MSci PolySci-Lab Modul P104. ie i. P104/16: Mechanics

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1 MSci PolySci-Lab Modul P104 Pol yyme rrma tte rr iai l ieni & Pol yyme rr ttechno logl ie i P104/16: Mechanics 1 Introduction In the laboratory P104/16: Mechanics the tensile properties of various polymeric materials and the influence of different testing conditions are determined. In the course the students should perform the test and evaluate the data by their own. In addition to the tensile testing the fracture surfaces of the polymeric materials should be characterised with scanning electron microscopy (SEM). The aim is a correlation between morphology and properties. 2 Literature 1. W. Grellmann, S. Seidler, Polymer Testing, Hanser-Verlag München T. A. Osswald, Polymer Processing Fundamentals, Hanser-Verlag München N. G. McCrum, C. P. Buckley, C. B. Bucknall, Principles of Polymer Engineering, 2 nd Ed. Oxford University Press, Oxford O. Schwarz, F.-W. Ebeling, B. Furth, Kunststoffverarbeitung, 7. Aufl., Vogel Würzburg, W. Kaiser, "Kunststoffchemie für Ingenieure", Hanser Verlag, München, Lecture "Polymertechnologie", BSci PolKol, 6. Sem., Prof. V. Altstädt G.H. Michler, Electron Microscopy, Hanser-Verlag München 2008

2 P104/16: Mechanics 3 3 Keyword Glossary backscattered electrons bending moment brittle conductive cross-head speed ductile electrical charges electron gun fibreglass load-extension diagram macro displacement transducer necking scanning electron microscope (SEM) secondary electron signal amplifier specimen strain gauge stress-strain diagram tensile strength tensile test tungsten x-ray yielding Young s modulus Rückstreuelektronen Biegemoment spröde leitfähig Traversengeschwindigkeit zäh Elektrische Ladungen Elektronenkanone Glasfaser Kraft-Weg-Diagramm Makro-Wegaufnehmer Einschnürung Rasterelektronenmikroskop (REM) Sekundärelektronen (Signal-) Verstärker Prüfkörper Dehnmessstreifen Spannungs-Dehnungs-Diagramm Zugfestigkeit Zugversuch Wolfram Röntgenstrahlung Fließen Elastizitätsmodul 4 Procedure 4.1 Mechanics Tensile Test With in mechanical testing the tensile test is regarded as a fundamental test and is used in material characterisation, quality assurance and construction. The tensile test is a static and quasi-static test, respectively. The test method and all used test specimens are defined in DIN EN ISO 527 (Fig. 1).

3 P104/16: Mechanics 4 Fig. 1: Specimens for tensile tests [1] The specimens are tested with a constant cross-head speed. Loading must be applied without impact and increase slowly and steadily until the fracture of the specimen occurs to provide a static and quasi-static test condition respectively. A uniaxial loading and stress state should be generated in the specimen. During the test a load-extension diagram is recorded and converted to a stress-strain diagram. The equations to determine stress and strain are given below (Equ. 1, Equ. 2). σ = F/(b 0 * d 0 ) = F/A 0 (1) σ stress [MPa] F force [N] b 0 width [mm] d 0 thickness [mm] A 0 Area [mm 2 ] ε =Δl/l 0 * 100% (2) ε strain [%] Δl elongation [mm] l 0 initial length [mm The elongation Δl is determined either with strain gauge or macro displacement transducer or laser displacement transducer. There are many factors that influence the values obtained for the mechanical properties. Therefore, an adequate number of tensile tests should be performed in order to determine the degree of variability of these properties. The material stress-strain behaviour depends on chemical structure of the polymer, processing history and testing condition. For example, in figure 2 Young s modulus E and tensile strength σ M of selected polymers are summarised.

4 P104/16: Mechanics 5 Fig. 2: Young modulus and tensile strength [1] In figure 3 a generalised stress-strain is mapped. All material equal whether ductile or brittle exhibits a linear elastic region (Fig. 3, region I). Depending on the ductility, the material reaches after the linear elastic region the linear viscoelastic region (Fig 3 region II) and the non-linear viscoelastic region (Fig. 3, region III). During the non-linear viscoelastic deformation the first damage of the material occurs. The regions I, II and III are characterised by constant deformation of the whole specimen. After reaching the yield strength the material shows plastic deformation (Fig. 3, regions IV-VI). Under the continuous loading the test specimen develops necking (Fig. 3, region IV). The region V is described as stationary plastic deformation. Before the break, so-called cold flow occurs. This behaviour is distinguished by the rise of the stress in the stress-strain diagram. Fig. 3: Characteristic deformation regions of a specimen during the tensile test [1]

5 P104/16: Mechanics 6 With in the stress-strain diagram the material parameter are determined: Strength parameter (Young s modulus, Stress at x % of strain, yield stress and tensile strength) Deformation parameter (breaking strain and breaking stress) All parameters are illustrated in figure 4. Definitions: Young s modulus E The slope of the elastic region (Fig. 3m region I) is called the modulus of elasticity or Young s modulus. This property represents the stiffness of the material and is important in determining the deflection of structures when loaded in the elastic region. The Young s modulus are described by Hooke s law: σ = E* ε Yield stress σ x at x % strain If the stress-strain diagram exhibits no macroscopic visual yield stress, this parameter can be used. This value is the stress σ x at which strain ε x reaches the defined value x in %. Yield stress σ y The yield strength of a material is the stress level at which yielding begins to occur and is a measure of the material's resistance to plastic deformation. For most structural applications, yielding is undesirable. For some metals and plastics, there is a smooth transition from the elastic region to the plastic region. The transition point is called the proportional limit. If the position of this point cannot be determined precisely from the stress-strain curve, then by convention, then a stress σ x at x % of strain is defined. Tensile strength σ M The tensile strength of a material represents the maximum stress that a material can withstand. Depending on the material behaviour, this value can be identical to yield stress or tensile stress at break. Breaking stress σ B The breaking stress characterises the stress at break of the specimen. Breaking strain ε B The breaking strain characterises the strain at break of the specimen.

