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Structural Integrity
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Procedia Structural Integrity 2 (2016) 1077-1084
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21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy
Influence of Corrosive NaCl solution on Life times of 7075 Aluminum alloy under Combined Fatigue loading in the VHCF
Regime
M. Meischela, S.E. Stanzl-Tschegga'*, Ab. Arcarib, N. Iyyerb, N. Phanc
"University of Natural Resources and Life Saiences, BOKU Vienna, PeterJordan St. 82, 1190 Wien, Austria technical DataAnalysis, Inc.(TDA), 3190FaiCview Park Drive, Suite 650, Falls Church, VA 220)442, USA CUS Naval Air System Command Patuxent River, MD, USA
Abstract
The loading histories of aircrafts and rockets are complex and the components are exposed to combined loading conditions with high numbers of high-frequency cycles superimposed to lower-frequency carrier vibrations. The amplitudes of both frequency ranges usually are variable. "The small-amplitude cycles arise from several discrete sources suchas structural vibrations which are especially critical at special components and places of an aircraft. The environment plays another important role andambient air and sea-water have to be coneidered. Almost no knowledge existe of" the msi^te^^^ response in the VHCF aegime, especially cnhcerning the environmental influence. In this study, experiments were performed on aluminum alloy 7075-T651 in a 3.5%-oormm-chloride solutionr and in laboraeory ah o022 °C of 50% relative humidity ad high (10C - 108) and very high numbers of cycium (5 x 108 - 1010). The toading sequences consistedof a lowrfrequency square wave (0.4 to 1 Hz)a being superimposed with a high frequency 20 kHz random vibration. The random vibrations were simulated by aGauss distribution allowing stress ranges of 10 to 70 MPa or alternatively 50 to 90 MPh. Seeso/slroin vs. life time curves were measured. The reeults "were correlated with m-siiu microscopy obsentations of the specimen surfaces and wiih poei-experimental fdacture-surface imagee. This tedmology allowed identifying fatigue crnck initiation amd propagation stages bping needed ftr an SvOerpreration of the relevgni fatigue-life mechanisms under enviroIlmenCcl influences.
Copyrighi © 2(n6 The Authore, Published by Elseviec B.V. This is ao open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under re sposcibüity of the ¡Scientific Committee of ECF21 .
Keywords: Superimposed loading; 20 kHz-random fatigue; very high cycle fatigue; corrosive environments; 7075 Al alloy.
* Corresponding author. Tel.: +43 1 47L54 51L0; fax: +43 1 47L54 5159. E-mail address: Stefanie.tschegg@boku.ac.at
Copyright © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer review under responsibility of the Scientific Committee of ECF21.
10.1016/j.prostr.2016.06.138
1. Introduction
Fatigue loading of any automobile part consists roughly of two types. One is high-amplitude loading at a relatively low frequency, and the second is a high-frequency load with a small amplitude which is superimposed to low-frequency loads. Thus, a combined cyclic load (CCF) is generated (Stanzl-Tschegg et al., 2015). Since automobiles and also other moving vehicles are several years in service they are exposed to high or even very high numbers of cycles. Therefore testing of the material response in the HCF and VHCF regimes is indispensable. Experimental studies, however, are rare, and researchers tried to solve problems with models based on fatigue properties at lower numbers of cycles. Experimental testing is even more demanding if i.) super-imposed loads have to be simulated, if ii.) the amplitudes are varying and iii.) the environment is corrosive. Therefore, almost no experimental results can be found in the literature.
The presented combined-cyclic load experiments allow to simulate actually occurring loadings quite well. In addition, the prevailing environmental conditions have to be considered since most machine parts and materials are not used in laboratory atmosphere. Pronounced changed life-times have to be expected which has been shown by Schönbauer et al. (Schönbauer et al., 2014) for different steels. In this study, the influence of different environments on the service fatigue lives of 7075 Al-T651 was studied systematically. Large influences were also detected in measurements on other Al-alloys (Fitzka et al., 2014, Mayer et al., 2013, Mayer et al., 2014), Mg-alloys (Mayer et al., 1999) and Ti-alloys (Sarrazin-Baudoux et al., 2016).
