Abstract
In Part II of this work, quasi-static tensile properties of four aeronautical grade carbon-epoxy composite laminates, in both the as-received and pre-fatigued states, have been determined and compared. Quasi-static mechanical properties assessed were tensile strength and stiffness, tenacity (toughness) at the maximum load and for a 50% load drop-off. In general, as-molded unidirectional cross-ply carbon fiber (tape) reinforcements impregnated with either standard or rubber-toughened epoxy resin exhibited the maximum performance. The materials also displayed a significant tenacification (toughening) after exposed to cyclic loading, resulting from the increased stress (the so-called wear-in phenomenon) and/or strain at the maximum load capacity of the specimens. With no exceptions, two-dimensional woven textile (fabric) pre-forms fractured catastrophically under identical cyclic loading conditions imposed to the fiber tape architecture, thus preventing their residual properties from being determined.
Highlights
Part I of this study focused on the quasi-static and dynamic bending properties of several carbon-epoxy composite laminates extensively used in the Brazilian aeronautical industry[5]
The results described above indicate that, in a general sense, carbon fiber tape pre-form impregnated with standard epoxy grade may be the best choice among all possible fiber/resin combinations
The obtained results have been interpreted on the basis of microstructural nuances exhibited by the materials tested, e.g., carbon fiber pre-form, type of epoxy resin, as well as testing conditions applied on their mechanical assessment, e.g., monotonic strain-rate and cyclic loading frequency
Summary
Part I of this study focused on the quasi-static and dynamic bending properties of several carbon-epoxy composite laminates extensively used in the Brazilian aeronautical industry[5]. Part II describes quasi-static tensile properties of the same materials, with special emphasis in the residual strength of pre-fatigued testpieces. It is well known[1,2,3,4] that individual trans-laminar (matrix cracking) and inter-laminar (interior delaminations) damage are the earliest and most prevalent fracture modes in composites subject to fatigue. Thermosetting polymer matrix composite manufacturers claim that stronger and more ductile resin matrix, e.g., rubber-toughened epoxy resin grades, improves the laminates performance under cyclic loading through the reduction of matrix damage. This study intends to assess this statement by comparing more expensive rubber-toughened epoxy resin to a cheaper standard grade, both impregnating cross-ply tapes and bidirectional fabric pre-forms of high-performance reinforcing carbon fibers
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