The assessment of impact strength

It is probably most useful to consider toughness as a property of a plastics part under some specified conditions of service. Whilst it is possible to devise impact tests and to rank a series of plastics materials according to the results obtained in such tests it remains almost impossible to use such tests to try to predict whether or not an article made from a specific material will or will not be satisfactory in service. 

Amongst the factors that will influence service performance are the effect of additives and impurities, temperature, detailed geometric size and shape, orientation and morphology, surface condition, energy and speed of any impacting blow, the shape of the impacting instrument, the environment, and strains in the article due to external loads. For this reason it is desirable, but not always feasible, to test prototype articles under conditions as close to service conditions as possible. 

Impact tests are, however, used to try to compare the impact strength of different materials. Of these tests four require specific mention. These are the Izod test, the Charpy test, the falling weight tests and the tensile impact test. 

Of these the most well known is the Izod test. This consists of a bar, one end of which is held in a vice, the sample being held vertically. The bar is then struck by a striking device under controlled conditions at a specified point above the vice. The energy required to break the sample is noted. It is common to have a notch in the bar which is located during the test at the top of the vice and on the 

Whilst Izod tests are, perhaps surprisingly, generally reproducible it has been found impossible to scale up the results and thus predict the energy required to break a bar of large cross-section from results obtained with a bar of smaller cross-section. This has led to some uncertainty in the method of quoting results and the following approaches are used  

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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 pioneering approach to impact assessment, the Leopold Matrix, was developed by Dr. Luna Leopold and others of the United States Geological Survey [6,10,74-75]. The matrix was designed for the assessment of impacts associated with almost any type of construction project. Its main strength is as a checklist that incorporates qualitative information on cause-and-effect relationships, but it is also useful for commimicating results. 

The Izod test is used mostly in the United States and the United Kingdom [British Standard (BS) 2782, Method 306A]. The notched specimen is firmly clamped in a vertical position in a vise fixed in the base of the apparatus (Fig. 1). A pendulum fitted with a striker head of 3.2-mm radius, falling from a height of 0.61 m (ASTM) or 0.31 m (BS), hits the specimen horizontally at a point above the notch. At least 10 samples are required to obtain a satisfactory assessment of impact strength. 

In a series of reviews [244-246] the models proposed for the assessment of the effect of fillers on the complex of PCM properties are discussed. Analysis of the models shows that, for a fixed filler content, the strength must be higher in compositions with fillers featuring the absolute adhesion to the matrix than in systems with little or no adhesion. The relative elongation and specific impact strength must, on the contrary, go up with the increasing adhesion. 

To provide a better understanding of toxic impacts on aquatic ecosystems, cause-effect relationships between changes in biodiversity and the impact of environmental pollution as causative factor as well as the underlying processes. This included the assessment of sub-lethal effects in vitro and in vivo as early warning strategies and of their strength to predict potential hazards to the ecosystem. 

Toughness tests are traditionally carried out at 20°C (-bS F) or dO C (68°F). This quantity is related to the notched Izod impact strength. It includes an assessment of the overall fracture toughness of the polymer. 

It is critical that surface treatment conditions be optimized to composite properties since overtreatment as well as undertreatment will degrade composite properties. Typically composite interlaminar shear strength (ILSS), in-plane shear, and transverse tension ate used to assess the effectiveness of surface treatment. More recently damage tolerance properties such as edge delamination strength, open hole compression, and compression after impact have become more important in evaluating the toughness of composite parts. 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

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