Former investigations on the 7075-T651 alloy (Arcari et al., 2015, Meischel et al., 2015, Stanzl-Tschegg et al., 2015) were performed in laboratory air, whereas fatigue loading of 7075-T651 in 3.5% NaCl solution is main issue of the present study.
In this paper, material, experimental set-up, measuring and evaluation procedure are described and some details of the results are reported and shortly discussed. A few conclusions are drawn finally.
2. Material and Experimental Details
2.1. Material and Specimen Preparation
The material was delivered in form of 20 mm thick plates which had been heat treated according to T651. The chemical composition was (in wt.%): 0.11 Si, 0.16 Fe, 1.5 Cu, 0.083 Mn, 2.6 Mg, 0.18 Cr, 0.005 Ni, 5.73 Zn, 0.033 Ti, 0.013 Ga, 0.015 V and REM Al. The mechanical properties were: Modulus of elasticity: 72 GPa, tensile stress: 540 MPa, yield stress: 470 MPa, fracture strain 12% and hardness 163 HV.
The material was machined with a high-precision automatic lathe to shapes as shown in Fig. 1(a) the specimens were polished to grade #600 parallel to their length axis afterwards. Fig. 1(a) shows that the central part of the dumbbell-shaped cylindrical specimen with a total length of 54.8 mm and a central cylindrical part with a length of 10 mm. The diameter of this part is 4 mm.
2.2. Experimental Setup and Process Description
In the experiments, high-frequency cyclic vibrations were superimposed to such of low frequency. The low-frequency wave was produced by a servo-hydraulic testing machine (MTS TestStar) at a frequency of 0.4, 0.5 or 1 Hz and was rectangular-shaped. The high-frequency vibration was generated by an ultrasonic-fatigue machine which was operated in resonance at 20 kHz (Mayer, 1999, Stanzl, 1981). As shown in Fig. 1(b), ten ultrasonic blocks, each of which comprises several thousand cycles were superimposed to one vibration of the low-frequency wave. The ultrasonic blocks were 100, 200 or 250 ms long which is equivalent to 2000, 4000 or 5000 cycles. The amplitudes of each ultrasonic block were quasi-randomly distributed according to a Gauss distribution (^ = 0.0, g = 0.3477, Fig. 2).
H FF es—^ B-
co c/) CD
•if "a,hf. ill 1
^min Villi '
Fig. 1. (a) Specimen shape; (b) Schematic of superimposed loading consisting of constant-amplitude (CA)-square-shaped low-frequency carrier
wave and randomly varying 20 kHz blocks.
The total minimum stress omm was 20 MPa in all tests and the total maximum stresses omax were 340, 360, 380 or 400 MPa. The amplitude of the high-frequency amplitudes varied according to a Gauss distribution between 90 and 50 MPa or, respectively, between 70 and 10 MPa. Distribution and cumulative frequency of the amplitudes are shown in Fig. 2(a) and (b).
Fig. 2. (a) Distribution of ultrasonic random signals with amplitudes between 90 and 50 MPa; (b) Cumulative frequency of amplitudes.
It is not possible to measure the amplitude of each 20 kHz cycle during the experiment. Therefore, prior calibration values are used that were obtained with strain gauges which recorded the strains at defined vibration amplitudes. From this calibration, the stress values are derived by multiplying the strain with the modulus of elasticity (assuming elastic loading).
For the evaluation, the low-frequency signal and the envelope curve of the positive ultrasound signals are recorded for each single block. These data are stored as binary-coded National Instruments TDMS files. Subsequently histograms as shown in Fig. 3 are generated.
Fig. 3(a) shows the histogram of the superimposed 20 kHz stress amplitudes. The idealized curve is shown with vertical green lines. Idealized means that pre-given stress-amplitude signals are evaluated. In other words, the curve would be like this if the used equipment would work without deviation. The actually realized values are shown in a
{OQ7{zyP{zy7{zzy7
dashed manner. In Fig. 3(b), the cumulative frequency of idealized and realized stress amplitudes is plotted. They are almost identical.
