Mechanical Tests

The testing for physical properties using mechanical testing of plastics is among the most important. These test determine the strength, stability, and hardness of materials. The literature from materials suppliers and the manufacturer's requirement for incoming inspection of materials often quotes the qualities of plastics in terms of tensile strength, modulus, elongation, impact strength, and hardness. These qualities outline the plastic's characteristics under tension and its resistance to any change in shape. The four most important mechanical test are Tensile (ASTM D638) Izod Impact (ASTM D256)Rockwell Hardness (ASTM D785) and Durometer Hardness (ASTM D2240).

A word of caution is important here. The test specimens are molded, normally in a master unit die (MUD mold), and therefore are affected by the molding and operating conditions inherent in their preparation. The test data is then gathered from test specimens that undergo steady deterioration. Since testing conditions are rarely long-lived, the material seldom undergoes the steady deterioration it would over a longer period. The molded product will ordinarily also be subjected to stress, cold flow, creep, chemical attack, aging, and changes in environment not reproducible under testing procedures. In selecting a plastic material the product designer should have a through understanding of the specimen preparation, specimen conditioning, and test method. With this understanding the designer will select the material most suitable for the product itself as well as the environmental effects.

Tensile Properties ASTM D638

The tensile test measure the strength of a material in resisting being pulled apart. The tensile test pulls on both ends of a 8-1/2" long coupon that resembles a dog bone. The testing coupon (test specimen) is injection molded in a mast unit die, more often called a mud mold (Figure 3-1). A specimen is typically 1/8" thick with a 3/4" width on both ends with a 1/2" wide in the middle. Test specimens of half this size are molded when smaller tensile machine are used. Since the specimen looks like a "dog bone," this term is used to refer to them.

The procedures for tensile strength is simple: both ends of the specimen are firmly clamped in the jaws of the testing machine, a force (stress) in pounds is applied at one of four rates, 0.2, 0.5, 2, or 20 inches a minute, pulling the sample from both ends. Gauge marks, small lines or points drawn one inch apart at the center of the specimen are used to measure the elongation (strain) of the specimen. The more accurate elongation of the specimen is measured by the use of a extensometer (strain gauge) attached to the center of the specimen that electronically graphs change in length in relation to force applied. A tensile specimen is illustrated in Figure 3-2. The stress applied is in pounds. The value is reported in pounds per square inch (PSI) and calculated from the cross section area of the center of the test specimen. The value is generated by dividing the cross section area into the stress in pounds to get PSI. Strain is a value measured in change in the length of the specimen and reported in inches per inch. Both are plotted on graph paper reflecting a stress-strain diagram. Figure 3-3 illustrates a typical tensile testing stress-strain diagram. Normally this procedure is completed automatically by the tensile testing machine. A typical tensile test machine is shown in Figure 3-4.

The properties derived from the tensile test are the most important indications of the strength of a material. The force necessary to pull a specimen apart, as well as along with how much the material stretches before breaking indicates how tough or brittle the material may be. Not all plastic materials produce stress-strain diagrams that offer clear indications of the various points of yielding as the load is increased. However, there are seven important references that can be derived from a stress strain diagram.

Stress

Stress is the amount of force applied to the test specimen is rated in pounds per square inch. It is illustrated along the vertical left side of the diagram (Figure 3-3). Force is increased from bottom to the top of the graph and is related to the amount of load that a material can bear. By relating stress to strain, how much the material changes in length, provides information on the rigidity of the material.

Strain

The change in length in inches in relation to the applied force is reported as strain. Strain is calculate in inches per inch in relation to applied force. Strain is recorded on along the bottom of the stress-strain diagram.

Elastic Limit

The elastic limit is best understood as that point of load at which the material under stress can recover its approximate original dimensions. The elastic limit marks the highest tensile force applied prior to permanently deforming the material. Tensile force that is release prior to reaching the elastic limit will stretch the material, but not damage it. The elastic limit, on the stress-strain diagram, is the end of the straight line on the first portion of the graph. Below the elastic limit the applied load (stress) is proportional to its change in length (strain). If the specimen load does not exceed the elastic limit then the specimen will return to it original length.

Plastic materials, in contrast to metals and ceramics, respond differently when undergoing stress. Metal and ceramics are true elastic materials. True elastic materials act like a spring when a load is applied below their elastic limit. After the applied force is removed they return to their original shape without deforming. Plastics, however, always undergo deformation (permanent strain) when a load is applied. In some literature this has been referred to a permanent set or in the product form as cold flow. In designing plastic products, the materials selected should have load characteristics well within the elastic limit to meet products service requirement. The plastic product are design so that the applied force does not exceed 75% of the materials elastic limit.

