18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS EVALUATION OF THE INTERFACE STRENGTH IN METAL/POLYMER COMPOSITE SYSTEMS S. Charca 1 , O. T. Thomsen 2 Department of Mechanical and Manufacturing Engineering Aalborg University, Pontoppidanstræde 101, DK-9220 Aalborg East, Denmark 1 Email: sch@m-tech.aau.dk, 2 Email: ott@m-tech.aau.dkweb page: http:// www.m-tech.aau.dk / Keywords : Fragmentation, Shear stress, critical length, shear strength, Finite element that can be obtained from one single test specimen is Abstract typically large, thus enabling a complete statistical Among the several test methods to characterize the analysis [1-3]. There are several studies completed fibre-matrix interface in fibrous composite materials, with the purpose to describe the stress transfer the single fibre fragmentation test is one of the most between resin and the fibres, especially for the simple in terms of experimental setup, and at the fragmentation test [5]. The fragmentation of the same time the amount of data that can be extracted fibres initially was observed and reported by Kelly from one single test is significant. In this work, the and Tyson (1965) in tungsten fibres embedded in a single fibre fragmentation test method was copper matrix. Based on these observations and implemented to assess the interface shear strength using a simple in-plane force balance they proposed obtained for a single steel filament embedded in an a simple equation to estimate the average shear unsaturated polyester resin. Furthermore the single stress, which in turn is interpreted as the interface fibre fragmentation test result was modelled and the shear strength � � � � �� �/2� � [4, 5], where � � �� is the results substantiated using finite element analysis tensile strength of the fibre at the fragment length, � (FEA) considering the material behaviour as being is the fibre diameter and � � is the critical fragment linear elastic. The stress distributions obtained from length. In the Kelly and Tyson relation only the the FEA and the one-dimensional Cox model were tensile strength of the fibre and the fragment length analyzed and compared. A comparison of obtained are used as model parameters. However, as the average interface shear stresses at filament failure single fibre fragmentation test in reality involves shows, that the different models have certain and complex 3D phenomena that are not addressed in the different ranges of applicability, which in some model, the Kelly and Tyson relation equation may cases can lead to misinterpretations and erroneous underestimate or overestimate the interface shear conclusions regarding the filament/resin interface strength. An alternative one dimensional elastic properties. stress transfer model (shear lag model) was proposed by Cox in order to study the matrix-fibres interaction 1 Introduction [6]. Cox developed the model based on the The performance of polymer composites materials is following assumptions: linear elastic material to a large extent determined by the fibre resin properties, the matrix material transfers shear and interface properties. There are several methods, tensile stresses, the fibres and the matrix share only which can be used to characterize the interface loading/stresses along the fibre direction, which properties including the single fibre pull out, micro- implies that the matrix strain is uniform, and that the tension, micro-indentation, and single fibre stress transfer between matrix and fibre depends of fragmentation tests [1-5]. Single fibre fragmentation the displacement difference of one point in the fibre tests are frequently used to characterize the interface and the same point expressed in the matrix in mechanical properties between glass or carbon fibres absence of the fibre [1, 6]. For the single fibre and the polymer resin in composite materials due to composite the Cox equation can be expressed by: its relative simplicity in the testing setup. �/� � � � � � � � ���� � ��� ��/�� � (1) Furthermore it offers the advantage over other competing methods that the number of fragments
���� ���/�� is considered, and consequently the fragmentation � � � � � � �1 � ���� ���/�� � (2) test sample dimensions should be redesigned. Assuming linear elastic behaviour for the steel � ���� ���/�� � � � �� � � � ���� ���/�� (3) filament and the polyester resin, and using the rule of mixtures, the minimum cross sectional area to where: � � and � � is the fibre and matrix modulus, obtain fragmentation is ~2.33mm 2 . However, to � � is the matrix Poison’s ratio, R and r is the fibre avoid premature failure due to manufacturing and matrix diameter, � � is the far field strain applied defects, a cross sectional area of 90mm 2 was used. to the single fibre composites. Dogbone samples with a steel filament inserted were Several types of fibres are being used as manufactured by casting resin into a silicone rubber reinforcement in polymer composite materials [1, 3]. mould. To avoid filament misalignment, the The most widely used fibres are carbon and glass filaments were pre-strained using 200g of weight. fibres, but metal fibres are also being introduced as a The effective length of the dogbone samples was potential replacement for carbon and glass fibres for 220mm. some specific applications [7]. The low cost and All the fragmentation tests were performed using an high mechanical properties (including ductility) of electro-mechanical Zwick/Z100 tensile testing steel filaments (or cords composed of tows of steel machine operated in deformation control. Based on filaments) compared to the traditional reinforcement preliminary tests a loading rate of 0.05mm/min was systems is the main motivation to explore the demonstrated to be appropriate to achieve filament potential and reliable application of polymers fragmentation. During the testing the filament reinforced by steel filaments/cords for civil damage and failure process was monitored using a engineering, automotive, and others applications. A 50× magnification optical stereomicroscope. significant “challenge” when using polymers Moreover, as the unsaturated polyester resin is reinforced by steel filaments/cords is the polymer transparent a planar photoelasticity technique was matrix-steel interface properties. In this work the employed to observe the birefringence phenomena matrix steel-filament interface properties are caused by the developing local stress concentrations investigated using the single fibre/filament around the filament fragment ends, and to localize fragmentation test method in which a single steel the areas/points where the filament fragmentations filament is embedded in a polymer matrix. occurred. Fig 1 shows the experimental setup used. Furthermore, the observed damage and failure mechanisms in the fragmentation process will be 3 Finite element modelling discussed. In order to validate the fragmentation test To validate the experimental results, a single steel results, linear elastic finite element modelling has filament embedded in polyester resin was modelled conducted including the resin curing shrinkage using the commercial FEA code ANSYS. Material induced during the manufacturing of the test linear elasticity was assumed, and the problem was specimen. Moreover the interface shear strengths simplified by assuming axi-symmetry (32000 axi- obtained using the Kelly and Tyson, Cox and FEA symmetric 2D plane183 elements were used). Fig. 2 models will be compared shows the model geometry (OEFA is the steel 2 Materials and experimental setup filament). OD is a symmetry axis, OB is the axis of axi-symmetry, the load is applied to the CB edge as In this study Zinc coated high strength single steel a prescribed axial strain of 3.54%, and the AFF’B is filaments embedded in an unsaturated polyester an empty volume representing the cavity created by resin have been considered. The mechanical the fracture of the filament. Perfect interface properties of the constituent materials are shown in bonding is assumed (line EF). The polyester resin Table 1. Traditionally, the single fibre displays a high volumetric shrinkage ( V shr = 8%), fragmentation test has been used successfully to and a thermal analogy was used to simulate the characterize the interface shear strength for manufacturing shrinkage pressure acting on the steel composite systems utilizing carbon, glass and other filament. For the thermal analogy, an equivalent fibres which diameters that are lower than 20µm. In thermal expansion coefficient was estimated using this study steel filaments with a diameter of 100 m the average elasticity modulus over the time
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