Impact strength defined

It may be seen from Fig. 2.80 that some plastics experience the change from ductile to brittle behaviour over a relatively narrow temperature range. This permits a tough/brittle transition temperature to be quoted. In other plastics this transition is much more gradual so that it is not possible to attribute it to a single value of temperature. In these circumstances it is common to quote a Brittleness Temperature, rg(l/4). This temperature is defined as the value at which the impact strength of the material with a sharp notch (1/4 mm tip radius) equals 10 kJ/m. This temperature, when quoted, gives an indication of the temperature above which there should be no problems with impact failures. It does not mean that the material should never be used below Tb(1/4) because by definition it refers only to the sharp notch case. When the material has a blunt notch or is un-notched its behaviour may still be satisfactory well below Tb(1/4). 

The study of the blends has shown a lack of compatibility between PVC and CPVC and a substantial decay of all the physical properties with the exception of the impact strength. This demonstrates the need to specify at well-defined limits the amount of PVC used as an aid in processing. 

The protective medium against fire discussed here is a water-insoluble mastic. Mastic is defined as a material of heavy paste consistency which can be applied with a putty knife, trowel, brush, or heavy duty spray equipment to yield a coating from 1/32 inch to one inch or more. These mastics are of mineral composition, bound by an organic resinous medium, and dry to ultimate hardness by the volatilization of an organic solvent. They are further characterized by high impact strength, and assistance to abrasion, water, and many chemicals. 

The mechanical properties of polymers can be expressed in a number of ways. Tensile modulus describes the stiffness of a material and provides information about its potential usefulness. Toughness is defined as the work required to elongate a sample to the breaking point. Polymers with high impact strength resist cracking or breaking when struck by a hard object. 

Impact strength is the resistance to breakage under high-velocity impact conditions. This property is of great practical importance, but extremely difficult to define in scientific terms. 

The test may be carried out on plain rectangular bars, but most often a carefully defined notch is molded or machined into the face to be struck (Figure 12.8). The impact tests are often regarded as a means of assessing the resistance of a material to shock where notches or stress raisers generally are present. The ratio of impact strength of unnotched to that of notched specimen is sometimes regarded as a measure of the notch sensitivity of a material. 

The Izod impact strength (r) of binary and ternary blends containing Epcar (solid lines) and Dutral (dotted lines) copolymers are shown in Fig. 1, as a function of the binary HDPE weight ratio defined as ... 

Somewhat more sophisticated instruments are frequently used for so-called instrumented impact tests. These instruments carry devices for measuring quantities such as the force applied to the sample and its displacement as a function of time and they allow the impact energy to be obtained more directly. Bending tests are, however, frequently carried out in a less sophisticated way using specimens without notches. The energy of fracture divided by the area of fracture is then called the impact strength. This quantity cannot be simply related to the quantities such as the critical strain-energy-release rate defined in fracture mechanics. 

Impact Resistance of Plastic Film by the Free-Falling Dart Method. Primary Film Test Method ASTM D1709 Additional Test Method ISO 7765-1. A weighted dart is dropped from a standard height onto a taut sample. Depending upon the expected impact strength of the test sample, either method A or method B is chosen. The method defines the dart size and the drop height for the dart. Test method A specifies a dart with a 38 mm diameter dropped from 0.66 m. Test method B specifies a dart with a 51 mm diameter dropped from 1.5 m. 

The impact strength of the polymeric materials is directly related to the overall toughness of the material. Toughness is defined as the ability of the polymer to absorb applied energy. The incorporation of elastomeric phase, such... 

The most common tests for impact strength are the Izod and Charpy tests. In these tests, a short beam specimen, which may be notched, is struck and broken by a swinging pendulum. The impact strength is defined as the energy needed to break the specimen. Both these tests are arbitrary in their specification. By changing the test conditions quite different results may be obtained and therefore the tests are usually used for quality control and comparison of different materials only. 

TPO materials are defined as compounds (mixtures) of various polyolefin polymers, semicrystalline thermoplastics, and amorphous elastomers. Most TPOs are composed of polypropylene and a copolymer of ethylene and propylene called ethylene—propylene rubber (EPR) [2]. A common rubber of this type is called ethylene propylene diene monomer rubber (EPDM), which has a small amount of a third monomer, a diene (two carbon-carbon double bonds in it). The diene monomer leaves a small amount of unsaturation in the polymer chain that can be used for sulfur cross-linking. Like most TPEs, TPO products are composed of hard and soft segments. TPO compounds include fillers, reinforcements, lubricants, heat stabilizers, antioxidants, UV stabilizers, colorants, and processing aids. They are characterized by high impact strength, low density, and good chemical resistance they are used when durability and reliability are primary concerns. 

Standard test methods for the determination of Charpy impact strength are Usted in Table 10.11. For example, BS 2782 (Method 351A) uses a rectangular, notched or unnotched, bar supported at both ends in such a position that the pendulum strikes it in the center, directly behind the notch. The energy absorbed in the impact is read directly by means of a pointer from a scale calibrated to allow for frictional and windage errors. Three standard test pieces are defined - the preferred being 120 x 15 x 10 mm with a span of 70 mm. The standard impact energies are 0.5, 1,4, 15, and 50 J. 

---