Rapid loading

Different behavioral characteristics for a wide range of loading rates have been reviewed. This review concerns load or strain duration that are much shorter than those reviewed that are usually referred to as being rapid impact loading. They range from a second or less (Fig. 2.22). There are a number of basic forms of rapid impact loading or impingement on products to which plastics react in a manner different from other materials. These dynamic stresses include loading due to direct impact, impulse, puncture, frictional, hydrostatic, and erosion. They have a difference in response and degree of response to other forms of stress.

The concept of a ductile-to-brittle transition temperature in plastics is well known in metals where notched metal parts cause brittle failure when compared to unnotched specimens. There are differences such as the short time moduli of many plastics compared with those in metals that may be 200 MPa (29 x 106 psi). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck.

Rapid loading velocity (Courtesy of Plastics FALLO)

-FIRED PROJECTILE -BATTED BASEBALL —PITCHED BASEBALL -FOOTBALL HELMET -TEN-FOOT FAIL — IZflD IMPACT TEST —REFRIGERATOR DOOR-SLAM

-HOUSE DOOR-SLAM

-CONVENTIONAL TENSILE STRENGTH

Impact

Impact loading analysis may take the form of design against impact damage requiring an analysis under high-rate loading or design for acceptable energy absorption, or a combination of the two. Impact resistance of a structure is defined as its ability to absorb and dissipate the energy delivered to it during relatively high speed collisions with other objects without sustaining damage that would damage its intended performance.

To determine whether failure will occur the acceptable energy absorption case requires an analysis of the stress and strain distribution during the impact loading followed by comparison with materials impact failure data. Whenever a product is loaded rapidly, it is subjected to impact loading. Any product that is moving has kinetic energy. When this motion is somehow stopped because of a collision, its energy must be dissipated. The ability of a plastic product to absorb energy is determined by such factors as its shape, size, thickness, type of material, method of processing, and environmental conditions of temperature, moisture, and/or others.

Temperature conditions effect impact strength. The impact strength of plastics is reduced drastically at low temperatures with the exception of fibrous filled materials that improve in impact strength at low temperature. The reduction in impact strength is especially severe if the material undergoes a glass transition where the reduction in impact strength is usually an order of magnitude.

From a design approach several design features affect impact resistance. For example, rigidizing elements such as ribs may decrease a part's impact resistance, while less-rigid sections may absorb more impact energy without damage by deflecting elastically. Dead sharp corners or notches subjected to tensile loads during impact may decrease the impact resistance of a product by acting as stress concentrators, whereas generous radii in these areas may distribute the tensile load and enhance the impact resistance. This factor is particularly important for products comprised of materials whose intrinsic impact resistance is a strong function of a notch radius. An impact resistance that decreases drastically with notch radius characterizes such notch sensitive materials. Wall thickness may also affect impact resistance. Some materials have a critical thickness above which the intrinsic impact resistance decreases dramatically.

There are different methods used to determine the impact resistance of plastics. They include pendulum methods (Izod, Charpy, tensile impact, falling dart, Gardner, Dynatup, etc.) and instrumented techniques. In the case of the Izod test, what is measured is the energy required to break a test specimen transversely struck (the test can be done either with the specimen notched or unnotched). The tensile impact test has a bar loaded in tension and the striking force tends to elongate the bar.

Impact strengths of plastics are widely reported, these properties have no particular design value. However, they are important, because they can be used to provide an initial comparison of the relative responses of materials. With limitations, the impact value of a material can broadly separate those that can withstand shock loading from those that are poorly in this response. The results provide guidelines that will be more meaningful and empirical to the designer. To eliminate broad generalizations, the target is to conduct impact tests on the final product or, if possible, at least on its components.

An impact test on products requires setting up an approach on how it should be conducted. The real test is after the product has been in service and field reports are returned for evaluation. Regardless, the usual impact tests conducted on test samples can be useful if they are properly related with product requirements.

Test and service data with PVC both rate low in notched Izod impact tests and performs well in normal service applications that involve impact loading. Another example is with some grades of rubber-modified high impact PSs that show up well in the Izod test fail on impact under field test conditions. These results have led to continual reexamination of the tests used to determine the toughness of plastics.

There are thermoplastics that tend to be very notch sensitive on impact. This is apparent from the molecular structure of the TP that consist of random arrangements of plastic chains (Chapter 1). If the material exists in the glassy state at room temperature the notch effect is to cut the chains locally and increase the stress on the adjacent molecular chains which will scission and propagate the effect through the material. At the high loading rate encountered in impact loading the only form of molecular response is the chain bending reaction which is limited in extent and generally low in magnitude compared to the viscoelastic response which responds at longer loading times.