Modulus of elasticity

The modulus of elasticity or tensile modulus, also called Young's modulus, is the ratio of stress to strain, below the elastic limit. It is expressed in pounds per square inch and is a measure of a material's stiffness. The modulus is calculated by dividing the strain into stress. This value is recorded as the E value. Materials with high E values are rigid stiff materials while materials with low E values are rubber like. Plastics that react like rubbers have high elongation (change in length) to low stress (force applied) and their E values rang in the two hundreds. Metals, much stronger and stiffer materials, have E values that range from six million to eighty million. The a comparison of the properties of plastics and metals, including the modulus of elasticity or E value are illustrates in Table 3-1.

Tensile Modulus

Plastic materials often do not conform to a theoretical stress strain curves as seen in Figure 3-3. More often they resemble those shown in Figure 3-5. A tensile modulus is a calculated modulus taken from several test with an average tensile modulus reported. The procedure for calculating tensile modulus is illustrated in Figure 3-5.

Elongation

An increase in the length of the specimen for a given load (stress) is reported as elongation. Elongation is a measure of the materials ductility and can be related to toughness in plastics materials. If the plastics part is designed to withstand a change in shape, then a stress strain diagram is very useful. Since it measures the material's ability to absorb rapid impact. The stress-strain diagram elongation area, under the curved line, indicates the material's toughness. Brittle material that exhibits high tensile strength with little elongation exhibit small areas of elongation under the stress-strain diagram. The shape and size of the stress strain diagram can be compared between materials to establish relative impact properties. Figure 3-6 illustrates the stress strain diagram for five common plastic materials. Notice the change in the area under the curve between a hard brittle Phenolic and soft yet tough polyethylene. The toughness of polycarbonate compared to brittle acrylic is a strong indicator of why polycarbonate is chosen when a transparent product is designed to withstand high impact.

Yield Point

The yield point is the first point beyond the elastic limit where the plastic material begins to loose it ability to resist the force applied to it and begins to stretch. The molecular structure can no longer resistance the applied force and begins to breakdown.

Yield Strength

This is the stress at which the material under going a load exhibits deformation behavior found at the yield point (see above). Unless otherwise specified it is reported as the same as the yield point. However, often the yield strength is reported as a percentage offset from the yield point. In this case the yield strength is the stress at 0.2% or 0.5% offset toward the break point of the material.

Ultimate Strength

The ultimate strength is the maximum force (stress) that a material can withstand. The "ultimate strength" for plastic materials is the same or higher than the "yield point". Some plastic materials under elongation will gain in strength, their resistance to stress becomes greater and the material becomes stronger. This hardening is common in amorphous plastics. It is due to the alignment of the amorphous molecular structure in the direction of applied force. As the molecular structure aligns, a greater amount of the covalent bonds begin to resist the load. The weaker Van der Waal's force between molecules are forced closer together which increases their ultimate strength. These plastic materials are generally referred to as the "soft through" materials.

Although the stress strain diagram used to test plastic materials appears simple, it reflects three type of molecular deformation. When a plastic material is deformed, the plastic will respond with three types of movement. First, the carbon atom to the carbon atom bond stretches out. As you can recall from the last chapter these bonds induce a molecular chain to twist. When a force is applied instantaneously, the plastics acts like a spring. Initially, it will deform and then spring back to it original dimension. If force is applied for a longer period of time then the amorphous areas of the molecule follows the direction of the force and begin to strengthen out and induce a small permanent set in the plastic material. If the force continues, then the straightened molecules will slip which leads to permanent changes in the material. Amorphous materials will easily exhibit all three types of molecule extension, while crystalline materials with fewer amorphous areas will resist molecular uncoiling.

The data derived from tensile tests are fundamentally important for product designers. However, the tensile test does not take into consideration the changes in temperature, environment, and molded in stress, that are inherent in the product under actual use conditions. It is true that control chamber can be attached to the tensile test apparatus and the temperatures around the molded specimen can be control. However, the test coupon and modified apparatus can not duplicate the unique molecular orientation found in the molded part nor can we predict with any certainty the temperature changes during the products life.

