Fracture Mechanisms of a Thin Elastic Plastic Laminate

The fracture toughness of a polymer-metal laminate composite is obtained by mechanical testing of a specimen containing a pre-crack. The laminate is a material used for packaging. It consists of a thin aluminium foil and a polymer coating. A centre cracked panel test geometry is used. Each of the layers forming the laminate is also tested separately. The result is compared with the measured fracture strength of the individual layers. It is observed that the load carrying capacity increases dramatically for the laminate. At the strain when peak load is reached for the laminate only aluminium is expected to carry any substantial load because of the low stiffness of the LDPE. However, the strength of the laminate is almost twice the strength of the aluminium foil. The reason seems to be that the aluminium forces the polymer to absorb large quantities of energy at small nominal strain. The toughness compares well with the accumulated toughness of all involved layers. Possible fracture of the interface between the layers is discussed. Introduction Liquid food packages are often made of packaging materials consisting of different material layers to fulfil several requirements of the package. It is very important to ensure that every layer maintains its function during the forming, filling and transportation processes. As an example here a liquid food packaging material is considered. This is a laminate consisting of LDPE (Low Density Polyethylene) and an aluminium foil. Several studies of different mechanical properties of these materials have been performed [1-5]. It was found in [2] that aluminium foil and LDPE laminated together provide significantly higher stress and strain at fracture as compared with the simplified analytical prediction. Related works can also be found in [6], [7], [8] where notched tensile strength, fracture toughness as well as fatigue resistance for fiber/aluminium composite laminate were studied. The purpose of this work is to study the fracture toughness of a laminated material in relation to the adhesion between the layers. Load and extension were measured for a two-layer laminate specimen with a pre-crack as well as for the individual layers of the laminate. The same specimen geometry was used in all tests. For comparison, measurements were also done for the laminate without any adhesion between the layers. Materials Laminate in this work consists of an aluminium foil and a Low Density Polyethylene (LDPE) foil. A fully annealed AA1200 aluminium foil and LDPE with the product name LD270 is used. Load versus extension were measured for the following combinations: 1. aluminium foil with the thickness 8.98 μm, 2. LDPE with the thickness 27.30 μm, 3. Al-foil coated by LDPE giving the total thickness 36.28 μm and 4. aluminium foil and LDPE joined together without adhesion between the layers and the total thickness is 36.28 μm. For the 3:rd case pieces of aluminium foil were cut from a roll of fabricated material. The laminate was then prepared in a Haake film extruder with a 36 μm Polyester (PET) as carrier. The foil was mounted on the PET carrier while LDPE was extruded and coated on aluminium foil at a melting temperature of 278 C. A nip with the pressure 202 bar was used to press the layers together. The laminated specimen for case 3 was then cut from roll including PET, aluminium foil and LDPE. After producing the material for case 3, LDPE was continuously extruded on the PET carrier under the same conditions but without the aluminium foil. By peeling off the PET carrier, the LDPE produced here was used for making the specimen for cases 2 and 4. Finally the specimen for case 1 is taken from the aluminium foil roll of the same direction as specimens for all other cases. Figure 1 a. shows schematically the geometry used for all cases 1 to 4. The material properties are taken from [1] and [2]. To simplify the analyses the materials are approximated as perfectly plastic with the following properties for the aluminium foil, the modulus of elasticity E = 34 GPa and the yield stress σo = 49 MPa. For the LDPE the modulus of elasticity was assumed to be E = 136 MPa with the yields stress σo = 2 MPa. Poisson's ratio, ν, was not measured. Here it is, for both materials, assumed to be ν = 0. Both materials are known to have a fairly high fracture toughness in plane strain so that fracture occur in plane stress for the thicknesses used here. The Experiment Centre cracked panels, as shown in Fig.1 a, are used for evaluating the fracture toughness of the laminated composite and components of it. Pre-fabricated cracks are manually cut, using a razor blade, to a total slit length of 2a = 45 mm. The width and gauge length of the specimens was 2W = 95 mm and 2h = 230 mm, respectively. A pair of wide clamps is utilised as shown in Fig. 1 b. The tests are made in a MTS Universal Testing Machine. The upper clamp is attached to a 2.5 kN load cell as well as a crosshead in the MTS-machine. Since the specimen is mounted vertically, the clamps are equipped with needles to facilitate a correct positioning. After the positioning of a sample the upper and lower clamps are closed and the pressure is applied by tightening four equally spaced quick-acting locking nuts along the front of each clamp as Fig. 1 b shows. Locking pins at the centre of the front jaws keep the clamps in an open position during mounting. Specimens are tested by traversing the upper crosshead up to move the sample under increasing tension u at a constant crosshead speed of 9.2 mm/min. The software TestWorks is used to control the load frame and also to record data. During testing both displacement between the crossheads and load is monitored and recorded. The laminate tests are run until the entire cross section has fractured. The experimental result is collected in Fig. 2. The differences between the load carrying capacity of the laminate and that of the components are surprisingly large. The peak force for the aluminium foil is 13 N and is reached when the foil is extended a little less than 1 mm. At that extension the LDPE-foil carries not more than 1 N. Still the composite of the to foils carries around 20 N at the same extension. When extended 2 mm the laminate carries around 23 N whereas the LDPE foil alone, carries 2 N and the aluminium foil does not contribute at all while being fractured. which means that the load carrying capacity i multiplied 10 times.