Imperfection Insensitivity Analyses of Advanced Composite Tow-Steered Shells

Wu, Keyu; Farrokh, Babak; Stanford, Bret

doi:10.2514/6.2016-1498

Cited by 8 publications

(7 citation statements)

References 8 publications

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“…If this is indeed the case, then tailoring stress fields in a particular (non-uniform) way should result in better correlations between predictions and experiments; the reasoning being that the symmetry of a circumferentially uniform stress field can be broken by a larger set of imperfections than an a priori tailored, non-uniform stress field. This could explain the recent success of researchers in deterministically tailoring and predicting the precise post-buckling behaviour of shells with engineered imperfections [8,54] and also with varying material properties [55]. Especially, the latter work may provide an impetus for future work on tailoring cylinders with modern materials technology, such as tow-steered composites.…”

Section: Resultsmentioning

confidence: 99%

On the role of localizations in buckling of axially compressed cylinders

Groh

Pirrera

2019

Proc. R. Soc. A.

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The collapse of axially compressed cylinders by buckling instability is a classic problem in engineering mechanics. We revisit the problem by considering fully localized post-buckling states in the form of one or multiple dimples. Using nonlinear finite-element methods and numerical continuation algorithms, we trace the evolution of odd and even dimples into one axially localized ring of circumferentially periodic diamond-shaped waves. The growth of the post-buckling pattern with varying compression is driven by homoclinic snaking with even- and odd-dimple solutions intertwined. When the axially localized ring of diamond-shaped buckles destabilizes, additional circumferential snaking sequences ensue that lead to the Yoshimura buckling pattern. The unstable single-dimple state is a mountain-pass point in the energy landscape and therefore forms the smallest energy barrier between the pre-buckling and post-buckling regimes. The small energy barrier associated with the mountain-pass point means that the compressed, pre-buckled cylinder is exceedingly sensitive to perturbations once the mountain-pass point exists. We parameterize the compressive onset of the single-dimple mountain-pass point with a single non-dimensional parameter, and compare the lower-bound buckling load suggested by this parameter with over 100 experimental data points from the literature. Good correlation suggests that the derived knockdown factor provides a less conservative design load than NASA's SP-8007 guideline.

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Section: Resultsmentioning

confidence: 99%

On the role of localizations in buckling of axially compressed cylinders

Groh

Pirrera

2019

Proc. R. Soc. A.

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“…For the tests with cut-outs good agreement was found with the linear bifurcation buckling loads. Since it is known that the buckling load of unstiffened CS cylinders and plates are sensitive to geometric imperfection recently, a numerical study on the influence of imperfections on the performance of VS cylinders has been performed [64]. Results showed what was expected based on the previous results: the imperfections have a limited influence.…”

Section: Introductionmentioning

confidence: 61%

“…Hence, the location of the buckling is not influenced by imperfections. This implies that the load redistribution is hardly influenced by the imperfections, and thus the effect of these imperfections is small [64].…”

Section: Introductionmentioning

confidence: 99%

Optimal design, manufacturing and testing of non-conventional laminates

Peeters

Irisarri

Groenendijk

et al. 2019

Composite Structures

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Composite materials are finding increasing application, for example in commercial aircraft. Traditionally fiber angles are restricted to 0 • , ±45 • and 90 •. The current work exploits the possibility of using multiple 'non-conventional' laminates where either fiber steering ('variable stiffness'), ply drops ('variable thickness'), or a combination of both is used. This leads to varying mechanical properties which means the load is being redistributed, increasing the overall buckling load. A flat panel of 400 × 600 mm loaded in uni-axial compression is optimized in the current work. As a benchmark a conventional laminate is used. The non-conventional laminates are 15% lighter to emphasize the possible weight savings. Only using variable stiffness or variable thickness is experimentally shown to not be sufficient to match the buckling load of the benchmark panel. However, using a combination of both, a 10% increase in the buckling load was found for a panel that is 15% lighter. This highlights the potential of non-conventional laminates.

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“…This explanation is not considered to be likely for two reasons. First, the pristine versions of Shell A and Shell B (without cutouts) were previously shown to be insensitive to imperfections [51], with good correlations between linearized buckling analyses and test results. Second, the buckling load predictions of the nonlinear dynamic analyses, which follow in the next section, correlate closely with the experimental test results and also do not account for geometric/loading imperfections.…”

Section: A Linearized Buckling Analysismentioning

confidence: 99%

“…In contrast to a constant-stiffness cylinder (e.g., an isotropic or straight-fiber composite design), in which the strain energy associated with a single buckle in the cylinder wall is invariant to circumferential translations, the nonuniform stiffness distribution in tow-steered cylinders favors the formation and "trapping" of buckles in the most highly-loaded regions around the circumference. This characteristic of tow-steered cylinders may explain their relatively low imperfection sensitivity when compared to isotropic or quasi-isotropic cylinders [35,51], as the adverse effect of geometric imperfections is limited to a small portion of the total cylinder surface.…”

mentioning

confidence: 99%

Nonlinear Buckling and Postbuckling Analysis of Tow-Steered Composite Cylinders with Cutouts

Groh

2022

AIAA Journal

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The buckling and postbuckling behavior of two composite tow-steered shells with cutouts of different sizes is assessed using nonlinear finite element (FE) analysis and compared to experimental measurements. The cylindrical shells are manufactured using an automated fiber placement system, where the shells' fiber orientation angles vary continuously around the shell circumference from ±10 degrees on the crown and keel to ±45 degrees on the sides. One shell features thickness variations due to tow overlaps that result from application of all 24tows during each pass of the fiber placement system. The second shell uses the system's tow drop/add capability to achieve a more uniform wall thickness without overlaps. Unreinforced cutouts of two different sizes-the first smaller cutout representing a passenger door on a commercial aircraft and the second larger cutout a cargo door-were machined into each of the two cylinders resulting in a total of four test cases. These cylinders were tested in axial compression and buckled elastically in previous work and are now analyzed using nonlinear FE models to compare bifurcation buckling loads as well as the load-displacement response in the prebuckling and postbuckling regimes. For all four shells analyzed, the prebuckling stiffness, buckling load, and deformation mode sequence throughout the loading-unloading cycle is accurately reproduced by the models. In particular, the shells first buckle locally around the cutouts in a stable (super-critical) manner with only a slight decrease in axial stiffness, which occurs due to the favorable load redistribution facilitated by tow steering. The shells then buckle globally in an unstable (sub-critical) manner with diamond-shaped buckles forming to the left and right (circumferential direction) of the cutouts. The buckling load of all shells with cutouts is at least 82% of the buckling load of the pristine shells without cutouts. Overall, the ability to sustain local buckling phenomena, and the relatively small reductions in global buckling load compared to pristine shells without cutouts, demonstrates the great potential of using tow

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Imperfection Insensitivity Analyses of Advanced Composite Tow-Steered Shells

Cited by 8 publications

References 8 publications

On the role of localizations in buckling of axially compressed cylinders

On the role of localizations in buckling of axially compressed cylinders

Optimal design, manufacturing and testing of non-conventional laminates

Nonlinear Buckling and Postbuckling Analysis of Tow-Steered Composite Cylinders with Cutouts

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