. Due to misalignment in the clamps the stress distribution may be differ-ent. The specimens are carefully oriented in the loading direction. Smallmisalignments affect the fatigue life.

5. Metallurgical variations. Faults in the micro structure affect the fatiguelife. Micro structure is always different in each specimen. It is not possibleto take these variations into account, when performing fatigue tests.

As seen above, it is impossible to control all parameters. But addressing someof these parameters will decrease the fatigue life scatter.

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Chapter 4

Microscopic analysis of thefractured samples

4.1 Microscopy methods

Optical microscopy

An optical microscope provides the means of creating a magnified image. Theease of use is an advantage of this type of microscopes. It creates instantaneouslyan image. The main disadvantage is its lack in depth resolution.

Electron microscopy

Environmental Scanning Electron Microscopy (ESEM) was applied in orderto make observations of the fracture surface appearance of the fatigue speci-mens. In addition, the ESEM was also used to determine chemical propertiesof AA2024-T3 by using Energy Dispersive X-ray (EDX).

After fatiguing the samples, microcracks on the surface are visible with a mi-croscope. Figures 4.1 and 4.2 show an optical and an ESEM image at the samespot. The main difference between the images is the lack of axial depth res-olution in the optical image. Due to the grooves in the rolling direction thematerial has a relatively rough surface. This will certainly be a problem whenone wants to look at the fracture surface. Since an optical microscope is notsuitable to cope with larger depth differences, the ESEM will mostly be usedto obtain images.

4.2 Surface condition

The surface of AA2024-T3 shows several phenomena, which will be discussedhere.

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Figure 4.1: Optical image Figure 4.2: ESEM image

Particles at the surface

AA2024-T3 as received shows several particles at the surface (like the brighterspots in figure 4.3). The particles are examined.

Figure 4.3: SE image Figure 4.4: EDX image, Al (blue),Cu (yellow), Mg (green)

Figure 4.4 shows clearly that the particle consist mostly of Cu. Less remarkablein figure 4.3, however present in figure 4.4 is Mg. The bright vertical line on theleft side is an error of the system. The cause of this error is unknown.

Grain boundaries

Figure 4.5 appears to show the grain boundaries at the surface as a result ofplastic deformation. Due to the rolling grooves it is difficult to see the grainboundaries. As indicated before, polishing will improve the visibility of thegrain boundaries.

Voids

As indicated before, surface effects (e.g. scratches) limit the fatigue life. Thesurface of the AA2024-T3 exhibits small voids of≈ 5µm in size, figure 4.6. Thesevoids are points of stress concentration and possibly crack initiation. Indeed,several micro cracks emanating from such a void can be observed. These voids

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Figure 4.5: Grain boundaries of AA6082-T6, the white lines indicates possiblegrain boundaries, the vertical lines are grooves caused during rolling

appear not only occur at the surface, but also in the bulk of the alloy. The ESEMimage in figure 4.8 shows similar voids on the final fracture surface. They areof the same size as the voids in figure 4.6. The numerous cup-like depressionsare dimples formed by ductile fracture. Similar voids are also present on thesurface of the sheet metal as received. It is not clear whether the initiation of amacroscopic crack is the result of micro cracking at the surface voids.

Figure 4.6: Surface voids

In attempt to trace the origin of the voids an element analysis has been car-ried out. Figure 4.9 shows several elements around this void. Clearly visible isthe particle right of the void, which consists of Cu-Mn-Fe. Also the larger Cuparticle is visible.

No distinct elements are present in the void. This image however, gives a dis-torted view. Because of the contrast difference it looks as if there is less alu-minium in the void, since less information from the void reaches the detector.It shows only the density of elements ∼ 2-5 µm below the surface.

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Figure 4.7: Focus of one void

Figure 4.8: Voids on the ductile fracture surface

4.3 The fracture surface

Looking at the fracture surfaces of each failed specimen one can see the shapeillustrated in figure 2.1. The two main parts of the surfaces as indicated inthis figure can clearly be distinguished in the electron micrographs of figures4.10 and 4.11. The darker surface represents the fatigue crack, while the lightersurface represents ductile fracture.

