טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBarak Yali
SubjectFracture Toughness and Surface Roughness: Do They Scale?
DepartmentDepartment of Mechanical Engineering
Supervisor Dr. Shmuel Osovski
Full Thesis textFull thesis text - English Version


Abstract

The fracture toughness of a material depends on its microstructure as well as the imposed loading conditions. Intuitively, the resultant fracture surfaces must contain information about the interlacing of these intrinsic (microstructure) and extrinsic (imposed loading) characteristics. Mandelbrot’s revelation that fracture surfaces are fractals, excited both the scientific and engineering communities, spurring a series of works focused at correlating the fracture toughness and the fractal dimension emerging from the fracture surface, representing the roughness. Unfortunately, these studies remained inconclusive, as both positive and negative correlations were reported along other observations where no correlation what so ever was found. Later, it was shown that the fractal dimension of the fracture surface is in fact a universal value. Here, we show that by going beyond universality, a definite correlation between the fracture toughness and the fracture surface roughness is obtained. Double edge notched specimens were machined out of an Aluminum 6061 alloy in the T6 condition. The raw material was characterized by means of optical and scanning electron microscopy, and the material’s intrinsic length-scale was extracted. Fracture experiments were held at varying loading rates spanning 7 orders of magnitude and the fracture toughness was extracted by means of digital image correlation (DIC). Scanning electron microscopy was used to create 3D reconstructed fracture surfaces, which were then analyzed to obtain the correlation lengths along the crack propagation direction. Both the lateral correlation length and the height difference associated with it, are observed to vary linearly with the measured fracture toughness. Our experimental results are in full agreement with recently published calculations, where a similar correlation was observed. Based on our results and the available literature on the topic we propose a mechanistic interpretation of the observed correlation, tying the extracted length-scales to the extent of the process zone. Our interpretation of the results explain both the appearance of rougher fracture surfaces at high loading rates, as well as the observed increase in fracture toughness. The correlation unravelled in our work, along with the proposed mechanistic interpretation, revives the hope of correlating fracture toughness and fracture surface roughness, allowing quantitative failure analysis and potential reconstructive approaches to designing fracture resistant materials.