Materials II Properties and Mechanics Module 1 Properties and Tests

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.

ASTM Standard

ASTM Standard

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

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

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

Mechanical Properties Stress-Strain Curve for a ductile plastic

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

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

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

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

Mechanical Properties Tensometer and Specimen

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

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

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

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.

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

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.

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.

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

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

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

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

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

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

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

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

Physical Properties Glass transition temperature Melt 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.

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

Thermal Properties Thermal conductivity Specific heat or heat capacity 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

Thermal Properties Heat Deflection Temperature Softening Point 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

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 %

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.

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

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

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

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

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

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

Optical Properties Specular gloss Luminous transmittance 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

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

Optical Properties Color Color defined by three terms Value Hue Chroma 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

Optical Properties Value

Optical Properties Hue

Optical Properties Chroma

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

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

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 red object = reflectance % wavelength (nm.)

Optical Properties daylight incandescent 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

Flammability Need to understand how polymers burn When exposed to a flame or when it starts to burn the material decomposes or the molecular chains start to come apart This produces volatile polymer fragment or short polymer chains on the surface of the part These short chains are fuel which goes to the flame front At the flame front it mixes with the oxygen in the atmosphere and produces more heat and more fire

Flammability Flammability of a material is reduced by breaking the cycle Additives to disrupt the flame generation Additives to promote the retention of the fuel Additive that act as a heat sink like hydrated alumina

Flammability Materials flammability is based on the following criteria Ease of ignition Flame spread – spreads across a surface Fire endurance - penetrates Rate of heat release – how much and quickly Ease of extinction Smoke evolution Toxic gas generation

Analytical Rheology As stated earlier melt index measure at a given temperature and a specific flow rate Rheology is the study of flow and viscosity is the resistance to flow due to friction between layers The more friction between layer the greater the resistance to flow The more force is needed to “move” the material, this force is referred to as shear

Analytical Rheology Shearing occurs when the fluid is poured or mixed The rate of speed that the layers move is called the velocity gradient and is called the shear rate Shear stress is the stress caused from the layers moving When the shear rate increases at the same rate as the shear stress the fluid is considered a Newtonian fluid Water is Newtonian Plastic materials are non-Newtonian

Analytical Rheology Rheology takes into account this shear stress and measures flow of a material at different shear rates (variations in injection pressure) Different types of rheometers Torque rheometers Rotational rheometers Capillary rheometers

Electrical Properties Arc resistance Measure of time for the plastic material to arc or to short Resistivity The resistance of two conductors with an insulator of the given material between them Dissipation factor Measures the power lost in the in the plastic insulator

Electrical Properties Dielectric strength The electrical voltage required to break down or arc through a test sample of plastic Dielectric constant Measure the ability of the material to store electricity

Data Sheets Contain the property values for the specific materials along with other pertinent information concerning the material Melt flow rate Specific gravity Melt temperature Drying conditions Typically received from the material supplier but can also be found on the internet and other sources