The mechanisms of crack growth in stress corrosion cracking of aluminium alloys

Modern understanding of possible mechanisms of crack growth in corrosion cracking of aluminium alloys immersed in various corrosion media such as aqueous solutions, saturated and undersaturated vapour, gaseous hydrogen, etc. is discussed in this review. The experimental data used as a basis to conclusions about the mechanisms of corrosion cracking in aluminium alloys are critically examined. A special emphasis is given to new methods to determining the effect of local anodic dissolution and hydrogen embrittlement in crack growth, i.e. the method of comparative tension and torsion tests deformations of types I and III, and the method based on evaluating ambiguous effect of cathodic polarization on crack growth.     

Rate-determining processes for corrosion fatigue crack growth in ferritic steels in seawater

The results of recent experiments on measuring corrosion fatigue crack propagation rates in structural steel immersed in seawater with and without cathodic polarisation are reported. These include measurements of electrochemical potentials near the growing crack tip. The experiments have been designed to learn more about the mechanisms and rate-determining processes influencing the rate of crack growth. From these and other results in the literature, deductions are made about the relative importance of mechanical and electrochemical limitations on the rate of crack propagation.     

Effects of dislocation dynamics and microstructure on crack growth mechanisms in two-phase titanium aluminide alloys

The fracture behaviour of two-phase titanium aluminide alloys was characterized by fracture toughness tests performed in a wide temperature range on chevronnotched three point bending bars. Temperature and rate dependent deformation processes were characterized by temperature and strain rate cycling tests. The alloy investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The temperature dependence of the fracture toughness was rationalized in terms of micro-processes which determine the glide resistance of the dislocations in the plastic zone of crack tips. The implications of such observations for the engineering application of the materials are addressed briefly.     

Superior fatigue crack growth resistance, irreversibility, and fatigue crack growth–microstructure relationship of nanocrystalline alloys

While previous studies have reported that nanocrystalline materials exhibit poor resistance to fatigue crack growth (FCG), the electro-deposited nanocrystalline Ni–Co alloys tested in this paper show superior resistance to FCG. The high damage tolerance of our alloy is attributed to the following: alloying with Co, low internal stresses resulting in stability of the microstructure, and a combination of high strength and ductility. The high density of grain boundaries interact with the dislocations emitted from the crack tip, which impedes FCG, as predicted by the present model and measured experimentally by digital image correlation. Further, the addition of Co increases the strength of the material by refining the grain size, reducing the fraction of low angle grain boundaries, and reducing the stacking fault energy of the material, thereby increasing the prevalence of twinning. The microstructure is stabilized by minimizing the internal stress during a stress relief heat treatment following the electro-deposition process. As a result grain growth does not occur during deformation, leaving dislocation-mediated plasticity as the primary deformation mechanism. The low internal stresses and nanoscale twins preserve the ductility of the material, thereby reaching a balance between strength and ductility, which results in a superior resistance to FCG.     

Predicting the mechanisms and crack growth rates of pipelines undergoing stress corrosion cracking at high pH

A fundamentally based mathematical model was developed with the goal to predict, as a first step, the crack growth rate (CGR) of high pH stress corrosion cracking (SCC) of buried steel pipelines. Two methods were used to predict CGRs and for both methods the model has included the film rupture and repassivation mechanism. The two methods are distinguished by the expression used to determine the active anodic current density at the crack tip. In the first method, this current density is expressed by the anodic polarization curve with a large peak current density and the prediction tends to yield a larger CGR and a lower pH at the crack tip. By contrast, when the Butler–Volmer equation is used to express the crack tip anodic current density, with a predicted low CGR the chemistry at the tip does not appear to have any significant change due to the high buffer of the solution.The predicted mechanism responsible for the steady-state crack growth is shown to be the balance between the increasing stress intensity factor as the crack grows, which tends to increase the crack tip strain rate and thus the CGR, and the change of the crack tip condition, which, for large CGRs, is the significant shift in the more negative direction of the crack tip potential, and for low CGRs, the increase of ferrous ion concentration, and either tends to decrease CGR.Limitations currently existing in the model and proposal for further development of the model are discussed.     

Subsurface crack initiation and propagation mechanisms in gigacycle fatigue

In the very high cycle regime (N_f > 10⁷ cycles) cracks can nucleate on inclusions, “supergrains” and pores, which leads to fish-eye propagation around the defect. The initiation from an inclusion or other defect is almost equal to the total crack growth lifetime, perhaps much more than 99% of this lifetime in many cases. Integration of the Paris law allows one to predict the number of cycles to crack initiation. A cyclic plastic zone around the crack exists, and recording the surface temperature of the sample during the test may allow one to follow crack propagation and determine the number of cycles to crack initiation. A thermo-mechanical model has been developed. In this study several fish-eyes from various materials have been observed by scanning electron microscopy, and the fractographic results analyzed as they related to the mechanical and thermo-mechanical models.     

