A. B. O. Soboyejo

Micromechanisms of fatigue crack growth in a forged Inconel 718 nickel-based superalloy

Abstract The micromechanisms of fatigue crack propagation in a forged, polycrystalline IN 718 nickel-based superalloy are evaluated. Fracture modes under cyclic loading were established by scanning electron microscopy analysis. The results of the fractographic analysis are presented on a fracture mechanism map that shows the dependence of fracture modes on the maximum stress intensity factor, K max , and the stress intensity factor range, Δ K . Plastic deformation associated with fatigue crack growth was studied using transmission electron microscopy. The effects of Δ K and K max on the mechanisms of fatigue crack growth in this alloy are discussed within the context of a two-parameter crack growth law. Possible extensions to the Paris law are also proposed for crack growth in the near-threshold and high Δ K regimes.

Some engineering properties of limestone concrete

Abstract In order to reduce energy consumption and CO2 emission and increase production, cement manufacturers are blending or intergrinding mineral additives such as slag, natural pozzolans, sand, and limestone. The reduced cost of limestone is mainly due to energy savings by substitution of a portion of the calcined clinker with a small amount of limestone and to the presence of limestone deposits near cement kilns, and hence, reduced transportation costs. This paper reports on a preliminary study underway on the performance of limestone cement mortar and concrete. The effect of different levels of limestone cement replacement (0% to 35%) on physical and mechanical properties of cement mortar is reported, as well as the effect of fineness of both clinker and limestone. From the test results, it was found that it is possible to manufacture cement with limestone addition with comparable or superior performance to that of ordinary Portland cement, provided that proper limestone quality is selected with optimum content and the optimum levels of fineness of both limestone and clinker are used. However, further research is needed to determine long-term performance, especially in marine and hot environments.

Reliability assessment of polysilicon MEMS structures under mechanical fatigue loading

This paper examines the current status and methodologies of study of material and system reliability in Microelectromechanical Systems (MEMS). This includes: a review of the current literature in the area of MEMS regarding failure analysis experimental investigations; testing methods and philosophies for material characterization and possible mechanistic analytical solutions for estimating material properties. The paper proposes a reliability framework that encompasses all the available information. This statistical platform will enable the MEMS design engineer to distill all the available information in the literature into a stand-alone semi-empirical material reliability model, and a holistic system-level model for a complete system.

Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy

The fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K[sub max], and the stress intensity factor range, [Delta]K. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative effects of [Delta]K and K[sub max] on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The influence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.

Chemically and Thermally Treated Vegetable Fibers for Reinforcement of Cement-Based Composites

This paper presents the effects of chemical and thermal treatments on the strength of vegetable fibers that were being considered as reinforcements in cementitious matrix composites for affordable housing. These include vegetable fibers extracted from banana trunks and sugar cane residues. The results suggested that pyrolysis increased the fiber strength, at least by a factor three. Acid attack generally degraded the strengths, while alkali attack had only a limited effect. Pyrolyzed banana leaves, coconut coir, and coconut sheaths fibers were also tested. The implications of the results were discussed for the development of strong cementitious matrix composites.

Porosity, Flow, and Filtration Characteristics of Frustum-Shaped Ceramic Water Filters

AbstractThis paper presents the results of an experimental study of the effects of porosity on the flow rate and Escherichia coli (E. coli) filtration characteristics of porous ceramic water filters (CWFs) prepared without a coating of silver. Clay-based CWFs were fabricated by sintering composites of redart clay and fine woodchips (sawdust) in three different proportions by volume, viz: 50∶50, 65∶35, and 75∶25. Sintering the greenware below 1,000°C produced reddish colored pot of three different degrees of porosity and micro- and nanoscale pores, which are the key to efficient filtration. The porosities and pore size distribution frequencies of the sintered clay ceramics were characterized using mercury intrusion porosimetry (MIP). The porosity of the CWFs ranged from ∼36% to ∼47% and increased with increasing sawdust content in a linear fashion, and the pore size varied from ∼10  nm to ∼100  μm. The volume flow rates of water through the CWFs were investigated by measuring the cumulative amount of water...

