Stress-strain curves compressive loading

Figure C2.1.17. Stress-strain curve measured from plane-strain compression of bisphenol-A polycarbonate at 25 ° C. The sample was loaded to a maximum strain and then rapidly unloaded. After unloading, most of the defonnation remains.

In compression, a single large flaw is not fatal (as it is tension). As explained in Chapter 17, cracks at an angle to the compression axis propagate in a stable way (requiring a progressive increase in load to make them propagate further). And they bend so that they run parallel to the compression axis (Fig. 20.7). The stress-strain curve therefore rises (Fig. 20.8), and finally reaches a maximum when the density of... 

The test can provide compressive stress, compressive yield, and modulus. Many plastics do not show a true compressive modulus of elasticity. When loaded in compression, they display a deformation, but show almost no elastic portion on a stress-strain curve those types of materials should be compressed with light loads. The data are derived in the same manner as in the tensile test. Compression test specimen usually requires careful edge loading of the test specimens otherwise the edges tend to flour/spread out resulting in inacturate test result readings (2-19). 

PP bead foams of a range of densities were compressed using impact and creep loading in an Instron test machine. The stress-strain curves were analysed to determine the effective cell gas pressure as a function of time under load. Creep was controlled by the polymer linear viscoelastic response if the applied stress was low but, at stresses above the foam yield stress, the creep was more rapid until compressed cell gas took the majority of the load. Air was lost from the cells by diffusion through the cell faces, this creep mechanism being more rapid than in extruded foams, because of the small bead size and the open channels at the bead bonndaries. The foam permeability to air conld be related to the PP permeability and the foam density. 15 refs. 

Some plastic materials have different tensile and compressive characteristics. For example, polystyrene is tough under compressive load but very brittle in tension. However, for most elastoplastic materials, the stress-strain curves in compression are the same as in tension. Hence, the deformation properties of these materials in tension may also be applied to those in compression, which is of great interest to gas-solid flows. 

Fig. 5.17 Unconfined compression stress-strain curves and experimentally measured temperature increase ATa as a function of strain for PS (Dow 685), LDPE (Dow 640), and PP (LG H670). The initial test specimen was at 26°C and the crosshead speed of the compressing har with the load cell was 25.4 mm/min. The specimen dimensions were 101 mm diameter and 71 mm height. [Reprinted by permission from M. H. Kim, Ph.D Thesis, Department of Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ (1999).]...

Both destructive and nondestructive measurements can be done on an Instron Material Tester. In this system, the sample is loaded in a test cell, and the compression or tension force is measured when the upper part of the cell is moved over a given distance (time). Within the elastic limit of the gel, the elastic modulus E (or gel strength) is obtained from the initial slope of the nondestructive stress/strain curve additional deformation results in the breakage of the sample, giving the characteristic parameters—yield stress and breaking strain. 

Figure 10.7 (a) Failure lines for grouted and ungrouted granular soils, (b) Drained triaxial test results for silicate grouted coarse and medium sands. (From Ref. 11.15.) (c) Typical stress-strain curve from unconfined compression test on chemically grouted sand, (d) Compression versus time data for creep test on chemically grouted sand, under constant load, (e) Failure time versus percent of unconfined compression failure load. (+) indicates unconfined compression tests, and ( ) indicates triaxial tests with S3 = 25% of Si. 

In contrast to the magnetic mPVA gels, compression of the carbonyl iron-loaded mPDMS network results in a break-point in the stress-strain curve if the deformation of the sample is parallel to the pearl chain structure. Figure 16b shows that the nominal stress increases with the compression up to a deformation ratio of 0.95 in every case. On increasing the compression above this ratio, the columnar structures of the iron particles are destroyed (Fig. 18). 

The gels were tested in a StevensjLFRA Texture Analyzer. The modulus of rigidity, E,Pa(N/m ), was calculated from the load causing compression of one or two millimeters, in which range the stress-strain curve was linear. The breaking load (in grams) was also determine. The... 

Compressive properties include compressive strength, modulus of elasticity, yield stress, and deformation beyond yield point. The ASTM procedure covers determinations of all of them. In all cases, tested specimens are loaded in compression at relatively low uniform rates of straining or loading. Compressive yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress. In other words, it is the load under which the specimen starts to move continuously without an increase in the load. Also, many plastic materials will continue to deform in compression until a flat disk is produced, without breaking of the specimen. In those cases the compressive stress (nominal) increases steadily in the process, without failure of the material. Compressive strength typically has no meaning in such cases. 

There remains the question of whether the drop in load observed at yielding arises from the purely geometrical strain softening associated with a true-stress-strain curve of the form shown in fig. 8.4(c), where there is no drop in the true stress but merely a reduction in slope of the stress train curve, or whether there is actually a maximum in the true-stress strain curve as shown in fig. 8.4(d). Experiments on polystyrene and PMMA in compression, under which the geometrical effect cannot take place, show that a drop in load is still observed. Results from extensive studies of PET under a variety of loading conditions also support the idea that a maximum in the true-stress train curve may occur in a number of polymers. 

The resistivity data are shown in Fig. 4 for the second specimen under cyclic tension-compression mode with 0.2% constant strain amplitude. In the first 50 cycles the rate of increase, dpm(e)/dN, at 5 T is about 8x 10 nfi-cm/cycle. At relatively low fields (S = 1 to 2 T), the rate of increase was about 80% of that at 5 T. This means dpm e)/dN was rather insensitive to magnetic field. Between 50 and 400 cycles, dpm(s) dN was reduced to 1.7x 10" nfi-cm/cycle at 5 T. After 400 cycles, the rate was reduced even further. The cyclic strain was not continued to the point of saturation of the resistivity. Increasing the number of cycles, the mode of the stress-strain curve was changed from almost plastic to elastic. At N = 600, the peak load values of tension and compression increased from about 29 to 65 MN/rn. 

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