2.6 IMPACT PROPETIES:
2.6.1 INTRODUCTION:
The impact properties of the polymeric materials are directly related to the overall toughness of the material. Toughness is defined as the ability of the polymer to absorb applied energy. The area under the stress strain curve is directly proportional to the toughness of the material. The higher the impact energy of the material, the higher the toughness of the material and vice versa. Impact resistance is the ability of a material to resist breaking under a shock loading or the ability to resist the fracture under stress applied at high speed. The theory behind toughness and brittleness of the polymers is very complex and therefore difficult to understand. The molecular flexibility plays an important role in determining the relative brittleness of toughness of the material. For example, in stiff polymers like polystyrene and acrylics, the molecular segment are unable to disentangle and respect to the rapid application of mechanical stress and the impact produces brittle failure. In contrast, flexible polymers such as plasticized vinyls have high impact behavior due to the ability of the large segment of molecules to disentangle and respond rapidly to mechanical stress.
Impact properties of the polymer are often modified simply by adding an impact modifier such as butadiene rubber of certain acrylic polymers. The addition of a plasticizer also improves the impact behavior at the cost of rigidity. Another way to improve impact properties is to use fibrous fillers that appear to act as stress transfer agent. Most polymers when subjected to the impact loading seem to fracture in a characteristic fashion. The crack is initiated on a polymer surface due to the impact loading. The energy required to initiate such a crack is called the crack initiation energy. If the load exceeds beyond the crack initiation energy then crack continue to propagate. A complete failure occurs when the energy applied goes beyond the crack propagation energy. Thus, both crack initiation as well as the crack propagation contributes to the impact property measurement. There are four types of failure mode when material is given an impact load[2,12].
BRITTLE FAILURE: In this type of failure, the part fractures extensively with out yielding. A catastrophic mechanical failure such as the one in the general-purpose polystyrene is observed.
SLIGHT CRACKING: the part shows slight cracking and yielding with out losing its shape or integrity.
YIELDING: The part actually yields showing obvious deformation and stress whitening but no cracking-taking place.
DUCTILE FAILURE: This type of failure is characterized by a definite yielding of material along with cracking. Polycarbonate is considered a ductile material.
Impact properties are one of the wildly specified mechanical properties of the polymeric materials. However, it is also one of the least understood properties. Predicting the impact resistance of polymers remains one of the most troublesome areas of product design. One of the with some earlier Izod and Charpy impact tests was that the tests were adopted by the plastic industry from metallurgist. The principles of impact mechanism as applied to metals do not seem to work satisfactorily with plastic because of the plastic complex structure.
2.6.2 TYPES OF IMPACT TESTS:
In the last decade, tremendous amount of money and time has been spent on the research and development of various type of impact test by organization throughout the world. Attempts have been made to develop different sizes and shape of specimen as well as impact tester. The specimens have been subjected to a variety of impact loads including tensile, compression, flexural and torsion impacts. Impact loading has been applied using every thing from and hammer, punches, and pendulums of falling balls and bullets. Unfortunately, very little correlation exists, if any between the types of tests developed so far. Numerous technical papers and articles have been written on the subject of the advantage of one method over other. To this date, no industry wide consensus exists regarding an ideal impact test method. Impact testing is divided in to three major classes and subdivided in to several classes as follows,
Fig. 2.4 Classification of Impact Testing
PENDULUM IMPACT TEST:
IZOD IMPACT TEST (ASTM D-256)
fig 2.5 Izod Specimen Positioning
The objective of the above test is to measure the relative susceptibility of a standard specimen to the pendulum type impact loading. The results are expressed in terms of kinetic energy consumed by the pendulum in order to break the specimen. The energy required to break a standard specimen is actually the sum of energies needed to deform it, to initiate its fracture, and to propagate the fracture across it, and the energy expended in tossing the broken ends of specimen. This is called “toss factor”. The energy lost through the friction and vibration of the equipment is minimum for all practical application and usually neglected.
The specimen used in Izod test must be notched. The reason behind notching the specimen is to generate a stress concentration area that promotes a brittle rather than a ductile failure. A plastic deformation is prevented in the specimen by the presence of a notch. The impact values are severally affected because of the notch sensitivity of a certain plastic material. The Izod test requires a specimen to be clamped vertically as a cantilever beam. The specimen is struck by a swing of a pendulum released from a fixed clamp from the specimen clamp.
