Materials II Properties and Mechanics

1. Materials II Properties and Mechanics Module 1 Properties and Tests

2. Why Do We Look at Tests • The importance of understanding the test is at the heart of understanding the materials and what they are capable of performing • Data sheets describe the material properties but do not describe the test methods. • Tests must be regulated to ensure accuracy in the testing of the materials.

3. ASTM Standard

4. ASTM Standard

5. Mechanical Properties • Mechanical properties are the most important properties looked at when considering a given material because virtually all service conditions and the majority of the end-use applications involve some form of mechanical loading • These values are almost always listed on material data sheets, it is important to remember that the recoded values are typically at room temperature and do not represent the different effect of temperature and other environmental changes • Also important to remember that the product is typically subjected to more than one type of deformation at once

6. Mechanical Properties • Describe how the material acts with applications of force or load • There are three different types of force Tension Compression Shear

7. Mechanical Properties • Comprised of three parts • Stress, the force over the original cross sectional area • Strain, the deformation in a percentage of the change in length compared to the original length • Modulus, is the toughness of the material and is the ratio stress/strain • When reviewing stress/strain curves the more area under the curve the tougher the material

8. Mechanical Properties • Stress-Strain Curve for a ductile plastic

9. Mechanical Properties • Stress – force applied to a given area to produce deformation • Strain – change in length per unit of the original length • Elongation – the increase in length produced by a tensile load • Yield point – the first point on the stress-strain curve where an increase in strain occurs with out an increase in stress • Yield strength – the stress at the yield point

10. Mechanical Properties • Proportional limit – the greatest stress a material is capable of without any deviation from proportionality of stress to strain • Modulus of elasticity – the ratio of stress to strain under the proportional limit of the curve • Ultimate strength – the maximum unit of stress a material will withstand when subjected to an applied load in compression, tension, or shear • Secant modulus – ratio of the total stress to corresponding strain a specific point on the curve

11. Mechanical Properties • Stress-strain curves are used to classify the general properties of a material • Soft and weak • Hard and brittle • Soft and tough • Hard and strong • Hard and tough

12. Mechanical Properties • Tensile Strength • Single most important indication of strength in a material • The force necessary to pull apart a specimen along with how much the material stretches before breaking

13. Mechanical Properties Tensometer and Specimen

14. Mechanical Properties • Flexural Strength • Stress-strain in flexure is also important to designers of plastic parts • It is the ability of a material to withstand bending forces applied perpendicular to the part • Stresses are a combination of compression and tension • The result of a flexural test is the maximum stress and strain that occur on the outer surface that is in tension

15. Mechanical Properties • Flexural Strength • Most polymers do not break therefore many times the test is maximum stress when the strain for the outer surface is 5% • This test is good because it represents beams or similar structures

16. Mechanical Properties • Shear Strength • Force needed to produce a fracture by a shearing action, an example is scissors • Force over the cross sectional area being sheared. Expressed in force per area (psi) based on the area of the sheared edge • Test specimen and apparatus • The specimen is a round or square washer approximately two inches in diameter or square with a 7/16 diameter thru hole in the center

17. Mechanical Properties • Impact strength • Indicates the amount of energy required to break a given material • Impact strength is directly related to the ability of a material to absorb and distribute energy. • Impact strength is directly related to the chemical structure of the polymeric material.

18. Mechanical Properties • Impact strength • Two main types of impact testing • The falling mass test consists of dropping a ball shaped mass onto the test sample • Pendulum test consists of dropping a pendulum into a test specimen • Charpy • Izod • Notched Izod

19. Mechanical Properties • Falling Mass Impact strength • Impact must be on a flat surface • Indicates a good for direct indicator • Does not take into consideration the design or built in stresses. • Does not take into account velocity increases.

20. Mechanical Properties • Notched Izod Impact strength • Utilizes a vertically placed specimen in a direct path to the pendulum • A 90 degree is notched into the speciman where the point of impact is going to occur • The 90 degree notch will induce failure, important for design consideration because of material notch sensitivity.

21. Mechanical Properties • Fatigue and Flexing • Fatigue life is defined as the number of cycles of deformation required to bring a part to failure under a given set of conditions • Materials strength is greatly reduce by cyclic loading • Since many materials see this kind of force this type of test is a popular one • Failures occur from repeated applications of stress in different directions • Values are normally given in numbers of cycles to failure at a given stress level

22. Mechanical Properties • Hardness • Resistance to deformation particularly permanent, indentation or scratching • Is a relative term, no units but a scale “relative hardness” • Two main tests Rockwell for relative harder materials Durometer for relatively softer materials

23. Mechanical Properties • Rockwell hardness • For relative harder materials such as nylons, acetals, polycarbonates, and acrylics • M scale very hard • R scale hard • C scale is used for metals

24. Mechanical Properties • Durometer hardness • For relatively softer materials • Shore scale D for harder • Shore scale A for soft rubbery types • Based on the penetration of a specific indenter under certain conditions • Indenter is spring loaded and protrudes from a base • Sharper indenter used for harder materials • Larger flat on point used for softer materials

25. Mechanical Properties • Abrasion resistance • Abrasion is related to force, load, and area of contact • The hardness of material also has a big affect • Abrasion resistance is the ability to withstand mechanical action such as rubbing, scraping, or erosion • The test is complicated by the fact that as the material is abraded friction will cause the material to heat up which gives it different characteristics

