Fracture properties of a fiber-metal laminates based on magnesium alloy

Abstract

Fiber-metal laminates (FMLs) are high performance laminated structures based on stacked arrangements of composite material and aluminum alloy. Currently, a glass fiber reinforced epoxy/aluminum alloy FML (GLARE) is being considered for use in the manufacture of the upper fuselage of the A380 Airbus aircraft [1]. Previous work on FMLs has shown that they combine the excellent durability and machinability common to many metals with the superior fatigue and fracture properties offered by many fiber-reinforced composites [2–4]. Krishnakumar [2] tested a Kevlar fiber FML (ARALL) and showed that its ultimate tensile strength is considerably greater than that of a conventional aluminum alloy. Vogelsang [3] conducted tension-tension fatigue tests on a number of FML systems as well as a plain aluminum alloy and showed that crack growth rates in the former were significantly less than those in the plain aluminum alloy. Several workers have investigated the impact response of aerospace-grade fiber-metal laminates [5, 6]. Vlot et al. [5] conducted low and high velocity impact tests on a GLARE system, a plain aluminum alloy and a carbon fiber reinforced thermoplastic. Their results showed that the FML offered the highest damage threshold energy of all the systems considered. The aim of the present work is to investigate the fracture properties of a novel fiber-metal laminate based on a lightweight magnesium alloy and a carbon fiber reinforced plastic. Magnesium alloys offer a number of advantages over many metals including their low density (thirty percent lower than an aluminum alloy), their superior corrosion resistance and their excellent electromagnetic shielding ability. Currently, magnesium alloys are being used in the automotive industry in the manufacture of transmission casings and in the aerospace industry for the manufacture of gearboxes and other lightweight components. The fiber-metal laminates examined in this study were based on 0.5 mm thick magnesium alloy sheets (AZ31 alloy from Advance Metals International Ltd.) and a woven carbon fiber reinforced toughened epoxy (Stesapreg EP121-C15-53 from Stesalit Ltd, Switzerland). The composite and metal plies were placed in a picture frame mold with dimensions 240 × 200 mm and cured for 4 h at 125 ◦C. All of the laminates tested in this research project were based on a 2/1 configuration (two magnesium alloy skins either side of a carbon fiber reinforced epoxy core). Table I summarizes the stacking configurations investigated in this study. The composite volume fraction within the FMLs was varied by increasing the number of composite plies between the two outermost magnesium alloy skins from two to eight. The tensile properties of the FMLs and the magnesium alloy were evaluated using 20 mm wide rectangular samples at a crosshead displacement rate of 2 mm/min. The initial strain history in each sample was recorded using a clip-on extensometer fixed to the specimen edge. The extensometer was incapable of measuring large strains. In such cases, the crosshead displacement was normalized by the length of the working section to yield a nominal strain (this was only undertaken when a complete stress-strain curve was required). The energy-absorbing properties of the fiber-metal laminates were evaluated through a series of single edge notch bend (SENB) tests on specimens with dimensions 100 × 18 mm × thickness. Prior to testing, a 9 mm long pre-crack was introduced at the mid-span. The precrack was sharpened by swiping a sharp razor blade along the tip of the stress concentration. The SENB specimens were positioned on two simple supports positioned 72 mm apart and loaded at a crosshead displacement rate of 5 mm/min. The work of fracture was then determined by dividing the area (energy) under the load-displacement curve by the cross-sectional area of the fractured ligament. Fig. 1 shows typical stress-strain (nominal) curves for the three FMLs and the plain magnesium alloy. The stress-strain curve for the plain magnesium alloy indicates that although its tensile strength is below that of the FMLs, it does exhibit quite a high degree of ductilty beyond the elastic limit. The stress-strain curves for all of the FMLs exhibited an almost linear response up to maximum stress at which point the composite layers fractured in a brittle manner. An examination