Corrosion fatigue specimen types

Types of tests. Laboratory corrosion fatigue tests can be classified as either cycles-to-failure (complete fracture) or crack propagation (crack growth) test. In cycles-to-failure testing, specimens or parts are subjected to a sufficient number of stress cycles to initiate and propagate cracks until complete fracture occurs. Such data are usually obtained by... 

Types of specimens for investigating stress-corrosion crackin (SCC) and corrosion fatigue. Source Ref 109... 

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. 

In contrast to stress corrosion cracking, corrosion fatigue is not just limited to specific metal-environment systems, but affects all metals that are subjected to cyclic tensile stresses in a corrosive environment. Figure 11.46 illustrates the non-specificity of corrosion fatigue [24]. The number of cycles to failure is plotted as a function of the amplitude of the applied stress for duplex stainless steel samples exposed to air, or immersed in a solution of either 3% NaCl or 0.05M H2SO4. The specimens degrade more rapidly in sulfuric acid in spite of the fact that this solution does not cause stress corrosion cracking for this type of steel, contrary to the NaCl solution. 

While many laboratory tests for resistance of metals to stress corrosion cracking have been developed, only a few tests are amenable to actual in-situ testing in seawater. These primarily consist of the exposure of statically stressed type test specimens such as described in ASTM G 30, G 38, G 39, and ISO 7539-2, ISO 7539-3, and ISO 7539-5. In addition, welded specimens such as described in ASTM G 58 are excellent for evaluation of the stress corrosion resistance of weldments in simple immersion tests. Evaluation of corrosion fatigue is usually limited to laboratory testing. 

Considerable attention has been devoted to the interaction between mechanical loading and corrosion phenomena. Corrosion fatigue has been investigated with sinusoidal loading [26] and with force patterns more representative of actual in vivo stresses [27]. Both specimens of simple geometry [28] and actual prosthetic devices [29] have been tested. SCC has been studied using U-bend [30], stress ring [37], bent beam [37], and fracture mechanics [32] type... 

In plain carbon steels and titanium alloys with body-centred cubic lattice, there is a true fatigue limit with a horizontal S-N curve at a number of cycles beyond 2 X 10 to 10 [130] (type I, figure 10.19(a)). This, however, is not true for notched specimens (and thus also for components) or if corrosion or oxidation occur during the experiment. 

Fig. 14.16 SEM examination of a CT specimen fracture surface of type 304 SS showing both a TG fatigue and b IG stress corrosion crack growth [23]...

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