Modelling of environment assisted cracking

Fracture and 'slow' crack growth reflect the response of a material (i.e. its microstructure) to the conjoint actions of mechanical and chemical driving forces and are affected by temperature. There is therefore a need for quantitative understanding and modeling of the influences of chemical and thermal environments and of microstructure, in terms of the key internal and external variables, and for their incorporation into design and probabilistic implications. This text, which the author has used in a fracture mechanics course for advanced undergraduate and graduate students, is based on the work of the author's Lehigh University team whose integrative research combined fracture mechanics, surface and electrochemistry, materials science, and probability and statistics to address a range of fracture safety and durability issues on aluminum, ferrous, nickel, and titanium alloys and ceramics. Examples are included to highlight the approach and applicability of the findings in practical durability and reliability problems.

Fracture mechanics : integration of mechanics, materials science, and chemistry

The effect of laboratory air, water vapor and oxygen on fatigue crack propagation behavior of aluminum alloy 5456 has been investigated over a range of growth rates spanning five orders of magnitude. Environmental interactions in the intermediate region are found to be associated with microscopic fracture features and mechanisms governing degradation. The alloy shows significantly enhanced crack growth rates in the environments of laboratory air and water vapor than in the oxygen environment. Mechanisms contributing to the enhanced crack growth rate are examined and discussed in light of constituents in the environment, fracture surface features and microstructure.

The effect of environment on fatigue crack growth behavior of aluminum alloy 5456

A mechanistically based approach to probability modeling for corrosion fatigue crack growth

Crack-propagation tests on a bulk metallic glass (BMG), Zr55Cu30Ni5Al10, were conducted either in aqueous sodium chloride (NaCl) solutions or in high-purity water under sinusoidal cyclic loading or sustained loading Although the crack growth rate in high-purity water was almost identical to that in air, the rate in the NaCl solution was much higher than that in air, even in a very low concentration of NaCl such as 001 mass pct In a 35 mass pct NaCl solution, the time-based crack growth rate during cyclic loading, da/dt, was determined by the maximum stress-intensity factor, K
 max, but was almost independent of the loading frequency and the stress ratio, and the rate was close to that of stress corrosion cracking (SCC) under a sustained loading

Stress Corrosion and Corrosion Fatigue Crack Growth of Zr-Based Bulk Metallic Glass in Aqueous Solutions

The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range (ΔK = 31 MPa-m1/2) in deaerated deionized water and 0.3 M Na2SO4 solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions of bare steel surfaces with the deaerated 0.3 M Na2SO4 solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent activation energy of about 40 kJ/M.

A. Alavi

Papers

Effects of frequency and temperature on short fatigue crack growth in aqueous environments