Force versus compression

We prepare force versus compression plots in a similar fashion to force versus elongation plots. We generally perform compressive testing over a much more limited range of strain than tensile testing. Samples typically take the form of thick pads, which do not break in the same manner as tensile specimens. The limit of compressive strain can approach 100% for low density foams, but is much less for other samples. The most common property that we obtain from this mode of testing is compressive modulus. 

FIGURE 5 Adhesion force versus compression force profile for a formulation at different levels of magnesium stearate. Source Adapted from Ref. IS. 

Resilience of textile fabrics when compressed in the bent state is related to wrinkle resistance and retention of shape, drape, and hand. Resilience is an important parameter for evaluating blankets, wearing apparel in which warmth is a factor, pUe fabrics including carpets, and bulk fiber utilization in mattresses, cushions, etc. The general method for determining compressional resilience is to compress and unload the material cycHcahy, creating a plot of compressive force versus fabric thickness. 

The classic way that we perform force versus deformation measurements is to deform a sample at a constant rate, while we record the force induced within it. We normally carry out such tests in one of three configurations tensile, compressive, or flexural, which are illustrated in Fig. 8.1. We can also test samples in torsion or in a combination of two or more loading configurations. For the sake of simplicity, most tests are uni-axial in nature, but we can employ bi-axial or multi-axial modes when needed,... 

The TS of the compacted samples was determined by transverse compression with a custom-built tensile tester. Tensile failure was observed for all the rectangular compacts when compressed between flat-faced platens at a speed ranging between 0.006 and 0.016 mm/sec. Platen speed was adjusted between materials to maintain a time constant of 15 2 seconds to account for viscoelastic differences the constant is the time between the sample break point and when the measured force equals Fbreak/e in the force versus time profile, where the denominator is the mathematical e. Specially modified punch and die sets permitted the formation of square compacts with a centrally located hole (0.11 cm diameter) that acted as a stress concentrator during tensile testing. This capability permitted the determination of a compromised compact TS and thus facilitated an assessment of the defect sensitivity of each compacted material. At least two replicate determinations were performed for each mechanical testing procedure and mean values are reported. 

Fig. 22 shows the same data of Fig. 21 plotted as force versus deformation ratio. If the data in compression are taken to follow Gaussian behaviour it is seen that a negative strain-dependent correction is needed... 

Fig. 22, The same data as in Fig. 21 plotted as force versus deformation ratio. If one identifies the compression modulus with the Gaussian constant 3 (A vk T/Lt) (( >o) the experimental curve in extension lies below the Gaussian curve. The C part of the Mooney-Rivlin curve in extension lies again below the experimental curve...

Some particles can be viscoelastic or lose liquid under compression, for example, hydrogels and plant cells, and the force versus displacement data may therefore vary with the compression speed. 

Figure 13 Force versus sampling time for compressing (curve AB) and holding (BC) a single tomato plant cell. The plant cell was 70 pm in diameter and was compressed at 23 pm s-1 (reproduced from Thomas et al, 2000). Permitted by Springer.

Figure 14 Comparison of experimental force versus displacement/time data with fitting by ABACUS. The microsphere diameter was 125 pm and compression speed 800 pm s1 (Nguyen et al., 2009). Permitted by Elsevier.

Figure 15 Comparison of force versus time data for compression of a calcium-shellac particle of 40 pm in diameter. The compression speed was 2pms 1. Curve AB corresponds to compression and BC holding. The force relaxation data were fitted by Equation (10) with / = 2, Fca= 14.3 pN, F = 8352 pN, Ti = 0.33 s, F2 — 6.0 pN and t2 = 4.5 s (Xue, 2008).

Fig. 11.43 The compressive force versus displacement of stacked PS and LLDPE cylinders at 25°C. Note that initially the stack deformation force tracks that of the more deformable LLDPE, before, at 3 mm, beginning to deform the PS cylinder. [Reprinted by permission from B. Qian, D. B. Todd, and C. G. Gogos, Plastic Energy Dissipation and Its Role on Heating/melting of Single-component Polymers and Multi-component Polymer Blends, Adv. Polym. Technol., 22, 85 (2003).]...

