A model on fatigue crack growth rates in the threshold region in polycrystalline metallic materials and analysis of crack closure

Over eighty percent of all mechanical failures have their origin fatigue. Fatigue is a very important phenomenon to be considered in the design and application of materials. Study of crack growth under fatigue conditions is a very broad subject since various microstructural mechanical, and environmental conditions affect fatigue crack growth in a compounding manner. Most of any material's fatigue life is spent in the threshold region of crack growth process. The current study is a major attempt to quantify fatigue crack growth in the threshold region in polycrystalline metallic materials based on fundamental microstructural and mechanical properties.; An analytical model has been developed for quantifying the fatigue crack growth rates in the threshold region. This model lays emphasis on material's toughness to resist fatigue crack growth. This model has been applied to two different materials, AISI 1045 steel and INCOLOY 825 which further has two categories mainly classified by the imparted heat treatment. Model has been found to successfully predict fatigue crack growth rates in the threshold region. The predicted crack growth rates agree very closely with the experimental values. The areas of demarcation of the physical crack growth process between the threshold and linear region can be represented by a characteristic fatigue toughness parameter which is a stress intensity factor range corresponding to the plastic zone size equaling the grain size. This parameter has been proposed as an alternate measure of fatigue strength.; Crack closure loads have been experimentally measured in all the materials studied at load ratios 0.1, 0.3, 0.5. Crack closure influence on the load ratio effect on the fatigue crack growth rates in the threshold region has been explained. The asymptotic nature of the fatigue crack growth curve has been attributed to crack closure. This is evidenced by the fact that both in model predicted and experimental crack growth curves, the crack growth rates for all load ratios converge to a band.