Tensile Test (Plastic) - PST472 PDF

PST472: POLYMER PHYSICAL TESTING & ANALYSIS TENSILE TESTING (PLASTIC) PREPARED BY: NOR MAZLINA ABDUL WAHAB (UITMPS)  2 Upon completion of this chapter, you should be able to: Describe...

PST472: POLYMER PHYSICAL TESTING & ANALYSIS TENSILE TESTING (PLASTIC) PREPARED BY: NOR MAZLINA ABDUL WAHAB (UITMPS)  2 Upon completion of this chapter, you should be able to: Describe the tensile testing of polymer. Sketch and label typical stress-strain LEARNING curve for tensile testing. Describe the tensile testing OUTCOME procedures. S: Define the related terms in tensile testing of polymer. Sketch and explain typical shape of the stress-strain behaviour of different polymer properties. Calculate the tensile strength, % elongation, and Young’s Modulus of polymeric materials.  3 Introduction Tensile specimen Testing apparatus-testing machine Conditioning CONTENT Test procedure S Typical stress strain-curve Terms and symbols relating to tensile test Typical shape of the stress-strain behaviour Factors affecting the test result  4 Tensile testing is probably the most widely used short-term mechanical test of all. This is because it is relatively easy to perform, gives reasonably reproducible results INTRODUCTI and yields a great deal of ON information. From this test, one can obtain not only tensile strength but also elongation and modulus. Such machines are usually constructed so that as the test specimen is deformed at standard cross head speed, the resistance to deformation and the amount of extension is measured.  5 Tensile tests measure the force required to break specimen and the extend to which the specimen stretches or elongates to that breaking point. Tensile test produce a stress- INTRODUCTIO strain diagram, which is used N to determine tensile modulus. The data is often used to specify a material, to design parts to withstand application force and as a quality control check of materials.  6 INTRODUCTION  The sample dimensions in the waist region (gauge section) are first measured with a micrometer or dial gauge.  Next, the sample is firmly gripped in the jaws of the machine.  An extensometer may then be clipped to the sample.  One jaw of the machine is drawn away from the other (usually by an electric motor) at the specified speed.  The resistance to deformation is usually measured by a load cell which is connected to one of the jaws.  A load/ extension curve is usually produced automatically by the machine.  Standard test method for tensile is ASTM D 638.  7 Sample preparation:  Injection molded or compression molded specimens.  Also can be prepared by machining operations from TENSILE materials in sheet, plate or SPECIMEN slab.  The sample is usually dumb- bell / dog-bone shaped or waisted in order to ensure that breaking points not occur in the grips, but in a specified region.  8 TENSILE SPECIMEN Various types of tensile specimen are shown below:: ASTM D638: Type I tensile bar ASTM D882: Specimen for thin sheet or film  ASTM D412 - Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension  1 TENSILE 0 SPECIMEN The sample dimensions in the waist region (gauge section) are first measured with a Measuring the gauge length micrometer or dial guage and then the sample is firmly gripped in the jaws of the machine. Gauge length: A portion of the narrowed section of the sample. Micrometer Dial gauge  1 1 Testing apparatus- testing machine INSTRON UNIVERSAL TESTER MACHINE  12 Testing apparatus-testing machine  1 3 Testing apparatus- testing machine CONDITIONING 1 4  Conditioned using standard conditioning procedures. (Procedure A of ASTM methods D 618)  Since the tensile properties of some plastics change rapidly with small changes in temperature, it is recommended that test be conducted in the standard lab temperature i.e 23±2°C and 50±5 percent relative humidity. TEST 1 5 PROCEDURE (ASTM D638) 1. The width and thickness of specimens are measured with thickness gauge. 2. The test specimen is positioned vertically in the grips of the testing machine (Instron). 3. The grips are tightened evenly & firmly to prevent any slippage.  1 6 The speed of testing is set at the proper rate Plastics & the machine specimen: is started. Rubber specimen: 50mm/min, 100mm/min 500mm/min (2 kN) (30kN) TEST PROCEDU As the specimen elongates, the resistance of the specimen increases and is detected by a RE (ASTM load cell. D638) This load value (force) is recorded by the instrument. TYPICAL STRESS-STRAIN CURVE17 Break / failure Yield Elastic region (Stress is proportional to strain). Generalized tensile stress-strain curve for polymeric materials.  1 TYPICAL 8 STRESS-STRAIN CURVE  There are three typical curves for polymers:  Brittle polymers fail in elastic region  Ductile polymers have an elastic region, yielding and a plastic region  Highly elastic polymers such as rubbers  1 Typical stress- 9 strain curve for ductile materials:  2 0 Typical stress-strain curve  2 Typical stress-strain curve for semicrystalline 1 polymers:  I: Linear elastic deformation  II: Homogeneous plastic deformation  III: Necking forms and grows unstably (inhomogeneous plastic deformation  IV: Neck stabilizes  V: Cold drawing, neck increases in length  VI: Entire sample is drawn  VII: Begin stretching of completely drawn sample  VIII: Fracture  2 Typical 2 stress-strain curve: LDPE Plastic  2 3 Terms and symbols relating to tensile test:  24 Terms and symbols relating to tensile test: Fracture  25 Terms and symbols relating to tensile test: TERMS DEFINITION Stress The tensile load per unit of the original cross- sectional area at a given moment. Ft Standard unit : pascal, Pa (N/m2) @ Mpa (N/mm2) Area, A Stress, σ = F/A F = Force, N Ft A = The original cross sectional area, mm2 Strain The ratio of elongation or deformation to the gauge length of the test specimen, that is the change in length per unit of original length. Terms and symbols relating to tensile test: TERMS DEFINITION Engineering stress (nominal stress): the applied load divided by the original cross – sectional area of a material True stress – applied load divided by the actual cross-sectional area (the changing area with respect to time) of the specimen at that load Elongation The increase in the length of a test specimen produced by a tensile load.  27 : Terms and symbols relating to tensile test TERMS DEFINITION Yield The stress at which material strain changes from strength elastic deformation to plastic deformation, causing it to deform permanently.  