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DETERMINATION OF INITIAL PLY FAILURELOAD BY THE VARIOUS THEORIES OF

FAILURE

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PROJECT GUIDEDr.B.Prabhu

TEAM MEMBERS

Dinesh.SNarendran.MSilambarasan.S

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OBJECTIVEy For an orthotropic material, strengthproperties are necessary

(not always sufficient though), tensile and compressive strengthsin the three principal directions of the material and shearstrengths in the three directions. Some criteria may require moreproperties that are more difficult to measure than these andstandardprocedures are often not f ound.

y The difficulties in establishing and validating failure criteria f orcomposites are associated with the definition of failure incomposites, which is not always absolutely clear. Hence weresolve to micromechanical analysis.

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METHOD OF APPROACHA s stated in theo bjective, we come to know that failure incomposites shows a much wider varietyof mechanisms than thatin metals. There are many different modes of failure. It depends on

the failure criterion adopted. However, f or an orthotropicmaterial strengthproperties are necessary (not always sufficientthough), tensile and compressive strengths in the threeprincipaldirections of the material and shear strengths in the threedirections. These properties are easily measurable and there arestandardprocedures f or measuring them. Some criteria mayrequire more properties that are more difficult to measure thanthese and standardprocedures areoften not f ound

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A s composites are mostly used in f orms of laminates, 2Dstress states areoften sufficient f or most applications. Beloware listed the 9 failureproperties reduced to tensile orcompressive and/or shear stresses f or one dimensional andf or 2D pro blems.

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The difficulties in establishing and validating failure criteriaf or composites are associated with the definition of failure incomposites, which is not always absolutely clear. Forexample, in a laminate,one or more laminate may be clearlyfailing, e.g. cracked, yet the laminate could keep on takingload to much higher level. When the laminate is unloadedfrom such a damaged state, a full recovery of the shape of thelaminate is often o bserved. In composites, material failuresometimes shows structural behavior, e.g. c ompression alongfibres, in fact along plies.

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y A nd this structural behavior along the plies of a laminateduring a compression test is done usingFEA (Finite ElementA nalysis).

y

By this we findout the appropriate failure theory which givesus the closest value and thereby indicating the initialplyfailure load f or the given composite.

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What is a composite Material?

y A broad definition of composite is: Two or more chemicallydistinct materials which when combined have improvedproperties over the individual materials. Composites could be natural or synthetic.

y Wood is a good example of a natural composite, combinationof cellulose fiber and lignin. The cellulose fiberprovidesstrength and the lignin is the "glue" that bonds and stabilizesthe fiber

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y Composites are combinations of two materials in whichone of

the material is called ther einfo rc ing p ha se, is in the f orm of fibers, sheets,or particles, and is embedded in theother materialcalled thema t r ix p ha se

y Typically, reinf orcing materials are strong with low densitieswhile the matrix is usually a ductileor tough material. If thecomposite is designed and fabricated correctly, it combines thestrength of the reinf orcement with the toughnessof the matrix

to achieve a combination of desirableproperties not available inany single conventional material.

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Composites Offery High strength-to-weight ratioy Directional strengthy Corrosion resistancey Weather resistancey Dimensional stabilityy low thermal conductivityy low coefficientof thermal expansiony Radar transparencyy Non-magneticy High impact strengthy High dielectric strength (insulator)y Low maintenancey Long term durabilityy Part consolidationy Small to large part geometry possibley Tailored surface finish

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