Strain-Rate Sensitivity

Both % El and % RA are frequendy used as a measure of workabifity. Workabifity information also is obtained from parameters such as strain hardening, yield strength, ultimate tensile strength, area under the stress—strain diagram, and strain-rate sensitivity. 

A ductile material can be stretched uniformly only when stable flow occurs. The stable flow of materials has been investigated by Hart who described the transition from the stable to unstable flow. The beginning of geometrical instability and localisation of strain is the limit of the stable flow. At temperatures above 0.5 T (at equilibrium between recovery and hardening) the strain rate sensitivity parameter "m" may be derived from the expression ... 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

As a result of simultaneous introduction of elastic, viscous and plastic properties of a material, a description of the actual state functions involves the history of the local configuration expressed as a function of the time and of the path. The restrictions, which impose the second law of thermodynamics and the principle of material objectivity, have been analyzed. Among others, a viscoplastic material of the rate type and a strain-rate sensitive material have been examined. 

The blend exhibited reversible strain only up to 2%. Below 20%, the deformation appeared to be homogeneous. Beyond this region, the stress derivative with strain decreased significantly. The decrease in stress derivative was strain rate-sensitive. The stress-strain behavior for the blends containing different SBRs are given in Figure 11.18. Both modulus and yield stress increased with increase in PP content. Neither modulus nor yield stress varied strongly with SBR type. On the other hand, aU the failure properties increased with increase in PP content. 

Sung N.H., Jones T.J. and Suh N.P. (1977). Strain-rate sensitive tough fiber reinforced composites. J. Mater. Sci. 12, 239-250. 

As shown in Figure 27, it is apparent that when the roller compaction force was scaled-up, the compactibility curve of the tablets shifted downward. Ideally, a formulator would prefer a 60 N hardness window of acceptable force. In this example, the flat part of the curve had no effect on friability and dissolution and thus could be processed. The example does show that for a roller compaction process, a scale-up factor must be considered for the roller compaction force early on in development. Factors that affect the tablet compactibility curve at scale-up include the roller compaction force and strain-rate sensitivity of the compact. 

Strain rate sensitivity of (or the effect of press speed on) the formulation is of primary concern in scale-up. Whether the product development work was performed on a single-stroke press or a smaller rotary press, the objective in operations will be to increase efficiency, in this case the tablet output rate and, therefore, the speed of the press. For a material that deforms exclusively by brittle fracture, there will be no concern. Materials that exhibit plastic deformation, which is a kinetic phenomenon, do exhibit strain rate sensitivity, and the effect of press speed will be significant. One must be aware that although specific ingredients (such as calcium phosphate and lactose) may exhibit predominately brittle fracture behavior, almost everything has some plastic deformation component, and for some materials (such as microcrystalline cellulose) plastic deformation is the predominant behavior. The usual parameter indication is that target tablet hardness cannot be achieved at the faster press speed. Slowing the press may be the only option to correct the problem. 

Both calcium phosphates have good flowability. The predominant deformation mechanism for both forms under compression is brittle fracture, which reduces the strain-rate sensitivity of the materials.49 However, unlike the hydrate form, anhydrous dicalcium phosphate, when compacted under high-pressure, can exhibit lamination and capping. 

Slow strain test. The strain rate chosen frequently for the tests, based on several studies, indicates important susceptibility to cracking at about 2 x 10 6 s 1 for steels, aluminum and magnesium alloys. However, the tests refer to open-circuit conditions and the strain rate sensitivity of cracking is dependent upon potential as well as solution composition. Where necessary the potential of the specimens can be controlled using a potentiostat during slow-strain-rate tensile testing.171 The reduction of area is a simple and appropriate way to quantify the susceptibility to SCC. 

With increasing plastic strain, the shear activation colume v decreases, giving increased strain-rate sensitivity, and increased pressure depmidence (since the latter depends on Q/v, where 1 remains constant). 

An upper bound analysis of the hydrostatic extrusion of linear polyethylene has been given by Gupta and McCormick This followed the approach outlined above for the flow stress (Eq. (4)) taking results frcnn Coates and Ward to describe the increase in strain rate sensitivity by a reduction in the shear activation volume. Reasonable fits were obtained for experimental data up to comparatively low deformation ratios (R = 9), giving further support for the more extensive treatments presented by Ward and coworkers. 

Actually, the strain-rate sensitivity is often used as a usefiil basis for examining different aspects of deformation. From the impact deformation standpoint, the value of strain-rate... 

According to the definition of strain rate sensitivity maitioned above, the values of strain rate sensitivity in both low and dynamic strain rate regions are calculated and listed in Table 2. It is found that dynamic strain rate sensitivity decreases with plastic strain, but low strain rate sensitivity is independent of plastic strain. Furthermore, the dynamic strain rate sensitivity is generally higher than low strain rate sensitivity for eaeh true strain level. 

Drug properties (inputs) Solubility (mgml ) Contact angle Yield pressure (MPa) Strain rate sensitivity (%) +Excipient compatibilities Formulation 0.1 82° 50 50 ... 

Many investigators have reported the effect of dwell times and strain rate sensitivity on the compaction of... 

Anhydrous dibasic calcium phosphate is used both as an excipient and as a source of calcium in nutritional supplements. It is used particularly in the nutritional/health food sectors. It is also used in pharmaceutical products because of its compaction properties, and the good flow properties of the coarse-grade material.The predominant deformation mechanism of anhydrous dibasic calcium phosphate coarse-grade is brittle fracture and this reduces the strain-rate sensitivity of the material, thus allowing easier transition from the laboratory to production scale. However, unlike the dihydrate, anhydrous dibasic calcium phosphate when compacted at higher pressures can exhibit lamination and capping. This phenomenon can be observed when the material represents a substantial proportion of the formulation and is exacerbated by the use of deep concave tooling. This phenomenon also appears to be independent of rate of compaction. 

Mayo, M.J. Nix, W.D. (1988) Measuring and Understanding Strain Rate-Sensitive Deformation with the Nanoindenter, Strength of Metals and Alloys, ICSMA 8 (Kettunen, P.O., Lepisto, T.K. Lehtonen, M.E., eds.) Pergamon, London, p. 1415. 

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