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Constant Load/Deflection Tests


Abstract:
One of the most common and straight forward methods utilized in EAC tests is the use of a constant, applied tensile load that acts as the driving force to EAC. Typically, a tension specimen is employed and specimens are loaded to various levels of applied stress as defined by ASTM G-49. A typical tension specimen and exposure cell is shown in Fig. 1. A distinction is usually made between the procedure of running these types of tension tests with regard to the methods employed for applying the load. A constant load is usually applied using a dead weight fixture. In its simplest form, a hanging weight is suspended from the specimen if small loads are required.
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One of the most common and straight forward methods utilized in EAC tests is the use of a constant, applied tensile load that acts as the driving force to EAC. Typically, a tension specimen is employed and specimens are loaded to various levels of applied stress as defined by ASTM G-49. A typical tension specimen and exposure cell is shown in Fig. 1. A distinction is usually made between the procedure of running these types of tension tests with regard to the methods employed for applying the load. A constant load is usually applied using a dead weight fixture. In its simplest form, a hanging weight is suspended from the specimen if small loads are required.

Alternately, for higher loads, a simple lever can be utilized to magnify the applied load. The stress (S) on an unnotched specimen is calculated with the following formula: S = P/A where P = load on the specimen and A= specimen cross-sectional area. For the case of dead weight loading, a constant load is produced on the specimen. However. once cracking initiates in the specimen, the cross-sectional area is reduced so the applied stress actually increases. Therefore. in these types of tests on susceptible material, the specimen often fails soon after initiation of cracking and little information on crack propagation is obtained. The effects of the corrosion and stress can be focused at a single location by the use of a single notch in the specimen gage section.

An alternative to the constant load tensile test is the constant deflection tensile test. In this case, the dead weight loading mechanisms is replaced by a spring, proof ring, or other compliant device. This type of loading configuration is usually much simpler and allows for more specimens to be tested in a limited area such as a laboratory exhaust hood. However, in this case the load will decrease as the crack propagates through the specimen. If insufficient compliance is available in the loading fixture, the crack will stop prior to specimen fracture. Therefore, for deflection-controlled tests. it is important to obtain information regarding the load/deflection relationship for the geometry of fixture used. Normally, the important aspects of testing are as follows:

  1. Check deflections both before and after tests.

  2. Examine the specimens closely for sub-critical crack growth.

  3. Calibrate the fixture periodically.

  4. If only limited compliance is available, break specimens in air after testing to locate any sub-critical cracks and the reduction in load-carrying capacity they cause.

Other Constant Deflection Specimens

There are a variety of specimens that utilized constant deflection. These include

  1. Bent beam specimens (2-, 3-, and 4-point loading) per ASTM G38 (see Fig. 2)

  2. C-ring specimens per ASTM G3R (see Fig. 3)

  3. U-bend specimens per ASTM G30 (see Fig. 4)

Each type of specimen has a compliance dictated by the specimen dimensions and the modulus of elasticity of the material bring tested. For purely elastic deflections, simple stressing equations can be utilized to relate applied tensile stress to specimen deflection. These are provided in the standard test method documents. One of the limitations inherent to deflection-controlled specimens is the nonlinearity in the stress vs. strain relationship once the elastic limit is surpassed. For determination of deflections on specimens stressed to high percentages of the engineering yield strength (i.e., beyond the yield point of the material), it is common to utilize a strain gauged calibration specimen. This specimen can be used to measure the exact stress/deflection relationship for the combination of material and specimen geometry being used.

Once the stress/deflection relationship for the specimens to be used is determined, it is typical to stress several specimens at various levels of stress. This allows for assessment of the susceptibility to cracking as a function of applied stress. This can be performed either by monitoring time to failure as a function of applied stress or by evaluation of the failure/no-failure performance at a fixed test duration and level of applied stress. This latter technique is particularly useful in quality assurance or lot release testing where the combination of service experience and laboratory testing has provided information regarding the level of laboratory performance required to determine acceptable service performance. In some cases, constant deflection specimens such as bent beams and C rings are not as severe as dead weight-loaded specimens. This is usually due either to variations in susceptibility with orientation or to limited compliance reducing the driving force for crack, propagation.

U-bend specimens are constant deflection specimens hut are normally not stressed to various levels of deflections or load. They are severely plastically deformed in their fabrication by bending a strip of material around a mandrel. Usually, multiple specimens are utilized and failure/no-failure is monitored after various durations of exposure. The plastic deformation in most cases provides a mechanical inducement to the initiation of SCC. Therefore, it can accelerate SCC in some systems that normally require an unacceptably long time of crack initiation on other types of specimens.










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