Testing precracked specimens

The most widely used corrodent for testing precracked specimens is 3.5% NaCl solution applied dropwise to the precrack two or (usually) three times daily (Ref 43-46). This intermittent wetting technique accelerates SCC growth, but it also causes troublesome corrosion of the mechanical precrack. Less corrosive corrodents that have been used include substitute ocean water (ASTM D 1141) and an inhibited salt solution containing 0.06 M NaCl, 0.02 M sodium dichromate, 0.07 M sodium acetate, and acetic acid to pH 4 (Ref 45, 46, 50). Some investigators have tested 7000 series alloys in distilled water (Ref 47) and in water v or at 40 °C (104 °F) (Ref 54). Topical test durations thk have been used range from 200 to 2500 h. 

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. 

The equipment required for slow strain-rate testing is simply a device that permits a selection of deflection rates whilst being powerful enough to cope with the loads generated. Plain or precracked specimens in tension may be used but if the cross-section of these needs to be large or the loads high for any reason, cantilever bend specimens with the beam deflected at appropriate rates may be used. It is important to appreciate that the same deflection rate does not produce the same response in all systems and that the rate has to be chosen in relation to the particular system studied (see Section 8.1). 

Smith, H. R. and Piper, D. E., Stress Corrosion Testing with Precracked Specimens, The Boeing Co, D6-24872, ARPA 878, June (1970)... 

Method for the preparation and use of precracked specimens Method for slow strain rate testing... 

Precracked specimen configurations for stress-corrosion testing... 

Predictions of the corrosion performance should be obtained from published data and through testing.The essential requirements of accelerated testing are that the acceleration should produce the same mode of failure and reflect at least a known order of resistance of some alloys in service media.168 The most common approaches employed to achieve testing objectives in SCC are the use of high stresses, slow continuous straining, precracked specimens, higher concentration of species in the test environment than in the service environment, increased temperature, and electrochemical stimulation.169 For electrochemical corrosion, the properties of the medium at the interface should be considered in accelerated tests. 

Another polymer that has received considerable attention in the literature concerning effects of M on fatigue performance is PVC. In tests on unnotched specimens at 3.5 Hi, tested in the presence of nitrogen gas or ethanol vapor, specimens having M = 51,000 fractured within a few seconds while specimens having M = 220,000 develop l no fractures even after 1,000 min Extensive tests have also been made on precracked specimens of PVC, both plasticized and unplasticized As M... 

After precracking, specimens were kept in environment of DT tests for about 20 hours, and then DT tests were carried out. In this study, in order to avoid hysteresis of the load-relaxation method (Sano, 1988), all of... 

An important aspect of EIC experiments concerns the shape of the test specimens and whether they contain a crack or not at the start of an experiment. One distinguishes two broad categories precracked specimens and non-precracked specimens. Precracking is accomplished by subjecting the test specimen to a cychc stress by means of a tensile test machine in order to initiate a crack. The precracked specimen is then subjected to the chosen test conditions for the study of the rate of crack growth. In non-precracked specimens both crack initiation and crack growth occur during the actual test. 

Static tests carried out with non-precracked specimens allow us to characterize, for a given environment, the sensitivity of an alloy to stress corrosion cracking and hydrogen embrittlement. TyfpicaUy, the test specimen subjected to a constant load or a constant strain is exposed to a corrosive environment and one measures the time to failure. The test specimens sometimes contain a notch—not to be confused with precracking—that fixes the location where failure will occur. In constant strain tests, the time to failure corresponds to the appearance of the first cracks. This kind of experiment thus indicates the crack initiation time. In constant load tests, on the other hand, the time to failure is the time leading to fracture of the specimen. It corresponds to the sum of the crack initiation and the crack propagation times. 

Static tests with precracked specimens Principle of the method... 

Corrosion fatigue testing with non-precracked specimens... 

In fatigue tests with non-precracked specimens one measures the time to failure tf, which is usually expressed in number of cycles ... 

The time to failure observed in corrosion fatigue testing with non-precracked specimens not only depends on the applied mechanical load and the corrosivity of the environment but also on other variables such as ... 

In practice, different types of fatigue testing machines are used for corrosion fatigue experiments, using either traction or bending. The time to failure varies between different experimental arrangements. A direct comparison of test results obtained with non-precracked specimens is possible only if the conditions with respect to all of the discussed variables are the same. 

The time to failure measured on non-precracked specimens represents the sum of the initiation and propagation times of a crack. Unfortunately, crack initiation, which often is a slow step, is a random phenomenon. As a result, the reproducibility of corrosion fatigue tests earried out with non-precracked specimens is usually mediocre. Furthermore, to determine a Wohler eurve, a large number of specimens is needed. Statistical methods are usually applied to determine the endurance limit in corrosion fatigue tests. 

In general, the importance of corrosion fatigue in a given environment increases with the intrinsic corrosion rate of the metal. In Figure 11.46, for example, the rate of uniform corrosion of the stainless steel is more important in sulfuric acid than in neutral NaCl solution. The time to failure for the specimen in sulfuric acid is therefore shorter. The results of Figure 11.47 confirm the importance of general corrosion [25]. Corrosion-fatigue tests were carried out on non-precracked specimens of carbon steel in a solution of 3% NaCl. Different anodic current densities were applied in order to... 

The mechanisms that control crack initiation in corrosion fatigue are poorly understood. It is generally believed, however, that corrosion in its various forms (pitting, intergranular, acid, at inclusions etc.) plays a dominant role. The observed effect of the general corrosivity of the environment on the time to failure in Figure 11.46 supports this statement, because the results were obtained on non-precracked specimens and this kind of test provides mostly an indication of the crack initiation time. 

---