109 » 1°8
>• 7
J 106 105
ideal amplitude realized amplitude
ê 10£
100 200 300 400 Stress Amplitude in MPa
ideal amplitude
--- realized amplitude
100 200 300 400 Stress Amplitude in MPa
Fig. 3. (a) Histogram of an experiment with pre-given and realized stress amplitudes; (b) Cumulative frequency of stress amplitudes (shown in
2.3. Measurements in Corrosive Environment
Part of the experiments was performed in a 3.5% NaCl solution. For this, a corrosion device, as shown in Fig. 4(a) was used (Schönbauer et al., 2014). Test chamber and solution reservoir are made of stainless 316L und 316Ti steel. The front plate of the test chamber is made from Polycarbonate in order to make a visual observation of the specimen possible. The facility contains sensors for measurement of temperature, electric conductivity and oxygen content as well as of the corrosion potential. The reservoir contains 30 l of the NaCl solution. For more details see (Schönbauer et al., 2014). In Fig. 4(b), the impact of the corrosive attack of the NaCl solution is visible. The right specimen was stored for 48 hours without applying a load in the 3.5% NaCl solution.
Fig. 4. (a) Corrosion facility (Schönbauer et al., 2014); (b) Surface of a polished reference specimen (left) and a specimen exposed without load
application to 3.5.% NaCl solution during 48 hours (right).
2.4. Testing Program
The testing program comprises three parameters that can be varied. One is variation of environment such that, at the begin laboratory air (23 °C and 50% relative air humidity (RH)) is used and afterwards laboratory air or 3.5% NaCl solution.
Second parameter is the variation of two random sequences of the high-frequency load as described in section 2.2. In the first sequence, cra,hf varies between 10 and 70 MPa and in the second one between 50 and 90 MPa.
Third parameter is the frequency of the low-frequency rectangular-shaped load, ff which means that the block-lengths of the high-frequency vibration are varied. The ff were chosen to be 0.4, 0.5 und 1 Hz so that, block lengths of 250, 200 and 100 ms resulted.
With this, 12 life-time curves were obtained. The total minimum stress amplitude omm was 20 MPa and the total maxima cw were 340, 360, 380 und 400 MPa.
3. Results
3.1. 90/50 MPa AIR
The life-times of the super-imposed loading sequences with stress amplitudes varying between the stress amplitude oa,hf = 50 and 90 MPa. The minimum stress omm was 20 MPa. In Fig. 5(a) the results obtained in laboratory air are shown and in Fig. 5(b) those in 3.5% NaCl solution are presented. Scatter of the results is pronounced, and 50% fracture-probability lines are plotted. These lines indicate that the life-times tend to be shorter for shorter block-lengths or, in other words, fewer low-frequency vibrations. The data for ccmax = 400 MPa do not follow this trend.
3.2. 90/50 MPa NaCl
The measurements in the 3.5% NaCl solution (Fig. 5(b)) likewise show a trend of shorter life-times for shorter block-lengths, i.e. fewer low-frequency vibrations. Scatter of the results is smaller than in air.
Cycles to failure Cycles to failure
Fig. 5. Life-times for superimposed loading: 10 high-frequency variable-amplitude sequences with ca,hf = 90/50 MPa and cmi„ = 20 MPa superimposed each to one constant low-frequency square wave: (a) Laboratory air (23 °C, 50% RH); (b) 3.5% NaCl solution.
3.3. 70/10 MPa, Air
Non-fracture was observed for 1 Hz carrier waves only at the lower total maximum stress omax in contrast to those of ff = 0.4 or 0.5 Hz. In addition, the values for ff = 0.4 Hz, i.e. block lengths of 250 ms look strange. Fractures and run-throughs occurred at all stress levels.
3.4. 7(/1(MPa, NaCl
Since fracture occurred only after more than 109 cycles in three cases and all other specimens survived at the second highest total stress level (omax = 380 MPa) no additional tests were performed at lower total stress levels. The results for block lengths of 250 ms look strange (Fig. 6(b)) which is a similar behavior as in air environment.