TPs impact properties can be improved if the material selected does not have sufficient impact strength. One method is by altering the composition of the material so that it is no longer a glassy plastic at the operating temperature of the product. In the case of PVC this is done by the addition of an impact modifier which can be a compatible plastic such as an acrylic or a nitrile rubber. The addition of such a material lowers the Tg (glass transition temperature) and the material becomes a rubbery viscoelastic plastic with improved impact properties (Chapter 1).

Molecular orientation can improve impact TP properties. As an example nylon has a fair impact strength but oriented nylon has a very high transverse impact strength. The intrinsic impact strength of the nylon comes from the polar structure of the material and the fact that the polymer is crystalline. The substantial increase in impact strength as a result of the orientation results from the molecular chains being aligned. This makes them very difficult to break and, in addition, the alignment improves the polar interaction between the chains so that even when there is a chain break the adjacent chains hold the broken chain and resist parting of die structure. The crystalline nature of the nylon material also means that there is a larger stress capability at rapid loading since the crystalline areas react much more elastically than the amorphous glassy materials.

Other methods in which impact strength can be substantially improved are by the use of fibrous reinforcing fillers and product design. With reinforcements materials act as a stress transfer agent around the region that is highly stressed by the impact load. Since most of the fibrous fillers such as glass have high elastic moduli, they are capable of responding elastically at the high loading rates encountered in impact loading. Designwise prevent the formation of notched areas that act as stress risers. Especially under impact conditions the possibility of localized stress intensification can lead to product failure. In almost every case the notched strength is substantially less than the unnotched strength.

Impulse

Impulse loading differs from impact loading. The load of two billiard balls striking is an impact condition. The load applied to an automobile brake shoe when the brake load is applied or the load applied to a fishing line when a strike is made is an impulse load. The time constants are short but not as short as the impact load and the entire structural element is subjected to the stress.

It is difficult to generalize as to whether a plastic is stronger under impulse loading than under impact loading. Since the entire load is applied to the elastic elements in the structure the plastic will exhibit a high elastic modulus and much lower strain to rupture. For example acrylic and rigid PVC (polyvinyl chloride) that appear to be brittie under normal loading conditions, exhibit high strength under impulse loading conditions. Rubbery materials such as TP polyurethane elastomers and other elastomers behave like brittle materials under impulse loading. This is an apparently unexpected result that upon analysis is obvious because the elastomeric rubbery response is a long time constant response and the rigid connecdng polymer segments that are brittle are the ones that respond at high loading rates.

Impact loading implies striking the object and consequently there is a severe surface stress condition present before the stress is transferred to the bulk of the material. The impact load is applied instandy limiting the straining rate only by the elastic constants of the material being struck. A significant portion of the energy of impact is converted to heat at the point of impact and complicates any analytically exact treatment of the mechanics of impact. With impulse loading the load is applied at very high rates of speed limited by the member applying the load. However, the loading is not generally localized and the heat effects are similar to conventional dynamic loading in that the hysteresis characteristics of the material determines the extent of heating and the effects can be analyzed with reasonable accuracy.

Plastics generally behave in a much different manner under impulse loading than they do under loading at normal straining rates. Some of the same conditions occur as under impact loading where the primary response to load is an elastic one because there is not sufficient time for the viscoelastic elements to operate. The primary structural response in thermoplastic is by chain bending and by stressing of the crystalline areas of crystalline polymers. The response to loading is almost completely elastic for most materials, particularly when the time of loading is of the order of milliseconds.

Improvements made with respect to impact loading for structures such as fibers and orientations apply equally to impulse loading conditions. Crystalline polymers generally perform well under impulse loading, especially polar materials with high interchain coupling.

To design products subjected to impulse loading requires obtaining applicable data. High-speed testing machines are used to determine the response of materials at millisecond loading rates. If this type data is not available evaluation can be done from the results of the tensile impact test. The test should be done with a series of loads below break load, through the break load, and then estimating the energy of impact under the non-break conditions as well as the tensile impact break energy. Recognize that brittle plastics perform well and rubbery materials that would seem to be a natural for impulse loading are britde.

Puncture

Puncture loading is very applicable in applications with sheet and film as well as thin-walled tubing or molding, surface skins of sandwich panels, and other membrane type loaded structures. The test involves a localized force that is applied by a relatively sharp object perpendicular to the plane of the plastic being stressed. In the case of a thin sheet or film the stresses cause the material to be (1) displaced completely away from the plane of the sheet (compressive stress under the point of the puncturing member) and (2) the restraint is by tensile stress in the sheet and by hoop stress around the puncturing member (part of the hoop stress is compressive adjacent to the point which changes to tensile stress to contain the displacing forces). Most cases fall somewhere between these extremes, but the most important conditions in practice involve the second condition to a larger degree than the first condition.