A physically-based model for the prediction of long fatigue crack growth in Ti–6Al–4V

Abstract A physically-based model is presented for the prediction of fatigue crack growth in Ti–6Al–4V. The model assumes that the crack extension per cycle is directly proportional to the change in the crack-tip opening displacement, during cyclic loading between the maximum and minimum stress intensity factor. The extent of irreversibility is also assumed to exhibit a power law dependence on the effective stress intensity factor range. A simple power law equation is then derived for the prediction of fatigue crack growth as a function of the effective stress intensity factor range. The model is validated for fatigue crack growth in the near-threshold, Paris and high-Δ K regimes. The fatigue crack growth mechanisms associated with the parametric combinations of stress intensity factor ranges and maximum stress intensity factor are then summarized on fatigue mechanism maps. Mechanistically-based fatigue crack growth relationships are thus obtained for the prediction of fatigue crack growth in the near-threshold, Paris and high-Δ K regimes.

An investigation of the effects of microstructure and stress ratio on fatigue crack growth in Ti–6Al–4V with colony α/β microstructures

Abstract This article examines the combined effects of colony microstructure and stress ratio on long fatigue crack growth in Ti–6Al–4V. The fatigue crack growth rates are compared in lamellar microstructures with fine and coarse lath dimensions produced by controlled cooling (rate) from the β phase field. The effects of β volume fraction are also investigated at stress ratios, R=Kmin/Kmax, between 0.1 and 0.8. The combined effects of the different variables are then modeled using multiparameter concepts. The coarser lamellar microstructures are shown to have better fatigue crack growth resistance than finer lamellar structures. However, β volume fraction is shown to have only a limited effect on fatigue crack growth resistance. Finally, the implications of the results are discussed for microstructural design and fatigue crack growth prediction.

Probabilistic modeling of fatigue crack growth in Ti-6Al-4V

Abstract This paper proposes an approximate approach to efficient estimation of some variabilities caused by the material microstructural inhomogeneities. The approach is based on the results of a combined experimental and analytical study of the probabilistic nature of fatigue crack growth in Ti–6Al–4V. A simplified experimental fracture mechanics framework is presented for the determination of statistical fatigue crack growth parameters from two fatigue tests. The experimental studies suggest that the variabilities in long fatigue crack growth rate data and the Paris coefficient are well described by the log-normal distributions. The variabilities in the Paris exponent are also shown to be well characterized by a normal distribution. The measured statistical distributions are incorporated into a probabilistic fracture mechanics framework for the estimation of material reliability. The implications of the results are discussed for the probabilistic analysis of fatigue crack growth.

An investigation on fatigue and dwell-fatigue crack growth in Ti-6Al-2Sn-4Zr-2Mo-0.1Si

Abstract This paper presents the results of a combined experimental and analytical study of fatigue crack growth and dwell-fatigue crack growth in forged Ti–6Al–2Sn–4Zr–2Mo–0.2%Si (Ti-6242). Following an initial characterization of microstructures and basic mechanical properties, the micromechanisms of long fatigue crack growth are presented for three microstructures. These include: a duplex α/β structure, an elongated α/β structure, and a colony α/β microstructure. The colony microstructure is shown to have the best resistance to fatigue crack growth. The elongated α structure has intermediate resistance, while the equiaxed α structure exhibits the fastest fatigue crack growth rates. The fatigue crack growth rates in the near-threshold, Paris and high ΔK regimes are then characterized with empirical crack growth laws that relate the crack growth rates to the stress intensity factor range and key parameters on the fatigue crack growth curve. Finally, the results of dwell-fatigue crack growth experiments are presented for the three microstructures. The dwell-fatigue crack growth rates are shown to be almost identical to the fatigue crack growth rates in the intermediate ΔK regime. However, the fatigue crack growth rates are faster at higher stress intensity factor ranges. The underlying mechanisms of dwell crack growth are compared with the mechanisms of fatigue crack growth before discussing the implications of the work for the prediction of dwell or fatigue crack growth in Ti-6242. The effects of cyclic frequency on fatigue crack growth are also explored.