Compression stress-strain

Figure 6.14 shows the reload compressive stress-strain response of shock-loaded copper as a function of pulse duration [40]. For copper shock loaded to 10 GPa the yield strength is observed to increase with increasing pulse... 

Fig. 25.9. The compressive stress-strain curve for a polymeric foam. Very large compressive strains ore possible, so the foam absorbs a lot of energy when it is crushed.

There are a number of different modes of stress-strain that can be taken into account by the designer. They include tensile stress-strain, flexural stress-strain, compression stress-strain, and shear stress-strain. 

Table 2-5 Room temperature flexural and compressive stress-strain data for several plastics and other materials...

In general, the compressive strength of a non-reinforced plastic or a mat-based RP laminate is usually greater than its tensile strength. The compressive strength of a unidirectional fiber-reinforced plastic is usually slightly lower than its tensile strength. Room-temperature compressive stress-strain data obtained per ASTM for several plastics are shown in Table 2-5. 

The majority of tests to evaluate the characteristics of plastics are performed in tension or flexure hence, the compressive stress-strain behavior of many plastics is not well described. Generally, the behavior in compression is different from that in tension, but the stress-strain response in compression is usually close enough to that of tension so that possible differences can be neglected (Fig. 2-19). The compression modulus is not always reported, since defining a stress at... 

Fig. 2-19 Comparison of tensile and compression stress-strain behavior of TPs.

The determination of the relationship of stress to strain when a rubber is deformed, the result being shown in the form of a stress-strain curve unless compression stress-strain is specifically stated, the expression normally applies to the tensile characteristics of a rubber. 

Tear strength is only applicable to flexible materials and is very little used to monitor ageing simply because tensile strength will serve perfectly well. There are circumstances where compression stress-strain properties would be relevant but the relatively bulky test pieces will be subject to the limitation of oxygen diffusion in any accelerated tests and changes can probably be estimated from tensile measurements. Similarly, shear stress-strain is very rarely used for monitoring ageing. 

Tear measurement is covered by ISO 6383 [38] and compression stress strain by ISO 604 [39] but there are no ISO shear methods for plastics. 

ISO 844 2001 Rigid cellular plastics - Determination of compression properties ISO 3386-1 1986 Polymeric materials, cellular flexible - Determination of stress-strain characteristics in compression - Part 1 Low-density materials ISO 3386-2 1997 Flexible cellular polymeric materials - Determination of stress-strain characteristics in compression - Part 2 High-density materials ISO 5893 2002 Rubber and plastics test equipment - Tensile, flexural and compression types (constant rate of traverse) - Specification ISO 7743 2004 Rubber, vulcanized or thermoplastic - Determination of compression stress-strain properties... 

ISO 7619-2 2004 Rubber, vulcanized or thermoplastic - Determination of indentation hardness - Part 2 IRHD pocket meter method ISO 7743 2004 Rubber, vulcanized or thermoplastic - Determination of compression stress-strain properties... 

For quality cured thermoset resins, approximately one percent of the mass is soluble when subjected to long-term leaching with tetrahydrofuran. Equilibrium is approached in two weeks resin swell is not visually noticeable. The monomeric, chemical structures are such that the hydrocarbon resins exhibit more pronounced viscoelastic properties whereas, the epoxy resins are similar to elastic bodies when subjected to tensile testing at room temperature. Therein, LRF 216 is less sensitive to flaws and is more nonlinear in tensile or compressive stress-strain analysis. 

Mills and Gilchrist (270) analysed the heat transfer that occurs when closed cell foams are subjected to impact, to predict the effect on the uniaxial compression stress-strain curve. Transient heat conduction from the hot compressed gas to the cell walls occurs on the 10 ms... 

The effect of gas compression on the uniaxial compression stress-strain curve of closed-cell polymer foams was analysed. The elastic contribution of cell faces to the compressive stress-strain curve is predicted quantitatively, and the effect on the initial Young s modulus is said to be large. The polymer contribution was analysed using a tetrakaidecahedral cell model. It is demonstrated that the cell faces contribute linearly to the Young s modulus, but compressive yielding involves non-linear viscoelastic deformation. 3 refs. 