Certain plastics easily undergo both creep and cold flow with changes in temperature and load. Additionally, plastic materials in product applications can undergo molecular change through aging and weathering. Changes in load over a long period of time will also induce changes in molecular structure and a relative reducing of physical properties or brittle effects. Other test that report rates of elastic recovery, permanent set and deformation under varying loads and temperatures must also be considered.

Izod Impact, D256

The Izod Impact tests is one of many test developed to measure the crack sensitivity of a plastic material and is related to the material's overall toughness. The test procedure is to clamp a notched specimen (Figure 3-7) into a heavy base pendulum testing machine (Figure 3-8) so that the specimen is vertical and facing the impact pendulum. The pendulum is held at a prescribed height; when it is released it swings down and breaks the specimen, then swings past it to a point were the energy in the pendulum reaches zero. The force exhibited in breaking the sample is calculated from the height that the pendulum reaches on the follow-through; it is and reported in foot pound per square inch of the notched specimen. The specimen used is normally 1" x 1/8" thick so it is necessary to multiply the follow-through pendulum results by eight to attain the results for a square inch sample.

The specimen for testing is molded at the same time with the tensile test specimen in a master unit die (MUD) mold, however some are machine or compression molded. The injection molded specimen yields higher impact properties than either machined or compression-molded specimens. The nature of injection molding causes the plastic to melt and then freeze rapidly in contact with the mold cavity wall. The molecular structure then aligns itself against the force of impact. The notch in the Izod test specimen acts to focuses the energy of the impact at the location of the notch. Figure 3-7 illustrates a Izod test specimen.

Plastic materials which have crystalline molecular structures are more sensitive to impact and thus more brittle than amorphous plastics. This property is due to the inability of the tightly packed crystalline areas to flex and absorb the impact energy. The amorphous areas can flex without breaking apart. Plastic materials fail usually because surface cracks start at a sharp corner. A failure will also take place were the filling fronts come together inside a mold cavity and fail to fuse. This type of crack is called a weld line. Sometime , other surface defects caused by aging, weathering, or molded in stress also create surface cracks. The Izod Impact specimen is created with a prescribed notch to facilitate the measurement of a plastic material's toughness in relation to cracks. All plastic materials are notch sensitive. This means that plastic parts will normally fail at surface cracks. Thus, plastics require the elimination of sharp corners, filled radiuses, and rounded threads in any product design. The Izod impact test is used to separate materials into degrees of crack sensitivity. (This only applies if the material is above it glass transition temperature.

Plastics with glass transitions above room temperature are brittle materials regardless of the molecular structure. This property is readily seen in both the amorphous plastics of acrylic and transparent polystyrene which have molecular structures that are more than 90% amorphous. Both materials are brittle at room temperature because their glass transition temperatures exceed 200 degrees F. Additionally, a plastic material that contains glass fiber to improve its strength will also exhibit brittle properties due to the inability of the materials to absorb impact.

The Izod Impact results are particularly useful in comparing various types or grades of a plastic, and comparing one plastic with another to selection materials for a product design. The test may also be used to identify the uniformity of glass distribution in glass filled plastic materials. In some instance Izod specimens have been machined out of actual parts and carefully notched. However, the Izod test is not a reliable indicator of overall toughness or impact strength of the material in product appellation

All plastics are notch sensitive, the notch specimen provides greater concentrations of stress than is normally found in a well designed plastic product. Additionally, machine notched specimens may induce heat stressing on the surface of the notch or incur additional small micro scratches on the surface that will greatly influence the crack sensitivity of the plastic. The Izod impact test does indicate the need for avoiding sharp corners in any sensitive plastic materials. Nylon, polycarbonate and Acetal plastics (some of the toughest plastic products) are notch sensitive and register relatively low values on the notched Izod impact test.

The Izod Impact can also be used to test compression. Using the notch facing away from the impact (an "unnotched test), data can be gathered about compression rather than crack propagation. A third test, Charpy Impact test, clamps a specimen at each end horizontally. The impact pendulum strikes the specimen at the center. The Charpy tests the toughness of a materials in both tension and compression. The test specimen, when struck at the center, causes the bar to undergo tension on the back surface and compression on the front surface. The result of this test method is often reported; however, it adds little understanding of the materials. Other test developed such as the chip impact, tensile impact, and falling ball impact test also provide some comparatively useful information. But, an explanation of these and other product impact test are beyond the scope of this text, because they are used so rarely.

