SYMPOSIA PAPER Published: 01 January 1985

Fatigue Response of Notched Graphite/Epoxy Laminates


Although composites have gained widespread use, we do not, as yet, have a precise and complete understanding of the mechanisms of damage development in composite materials. Recent research results have pointed out the need to treat damage as a collective condition—that is, a damage state, rather than as an assembly of discrete and independent damage modes. This paper presents the results of an investigation to determine the damage states which develop in two quasi-isotropic graphite/epoxy laminates with center holes due to tension-tension cyclic loads, to determine the influence of stacking sequence on the initiation and interaction of damage modes and the process of damage development, and to establish the relationships between the damage states and the strength, stiffness, and life of the laminates. Damage was monitored nondestructively throughout the loading history using X-ray radiography, moiré interferometry, and stiffness change. Some specimens were deplied after specific numbers of cycles to determine the distribution of damage in each ply around the hole and to confirm the components and size of the damage state observed nondestructively. Fatigue life and residual strength tests were also performed.

Fatigue damage in the two laminates included matrix cracks in all plies followed by delaminations. The density of matrix cracks and the distribution of the damage zone (matrix cracks plus delaminations) in laminates cycled at the same percent of notched tensile strength were strongly dependent on the local constraint and distribution of interlaminar stresses as governed by the stacking sequence. The distinctly different damage states which developed in the two initially quasi-isotropic laminates due to similar load histories produced stiffness changes of 15% to 20%, different rates of change in residual strength (an initial increase followed by a decrease), and a factor of two to four difference in fatigue life.

Author Information

Kress, GR
Institute for Structural Mechanics, DFVLR, Braunschweig, VA, West Germany
Stinchcomb, WW
Institute for Structural Mechanics, DFVLR, Braunschweig, VA, West Germany
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Developed by Committee: D30
Pages: 173–196
DOI: 10.1520/STP32788S
ISBN-EB: 978-0-8031-4929-8
ISBN-13: 978-0-8031-0436-5