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Damage tolerance behaviour of integral aircraft structures obtained by stationary shoulder friction stir welding
Citation Link: https://doi.org/10.15480/882.3558
Publikationstyp
Doctoral Thesis
Date Issued
2021
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2020-07-03
TORE-DOI
TORE-URI
Citation
Technische Universität Hamburg (2021)
Peer Reviewed
true
Research focusing on new ways to achieve weight reduction without sacrificing the safety performances of an aircraft is the subject of a constant push in the aeronautical industry. The use of riveting to join skin and stringer is highly optimised but in order to reduce structural weight, aside from the continuous development of new materials, it is important to use the already available materials in an efficient way. For this reason, friction stir welding (FSW) was looked at for many years as a suitable replacement alternative for riveting but the focus of the research was mostly driven with a material science point of view. Therefore, the development of a new solid-state welding technology ensuring good material properties and, at the same time, being investigated as a structural joint is still a necessity.
The present-study addresses the development of a new procedure for joining skin and stringers made of dissimilar aluminium alloys by stationary shoulder friction stir welding (SSFSW), which is to be applied in primary aircraft structures. Three different joint configurations were developed, and their performances were compared. At first, a fundamental analysis of the correlation between the two main process parameters, welding speed and rotational speed on heat development and joint formation, was conducted. The correlation between the generated microstructure and the mechanical properties of the three configurations was investigated on coupon-size specimens. In the following step, the fatigue behaviour in sub-component scale was studied in terms of fatigue life and crack initiation for two of the joint configurations. In the last phase of the work, 5-stringer panels were realised by SSFSW using the joint configuration, which showed the best properties in the performed analysis. These panels were investigated in the context of a damage-tolerant environment to understand fatigue crack growth and residual strength behaviour in comparison to conventional joining technologies.
At the coupon level, the three joint configurations showed higher mechanical performances for different loading conditions with respect to a conventionally riveted skin and stringer. The interface between the skin and the stringer’s alloys proved to play the largest influence on the strength of the joints. The two welding parameters control the geometry of this by influencing the material flow and heat generation. The notch-skin double pass (NS-DP) configuration proved to have limitations in terms of process development and were excluded from the study for up-scaled components. The sub-components realised by SSFSW with the two remaining joint variants showed higher fatigue performances in hoop-directions with respect to riveted structures. The SSFSW technology leads to half the stress concentration of a riveted T-joint and an increase of 50% in the fatigue limit. In spite of a similar fatigue life, in the NS-SP joints, crack turning and unwanted delamination of the stringer was observed. In the longitudinal load direction, a higher fatigue life was recorded for the 3-Part joints in the high-cycle area with respect to the NS-SP variant. From the fatigue crack growth (FCG) tests of the 5-stringer panels, the optimum load redistribution between skin and stringers joined by the SSFSW led to a reduction of the FCG rate for short cracks away from the stringers. This structural behaviour improves the FCG with respect to riveted and adhesive bonded structures. An increase in lifespan over 62% for a crack length that reaches the two-bay width has been shown. The residual strength of the SSFSW panels was measured for a two-bay crack showing a significant improvement over conventional joining techniques and confirming the superior quality of structures realised by SSFSW in a damage-tolerant environment.
The present-study addresses the development of a new procedure for joining skin and stringers made of dissimilar aluminium alloys by stationary shoulder friction stir welding (SSFSW), which is to be applied in primary aircraft structures. Three different joint configurations were developed, and their performances were compared. At first, a fundamental analysis of the correlation between the two main process parameters, welding speed and rotational speed on heat development and joint formation, was conducted. The correlation between the generated microstructure and the mechanical properties of the three configurations was investigated on coupon-size specimens. In the following step, the fatigue behaviour in sub-component scale was studied in terms of fatigue life and crack initiation for two of the joint configurations. In the last phase of the work, 5-stringer panels were realised by SSFSW using the joint configuration, which showed the best properties in the performed analysis. These panels were investigated in the context of a damage-tolerant environment to understand fatigue crack growth and residual strength behaviour in comparison to conventional joining technologies.
At the coupon level, the three joint configurations showed higher mechanical performances for different loading conditions with respect to a conventionally riveted skin and stringer. The interface between the skin and the stringer’s alloys proved to play the largest influence on the strength of the joints. The two welding parameters control the geometry of this by influencing the material flow and heat generation. The notch-skin double pass (NS-DP) configuration proved to have limitations in terms of process development and were excluded from the study for up-scaled components. The sub-components realised by SSFSW with the two remaining joint variants showed higher fatigue performances in hoop-directions with respect to riveted structures. The SSFSW technology leads to half the stress concentration of a riveted T-joint and an increase of 50% in the fatigue limit. In spite of a similar fatigue life, in the NS-SP joints, crack turning and unwanted delamination of the stringer was observed. In the longitudinal load direction, a higher fatigue life was recorded for the 3-Part joints in the high-cycle area with respect to the NS-SP variant. From the fatigue crack growth (FCG) tests of the 5-stringer panels, the optimum load redistribution between skin and stringers joined by the SSFSW led to a reduction of the FCG rate for short cracks away from the stringers. This structural behaviour improves the FCG with respect to riveted and adhesive bonded structures. An increase in lifespan over 62% for a crack length that reaches the two-bay width has been shown. The residual strength of the SSFSW panels was measured for a two-bay crack showing a significant improvement over conventional joining techniques and confirming the superior quality of structures realised by SSFSW in a damage-tolerant environment.
Subjects
Friction Stir Processing
fatigue crack propagation
T-joint
Damage Tolerance
Fatigue assessment
DDC Class
600: Technik
More Funding Information
LISA (Lightweight Integral Structures for future generation Aircraft) project
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