Fatigue Crack Growth and Life Prediction Methods I
Chair: Hiroyuki Terada
DETAILED STRAIN FIELD ANALYSES DURING FATIGUE CRACK PROPAGATION IN FRICTION STIR WELDED JOINTS|
Rik Jan Lemmen, Rene Alderliesten, Rinze Benedictus
Friction Stir (FS) welding has a huge potential for the aerospace industry because it can reduce lead times, manufacturing costs and structural weight. Since FS welding is a solid state process, it is possible to join high strength aluminum alloys which are considered to be non-weldable. Moreover, FS welding is a robust process without emission of dangerous gasses or radiations for which protection is required.
FS welding is performed by pressing a non-consumable, rotating tool at the work piece. The FS weld tool consists of a non-consumable rotating cylinder with a ‘pin’ at the centre of the lower circular surface (the ‘shoulder’). The frictional heat generated between the shoulder and the work piece enables it for the pin to stir two parts together. After welding, different zones can be distinguished in the weld with their individual material properties, which can be significantly different from the base material properties.
Before an FS weld can be applied in a damage tolerant fuselage structure, understanding of the fatigue behavior of an FS weld is a key issue. This includes fatigue crack initiation, fatigue crack growth and final failure. In previous research interesting crack growth behavior was observed like turning of fatigue cracks and crack path deviation in the weld structure. This behavior is attributed to both the reduced yield strength and the residual stresses in the FS weld. The yield strength influences the size and orientation of the plastic zone in front of the crack tip, while the residual stresses influence the magnitude and the orientation of the main stress at the crack tip.
The yield strength was measured in a previous test program to obtain the yield strength profile of FS welds in three alloys, AA2024-T3, AA7075-T6 and AA6013 T4. The local yield strength profiles were obtained using a newly developed technology; Digital Image Correlation (DIC). DIC is a measuring technology which uses images of a test specimen to analyse the deformations during a (fatigue) test. DIC is a flexible method because it is a non-contact measurement system and the detail of the measurement can easily be adjusted by changing the zoom factor. Beside the yield strength, also the residual stress profiles in the FS welds were measured using X-ray diffraction.
This paper presents the results of fatigue crack growth experiments performed with the goal to obtain the deformations and plasticity in front of a fatigue crack, propagating through an FS weld. Digital image correlation was used to visualize and measure the strain distribution and the size and shape of the plastic zones in the vicinity of the crack tip.
For this research, centre crack specimens were prepared for three alloys, AA2024-T3, AA7075-T6 and AA6013-T4, containing FS welds under an angle of 0° (parallel to the load), 45° and 90° (perpendicular to the load). In the specimens with the weld perpendicular to the load, the initial crack (2a = 5mm) was located at different locations in the FS weld, i.e. 0.0, 3.0 and 6.5 mm from the weld centre. In the specimens with the weld parallel and under an angel of 45° to the load, the initial crack was placed outside the weld such that one crack tip crosses the weld during the test. At certain fatigue intervals the test was interrupted to measure the crack length and take images from the surface of the specimen while maximum load was applied. Afterwards the images were analyzed using the DIC tool to evaluate the strain field. To obtain high accuracy, a thin paint layer was applied over the surface of the specimens consisting of fine black and white droplets, which created a so-called ‘speckle pattern’.
The most interesting results were found when the fatigue crack entered the weld under 45°. Crack turning was observed in different grades for all 45° specimens, but one specimen showed a rotation of the crack of 90° from mode I crack propagation into pure mode II crack propagation. Other effects, like crack splitting and combined mode I and II crack growth, were also observed during the tests.
The results of the DIC measurements clearly showed how the geometry of the plastic zone in front of the crack tip was highly affected by the presence and orientation of the FS weld. For the specimens with the fatigue crack parallel to the FS weld, the DIC results showed a layered plastic zone due to the variation in yield strength.
The results presented in this paper give more understanding of fatigue crack propagation in FS welded joints. Especially the influence of the yield strength on the magnitude and geometry of the plastic zone in front of the crack tip is quantified using the results obtained by digital image correlation.
