Fatigue crack propagation in UFG Ti grade 4 processed by severe plastic deformation

• Published in: International journal of fatigue 2017-05, Vol.98, p.187-194
• Main Authors: Fintová, S., Arzaghi, M., Kuběna, I., Kunz, L., Sarrazin-Baudoux, Ch
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Cold drawing; Cold pressing; Crack closure; Crack growth; Crack propagation; Deformation mechanisms; Engineering Sciences; Environmental Sciences; Equal channel angular drawing; Equal channel angular pressing; Equal channel angular processing; Fatigue; Fatigue cracks; Fatigue failure; Fracture mechanics; Fracture surfaces; Growth rate; Materials fatigue; Mathematics; Morphology; Physics; Plastic deformation; Plastic properties; Stress intensity; Stress intensity factors; Studies; Threshold stress; Titanium
• ISSN: 0142-1123; 1879-3452
• DOI: 10.1016/j.ijfatigue.2017.01.028

•Experimental study of fatigue crack growth rate and threshold in UFG Ti.•Investigation of crack closure effect in UFG Ti.•Influence of cyclic loading on microstructure of UFG Ti.•Comparison of experimentally and theoretically determined closure. Fatigue crack growth was investigated in commercially pure Ti grade 4 processed by severe plastic deformation. The ultrafine-grained structure was prepared by means of equal channel angular pressing technique followed by cold drawing. The growth of fatigue cracks was investigated also in the as-received coarse-grained state of material for comparison. The fatigue crack growth rate in the ultrafine-grained material was found to be higher and the threshold stress intensity range ΔKth for crack growth lower than in the coarse-grained Ti, 2.5 and 4.7MPam1/2, respectively. A combination of transcrystaline and intercrystaline crack growth was typical for all crack growth rates in the coarse-grained material. Contrary to the combined fracture mechanism in coarse-grained Ti only transcrystalline crack growth was acharacteristic feature of crack propagation in the ultrafine-grained material. It has been found that the experimentally and theoretically determined values of closure are in reasonable agreement. Crack closure calculated according to the Newman model taking into account only plasticity induced closure was in the range from 67% to 69% of Kmax for as-received and 70% for UFG state of material. Crack closure determined experimentally was 67% of Kmax for as-received and 67% for UFG CP Ti grade 4. Investigation of fracture surfaces did not bring any evident signs of the effects of fracture surface morphology resulting in roughness induced closure or oxide induced closure under testing conditions used. This indicates that plasticity induced closure seems to be a dominant closure mechanism in both states of the investigated Ti grade 4.