28 : Terms and symbols relating to tensile test TERMS DEFINITION Tensile The maximum tensile stress (nominal) sustained strength by a test piece during a tension test. Tensile Tensile stress corresponding to the point of Strength rupture. at Break Terms and symbols relating to tensile test: 29 TERMS DEFINITION Tensile The tensile stress corresponding to the yield strength point. at yield Gauge The original length between 2 marks on the test length piece over which the change in length is determined. Terms and symbols relating to tensile 30 test: TERMS DEFINITION Proportional The greatest stress at which a material is capable limit of sustaining the applied load without any deviation from proportionality of stress to strain (Hooke's Law). Modulus of The ratio of stress to corresponding strain below elasticity the proportional limit of a material. It is expressed in F/A. This is also known as Young's modulus. A modulus is a measure of material's stiffness. Stiffness is the resistance of an elastic body to deflection or deformation by an applied force. The stiffness of a thermoplastic is indicated by its tensile modulus.  31 Terms and symbols relating to tensile test: TERMS DEFINITION Necking At the yield point there is a local decrease in cross- sectional area at a point along the length of the sample. Cold Draw The process by which the neck extends. During cold draw, the carbon chains become elongated. Aligning themselves parallel to the direction of the applied stress.  3 Terms and symbols relating to tensile 2 test: Toughness:  The toughness of a thermoplastic is quantified by determining the area under the material's stress-strain curve.  3 Terms and symbols relating to tensile 3 test: Stiffness:  The stiffness of a thermoplastic is indicated by its tensile modulus (elastic / Young’s modulus).  3 Typical shape of 4 stress-strain behaviour  The polymeric materials can be broadly classified in terms of their relative softness, brittleness, hardness and toughness. Tensile stress-strain curves for four types of polymeric  3 Factors Affecting the Test Results 5 1. Specimen preparation & specimen size 2. Rate of straining 3. Temperature  Factors Affecting the Test Results 36 1. Specimen preparation: Molecular Orientation- Molecular orientation has a significant effect on tensile strength values. A load applied parallel to the direction of molecular orientation may yield higher values than the load applied perpendicular to the orientation.The opposite is true for elongation. The Process of Sample Preparation-For example, injection molded specimens generally yield higher tensile strength values than compression molded specimens. Machining usually lowers the tensile and elongation values because of small irregularities introduced into the machined specimen. Size and Gate location- This is especially true in the case of glassfiber-reinforced specimens. A large gate located on top of the tensile bar will orient the fibers parallel to the applied load, yielding higher tensile strength. A gate located on one side of the tensile bar will disperse the fiber in a random fashion. Tensile properties should only be compared for equivalent sample sizes and geometry.  37 Factors Affecting the Test Results 2) Strain rate: As the rate of elongation increases, the tensile strength and the modulus also increase. However, the elongation is inversely proportional to the strain rate. The effect of the strain rate on sress-strain behaviour of plastic  Factors Affecting the Test Results 38 (MPa) 80 4°C Data for the semicrystalline 60 polymer: PMMA 20°C (Plexiglas) 40 40°C 20 to 1.3 60°C 0 0 0.1 0.2  0.3 3. Temperature: The modulus, yield strength and tensile strength generally increase as the temperature decreases. 3 9  40 1. A dumb bell tensile test piece was tested in accordance with ISO 37. Using data given below, calculate the tensile strength at break and percentage elongation at break of the test piece. thickness = 2mm width= 4mm gauge length = 20mm breaking length= 131.9mm Exercise: load at break= 190.4N 2. Define the following terms; a) Tensile stress b) Tensile strain c) Cold draw d) Gauge length 3. State three (3) factors which affecting the plastics tensile test. Explain one (1) of the factors.  4 1 The following is the data obtained from a polypropylene dumbbell test-piece that underwent stress-strain test. Width(W)=12.61mm, thickness(t) = 3.47mm, initial gauge length (L0)=50mm. The maximum force to fracture the sample = 1290N and length between gauge marks at break(L ) = 97mm. Calculate the tensile strength and tensile strain of the polymer under test.  