Three-point bending clamp

Single- and dual-cantilever clamps Three-point bending clamp Shear sandwich clamp... 

We perform flexural testing on polymer rods or beams in the same basic apparatus that we use for tensile or compressive testing. Figure 8.6 illustrates two of the most common flexural testing configurations. In two-point bending, shown in Fig. 8,6 a), we clamp the sample by one end and apply a flexural load to the other. In three-point bending, shown in Fig. 8.6 b), we place the sample across two parallel supports and apply a flexural load to its center. 

Commercial DMA instruments vary in their design. One commercial instrument is shown in Fig. 16.36, set up for a three-point bend test under dynamic load. A different commercial instrument schematic. Fig. 16.37 shows a sample clamped between two arms that are free to move about the pivot points [Fig. 16.37(a)] the electromagnetic drive and arm/ sample assembly are shown in Fig. 16.37(b). The electromagnetic motor oscillates the arm/sample system and drives the arm/sample system to a preselected amplitude (strain). The sample undergoes a flexural deformation as seen in Fig. 16.37(a). An LVDT on the driver arm measures the sample s response to the applied stress, calculates the modulus (stiffness) and the damping properties (energy dissipation) of the material. 

Three-Point Bending. Here the sample is supported at both ends but not clamped, so clamping effects are eliminated. The strain is applied at the middle by a blunt oscillating probe. 

S.2. Composites Sample end Corrections, Composite Specimen Geometry Effects Sample geometry modes with clamped samples (shear and three-point bending are not clamped) will exhibit errors due to clamping of the sample. Generally speaking, tension suffers the least error, compression... 

Figure 5,55. Effect on modulus measured in DMA due to fiber orientation of a unidirectional fiber composite relative to sample clamping in three-point bending and torsion (from Gerrard, et al., 1990, reprinted with permisssion of the Society of Plastics Engineers).

FIGURE 16.8 Dynamic mechanical analysis testing clamps (used with permission from TA Instruments, New Castle, DE). Different clamp setups can he used for tension, compression, three-point bending, and shear tests. The application of stress (or strain) is sinusoidal. 

Simple shear > tension > clamped bending > three-point bending... 

Figure 4.5 Schematic of available sample geometries, (a) Top left - single cantilever bending (b) top right-dual cantilever bending (c) middle left - tension (d) middle right - compression (e) bottom left-three-point bending (f) bottom right - shear. Note that for these definitions the sample length, /, is always taken as the distance between the fixed clamp and the driveshaft clamp.

Three-point bending is the best choice of geometry for acciuate modulus determination. In fact, testing an accurately machined steel sample (ordinary steel, not stainless) is certainly the best way of verifying a DMAs modulus measuring accuracy. Despite this, a poor choice of sample dimensions or displacement amplitude can yield results which are wrong. First, it is best that no sample is over 5 mm in width. It is very hard to clamp or uniformly load wide... 

A small shear correction is normally apphed to clamped and three-point bending data by dividing fc by 1 +2.9 t/l) for clamped bending and l + 2.9 t/21) for three-point bending. This correction is based on an average Poisson ratio of 0.33 for glassy polymers and 0.5 for rubbers, namely 0.45. 

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