Robert P. Wei

Microconstituent-Induced Pitting Corrosion in Aluminum Alloy 2024-T3

Abstract Free corrosion immersion experiments were conducted on a commercial airframe material, Al 2024-T3 (UNS A92024), in 0.5 M sodium chloride (NaCl) solution to investigate the role of microcon...

Some aspects of environment-enhanced fatigue-crack growth

Abstract Metal fatigue as an engineering problem has been well recognized. Many experiments have been carried out in recent years to assess the effects of load amplitude, mean load, load range and test frequency on the rate of fatigue-crack growth to characterize material response to different load profiles and to develop methods for estimating the service behavior of structural components. Recent studies have shown that fatigue-crack growth can be influenced strongly by the test environment; the effect being dependent on the particular material-environment involved. These results suggest that careful reassessments of the data of previous studies and of the proper direction for future fatigue research will be required. Hence, a critical review and analysis of existing fatigue-crack-growth data on high-strength aluminum and titanium alloys and high-strength steels has been made. The influences of load profile, test frequency, temperature, test environment and state-of-stress on fatigue-crack growth are discussed. Correlation with environment-enhanced crack growth under sustained loads and the associated changes in material response are examined. Possible mechanisms, synergistic effects and implications regarding methods for service-life estimation are considered.

A study of crack closure in fatigue

Abstract Crack closure phenomenon in fatigue was studied by using a Ti-6Al-4V titanium alloy. The occurrence of crack closure was directly measured by an electrical potential method, and indirectly by load-strain measurement. The experimental results showed that the onset of crack closure depends on both the stress ratio, R , and the maximum stress intensity factor, K max . Crack closure was not observed for stress ratio, R , greater than 0.3 in this alloy. A two-dimensional elastic model was used to explain the behavior of the recorded load-strain curves. Closure force was estimated by using this model. Based on the estimated closure force, the crack opening displacement was calculated. This result showed that onset of crack closure detected by electrical-potential measurement and crack-opening-displacement measurement is the same. The implications of crack closure on fatigue crack are considered. The experimental results show that crack closure cannot fully account for the effect of stress ratio, R , on crack growth, and that it cannot be regarded as the sole cause for delay.

Transition from pitting to fatigue crack growth—modeling of corrosion fatigue crack nucleation in a 2024-T3 aluminum alloy

Abstract The nucleation of fatigue cracks from corrosion pits was investigated by conducting fatigue experiments on open-hole specimens of a 2024-T3 aluminum (bare) alloy in 0.5 M NaCl solution at room temperature and different load frequencies from 0.1 to 20 Hz. The maximum cyclic stresses applied at the hole ranged from 144 to 288 MPa and the load ratio, R , was 0.1. A specimen subjected to pre-corrosion in the NaCl solution prior to corrosion fatigue was also investigated. Pitting was found to be associated with constituent particles in the hole and pit growth often involved coalescence of individual particle-nucleated pits. Fatigue cracks typically nucleated from one or two of the larger pits, and the size of the pit at which the fatigue crack nucleates is a function of stress level and load frequency. The observations indicate that the nucleation of corrosion fatigue cracks essentially results from a competition between the processes of pitting and crack growth. Pitting predominates in the early stage of the corrosion fatigue process, and is replaced by corrosion fatigue crack growth. Based on these results, two criteria are proposed to describe the transition from pit growth to fatigue crack growth: (1) the stress intensity factor of the equivalent surface crack has to reach the threshold stress intensity factor, Δ K th , for fatigue crack growth, assuming that a corrosion pit may be modeled by an equivalent semi-elliptical surface crack, and (2) the time-based corrosion fatigue crack growth rate also exceeds the pit growth rate.

Environmental fatigue crack propagation of aluminum alloys at low stress intensity levels

Environmental fatigue crack propagation in 2024-T3, 7075-T6, and 7178-T6 has been studied at low levels of cyclic amplitude of stress intensity, ΔK. Both wedge force loading and remote loading techniques were employed to achieve the desired ΔK levels, and preliminary experiments were designed to test their compatibility. Testing was carried out in humid air, distilled water, and 3.5 pct sodium chloride solution, and the observed crack growth rates compared with those in desiccated air. Later studies were also conducted in an inert reference environment with a total water content of less than 2 ppm. When the data are plotted as log ΔK vs log d2a /dN, alloy 2024-T3 exhibits a marked slope transition, alloy 7075-T6 a slight slope transition, and alloy 7178-T6 a rectilinear behavior throughout the whole range of ΔK studied. The basic shape of these curves is discussed in terms of state-of-stress conditions at the crack tip, frequency effects, environmental effects, strain rate sensitivity, and metallurgical structure. An attempt is also made to correlate the rate of fatigue crack propagation in a particular environment and at a particular ΔK level with the fracture topography.

