Civil Engineering Conference Innovation Without Limit (CEC-09), September 18-19, 2009, NIT Hamirpur, pp. 89-96.

LAMINATED COMPOSITE STIFFENED PANELS: APPLICATIONSAND BEHAVIOUR

Hemendra Kumar JainM.Tech. Student, Deptt. of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee – 247667.Email: hkj401@gmail.com

Akhil UpadhyayAssociate Professor, Deptt. of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee – 247667. Email: akhilfce@iitr.ernet.in

ABSTRACT

Composite materials are formed by the combination of two or more materials that retain their respective characteristics when combined together to achieve desired properties (physical, chemical, etc.) that are superior to those of individual constituents. The main components of composites are reinforcing agents and matrix. Composites have high strength-to-weight ratio and high stiffness-to-weight ratio as compared to conventional materials. Composite are used in many sectors like civil, aerospace, automobiles, marine, medical and power transmission etc., but in this paper more emphasis is given on structural applications.

This paper presents the literature review of application and behaviour of laminated composite stiffened panels in structural members. Being a thin walled structure, their behaviour is governed by stability criteria.Here brief review of stability behaviour of laminated composite stiffened panels is presented along withaddition the knowledge are also identified into which more comprehensive analysis is needed.

INTRODUCTION

Since the early dawn of civilization, the strong and light material has always fascinated mankind for typical applications. The idea of composite materials are formed by the combination of two or more materials that retain their respective characteristics when combined together to achieve desired properties (physical, chemical, etc.) that are superior to those of individual constituents. The main components of composites arereinforcing agents and matrix. The fibers, particulates & whiskers act as the reinforcement and provide most of the stiffness & strength. The matrix binds the reinforcement together thus effecting the load transfer from matrix to reinforcement. Composites are light weight possess high strength-to-weight ratio and high stiffness-to-weight ratio as compared to conventional materials. Laminated composites are a special form of FRP which belongs to the new generation of energy efficient materials, almost dominating over the metallic materials. The potential of laminated composites offer several possibilities but on the other hand themechanical characterization of a composite structure is more complex than that of metal structures.

Laminated composite stiffened panels, which are anisotropic and orthotropic in nature, are gaining popularity in structural applications such as long span bridge decks, ship deck hulls, buildings, water tank and superstructure of offshore oil platforms etc. The use of laminated composite provides flexibility to tailor different properties of the structural elements to achieve strength and stiffness requirements.

Many composite structure are thin-walled in nature, the assessment of their buckling behaviour is a predominant aspect of analysis and design. Mallela U. K. and Upadhyay A. (2006) present parametric studies on simply supported laminated composite blade-stiffened panels subject to in-plane shear loading. Knippers J. and Gabler M.(2007) are offer new option especially bridge deck systems made out of fibre reinforced polymers (FRP), hybrid structures are competitive, i.e. steel girders combined with a pultruded FRP bridge deck. Based on this technology the design of a highway flyover was developed. The bridge, to be constructed during 2006-07, will be the first major FRP road bridge in Germany. The innovative technology, its economical aspects and the design of the bridge are highlighted in this paper. Upadhyay A. and Kalyanaraman (2003) discussed the behavior of FRP box girders and proposed a simplified computationally efficient method for the analysis of single cell FRP box – girder bridges made of blade, angle or T stiffened panels.

Civil Engineering Conference Innovation Without Limit (CEC-09), September 18-19, 2009, NIT Hamirpur, pp. 89-96.

LAMINATED COMPOSITE STIFFENED PANELS

Laminated composite stiffened panels are generic structural elements in weight sensitive structureapplications, which are made by adjoin of laminated composite plate and stiffeners. Laminated composite plates are made by stacking of lamina with various orientation of the principle material direction. Stiffened panel consist of laminated composite plate provided with stiffeners in the longitudinal and/or transverse direction. Composite plate stiffened by different type of stiffeners. These are broadly classified as open type and closed type or box type. Open type are torsionally week while closed type or box type are torsionally stiff. Fig.1 shows a schematic representation of different type of stiffeners. Some various shaped stiffening members commonly used for panel structural concepts are “T”, “Z”, “I”, “C”, “J”, and hat. One very common type of stiffener is the so-called blade stiffener which actually is a plate perpendicularly attached to the composite plate. A typical arrangement of this class can be found in Fig. 2. These panels are becoming increasingly used in structural applications because of their high specific stiffness (stiffness per unit weight) and specific strength (strength per unit weight). The stiffened elements representing a relatively small part of the total weight of the structure substantially influence their stiffness and stability.

