Constant strain tester

Table 4 shows comparative data obtained from plant tests on various types of board. Note that the standard deviations are very similar on the two machines. Possibly the constant strain tester is slightly more precise, but it is apparent that most of the variation is in the products. 

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). 

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 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. 

Another fatigue-testing device is the (Monsanto) Fatigue to Failure Tester. This instrument measures the ultimate fatigue fife as cycles to failure. Tensile-like samples are stretched at 100 cycles per minute at a preselected extension ratio. Samples can be strained over a range of 10 to 120 percent. The fatigue performance of compounds can be measured (number of cycles to failure, i.e., rupture) and compared either at constant extension ratio (strain) or at constant strain energies (work input). 

At small deformations, viscoelastic information can in principle be obtained from stress-strain measurements at a constant strain rate, as shown for shear deformations in equations S6 to S9 of Chapter 3. Such experiments are often made in simple extension, but the deformations can become rather large so there are marked deviations from linear viscoelastic behavior. The most commonly used instrument is the Instron tester other carefully designed devices have been described. - The sample is usually a dumbbell or a ring. In the former case, the strain in the narrow section as checked by separations of several fiducial marks can be calculated from the separation between the clamps by a suitable multiplication factor. In... 

Despite the consolidation and relaxation behaviour the behaviour after a relief of the sample will deliver additional information. Relief tests in the biaxial tester include four steps 1) biaxial consolidation to a prior defined stress with a constant strain rate, 2) relaxation, 3) biaxial relief to a prior defined strain with a constant strain rate and 4) relaxation. A relaxation phase between the consolidation and the relief is necessary because the stress decrease due to relaxation is superimposed by the stress decrease due to the relief and both effects cannot be separated. The relaxation phase after the relief is not obligatory but gives mores information on the samples behaviour at a constant volume after the relief. The stress decrease due to a relief is extremely quick, e.g. a biaxial relief of only 0,5% (volume percent) causes a stress decrease from lOkPa to 2kPa for a strain rate of 0,1%/min. In comparison a stress increase of the same value during a first consolidation would require a strain of approx. 15%. It is a well known fact [4, 5, 6J that the stiffness of bulk solids is much higher for relief tests than for consolidation tests. 

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). 

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. 

A useful approximation of B for a conical hopper is B = 22f/a, where a is the bulk density of the stored product. The apparatus for determining the properties of solids has been developed and is offered for sale by the consulting firm of Jenike and Johansen, Winchester, Massachusetts, which also performs these tests on a contract basis. The flow-factor FF tester, a constant-rate-of-strain, direct-shear-type machine, gives the locus of points for the FF cui ve as well as ( ), the... 

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 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. 

Note AMPT, asphalt mixture performance tester APA, asphalt pavement analyser FN, flow number HWTT, Hamburg wheel track tester IDTHT, indirect tensile strength at high-temperature test MPSS, maximum permanent shear strain SST/ RSCH, superpave shear tester/repeated shear constant height. 

Turkey feathers with their inner quills removed and consisting of pennaceous and plumulaceous fibers attached to the outer quill were supplied by MaXim LLC (Pasadena, CA). Two different processes were used to convert these into fabric, depending on whether they were going to be converted into yam then knit fabric or converted directly into nonwoven fabric. Before processing commenced, the fibers were characterized in terms of length, tenacity, strain, and modulus. One hundred plumulaceous fibers and one hundred pennaceous fibers were tested with an Instron Model 1125 constant-rate-of-extension tester interfaced with an IBM Personal System/2 Model 55 SX computer equipped with Labvantage Series IV software. "... 

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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