Striations on fatigue surface

At the scale of microns both alloys show striations on the fatigue surface. Thespacing of the striations is not the same in the two materials (as can be seen infigures 4.12 and 4.13).

As indicated before, striations are formed during of stage II of the fatigue life.

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Figure 4.9: EDX image of a void

The spacing between each striation tells something about the fatigue crackgrowth life. For AA2024-T3 a rough estimate can be made as follows:

fatigue fracture surface lengthspacing of the striations

≈ 12500.2

µmµm

= 6250 (4.1)

These numbers have been measured in figure 4.11 and 4.13. This specimen hada fatigue life of 125019 cycles. The number determined from equation (4.1) is anindication of the crack propagation life. Roughly 6250 cycles before final fracturethe crack begins to propagate through the specimen, which means that only asmall fraction of the total fatigue life shows crack propagation and most of itis taken up by crack initiation. It should be noted that the spacing increasesas the distance from the crack initiation site increases [3]. So this number isactually smaller. For a given specimen, this estimate of the crack growth life,combined with the total fatigue life, can be used to determine the number ofcycles after which a crack was initiated. However, given the large scatter inthe total fatigue lives obtained, predicting crack initiation with a reasonableaccuracy is impossible via the method.

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Figure 4.10: Failure surface of AA6082-T6. The bumpy surface on the left istape used to fixate the specimen in the ESEM.

Figure 4.11: Failure surface of AA2024-T3. The bumpy surface below is tapeused to fixate the specimen in the ESEM.

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Figure 4.12: Striations on the failure surface of AA6082-T6

Figure 4.13: Striations on the failure surface of AA2024-T3

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Chapter 5

Conclusion andrecommendations

Conclusion

Using the MTS 810 Elastomer Test System for high cycle fatigue testing proveduseful, but the valves of the hydraulic system are not accurate enough to copewith small and rapid displacements. This limits the frequency used in our ex-periments to 0.5 Hz. Smaller and more accurate valves make higher frequenciespossible. The experiments have a relatively large scatter. The main causes aresurface effects and test system instability.

Some effects caused by fatigue are clearly visible using an optical microscopeand an ESEM, nevertheless the ESEM provides better contrast and resolution.Striations on the fracture surface have been made visible using an ESEM. Alsograin boundaries caused by plastic deformation are visible. Due the surfaceeffects is difficult to notice the boundaries.

AA2024-T3 shows voids of ≈ 5 µm through the entire alloy. These voids arepoints of stress concentration. EDX analysis shows no distinct elements presentin these voids. However, microscopic analysis shows cracks around these voids.It is unclear whether the final, macroscopic crack originates from these micro-cracks

Recommendations

1. Replace the hydraulic valves of the test system. More experiments canbe done in the same amount of time. If this is not sufficient, anothertest system which is more suitable for fatigue testing of stiff materials isnecessary.

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2. To look at crack initiation it is important to reduce scatter. Polishing thesurface before testing. This removes surface scratches and dents, which re-duces the scatter of fatigue life. Grains which slipped out the surface planeand other effects of testing are more visible using microscopic analysis.

3. Determine the crack initiation by means of striation spacing or wait till thefirst visible crack arises. Next analyze the crack initiation and propagation.

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Bibliography

[1] S. Suresh Fatigue of Materials (1998), Cambridge University Press, Cam-bridge

[2] William D. Callister, Jr. Materials Science and Engineering, an Introduction(2003), John Wiley and Sons Inc, New York

[3] Youri N. Lenets and Richard S. Bellows Crack propagation life predictionfor Ti-6Al-4V based on striation spacing measurements (2000), pp. 521-529International Journal of Fatigue

[4] http://aluminium.matter.org.uk/aluselect/

[5] http://www.vibanalysis.co.uk/technical/FatigueFracture.pdf

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Appendix A

ESEM images

This appendix shows several ESEM images which are not useful in the reportbut give a clear view of some fatigue effects.

Figure A.1: Crack at the surface.The Cu particles are clearly visible

Figure A.2: Some grains which arecoming out of the surface plane

Figure A.3: Small crack at the sur-face

Figure A.4: The crack initiation site

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