Compressibility strain rate sensitivity

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. 

The degree of fragmentation was found to diminish with smaller sieve fractions at the same compression load when several sieve fractions of unmilled crystalline a-lactose monohydrate was used. The authors concluded that particle fragmentation would reduce as porosity approached zero and elastic behavior would start to dominate the consolidation process (43). With a decrease in particle size, yield pressure decreased and the strain rate sensitivity index increased (44) which suggested a reduction in the extent of fragmentation. The transition from brittle to ductile material was thought to occur for a median particle size of around 20 pm (45). 

The effect of compression speed on the yield pressure of a material has been suggested as a method of determining the time-dependent nature of materials compression properties (Roberts and Rowe 1985). Heckel plots are produced at two punch velocities, 0.03 and 300 mm sec, and the yield pressures determined. The strain rate sensitivity (SRS) is calculated as ... 

Shown in Eig. 3 are yield stress data for 95.5Sn-3.9Ag-0.6Cu (wt%) solder that were obtained by compression testing, using several strain rates (Ref 18, 21). The nominal sample dimensions were 19 mm (0.74 in.) length and 10 mm (0.39 in.) diameter and were tested in the as-cast condition. Minimum and maximum values of the duplicate tests were within 5% about the mean value. The plot shows the expected decrease of yield stress with increased test temperature. The strain rate sensitivity of the yield stress is most distinct at the lowest temperatures, and then diminishes somewhat at the higher test temperatures. The yield stresses that were measured at the lowest strain rates, provide... 

In equation (1), Tocto corresponds to the shear yield stress under zero pressure and a is a pressure coefficient, which quantifies the yield stress sensitivity to pressure. Such a yield criterion has previously been shown to hold for epoxy resins under a wide range of pressure, temperature and strain rate conditions [10, 11]. The two parameters, Tocto and a were found to be 44 MPa and 0.173 respectively from the uniaxial and plane strain compression results reported in table I. 

The viscosity behavior of plastics makes them sensitive to strain rates as well as temperatures (see Chapters 2, 3, and 4). It therefore becomes important to define the rate, magnitude, duration, and type of mechanical stress and strain loading (i.e., tension, compression, flexure, and shear) along with temperatures during loading. The rate and duration of loading also determine whether creep or impact will be a factor in a given part s mechanical response [1, 2, 5-14, 29, 33,40-43, 55-68, 152, 202, 225, 235, 250, 270-74, 808]. 

Leroueil, S., Kabbaj, M., Tavenas, F. and Bouchard, R. Stress tiain strain rate relation for the compressibility of sensitive natural clays. Geotechnique, 35(2) 159 180,1985. 

External factors that will influence polymer mechanical properties are temperature or thermal treatment, temperature history, large differences in pressure, and environmental factors such as humidity, solar radiation, or other types of radiation. The mechanical properties of polymer are also sensitive to the methods and variables used for testing, such as strain deformation as well as the rate at which the strain is performed. Finally, the mechanical behavior of polymeric materials and the values of their mechanical properties will be sensitive to the kind of strain that is imposed by the applied force, namely, tension, compression, biaxial, or shear. 

Mechanical Load. Static mechanical load by strain leads to stretching of random-coil polymer chains in the direction of sample elongation and chain compression in the orthogonal directions. The value of the residual dipolar and quadrupolar couplings is increased by the mechanical load, and moreover, the distribution of the correlation times is also modifled. Therefore, many NMR parameters sensitive to the residual dipolar couplings and slow motions can be used for characterization of the local strain-stress effects in heterogeneous elastomers (158,160,161,179). Dynamics mechanical load leads to sample heating where the temperature distribution in dynamic equilibrium is determined by the temperature-dependent loss-modulus and the thermal conductivity of the sample. Because transverse relaxation rate (approximated by the T2 relaxation) scales with the temperature for carbon fllled SBR, a T2 map provides a temperature map of the sample. Such temperature maps have been measured for carbon-black filled SBR cylinders for different filler contents and mechanical load (180). 

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