Copyright Joseph Greene Properties and Standard Testing Chapter 4 Professor Joe Greene CSU, CHICO

Copyright Joseph Greene Properties and Standard Testing Materials and Test Specimens Short term conventional testing –Tensile, flexural, compression, torsion, shear, –Tear, biaxial stress Hardness –Rockwell, Shore, ball, Barcol Friction, Wear, and Abrasion Impact Cyclic loading –Dynamic testing and fatigue Long term static loading –Creep and static fatigue

Copyright Joseph Greene Properties and Standard Testing High temperature softening –Heat deflection temperature and Vicat softening point –Transition temperature and Dynamic mechanical testing Low temperature stiffening –Stiffness, static and dynamic brittleness Flammability and combustion Electrical properties Thermal properties

Copyright Joseph Greene Mechanical Test Considerations Principle factors are in three main areas –manner in which the load is applied –condition of material specimen at time of test –surrounding conditions (environment) during testing Tests classification- load application –kind of stress induced. Single load or Multiple loads –rate at which stress is developed: static versus dynamic –number of cycles of load application: single versus fatigue Primary types of loading tension compression shear torsion flexure

Copyright Joseph Greene Testing Procedure Tensile tests yield a tensile strain, yield strength, and a yield stress Tensile modulus or Young’s modulus or modulus of elasticity –Slope of stress/strain –Yield stress –point where plastic deformation occurs –Some materials do not have a distinct yield point so an offset method is used Stress Strain in/in 1000 psi Yield stress Yield strength Slope=modulus

Copyright Joseph Greene Expected Results Stress is measured load / original cross-sectional area. True stress is load / actual area. True stress is impractical to use since area is changing. Engineering stress or stress is most common. Strain is elongation / original length. Modulus of elasticity is stress / strain in the linear region Note: the nominal stress (engineering) stress equals true stress, except where large plastic deformation occurs. Ductile materials can endure a large strain before rupture Brittle materials endure a small strain before rupture Toughness is the area under a stress strain curve

Copyright Joseph Greene Rockwell Hardness Hardness is a function of the degree of indentation of the test piece by action of an indenter under a given static load (similar to the Brinell test) Rockwell test has a choice of 3 different loads and three different indenters The loads are smaller and the indentation is shallower than the Brinell test Rockwell test is applicable to testing materials beyond the scope of the Brinell test Rockwell test is faster because it gives readings that do not require calculations and whose values can be compared to tables of results (ASTM E 18)

Copyright Joseph Greene Rockwell Test Description Specially designed machine that applies load through a system of weights and levers –Indenter can be 1/16 in hardened steel ball, 1/8 in steel ball, or 120° diamond cone with a somewhat rounded point (brale) –Hardness number is an arbitrary value that is inversely related to the depth of indentation –Scale used is a function of load applied and the indenter Rockwell B- 1/16in ball with a 100 kg load Rockwell C- Brale is used with the 150 kg load –Operation Minor load is applied (10 kg) to set the indenter in material Dial is set and the major load applied (60 to 100 kg) Hardness reading is measured Rockwell hardness includes the value and the scale letter

Copyright Joseph Greene Rockwell Values B Scale: Materials of medium hardness (0 to 100HR B ) Most Common C Scale: Materials of harder materials (> 100HR B ) Most Common Rockwell scales divided into 100 divisions with each division (point of hardness) equal to 0.002mm in indentation. Thus difference between a HR B 51 and HR B 54 is 3 x mm mm indentation The higher the number the harder the number

Copyright Joseph Greene Hardness Hardness is measured as the resistance to penetration of a material by an indentor (impressor) which can have a variety of shapes. Shore hardness: Fig 4.21 and Fig 4.23 –Associated with a Durometer instrument Features a pin-shaped indentors of two types (A or D) which protrude from a flat surface and loaded by a calibrated spring. Instrument is pressed against the polymer specimen until the surfaces come in close contact. “A” scale with a blunt indentor is suitable for soft materials, e.g., elastomers and covers the range from 20A-95A “D” scale with a pointed indentor is intended for harder materials, e.g., soft plastics and covers a range from 40D-90D. Soft materials: 20A-30A are for rubber bands and soft erasers. Semi-rigid materials: 45A is for LDPE, 65D for HDPE, and 55A-70A for tire treads. Hard materials: 90A for typewriter rolls. Very hard materials: 90D for bowling balls.

