Mechanical tests and behaviour

Fracture Mechanics Tests One problem of both sustained load and slow strain-rate tests is that they do not provide a means of predicting the behaviour of components containing defects (other than the inherent defect associated with the notch in a sustained load test). Fracture mechanics provides a basis for such tests (Section 8.9), and measurements of crack velocity as a function of stress intensity factor, K, are widely used. A typical graph of crack velocity as a function of K is shown in Fig. 8.48. Several regions may be seen on this curve. At low stress intensity factors no crack growth is... 

ApA < 1. In Fig. 2 the region of curvature is much broader and extends beyond — 4 < ApA < + 4. One explanation for the poor agreement between the predictions in Fig. 3 and the behaviour observed for ionisation of acetic acid is that in the region around ApA = 0, the proton-transfer step in mechanism (8) is kinetically significant. In order to test this hypothesis and attempt to fit (9) and (10) to experimental data, it is necessary to assume values for the rate coefficients for the formation and breakdown of the hydrogen-bonded complexes in mechanism (8) and to propose a suitable relationship between the rate coefficients of the proton-transfer step and the equilibrium constant for the reaction. There are various ways in which the latter can be achieved. Experimental data for proton-transfer reactions are usually fitted quite well by the Bronsted relation (17). In (17), GB is a... 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

The term dynamic test is used here to describe the type of mechanical test in which the rubber is subjected to a cyclic deformation pattern from which the stress strain behaviour is calculated. It does not include cyclic tests in which the main objective is to fatigue the rubber, as these are considered in Chapter 12. Dynamic properties are important in a large number of engineering applications of rubber including springs and dampers and are generally much more useful from a design point of view than the results of many of the simpler static tests considered in Chapter 8. Nevertheless, they are even today very much less used than the "static" tests, principally because of the increased complexity and apparatus cost. 

As the previous sections have shown, there are a large number of low temperature tests in existence. Even when ad hoc bending tests are disregarded, together with the use of the normal range of physical tests, such as tensile modulus and resilience, and the automation of a mechanical test by thermal analysis, there remain several types of specially developed low temperature tests. The various tests do not all have equal relevance to a given product. A test, or tests, should wherever possible, be chosen to provide the information most relevant to the particular application, but for many quality control purposes a test is used simply as a general indication of low temperature behaviour. Whatever the relative merits of the different methods in any situation, the question of correlation between the methods is frequently asked. 

In the food industry a range of practical or descriptive tests are used to evaluate product quality and the stability of whippable emulsions. Using such methods a number of reliable and commercially valuable whippable emulsions have been developed over the years. To develop new whippable emulsion systems which are more difficult to stabilize, i.e. primarily low-fat products, more advanced physical methods have been used to elucidate the fundamental mechanisms behind the behaviour of whippable emulsions. 

For the reasons indicated it is not possible here to cover each material in detail and before going on to make some general observations about basic techniques for finishing one should emphasize that trials in advance with the actual substances to be worked usually are necessary. The testing should cover every aspect relevant to what is required of a component, and obviously it is helpful in this regard if reference can be made to a written Standard or specification. Tests may be of chemical behaviour, electrical, mechanical, physical, and thermal properties, or of some other aspect like performance when exposed to fire or to strong sunlight. The tests should be on samples that are representative of the production, should be appropriate for the parameters required, and be carried out under proper supervision. Reference samples and... 

Sheppard, A., Kelly, D., Tong, L., (1998), Int. J. of Adhesion and Adhesives 18, 385. Wemersson, H., (1994), Fracture characterization of wood adhesive joints. Report TVSM-1006, Lund University, Division of Structural Mechanics, Lund, Sweden, Simon, F., Morel, S., Valentin, G. (1997). In Proceedings of the Euromech Colloquium 358, Mechanical behaviour of adhesive joints, analysis, testing and design, Pluralis, Paris, pp. 341-351. 

The question rise whether the behaviour described can be evaluated on a fracture mechanics basis. The critical energy release rate associated with the formation of a transverse crack is technically difficult to measure. As a first approximation, it is possible to use the critical energy release rate obtained from a double cantilever beam fracture mechanics test (DCB). This test concerns the growth of a delamination between two layers (mostly oriented ai 0°) in opening mode I. Tests performed on the same carbon-polyetherimide at 0°/0° interface as in this study were reported recently [9] and gave a value of 1200 J/m. ... 

Hardness was estimated by Vickers indentation on sintered gradient samples, thermal expansion coefficient measurements and elastic behaviour was tested on the different dispersions. Preliminary mechanical testing (single edge notched beam, 3PB-SENB) and oxidation tests were performed on sintered FGM s. 

To investigate the behaviour observed in both the Isothermal Test and Buffer Container Experiment numerical simulations have been undertaken. In particular coupled hydraulic-mechanical analyses of the Isothermal test and thermal-hydraulic-mechanical analyses of the Buffer Container Experiment have been undertaken. These analyses have been performed via a finite element computer code, COMPASS (Thomas and He, 1997 1998). 

It is of a great significance to understand how the mechanical behaviours and properties of rock masses change with temperature, such as for nuclear waste repositories and deep mining at certain temperatures. The key to this problem is how to make predictions to long-term response of rocks based on mechanical models and test results within a short time of experiments. It is put forward in this paper that the problem can be resolved by means of time-temperature equivalent principle for rocks. 

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