Fatigue failure of adhesively patched 2024-T3 and 7075-T6 clad and bare aluminium alloys

The fatigue behaviour of adhesive patches used for repairing aircraft components was investigated. Adhesive patches were simulated using single‐lap shear specimens on clad and bare 7075‐T6 and 2024‐T3 aluminium alloy substrates. Stress–life curves were generated under constant amplitude loading at three stress ratios: R=?1, 0 and 0.5. In the bare materials, failure always occurred in the adhesive itself leaving the substrates intact. At fatigue lives below about 100 000 cycles, the clad alloy specimens also failed in this manner. However, at lower stress levels, the clad alloys failed by cracks initiating in the cladding layer along the end of the lap and subsequently propagating through the substrate. The fatigue strength of the substrate, due to the adhesive patch on the clad materials, was reduced by an order of magnitude compared to the Military Handbook values.     

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

Fatigue of shot peened 7075-T7351 SENB specimen – A 3-D analysis

As‐received or shot peened 7075‐T7351 single‐edged notch bend (SENB) specimens, 8.1‐mm thick, were fatigued at a constant maximum load and at stress ratios of R= 0.1 and 0.8 to predetermined numbers of fatigue cycles or to failure. The SENB specimens were then fractured by overload and the tunnelling crack profiles were recorded. The crack‐growth rate, da/dN, after crack initiation at the notch was determined by crack‐profile measurement and fractography at various fatigue cycles. The shot peened surface topography and roughness was also evaluated by three‐dimensional (3‐D) laser scanning microscopy. Residual stresses in the as‐received specimens and those generated by shot peening at Almen scales of 0.004A, 0.008A, 0.012A and 0.016A, were measured by an X‐ray diffraction stress analyser with an X‐ray target, CrK, every 0.1 mm to a depth of 1 mm. The 3‐D stress intensity factor of the curved crack front was determined by the superposition of the 3‐D finite element solutions of the stress intensity factor of the loaded SENB specimen without the residual stress and the stress intensity factor of the unloaded SENB specimen with a prescribed residual stress distribution. da/dN versus the resultant stress intensity factor amplitude, ΔK_I, plots showed that while the residual stress locally retarded the crack‐growth rate it had no effect on the overall crack‐propagation rate.     

A third-order state-space model for fatigue crack growth

A second‐order state‐space model of fatigue crack growth in ductile alloys was presented by Patankar et al., 1 - 4 where the crack length and the crack opening stress were treated as two state variables. Simulation results showed that this model gave good predictions when compared with experimental data for aluminium alloy 7075‐T6 and 2024‐T3 at constant‐amplitude load as well as with overloads. These model predictions were, however, poor for cases with over/under load or under/over load sequences where load excursion effects were underestimated. A third‐order state‐space model is presented is this paper that is believed to be more accurate for predictions of fatigue crack growth for ductile alloys under various loadings. The constraint factor calculated from an algebraic equation in the second‐order state‐space model is treated as the third state variable in this model. Through simulations, it is shown that the third‐order state‐space model gives better predictions than the second‐order state‐space model and FASTRAN II, especially when the effects of over/under load and under/over load are necessary considerations.     

Einfluß von Überlasten auf die Ausbreitung von Ermüdungsrissen in der Aluminiumlegierung 7075 T 7351

Influence of overloads on the propagation of fatigue cracks in the aluminium alloy 7075 T 7351 For realistic lifetime predictions the crack propagation behaviour under variable amplitude spectrum loads has to be known. In this work the influence of single overloads on the fatigue crack propagation in the alloy 7075 T 7351 was investigated at a constant loading level (K_max and ΔK = const.). The experiments showed, that a single overload causes a short crack acceleration followed by a pronounced deceleration of the crack propagation rate. After this deceleration at higher overloads a zone where the crack propagation rate is slightly increased (lost retardation) was observed. It has been shown, that the minimum crack propagation rate as well as the length of the influenced region depends on the height of the overload. The observed crack acceleration, the deceleration as well as the lost retardation can be explained by internal stresses caused by those overloads which have to be added to the external stresses.     

Dauerfestigkeit und Kerbempfindlichkeit der Legierungen AlMgSi1, AlCuMg2 und AlZnMgCu1,5

The Fatigue Strength and Notch Sensitivity of Al-Alloys 6061, 2024 and 7075. Comparison of the fatigue limits and fatigue notch factors for internally-notched sheet specimens of 6061-T4, 2024-T3-Alclad and 7075-T6-Alclad with k_T = 2 to 4,6. Effect of clad on the fatigue strength of notched specimens. Notch sensitivity k_F/k_T as a function of notch radius. Notch sensitivity of the investigated Al-alloys for various stress concentration factors k_T and stress ratios. Calculation of the fatigue notch factors of the investigated alloys with the aid of a two-parameter formula.     

Fatigue crack propagation under variable amplitude loading in PMMA and bone cement

Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements.     