Fig.1.Example for stiffeners cross section

Fig.2.Composite blade stiffened panel (open type)

ADVANTAGE OF LAMINATED COMPOSITE STIFFENED PANELS

Laminated composite stiffened panels have the advantage of both features structural form and material.A few significant advantages are:

Composites can provide structures that are 25-45 % lighter than the conventional structures designed to meet the same functional requirements.

Tensile strength of composites is four to six times greater than that of steel or aluminium. Unidirectional fiber composites have specific modulus (ratio of material stiffness to density) about 3

to 5 times greater than that of steel and aluminium. Fatigue endurance limit of composites may approach 60% of their ultimate tensile strength. For steel

and aluminium, this value is considerably lower. Corrosion resistance of fiber composites leads to reduced life cycle cost. Unlimited capacity to be molded (fabric impregnated in situ) and perfect adaptability to the original

shape of the structural element that needs to be reinforced, stiffened or integrated. Simple application methods with absence of complex preparation. Better durability, excellent damage tolerance, non invasive application, improved torsional stiffness

and impact properties. Composites are less noisy while in operation and provide lower vibration transmission than metals. Composites are more versatile than metals and can be tailored to meet performance needs and

complex design requirements. Excellent fatigue, impact, environmental resistance and reduced maintenance. Composites enjoy reduced life cycle cost compared to metals.

Civil Engineering Conference Innovation Without Limit (CEC-09), September 18-19, 2009, NIT Hamirpur, pp. 89-96.

Composites exhibit excellent corrosion resistance and fire retardancy. Improved appearance with smooth surfaces and readily incorporable integral decorative melamine

are other characteristics of composites. Composite parts can eliminate joints/fasteners, providing part simplification and integrated design

compared to conventional metallic parts.

In addition to above, From an economic point of view, the main factors contributing to their competitiveness with respect to conventional materials are time saving, flexibility, low labor costs, low tooling and machinery costs on the construction site because of the light weight and manageability of tools, possibility of restoring a structure without interrupting its utilization by users and durability. In spite of many advantages of laminated composites over traditional materials, complex mechanics involved in laminated fibrous composites poses new challenges for the construction industry.

The downside of composites is usually the cost. Although manufacturing processes are often more efficient when composites are used, the raw materials are expensive. At present composites can not totally replace traditional materials like steel, but in many cases they are just what we need. And no doubt new uses will be found as the technology evolves.

WHY USE THE LAMINATED COMPOSITE STIFFENED PANELS?

In all cases of buckling of plates, critical loads are proportional to the flexural rigidity of the plates. The stiffeners are of very less weight but at the same time enhance the in-plane critical load carrying capacity when compared with unstiffened panels. Stability of the plate increases with the increase in the thickness of the plate but a more economical solution is obtained by keeping the thickness of the plate as small as possible and increasing the stability by introducing stiffeners. Being a thin walled structure the design of stiffened plates is governed both by stability and strength criterion.

These panels are becoming increasingly used in structural applications because of their high specific stiffness (stiffness per unit weight) and specific strength (strength per unit weight).The laminated composite panels are high strength material so thickness requirement is less as compare to conventional material as well as thickness requirement is less from strain point of view so analysis and design is governed by stiffness and stability consideration.

Many composite structure are thin-walled in nature, the assessment of their buckling behaviour is a predominant aspect of analysis and design. Composite possess high in plane strength so this is not a major issue of analysis or designing but stability need proper attention and efficient structural form, which is provided by stiffened panels. Sufficient weight reduction can be achieved by the use of stiffened panels.

APPLICATION OF LAMINATED COMPOSITE STIFFENED PANELS

Laminated composite stiffened panels are widely employed in many engineering application. Such structures are usually thin-walled and therefore susceptible to buckling. Stiffened panels are generic structural elements in weight sensitive structural, civil, aerospace and marine applications. A laminated composite stiffened panel, which is anisotropic and orthotropic, are gaining popularity in structural applications such as long span bridge decks, ship deck hulls and superstructure of offshore oil platforms etc.

APPLICATION OF LAMINATED COMPOSITE PANELS IN MARINE ENGINEERING

With composites exhibiting excellent resistance to the marine environment, their applications have made good inroads in the marine sector worldwide. Complex configurations & the advantages of seamless hulls were the main driving factors in the development of FRP boats. Racing power boats employ advanced & hybrid composites for a higher performance craft and driver safety. Major structural elements viz. deckhouses, hatch covers, kings posts & bow modules, high speed boats, naval vessels, sail boats, fishing boats, high capacity trawlers, barges & other ship components appears to be very well suited for FRP construction.