Tensile elongation and tensile 4 modulus measurements are among the most important 2 indications of strength in a material and are the most widely specified properties of plastic materials. Tensile test, in a broad sense, is a measurement of the CONCLUSIO ability of a material to withstand forces that tend to N pull it apart and to determine to what extent the material stretches before breaking. Tensile modulus, an indication of the relative stiffness of a material, can be determined from a stress-strain diagram.  4 Different types of plastic materials are often compared on the basis of tensile strength, elongation, and tensile modulus data. 3 Many plastics are very sensitive to the rate of straining and environmental conditions. CONCLUSIO N Therefore, the data obtained by this method cannot be considered valid for applications involving load time scales or environments widely different from this method. The tensile property data are more useful in preferential selection of a particular type of plastic from a large group of plastic materials and such data are of limited use in actual design of the product. POLYMER PHYSICAL TESTING & ANALYSIS TENSILE TESTING (RUBBER/ELASTOMER)  45 LEARNING OUTCOMES: Upon completion of this chapter, you should be able to: Describe the tensile testing of rubber. Sketch the typical stress-strain curve for rubber tensile testing. Describe the tensile testing procedures for rubber. State and describe the rubber specimen for tensile test. Discuss the advantages and disadvantages of ring and dumb-bell shape specimen. Explain the significance of rubber tensile test.  46  ASTM D412 (ISO 37) specify a method for the determination of tensile stress – strain properties of vulcanized rubbers.  Tensile tests measure the force required to break a specimen and the extent to INTRODUCTI which the specimen stretches ON or elongates to that breaking point.  The data is often used to specify material, to design parts to withstand application forces and as a quality control check of materials.  4 7  Tensile test pieces are in the form of either dumb-bells or rings with the former more commonly used.  Tensile testing is accomplished by first molding a flat sheet of rubber about 2 TEST mm thick, from which dumbbell shaped pieces are die cut. SPECIME  The international standard ISO 37 specifies three sizes of dumb-bell & two N sizes of ring.  4 8 TEST SPECIMEN i) Ring test piece (normal size):  Internal diameter = 44.6 mm  External diameter = 52.6 mm  Thickness = 4 ± 0.2 mm  The circumference is = Π x 48.6 mm = 152 mm O-ring Tensile Testing Using Rotating Roller Grip  4 9 i) Ring test piece (normal size): Tensile strength: F/2A Modulus: F/2A The % elongation at break: [(I / I0)/ Io] x 100 TEST Where I = the internal SPECIMEN circumference, in mm, at break I0 = the initial internal circumference, in mm  50 TEST SPECIMEN ii) Dumb-bell test piece: ASTM D412: Tensile Specimen for Rubber and Elastomers  Type I:  TEST SPECIMEN Dumb-bell Ring Advantages: Advantages: Stress and strain is uniform No gripping problems throughout the central Elongation is easily parallel portion of the test measured piece Grain effects can be studied by cutting the test-piece in different direction Disadvantages: Disadvantages: Difficult to grip Strain distribution is not Elongation cannot be easily uniform taken from grip separation More difficult to prepare a The strain along the whole good ring test pieces is not uniform Tensile result much lower  5 2 TEST PROCEDURE Place the test pieces in the grips of the Instron at a specified gage length. The test pieces are then stretched in a tensile testing machine and the force required to stretch the samples is measured. Test speed: 500mm/min Values of stress (force divided by the unstretched cross sectional area of the straight portion of the dumbbell) are recorded at various levels of extension, up to the break point.  53 Typical stress-strain curve The stress vs strain for elastomers is a curve like below:  Typical stress-strain curve 54 The stress vs strain curve for elastomer and other type of materials (a comparison):  5 Expression of Results 5  The tensile strength is calculated as: F/A (MPa)  The extension is measured as percent elongation and is defined as:  Tensile values before the sample breaks, give the modulus of the sample. (Modulus = F/A)  56 Expression of Results  For the rubber chemist, modulus means the tensile value (stress) at a given elongation.  Modulus numbers at l00% (M100), 200% (M200) and 300% (M300) elongation are commonly measured.  Note that the modulus, as defined here, is not equivalent to the modulus as understood by an engineer, which is equal to stress over strain. M100 : modulus at 100% strain (stress measured at 100% elongation) M300: Modulus at 300% strain (stress measured at 300% elongation).  5 7  An important function of tensile testing is to determine how well ingredients are Significanc dispersed in the rubber compound, during the mixing stage. e of  For example, if carbon black is poorly Tensile dispersed, the tensile strength (at break) Test of the cured compound will be lower than it should be.  A low state of cure, due to insufficient curative, as well as inadequate cure time or temperature, will also give a lowered tensile strength.  If a compound has too much carbon black, not enough oil, or too high a state of cure, perhaps due to excessive sulfur or accelerator, it will be reflected in a higher modulus value.  A severely over processed NR 5 compound might have a lowered 8 modulus value. Compounds used in the rubber industry have tensile strengths from less than 7 MPa to around 28 MPa. Significanc e of Urethanes can have even higher tensile Tensile strengths. Test There are cases where tensile strength is specifically relevant to an application, for example an elastic band. A higher tensile strength is also preferred for highly dynamic applications.  59 THANK YOU