Introduction to Composite - NPTEL

1 IntroductiontoCompositeIntroduction to Composite Materials and StructuresNachiketaTiwariNachiketa TiwariIndian Institute of Technology KanpurLecture 28 Failure of Unidirectional CompositesIntroductionIntroduction Failure of composites is, unlike metals, a complex multi stage process. Failure of a Composite sample may get triggered in a certain mode , but its propagation and final failure modes may be significantly different. In a large number of cases, Composite failure gets initiated internally, and it is only once failure has propagated beyond a certain extent, that changesincomposite in Composite s behavior and appearance are observed. The internal failure of a Composite sample could manifest as:Bkiffib Breaking of fibers Development of micro cracks in matrix Debonding between fibers and matrix Delaminationieseparationofdifferentlayer sofalaminate Delamination, separation of different layers of a laminate Here, we explore different failure mechanisms of composites, first at ply levelandsubsequentlyatlaminatelevellevel , and subsequently at laminate of Unidirectional Layers In many Composite samples, before failure occurs at macroscopic scale, ypp,p,the material response changes significantly.

2 This change in material response, which precedes actual material failure, is a manifestation of a largenumberof micro failures aswellasnon linearmaterialbehaviorlarge number of microfailures as well as nonlinear material behavior. For unidirectional laminates, the stress strain behavior is typically linear. However, as externally applied load exceeds a certain threshold, the stress strain response curve maybecome increasingly nonlinear. This threshold could be one definition of the failure load of unidirectional lhfilldfidi i llldbhl Alternatively, the failure load of a unidirectional ply could be the actual load at which the Composite fractures. FailureofUnidirectionalLayersFailure of Unidirectional Layers If failure in unidirectional lamina is considered to be Composite material s proportional limit, then such a failure criteria ensures a significant margin of safety.

3 However, in a very large number of composites the stress strain responses curve is fairly straight to the point of failure. This is so, because of a combinationoftworeasons:combination of two reasons: Most of the fibers used in modern Composite laminates exhibit brittle failure. The fiber volume fraction in a very large number of composites is maximized, and hence, the fiber failure and Composite failure occur at same load level. However, in materials which have either very low fiber volume fraction or where matrix material is very stiff (vis vis fiber), the overall stress strain response of Composite may be considered as its proportional limit. Infollowingslides,differentfailuremodeso funidirectionallayershaveIn following slides, different failure modes of unidirectional layers have been of Unidirectional Layer due to Longitudinal Tension When a unidirectional lamina is subjected to increasing tensile force in jglongitudinal direction, all fibers do not fail at a specific load level.

4 Rather, fibers fail in a series of failure events, with the weakest fiber breaking first. Thisisbecausefiberstrengthisastatistical lydistributedfunctionThis is because, fiber strength is a statistically distributed function. Because of this reason, fiber breakage may initiate at loads sometimes lthhlfflti ttiltthflieven less than half of ultimate tensile strength of lamina. Load borne by broken fibers gets redistributed in the Composite , thereby increasing stresses in unbroken fibers. Dtthidldtiildthbffib Due to this reason, and also due to increasing load, the number of fibers breaking at a given load increases. This phenomenon is shown in Fig. of Unidirectional Layer due to Longitudinal TensionFig28 1:CumulativeNumberofFiberBreakswithIncre asingLongitudinalLoadFig.

5 : Cumulative Number of Fiber Breaks with Increasing Longitudinal LoadFailure of Unidirectional Layer due to Longitudinal Tension Once number of broken fibers exceeds a certain threshold, the Composite may become too weak at a specific cross section to bear any further in load. At this point, the unidirectional lamina breaks into two parts. Such a filitltthdiff tfildThfailure can occur in at least three different failure modes. These are: Brittle failure of fibers: In such a mode, the fiber exhibits brittle failure. Brittle failure of fibers with fiber pullout: In such a mode, the fiber not only exhibits brittle failure, but also pulls out of matrix due to debonding of fiber and Brittle failure of fibers accompanied with fiber pullout, as well as shear failure at interface matrix location, and/or matrix debonding from A schematic depiction of these three failure modes are shown in Fig.

6 Of Unidirectional Layer due to Longitudinal TensionFig28 2:DifferentFailureModesofaUnidirectional LaminainLongitudinalTensionFig. : Different Failure Modes of a Unidirectional Lamina in Longitudinal Tension(Left: Brittle Failure, Center: Brittle Failure with Fiber Pullout, Right: Brittle Failure with Debonding and/or Matrix Cracking)Failure of Unidirectional Layer due to Longitudinal Tension Glass fibers are very brittle, and their composites, especially with low fiber volume fractions tend to exhibit brittle fiber failure. Glass fibers with moderate fiber volume fraction (between 40% and 65%) exhibit brittle fiber failure with fiber pullout. Glass fiber composites with high fiber volume fractions exhibit the third modeoffailureiebrittlefiberfailureaccomp aniedwithfiberpulloutasmode of failure, brittle fiber failure accompanied with fiber pullout, as well as shear failure at interface matrix location, and/or matrix debondingfrom fibers.

7 Graphite fiber composites however exhibit second and third modes of Analysis and Performance of Fiber Composites, Agarwal,BDdBtLJJhWil& , ,JohnWiley& ofCompositeMaterials,Jones, ,Mc Engineering Mechanics of Composite Materials, Daniel, ,O.,OxfordUniversityPress.