Deck and ship bottom structures are modelled as assemblies of laminated composite stiffened panels. The layered configuration of laminated structures allows opportunities to embed and integrate specialized materials into the composite lay-up which provide improved electromagnetic performance. U.S. shipbuilding industry recently has started incorporating composite materials as well as laminated composited stiffened

Civil Engineering Conference Innovation Without Limit (CEC-09), September 18-19, 2009, NIT Hamirpur, pp. 89-96.

panels in the construction of both military and commercial ships due to the advantages of composite construction. Fig.3 and Fig.4 shows application of FRP panels in boats.

Sarah E Mouring(1998) focuses on different fiber orientations for the frames and summarizes preliminary results of the testing of composite stiffened panels with preform frames under in-plane uniaxial compressive loads. Such loads are representative of the forces caused by wave bending moments which develop from alternative hogging and sagging wave conditions. Biaxial (0, 90), quadaxial (0, 90,+45,−45), and triaxial (+45,−45,0) laminates were used in the frames.

Fig.3.Lay-up configuration for advanced material transporter Fig.4.Laminated hat stiffened deck house (AMT) validation model [Nguyen, 93 Sml Boat] panels test element [Scoot Bartlett, NSWC]

APPLICATION OF LAMINATED COMPOSITE PANELS IN AEROSPACE ENGINEERING

The Air Force Research Laboratory Space Vehicles Directorate (AFRL/VS) USA is exploring new structural configurations in the form of advanced grid composite stiffened panel and corresponding methods for fabricating launch vehicle fairings. The aerospace industry began to use composites in pressure vessels, containers, and non -structural aircraft components. A structurally efficient hat-stiffened panel concept that utilizes structural foam as a stiffener core material has been designed and developed for aircraft primary structural applications. This hat-stiffener concept is structurally more efficient than most other prismatically stiffened panel configurations in a load range that is typical for both fuselage and wing structures. Fig.5 shows application of stiffened panels in jet.

Fig.5.The entire fuselage of the boeing’s 787 Dreomliner jet built with carbon fiber

York et al. (1993) have designed eight benchmark stiffened laminated wing panels for aircraft. These benchmark panels had three identical and equally spaced blade stiffeners. Fig.6 shows an isometric view of the components of the Dornier wing and also shows a plan view of its top skin and stacking sequence.

Civil Engineering Conference Innovation Without Limit (CEC-09), September 18-19, 2009, NIT Hamirpur, pp. 89-96.

Fig.6.(a) Isometric view of the components of the Dornier wing (b) Plan view of the top skin (c) Staking sequence of benchmark panels

APPLICATION OF LAMINATED COMPOSITE PANELS IN CIVIL ENGINEERING

Laminated composite stiffened panels have long been used in the construction industry. Applications range from non-structural gratings and claddings to full structural systems for industrial supports, buildings, long span roof structures, tanks, bridge components and complete bridge systems.

LAMINATED COMPOSITE PANELS IN BRIDGES

Civil engineers are beginning to gain confidence and experience in applying this technology to civil structures. In October 1996, the Federal Highway Administration organized and sponsored a scanning tour on advanced composites in bridges to three selected European countries and Japan. The purpose of the tour was to assess the state-of-the-art in the use of composites stiffened panels in bridge construction. The technical findings from the scanning team are basically categorized into new bridge construction, and strengthening of existing bridges.

With the ageing of (more than 50 years) many bridges in the US, alternative options are being explored to retrofit the bridges or replace the old decks with lighter decks which meet the service criteria of the existing decks and even replace the bridges itself. Considering, the sheer number (almost 23% 0f 600,000 bridges) of bridges which are structurally deficient and functionally obsolete (Hwang-Chu Wu et al. 2006 and Halvard E. Nystrom et al. 2003). According to a data base recently compiled by the Composites Institute, there are more than 80 bridge projects worldwide using FRP composites panels. The U.S. has a modest beginning with 30 projects, 26 of which were built within the last 4 years. The remaining discussion in this will focus on some of the initial successful bridge applications in the U.S. using FRP composites panels. The discussion will also include the advantages, characteristics, concerns, and future needs to advance the composite technology into the civil infrastructure. In new bridge construction, the FRP composite panels may be used in the entire structure, or they could be used as structural members or components. Some of example illustrated in Fig.7.

Fig.7.FRP Bridge Decks: Panels being placed into position

A key factor behind bridge deterioration is the corrosion of reinforcing steel in concrete decks, exacerbated by road salt used to combat winter ice and snow. FRP panels are expected to be much more durable than ordinary concrete and steel. A rapid growth of interest in FRP composites panels for bridge construction has resulted in a multitude of recent research and development initiatives and demonstration projects. The high strength, high fatigue resistance, low density, and excellent corrosion resistance of composites are desirable

Civil Engineering Conference Innovation Without Limit (CEC-09), September 18-19, 2009, NIT Hamirpur, pp. 89-96.

characteristics for bridge application, especially for decks. Bridge decks are subjected to severe environmental conditions and heavy traffic loads. They sometimes account for a major percentage of a bridge structure’s dead load. FRP deck weighs approximately 80% less than a concrete deck. Reducing the dead load will increase the allowable live load capacity of the bridge without significant repair to the existing superstructure, thus lengthening its service life. Hence, lightweight and durable FRPs can be an excellent candidate for replacing concrete decks.