Gaseous hydrogen embrittlement of high strength steels

The kinetics of sustained-load subcritical crack growth in hydrogen were determined for 18Ni(200) and 18Ni(250) maraging steels over a range of hydrogen pressures and temperatures. Crack growth in each steel was characterized by an apparent threshold stress intensity, a domain where the growth rate increased sharply with stress intensity (K) (Stage I), and a range where the growth rate was independent ofK (Stage II). The rate-limited Stage II crack growth in these steels exhibited three distinct regions of temperature dependency, with a different isothermal pressure dependence in each region. In the low temperature region, Stage II crack growth was thermally activated with δH = 18.2 ±1.7 kj/mol; (δH being independent of hydrogen pressure and yield strength). The growth rates at a givenK were proportional to the square root of hydrogen pressure. In the intermediate temperature region, Stage II growth rates increased at slower rates, passed through a maximum and then decreased with increasing temperature. Within this region, the pressure dependence for crack growth increased from 1/2-power to 2.0-power with increasing temperature. Above a transition temperature, each grade of maraging steel became essentially immune to gaseous hydrogen embrittlement for the hydrogen pressure range considered. The transition temperature was strongly affected by yield strength and hydrogen pressure. Plausible explanations for these phenomenological results are considered.

A PROBABILITY MODEL FOR THE GROWTH OF CORROSION PITS IN ALUMINUM ALLOYS INDUCED BY CONSTITUENT PARTICLES

Abstract Localized corrosion in the form of pitting in certain classes of aluminum alloys, e.g. 2024-T3 and 7075-T6, is recognized as one of the degradation mechanisms that affect the durability and integrity of structures, and it is a concern for commercial transport and military aircraft. Pitting has been shown to initiate at constituent particles, which are either anodic or cathodic relative to the matrix, and involves complex electrochemical processes. Furthermore, local interactions between particles and the matrix enhance the rate of pit growth. Probabilistic modeling for the growth of corrosion pits in aluminum alloys in aqueous environments in presented. The focus of the growth process is specifically the role of clustered particles. The purpose of the effort is to estimate the cumulative distribution function ( cdf ) for the size of corrosion pits at a given time for use in multi-site damage and crack growth analyses. The model incorporates the evolution of local damage at each particle and the corrosion pit growth involving interactions with neighboring particles. Some of the key random variables included are the size, density and location of both the anodic and cathodic particles. Statistical estimates for the distribution functions of the underlying random variables are based on experimental observations.

Modelling of small fatigue crack growth interacting with grain boundary

Abstract The slip band at the tip of a small fatigue crack interacting with grain boundaries is modelled for four cases: a slip band not reaching the grain boundary, a slip band blocked by the grain boundary, a slip band propagated into an adjacent grain, and a slip band propagated through one and then blocked by a second grain boundary. The theory for continuously distributed dislocations is used to calculate the crack-tip sliding or opening displacement and the microscopic stress intensity factor under tensile and shear loading. Assuming that the range of the tip displacement directly determines the propagation rate of both Stage I and II cracks, prediction of the propagation behavior of a small crack is made as a function of the distance between the crack tip and the grain boundary, and of the difficulty to propagate slip into adjacent grains, as well as a function of crack length and stress level. The directions for further development of modelling are discussed.

Fracture mechanics and surface chemistry studies of subcritical crack growth in AISI 4340 steel

Coordinated fracture mechanics and surface chemistry experiments were carried out to develop further understanding of environment enhanced subcritical crack growth in high strength steels. The kinetics of crack growth were determined for an AISI 4340 steel (tempered at 204°C) in hydrogen and in water, and the kinetics for the reactions of water with the same steel were also determined. A regime of rate limited (Stage II) crack growth was observed in each of the environments. Stage II crack growth was found to be thermally activated, with an apparent activation energy of 14.7 ±2.9 kJ/mole for crack growth in hydrogen, and 33.5 ± 5.0 kJ/mole in water. Fractographic evidence indicated that the fracture path through the microstructure was the same for these environments, and suggested hydrogen to be the embrittling species for environment enhanced crack growth in hydrogen and in water/water vapor. A slow step in the surface reaction of water vapor with steel was identified, and exhibited an activation energy of 36 ± 14 kJ/ mole. This reaction step was identified to be that for the nucleation and growth of oxide. The hydrogen responsible for embrittlement was presumed to be produced during this reaction. On the basis of a comparison of the activation energies, in conjunction with other supporting data, this slow step in the water/metal surface reaction was unambiguously identified as the rate controlling process for crack growth in water/water vapor. The inhibiting effect of oxygen and the influence of water vapor pressure on environment enhanced subcritical crack growth were considered. The influence of segregation of alloying and residual impurity elements on crack growth was also considered.

In-Situ Monitoring of Pitting Corrosion in Aluminum Alloy 2024

Abstract An in-situ monitoring method was used to observe the initiation sites and the processes of pitting corrosion in aluminum alloy 2024-T3 (UNS A92024) in real time. These observations complemented more detailed information about composition and distribution of constituent particles, pre- and post-corrosion surface morphology, and the internal morphology of corrosion pits. In-situ observations provided a comprehensive view of the development of localized corrosion in real time. Results confirmed the importance of intermetallic constituent particles in promoting initiation and growth of pits in aluminum alloys. Heterogeneous distribution of these particles served to define the location and extent (or severity) of pitting. A conceptual model was proposed as a framework for continued research. The formation of occluded cells under corrosion product domes over severe pits was observed. This formation will be incorporated into an overall reconsideration and modeling of the processes of pitting corrosion o...