Test methods constant-load tests

Turnbull A, Maxwell AS, Pillai S (2000) Comparative assessment of slow strain rate, 4-pt bend and constant load test methods for measuring environment stress cracking of polymers. Polym Test 19(2) 117—129... 

Crooker, T. W., Bogar, F. D., and Yoder, G. R., Standard Method of Test for Constant-Load-Amplitude Fatigue Crack Growth Rates in Marine Environments, Naval Research Laboratory Report 4594, Washington, DC, 1981. 

A variety of specimen shapes and sizes can be used but the most common is a smooth-bar tensile coupon, as described in ASTM E 8, Test Method for Tension Testing of Metallic Materials." In smooth-bar tests, the changes are measured in terms of time to failure, ductility (elongation or reduction-in-area), maximum load achieved, and area bounded by a nominal stress-elongation curve or a true stress/ true strain curve, which are often supported by fractogra-phic examination. The specimen is exposed to the environment while it is stressed under a constant displacement rate. 

Ultrasonic Microhardness. A new microhardness test using ultrasonic vibrations has been developed and offers some advantages over conventional microhardness tests that rely on physical measurement of the remaining indentation size (6). The ultrasonic method uses the DPH diamond indenter under a constant load of 7.8 N (800 gf) or less. The hardness number is derived from a comparison of the natural frequency of the diamond indenter when free or loaded. Knowledge of the modulus of elasticity of the material under test and a smooth surface finish is required. The technique is fast and direct-reading, making it useful for production testing of similarly shaped parts. 

As was cited in the case of immersion testing, most SCC test work is accomplished using mechanical, nonelecdrochemical methods. It has been estimated that 90 percent of all SCC testing is handled by one of the following methods (1) constant strain, (2) constant load, or (3) precracked specimens. Prestressed samples, such as are shown in Fig. 28-18, have been used for laboratory and field SCC testing. The variable observed is time to failure or visible cracldng. Unfortunately, such tests do not provide acceleration of failure. 

In the constant-strain method, the specimen is stretched or bent to a fixed position at the start of the test. The most common shape of the specimens used for constant-strain testing is the U-beud, hairpin, or horseshoe type. A bolt is placed through holes in the legs of the specimen, and it is loaded by tightening a nut on the bolt. In some cases, the stress may be reduced during the test as a result of creep. In the constant-load test the specimen is supported horizontally at each end... 

In order to accurately model the fatigue behavior of rubber, fatigue analysis methods must account for various effects observed for rubber during constant amplitude testing. Effects associated with load level, 7 -ratio (ratio of minimum to maximum loading level), and crack closure are presented in this section. 

Analysis of Table II shows discrepancies in the hardness and stress behavior of a-C(N) H films. Although all the works reported a clear stress reduction upon nitrogen incorporation, the hardness sometimes is quoted as almost constant, or on the other hand clearly decreasing. In addition to the possible effect of different deposition methods and conditions, it can be easily seen that the differences in hardness testing methods are the major source for discrepancies. Constant hardness behavior is only reported with the use microindentation methods, like Vickers and Knoop microhardness. On the other hand, the use of low-load nanoindentation methods always led to a nitrogen-induced decrease in hardness. This is basically the consequence of two factors. The first one is the higher penetration... 

A test method to evaluate the shear stress capability of a seal material is reported [36], An electrolyte-anode-electrolyte trilayer was glass sealed to two metal interconnect plates as shown in Figure 5.11. Shear testing was done in two different modes, constant loading rate and constant displacement rate, to determine the shear modulus and viscosity. 

The softening point is generally determined by slowly heating a test piece under constant load until it experiences a certain deformation. The temperature at which this occurs is known as the softening point. Since the methods used involve empirical and arbitrarily chosen test parameters, the softening point is... 

Determination of tension set under constant elongation, and of tension set, elongation, and creep under constant tensile load Standard test methods for vulcanized rubber and thermoplastic elastomers-tension Testing of rubber and elastomers... 

Tests with Progressive Load In this context a method was developed in the DuPont Marshall Lab. (Philadelphia) that generates and evaluates a single scratch on a surface (nanoscratch method). This method is used for tests along with development work for clearcoats and it is constantly being improved. 

Semi-solid foods, such as soft butter and some cheeses, cannot be formed into samples capable of supporting their own weight. For such foods, compression testing takes the form of cone or die penetrometry, in which a cone, die, needle or sphere is made to penetrate the sample (held in a suitable container) either under constant load or at constant speed, and the penetration depth measured as a function of time. Standard methods for penetrometry of fats are published by the AOCS (AOCS Official Method Cc 16-60, Firestone, 1998) and the British Standards Institution (BS 684 Section 1.11 1976, BSI, 1976c). 

In addition to those standardized tests, two other test methods, monotonic creep and microhardness, have been developed by Hough and Wright [48]. In the monotonic creep test, the strain response to a constant stress rate is monitored. The deviation of the stress-strain characteristics in air and in the fluid of interest is taken to be the initiation of ESC. This method is shown to differentiate to a high resolution between polymers, and in the short term, the ESCR of polymer/fluid pairs that exhibit mild/weak interactions can be distinguished. The microhardness method, in which a pyramidal diamond indentor is pressed into the surface of the polymer component at a known load and for a known time, has the potential for mass screening of plastic/fluid compatibility, including extraction as well as absorption, and should be of interest to polymer suppliers. 

A further development of this procedure is fatigue crack propagation (FCP) experiments in the presence of a stress cracking environment. While in the fracture mechanics test methods described above the specimen is under constant load, in a FCP experiment the specimen is tested under cyclic loading conditions in the presence of a sensitizing medium. 

The results of the ESC experiments obtained with the notched specimen in air, water and phosphoric acid solution are depicted in Fig. 3. The TTF is plotted against the nominal stress, and the dashed lines at 8 and 276 hours denote the respective saturation and the time to hydrolysis (50% chemical degradation see Material and Hydrolysis ). The time to hydrolysis and the TTF cannot be directly compared because they are the result of two totally different test methods, i.e. a tensile test and a constant-load test. However the fracture stress as a function of the exposition time shows a sharp decline around the time to hydrolysis, while there is hardly any influence at shorter exposition times. Therefore a significant influence of only hydrolysis on the TTF can only be expected around and after the time to hydrolysis. 

Thermomechanical analysis methods are used in geometries more commonly associated with traditional mechanical testing to increase sensitivity or to mimic other tests. The most common of these are the flexural and penetration modes. Flexure studies involve loading a thin beam, often a splinter of material, with a constant load of lOOmN or more and heating until... 

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