Cycles to failure Cycles to failure
Fig. 6. Life-times for superimposed loading: 10 high-frequency variable-amplitude sequences with oa,hf = 70/10 MPa and omin = 20 MPa superimposed each to one constant low-frequency square wave: (a) Laboratory (23 °C, 50% RH) air; (b) 3.5% NaCl solution. For data points 1, 2 with green circles see fracture surfaces in Figs. 8(b), 8(c) and Figs. 9(b) and 9(c).
4. Discussion
Plotting the 50% survival lines of Figs. 5 and 6 without data points makes a comparison and interpretation of the fatigue response to superimposed loading in the VHCF regime easier, see Fig. 7.
Fig.7. Comparison of 50% survival lines for three different block lengths. (a) block length 100 ms; (b) block length 200 ms; (c) block length 250 ms. Each diagram contains the results with 90/50 MPa sequences in air, 70/10 MPa sequences in air and 70/10 MPa sequences in 3.5% NaCl
solution. In (b) and (c) also the 90/50 MPa sequence results are plotted.
Survival is most probable if the specimens are random-loaded with stresses between 70 and 10 MPa in NaCl solution. For block lengths of 250 ms, almost no differences between measurements in air and NaCl solution can be recognized. For 100 and 200 ms, significant differences are visible.
In the following, fractographic structures which were obtained with SEM in the SE and BS mode together with EDX studies are discussed. Fig. 8 visualizes the features for loading in laboratory air and Fig. 9 those for 3.5% NaCl solution. Loading of the specimen in Fig. 8 was crmax = 400 MPa, oa,hf = 70/10 MPa and the block length was 200 ms, see green circle with the number 1 in Fig. 6(a). The specimen broke after 8.96 x 107 cycles with crack initiation from an inclusion. Fig. 8(a) gives an overview showing different fracture areas. Figs. 8(b) and 8(c) were taken in the back-scatter mode with two different magnifications in order to make the inclusions better visible. The size of the inclusion is about 20 ^m, and EDX studies on this and all other specimens showed the elements Al, Fe, Zn and Cu representing intermetallic inclusions. No oxides could be found. Figs. 8(b) and 8(c) show a transcrystalline fracture mode, Fig. 8(b) makes the crack advance in parallel bands visible that reflect the block by block-load sequences. In Fig. 8(c), secondary cracks perpendicular to the main crack and image plane may be recognized.
Fig.8. Fracture surface of specimen 1 (see Fig. 6(a)) after superimposed loading during 8.96 x 107cycles with omax = 400 MPa, and oa,hf = 70/10 MPa and a block length of 200 ms; (a) overview; (b) and (c) magnifications of (a).
A similar evaluation of the fracture surfaces was performed on specimens that were fatigued in 3.5% NaCl-solution. The fracture surface of specimen 2 (Fig. 6(b)) is shown in Fig. 9. It was loaded with o-max = 400 MPa and oa,hf = 70/10 MPa and a block length of 200 ms and broke after 2.30 x 108 cycles. Fig. 9(a) is an overview of the different fracture areas. Mainly two fatigue-crack initiation and propagation areas are visible which both started from the specimen surface. One crack started from the top in Fig. 9(a) and the second one at the right side. The final fracture surfaces are rougher.
Fig. 9. Fracture surface of specimen 2 (see Fig. 6(b)) after superimposed loading during 2.30 x 108cycles with omax = 400 MPa, oa,hf = 70/10 MPa and a block length of 200 ms; (a) overview: Two crack initiation sites; (b) and (c) magnifications of (a) showing the two crack initiation sites.
In summary, the following conclusions may be drawn:
A corrosive attack of the NaCl solution takes place at the specimen surfaces which causes surface crack initiation and no interior cracks. One or two surface cracks were also formed perpendicular to the main fracture surface. It is assumed that, NaCl solution penetrating along grain boundaries is responsible for this. The fracture surfaces are rougher, more brittle and show deeper and secondary cracks to a greater extent. These secondary cracks are probably responsible for the observed fatigue-crack growth retardation.
Acknowledgements
This work is based on research funded by US Naval Air System Command under guidance of Dr. Nam Phan. Special thanks go to Dr. Bernd Schönbauer for providing the corrosion chamber and Dr. Michael Fitzka for experimental support with the equipment.
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