Influence of uncertainties on the response and reliability of self-adaptive composite rotors

doi:10.1016/j.compstruct.2011.07.011

Composite Structures

Volume 94, Issue 1, December 2011, Pages 114-120

https://doi.org/10.1016/j.compstruct.2011.07.011 Get rights and content

Abstract

Advanced composite structures are becoming increasingly popular because of their high specific strength and stiffness, as well as ability to provide improved performance through passive morphing via intrinsic bend–twist deformation coupling. Self-adaptive composite structures tend to be more susceptible to geometric, material, and loading uncertainties because of their complex configuration, manufacturing process, and dependence on fluid–structure interaction (FSI) response. The objective of this work is to quantify the effects of material, geometric, and loading uncertainties on the response of self-adaptive composite propellers and overall system reliability. A fully-coupled, 3-D boundary element method–finite element method is used to compute the dynamic FSI response. Variability in propeller performance is estimated by considering variations in operating conditions, as well as blade geometry and stiffness. Modeling uncertainties are considered by employing various mechanistic-based failure initiation models. Random variations in material strengths are implemented and an estimate of the structural reliability is determined. The results indicate that adaptive composite structures that depend on FSI are more sensitive to natural, random variations than equivalent rigid, isotropic structures. Therefore, it is necessary to quantify the effects of material, geometric, and loading uncertainties on the responses, safe operating envelopes, and reliability of self-adaptive composite structures.

Highlights

► Investigated the effect of material, geometric, loading uncertainty on the response and reliability of self-adaptive composite structures. ► Demonstrated that a self-adaptive structure that interacts with the flow is more susceptible to performance variability. ► Demonstrated that variability in material strength parameters and failure models significantly influence reliability estimates. ► Developed a probability-based analysis framework that is generally applicable to any self-adaptive composite structures.

Introduction

Recently, advanced composite materials have become a popular alternative to traditional metallic alloys in marine applications, including marine rotors such as propellers and turbines. A typical marine rotor is constructed from nickel–aluminum–bronze (NAB) or manganese–aluminum–bronze (MAB) alloys because of their superior yield strength, reliability, and affordability. Composite materials, on the other hand, are significantly lighter and provide improved corrosion resistance. In addition, composite rotors can provide improved hydroelastic performance through passive tailoring of the coupled bend–twist deformations. Through proper design, the load-dependent deformations of a composite rotor can be tailored to maintain a more optimal pitch angle distribution over a range of flow conditions, and hence achieve improved performance compared to its rigid metallic counterpart [1], [2], [3], [4], [5].

Compared to metallic alloys, composite materials tend to be more susceptible to geometric and material imperfections due to the complex manufacturing process [6], [7]. Moreover, the geometry of a rotor, particularly marine propellers, can be highly complex. Slight variations in the blade pitch, rake, and skew distributions can affect overall performance. For a composite rotor, random variations due to fiber misalignments, voids, and laminate properties add another level of variation in the overall performance. Consequently, it is necessary to quantify the influence of geometric, material, and loading uncertainties on the response, safe operating envelope, and reliability of self-adaptive structures.

There are many different strategies for estimating the stochastic response and developing reliability estimates of composite structures considering both first-ply and progressive criteria (e.g. [8], [9], [10], [11], [12], [13]) in addition to considerations for residual strength due to load sequence and damage accumulation effects (e.g. [14], [15], [16], [17], [18], [19]). There are very few references, however, that address the stochastic response and reliability of self-adaptive composite structures (e.g. [5], [20]). In addition to material and geometric uncertainties, there is a level of modeling uncertainty when quantifying the initiation and evolution of material failures due to the complex, multi-scale failure mechanisms. There are many different failure models for composite structures and selection of an appropriate model is not trivial. A detailed summary of the more common composite failure models can be found in [21].

The objective of this work is to quantify the effects of material, geometric, and loading uncertainties on the response of self-adaptive composite propellers and overall system reliability. In doing so, safe operating envelopes can be developed and design tolerances can be recommended for a safe and reliable structure. The proposed framework can be generally applied for any self-adaptive composite structure that is required to operate over a wide range of loading conditions. To demonstrate the methodology, results are shown for a composite marine propeller.

Section snippets

Propeller analysis

Propeller performance is analyzed using a previously developed, 3-D coupled boundary element method–finite element method (BEM–FEM) that considers the effects of nonlinear geometric coupling, spatially varying flow, transient fluid sheet cavitation, and fluid–structure interactions. The fluid behavior is assumed to be governed by the incompressible Euler equations in a blade-fixed rotating coordinate system. The total velocity is decomposed into an effective inflow velocity that accounts for

Propeller geometry and probabilistic operational space

The performance of an adaptive propeller is dependent on the total load acting on the blades, which is directly dependent on the propeller operating conditions. In previous works [26], [27], the authors designed two pairs of rigid (NAB) and self-adaptive composite propellers for a twin-shafted naval combatant craft. Both the rigid and adaptive propellers were designed to maximize the lifetime efficiency, i.e. the integrated efficiency over the probabilistic operational space for the design life

Uncertainty analysis

The objective of this work is to quantify the effects of material, geometric, and loading uncertainties on the response and overall system reliability of self-adaptive composite propellers. For the purposes of this research, the analysis of uncertainties on propeller performance will be separated into two categories: (1) stiffness and geometric uncertainties and (2) strength uncertainties. The separation is reasonable because stiffness and geometric uncertainties affect the linear elastic

Conclusions

This paper aims to quantify the effects of material, geometric, and loading uncertainties on the response of self-adaptive composite propellers and overall system reliability within a probabilistic design space. Applying assumed random variations in geometric, stiffness, and strength parameters, structural reliability is estimated for a previously optimized adaptive composite propeller. Safe operating envelopes and design tolerance issues are discussed.

The increased flexibility associated with

Acknowledgments

The authors are grateful to the Office of Naval Research (ONR) and Dr. Ki-Han Kim (program manager) through Grant Nos. N00014-09-1-1204 and N00014-10-1-0170, and the Naval Engineering Education Center through Award No. N65540-10-C-0003, for their financial support.

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Influence of uncertainties on the response and reliability of self-adaptive composite rotors

Abstract

Highlights

Introduction

Section snippets

Propeller analysis

Propeller geometry and probabilistic operational space

Uncertainty analysis

Conclusions

Acknowledgments

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