Copyright Joseph Greene Hardness Barcol hardness: Fig 4.21 and Fig 4.24 –For rigid plastics, e.g., fiber reinforced plastics and composites. –Barcol (Barber and Coleman) impressor uses an indentor similar to that used in the Shore D method, except for a flat tip instead of a rounded one. Minimum recommended test specimen thickness is about 1.5mm –Indentor protrudes from a surface that is pressed against the polymer specimen. Indentor is loaded with a calibrated spring and its penetration is related to a Barcol hardness number that is on a dial on the tester. Usual range is about 50B-90B Barcol 60B is equivalent to Shore 80D and to Rockwell M100

Copyright Joseph Greene Friction, Wear, and Abrasion Tribology: study of the frictional characteristics of materials –Applications include roller bearing, bushings, bearing pads, cams, piston rings, and gears. Coefficient of friction –Primary tribological characteristic is the coefficient of friction between two surfaces. The coefficient, µ, is defined as –The ratio of the sliding force, F s, to the normal force, F N. –The ratio of the shear stress, S, to the normal pressure, P N. –Well-defined for metallic materials –Is significant for polymers and polymers with another material Two types –Static (starting) coefficient is generally higher than, –Dynamic (sliding) coefficient

Copyright Joseph Greene Friction, Wear, and Abrasion Factors affecting friction –Dynamic friction- Typical testing velocity is 25 cm/sec (50 ft/min) –Typical normal or contact pressure, P N, is about 0.2 MPa (30psi) –Surface finish and Lubricants affects friction –Additives can reduce friction, including Fluorocarbon polymers, bronze, brass, graphite, molybdenum sulfide –Method for determining static and dynamic coefficients of friction An inclined plate (Figure 4.25) with gravity being responsible for the sliding force as well as the normal force Values for plastics range from 0.05 for PTFE to 2.5 for rubbers Engineering plastics used for frictional applications (PMO and PA) have values less than 0.3

Copyright Joseph Greene Friction, Wear, and Abrasion Wear

Copyright Joseph Greene Energy Capacity Energy Capacity: ability of a material to absorb and store energy. Energy is work. Energy = (force) x (distance) Energy capacity is the area under the stress-strain curve. Hysteresis: energy that is lost after repeated loadings. The loading exceeds the elastic limit. Stress Strain Stress Strain Elastic strainInelastic strain

Copyright Joseph Greene Impact Testing Two Basic Methods- notched or unnotched samples –Izod (vertical beam) –Charpy (horizontal beam)

Copyright Joseph Greene Cyclic loading Tensile, flexural, compression, torsion, shear, Tear, biaxial stress

Copyright Joseph Greene Long Term Static Loading: Creep Creep –Measures the effects of long-term application of loads that are below the elastic limit if the material being tested. –Creep is the plastic deformation resulting from the application of a long-term load. –Creep is affected by temperature Creep procedure –Hold a specimen at a constant elevated temperature under a fixed applied stress and observe the strain produced. –Test that extend beyond 10% of the life expectancy of the material in service are preferred. –Mark the sample in two locations for a length dimension. –Apply a load –Measure the marks over a time period and record deformation.

Copyright Joseph Greene Creep Results Creep versus time Creep (in/in) Time (hours) Primary Creep Secondary Creep Tertiary Creep l0l0 lFlF Constant Load Fixed

Copyright Joseph Greene Short Term Conventional Testing Tear –Flexible plastics and elastomers often fail in a tearing mode and their resistance to tearing is often inadequately reflected in tensile strength –Standard tear tests involve a variety of test specimen geometries (angle tear, trouser tear, etc.) Figure 4.12 Conducted on a Universal testing machine or specialized equip Involve a cut, slit, or nick which is made before the test. Biaxial stress –Developed when a circular diaphragm, pipe, or container is subjected to pressure (Fig 4.13) Basis for quick-burst tests. The pressure at failure (rupture), or the stress is measured

Copyright Joseph Greene High Temperature Softening –HDT

Copyright Joseph Greene Low Temperature Stiffening Low temperature stiffening –Stiffness, static and dynamic brittleness

Copyright Joseph Greene Electrical Properties Electrical properties Thermal properties

Copyright Joseph Greene Thermal Testing Five Thermal Properties –Melting Point –Heat deflection temperature –Vicant Softening Temperature –Flamability –Oxygen Index

Copyright Joseph Greene Thermal Testing Five Thermal Properties –Melting Point, Tm, and Glass Transition, Tg (DSC) Measures the temperature difference, and energy necessary to establish a “zero” temperature difference, between as sample and a reference sample. Figures a and b –Heat Deflection Temperature (HDT) 3-Point bending test on a sample in a temperature environment. Temperature at which the sample deflects at specified amount. –Vicant Softening Temperature Similar to the HDT test except the sample is not supported, but placed flat at the base of the apparatus, which is placed in a temperature environment. Temp is raised until needle penetrates sample a given amount.

Copyright Joseph Greene Thermal Testing Five Thermal Properties –Flammability Measures the condition of the sample as it is exposed to an ignition source. Dripping, smoking, or other condition is recorded, as well as the speed and distance the flame travels. –Limited Oxygen Index (LOI) Measure the minimum amount of oxygen that will support flaming combustion of a plastic product. Specimen is ignited with a flame source, then the source is removed. The oxygen level is adjusted upward or downward to determine the minimum level that will sustain burning. The level is stated as the percentage of oxygen contained in the airstream

Copyright Joseph Greene DSC and TGA DSC Measures –Tg –Tm –Crystallinity Thermogravimetric Analysis (TGA) Measures –Filler content, resin content, additives content –Place small specimen in a chamber that is part of the TGA apparatus –The chamber weighs sample as it is slowly heated to 1000F and sample decomposes –As the sample is heated the sample slowly burns and the weight is reduced and measured.