Stress fields in composite laminates

Results of a study on the energy absorption characteristics of selected composite material systems are presented and the results compared with aluminum. Composite compression tube specimens were fabricated with both tape and woven fabric prepreg using graphite/epoxy (Gr/E), Kevlar (TM)/epoxy (K/E) and glass/epoxy (Gl/E). Chamfering and notching one end of the composite tube specimen reduced the peak load at initial failure without altering the sustained crushing load, and prevented catastrophic failure. Static compression and vertical impact tests were performed on 128 tubes. The results varied significantly as a function of material type and ply orientation. In general, the Gr/E tubes absorbed more energy than the Gl/E or K/E tubes for the same ply orientation. The 0/ + or - 15 Gr/E tubes absorbed more energy than the aluminum tubes. Gr/E and Gl/E tubes failed in a brittle mode and had negligible post crushing integrity, whereas the K/E tubes failed in an accordian buckling mode similar to the aluminum tubes. The energy absorption and post crushing integrity of hybrid composite tubes were not significantly better than that of the single material tubes.

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Energy Absorption of Composite Materials.

Compressive failure of composites, part I: Testing and micromechanical theories

The mechanical anisotropy of an aromatic polyamide fibre, Kevlar 49, was studied in tension, compression and torsion. A new technique involved applying small and defined compressive strains to filaments by bonding them to one side of a beam which is subsequently bent to compress the fibres. Using scanning electron and optical microscopy, fibres were shown to form regularly-spaced helical kink bands at 50 to 60° to the fibre axis after the application of small axial compressive strains. Tensile tests of previously-compressed fibres revealed only a 10% loss in tensile strength, after application of as much as 3% compressive strain. A torsion pendulum apparatus was used to measure the shear modulus and an apparent shear strength of fibres. A loss of tensile strength after the application of large (> 10%) torsional shear strains coincided with a loss in recoverable shear strain due to longitudinal fibre splitting. Ratios of tensile-to-compressive strength, tensile-to-shear strength and tensile-to-shear moduli of 5∶1, 17∶1, and 70∶1, respectively, were measured for Kevlar 49.

Compressive and torsional behaviour of Kevlar 49 fibre

Characterization of Kevlar Fiber and Its Composites: A Review☆

An investigation of the compressive strength of Kevlar 49/epoxy composites

The development of unidirectional fiber composite materials is discussed. The mechanical and physical properties of the materials are described. Emphasis is placed in analyzing the compressive behavior of composite materials and developing methods for increasing compressive strength. The test program for evaluating the various procedures for improving compressive strength are reported.

S. V. Kulkarni

Papers

An investigation of the compressive strength of Kevlar 49/epoxy composites

An investigation of the compressive strength of PRD-49-3/Epoxy composites