PSI - Issue 4

Stefan Kolitsch et al. / Procedia Structural Integrity 4 (2017) 95–105 Stefan Kolitsch/ Structural Integrity Procedia 00 (2017) 000 – 000

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Four materials with different microstructure and strengths have been compared:  pearlite with a tensile strength of 1070 MPa,  fine-pearlite with a tensile strength of 1120 MPa,  bainite with a tensile strength of 1120 MPa,  ferrite-martensite with a tensile strength of 1510 MPa

The damage tolerant design is distinguished into two areas, the manufacturing process and the fatigue endurance. Due to high applied strains during the bending process the static fracture is the limiting factor. Common standards prescribe a fracture strain of polished specimens. Furthermore a maximum failure curve depending on defect sizes was investigated for the different materials. Using a common stress based design for cyclically loaded components, high strength materials have a higher endurance limit; however, this endurance limit is lowered markedly even by small flaws. To guarantee the application of high strength materials by considering small defects a fracture mechanics approach is investigated and additionally transformed into the commonly used Smith diagram.

Nomenclature a

total crack length [mm]

material constant of the Ramberg-Osgood hardening equation [-]



a 0 a pl

initial crack length [mm]

transition point between linear-elastic and elastic-plastic fracture mechanics [-]

c

half width of the semi-elliptical crack front [mm]

D

material constant to calculate the Q-stress depending on the crack length [-]

Δ a Δ K

crack extension [mm]

stress intensity factor (SIF) range [MPa√m]

Δ K th

threshold of the SIF range for fatigue crack growth (FCG) [MPa√m]

Δ K th,eff effective FCG threshold [MPa√m] Δ K th,lc long crack FCG threshold [MPa√m] Δ σ stress range [MPa] Δ σ e

endurable stress range (fatigue endurance limit) [MPa]

E ε 0

Young’s modulus [MPa]

strain at the yield stress in the Ramberg-Osgood hardening equation [-]

ε bending strain in the outer fibre due to a bending load [-] ε f failure strain [-] ε f-exp failure strain from the tensile experiment [-]  plastic support factor [-] F ( R , Δ a ) NASGRO crack growth rate factor [-] f w,zd η pl

material constant to calculate the critical J-Integral depending on the Q-stress [-]

fatigue strength factor for tension and compression [-]

J c

critical J-Integral [J/mm]

k

material constant to calculate the Q-stress depending on the crack length [-]

K AK ( R ) mean stress factor [-] K Ic fracture toughness [MPa√m] K surface surface roughness factor [-] l i

length scale for the build-up of crack closure [mm]

m

material constant to calculate the Q-stress depending on the crack length [-]

Poisson’s ratio [ -]

  i

material constant for the cyclic crack resistance curve [-]

n σ

elastic support factor [-]

p

material constant to calculate the critical J-Integral depending on the Q-stress [-]

Q R

Q-stress [-] stress ratio [-]