Constant stress tester

Constant deflection or strain testers have a disadvantage in that once a large crack develops, the stress level drops below the fatigue endurance limit and the specimen does not fail for quite some time. However, this test allows one to observe crack propagation in the specimen due to slow rate of failure. In contrast, in the constant stress tester, once the crack develops, the amplitude of deformation increases and failure occurs very rapidly (54). 

A method for measuring the uniaxial extensional viscosity of polymer soHds and melts uses a tensile tester in a Hquid oil bath to remove effects of gravity and provide temperature control cylindrical rods are used as specimens (218,219). The rod extmder may be part of the apparatus and may be combined with a device for clamping the extmded material (220). However, most of the mote recent versions use prepared rods, which are placed in the apparatus and heated to soften or melt the polymer (103,111,221—223). A constant stress or a constant strain rate is appHed, and the resultant extensional strain rate or stress, respectively, is measured. Similar techniques are used to study biaxial extension (101). 

The slope of the first part of the stress-strain curve is a measure of the flexibility of a board. Numerous attempts have been made to measure this property by measuring load versus deflection on the constant stress type of tester, but it is not practical on this type of machine. The area under the curve measures the total work required to break the board or the toughness of the board. 

Middleman [M24], Goldstein [G8], Furuta et al. [Fll], Lobe and White [LI4], Toki and White [T7], Montes et al. [M37], Osanaiye et al. [OlO], and K. J. Kim and White [K8a]. The apparatus (with constant-temperature chamber) may be placed in a tensile tester and operated in a mode with a fixed velocity V giving a constant shear rate. It may, on the other hand, be used in a creep mode with hanging weights. This provides constant stress experiments. At low stress levels one needs to compensate for the weight of the central member which exerts a gravitational stress [OlO]. At very low stresses one may accurately determine the yield value of rubber-carbon black compounds. Osanaiye et al. [OlO] have made measurements at shear stresses below the yield value. 

The following is the summary of the results of the studies (31) on the effect of stereoregularity on stress-induced crystallization and mechanical properties of the vulcanlzates of propylene oxide copolymers. In the studies on stress-induced crystallization, stress (O ) and birefringence ( ) were measured as a function of temperature using an Instron tester fitted with a thermostatted Insulated chamber and an optical system. In an amorphous rubber, the quantity (T Vo) (where T is absolute temperature) is almost constant. Stress-induced crystallization leads to the formation of crystals oriented in the stretching direction which substantially Increase the birefringence. 

Tensile Testing. The most widely used instmment for measuring the viscoelastic properties of soHds is the tensile tester or stress—strain instmment, which extends a sample at constant rate and records the stress. Creep and stress—relaxation can also be measured. Numerous commercial instmments of various sizes and capacities are available. They vary greatiy in terms of automation, from manually operated to completely computer controlled. Some have temperature chambers, which allow measurements over a range of temperatures. Manufacturers include Instron, MTS, Tinius Olsen, Apphed Test Systems, Thwing-Albert, Shimadzu, GRC Instmments, SATEC Systems, Inc., and Monsanto. 

The method makes use of the principle that a constant ratio of induced stress. s in the stored contents to the consolidating pressure p exists. Thus, for any hopper design for which th.eff curve is available, the shear-tester results can be potted, and the point where/= s is located. Since the distance at which this occurs above the hopper vertex is also known, these values become the hopper dimensions at that point. 

We examined the role of vector percolation in the fracture of model nets at constant strain and subjected to random bond scission, as shown in Fig. 11 [1,2]. In this experiment, a metal net of modulus Eo containing No = 10" bonds was stressed and held at constant strain (ca. 2%) on a tensile tester. A computer randomly selected a bond, which was manually cut, and the relaxation of the net modulus was measured. The initial relaxation process as a function of the number of bonds cut N, could be well described by the effective medium theory (EMT) via... 

Constant strain for stress relaxation tests and constant load creep tests may be conducted in simple devices. Temperature control is critical since the results are usually applied as a spectral representation for structural analysis or research purposes. Figure 8 illustrates a multistation creep tester with automated data recorders. Strain and load endurance tests are conducted in similar devices, but the conditions existing at failure and time to failure are normally the only data required. The endurance tests are used frequently to supplement the constant displacement rate tests for routine evaluation. 

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. 

Typically, in compression tests a cylindrical piece of the test sample is compressed between smooth plates using a Material Tester. Assuming constant volume, the stress and strain (Hencky strain) are calculated from the force, displacement data. However,... 

Many types of fatigue tester are used (flexing beams, rotating beams, constant amplitude of cyclic stress or strain, constant rate of increase in amplitude of stress or strain, etc.). 

The use of the stress-strain curve recorder does not give much additional information with this test. However, the timesaving that is possible would recommend the adoption of the constant strain type of tester by manufacturing plants, testing agencies, and ASTM. 

A flexible boundary cubical triaxial test is another commonly used test for compression studies (Kamath et al., 1993 Li and Puri, 1996). A picture of a triaxial compression tester is shown in Figure 8. It allows not only the application of the three principal stresses independently, but also constant monitoring of the volumetric deformation and deformations in three principal directions. In a triaxial compression test, the specimen is at an initial isotropic state of stress then the three pressure lines apply the same pressure at the same rate to all six faces thus pressure is the same in all three directions (i.e., cti = 02 = 03). 

Stress-strain behavior represents the response of a material to loading. Tests are performed on a universal testing machine (UTM), sometimes referred to as a tensile tester because of the primary mode of deformation used to characterize this form of behavior. Specimens are typically deformed at a constant speed, for reasons explained later. Since the properties vary significantly with temperature, tests may be conducted within an environmental chamber to obtain data at elevated and subambient temperatures. The most common information obtained from these tests are the modulus and tensile strength. 

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