A STUDY OF INTERACTION AND COALESCENCE OF MICRO SURFACE FATIGUE CRACKS IN ALUMINIUM 7050|
Weiping Hu, Qianchu Liu, Simon Barter
Abstract: Multi-site cracks may occur in structural components with shallow notches in the early stages of fatigue damage, in corroded parts, or they can occur as a result of multiple notches such as in fuselage lap joints. In these cases, the development, interaction and coalescence of these multiple cracks poses a challenge to the accurate prediction of the rate of fatigue crack growth and the residual strength, as the interaction of cracks tend to accelerate the growth, and the resulting combined crack usually leads to a lower residual strength than the individual cracks. It is, therefore, important to gain an insight into the nature of interaction and coalescence of multiple small surface cracks and to investigate the criteria by which coalescence takes place.
In this paper we analyse the interaction and coalescence of micro multiple surface cracks in plates of 2024-T3 and 7050-T7451 aluminium alloys. Earlier studies show that for 7050-T7451, which is used on combat aircraft such as F/A-18, the initiation and growth of micro-cracks take up to 50~90% of the total fatigue life, hence it is vital to accurately assess the evolution of these micro-cracks and their link-up to form primary long cracks. Experiments have been carried out under constant amplitude cyclic loading and spectrum loading using 2024-T3 and 7050-T7451 plate specimens containing artificially introduced micro-cracks of the order of ~50 μm. These parallel shallow surface cracks were arranged co-linearly or with an offset, with varying distances between inner tips and offsets. The crack length and the coalescence of cracks were measured using quantitative fractography, as shown in the Figure 1 below.
Based on linear elastic fracture mechanics, finite element analyses were performed to determine the extent of interaction between cracks and the condition under which the two cracks may be considered as coalesced, for each arrangement of multiple cracks, by varying the distance between the inner crack tips and the offset. Some results are shown in Figure 2. The plasticity-induced crack closure model was modified to allow the use of El Haddad model to modify the stress intensity factor solution to account for plasticity. Fatigue crack growth prediction was made for different service load spectra, and compared to the experimental results.
It is envisaged that the interaction and coalescence model developed in this work will help to improve fatigue life assessment in the short crack regime for the aluminium alloys considered, thus lead to improvement in the technical advice provided to the Royal Australian Air Force.
FRACTURE MECHANICS OF SKIN MATERIALS IN CIVIL AIRPLANE STRUCTURES|
Grigory Nesterenko, Boris Nesterenko, Valentin Basov
Abstract: This paper presents test-analytical results of the research in the field of fracture mechanics in
aluminum alloys applied in the skin of lower and upper wing surface and fuselage in transport airplanes.
Mechanic properties, characteristics of fatigue, crack growth and fracture toughness have been investigated.
Fatigue and crack growth rates have been studied at regular cyclic loads and under random in-service load spectra. The truncated TWIST spectrum, Boeing and TsAGI spectra were applied as random ones.
Fatigue analysis in case of random load spectra has been performed basing on linear hypothesis of damage accumulation. Crack growths were analyzed basing on linear models as well as using acceleration/retardation models.
Fracture properties have been studied for the materials applied in the transport structures of airplanes by Boeing, Airbus, Russian aircraft companies (Ilyushin, Tupolev, Yakovlev) and Ukrainian Antonov company. Fracture properties for old Al-alloys manufactured in 1950-1960ies have been defined alongside with fracture parameters of improved Al-alloys delivered in 1970-1990ies such as: 7075-T6, 7178-T6, 7075-T7751, 2024-T3, 2324-T39, 2524-T3, V95-T1, V95-ochT2, V96ts-3PchT12, D16AT, 1163AT, 1161T, 1163-T7.
Semi finished product properties have been studied in TsAGI on electro hydraulic test machines Schenk an MTS. Tests were conducted in accordance with Russian Aviation industry standards and similar USA regulations (ASTM).
Summary research results are presented in the tables containing the following fracture parameters: N,da/dN, Kapp.. The values of accumulated fatigue damages Σ ni/Ni. at fracture are given. The figures give the comparison of analytical and experimental crack growth durations under random loading spectra.
Stated in the summary:
• Good parameters of strength, fatigue and crack resistance in the improved Al-alloys are the result of
o Zirconium introduction as alloying elements;
o Alloy purity enhancement in terms of silicium and iron additives;
o Improvement of alloy production technology.
• Analytical results for fatigue under random loading spectra should be corrected by experimental data
• Crack growth rate calculation models should be improved under in-service loading spectra.