Cohesiveness and tensile strength

1 Cohesiveness and adhesion Cohesiveness of a powder is the ability of individual particles to stick together. This attraction may be caused by the presence of moisture, electrostatic charges or the fineness of the particles within a powder. Fine powders generally have a high surface area, which is usually associated with an increase in the surface energies within the powder and thus enhancement of the binding forces between particles (Stanley Wood et al. 1990). 

Powder cohesion is relevant to powder flow, mixing of powders and in the strength that a powder exhibits when subjected to a tensile stress. It can be argued that the only direct way to measure cohesion is via the determination of the minimum force needed to cause 

2 Tensile strength The tensile strength of a powder can be regarded as the completion of the yield locus in the shear stress-normal stress profile. The negative stress, at zero shear, to break apart the consolidated particulate powder is thus termed the tensile strength of a powder (T). 

Tensile strength is a fundamental failure property but tensile strength testing is dependent upon the direction of force necessary to cause separation of a bulk structure with respect to the direction of compaction or consolidation. Split cell testers pull the sample apart at 90° to the direction of compaction whilst the lifting lid or vertical shear testers pull in the same direction as the compaction/consolidation stress was applied. Results obtained from both methods differ greatly because tensile testing has a poor record of reproducibility, possibly due to the fact that consolidated powders in the tester cells may not be isotropic. 

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The mechanical phase of the computation has made use of the 3DEC distinct element code, which relies upon an explicit solution of motion equations based on small lime steps the ensuing computation cycles lead to achieving equilibrium. The blocks separated by discontinuities were considered as elastic in the FEBEX case. Joints respect the Coulomb slip criterion characterised by normal and shear stiffnesses, as well as friction, cohesion and tensile strength properties assigned to each joint. 

The layer sequence and the thickness of each layer used in the model are given in Table 1. Physical and mechanical properties selected to represent the coal and rock masses and to perform stress-deformation analysis are also shown in Table 1. The values selected to represent Young s Modulus, cohesion and tensile strength is about 30% less than the intact rock values. The value used to represent Poisson s ratio is about 20% higher than the intact rock value. Therefore, the selected mechanical property values represent the equivalent continuum properties of each rock mass. 

To obtain the proper initial geostatic stress state of the slope, static calculation is required before the dynamic analysis (Itasca Consulting Group, 1999). Two procedures are followed in the static calculation (a) set relevant mechanical parameters and take elastic model as material constitutive model, and then make the slope model balanced in gravity field (b) take Mohr-Coulomb model as the material constitutive model (Yan et al., 2011) and reset cohesion and tensile strength to their initial value, and then rebalance the model. This is just the slope model on which the dynamic load will be applied. The contour plot of initial vertical geostatic stresses in the slope (Figure 3) reflects the vertical stress state in which no dynamic load is applied on the slope. 

Figure 1.18 Schematic diagram to show the parameters of cohesion and tensile strength.

Although estimation of tensile strength, adhesion and cohesion from a Jenike shear test yield locus is the easiest and less demanding way of assessing powder stresses, there are other types of equipment which attempt to measure cohesion and tensile strength. 

The correlation between cohesion and tensile strength, at identical packing densities, appropriated to the relationship ... 

Figure 1.30 Relationship between cohesion and tensile strength for a range of industrial powders (Fadey Valentin 1967/68).

Eaves, T. Jones, T.M. (1971) Cohesion and tensile strength of bulk solids. Rheol. Acta, 10(1), 127-134. 

A further conclusion, however, remains to be drawn which is less familiar. The effect of the mutual attraction between molecules must be the same as that of a pressure existing in the liquid, and this is called the intrinsic pressure. A liquid must, therefore, oppose a resistance to forces tending to enlarge its volume or, in other words, must possess cohesion or tensile strength. We habitually overlook this fact, only because we handle liquids almost exclusively under conditions which change their shape, but do not alter their volume. If, however, we attempt to do the latter, the existence of cohesion or intrinsic pressure is easily demonstrated, and some experiments in this sense will be referred to below. 

Indices are dimensionless parameters derived from various mechanical and physical properties of the tablet blend and resulting compacts. Mechanical properties typically measured include indentation hardness (kinetic and static), elastic modulus, and tensile strength (10,11). Physical properties include particle size, shape, and size distribution, density (true, bulk, and tapped), flow properties and cohesive properties. 

Extended exposure of the uncured material to temperatures or conditions outside those recommended by the manufacturer will cause change in physical properties of the uncured material and will likely reduce its resulting cohesive and adhesive strength. The reactions that occur due to ambient storage conditions are described in Chap. 3. Figure 17.5 shows the effect of aging conditions on the tensile shear strength of an epoxy film adhesive. 

Farley and Valentin (1967/68) were the first to correlate the bulk powder properties of cohesion C, and tensile strength T, for five different powders, containing a range of particle sizes. It has been shown for a range of powders (calcium carbonate, precipitated and mined, aluminium oxide and zinc dust) that cohesion appears to be approximately equivalent to 2T... 

Parameters Sc and St denote the unconfined compressive and tensile strengths, respectively. Compressive strength Sc and tensile strength St can be expressed in terms of cohesion(C) and internal friction angle ( ), namely ... 

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