CHARPY IMPACT TEST (ASTM D-4812):
fig 2.6 Charpy Specimen Positioning
This test is conducted in a very similar manner to the Izod impact strength test. The only difference is the positioning of the specimen. In the test the specimen is mounted horizontally and supported unclamped at both ends. Only the specimen that breaks completely is considered acceptable. The Charpy impact strength is calculated by dividing the indicator reading by the thickness of the specimen. The results are reported in (feet-lbf/inch) of the specimen. The obvious advantage of the of the Charpy test over the Izod test is that the specimen does not have to be clamped and, therefore, it is free of variation in clamping pressure.
APPARTUS AND TEST SPECIMEN:
fig 2.7 Impact Testing Machine
The testing machine consisting of a heavy base with a vise for clamping the specimen in place during the test. In most of cases, the vise is designed so that the specimen can be clamped vertically for the Izod test or positioned horizontally for the Charpy test with out making any changes. A pendulum type hammer with an antifriction bearing is used. Additionally weights may be attached to the hammer for breaking tougher specimens. The pendulum is connected to a pointer and a dial mechanism that indicates the excess energy remaining in a pendulum after breaking the specimen. The dial is calibrated to read the impact values directly in inch. A hardened steel striking nose is attached to the pendulum. The Izod and Charpy test use different types of striking noses. The test specimen can be prepared either by molding or by cutting them from sheets. Izod test specimen are 2 ½ * ½ *1/8 (inch). A notch is cut into a specimen very carefully by a milling machine or a lathe. The recommended notch depth is 0.1 inch.
TEST PROCEDURE:
The test specimen is clamped into positioning so that the notched end of the specimen is facing the striking edge of the pendulum. The pendulum hammer is released, allowed to strike the specimen, and swing through. If the specimen does not break more weight is added to the hammer and test is repeated until failure occurs. The impact values are directly read from the scale. The impact values are calculated by dividing the impact values obtained from the scale by the thickness of the specimen. The reverse notch impact resistance is obtained by reversing the position of the notched specimen in the vise. In the case, the notch is subjected to compressive rather than tensile stress during impact.
Notching the specimen drastically reduces the energy loss due to deformation, and hence it can be neglected. Tough plastic material that have an Izod impact higher than .5 (feet-lb/inch) of notch seem to expand very little energy in tossing the broken end of the specimen.
2.6.3 EFFECT OF TEST VARIBLES AND LIMITATION:
NOTCH:
A slight variation in the radius and depth of a notch affects the impact strength results. Many other variables such as the cutter speed, sharpness of the cutting tooth, feed rate, type of plastic, and quality of the notch cutting equipment all seem to have a significant effect on the results. Such a variation is difficult to control and non-uniformity between the lots is quite common. Certain heat sensitive polymers are also affected by the high cutter speed that seems to contribute to the thermal degradation. The notch in the specimen tends to create a stress concentration area that produces unrealistically low impact values in crystalline plastics. Recommendations include, even time interval between notching and testing and waiting at least 40 hours after notching and before testing.
SPECIMEN THICKNESS:
Although the impact values reported are based on 1-inch thickness specimen, the actual thickness of the specimen used in the test influences the test results. This is especially in the case of polycarbonate.
SPECIMEN PREPARATION:
Injection molded specimens seem to yield higher impact strength values than compression molded specimens. This is due to the molecular orientation caused by the injection molding process. The location of the gate also has a significance effect on the test specimen’s results, particularly in the case of fiber-reinforced specimens.
TEMPERATURE:
Impact values increases with increase in the temperature and vice versa.
FILLERS AND OTHER ADDITIVES:
Fillers and reinforcements have pronounced effect on the test results. Fillers and reinforcement agents generally lower the impact values.
2.6.4 LIMITATION:
The results obtained from Izod and Charpy method cannot be directly used for part design these results does not give the true energy required to break the specimen. The notched Izod impact test measures only the notch sensitivity of the different polymers and not the toughness.
2.7 FLEXURAL PROPERTIES (ASTM D-790):
2.7.1 INTRODUCTION:
The stress strain behavior of polymers in flexure is of interest to a designer as well as a polymer manufacturer. Flexural strength is the ability of the material to with stand bending forces applied perpendicular to its longitudinal axis. The stress induced by the flexural load is a combination of compressive and tensile stresses. Flexural properties are reported and calculated in terms of the maximum stress and strain that occurs at the outside surface of the test bar. Many polymers do not break under flexure even after a large deflection that makes deformation of the ultimate flexural strength impractical for many polymers. In such cases, the common practice is to report flexural yield strength when the maximum strain in the outer fiber or the specimen has reached 5 percent. For polymeric materials that break easily under flexural load, the specimen is deflected until rupture occurs in the outer fibers.