If the plastic is thick compared to the area of application of the stress, it is effectively a localized compression stress with some shear effects as the material is deformed below the surface of the sheet.

Plastics that are biaxially oriented have good puncture resistance. Highly polar polymers would be resistant to puncture failure because of their tendency to increase in strengdi when stretched. The addition of randomly dispersed fibrous filler will also add resistance to puncture loads.

Anisotropic materials will have a more complicated force pattern. Uniaxially oriented materials will split rather than puncture under \puncturing loading. To improve the puncture resistance materials are needed with high tensile strength. In addition, the material should have a high compression modulus to resist the point penetration into the material. Resistance to notch loading is also important.

Friction

Friction is the opposing force that develops when two surfaces move relative to each other. Basically there are two frictional properties exhibited by any surface; static friction and kinetic friction. The ranges of friction properties are rather extensive. Frictional properties of plastics are important in applications such as machine products and in sliding applications such as belting and structural units such as sliding doors. In friction applications suggested as well as in many others, there are important areas that concern their design approach.

It starts in plastic selection and modification to provide either high or low friction as required by the application. There is also determining the required geometry to supply the frictional force level needed by controlling contact area and surface quality to provide friction level. A controlling factor limiting any particular friction force application is heat dissipation. This is true if the application of the friction loads is either a continuous process or a repetitive process with a high duty cycle. The use of cooling structures either incorporated into the products or by the use of external cooling devices such as coolants or airflow should be a design consideration. For successful design the heat generated by the friction must be dissipated as fast as it is generated to avoid overheating and failure.

The relationship between the normal force and the friction force is used to define the coefficient of static friction. Coefficient of friction is the ratio of the force that is required to start the friction motion of one surface against another to the force acting perpendicular to the two surfaces in contact. Friction coefficients will vary for a particular plastic from the value just as motion starts to the value it attains in motion. The coefficient depends on the surface of the material, whether rough or smooth. These variations and others make it necessary to do careful testing for an application which relies on the friction characteristics of plastics. Once the friction characteristics are defined, however, they are stable for a particular material fabricated in a prescribed method.

The molecular level characteristics that create friction forces are the intermolecular attraction forces of adhesion. If the two materials that make up the sliding surfaces in contact have a high degree of attraction for each other, the coefficient of friction is high. This effect is modified by surface conditions and the mechanical properties of the materials. If the material is rough there is a mechanical locking interaction that adds to the friction effect. Sliding under these conditions actually breaks off material and the shear strength of the material is an important factor in the friction properties. If the surface is polished smooth the governing factor induced by the surface conditions is the amount of area in contact between the surfaces. In a condition of large area contact and good adhesion, the coefficient of friction is high since there is intimate surface contact. It is possible by the addition of surface materials that have high adhesion to increase the coefficient of friction.

If one or both of the contacting surfaces have a low compression modulus it is possible to make intimate contact between the surfaces which will lead to high friction forces in the case of plastics having good adhesion. It can add to the friction forces in another way. The displacement of material in front of the moving object adds a mechanical element to the friction forces.

In regard to surface contamination, if the surface is covered with a material that prevents the adhesive forces from acting, the coefficient is reduced. If the material is a liquid, which has low shear viscosity, the condition exists of lubricated sliding where the characteristics of the liquid control the friction rather than the surface friction characteristics of the plastics.

The use of plastics for gears and bearings is the area in which friction characteristics have been examined most carefully. As an example highly polar plastic such as nylons and the TP polyesters have, as a result of the surface forces on the material, relatively low adhesion for themselves and such sliding surfaces as steel. Laminated plastics make excellent gears and bearings. The typical coefficient of friction for such materials is 0.1 to 0.2. When they are injection molded (IM) the skin formed when the plastic cools against the mold tends to be harder and smoother than a cut surface so that the molded product exhibit lower sliding friction and are excellent for this type of application. Good design for this type of application is to make the surfaces as smooth as possible without making them glass smooth which tends to increase the intimacy of contact and to increase the friction above that of a fine surface.

To reduce friction, lubricants are available that will lower the friction and help to remove heat. Mixing of slightly incompatible additive materials such as silicone oil into an IM plastic are used. After IM the additive migrates to the surface of the product and acts as a renewable source of lubricant for the product. In the case of bearings it is carried still further by making the bearing plastic porous and filling it with a lubricating material in a manner similar to sintered metal bearings, graphite, and molybdenum sulfide are also incorporated as solid lubricants.