Load interaction effects on fatigue crack propagation in 2024-T3 aluminum alloy

Variable amplitude fatigue studies have been conducted within a linear-elastic fracture mechanics framework in order to systematically examine the effect of complex loading on fatigue crack retardation in 2024-T3 aluminum alloy. Complex loading conditions were simulated by introducing a second tensile or compressive peak load after the crack had extended various distances, a', into the region affected by a previously applied high load excursion.

Maximum interaction between single peak overloads resulted when the two peak load cycles were separated by a small distance, a' _min, where the fatigue crack propagation rate resulting from a single overload reached a minimum. This behavior was attributed in part to interference of tensile displacements produced during the first peak load cycle which was verified from fractographic observations. Crack growth rate retardation was related also to the development of a favorable compressive residual stress at the crack tip. Peak loads were found to act as completely isolated events only when they were separated by a distance approximately three times the plastic zone size resulting from a single overload. Comparable findings resulted when 10 cycle block overloads were employed in place of single peak excursions.

When a single peak overload was followed by a compressive cycle, retardation was found to decrease to a minimum; however, when the loading sequence was reversed, the effect was less damaging. In addition, as the distance between positive and negative peak loads was increased, the number of delay cycles quickly approached that associated with a single high load excursion.     

COMPARISON OF SHORT AND LONG FATIGUE CRACK GROWTH IN 7075-T6 ALUMINUM

The fatigue crack growth rates of physically-short cracks (0.5 ≤a≤ 1.0 mm), intermediate cracks (1 < a≤ 2 mm) and long cracks (7 < a < 25 mm) were compared using SEN type tensile specimens in 7075-T6 aluminum alloy with load ratios, R, of 0.05, ? 1 and 0.5 under constant amplitude testing at room temperature. It was found that the short cracks grew much faster than long cracks based on applied δK with da/dN≤ 10^?7 m/cycle. Even the intermediate cracks grew faster than the long cracks below 10^?7 m/cycle. The transition crack lengths where similitude with δK existed was between 1 and 2 mm. Mean stress effects were similar for R= 0.05 and ? t, but R= 0.5 caused increased crack growth rates. The above differences are partially attributed to crack closure effects. Based upon plastic zone sizes, LEFM was justifiable with all the experiments.     

Fracture of laminates combining 2024-T3 and 7075-T6 aluminum alloys

Crack growth resistance curves have been determined for crack-divider laminates in which layers of 2024-T3 aluminum alloy are adhesively bonded to layers of 7075-T6 alloy. Results are compared with the fracture resistance of laminates consisting wholly of each material, the layer thickness being the same (1.54 mm) in all cases. The initial portions of the resistance curves are similar for both alloys; however those for 2024-T3 have steeper slopes at longer effective crack lengths. As a result, laminates consisting entirely of 2024-T3 alloy exhibit greater amounts of stable crack extension and higher toughnesses at instability. This is attributed in part to the greater strain hardening rate in 2024-T3 material. Laminates combining 2024-T3 and 7075-T6 layers are intermediate between those consisting entirely of one or the other alloy.     

Mechanisms of overload effect on fatigue crack propagation in aluminium alloys

Fatigue crack growth rate is retarded after application of one or several tensile overloads. This effect has been studied m 2024 and 2618 aluminum alloys. The main parameters which control the retardation are the rate of overload and its intensity, but other factors such as the number of overloads and the frequency of the test are to be considered. The delay effect is notable in aluminum alloys and might reach millions of cycles. A mechanism of retardation is proposed, deriving from metallographic observations.     

Mechanical response of 2024-7075 aluminium alloys joined by Friction Stir Welding

The mechanical and microstructural properties of 2024 and 7075 aluminium alloys joined together by friction stir welding were analysed in the present study. The two materials were welded with perpendicular rolling direction and after were tested in tension at room temperature in order to analyse the mechanical response and to observe the differences with the parent materials, the tensile response of the material in longitudinal direction revealed an increase in strength respect to the transverse one. The cyclic fatigue tests were conducted in the axial direction with R = σ_min/σ_max = 0.1). The microstructure resulting from the FSW process was studied by employing optical and scanning electron microscopy.     

Effect of friction stir welding process parameters on the tensile strength of dissimilar aluminum alloy AA2024-T3 and AA7075-T6 joints

This work investigates the influence of friction stir welding parameters on the mechanical properties of the dissimilar joint between AA2024-T3 and AA7075-T6. Experiments are conducted consistent with the three-level face-centered composite design. Response surface methodology is used to develop the regression model for predicting the tensile strength of the joints. The analysis of variance technique is used to access the adequacy of the developed model. The model is used to study the effect of key operating process parameters namely, tool rotation speed, welding speed and shoulder diameter on the tensile strength of the joints. The results indicate that friction stir welding of aluminum alloys at a tool rotation speed of 1050 min⁻¹, welding speed of 40 mm/min and a shoulder diameter of 17.5 mm would produce defect less joint with high tensile strength.     

