Low velocity impact response of novel fiber-reinforced aluminum foam sandwich structures

Abstract

Sandwich structures based on high performance skins are currently finding widespread use in a number of aerospace, marine and automotive applications, where low weight, high strength and high stiffness are important design requirements. Many sandwich structures are based on polymeric foams such as PUR, PEI and PVC and aluminum honeycomb. Currently, most of such structures are manufactured using low curing temperature thermosetting-based composites. One of the disadvantages associated with the aforementioned polymeric foams is their extreme temperature sensitivity, an effect which precludes them from being used in conjunction with most tough thermoplastic composites. This is unfortunate, since many thermoplastic-based composites can be shaped and molded in a simple, cost effective manufacturing operation. Honeycombcore sandwich structures offer the lowest weight for a given structural requirement but also suffer several limitations [1]. Durability problems have been associated with water intrusion into panels. Forming complex curved shapes is difficult and the cores themselves are highly anisotropic. At present, there is an increasing level of interest in investigating the potential offered by lightweight closed-cell metallic foams for use in a range of high performance structural applications. Preliminary tests on closed-cell aluminum foams have shown that these materials offer improved soundproofing characteristics, low thermal conductivity characteristics, low toxicity under fire conditions, excellent toughness and impressive impact energy absorbing capabilities [2–4]. Recently, a range of high performance aluminum foams have been developed that retain many of their key mechanical properties at temperatures in excess of 300 ◦C [2, 4]. The availability of such metallic foams opens up many exciting new avenues in the development of novel lightweight cost-effective sandwich structures. Indeed, it should be possible to use a simple cold stamping manufacturing procedure in order to manufacture a range of aluminum foam sandwich structures based on fiber reinforced thermoplastic skins. Using such a manufacturing procedure, it should be possible to reduce the processing cycle from many hours to several minutes. The aim of this work is to characterize the fracture properties and low velocity impact response of lightweight sandwich structures based on glass fiber reinforced polypropylene skins and a high performance closed-cell aluminum foam. Initial attention will focus on optimizing and characterizing the level of adhesion between the composite skins and core materials over a range of loading rates. Once this has been achieved, the low velocity impact response of the sandwich structures will be investigated through a series of low velocity impact tests. Sandwich panels with dimensions 240 × 200 mm were manufactured using a cold stamping procedure similar to that used to manufacture thermoplastic-based fiber-metal laminates [5]. Here, sheets of glass fiber reinforced polypropylene prepreg were stacked either side of a 10 mm thick closed cell aluminum foam block (Alporas foam from the Shinko Wire Company, Japan) and placed in a picture frame mold. Initially, tests were undertaken on sandwich structures based on plies of unidirectional glass fiber reinforced polypropylene (Plytron from Borealis) with a 0◦/90◦/0◦/90◦ lay-up. Following this, structures consisting of woven glass fibre/polypropylene skins (Twintex PP60 from Vetrotex Ltd.) were manufactured and investigated. For simplicity, the former will be referred to as the unidirectional sandwich structure and the latter the woven sandwich structure. A 0.14 mm thick layer of polypropylene (XAF 2311 from Xiro Ltd.) was incorporated between the composite and foam materials in to ensure a high degree of adhesion across the skin-core interface. The mold was placed in an air-circulating oven, heated to 185 ◦C and then removed and stamped in a cold press. Once the mold had cooled to below 60 ◦C (typically within five minutes), the panel was removed from the mold and visually inspected for defects. The interfacial fracture properties of the skin-core interface were characterized using the single cantilever beam (SCB) sandwich specimen shown in Fig. 1. Here, a folded aluminum foil starter defect was positioned at one end of the prior to lamination. Prior to testing, the core and the upper skin directly above the aluminum starter defect were removed from the specimen to leave protruding skin as shown in Fig. 1. Beams with dimensions 200 mm × 20 mm were clamped in a steel frame and tested on an Instron 4505 universal testing machine at crosshead displacement rates between 1 and 1000 mm/minute. Dynamic tests were undertaken using an instrumented falling weight impact tower. During testing, the impact force was measured using a piezoelectric load cell positioned above the 10 mm diameter hemispherical steel indenter and the carriage displacement using a laser Doppler velocimeter. During the tests, the load and displacement data were recorded. Crack propagation during tests at up to 10 mm/min