Abstract
This study analyzes the relationship between fracture toughness and the fracture surface fractal dimension for a set of twenty-four CT-type AISI 4340 steel specimens heat treated to a variety of tensile strengths. Specimens were tested in accordance with ASTM E 399. Their respective fracture surfaces were plated, polished, photographed under an SEM in BSE mode and measured according to the Richardson method to obtain fractal dimensions. For brittle materials the limited results are consistent with previous literature: increasing fractal dimension with increasing toughness. For partially or fully ductile materials the results indicate a decrease in fractal dimension with an increase in fracture toughness. The data are modeled using a variation of the function applied to ceramics. Fracture in a ductile mode is characterized by the formation of dimples which exhibit fractal characteristics. The results are discussed in terms of the micromechanisms of fracture.
Highlights
In recent years, a great deal of interest has developed in the area of quantitative fractography—the science of relating fracture surface features to material properties or behavior
This study analyzes the relationship between fracture toughness and the fracture surface fractal dimension for a set of twenty-four compact tension (CT)-type AISI 4340 steel specimens heat treated to a variety of tensile strengths
At the high fracture toughness end, the D* values were observed to rise above the downward trend
Summary
A great deal of interest has developed in the area of quantitative fractography—the science of relating fracture surface features to material properties or behavior. Investigations into the various modes of failure have produced significant advances in the qualitative and quantitative understanding of how and why fractures develop [1,2,3]. A continuation of studies of this kind is expected to provide a better understanding of the fracture process, lead to the development of tougher materials and establish tools and procedures for the measurement of toughness properties from fracture surfaces alone. Fractal geometric analysis, when applied to a fracture surface, provides a measure of its irregularity which can be correlated to the plane-strain fracture toughness, KIC. In the particular case of irregular surfaces, for instance, it is possible to obtain fractal dimensions between 2.0 and 3.0. Fractal objects possess at least these two important qualities [6]: scale invariance and self-similarity: fractal objects display identical levels of irregularity no matter what the magnification of the object
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