Fillers can be used to increase the thermal conductivity of the material such as glass and metal fibers. The filter can be a material like PTFE (polytetrafluoroethylene) plastic that has a much lower coefficient of friction and the surface exposed material will reduce the friction.

With sliding doors or conveyor belts sliding on support surfaces different type of low friction or low drag application is encountered. The normal forces are generally small and the friction load problems are of the adhering type. Some plastics exhibit excellent surfaces for this type of application. PTFEs (tetrafluoroethylene) have the lowest coefficient of any solid material and represent one of the most slippery surfaces known. The major problem with PTFE is that its abrasion resistance is low so that most of the applications utilize filled compositions with ceramic filler materials to improve the abrasion resistance.

In addition to PTFE in reducing friction using solid materials as well as films and coatings there are other materials with excellent properties for surface sliding. Polyethylene and the polyolefins in general have low surface friction, especially against metallic surfaces. UHMWPE (ultra high molecular weight polyethylene) has an added advantage in that it has much better abrasion resistance and is preferred for conveyor applicadons and applications involving materials sliding over the product. In the textile industry loom products also use this material extensively because it can handle the effects of the thread and fiber passing over the surface with low friction and relatively low wear.

There are applications where high frictions have applications such as in torque surfaces in clutchcs and brakes. Some plastics such as poly-urethanes and plasticized vinyl compositions have very high friction coefficients. These materials make excellent tracdon surfaces for products ranging from power belts to drive rollers where the plasdcs either drives or is driven by another member. Conveyor belts made of oriented nylon and woven fabrics are coated with polyurethane elastomer compounds to supply both the driving traction and to move the objects being conveyed up fairly steep inclines because of the high friction generated. Drive rollers for moving paper through printing presses, copy machines, and business machines are frequently covered with either urethane or vinyl to act as the driver members with minimum slippage.

Erosion

Friction in basically the effect of erosion forces such as wind driven sand or water, underwater flows of solids past plastic surfaces, and even the effects of high velocity flows causing cavitation effects on material surfaces. One major area for the utilization of plastics is on the outside of moving objects that range from the front of automobiles to boats, aircraft, missiles, and submarine craft. In each case the impact effects of the velocity driven particulate matter can cause surface damage to plastics. Stationary objects such as radomes and buildings exposed to the weather in regions with high and frequent winds are also exposed to this type of effect.

Hydrostatic

In applications where water is involved if the water does not wet the surface, the tendency will be to have the droplets that do not impact close to the perpendicular direction bounce off the surface with considerably less energy transfer to the surface. Non-wetting coatings reduce the effect of wind and rain erosion. Impact of air-carried solid particulate matter is more closely analogous to straight impact loading sincc the particles do not become disrupted by the impact. The main characteristic required of the material, in addition to not becoming brittle under high rate loading is resistance to notch fracture.

The ability to absorb energy by hysteresis effects is important, as is the case with the water. In many cases the best type of surface is an elastomer with good damping properties and good surface abrasion resistance. An example is polyurethane coatings and products that are excellent for both water and particulate matter that is air-driven. Besides such applications as vehicles, these materials are used in the interior of sand and shot blast cabinets where they are constandy exposed to this type of stress. These materials are fabricated into liners in hoses for carrying pneumatically conveyed materials such as sand blasting hoses and for conveyor hose for a wide variety of materials such as sand, grain, and plastics pellets.

The method of minimizing the effects of erosion produced when the surface impact loading by fluid-borne particulate matter, liquid or solid, or cavitation loading is encountered, relates to material selection and modification. The plastics used should be ductile at impulse loading rates and capable of absorbing the impulse energy and dissipating it as heat by hysteresis effects. The surface characteristics of the materials in terms of wettability by the fluid and frictional interaction with the solids also play a role. In this type of application the general data available for materials should be supplemented by that obtained under simulated use conditions since the properties needed to perform are not readily predictable.

Cavitation

Another rapid loading condition in underwater applications is the application of external hydrostatic stress to plastic structures (also steel, etc.). Internal pressure applications such as those encountered in pipe and tubing or in pressure vessels such as aerosol containers are easily treated using tensile stress and creep properties of the plastic with the appropriate relationships for hoop and membrane stresses. The application of external pressure, especially high static pressure, has a rather unique effect on plastics. The stress analysis for thick walled spherical and tubular structures under external pressure is available.