FIGURE 10.14 Compression force versus gap of EVA/PA6/APP-based intumescent formulations measured at400°C without (REF) and with nanoparticles (Si02, silica OMMT, organomodified montmorillonite LDH, lamellar double hydroxide). 

The cut-off distance Dc for rigid surfaces is sufficiently so small that it is commonly neglected in the display of force versus separation curves. This cut-off distance may not be neghgible in the presence of absorbed layers of surfactant or polymers, compressed between the surfaces thus it is common to report apparent separation distance on AFM measurements [27]. 

In each case, preconditioned material was placed in the hopper of an instrumented tablet machine and the performance of these two materials was compared at constant machine settings. The sample with 5.1% MC produced tablets that weighed slightly less (an effect on flow and bulk density), and as a result were exposed to lower compression force. However, the moist microcrystalline cellulose resulted in harder tablets, even though it had been exposed to lower compression force. The loci of points on the compression force versus hardness profile (Fig. 20) for the tablets indicate a different fundamental behavior for the two materials. 

FIGURE 36 Comparison of compression force versus crushing strength for tablets produced on a Presstcr. Korsch Ph 300e/29 (14.9 milliseconds dwell time) and a Feite PT 2090/29 (15.5 milli-second.s dwelt linte). 

Different vertical loads can be applied to a bulk solid sample of known mass, and compression of the sample is recorded electronically (Thomson, 1997). With these data, powder contact volume versus compressive force or stress can also be represented. Bulk density of a solid is a function of consolidation stress and changes during flow as the stress changes. Because the mass consolidating load and volume are known, the relationship can be plotted as shown in Figure 6. 

TMA has been used for determining softening in polymers by measuring the degree of probe penetration into a polymer at particular applied forces as a function of temperature. This technique allows determination of Tm and the evaluation of dimensional properties over the temperature range of use or under actual accelerated condition cycles (plots of temperature versus compression). 

Figure 4. Release constant versus compression force. Punch 0 core 7 mm coating 9 mm Coat (mg) 78 O 99 128 145.

Figure 3.18e shows the effective force law (force versus displacement) between two parallel dimers with aspect ratio = 1.3 undergoing compression for the micromechanical model in which (1) lobe interactions are multiply counted or (2) the interaction potential is given by Equation 3.15. The two force laws are the same as long as overlaps between lobes have not merged, or < 0.021 for the configuration in Figure 3.18a. Beyond Sm, the two force laws differ. The force law based on the total area of overlap converges to linear behavior f 5 more quickly than the one that multiply counts lobe interactions, for example, it is not sensitive to the formation of the fourth lobe contact at 8/a = 84/a = 0.075. In future studies, these results can be compared to finite element analyses of linear elastic particles with complex shapes.

Figure 2 Prediction of the mechanical performance of a polymer product. Left Compression testing, maximum in ioad found at different rates. Inset experimental and numerical force versus displacement curves. Right Creep experiments, time to faiiure at different ioads. Symbois indicate experiments and lines indicate full predictions, including the influence of the different processing conditions used during the maidng of the product ...

Data are generated by placing a test specimen between the two flat, parallel faces of a testing machine and then moving these faces together at a specified rate (ASTM D 695). A displacement transducer may be used to measure the compression of the specimen, while a load cell measures the compressive force exerted by the specimen on the testing machine. Stress and strain are computed from the measured compression load, and these are plotted as a compressive stress-versus-strain curve for the material at the temperature and strain rate employed for the test. 

Most mechanical and civil engineering applications involving elastomers use the elastomer in compression and/or shear. In compression, a parameter known as shape factor (S—the ratio of one loaded area to the total force-free area) is required as well as the material modulus to predict the stress versus strain properties. In most cases, elastomer components are bonded to metal-constraining plates, so that the shape factor S remains essentially constant during and after compression. For example, the compression modulus E. for a squat block will be... 

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