EFFECT OF LOAD ASYMMETRY AND MICROSTRUCTURE ORIENTATION ON FATIGUE CRACK GROWTH IN STABLE AND THRESHOLD REGIONS IN AN AIRCRAFT AL 2124-T851 ALLOY

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1 EFFECT OF LOAD ASYMMETRY AND MICROSTRUCTURE ORIENTATION ON FATIGUE CRACK GROWTH IN STABLE AND THRESHOLD REGIONS IN AN AIRCRAFT AL 2124-T851 ALLOY Ivo ČERNÝ SVÚM a.s., Podnikatelská 565, Praha 9, Czech Republic, EU, Abstract Results of an experimental programme aimed at evaluation of fatigue crack growth (FCG) rates in an aircraft Al 2124 alloy in T851 treatment conditions are presented and discussed. Measurement was performed in both regions of stable crack growth and threshold region. FCG rates were measured in both L-T and T-S directions to evaluate sensitivity of FCG resistance on different microstructure orientation. Different load asymmetry conditions R = F min / F max were used, namely R = 0.1 and 0.6. Repeated measurements in more than one specimen at each condition enabled to evaluate reproducibility, scatter of measurement and to perform regression analyses including confidence bands and tolerance limits. The reproducibility of measurement was particularly good for L-T orientation. In this case, threshold values were somewhat higher and FCG rates lower in comparison with the T-S orientation. The results are completed with statistical regression analyses, which enable to perform an engineering assessment of residual life in specific types of components. The results are also discussed considering microstructure aspects. Key words: Aircraft Al 2124 alloy, fatigue crack growth, threshold conditions, microstructure aspects 1. INTRODUCTION Al 2024 alloys are successfully being used in aircraft industry already for many years. Their low cost, lightweight, high strength and great fatigue resistance find excellent usage in parts and structures where a high strength-to-weight ratio is desirable. Al 2024 is a heat treatable alloy, and use in this condition is recommended. Its workability is good and it may easily be machined to high finishes. Its weldability is low, though it may be flash, spot, or seam welded if necessary. The corrosion resistance is rather poor, but in the "Alclad" condition, i.e. with Al cladding, its corrosion resistance is excellent 1. Dynamic components for aircraft structures have been typically designed for fatigue using the Palmgren/Miner rule. This rule, also known as the safe-life methodology, determines a safe-life for the component from an assumed usage spectrum, associated stress levels and the S/N curve for the component. However, the inability to quantify reliability, the cost of retiring parts that probably have no damage have led toward the damage tolerance design approach 2. The damage tolerance design philosophy has been already implemented into numerous aviation regulations. Damage tolerance design philosophy, accepting existing defects in structures or components provided that they can be reliably assessed not to cause unexpected failures, starts to be applied even in other industrial fields than aerospace, like automotive 3 ship building 4 or in structures with complicated, combined damage mechanisms like pitting, fatigue and corrosion fatigue 5. One of the reasons is a necessity to reduce costs of structures and their operation reducing not only amount of the material to be used, but also reducing the structure weight resulting in saving of fuel consumption in transport industry. Environmental aspects also have their importance. Note that the damage tolerance and reliability of such structures can

2 only be assured through appropriate materials selection, application of rigorous testing methodologies and through-life inspection and monitoring. This contribution contains results of a fairly comprehensive study of fatigue crack growth (FCG) in an Al 2124 alloy used for manufacture of certain components of aircraft with the aim to obtain necessary data to apply damage tolerance approach. 2. EXPERIMENTAL PROGRAMME Investigations and measurements of FCG was performed using Al 2124 T851 alloy. The Al 2124 alloy is a modification of the Al 2024 alloy. Their chemical composition is very close to each other, purity of Al 2124 being slightly higher. Therefore, particularly the version 2124 is being used in aircraft industry. The T851 state means solution heat treatment, rolling, cold working by stretching and artificial ageing. Standard chemical composition of Al 2024 alloy is in Table 1 6. The composition of Al 2124 differs just in Fe content ( 0.3) and Si content ( 0.2). Mutual comparison of basic mechanical properties of both the versions is in Table 2 6. Table 1 Chemical composition of Al 2024 alloy Element Al Cu Mg Si Mn Fe Zn Cr Ti Weight % Table 2 Basic mechanical properties of Al 2024 and Al 2124 alloys Alloy Strength (MPa) Yield strength (MPa) Elongation at break (%) HV Fatigue strength (MPa) Fracture toughness in L-T (MPa m 1/2 ) Al 2024-T Al 2124-T Hardness measurement carried out at SVÚM laboratory confirmed the declared value. Actual hardness HV10 was measured at nine point on different specimens and the actual values was Fig. 1a Microstructure at cross section of L-T specimen Fig. 1b Microstructure at longitudinal section of L-T specimen

