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Crack path of titanium alloy Ti-6Al-3Mo-0.4Si in high- and veryhigh- cycle-fatigue regime after tempering and hardening procedures of smooth and notched specimens
Last modified: 2011-02-25
Abstract
The influence of different technological procedures on titanium alloy VT3-1 behaviour has
investigated in the range of 104 – 5.107 cycles under the specimens’ tension with frequency 35Hr. Smooth and
notched round bar specimens have been tempered and hardened or subjected to tests without one or both
technological procedures. Tests were performed in the range of maximum stress level 140-920MPa at the Rratio
from -1.0 till + 0.67. It was revealed the stress range when in material behaviour takes place transition
from cracks origination at the surface to the subsurface. The subsurface area of the crack origination for
smooth and notched specimens is the facetted pattern of the material quasi-cleavage. The Mode-III mechanism
of twisting under three-axial compression is discussed to explain the way of subsurface origination of the first
smooth fracture facet (fatigue crack origin) followed by creating a plastic zone around the crack origin area,
weakened by the fist facet. Material cracking out-off subsurface origin determines by combined Modes I+III
mechanism. The critical values of w and w for the crack subsurface origination have been estimated and it
was shown that ratio w / w for the first weakened smooth facet of titanium alloy VT3-1 ranged between 2.37
and 2.6. The fatigue curves were reconstructed and bimodal distribution of material durability f N was
demonstrated. The crack growth period, p N , has unified description for all combinations of specimens surface
states and material conditions in term of p f N / N versus f N .
KEYWORDS. Titanium alloy; Tempering; Hardening; External tension; Bimodal distribution; Durability; Fatigue
cracking; Subsurface; Modes-III; Crack growth; Combined modes I+III; Fatigue striations.
investigated in the range of 104 – 5.107 cycles under the specimens’ tension with frequency 35Hr. Smooth and
notched round bar specimens have been tempered and hardened or subjected to tests without one or both
technological procedures. Tests were performed in the range of maximum stress level 140-920MPa at the Rratio
from -1.0 till + 0.67. It was revealed the stress range when in material behaviour takes place transition
from cracks origination at the surface to the subsurface. The subsurface area of the crack origination for
smooth and notched specimens is the facetted pattern of the material quasi-cleavage. The Mode-III mechanism
of twisting under three-axial compression is discussed to explain the way of subsurface origination of the first
smooth fracture facet (fatigue crack origin) followed by creating a plastic zone around the crack origin area,
weakened by the fist facet. Material cracking out-off subsurface origin determines by combined Modes I+III
mechanism. The critical values of w and w for the crack subsurface origination have been estimated and it
was shown that ratio w / w for the first weakened smooth facet of titanium alloy VT3-1 ranged between 2.37
and 2.6. The fatigue curves were reconstructed and bimodal distribution of material durability f N was
demonstrated. The crack growth period, p N , has unified description for all combinations of specimens surface
states and material conditions in term of p f N / N versus f N .
KEYWORDS. Titanium alloy; Tempering; Hardening; External tension; Bimodal distribution; Durability; Fatigue
cracking; Subsurface; Modes-III; Crack growth; Combined modes I+III; Fatigue striations.
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