Compressive measurement force/deformation curves

The particle deformation and breakage behavior can be illustrated by means of the measured force-displacement curves. A typical force-displacement curve for the investigated sugar pellets is shown in Fig. 9. The force F characterizes the contact force between stamp and particle. The distance 5 shows the corresponding displacement of the particle. AH compression tests were carried out up to a compressive strain of 20%. 

In addition to the compression loading, uniaxial extension of entangled PDMS chains have been investigated by pulling a small portion of the material and measuring elastic response before the rupture happens [419]. The multiple ruptures observed in the force-distance curves (Fig. 43) have been interpreted as fractures of an entangled network of PDMS chains formed between the tip and the silica grafted surface. At small deformations, also the capillary forces were shown to contribute in the force. The elastic part of the curves was described us-... 

The compression of uniform samples to the point where the force exceeds the structural capacity causes it to permanently deform and essentially break (4). A typical load-deformation curve can be used to derive values for yield stress, yield strain, and compressive yield work, and depending on the linearity of the onset of compression, a compressive modulus may be obtained (4). These measurements can be used to provide an index of hardness for fats, which have been successfully correlated to the textural attributes of hardness and spreadability obtained through sensory evaluation (4). Unfortunately, these tests are destructive in nature and yield minimal information about the native microstructure of the system. 

Punch displacement measurements are easily done on a single station press by attaching LVDT to the punch. On a rotary press, such measurements can be done by means of slip ring, telemetry, or instrumented punch. Punch displacement profiles may be used in conjunction with compression force to estimate work of compression and work of expansion (measure of elasticity). Because capping tendency increases with the punch penetration depth, it may be desirable to monitor actual punch movement into the die. The shape of a force-displacement curve is an indication of the relative elasticity or plasticity of the material whereas plastic deformation is desirable for stronger tablets, excess plasticity usually results in tablets that tend to cap and laminate. ... 

Typical surface-force isotherms, F(D)IR, for adsorbed PLL-g-PEG copolymer films measured in 1 mM KCl aqueous solution are shown in Fig. 2. The value D = 0 nm indicates the absolute location of the mica surfaces ( 0.2 nm, absolute). Under compression, D represents the polymer film thickness. Two curves are shown in curve a the adsorbed copolymer covers only one mica surface and in curve b both surfaces are covered by the adsorbed copolymer film. The measured force is chiefly repulsive, i.e., FjR > 0. The high-load regime can be well described by an exponential function. The film thickness of curve b is slightly less than twice that of curve a for a comparable compression. The exponential decay length is larger for the symmetric case. We note that the highly compressed copolymer film (FIR > 1 mN/m) deforms virtually free of hysteresis, which suggests an equilibrium... 

It has been shown [56] that if we measure the areas under the approach and retract curves of the force-distance plot we can get quantitative values of the resilience. Resilience is closely related to the ability of the polymer chain to rotate freely, and thus will be affected by rate and extent of deformation, as well as temperature. Different materials will respond differently to changes in these variables [46] hence, changing the conditions of testing will result in a change in absolute values of resilience and may even result in a change in ranking of the materials. Compared to more traditional methods of resilience measurement such as the rebound resiliometer or a tensUe/compression tester. 

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. 

From the initial region of the stress-strain curve, Young s modulus E and the shear modulus G can be obtained. Both are a measure of the stiffness of a given material, which mirrors the resistance of an elastic body against deflection of an applied force. The point where the stress-strain curve abmptly falls down is known as the fracture point where the sample ruptures. Fracture stress and fracture strain are defined as the maximal stress and deformation (elongation or compression) that a sample can withstand. Material toughness can also be calculated from the area under the stress-strain curve up to ultimate fracture point. It is defined as amount of energy per unit volume required to cause a fracture in a material. 

Mechanical Properties. The measurement of mechanical properties is concerned with load-deformation or stress-strain relationships. Forces may be applied as tension, shear, torsion, and compression and bending. Stress is the force divided by the cross-sectional area of the sample. Strain is the change in a physical dimension of the sample divided by the original dimension. The ratio of the stress to strain is referred to the modulus. Stress maybe applied continuously or periodically at varying rates for dilferent tests. The characteristic stress-strain curve, stress relaxation, or impact behavior is very important in determining the applications and limitations of a polsrmer. 

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