Effects of Notch Geometry and Moisture on Fracture Strength of Carbon/Epoxy and Carbon/Bismaleimide Laminates

Abstract

Composite panels of various ply orientations and 1 to 2 mm thickness were tested in tension with a center notch to determine the translaminar fracture toughness and in bending with no notch to determine the bulk flexural strength and modulus. Carbon/epoxy panels with a variety of notch lengths and widths were subjected to laboratory air conditions before testing. Carbon/bismaleimide panels with one basic notch configuration were tested in three pretest exposure conditions: laboratory air, a 400-h exposure in a controlled moisture chamber, and a 4000 h exposure in a natural tropical environment. The tests were performed to investigate the following effects on fracture and mechanical properties and on micro and macrofailure mechanisms of the laminates. (a) Effects of notch width and length and ply orientation on translaminar fracture toughness were studied in 0/90 and 0/±60 carbon/epoxy laminates, including effects of notches produced by ballistic penetration. (b) Effects of extended exposure to moisture and sunlight on fracture and mechanical properties were studied in various 0/90 layups of carbon/bismaleimide laminates. A direct comparison of controlled chamber and natural tropical moisture effects was made for two materials. (c) The load-deflection macrobehavior of notched panels was noted and analyzed. Linear-elastic and J-integral analyses were used to determine critical values of stress intensity factor, K, at the onset of self-similar translaminar fracture. A simple splitting model was used to describe the interlaminar failure that competed with and often prevented the translaminar fracture. (d) Scanning electron microscope fractography was used to investigate and contrast the microfailure mechanisms of the laminates. The differences in micromechanisms between carbon/epoxy and carbon/bismaleimide were related to differences in the macrofracture behavior of the materials. Analysis was performed of the test procedures for determination of fracture toughness and flexural strength and modulus. The method used for fracture toughness and associated K, J, and displacement expressions were proposed for more general use in measuring translaminar fracture toughness of composites. Use of the four-point bending test for flexural strength and modulus revealed an error that can occur in this type of test that is not fully addressed in ASTM Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials (D 790-86).

Determination of the crack resistance curve for intralaminar fiber tensile failure mode in polymer composites under high rate loading

“…For the measurement of the intralaminar fracture toughnesses for fiber tensile failure, the compact tension (CT) specimen, originally developed for the fracture toughness characterization in metals [11,12], is commonly used [13, 14,15,16,17,18,19]. In contrast to the centre-notched (CN) [20,21,22,23,24,25,26], the 10 double-edge notched (DEN) [27,28,29] and the four-point bending (4PB) [27] specimens, the CT specimen shows a stable crack propagation, which enables the determination of the R-curve. Unfortunately, the CT specimen has several limitations: i) the tendency of buckling/twisting at the back side (because of the reduced thickness of the specimen [30] or because of the high load that occurs 15 especially when testing high toughened material systems [31]); ii) failure at the back side (because the compressive stress reaches the compressive strength of the material); and iii) failure at the load introduction point.…”