The interesting aspect that plastics have in this situation is that the relatively high compressive stresses increase the resistance of plastic materials to failure. Glassy plastics under conditions of very high hydrostatic stress behave in some ways like a compressible fluid. The density of the material increases and the compressive strength are increased. In addition, the material undergoes sufficient internal flow to distribute the stresses uniformly throughout the product. As a consequence, the plastic products produced from such materials as acrylic and polycarbonate make excellent view windows for undersea vehicles that operate at extreme depths where the external pressures are 7MPa (1000 psi) and more.

With increasing ship speeds, the development of high-speed hydraulic equipment, and the variety of modem fluid-flow applications to which metal materials are being subjected, the problem of cavitation erosion becomes more important since it was first reported during 1873 (Chapter 8). Erosion may occur in either internal-flow systems, such as piping, pumps, and turbines, or in external ones like ships' propellers.

This erosion action occurs in a rapidly moving fluid when there is a decrease in pressure in the fluid below its vapor pressure and the presence of such nucleating sources as minute foreign particles or definite gas bubbles. Result is the formation of vapor bubble that continues to grow until it reaches a region of pressure higher than its own vapor pressure at which time it collapses. When these bubbles collapse near a boundary, the high-intensity shock waves (rapid loading) that are produced radiate to the boundary, resulting in mechanical damage to the material. The force of the shock wave or of the impinging may still be sufficient to cause a plastic flow or fatigue failure in a material after a number of cycles.

Materials, particularly steel, in cavitating fluids results in an erosion mechanism that includes mechanical erosion and electrochemical corrosion. Protection against cavitation is to use hardened materials, chromium, chrome-nickel compounds, or elastomeric plastics. Also used are methods to reduce the vapor pressure with additives, add air to act as a cushion for the collapsing bubbles, reduce the turbulence, and/ or change the liquid's temperature.

Rain

As it has been reported since the 1940s as one walks through a gende spring rain one seldom considers that raindrops can be small destructive "bullets" when they strike high-speed aircraft. These rapid loaded bullet-like raindrops can erode paint coatings, plastic products, and even steel, magnesium or aluminum leading edges to such an extent that the surfaces may appear to have been sandblasted. Even the structural integrity of the aircraft may be affected after several hours of flight through rain. Also affected are commercial aircraft, missiles, highspeed vehicles on the ground, spacecraft before and after a flight when rain is encountered, and even buildings or structures that encounter high-speed rainstorms. Critical situations can exist in flight vehicles, since flight performance can be affected to the extent that a vehicle can be destroyed.

First reports on rain erosion on aircraft were first reported during WW II when the B-29 bomber was flying over the Pacific Ocean. Aerodynamic RP radar wing-type shaped structure on the B-29 was flying at a so called (at that dme) high-speed was completely destroyed by rain erosion (DVR was a flight engineer on B-29). The "Eagle Wing" radome all-weather bomber airplanes were then capable of only flying at 400 mph. The aluminum aerodynamic leading edges of wings and particularly of the glass-fiber-reinforced TP polyester-nose radomes were pardcularly suscepdble to this form of degradadon. The problem continues to exist, as can be seen on the front of commercial and military airplanes with their neoprene protective coated RP radomes; the paint coadng over the rain erosion elastomeric plasdc erodes and then is repainted prior to the catastrophic damage of the rain erosion elastomeric coating.

Extensive flight tests conducted to determine the severity of the rain erosion were carried out in 1944. They established that aluminum and RP leading edges of airfoil shapes exhibited serious erosion after exposure to rainfall of only moderate intensity. Inasmuch as this problem originally arose with military aircraft, the U.S. Air Force initiated research studies at the Wright-Patterson Development Center's Materials Laboratory in Dayton, Ohio (DVR department involved; young lady physicist actually developed the theory of rain erosion that sdll applies). It resulted in applying an elastomeric neoprene coadng adhesively bonded to RP radomes. The usual 5 mil coadng of elastomeric material used literally bounces off raindrops, even from a supersonic airplane traveling through rain. Even though a slight loss (1%/mil of coating) of radar transmission occurred it was better than losing 100% when the radome was destroyed.

To determine the type of physical properdes materials used in this environment should have, it is necessary to examine the mechanics of the impact of the particulate matter on the surfaces. The high kinetic energy of the droplet is dissipated by shattering the drop, by indenting the surface, and by frictional heating effects. The loading rate is high as in impact and impulse loading, but it is neither as localized as the impact load nor as generalized as the impulse load. Material that can dissipate the locally high stresses through the bulk of the material will respond well under this type of load. The plastic should not exhibit brittle behavior at high loading rates.

In addition, it should exhibit a fairly high hysteresis level that would have the effect of dissipating the sharp mechanical impulse loads as heat. The material will develop heat due to the stress under cyclical load. Materials used are the elastomeric plastics used in the products or as a coating on products.

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