3 Metallographical analysis was carried out after sectioning one of the specimens after the FCG measurement, where FCG in L-T direction was evaluated. Microstructure in cross section of this specimen, i.e. in a plane parallel to fracture surface, is in Fig. 1. Microstructure in longitudinal section of the specimen is in Fig. 2. Microstructure is typical for rolled and artificially aged conditions, containing elongated grains and fairly coarse precipitates. Quite a lot of inclusions could be observed, which were locally accumulated (Fig. 2). FCG measurement was performed using three-point-bend specimens of total length L = 140 mm, width W = 30 mm and thickness B = 10 mm. Load span was 120 mm. Mechanical high-cycle fatigue machine SCHENCK PHG was used, test frequency 40 Hz. Crack length and growth was measured using DCPD device and method developed at SVÚM laboratory earlier 7, 8. The DCPD method modified at SVÚM and the device enable to perform the measurement very exactly, which has been recently confirmed during a Proficiency Test Programme organised by Exova / GE Aviation using an Al 7075 material. Results obtained at SVÚM laboratory were one of the most exact between all the worldwide participating laboratories 9. The specimen prepared for the FCG measurement is in Fig. 2. Fig. 2. Specimen prepared for the FCG measurement Measurement was performed at two different load asymmetry, namely R = 0.1 and R = 0.6. It was very important to evaluate effect of load asymmetry besides different crack growth directions, because strong stress ratio and material dependence effects on FCG in Al alloys are characteristic 10. The differences are caused mostly by crack closure effects, which occur at low R values unlike at high values of load asymmetry like R = EXPERIMENTAL RESULTS AND DISCUSSION Results of FCG measurements are shown in the following three diagrams. Fig. 3 shows a comparison of FCG rates at the load asymmetry R = 0.1 in the two directions, L-T and T-S, respectively. The next diagrams Figs. 4 and 5 show comparisons between FCG rates at load asymmetries R = 0.1 and R = 0.6 for L-T and T-S growth directions, respectively. Considering final fracture of specimens, namely the final K max value, the values are in a good agreement with the general ASM Aerospace Specification 6, where fracture toughness in L-T direction is 32 MPa m 1/2. K max values at break in Fig. 3 are 29.2 and 29.5 MPa m 1/2, which can be considered as a very good agreement.

4 da / dn (m/cycle) da / dn (m/cycle) da / dn (m/cycle) , Brno, Czech Republic, EU 1.E-05 1.E-06 Specimen LT1 Specimen TS1 Specimen LT3 Specimen TS3 K max = 35.0 K max = E-07 K max = 29.5 K max = E-08 1.E-09 K th = 2.93, 2.98 K th = 3. 71, E Delta K (MPa m 1/2 ) Fig. 3. Comparison of FCG rates in L-T and T-S directions at load asymmetry R = E-05 1.E-06 Specimen LT1 - R = 0.1 Specimen LT3 - R = 0.1 Specimen LT2 - R = E-07 1.E-08 1.E-09 1.E-10 K th = 2.6 K th = 3. 71, Delta K (MPa m 1/2 ) Fig. 4. Comparison of FCG rates in L-T direction at load asymmetries R = 0.1 and R = E-05 1.E-06 Specimen TS1, R=0.1 Specimen TS3, R=0.1 Specimen TS4, R=0.6 1.E-07 1.E-08 1.E-09 K th = 2.93, E Delta K (MPa m 1/2 ) Fig. 5. Comparison of FCG rates in T-S direction at load asymmetries R = 0.1 and R = 0.6

5 There are not many possibilities to compare the measured FCG rates in threshold regions and threshold values with the literature it is quite difficult to find such the data for the Al 2124 alloy in T851 treatment conditions. Literature data being at disposal are just those for the Al 2024 alloy in T3 conditions. The data obtained in this work are not far from them. FCG data for the Al 2024-T3 alloy loaded in ambient air at frequency 20 Hz are presented in 11. Threshold values are approximately 3 and 2 MPa m 1/2, respectively, at load asymmetries R = 0.05 and R = 0.5, respectively. These values correspond quite well with those obtained in this work, particularly in T-S direction. However, concerning the slope of the linear regression line in log-log coordinates (power dependence da/dn = C K m ), i.e. values of the coefficient m, the slope is not so steep in this presented work, which is much more realistic in comparison with 11. As already mentioned, in this work, final failure occurred at K max more than approximately 30 MPa m 1/2 and the corresponding FCG rates were around 10-6 m/cycle, whilst in 11, failure occurred at FCG rates 10-7 m/cycle and corresponding K max less than 10 MPa m 1/2. Values of the coefficients C and m evaluated for selected specimens is in Table 3. Table 3 Values of the coefficients C and m evaluated for selected specimens Specimen Coefficient C Coefficient m LT1 7.04E LT3 1.28E TS1 2.90E TS3 7.14E It can be pointed out that values of the coefficients are very reproducible and self consistent particularly in case of L-T FCG direction. Another phenomenon in the diagrams should be pointed out, namely reasonable differences between FCG rates in both threshold and stable growth regions at R = 0.1 and R = 0.6 in L-T direction (Fig. 4), but surprisingly no such the differences in T-S direction (Fig. 5). This behaviour cannot be easily explained without an evaluation of crack closure, which is likely a very important factor. Concerning some declinations of FCG values from the linear regression lines in T-S direction at fairly high K range Figs. 3 and 5, the local retardation can be connected with some microstructure irregularities observed, namely local coarse grain and clusters of inclusions. 4. CONCLUSIONS The most important results of a comprehensive experimental programme aimed at an evaluation of FCG rates in Al 2124-T851 alloy, in L-T and T-S directions, at load asymmetries R = 0.1 and R = 0.6 can be summarised as follows: Concerning final fracture of specimens, namely the final K max value, the values are in a good agreement with the general ASM Aerospace Specifications, being around 30 MPa m 1/2. Data from the near threshold region and threshold values of K th were in quite a good agreement with literature FCG data for a similar alloy, namely Al 2024-T3. The slope of the linear regression line in log-log coordinates (power dependence da/dn = C K m ) is not so steep in this presented work in comparison with the literature data of the Al 2024 alloy. Results obtained in this work look to be much more realistic in comparison with the literature ones. There were reasonable differences between FCG rates in both threshold and stable growth regions at R = 0.1 and R = 0.6 in L-T direction, but surprisingly no such the differences in T-S direction.

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