Slow crack growth resistance and bridging stress determination in alumina-rich magnesium aluminate spinel/tungsten composites

The slow crack growth (SCG) resistance (V–K_I diagrams) of magnesium aluminate spinel and its tungsten composites with different metallic content (7, 10, 14 and 22 vol.%) is reported. It is found that tungsten plays a crucial role in the composite by increasing crack resistance: the higher the W content, the higher the stress intensity factor needed for crack extension at a given rate. The reinforcement is due to the bridging mechanism performed by metal particles, as it strongly affects the compliance of cracked specimens. Its magnitude is estimated by a compliance function Φ(a) from a double torsion test. From the compliance function, R-curve behaviour is predicted for the composite with highest tungsten content. It explains the effect of metal particles on SCG curves. The W–MgAl₂O₄ interface is believed to influence the reinforcement mechanism.     

On the mechanisms of environment sensitive cyclic crack growth of nuclear reactor pressure vessel steels

An analysis of the cyclic crack growth rate data generated so far for pressure vessel materials in simulated light water reactor environments suggests a strong dependence on frequency, load ratio, waveform, temperature and material composition. To account for these observations a mechanistic crack growth model has been advanced based on hydrogen-induced cracking, where anodic dissolution creates the conditions for hydrogen absorption at the crack tip. Hydrogen-induced cracking starts from the manganese sulfide inclusions, which act as strong hydrogen traps. The hydrogen-induced crack growth around the manganese sulfide inclusions generally spans several prior austenite grains. At high hydrogen input rates brittle crack growth also occurs, this being unrelated to inclusions. When the crack growth exposes manganese sulfide inclusions, these dissolve and the crack tip environment becomes aggressive and conducive to hydrogen absorption. This hydrogen-induced cracking model explains why inclusions form a preferred crack path, and accounts for the effect of sulfur on crack growth rate both in PWR and BWR conditions. Based on the model, the observed crack growth rate dependence on different testing variables can also be explained.     

Influence of retained austenite on short fatigue crack growth and wear resistance of case carburized steel

The influence of the amount of retained austenite on short fatigue crack growth and wear resistance in carburized SAE 8620 steel was studied in this article. Different amounts of retained austenite in the microstructure of the carburized case were obtained through different heat treatment routes applied after the carburizing process. The wear tests were carried out using pin on disk equipment. After every 200 turns the weight loss was registered. Four point bend fatigue tests were carried out at room temperature, using three different levels of stress and R=0.1. Crack length versus number of cycles and crack growth rate versus mean crack length curves were analyzed. In both tests the results showed that the test pieces with higher levels of retained austenite in the carburized case exhibited longer fatigue life and better wear resistance.     

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.     

Corrosion-fatigue crack growth in titanium alloys

Fatigue crack growth data in the form of a da/dN-ΔK-curve can be used for component design and for prediction of residual lifetime. High-strength titanium alloys have proven themselves in aerospace applications and are being seriously considered for use in automotive technology. Depending on the type of alloy (α, (α +β)- or metastable β-), microstructure and/or texture development reacts differently to thermomechanical processing, which is reflected in the mechanical properties. Fatigue crack growth in titanium can be more strongly affected by changes in microstructure than in steels or aluminum alloys. This depends in part on the anisotropic nature of the hexagonal α-phase and in part on the extreme variation in microstructural morphologies (lamellar or equiaxed) which can be present in certain high-strength titanium alloys. Such effects can be exacerbated under the influence of a corrosive environment. The influence of loading frequency must be taken into account when corrosion-fatigue crack growth is considered. While high-strength (α + β)-alloys react sensitively to the effect of loading frequency, α- and metastable β-alloys tend to be insensitive. This behavior can be explained with the differing consequences of an increased level of hydrogen ahead of a crack tip.     

Fracture resistance evaluation of RuO2-based thick film resistor material by in situ crack extension observation in a scanning electron microscope

Parameters for the fracture mechanics of thick film materials are scarce in the literature. One reason is that for many such materials it is very difficult to produce a bulk specimen as required for most standard tests. This paper describes an alternative method for measuring the fracture resistance of a ruthenium dioxide (RuO₂)-based thick film resistor material for electronic applications. The method is based on an in situ investigation of crack propagation in the loaded material. The investigated material is printed as a thick film on a substrate of low-temperature-co-fired-ceramic. An initial crack in the film is introduced with a Vickers indenter. The crack is subsequently loaded with a four-point bending equipment in a scanning electron microscope, which allows for in situ crack length measurement. The crack growth measurements reveal that once a certain crack length is achieved the load required to extend the crack becomes independent of the crack length. Beyond this length, the crack propagates in the so-called steady-state region, which is used in the present method to estimate the fracture resistance of the film. Both tensile stresses resulting from bending and tensile residual stresses are taken into account. Although a brittle substrate was used, the crack did not penetrate into the substrate. The measured fracture resistance of 0.69 ± 0.14 MPa√m is found to be realistic for the investigated thick film material with high silicate glass content.