Flexure pivots

Figure 6-25. Schematic of an integral centering flexure pivot spring squeeze film damper.

The flexural pivot is free of the problems associated with the knife edge. However, it can act as a spring and therefore may add an extra force to the weighing process. This force may artificially subtract from an object s true mass and provide inaccurate readings. This effect may arise as a result of time or as temperature changes. Manufacturing design may decrease this effect, and it may also be... 

The flexural pivot may become more or less stiff due to time and/or temperature. 

One instmment capable of measuring the dynamic shear modulus is a dynamic mechanical analyzer (DMA). A DMA measures the viscoelastic properties of a material by measuring the mechanical response that is deformed under periodic stress. Operation of a DMA tool offered by TA Instruments is as follows The sample is clamped between the ends of two parallel arms, which are mounted on low-force flexure pivots, allowing motion only in the horizontal plane. The distance between the two arms is adjustable by means of a precision mechanical slide to accommodate a wide range of sample lengths (from < 1 mm up to 65 mm). An electromagnetic motor attached to one arm drives the arm/sample to a strain (amplitude) selected by the operator. As the arm/sample system is displaced, the sample undergoes a flexural deformation [as depicted schemati-... 

Figure 2-3 shows the configurations for two electronic analytical balances. In each, the pan is tethered to a system of constraints known collectively as a cell. The cell incorporates several flexures that permit limited movement of the pan and prevent torsional forces (resulting from off-eenter loading) from disturbing the alignment of the balance mechanism. At null, the beam is parallel to the gravitational horizon and each flexure pivot is in a relaxed position. 

Fig. 10. Cross section of an EMFC weighing cell. Item 1, pan 2, hanger 3, parallel guide 4, chassis 5, flexure 6, lever 7, flexible link 8, pivot point 9, position indicator 10, position sensor 11, permanent magnet 12, permanent magnet air gap 13, compensation cod. See text.

Another resonant frequency instmment is the TA Instmments dynamic mechanical analy2er (DMA). A bar-like specimen is clamped between two pivoted arms and sinusoidally oscillated at its resonant frequency with an ampHtude selected by the operator. An amount of energy equal to that dissipated by the specimen is added on each cycle to maintain a constant ampHtude. The flexural modulus, E is calculated from the resonant frequency, and the makeup energy represents a damping function, which can be related to the loss modulus, E". A newer version of this instmment, the TA Instmments 983 DMA, can also make measurements at fixed frequencies as weU as creep and stress—relaxation measurements. 

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. 

Figure 5.5. Sample under flexural load force is applied at a central point and the sample is supported at the ends on pivot points.

The above beneficial aspects of the wall behavior are due to the large horizontal dimension of the wall, combined with the no-tension feature of cracked concrete (similar to the interface between a footing and the ground) that causes the flexural rotation to take place about a pivot near the edge of the wall section. These phenomena are of purely geometric origin (due to coupling of the rotations with the vertical... 

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