Date Awarded

1997

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Applied Science

Abstract

A Trans-Laminar-Reinforced (TLR) composite is defined as composite laminate with up to five percent volume of fibrous reinforcement oriented in a "trans-laminar" fashion in the through-thickness direction. The TLR can be continuous threads as in "stitched laminates", or it can be discontinuous rods or pins as in "Z-Fiber{dollar}\sp{lcub}\rm TM{rcub}{dollar}" materials. Adding TLR to an otherwise two dimensional laminate results in the following advantages: substantially improved compression-after-impact response; considerably increased fracture toughness in mode I (double cantilever beam) and mode II (end notch flexure); and severely restricted size and growth of impact damage and edge delamination. TLR has also been used to eliminate catastrophic stiffener disbonding in stiffened structures. as little as one percent volume of TLR significantly alters the mechanical response of laminates.;The objective of this work was to characterize the effects of TLR on the in-plane and inter-laminar mechanical response of undamaged composite laminates. Detailed finite element models were made of "unit cells," or representative volumes, and an inter-laminar dominated problem of practical interest. The work was limited to the linear response of undamaged material with at least one ply interface. Adding a few percent TLR had a small negative effect on the in-plane extensional and shear moduli, E{dollar}\sb{lcub}\rm x{rcub}{dollar}, E{dollar}\sb{lcub}\rm y{rcub}{dollar} and G{dollar}\sb{lcub}\rm xy{rcub},{dollar} but had a large positive effect (up to 60 percent) on the thickness direction extensional modulus, E{dollar}\sb{lcub}\rm z{rcub}{dollar}. The out-of-plane shear moduli, G{dollar}\sb{lcub}\rm xz{rcub}{dollar} and G{dollar}\sb{lcub}\rm yz{rcub},{dollar} were significantly affected only with the use of a TLR with a shear modulus an order of magnitude greater than that of the composite lamina. A simple stiffness averaging method for calculating the elastic constants was found to compare closely with the finite element results, with the greatest difference being found in the inter-laminar shear moduli, G{dollar}\sb{lcub}\rm xz{rcub}{dollar} and G{dollar}\sb{lcub}\rm yz{rcub}.{dollar} Delamination initiation was studied with a strength of materials approach in the unit cell models and the flanged skin models. It was concluded that if the formation of a transverse crack is included as a source of delamination initiation, the addition of TLR will not be effective at preventing or delaying the onset of delamination. The many benefits of TLR may be accounted for by an increased resistance to delamination growth.

DOI

https://dx.doi.org/doi:10.21220/s2-8vqq-tj58

Rights

© The Author

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