Mechanical Testing of Hard Materials

In this contribution we review current knowledge of the mechanical properties of hard materials and the relation between these properties and their microstructure. However, a quantitative relationship is only meaningful to the extent that test methods are clearly defined and understood, while the test data are only useful within the context of the engineering requirements which determine material selection for a specific application. It is also important to define what is meant by a hard material . 

From the engineering perspective, a hardness test is an ideal method for monitoring the mechanical properties of hard materials, since minimal sample preparation is required and the test can be performed on actual components using simple apparatus operated at low loads. The hardness test may also be considered nondestructive , since components can often be put into service after testing. The mechanical performance of hard materials is often ranked by their indentation hardness, which in itself accounts for both the popularity and the technological success of this simple, cost effective test. 

Figure 8.11. Dynamic load-reversal mechanical test of hard-elastic PP film. As a function of the elapsed time, t, uauostructural parameters (diagram in the middle) and the macroscopic parameters elongation, e (top), and tensile stress, a (bottom) are displayed. The middle diagram shows the long period, L (solid line), the lateral extension (dashed) of the layers, and a quantity S/V (solid line with dots) which is a measure of the number of lamellae in the material. Vertical bai s indicate zones of strain-induced crystallization (dark gray) and relaxation-induced melting (light gray), respectively...

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

The mechanical properties of a material describe how it responds to the application of either a force or a load. When this is compared to an area, it is called stress, another term for pressure. Three types of mechanical stress can affect a material tension (pulling), compression (pushing), and shear (tearing). Figure 15.27 shows the direction of the forces for these stresses. The mechanical tests consider each of these forces individually or in some combination. For example, tensile, compression, and shear tests only measure those individual forces. Flexural, impact, and hardness tests involve two or more forces simultaneously. 

Chemical, Physical, and Mechanical Tests. Manufactured friction materials are characterized by various chemical, physical, and mechanical tests in addition to friction and wear testing. The chemical tests include thermogravimetric analysis (tga), differential thermal analysis (dta), pyrolysis gas chromatography (pgc), acetone extraction, liquid chromatography (lc), infrared analysis (ir), and x-ray or scanning electron microscope (sem) analysis. Physical and mechanical tests determine properties such as thermal conductivity, specific heat, tensile or flexural strength, and hardness. Much attention has been placed on noise /vibration characterization. The use of modal analysis and damping measurements has increased (see Noise POLLUTION AND ABATEMENT). 

In hardness estimation with a view to mechanical characterization of a material, mineralogy went no further than the pictorial scale devised 177 years ago by Mohs (1812), which is still used owing to its great testing simplicity and the extraordinarily apt choice of standard minerals (Fig. 1.1). 

In Fig. 2.2 an overview about some fundamental experimental methods in short-term mechanical testing of polymers and the related typical loading speeds are given which form the basis to determine basic material properties such as ductility, strength, stiffness, toughness and hardness to be explained more detailed in the following. 

A very convenient and simple way to compare the mechanical strength of two materials is to compare their hardness. The testing device used to measure the hardness is a Zwick type 3117 apparatus. The hardness is measured by pushing a needle into the material with a certain force (fixed value). The depth of the indentation is related to the hardness (hardness scale Shore D). Materials with a hardness between 70-80 Shore D are considered to he very hard. 

The indentation test is one of the simplest ways to measure mechanical properties of a material. The micromechanical behavior of polymers and the correlation with microstrnctnre and morphology have been widely investigated over the past two decades (23). Conventional microindentation instruments are based on the optical measnrement of the residual impression produced by a sharp indenter penetrating the specimen surface under a given load at a known rate. Microhardness is obtained by dividing the peak load by the contact area of impression. From a macroscopic point of view, hardness is directly correlated to the yield stress of the material, ie, the minimnm stress at which permanent strain is produced when the stress is snbseqnently removed. 

Indentation tests can be used to determine the hardness and the elastic modulus of a material (Tabor 2000). Several tests are proposed using different types of indenters such as spherical, three-sided pyramidal, or four-sided Vickers, and the tests are performed not only on a macroscopic scale but also on a microscopic one Measurement of the hardness of very small samples is called nanoindentation. The advantage of applying the indentation test is that only a small sample and portable equipment are needed to measure the mechanical properties of the material. 

Figure4.10 presents the nanostructure evolution during the mechanical test for the material injection molded from the hottest melt (235 °C). A plain microfibrillar pattern is observed from the beginning. During the test the microfibrils narrow (vertical bar-shaped reflections move closer to the meridian), and the intensity in the central meridional streak is changing. An ordinary nanostructure and its response to strain is not observed. An ordinary nanostructure would exhibit distinct peaks along the meridian that clearly move outward with increasing strain instead of a meridional streak. In other materials that are studied in straining tests (cf. Chaps. 5 and 6) we have observed such distinct peaks moving. They indicate a well-defined preferential distance between hard domains instead of an extremely broad distribution of distances. Nevertheless, let us relate the position of the peak maximum on the meridian...

The microhardness technique is used when the specimen size is small or when a spatial map of the mechanical properties of the material within the micron range is required. Forces of 0.05-2 N are usually applied, yielding indentation depths in the micron range. While microhardness determined from the residual indentation is associated with the permanent plastic deformation induced in the material (see section on Basic Aspects of Indentation), microindentation testing can also provide information about the elastic properties. Indeed, the hardness to Young s modulus ratio HIE has been shown to be directly proportional to the relative depth recovery of the impression in ceramics and metals (2). Moreover, a correlation between the impression dimensions of a rhombus-based pyramidal indentation and the HIE ratio has been found for a wide variety of isotropic poljuneric materials (3). In oriented polymers, the extent of elastic recovery of the imprint along the fiber axis has been correlated to Young s modulus values (4). 

Tests for indention under load are performed basically like the ASTM measure the hardness of other materials, such as metals and ceramics. There are at least four popular hardness scales in use. Shore A and Shore D is for soft to relatively hard plastics and elastomers. Barcol is used from the mid-range of Shore D to above it as well as RPs. Rockwell M is used for very hard plastics (Chapter 5, MECHANICAL PROPERTY, Hardness),... 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Determination of die mechanical properties of a cured polymer serves to characterize its macroscopic (bulk) features such as flexibility and hardness. Using standardized methods of the American Society for Testing and Materials (ASTM) and die International Standards Organization (ISO) allows direct comparison to otiier materials. The vast majority of polyurethane research and development is conducted in industry where mechanical properties are of vital importance because tins information is used to design, evaluate, and market products. General test categories are presented here with a few illustrative examples. 

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