Compressive creep strain

Fig. 6.6. Compressive creep strains in repair mortars, (a) Polymethyl methacrylate mortar, (ft) Polyvinyl acetate modihed mortar, (c) Magnesium phosphate modified mortar, d) Flowing concrete.

Fig. 9.29 Compression creep strain-time curves for one of the nano-nano composites (1 wt% Y2O3, 1600 °C/10 min sintered) [38]. With kind permission of John Wiley and Sons...

Most tests will be made on standard test pieces which may be pieces cut from a component or a sheet, or they may have been moulded separately from the same material. Where test pieces or sheet are produced for the trials it is important that they are produced in as near as possible the same way as the product and that the processing conditions are recorded. Different results can be expected from compression and injection moulding or from extrusion (where a choice is possible). Directional properties can result from the conditions of flowing and cooling in a mould. For example, in a study at ERA, the creep strain of unfilled HDPE, either individually moulded or cut from square plaques, varies by up to a factor of two depending on the orientation of flow [40]. This difference becomes even more marked with short fibre reinforcement. 

Deformation such as drawing, compression, annealing, strain, creep and stress relaxation of polymers including fibers may produce quite different orientational behavior, the results of which can be examined with solid-state NMR from both the static and dynamic viewpoints. The accurate model produced on the basis of atomic resolution of the local structure and the local dynamics can be built up in order to interpret the mechanical properties of polymers and the deformation mechanisms. 

Creep tests are made mostly in tension, but creep experiments can also be done in shear, torsion, flexure, or compression. Creep data provide important information for selecting a polymer that must sustain dead loads for long periods. The parameter of interest to the engineer is compliance (J), which is a time-dependent reciprocal of modulus. It is the ratio of the time-dependent strain to the applied constant stress [J(t) = e(t)/Oo]. Figure 13.3 shows creep curves for a typical polymeric material. 

Special requirements, with examples being stress relaxation, creep, wear, compression stress-strain behavior, dynamic properties, staining, and metal corrosion. 

For orthorhombic symmetry on the other hand, tensile creep and lateral compliance measurements on specimens cut from oriented sheet will yield only 6 of the 9 required creep functions those not accessible by this method being Suit), Sssit) and S66(t)- The two shear compliances 555(1) and Seeit) can be obtained by torsional creep experiments, but these need to be carefully designed and involve complex experimental procedures. The only possibility for measurement of Su(t) on sheet appears to be by compressive creep techniques, however, one would expect substantial experimental difficulties largely associated with strain measurement and specimen geometry. There appears to be no reported evaluation of the full characterisation of creep for the case of orthorhombic synunetry. 

Opposite to the regulations of the standard ISO 899-1 the compression-creep modulus is named with the symbol Fee- The frequently used values in the following Table are the modulus at 1 hour Ecd, 100 hours Fccioo and at 1000 hours Fcciooo- In the case of stress relaxation experiments the compression-relaxation modulus Ere can be determined from the time dependent stress constant strain level Sco-... 

The effect of a sustained compressive stress of 10 N/mm on the creep strains induced by 160 mm long by 40 mm square prisms of four different generic forms of repair material is shown in Fig. 6.6. The range of ultimate creep strains varies from over 4500 mierostrain with one resin-based system down to less than 400 mierostrain with some modified eementitious systems. These strains inelude the effect of any curing shrinkage/expansion which may also be occurring during the period under load. 

COPs are resilient, with low hysteresis and heat buildup for uses requiring rapid, repeated flexing. In their elastic, low-strain region, COPs have very good resistance to flex fatigue and to tensile and compressive creep. 

As in the previous discussion on single crystals, this section concludes with a consideration of creep in polycrystaUine SrTiOs. Experimental data on the high-temperature deformation of SiTiOa are fairly limited and most of the studies were conducted on single-crystal specimens. The deformation experiments presented below were performed at 1200-1345 °C by compression at strain rates in the range... 

Note that many early studies on the subject attributed a stress exponent, n, to some form of intragranular dislocation creep. At 9.4 %, yttria stabilizes cubic zirconia single crystals. The compressive creep characteristics tested in the [112] orientation appear in Fig. 6.46. Here, the strain is normalized for tests in the temperature range... 

Fig. 9.36 Creep strain (b) recorded as a function of time (a) and creep rate as a function of creep strain measured during compression creep test for composites with different amounts of CNTs [19]. With kind permission of Elsevier...

Compressive creep tests allow measurement of strain as a function of time when a constant stress is applied. These can be conducted at several stress levels for aerogel of various densities. Loads are removed at the end of the creep test, and strains as a function of time are monitored to determine the recovery behavior. Compressive relaxation tests can be conducted at different strain levels. The relaxation functions determined at the same strain level at different temperatures can be shifted horizontally to determine whether a master curve can be formed for use to determine the long-term behavior. Recovery behavior after relaxation can also be characterized by monitoring the stress as a function of time after removing partially the step strain. For aerogels that contain polymers such as X-aerogels... 

Mindel and Brown [27] performed a Sherby-Dom type of analysis on data for the compressive creep of polycarbonate. Superposition was achieved using an equation of the form of Equation (10.30) with an activation volume of 5.7 nm, which was very close to the values of the activation volume obtained from measurements of the strain rate dependence of the yield stress (Section 11.5.1). 

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