Vibrothermography and Ultrasonic Pulse-Echo Methods Applied to the Detection of Damage in Composite Lamintates

It has recently been shown in our laboratories that quasi-isotropic, graphite-epoxy, composite laminates develop a typical damage state that eventually leads to final failure. This damage state cannot be represented by a single through crack that propagates in a self-similar manner in the fashion ordained by fracture mechanics. To the contrary, the damage state is a complex one which begins by transverse cracking in the weakest lamina, continues by an increase in transverse crack density until a stable equilibrium spacing is achieved, proceeds by growth into the adjacent laminae.and ends by final, catastrophic failure. In certain stacking sequences, the damage state is further complicated by delamination. Several NDE methods are being developed in our laboratories specifically to identify and quantitatively describe this damage state. The vibrothermography technique uses low amplitude vibrations as a steady state energy source in the composite laminate. The mechanical energy is preferentially absorbed in the region of damage and converted to heat, which can then be detected by thermography. This technique is especially applicable to detecting delamination. An ultrasonic pulse-echo method utilizing a straightforward diffraction analysis is. being developed to detect the transverse cracks which, as they approach and attain an equilibrium spacing, present the appearance of a changing diffraction grating to the ultrasonic beam

Fatigue Damage in Notched Composite Laminates under Tension-Tension Cyclic Loads

Composite materials are established as reliable and efficient materials for a large number of structural applications. Although composites have gained widespread use, we do not, as of yet, have a precise and complete understanding of the mechanismsm of damage development in composite materials. Recent research results have pointed out the need to treat damage as a collective condition; i.e., a damage state, rather than as an assembly of discrete and independent damage modes. The process of the development of the damage state and the subsequent response of the composite laminate throughout the loading history can then be related. This report presents the results of an investigation to determine the damage states which develop in graphite epoxy laminates with center holes due to tensio-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.Two quasi-isotropic laminates were selected to give different distributions of interlaminar stresses around the hole. The laminates were tested under cyclic loads (R=0.1, 10 Hz) at maximum stresses ranging between 60 95 percent of the notched tensile strength. Damage was monitored nondestructively throughout the loading history using stereo X-ray radiography, acoustic emission recording, and stiffness change. Some specimens were deplied after specific numbers of cycles to determine the nature and distribution of damage in each ply araound 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-sitropic laminates due to similar load histories produced stiffness changes of 15-20 percent, different rates of residual strength degradation, and a factor of four difference in fatigue life. The results of this study are interpreted to establish relationships between the loading history, the progressive development of the damage state, and the response of the notched laminates