Constant deformation tests

ASTM retains a constant stress method for compression set. and one is also specified for electrical mats in BS 921. In these tests the set is expressed as a simple percentage of the original test piece thickness, and thus it is not unusual to see specification limits and test results that are numerically much smaller than those obtained in an equivalent constant deformation test. Such differences can sometimes lead to confusion and indeed alarm when confronted by users much more familiar with the tests of ISO 815. 

Fig. 9.2 C rings used in constant deformation tests [11], Reprinted, with permission, from ASTM G 38-01 Standard Practice for Making and Using C-Ring Stress-Corrosion Test Specimens, copyright ASTM international West Conshohocken, PA.

Figure 11.17 Effect of strain rate on the striction coefficient in constant deformation tests in inert surroundings, in the presence of stress corrosion cracking, and in the presence of hydrogen embrittlement.

Puncture probes are commonly used for fruits and vegetables, and allow for the determination of force at rupture of the cellular structure. The procedure outlined below is adapted from the method of Bourne (1979). Cone penetrometers are commonly employed for determining firmness and yield value for foods such as margarine and butter, which may be a reflection of the product s spreadability. Quite often it is desirable to use a testing system that provides a constant deformation rate. Additionally, a mechanical testing machine allows for production of a force/deformation curve to further analyze the data. 

Determination of tension set after constant deformation Physical testing of rubber part A5 Method for determination of tension set at normal and high temperatures... 

Two modes of operation are possible in applying a load to an agglomerate. With an automated tester the more common procedure is to move the platens at a constant rate this is a constant deformation rate tester (CDRT). A second mode is possible in which the compressive force on the agglomerate is increased at a constant rate this is a constant loading rate tester (CLRT). Cahn and Karp inski [29] provide an interesting comparison of these two modes of operation and conclude that the CLRT should be used when accuracy and precision are emphasized whereas the CDRT will suffice for routine testing. 

Constant-load SCC tests have been shown to be more severe than constant-deflection tests. Under a constant load, stress increases as the cross-section is reduced by cracking or corrosion. However, this condition produces decreasing stress when deflection is fixed. It has been suggested that SCC threshold stress is associated with the onset of plastic deformation, that is, the elastic limit of the alloy. The elastic limit is difficult to measure unambiguously, however, the stress at which 0.2% plastic deformation occurs is generally used. 

ISO 4600 details a ball or pin impression method for determining the ESCR. In this procedure, a hole of specified diameter is drilled in the plastic. An oversized ball or pin is inserted into the hole, and the polymer is exposed to a stress cracking agent. The applied deformation, given by the diameter of the ball or pin, is constant. The test is multiaxial, relatively easy to perform, and with not very well-defined specimens, and the influence of the surface is limited. Drawbacks are the small testing surface and the undefined stress state. After exposure, tensile or flexural tests may be performed on the specimens. This leads to the determination of either the residual tensile strength or the residual deformation at break. 

PS, and in the range 142-155 °C for PC. The following mechanical test has been adopted the specimens are stretched at constant elongational strain rate (0.05 s l for PS and 0.025 s l for PC) up to a final extension ratio L/Lq close to 2.5 for all of the samples. The stress is then allowed to relax at constant deformation. 

Using these two types of samples, we performed uniaxial compression tests under constant strain rates from 2.8x10 to 2.9x1 O s in a cold room at -10°C. The experimental conditions used in this study are summarized in Table 1. Nine series of the deformation tests were performed so as to study the dependencies on each physical parameter. 

The experimental research of RubCon deformation at uniaxial central compression was carried out. A special testing module consisting of three calibrated rods provided constant deformation speed (Figure 2.5). 

In experiments on the adhesion of a powder, HOPE modified by oil-soluble contact Cl containing salts of organic amines or sulfur acids with either carboxyl or hydroxyl groups has been used. The composition of HDPE - - Cl has been applied to a preliminary cleaned and degreased 100—pm-thick aluminum foil at a temperature of T = 155 5°C and a pressure of p = 7—10 MPa. The splices have then been subjected to thermal treatment in an oven at 200° C for one hour, which corresponded to the maximum strength of the adhesive joints. The strength has been estimated by delamination tests under a constant deformation rate. Not less than 20 samples were tested in each point to show 10-30% variation factor at a confidence interval of 0.95. 

The specimen in these tests is kept at constant deformation stress. The following shapes of... 

There exists a related but different German Standard DIN 53 442 which uses dumb-bell-shaped specimens differing from those used for tensile testing by a rounded middle section. Another difference in comparison with the above ASTM method is the use of constant deformation amplitude of the vibrations. This results in a stress amplitude decreasing with time due to stress relaxation. Apart from this, the stress amplitude diminishes also due to the heating of the specimen. The results are reported in a similar manner as required by the ASTM standard with the stress amplitude relating to the first cycle. 

The gradual decrease in stress with time under a constant deformation (strain) and temperature. Stress relaxation is determined in creep test. Data is often presented as stress vs. time plot. The stress relaxation rate is given by the slope of the curve at any point. 

Deformation tests are started at a certain isotropic stress state, the axial strain is increased with a constant strain rate (Fig. 11.40). The related evolution of volume, AE rate, ultrasonic wave velocities, and permeability is... 

The modes of operation of a DMA are varied. Using a multifrequency mode, the viscoelastic properties of the sample are studied as a function of frequency, with the oscillation amplitude held constant. These tests can be run at single or multiple frequencies, in time sweep, temperature ramp, or temperature step/hold experiments. In multistress/strain mode, frequency and temperature are held constant and the viscoelastic properties are studied as the stress or strain is varied. This mode is used to identify the LVR of the material. With creep relaxation, the stress is held constant and deformation is monitored as a function of time. In stress relaxation, the strain is held constant and the stress is monitored versus time. In the controlled force/strain rate mode, the tanperature is held constant, while stress or strain is ramped at a constant rate. This mode is used to generate stress/ strain plots to obtain Young s modulus. In isostrain mode, strain is held constant during a tanpera-ture ramp to assess shrinkage force in films and fibers. 

F ure 11.6 Real and nominal stress versus time in stress corrosion cracking tests at constant load and constant deformation. 

Figure 11.15 Variation of stress intensity at crack tip as a function of the testing time (a) constant load and (b) constant deformation. The labels 1 and 2 refer to two tests that differ by the magnitude of the applied load or deformation.

Constant-strain tests are more popular and simpler to use. In this case, sample discs of rubber are compressed between two polished steel plates to a predetermined strain by using spacers and bolts. The permanent set is usually quoted as the unrecovered percentage of the applied deformation, and is not related to the original sample height. The BS 903 rubber test specifies a cylindrical test piece 13 mm in diameter and 6.3 mm high. The sample dimensions are especially important in compression due to the shape factor effect. Recovery time before measurement of final set value is usually 30 min., and must be specified as this creep recovery is time-dependent. 

Another common transient test is the relaxation test, refer to Fig. 33. In this test, a constant deformation or strain is suddenly imposed on the material, and the resulting stress required to maintain the constant strain is measured. 

---