Gas compression in closed-cell polymer foams was analysed, and the effect on the uniaxial compression stress-strain curve predicted. Results were compared with experimental data for a foams with a range of cell sizes, and the heat transfer conditions inferred from the best fit with the simulations. The lateral expansion of the foam must be considered in the simulation, so in subsidiary experiments Poisson s ratio was measured at high compressive strains. 13 refs. 

The measurement of extension (or other mode of deformation) is an essential part of several tests, notably tensile or compression stress/strain properties and also thermal expansion. The precision required must be specified in the individual test method and is unlikely to be the same as that required for test piece dimensions. The method of measurement will also be dependent on the test in question and particular techniques will be given in most cases. Hence, the requirements for specific tests will be discussed in the relevant sections in later chapters. 

The fundamental shear and Young s moduli are the slopes of the shear and tension/compression stress/strain curves at the origin. The relationships given above are an attempt, with theoretical justification, to describe the shapes of the stress/strain curves at higher strains. Appreciation of this may avoid confusion between the absence of a single modulus figure for rubber whilst such values are quoted. 

A compression stress/strain test is in many ways easier to carry out than a tensile test, and in view of the large number of applications of rubber in compression, should be more often used. Frequently, it would be logical for the test piece to be the complete product and a compressive force applied as it would be in service. Usually a constant rate of deformation would be appropriate and the force and corresponding deformation recorded without attempts at calculating the resultant stresses and strains. 

It was not until 1989, which in relative terms is quite late, that an international standard for compression stress strain was published. This is perhaps a sad reflection on the order of priorities that existed within the standardisation of rubber testing. However, ISO 7743104 is now well established. 

If compression stress strain is used to obtain input data for finite element analysis, the tests would be made with lubricated platens. ISO 7743 does not mention that if the lubrication really is near perfect the test piece can have the unfortunate habit of slipping out of the platens. To prevent this, a small pin should project from the centre of one platen. 

Nuebel, C and Peleg, M. (1993). Compressive stress-strain relationships of two puffed... 

Fig. 10. Compression stress-strain properties of various elastomeric syntactic foams129> (l)urethane elastomer binder and glass microspheres y = 640 kg/m3, void fraction 0.321 (2) polysulfide elastomer binder and phenolic microspheres y = 1500 kg/m3, void fraction 0.133 (3) silicone elastomer binder and glass microspheres y = 610 kg/m3, void fraction 0.407...

The compression stress-strain curves obtained over quite a broad temperature range are shown in Fig. 15 [33]. It appears that the strain softening, weak and slightly temperature-dependent in the temperature range from 40 to 100 °C, increases when lower temperatures are considered. 

Fig. 16 Effect of strain rate on uni-axial compression stress-strain curves of PMMA a at - 50 °C and b at 50 °C (From [33])...

Fig. 68 Compression stress-strain curves for BPA-PC at various temperatures at a strain rate of 2 x 10-3 s 1 (From [53])...

The compression stress-strain graphs are normally determined using samples that are bonded to steel plates. If there is any lubrication, the results are completely different due to slippage that may take place. 

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).]...

Fig. 10.60 Compressive stress-strain behavior of PS and LLDPE at 25°C and crosshead speed of 25.4 mm/min. At a compressive stress level of 20 MPa the deformation of the soft LLDPE is large, in the dissipative region and nearly twenty times the PS deformation, which is of the order of 0.04, in the elastic nondissipative range. [Reprinted by permission from B. Qian, D. B. Todd, and C. G. Gogos, Plastic Energy Dissipation (PED) and its Role in Heating/Melting of Single Component Polymers and Multi-component Polymer Blends, Adv. Polym. Techn., 22, 85-95 (2003).]...

Fig. 28. Dependence of the compressive stress-strain curves on strain rate of silica filled epoxy [46]...

---