A review indicates that extensive research is being carried out both in laboratory as well as on ground to design the FRP decks and study the response to various external effects. Lot of on ground applications in the form of pedestrian bridges, hybrid bridges (with RC), all composite bridges and bridge decks as replacement in old bridges are underway in US, Europe, Australia, Korea etc. Two major types of FRP composite bridge decks are currently in use: sandwich type construction and cellular or stiffened structure.

LAMINATED COMPOSITE PANELS IN BUILDINGS

The composite panel is an ideal material for the manufacture of prefabricated, portable and modular buildings as well as for exterior cladding panels which can simulate masonry or stone. The all too familiar translucent roof sheeting is now supplied in a variety of colours and profiles to suit both commercial and domestic building needs. Stiffened panels are used in wall, ceiling, door, building blocks and primary load bearing structure etc. This area holds priority for the induction of composites in place of conventional materials being used as in doors & windows, paneling, furniture and other interiors. Components made of composite panelsfind extensive applications in shuttering supports, special architectural structures imparting aesthetic appearance etc. with the advantages like longer life, low maintenance, ease in workability, fire retardancy etc.

Fig.8.Site office/Visitor’s Centre at Severn Crossing is made by composite stiffened panels

Laminated composited stiffened panels are also used in commercial canop, water tank, daylighting matel building, power plant, automobile, railway, medical, sky bus coatch, Offshore Oil and Gas Industry.

BEHAVIOUR OF LAMINATED COMPOSITE STIFFENED PANELS

The laminated composite panels are high strength material so thickness requirement is less as compare to conventional material. There are two major categories leading failure of a component: material failure and structural instability, which is often called buckling. The load at which buckling occurs depends on the stiffness of the components, not upon strength of its material. Buckling refers to the loss of stability of components and independent of material strength. This loss of stability usually occurs in elastic rang of material. Buckling failure is primarily characterized by loss of structural stiffness and is not modeled by the usual linear finite element analysis, but by eigenvalue solution. Slender or thin walled component under compressive stress are susceptible to buckling. Laminated composited stiffened panels are thin walled structure so buckling is a major problem in its design.

Stiffened panels in real structures can be loaded under a combination of in-plane and out-plane loading. In plane loads include in-plane compression, in-plane shear and a combination of these loadings. Out-of-plane loading would include lateral pressure or bending about the transverse and longitudinal axes of the stiffened panel. Generally these plates are flexible when the loads are applied normal to the plane of the plate but it is very stiff when the loads are applied in the plane. It is this in-plane high stiffness which is utilized in thin-walled structures. The relatively high out of plate flexibility of the plate element make these elements more susceptible to the buckling under in-plane compression and in-plane shear. The buckling behaviour of stiffened panels is more complex when compared to unstiffened panels.

Civil Engineering Conference Innovation Without Limit (CEC-09), September 18-19, 2009, NIT Hamirpur, pp. 89-96.

Stiffened panels when subjected to these in-plane loading may buckle in local mode or global (general instability) mode. Stiffened panels are said to buckle locally when the plate between the stiffeners buckle. This termed as plate buckling. This mode of failure illustrated in Fig.9. Researchers reported that this failure result in more significant drop in load-carrying capacity after buckling then the overall buckling failure modes.

On the other hand, stiffened panel is said to buckle globally when the stiffener and the plate buckle simultaneously in such way that the half sign wave extends over more than one plate element between the stiffeners (Fig.10). Generally this failure is the most favourable mode of failure since it behaves in a more stable manner after buckling.

Fig.9.Local mode of buckling

Fig.10.Global mode of buckling

CONCLUSIONS

Laminated composite stiffened panels are used throughout the civil, marine and aerospace industry for numerous structure applications. Light weight composite stiffened panels may be utilized with great advantage in bridge decks as well as structural component of buildings. The technological advances in composite stiffened panels and new understanding of how panels behave under load have been crucial in the development for the structural application. It is essential to fully understand how these panels behave, because only that way we can built stronger and lighter structural components. Stiffened panel are relatively new material so no design codes, guidelines or specifications are available for the uses in structural application. There is need to develop data base which will be helpful to new designs.

REFERENCES

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