26. Mechanical Properties • Abrasion resistance • Abrasion resistance is typically measured by a weight loss when a material is abraded with a given abrader

27. Physical Properties • Specific gravity or density • Mass per unit volume lbs/in3 or kg/m3 • Relative density, ratio of mass of a given volume of material over the mass of an equal volume of water with a density of 1

28. Physical Properties • Tensile Creep • The mechanical tests that we have noted to this point measure the strength of plastic in a short period of time • Short time tests are irrelevant due to the fact that most plastics are in continuous use over a long period of time • Creep measures the deformation of a material over a period of time

29. Physical Properties • Glass transition temperature • The temperature when the material looses its rigidity and becomes pliable, all materials have a glass transition temperature, it is the window in which they exist is vastly different. (crystalline vs. amorphous) • Melt temperature • The temperature at which the material becomes liquid loosing 90% of its’ viscosity.

30. Thermal Properties • Thermal properties are how the temperature affects different mechanical, electrical, optical, and other properties • Different things effect how the temperature effects the properties • Molecular orientation • Orientation decreases thermal stability • Molecular weight • Low temperature flexibility and brittleness

31. Thermal Properties • Thermal conductivity • The rate at which the material will transmit heat • Given by a k factor, aluminum has a k factor of 122, it transfers heat very well, some plastic foams have a k factor of 0.01 • This is important in insulation materials • Specific heat or heat capacity • The amount of heat required to raise the temperature of one unit of mass by one degree Celsius

32. Thermal Properties • Heat Deflection Temperature • The highest continuous temperature that a material can withstand without deforming • Softening Point • This test is done by placing a needle against a sample of material, the temperature is increased 50 C per hour and when the needle penetrates the temperature is recorded • Thermal Expansion • A coefficient used to determine expansion in length, area, or volume

33. Thermal Properties • Mold Shrinkage • The amount by which a molded part is smaller than the cavity space where it was produced • Typically given in in/in, mm/mm, or %

34. Thermal Properties • Brittleness Temperature • At low temperatures the material approaches it’s glass transition temperature it becomes hard and brittle • The temperature at which a material exhibits a brittle failure in an impact test.

35. Environmental Properties • Environmental refers to the area that the plastics products are used in • The environment can have drastic effects on the properties and appearance of different materials • The major environmental properties are • Solar radiations • Caused from different type of solar energy • Ultraviolet radiation can cause fracture of the molecular chains which promotes thermal oxidative degradation • This degradation results in embrittlement, discoloration and loss of mechanical properties

36. Environmental Properties UV stabilizers are used to combat these effects • Absorbers are both organic and inorganic pigments that absorb the harmful radiation and dissipate it, a common one is carbon black • Stabilizers inhibit the rupture of the chains by chemical means, basically dissipating the energy to lower less damaging levels

37. Environmental Properties • Microorganisms, bacteria, fungus, and mold • Polymers by themselves are typically not effected by microorganisms but the lower molecular weight additives such as plasticizers, lubricants and stabilizers are • As these additives migrate to the surface of the part they can come under attack • Degradation can also show up as loss of aesthetics, mechanical properties, and increase of embrittlement

38. Environmental Properties • Weathering • Test sample are exposed to heat, sunlight, and humidity • Samples are rated on color change, gloss level, and loss of physical properties • Two main types environmental weathering and accelerated weathering

39. Environmental Properties • Ultraviolet resistance (ASTM D-2565, G-23) • Going with weatherability it’s the resistance or the effects of sunlight • Stress cracking • Stress cracking may be caused by solvents, radiation, or strain

40. Environmental Properties • Moisture content • All plastic materials either collect moisture or absorb it (hygroscopic) from the atmosphere • A moisture analyzer is a piece of equipment used for this test • Materials require drying prior to processing otherwise you will have poor properties and difficulty processing

41. Optical Properties • Specular gloss • Directs light at different angles (20, 45, 60) and compares the results to the reflection of a mirror • Luminous transmittance • Measure the clarity of the plastic

42. Optical Properties • Color • Color perception requires three things • Light source • Object • Observer • Color is created by the selective reflection and absorption of specific light waves • When light strikes an object the light waves that are reflected is the color that we see • Example an object absorbs all colors accept blue so we see the object as blue

43. Optical Properties • Color • Color defined by three terms • Value • Referred to as neutral colors, ranging from white to black • Also called lightness • Hue • The attribute of color perception • Red – blue – green – yellow • Chroma • Also referred to as saturation • How far the color is from the neutral axis

44. Optical Properties • Value

45. Optical Properties • Hue

46. Optical Properties • Chroma

47. Optical Properties • Color • Note that color is affected by the light source or the illuminant • CIE has standard illuminates • Daylight • Noon light • Florescent light • Ways to measure color • Tristimulus system or L, a, b, • L is where it lies on the neutral axis, 100 = perfect white and 1 = black • a is green verses red • b is blue verses yellow

48. L*=100 white + b* = yellow - a* = green + a* = red - b* = blue L*=0 black Optical Properties • Color • Tristimulus system or L, a, b,

49. Optical Properties • Color • Note that all different colors produce different series of light waves • Instrumented color measurement done with a spectrophotometer • Uses a specific light source • Gives a spectral read out = reflectance % red object wavelength (nm.)

50. Optical Properties • Color • Also must do visual color evaluation due to different light sources • Color appears different depending on light source • Referred to as Metamerism • Use a light booth for this evaluation daylight incandescent