Editing Composite material (section)

===Failure===
Shock, impact of varying speed, or repeated cyclic stresses can provoke the laminate to separate at the interface between two layers, a condition known as [[delamination]].<ref>{{cite journal |last1=Ma |first1=Binlin |last2=Cao |first2=Xiaofei |last3=Feng |first3=Yu |last4=Song |first4=Yujian |last5=Yang |first5=Fei |last6=Li |first6=Ying |last7=Zhang |first7=Deyue |last8=Wang |first8=Yipeng |last9=He |first9=Yuting |title=A comparative study on the low velocity impact behavior of UD, woven, and hybrid UD/woven FRP composite laminates |journal=Composites Part B: Engineering |date=February 2024 |volume=271 |pages=111133 |doi=10.1016/j.compositesb.2023.111133 }}</ref><ref>{{cite journal |last1=Sanchez-Saez |first1=S. |last2=Barbero |first2=E. |last3=Zaera |first3=R. |last4=Navarro |first4=C. |title=Compression after impact of thin composite laminates |journal=Composites Science and Technology |date=October 2005 |volume=65 |issue=13 |pages=1911–1919 |doi=10.1016/j.compscitech.2005.04.009 |hdl=10016/7498 |hdl-access=free }}</ref> Individual fibres can separate from the matrix, for example, [[fiber pull-out|fibre pull-out]].

Composites can fail on the [[macroscopic]] or [[microscopic]] scale. Compression failures can happen at both the macro scale or at each individual reinforcing fibre in compression buckling. Tension failures can be net section failures of the part or degradation of the composite at a microscopic scale where one or more of the layers in the composite fail in tension of the matrix or failure of the bond between the matrix and fibres.

Some composites are brittle and possess little reserve strength beyond the initial onset of failure while others may have large deformations and have reserve energy absorbing capacity past the onset of damage. The distinctions in fibres and matrices that are available and the [[mixture]]s that can be made with blends leave a very broad range of properties that can be designed into a composite structure. The most famous failure of a brittle ceramic matrix composite occurred when the carbon-carbon composite tile on the leading edge of the wing of the [[Space Shuttle Columbia]] fractured when impacted during take-off. It directed to the catastrophic break-up of the vehicle when it re-entered the Earth's atmosphere on 1 February 2003.

Composites have relatively poor bearing strength compared to metals.

[[File:Composite Strength as a Function of Fiber Misalignment.png|thumb|The graph depicts the three fracture modes a composite material may experience depending on the angle of misorientation relative to aligning fibres parallel to the applied stress.]]

Another failure mode is fiber tensile fracture, which becomes more likely when fibers are aligned with the loading direction, so is the possibility of fiber tensile fracture, assuming the tensile strength exceeds that of the matrix. When a fiber has some angle of misorientation θ, several fracture modes are possible. For small values of θ the stress required to initiate fracture is increased by a factor of (cos θ)<sup>−2</sup> due to the increased cross-sectional area (''A'' cos θ) of the fibre and reduced force (''F/''cos θ) experienced by the fiber, leading to a composite tensile strength of ''σ<sub>parallel </sub>/''cos<sup>2</sup> θ where ''σ<sub>parallel </sub>'' is the tensile strength of the composite with fibers aligned parallel with the applied force.

Intermediate angles of misorientation θ lead to matrix shear failure. Again the cross sectional area is modified but since [[shear stress]] is now the driving force for failure the area of the matrix parallel to the fibers is of interest, increasing by a factor of 1/sin θ. Similarly, the force parallel to this area again decreases (''F/''cos θ) leading to a total tensile strength of ''τ<sub>my</sub> /''sin&nbsp;θ cos&nbsp;θ where ''τ<sub>my</sub>'' is the matrix shear strength.

Finally, for large values of θ (near π/2) transverse matrix failure is the most likely to occur, since the fibers no longer carry the majority of the load. Still, the tensile strength will be greater than for the purely perpendicular orientation, since the force perpendicular to the fibers will decrease by a factor of 1/sin θ and the area decreases by a factor of 1/sin θ producing a composite tensile strength of ''σ<sub>perp</sub> /''sin<sup>2</sup>θ where ''σ<sub>perp </sub>'' is the tensile strength of the composite with fibers align perpendicular to the applied force.<ref>
{{cite book |last=Courtney |first=Thomas H. |date=2000 |title=Mechanical Behavior of Materials |edition= 2nd |publisher=Waveland Press, Inc. |location=Long Grove, IL |pages=263–265 |isbn=978-1-57766-425-3}}
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