The results from Izod Impact testing can vary greatly due to variables in specimen preparation, machine operation, and differences between materials. The three procedures in specimen preparation, injection molding, compression molding, or machining of samples all induce different surface variables and internal molecular structure that vary greatly. If the temperature is too high, or the cooling of the molded specimen occurs too fast, the material will exhibit brittle qualities. The preparation of the notch, its size, and shape affect the concentration of pendulum force. The consistency of positioning the test specimen during loading can greatly affect the outcome too. Upon impact, a person wearing a football helmet experiences 10 times more impact than the helmet would experience during the Izod impact test. The temperature conditioning of the materials, the proper drying of the materials, and the consistency of the molecular structure between materials specimens -- all have a great effect on the testing results. Ultimately the tests offer only partial information about toughness. They cannot insure suitability of the materials for an actual product design. Instead, the test results indicate the material notch sensitivity of a certain plastic only in relations to its specimen preparation, testing procedure, and testing conditions.

Rockwell Hardness, ASTM D785

The Rockwell Hardness (the machine is pictured in Figure 3-9) test measures a material's resistance to the indentation of a steel ball whose size and load specification are determined in relation to the hardness of the material. The test specimen for the Rockwell hardness is required to be 1/4" thick with parallel surfaces that lay flat on the testing anvil. Two 1/8" Izod Impact specimens placed on top of each other may be used. The steel ball acts as the indenter while a minor load is applied to the surface of the specimen. Figure 3-10 illustrates the testing sequence. The minor load indents the ball slightly to assure that contact is made. The machine gauge is set at zero and the lever at the top of the machine is flipped. This application last 15 seconds. After the major load is removed the original minor load is left in place. The indentation remaining after 15 seconds is read directly off the dial. This value is recorded as the Rockwell hardness values and is preceded by the Rockwell hardness letter scale which designates the type of ball and procedures used. Table 3-2 lists the hardness ratings for several plastic materials.

The Rockwell test, when used with plastic materials, is a better indicator of abrasion resistance than hardness. When the harder plastics are tested, acrylic, polyphelene oxide, or thermoset phenolic, a tested than the Rockwell test is a measure of the materials resistance to penetration. For true abrasion there are many test (ASTM D968, D5181, D658, D2486) that can be useful. Some rigid plastics like acrylic have very high Rockwell hardness values, yet are easy to scratch. Rockwell hardness tests are used in measuring a plastic rigidity. But since elastic recovery is involved as well as hardness, it is not valid to compare hardness or rigidity of various kinds of plastic entirely on the basis of this test. Load deflection test (ASTM D790) is more often used to measure rigidity. Plastic materials molecular structure allows the material to store deformation in the relatively spongy amorphous areas. Once the deformation force is removed some plastics spring back to their original condition.

The molded plastic parts react significantly different from the tested samples. Molded plastic parts have tighter molecular skin structures with molecules stretched and aligned in the direction of mold fill. These skin structures, whose thickness is determine by the molding conditions, will exhibit high resistance to indentation, yet will fail dramatically if penetrated. The molded-in skin structure allows the plastic product to exhibit various levels of hardness and elastic recovery not record able using this hardness test. Two other tests, Brinel and Mohs hardness, were specifically designed to test soft materials. Figure 3-11 compares Rockwell hardness scales with the scales for Brinel and Mohs hardness.

Durometer Hardness, ASTM D2240

The Durometer Hardness (Figure 3-12), sometimes referred to as shore hardness, measures a soft material's resistance to the indentation of a flat 0.79 mm (A type) or a pointed 0.100 mm (D type) instrument. The A indentor is used for soft materials, such as urethanes, while the D indentor is used for the harder materials such as nylon. The point, which is spring loaded, extends out of a holder in the apparatus. The distance that the A or B point penetrates the materials is indicate on the apparatus dial. Reading are taken from the dial within one second after loading. The Durometer test instrument is made by the Shore company and is sometimes referred to as the Shore A or Shore D test. The apparatus's dial indicator has two reading for the two different indentors. Durometer hardness reading for the A scale (softer materials such as rubber and soft wood) range from 20 up to 100. The Durometer reading for the D scale (harder materials such as neoprene, urethane, lead, nylon and acrylics) range from 40 to 100. The two scales overlap and are not equal. A 40 on the D scale is the same as an 80 on the A scale.

The test specimens are at least 1/4 inch thick. They are either molded in a MUD mold or cut from a sheet. The test is carried out by first placing the specimen on the table surface of the instrument. The indentor is pressed onto the specimen. It is important that the specimen lay flat so that the indentor enters tangent to the surface. To measure the relative softness of small tubing a hand held Durometer test instrument is available.