There are several advantages of flexural strength tests over tensile tests. If a material is used in the form of a beam and if the service failure occurs in the bending, then a flexural test is more relevant for a design or specification purpose than a tensile test, which may give strength value very different from the calculated strength of the outer fiber in the bent beam. The flexural specimen is comparatively easy to prepare with out residual strain. The specimen alignment is also difficult in the case of a tensile test. Also the tight clamping of specimen in tensile create stress raising points. One other advantage of the flexural is that a small strain, the actual deformations are sufficiently large to be measured accurately.
Two basic methods cover the determination of flexural properties of the polymer. Method 1 is a three point loading utilizing center loading on a simple supported beam. A bar of rectangular cross section rests on two supports and it is loaded by means of a loading nose at the center. The maximum axial fiber stress occurs on a line under the loading nose. Method 2 is a four point loading system utilizing two load points equally spaced from their adjacent supporting points, with a distance between loading point of one third of the support span. In this test, the test bars lie on a two support and is loaded at two points each an equal distance from the adjacent support points. Either method can be used with the two procedures. Method1 is used for materials that break at comparatively low loads. Method2 is used particularly for those materials that undergo large deflections during testing. The basic difference between the two procedures is the strain rate. Method1 is .01 (inch/inch/min) and Method2 is .1(inch/inch/min).
2.7.2 APPARATUS:
Quiet often the machine used for tensile testing is also used for flexural testing. The upper or lower portion of the movable crosshead can be used for flexural testing. The dual-purpose load cell that indicates the load applied in the tension as well as compression facilitates testing of the specimen in either tension or compression. The machine used for this purpose should operate are a constant rate of cross head motion over the entire range and the error in the load measuring system should not exceed. The loading nose and support must have cylindrical surfaces. The radius of the nose and the nose support should be at least 1/8 inch. To avoid excessive indentation or failure due to stress concentration directly under the loading nose. A strain gauge type of mechanism called a deflectometer or a compression is used to measure deflection in the specimen.
2.7.3 TEST SPECIMEN AND CONDITIONING:
The specimen used for the flexural testing are bars of rectangular cross section and are cut from sheets, plates, or molded shapes. The common practice is to mold the specimen to the desired finish dimensions. The specimens are conditioned in accordance with the procedure A of ASTM methods D 618 as explained. The specimen of size 1/8 * ½ * 4(inch) are the most commonly used.
2.7.4 TEST PROCEDURE AND CALCULATION:
The test is initiated by applying the load to the specimen at the specified crosshead rate. The deflection is measured either by a gauge under the specimen in contact with it in the center of the support span or by measurement of the motion of the loading nose relative to the supports. A load deflection curve is plotted if the determination of flexural modulus value is desired. The fiber stress is related to the load and sample dimensions and is calculated using the following equation
Method 1 S = 3PL/2bd2
Where S= stress (psi), P= load (lb), L= length of span (in.), b=width of specimen (in.), d= thickness of specimen (in.).
Flexural strength is equal to maximum stress in the outer fibers at the moment of the break. This value can be calculated by using the above stress equation by letting load value P equal to the load at the moment of the break.
For materials that do not break at outer fiber strains up to 5 percent, the flexural yield strength is calculated using the same equation. The load value P in this case is the maximum load at which there is no longer an increase in load with an increase in deflection. The maximum strain of the fibers, which also occurs at mid span, is calculated using the following equation (method 1).
r = 6Dd/ L2
Where r = strain (in./in.), D = deflection (in.), L = length of the span (in.), d = thickness of the specimen (in.).
The equations for calculating maximum fiber stress and maximum fiber strain are slightly different in method 2.
2.7.5 FACTORS AFFECTING THE TEST RESULTS:
SPECIMEN PREPERATION:
The molecular orientation in the specimen has a significant effect on the test results. For example, the specimen with a high degree of molecular orientation perpendicular to the applied load will show higher flexural values than that of the specimen with orientation parallel to the applied load. The injection-molded specimen usually shows a higher flexural value than a compression-molded specimen does.
TEMPERATURE:
The flexural strength and modulus values are inversely proportional to the temperature. At higher testing temperature, flexural strength and modulus values are significantly lower.
TEST CONDITIONS:
The strain rate, which depends upon testing speed, specimen thickness, and the distance between supports (span), can affect the results. At a give span a flexural strength increases as the specimen thickness is increased. The modulus of the material generally increases with increasing strain rate.
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