Practical Engineering Experience in Aircraft Structural Design Practical Engineering Experience in...
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Practical Engineering Experience in Aircraft Structural Design
Dr. Masoud Rais-Rohani, Mississippi State University
Masoud Rais-Rohani is the Associate Dean for Research and Graduate Studies and Professor of Aerospace Engineering in the Bagley College of Engineering at Mississippi State University. Masoud earned his PhD degree in aerospace engineering from Virginia Tech in 1991. He has taught courses mainly in the areas of aerospace structures, mechanics, and design optimization. He has made extensive use of experiential learning and computer applications in his courses, particularly the senior-level aerospace structural design course.
c©American Society for Engineering Education, 2015
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Practical Engineering Experience in Aircraft Structural Design Abstract Engineering analysis and design topics in a senior-level aerospace structural design course are supplemented with simple in-class demonstrations, hands-on experience in sheet-metal fabrication, and a comprehensive engineering project involving design, analysis, optimization, manufacturing, and testing of stiffened panels. Besides providing students with opportunities to gain a deeper understanding of the concepts discussed in the course, they are introduced to other important aspects of engineering such as teamwork, communication skills, time management, personal responsibility and ethics. Details of the individual experiential learning activities and the resulting outcomes are presented and discussed. 1. Introduction Experience shows that when students are engaged in activities that integrate both cognitive and sensory systems, they are more likely to maintain focus and have better retention of the subject presented. There is anecdotal and scientific evidence to support the positive impact of experiential learning that many experienced teachers know to be true through personal interactions with students. The continuing interest in active, collaborative, cooperative, and project-based learning1-3, and pedagogies of engagement4 is fueled by the need to modify, transform, or otherwise increase the effectiveness of lecture-based courses, particularly in STEM fields. Some engineering educators have relied on the use of physical models5-7 and hands-on classroom demonstrations8-11 as a way of increasing the impact of traditional lectures in structural mechanics. Others have attempted to broaden the design education through experiential learning opportunities in manufacturing12 and construction13 engineering as a result of the pioneering work by Kolb14. Strengthening the understanding of aircraft structural analysis and design through classroom demonstrations and team projects is the main focus of this paper, which extends the prior work on the subject15. In particular, it relies on the experience of the author in teaching the senior-level Aerospace Structural Design course in the Department of Aerospace Engineering at Mississippi State University for nearly two decades. The field of aerospace structures, as a subset of the broader area of structural mechanics, deals with topics—such as deformation, stress, instability, and failure—that are influenced by the physical and engineering properties of materials as well as the geometric attributes and boundary conditions of the structural system. The thin-walled nature of flight structures is a noteworthy feature that, combined with low-density materials, is critical in reducing the weight of aircraft and spacecraft. For example, the aluminum-alloy fuselage skins of most transport airplanes16 (e.g., B737 to B747) have a thickness in the order of 0.036" to 0.071", which is roughly 4 to 8 time greater than the 0.009" thick standard printer paper. The longitudinal stiffeners or stringers used to support the fuselage skin are also of similar thickness.
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The fact that such thin-walled structures are used in the construction of large transport aircraft has led to interesting classroom discussions. It has also provided learning opportunities ranging from classroom testing of simple cardboard models to a comprehensive engineering project involving design, analysis, optimization, manufacturing, and testing of stiffened aluminum alloy panels. Although the learning of aircraft structural design and analysis methods is essential, it is the experiential learning opportunities that help students gain practical engineering experience as future engineers. For example, without familiarity or direct experience with a particular manufacturing process, a student may suggest design features or dimensions that could not be produced. A design concept may be easily drawn on paper or captured using a CAD tool, but without adequate consideration of the manufacturing process, the result is likely to be a non- viable design. Similarly, other important factors, such as variability in the material properties or the manufacturing process and its potential impact on performance of the designed product, could be easily overlooked. Thus, a more comprehensive approach to engineering design education is one that involves manufacturing and testing. The paper aims to provide insight into the overall level of success in introducing a practical engineering experience into aerospace structural design course by describing each activity with pertinent detail. 2. Structural failure concepts of interest Instability and compressive failure of thin-walled structures are the principal topics of interest in the experiential learning activities described below. To provide proper context, a brief description of each failure concept is presented. 2.a Euler buckling When a structural member is placed under compression, it can experience global instability or buckling. For columns, buckling depends on the effective slenderness ratio !! !, and it could be either elastic or inelastic. In the elastic case, column-buckling (or critical) stress can be predicted using the Euler buckling formula given as
!!" = !!! !! ! !
where E is the Young’s modulus of the material, !! is the effective length of the column, which depends on the column length and its boundary conditions, and ! is the radius of gyration of the critical cross-sectional axis. Equation (1) is applicable to the so-called long columns when buckling occurs at a stress level below the proportional limit of the material. 2.b Local buckling This form of buckling occurs in thin-walled columns. Each flange or web member is susceptible to local buckling. The member with the lowest !! ! ! !value buckles first, where !! is the
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buckling coefficient that depends on the aspect ratio of the member and the rotational rigidity of its supported edges, t is the wall thickness, and b is the length of the loaded edge of the member. The general equation for elastic local buckling under uniaxial compression is given as
!!" = !!!!!
12 1− !! ! ! ! (2)
where ! is the Poisson’s ratio of the material. As in the case of global buckling, it is possible for local buckling to be inelastic. In that case, the Young’s modulus is replaced by the tangent modulus, !! found using the Ramberg-Osgood formula17 !! ! = 1+
3 7! ! !!.!
where n is a parameter that describes the shape of the stress-strain curve in the yield region with !!.! representing the stress corresponding to the secant line at a slope of 0.7E. The values of n and !!.! are tabulated quantities describing the properties of structural materials commonly used in the aircraft industry. 17 2.c Crippling Crippling is a form of local failure observed in thin-walled sections (i.e., short columns) under compression. Depending on the cross-sectional geometry, different semi-empirical equations have been developed by Gerard.17,18 As an example, for a multi-corner section, the crippling stress, !!" is found as
!!" = 0.56!!" !!! ! ! !!" !.! !.!"
(4) where !!" is the compressive yield strength of the material, A is the cross-sectional area of the section, and ! is a shape parameter with its value depending on the number of web and flange elements in the section. It is worth noting that the length of the loaded section does not appear in Eq. (4) because it is assumed to be small (i.e., of the same order of magnitude as the cross- sectional perimeter). For pure crippling to occur, the thin-walled short column has to have an effective slenderness ratio of less than 20. 2.d Johnson-Euler failure For thin-walled columns with effective slenderness ratio in the range of 20 ≤ !! ! ≤ !! ! ! , failure is a combination of crippling and inelastic buckling with the corresponding stress estimated using the Johnson-Euler formula17 given as !!" = !!" 1−
!!" 4!!! !! !
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The critical slenderness ratio !! ! ! corresponds to the transition point beyond which the column response is in the form of Euler buckling. The compressive strength at !! ! ! is equal to !!" 2. 2.e Stringers versus stiffened panels The failure modes described by Eqs. (1) through (5) apply equally to both stringers and stiffened panels under axial compression. Key differences are in the calculation of effective width in the case of stiffened panels where the skin is assumed to have a uniform stress equal to that experienced by the stringer attached to it. Additional consideration is given to panels with riveted stringers that can experience inter-rivet buckling. Further details can be found in many airframe structures textbooks, e.g., Bruhn17. 3. Cardboard column a