TRANSIENT FATIGUE CRACK GROWTH BEHAVIOUR FOLLOWING SINGLE OVERLOADS AT HIGH STRESS RATIOS

Fatigue crack growth tests with constant amplitude loading and single overload have been performed on a long mode I crack in 2017-T3 aluminium alloy at various stress ratios from 0 to 0.7. Two crack tip parameters of σ_op and σ_tt were evaluated using a finite element analysis for a growing crack under these loading conditions. The former is the crack opening stress and the latter is the applied stress level at which the stress at the crack tip becomes tensile. It was found that transient crack growth behaviour following single overloads at high stress ratios was significantly different from that at a low stress ratio: at higher stress ratios, following the application of the overload, there was a rapid retardation which was followed by an acceleration in growth rate and then a faster return to the steady state level at baseline loading. The experimentally observed transient post-overload behaviour is discussed in terms of the two effective stress range ratios of U_op and U_tt, which are determined from σ_op and σ_tt, respectively. For the stress ratios and overload ratios studied, the results indicate that the changes in U_tt with crack extension after the overload are reasonably consistent with the crack growth rate trends. The stress distribution at minimum applied stress would account for the transient changes in U_tt.     

Wettability of TiC by commercial aluminum alloys

The effect of alloying elements on the wettability of TiC by commercial aluminum alloys (1010, 2024, 6061 and 7075) was investigated at 900°C using a sessile drop technique. Wetting increased in the order 6061 < 7075 < 2024 < 1010 for both, static argon or vacuum atmospheres. Alloys 1010 and 2024 wet TiC under both atmospheres, leading to contact angles in the order of 60° and less, while 7075 only wets under vacuum, with the poorest wettability being exhibited by 6061. Evaporation of Zn and Mg under vacuum conditions contributed to the rupture of the oxide film covering the aluminum drop and thereby improving wetting and spreading. Continuous and isolate Al₄C₃ was detected in all the cases. CuAl₂ precipitation at the interface slightly decreased Al₄C₃ formation and increased the adhesion of 2024 to TiC.     

Three-dimensional finite element simulations of microstructurally small fatigue crack growth in 7075 aluminium alloy

Three‐dimensional finite element simulations were performed to study the growth of microstructurally small fatigue cracks in aluminium alloy 7075‐T651. Fatigue crack propagation through five different crystallographic orientations was simulated using crystal plasticity theory, and plasticity‐induced crack opening stresses were calculated. The computed crack opening stresses were used to construct small crack da/dN‐ΔK diagrams. The generated da/dN‐ΔK curves compared well with experimental small crack data from the literature. The variance observed among the da/dN‐ΔK results, which occurred as a consequence of the different crystallographic orientations employed, was found to be of the same order of magnitude as commonly observed variability in small fatigue crack growth data. This suggests that grain orientation is a major contributor to observed small fatigue crack data scatter.     

A study of corrosion fatigue behaviour of anodized and unanodized 2024-T3 aluminium alloy

The corrosion fatigue (CF) behaviour, under constant deflection bending conditions with a pulsating tension stress form, of 2024-T3 aluminium alloy, unanodized and anodized to form a thick porous film, in 3.5% NaCl solution has been investigated. It was found that E _corr varies very little until specimen fracture under low frequency CF conditions, whereas E _corr drops rapidly when approaching the later fracturing stage of the CF process under high-frequency conditions for unanodized specimens. However, a slow drop in E _corr was detected from the commencement of the CF process, and lasted up to a much more rapid drop at a later fracturing stage for the anodized specimen. This behaviour presumably can be explained by the cracking of the anodic film and the theory of imperfect recovery of the surface film. It is suggested that the E _corr monitoring technique may be useful for determining the remnant CF life for existing structural parts of this alloy or other aluminium alloys regardless of whether or not they are anodized. Furthermore, the T3 temper provides a microstructure which may retard main-crack formation and penetration in the CF process of the anodized alloy, thus mitigating partly the negative effect of the readily crackable anodic film.     

Retardation effects due to overloads in aluminium‐alloy aeronautical components

Fatigue data are generally derived under constant‐amplitude loading conditions, but aircraft components are subjected to variable‐amplitude loading. Without interaction effects, caused by overloads and underloads intermingled in a loading sequence, it could be relatively easy to establish a crack growth curve by means of a cycle‐by‐cycle integration. However, load‐spectrum effects largely complicate a crack growth under variable‐amplitude cycling. In this paper, fatigue crack growth behaviour of aeronautical aluminium alloy 2024‐T3 was studied. Effects of various loading conditions such as stress ratio and amplitude loadings were investigated. In particular, the effect of different overloads on the fatigue crack growth was simulated using Zencrack code. Preliminary analyses on Compact Tension (CT) specimens proved that the numerical results generated were in agreement with the results provided by an afgrow code for the same conditions. A case study was carried out on a helicopter component, undergoing repeated overloads, to compare numerical results obtained implementing yield zone models in Zencrack.