Impact Behavior of Plastics

There are other tests used to evaluate the impact behavior of plastics, including many special setups just for evaluating specific products. These special test setups are usually applicable only to one company or association that has found them to be proven useful. Some of these special tests have then become standards in their own industry, such as ASTM and UL guidelines. Among the more popular ASTM standards are its D 256, D 1709, D 1822, D 2289, D 3029, D 3099, D 3420, and a few others. In these tests including the special ones, a material is impacted by using various devices such as a ball on a pendulum or puncture tests (see Fig. 3-85), air-driven spherical or piercing-type missiles and others. 

The impact behavior of plastic materials is strongly dependent upon the temperature. At lower temperatures the impact resistance is reduced drastically. The reduction in impact is even more dramatic near the glass transition temperature. Conversely, at higher test temperatures, the impact energy is significantly improved. 

For many engineering applications, impact fracture behavior is of prime practical importance. While impact properties of plastics are usually characterized in terms of notched or un-notched impact fracture energies, there has been an increasing tendency to also apply fracture mechanics techniques over the last decade [1, 2 and 3]. For quasi-brittle fracture, a linear elastic fracture mechanics (LEFM) approach with a force based analysis (FBA) is frequently applied to determine fracture toughness values at moderate loading rates. 

Temperature strongly influences the impact behavior of toughened plastics. Charpy impact energy measurements at different temperatures in the case of HIPS containing various concentrations of PBD showed two transitions, at 233 and 273 K [Bucknall, 1988]. At these temperatures, the material exhibited transitions from brittle to semi-ductile and then to ductile. 

A new part to ISO 179 is under preparation that will cover the instrumentation of the Charpy pendulum so that force-time (and by integration, force-dcflection) curves can be obtained. This allows for a fuller characterization of the impact behavior of the plastic than can be derived from only the energy to break of the typical test. There has been an instrumented version of the falling weight impact test (see later in this section) for several years and the same principles apply to both. 

Temperature has a pronounced effect on the impact strength of plastics. In common with metals, many plastic materials exhibit a transition from ductile behavior to brittle as the temperature is reduced. The variation of impact strength with temperature for several common thermoplastics is shown in Figure 3.38. 

In some applications impact properties of plastics may not be critical, and only a general knowledge of their impact behavior is needed. In these circumstances the information provided in Table 3.1 would be adequate. The table lists the impact behavior of a number of commonly used thermoplastics over a range of temperatures in three broad categories [22]. 

Also, the AUC (area under curve) of the different materials represents their resilience. Cast iron and ceramics are very brittle steel, copper, and aluminum, as well as the thermoplastics PA and PP, are highly deformable and can therefore absorb large amounts of energy, for example from (impact) load application. It must be remembered here that the deformation behavior of plastics is highly dependent on time and temperature factors (see Fig. 15). Simplified explanations of deformation terminology follow. 

As with tensile and impact behavior of rubber-toughened plastics, a major energy-absorbing mechanism appears to be crazing. Thus, at least qualitatively, low-frequency fatigue behavior of rubber-modified plastics appears to involve the same phenomena as are seen in tensile and impact loadings. 

Standardized notched impact tests such as the Izod and Charpy tests (ASTM, ISO, DIN) are the most commonly used to characterize the impact strength of plastic materials. It is very difficult to use measured data from tests using idealized laboratory specimens to predict impact behavior of end-use polymeric material. The apparent lack of good correlation between measured impact fracture energy and end-use impact resistance is due to the extreme complexity of microscopic fracture processes. In particular, the influence of specimen geometry is sometimes poorly matched with the type of failure mechanism of defects present in the actual molded part subjected to end-use impact forces. 

It is important to note that certain properties of these blends are better than those of the parent homopolymers. Adding PE to PP allows improvement of the low temperature impact behavior of PP [12]. In turn, the PP content contributes to the increase of toughness, high modulus and heat resistance to PE [13]. The PP/LDPE blends can be used as engineering plastics, consumer items for household use, goods with improved performance, etc. 

The viscosity behavior of plastics makes them sensitive to strain rates as well as temperatures (see Chapters 2, 3, and 4). It therefore becomes important to define the rate, magnitude, duration, and type of mechanical stress and strain loading (i.e., tension, compression, flexure, and shear) along with temperatures during loading. The rate and duration of loading also determine whether creep or impact will be a factor in a given part s mechanical response [1, 2, 5-14, 29, 33,40-43, 55-68, 152, 202, 225, 235, 250, 270-74, 808]. 

Plastics are viscoelastic. Their behavior is partly elastic and partly that of a very viscous fluid. Properties of strength and rigidity vary with amount of stress, the rate of loading, and the temperature at which the stress is applied. Viscoelastic behavior requires performance tests to measure time dependence. The viscoelasticity of plastics also severely limits the usefulness of many short-time tests such as impact, tensile, and flexural strengths and modulus. Unfortunately, such test data are very widespread because they are easier and cheaper to obtain than time- and temperature-dependent information. These data can cause much confusion and disappointment when used for plastics. Short-time data are useful for quality control and specification purposes, and if properly interpreted, can shed some light on plastic performance. However, they cannot be used in design and are more often than not misleading because they do not account for the viscoelastic behavior of plastics. 

The following discussion is based on results obtained from the notched Izod or Charpy methods, as these are the most widely used tests to determine impact behavior in plastics. Even with the above-stated limitations, these protocols provide a useful method for discriminating between materials with different degrees of toughness. 

The properties presented in the table show average impact resistance values for engineering plastics and alloys. Final fabrication conditions can significantly affect their impact resistance, particularly where crystallinity is present, as is the case in most engineering polymers. Also noteworthy in this series is the impact behavior of polycarbonate, which responds in a ductile fashion in thin samples, even under notched conditions. Thicker specimens are brittle. Tougheners have to be incorporated to re-establish the original ductility. A similar behavior is observed for the other systems, although not to the same extent as in polycarbonate. 

ISO 6603 [320] Plastics - Determination of puncture impact behavior of rigid plastics... 

Impact behavior is one of the most widely specified mechanical properties of the polymeric materials. However, it is also one of the least understood properties. Predicting the impact resistance of plastics still remains one of the most troublesome areas of product design. One of the problems with some earlier Izod and Charpy impact tests was that the tests were adopted by the plastic industry from metallurgists. The principles of impact mechanisms as applied to metals do not seem to work satisfactorily with plastics because of the plastics complex structure. 

A notch in a test specimen or a sharp corner in a fabricated part drastically lowers the impact energy. A notch creates a localized stress concentration and hence the part failure under impact loading. All plastics are notch-sensitive. The rate of sensitivity varies with the type of plastics. Both notch depth and notch radius have an effect on the impact behavior of materials. For example, a larger radius of curvature at the base of the notch will have a lower stress concentration and, therefore, a higher impact energy of the base material. Thus it is obvious from the above discussion that while designing a plastic part, one should avoid notches, sharp corners, and other factors that act as stress concentrators. 

The elevated temperature performance of most plastics is dominated by the temperature of occurrence and the temperature range of the glass transition and, in the case of semicrystalline plastics, by the crystalline melting point. The glass transition has a profound negative effect on all mechanical properties except impact, and on certain thermal properties such as the coefficient of expansion. Therefore, a knowledge of the transitional behavior of plastics is necessary in order to understand their elevated temperature capabilities. This knowledge is best derived from... 

---