6 P104/16: Mechanics 7 Fig. 4: Characteristic material parameter regarding stress-strain diagram [1] The stress-strain behaviour depends on temperature and testing speed. Examples are pictured in figure 5. Fig. 5: Influence of testing speed (left) and temperature (right) on mechanical properties of a ductile thermoplastic polymer [1] In comparison to other materials the dependency of the properties on temperature and testing speed is strong distinctive for polymers, because polymers change their properties in the region of the room temperature. The outcome of this is, that with rising cross-head speed the stress increase and strain fall. Similar behaviour is observed with decreasing testing temperature. Only if the test is carried out under reproducible standard conditions, definite and comparable results could get. The binding and general accepted definitions of the tensile test are described in DIN EN ISO 527.

7 P104/16: Mechanics Characterisation of the fracture surface Scanning Electron Microscope (SEM) The Scanning Electron Microscope (SEM) is developed in the sixties of 20 th century. Nowadays, SEM is applied in a lot of fields in industry and research and development. The major advantages are the high resolution, the great depth of focus and the facility for X-ray microanalysis. In polymer science the SEM is often used to picture material surfaces, to analysis the morphology of material and determine the behaviour of material. The principle of a SEM is pictured in figure 6. Fig. 6: Structure of a scanning electron microscop [6] SEM is a microscope that uses electrons rather than light to form an image. The SEM consists of an electron gun (cathode, anode and Wehnelt cylinder) to generate the electrons, an electromagnetic lenses system (condenser, objective) to focus the electron beam on the sample and detectors (backscattered electron detector, secondary electron detector), signal amplifier and screen to create an image. The electron beam comes from a filament, made of various types of materials. The most common is the Tungsten hairpin gun. The tungsten loop that functions as cathode is heated up and electrons are released. The positive anode generates a powerful force for the electrons. This causes electrons to accelerate toward the anode. The focussed electron beam scans line by line over the sample surface in the evacuated microscope column. The electron beam interacts with the sample surface (atoms) and photon and electron signal are emitted from the surface, which are electronically detected and amplified by suitable equipment. In figure 7 the electron/sample surface interactions are illustrated. The signals are most commonly used are secondary electrons, backscattered electrons and X-ray.

8 P104/16: Mechanics 9 Fig. 7: Interaction of the electron beam with the sample surface [6] If the samples are not conductive, e.g. polymeric material, ceramics, plants, the samples need a special preparation to avoid electrical charges on the sample surface. Electrical charges cause false signals. The kind of preparation depends on the sample and on the electron microscopy method. To prevent electrical accumulation usually polymeric material are dried and coated with an ultra-thin electrical conducting material. The coating consists of gold, platinum or graphite. The SEM photograph in figure 8 shows a brittle/ductile transition. On the left hand side a ductile fracture surface is observed. The fracture structure is fine and round. In evidence on the right hand side a brittle fracture surface is visible. In comparison to the ductile surface the brittle surface is rough and larger smooth regions are visible, that means no significant plastic deformation in this region. 50 µm Fig. 8: Brittle/ductile fracture transition [6] In figure 9 a typical fracture surface of fibreglass reinforce polymer is pictured. Fibreglass are used to improve the mechanical properties of polymer. With in the SEM it is possible to observe the orientation of fibres in the matrix and the bonding of fibres to the matrix. The fibreglass in the matrix in this case polypropylene are good observable. A pullout of the fibre is clearly visible.

9 P104/16: Mechanics 10 1 mm Fig. 9: Polypropylene with fibreglass [6] 5 Questions 1. Outline the principle of the tensile test! 2. Explain the parameters by means of the stress-strain diagram! 3. Define the parameters! 4. Normally parts are constructed in that way, that only elastic loading occurs. Why is the knowledge of the deformation properties for the constructing engineer so important? 5. What is the difference between the nominal stress and the real stress in a stressstrain diagram? 6. Explain influence factors on the material parameter of tensile test! 6 Task 1. Familiarise oneself with tensile test with the aid of the testing standard DIN EN ISO 527 and the literature! The standard and literature you will find the library. 2. Answer the questions (see bullet 5)! 3. Achieve know-how in using tensile test as one of the most important mechanical testing method. 4. Determination and comparison of the strength and ductility of different polymers regarding the stress-strain diagram a) Perform and evaluate the tensile test regarding DIN EN ISO 527 b) Determination of the Young s modulus and the material parameter (yield strength, tensile strength, breaking strain with current the characteristic elongations) c) Determine the influence of the testing condition (cross-head speed) on the tensile properties! 5. Using SEM for Characterisation of the fracture surface. 6. Elaborate your results in a technical report! 7. Compare the generate data with the literature!

10 P104/16: Mechanics Give general information about the operational safety! Note: 6.1 and 6.2 have to be done before the laboratory course!