An experimental study on impact behavior of quasi-isotropic CFRP laminates

Abstract

This work was carried out to bring out the differences in drop weight low velocity impact behavior of two quasi-isotropic (QI) laminates. These laminates were made of AS4/914 carbon fiber reinforced plastic (CFRP). The layup sequence chosen was [0/45/-45/90]_2S and [0//90/45/-45]_2S. Two types of impactors were employed for the impact tests. One is a hemispherical-end and the other is of a conical-end. Coupon level specimens were supported on all four sides during the impact test. The impact energy was calculated for the laminates and the difference, as a comparison, in each case is presented. It has been observed that the layup sequence has a significant effect on the impact resistance of the laminates under hemispherical impactor. In the case of conical impactor, both the laminates were perforated while absorbing similar energy levels.

Introduction

The high strength to weight ratio and the ability to tailor the material mandates the composite materials in aerospace industries. Initially, the composite materials were used only in the secondary structures of the aircraft components. With the advances in the composite industries, now-a-days, even primary structures are also made by employing the composite materials. Reportedly, there are aircraft made with 50% by weight of composite materials [1]. The aircraft demands for thin skins is to reduce the weight while keeping the performance high. This requires highly efficient structural materials, like, CFRP.

Low velocity impact damage is a concern for aircraft right from taxiing, landing as well as during maintenance. Small debris from the ground may come and hit the wing skin during taxiing and tool drop during maintenance causes these low velocity impact damages. This kind of impact damages are again based on the nature of tool or the debris which are hitting the structures. Either it may have a sharp edge or a smooth blunt edge. The structures may lose up to 50% of their strength when facing low-velocity impact problems due to incidents during manufacturing or maintenance processes [2]. Conventional non-destructive testing techniques fail to detect the barely visible damages or subsurface defects, such as expected during low velocity impacts on CFRP composites.

Impact damage and impact behaviour depend on many parameters such as the impact velocity and energy, projectile/impactor shape, lay-up sequence and boundary conditions [3]. Composite fibre reinforced laminates have various damage modes such as matrix cracking, fibre breakage and delamination. These various damage modes appear together under impact loading [4]. Metals absorb energy in elastic and in plastic regions, whereas composite laminates absorb energy mostly in elastic deformation. Since most composites are brittle, the damage mechanism is not due to plastic deformation. Low velocity impacts can be defined as events, which can occur in the range 1–10 m/s depending on the material properties, projectile mass and stiffness [5]. A drop weight impact testing unit enables the simulation of a wide range of real-world impact conditions and collects detailed performance data [6].

For the low velocity impact test, the end profiles of the impactor/striker usually are blunt, conical or hemispherical. Blunt projectiles cause failure by shear plugging, conical projectiles cause petaling in thin laminates and ductile hole enlargement in thicker laminates, and hemispherical projectiles cause tensile stretching after severe indentation and thinning of the target laminates [7]. The conical projectiles require less perforation energy compared to blunt and hemispherical projectiles [8]. The specimen absorbs more energy when impacted by a conical impactor and hemispherical impactors produce the highest contact force and lowest contact time [9], [10]. The present work is intended to get an appreciation of low velocity impact resistance for both hemispherical and conical impactors with two different QI laminate sequences.