Measurement of mechanical properties

Strength has been widely measured for AB cements. It is formally defined as the force experienced by a material at the point where fracture occurs (Gillam, 1969). The study of strength is complicated in that fracture is a point of discontinuity, so cannot be readily interpreted in terms of events leading up to it. 

Strength can be measured in compression, in tension, in shear and transversely (flexural strength). However, if we exclude plastic flow as a means of failure, then materials can only fracture in one of two ways (1) by the pulling apart of planes of atoms, i.e. tensile failure, or (2) by the slippage of planes of atoms, i.e. shear failure. Strength is essentially a measure of fracture stress, which is the point of catastrophic and imcontrolled failure because the initiation of a crack takes place at excessive stress values. 

AB cements tend to be essentially brittle materials. This means that when subjected to mechanical loading, they tend to rupture suddenly with minimal deformation. There are a number of different types of strength which have been identified and have been determined for AB cements. These include compressive, tensile and flexural strengths. Which one is determined depends on the direction in which the fracturing force is applied. For full characterization, it is necessary to evaluate all of these parameters for a given material no one of them can be regarded as the sole criterion of strength. 

The most common mechanical property of cements that has been measured routinely is compressive strength (Polakowski Kipling, 1966). Measurement is easy to carry out but there are several reasons to consider that the results from the technique are unsatisfactory. Interpretation of results is uncertain because of the complexities in the mode of failure. Minor imperfections in the material lead to localized stress concentrations which affect the magnitude of the result. 

The variation in the mode of failure makes comparison of different types of cement quite impossible. As Darvell (1990) has pointed out, compressive strength is not a material property under any condition, but can only be used to compare materials of a very similar nature. 

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The measurement of mechanical properties is a major part of the domain of characterisation. The tensile test is the key procedure, and this in turn is linked with the various tests to measure fracture toughness... crudely speaking, the capacity to withstand the weakening effects of defects. Elaborate test procedures have been developed to examine resistance to high-speed impact of projectiles, a property of civil (birdstrike on aircraft) as well as military importance. Another kind of lest is needed to measure the elastic moduli in different directions of an anisotropic crystal this is, for instance, vital for the proper exploitation of quartz crystal slices in quartz watches. 

One of the more recently exploited forms of thermal analysis is the group of techniques known as thermomechanical analysis (TMA). These techniques are based on the measurement of mechanical properties such as expansion, contraction, extension or penetration of materials as a function of temperature. TMA curves obtained in this way are characteristic of the sample. The technique has obvious practical value in the study and assessment of the mechanical properties of materials. Measurements over the temperature range - 100°C to 1000°C may be made. Figure 11.19 shows a study of a polymeric material based upon linear expansion measurements. 

Thus, the introduction effect of long-chain alkyl groups into the DAP resins was reflected in the improved flexibiiity. In particular, LMA and SMA as comonomers showed a remarkable effect for example, the DAP resin cocured with 10 mol% of LMA had the tensile strength of 600 kg/cm and the elongation of 9.0%, although the DAP resin obtained by homopolymerization was quite brittle and, therefore, could not even be subjected to the measurement of mechanical properties as mentioned above. 

All deformations used in conventional measurements of mechanical properties are interpreted in terms of homogeneous deformations. 

The estimation of degree of dispersion can be made indirectly by measurement of electrical methods or measurement of mechanical properties. Boonstra54 used a coaxial electrode system to estimate dispersion form electrical resistivity whilst Belokur et al55 investigated the possibility of assessing dispersion from rheological measurements. 

There is at present available in the literature on polymers and on materials science a wealth of information regarding measurements of mechanical properties. These properties are dependent upon many relevant physical parameters and most measurements take this into account. There is also available a great deal of information regarding the relations between molecular structure and macroscopic physical properties and many calculations have been made. The bridge between these two extremes (the macro and the micro) is constructed primarily by the use of models of structure. 

For the materials scientist the measurement of mechanical properties provides information about internal structure and such measurements afford an opportunity to test theories of structure. In consequence a range of tests has been developed by materials scientists which supplement the, usually simpler, engineering tests and which in many cases provide information of no obvious value to the engineer. There are, however, many ad hoc tests used by the practical man which provide information, applicable by the materials scientist, but often in a confusing or complicated way. 

Benlahsen, M., Lepinoux, L. and Grilhe, J. (1993), Image forces on dislocations the elastic modulus effect , Materials Science and Engineering, A164, 428 132. Bolshakov, A., Oliver, W.C. and Pharr, G.M. (1996), Influences of stress on the measurement of mechanical properties using nanoindentation Part II. Finite element simulations , Journal of Materials Research, 11, 760-768. 

Dobraszczyk, B.J., and Vincent, J.F.V. (1999). Measurement of mechanical properties of food materials in relation to texture the materials approach. In A.J. Rosenthal (ed.). Food Texture Measurement and Perception, Aspen Publishers, Gaithersburg, MD, pp. 99-151. 

Mehregany, M., Allen, M. G., and Senturia, S. D., "Use of Micromachined Structures for the Measurement of Mechanical Properties and Adhesion of Thin Films", IEEE 1986 Workshop on Solid-State Sensors, Hilton Head NC, June 1986. 

Hardness testing, in the past, has been mainly used as a simple, rapid, nondestructive production control test, as an indication of cure of some thermosetting materials, and as a measure of mechanical properties affected by changes in chemical composition, microstructure and ageing. 

A comprehensive review of the measurement of mechanical properties such as fracture strength or adhesion strength can be found in the literature [13]. In general, the best way to determine mechanical properties is to test microma-chined structures that are as close to the actual design as possible using, for example, the micromanipulator developed at the University of Uppsala [14]. 

Preliminary measurements of mechanical properties of polypropylene (PP) - polycarbonate (PC) blends revealed that the blends containing about 10 wt.% PP maintain several properties at the level of pure PC. Some of the properties (e.g. tensile yield, Young s modulus or impact strength) of these blends are even higher than those of pure PC We have recently measured the electrical... 

Maxima of TSD current, attributed to Tg of PC, Tge vs. the composition of PP-PC blends, are shown in Fig. 8. The dynamic glass transition temperature, Tg, from measurements of mechanical properties is also ahown in Fig. 8 for the sake of comparison. It is... 

In spite of the visible heterogeneity of PP-PC blends, measurements of the electric properties confirm the existence of some interactions between the components. This finding agrees with the previous measurements of mechanical properties. 

As discussed in the Introduction, if equivalent information is obtained from both direct and indirect measurements of mechanical properties, then the fluctuation-dissipation theorem is applicable (1). The exact proof of the theorem has already been made. However, an experimental verification is still useful for experimental chemists in an understanding of this theorem. In order to carry out the verification, the spin-lattice relaxation times are con ared with corresponding mechanical properties for fresh silk san les. A linear relationship between the two measurements would then constitute an experimental verification, at least for these sanq>les. It may be noted that experimental verification does not imply a general proof because this is not a rigorous mathematical proof... 

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