Impact Response and Damage of Composite Shell with Various Curvatures

In this study, the impact responses and damage behaviors of CF/Epoxy(Carbon Fiber/Epoxy Resin) composite laminates have been studied considering the effects of various curvature radius. During the impact, the impactor’s kinetic energy is consumed by the rebound energy of the impactor, strain energy and damage energy. Most of the kinetic energy is consumed by delamination propagation including crack growth. Therefore the contact force-time and deflection-time histories were measured to determine the specimen response. All specimens were visually evaluated for damage after testing, and the non-destructive method(C-scan method) was used to determine the damage area. A device previously developed for impact testing of composite structures was used as the vertical free-falling testing machine. By experimenting with composite structures, we found that substantial differences in the responses of laminates subjected to either flat-plate or shell to transverse loading. And contact force, damage area and absorbed energy of the composite shells were a function of the curvature radius and impact energy. Introduction The rapidly expanding applications of composites recently have provided much optimism for the future of composite technology. Although man-made composites have existed for thousands of years, advanced composites have evolved in the aerospace industry only in the last fifty years [1]. Because of their high strength, stiffness, and low density, composites are currently being considered for many structural (aerospace vehicles, automobiles, trains and ships) applications. These properties can reduce structural weight. However, impacted composite structures are known have 50-75% reduced strength as compared to undamaged specimens. Over the past several years, researchers have extensively studied the effect of low-speed impact damage on the compression strength of laminated composite structures. Most of the studies on the effects of low-speed impact damage reported in the literature have focused on thicker flat plates that are typically used for wing structures. Structures with curvature and certain combinations of plate dimensions exhibit a nonlinear softening response when impacted with dropped-weight impactors [2]. Wardle and Laace [2] reported the differences in the composite shell and plate behavior, particulary damage resistance. Huang and Lee [4] investigated the dynamic response and damage of a composite shell under low velocity impact. The present work was undertaken to experimentally characterize and explore the response of composite structures with a wide range of structural configurations to both impact and quasi-static loading. Therefore, the objective of this study was to determine, in CF/Epoxy(Carbon Fiber/Epoxy Resin) composite laminates with various curvatures, a method for changing the curvature radius and to determine the effect of impact velocity on the contact force-deflection behavior, damage area and absorbed energy. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1911-1916 doi:10.4028/www.scientific.net/KEM.270-273.1911 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:38:15) Title of Publication (to be inserted by the publisher) Experimental Procedure The test specimens studied in this investigation were fabricated from Hankook Fiber Inc. CU125NS prepreg sheet. The specimens were cured using the resin manufacturer’s recommended curing procedure(the standard manufacturer’s cure cycle of a 90 minutes flow stage at 130°C). Typical mechanical properties for the CU125NS carbon-epoxy material are presented in Table 1. The nominal ply thickness of the material is 0.125mm. Specimens were fabricated from 8-ply-thick flat and curved plates with [02/902] S and [0/902/0]S laminates. The curvature radius of curved specimens is 100, 150, and 200mm. Both the flat and curved specimens are 100mm wide and 100mm long. Table 1 shows the details of the specimens. S represents the static test and D, the dynamic test. The curvature radius marks is R10, R15, R20 and RU. R10 is a specimen with 100mm curvature radius, and R15 and R20, 150mm and 200mm, respectively. The RU specimen is a plate shell, which is a shell with an unlimited curvature radius. Table 1. The details of the specimen Spec. Curvature [m-1] Span [m] Camber [m] Thickness [m] Mass [kg] Other variables RU 0 0.080 0 0.001 0.0150 R20 0.005 0.081 0.004 0.001 0.0156 R15 0.0067 0.081 0.005 0.001 0.0157 R10 0.01 0.082 0.008 0.001 0.0164 a.Interlaminar number b.Drop height The specimens were subjected to circular clamped boundary conditions under a test fixture described in Fig. 1. Fixtures for flat and curved specimens were manufactured by ASTM D-3763, and the diameter of these fixtures was 80mm. The static test was performed to examine the change in the interlaminar number and curvature, by using a UTM with a 5ton capacity. The load-displacement curves were recorded by using an automatic data acquisition system. In the case of the static test, all specimens were compressed at a rate of approximately 10mm/min., the static speed, until the fibers at the other face of the loading point were fractured. Each test was repeated at least five times and in some cases six or eight times. The vertical free falling testing machine consists of a drop weight, a jig for the specimen, a load measurement bar, guide bars, and frames. Fig. 2 shows the measurement system of the testing machine with a free-falling drop weight. During each impact test, the contact force-time and deflection-time histories were measure to determine the specimen response. The impact loads were obtained by converting the electrical resistance variations on the semiconductor strain gauges into the load measurement bar. The resistance variations of the semiconductor strain gauge going through the shield line and the bridge-circuit, were fed into a dynamic strain amplifier, which converted the resistance variations into voltage variations. Deformation of a specimen was measured by using a non-contacting optical deformation system, which detects the movement of the target on the drop weight. A load-deformation curve showing the response history was obtained by eliminating the time axis from each measured time-load and time-deformation curve. The specimens were subjected to a normal impact force by a low-velocity dropped weight. An instrumented 100g steel weight with a 10mm diameter hemispherical tip was used as the dropped-weight impactor. The dropped-weight impactor was dropped from a prescribed height(0.4, 0.6, 0.8, 1.0, and 1.2m).The impact-energy levels were increased from a low level to determine the impact energy necessary to initiate the damage in the specimens, and to cause barely visible damage on the surface of the specimens. Fig. 3 (a) is the load-deflection curve of the static test on the CF/Epoxy composite shells with an interlaminar number of 2. Fig. 3 (b) represents the load-time diagram of a specimen with an interlaminar number of 2 after impact testing. The shells were inspected non-destructively by a ultrasonic scanning technique 1912 Advances in Nondestructive Evaluation Title of Publication (to be inserted by the publisher) (C-scan) both before and after the static and impact tests. The C-scan measurements provided an overall area of the damage as viewed from the top of the test specimen. Specimen between rubbers Φ 80 hole Fixtures Fixtures Φ 80 hole Specimen between rubbers