Complex Analysis of the Impact Damages Growth in the Composite Element Under Cyclic Compression

Аuthors

Turbin N. V.^*, Kononov N. O.^**

Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

*e-mail: turbinnv@mai.ru
**e-mail: kononovno@mai.ru

Abstract

The studies of damage growth caused by the falling load shock were conducted on the test samples from laminate reinforced by the carbon fiber under the impact of the cyclic compression. The basic principle of the composite structures design with account for the fault tolerance is a principle of the damage non-growth, which means that damage, appeared while operation or manufacturing should not grow after its occurrence under static or cyclic loads in the specified environmental conditions. The supportive tools for compliance ensuring with damage permissible standards suppose the structure testing with artificially introduced damage zones. The scope of such tests is obviously limited by the project budget and the capabilities of the methods for predicting the structure critical zones. The objective of the study consists in revealing regularities of the impact damages growth in the composite element at cyclic loads based on the tests, non-destructive control and theoretical model of the imbedded stratification. The test samples are made of unidirectional polymer composite material with rigid stacking and cut by high-speed milling tool. Theoretical model includes expressions for computing critical delamination strains of an elliptical shape, as well as expressions for the quasi-equilibrium growth computing of delamination under the biaxial compressive load. To illustrate the theoretical model operation, the article presents the qualitative results obtained with the equations for critical delamination strains, quasi-equilibrium growth of delamination, and the equations for the durations of delamination growth. The size and depths of internal delaminations prior and after testing employing ultrasonic control were determined. The results of the non-destructive control after 40,000 load cycles revealed the growth of one delamination in the transverse direction relative to the load. Such an orientation of growth is typical for the uniaxial cyclic compression test after the impact, which was noted in several studies. The delamination activated under the load was near the back side of the sample and was oriented by the major axis of the ellipse along the 90° direction of the laminate. Dynamics of the composite damaged state detected by the ultrasonic testing corresponds to the prediction by the theoretical model. The tests were conducted on the SUNS-890-500 servo-hydraulic mechanical testing machine, which allows both static and dynamic testing. Cyclic tests revealed several stages of delamination growth and growth stunt before the end of the fatigue testing cycle base. Although the compliance monotonically increases during cycling, no step changes are being observed during the period of delamination growth. Thus, the occurrence and growth of delamination are not recorded when monitoring the sample compliance during testing, hence additional methods for monitoring damage growth are required. It follows from the experimental results processing that the presented theoretical model is able to qualitatively predict the location of damage initiation in the material corresponding to the experimental results. The technique for the activated delamination location determining is based on the comparison of the delamination initiation coefficients and critical deformations of the delamination stability loss. Thus, the possibility of the damage resistance computing of a composite element under cyclic loads is confirmed.

Keywords:

composite structures, damage tolerance, progressive delamination, compression fatigue after impact, mechanical testing; fracture mechanics; ultrasonic inspection

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