PSI - Issue 28

Stepan Major et al. / Procedia Structural Integrity 28 (2020) 561–576 Stepan Major/ Structural Integrity Procedia 00 (2019) 000–000

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of fracture structures in three dimensions and allowed their quantification. Other methods were later added to SEM. Nowadays various methods are used to reconstruct fracture surface and these three are most important SEM stereophogrammetry, optical methods based on chromatic aberration and laser triangulation.

Nomenclature a

side length of elementary square (with area S i ) on fracture surface area of elementary triangle in triangulation network

A i,j

fractal parameter Hurst exponent

D H

number of triangles in triangulation network on fracture surface number of squares in square network on fracture surface

i j

number of triangles in elementary square number of moving bands in Bandwidth method

g k

kurtosis

K L

true profile length projected profile length

L P L R m 0

loading ratio

number of peaks of the profile per unit length number of valid data for points on the profile

n

number of cycles to failure

N f i n  , T i j n  M

normal vector of elementary square

normal vector of elementary triangle in triangulation network

arithmetic roughness root mean square roughness

R a R q R Z

vertical range

skewness

S

area of elementary square on fracture surface (squares covering whole surface)

S i z i α

vertical coordinate on fracture surface

deviation angel between normal vector of elementary flat and sample axis

mean value of deviation angel in area of interest

α m

β deviation angel between normal vector of elementary flat and primary crack growth direction ε band width in bandwidth method σ a bending loading stress amplitude σ U ultimate strength σ y yield strength τ a torsion loading stress amplitude The quantitative fractography as a method of fracture morphology description enabled the transition from a qualitative description to more precise quantification of relationship between loading condition and fracture geometry. Currently, there are many different methods that allow quantitatively describe fracture morphology. At present time, we also have techniques that allow us to monitor the fracture process on an ongoing basis (i.g. Antunes et al. (2000); Kobayashi and Shockey (2001); Laushmann and Nedbal (2002); Taschl at al. (2000)). Most of the mentioned studies, however, are confined to uniaxial fatigue. It can be said, that the surface roughness is usually extremely extended when a high portion of a lower to medium amplitudes of torsion is applied. It is therefore loading state, which is characterized by significant share of shear loading modes II a III. In such cases the crack generally propagates in exceptionally complicated manner making local arrests and forming a branch/twist crack morphology or the so-called factory roofs, Pokluda and Pippan (2005), Pook (2002). A complex interaction between both mating fracture surfaces often makes even a thorough qualitative understanding of fatigue crack propagation difficult. This complex process represents