Abstract
Using the criterion that a crack will extend perpendicular to the maximum circumferential stress,σ θ, we show that the directional stability of crack growth is governed by the location of microcrack initiation ahead of the crack tip. At distances greater than a geometrical radiusr o, the maximum value ofσ θ deviates from the position of symmetry. Thus, if we assume that the physical processes involved in fracture lead to crack initiation at a distancer c ahead of the crack tip, the criterion for directional stability isr o>r c. Experimental and theoretical values ofr o verify that, asr o becomes small, the crack's directional stability deteriorates.
Observing that a lengthwise compressive stress increasesr o, a center-cracked specimen was developed which allows the application of controlled lengthwise compression independently of the opening-mode load. A detailed photoelastic analysis of the specimen has provided the value ofr o as a function of the crack length. The value ofr o is then compared with the expected microcrack initiation distances in ductile fracture.
By applying sufficient lengthwise compression, we are able to make the crack grow straight and obtain numerous data points from this specimen which would otherwise be directionally unstable. The results indicate that, as the total lengthwise tensile stress at the crack tip increases, the fracture toughness also increases. Using this information we can then adjustK Ic for zero lengthwise loading and obtain a geometry independent fracture toughness.
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Abbreviations
- A 2 :
-
constant second term of crack-tip stress series (Pa)
- CCS :
-
center-crack specimen
- CT :
-
compact tension
- DCB :
-
double-cantilever beam
- K 1 :
-
stress-intensity factors for Mode I loading. Singularity term of crack-tip stress-series solution (MPa-m1/2)
- Q(θ f):
-
function ofθ f for photoelastic use
- r, θ:
-
polar-coordinate system centered at crack tip. Direction of crack propagation along θ=0 deg
- r c :
-
critical distance in front of the crack tip where crack initiation occurs (m)
- r f :
-
photoelastic-fringe radius (m)
- r o :
-
maximum radius for which the maximumθ f lies on θ=0 deg (m)
- θ f :
-
photoelastic-fringe angle for (∂τm)/(∂θ)=0
- σ r,σ θ,τ θ :
-
crack-tip stresses referred to polar-coordinate system (Pa)
- τ m :
-
maximum shear stress for a given point (Pa)
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Streit, R., Finnie, I. An experimental investigation of crack-path directional stability. Experimental Mechanics 20, 17–23 (1980). https://doi.org/10.1007/BF02324422
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DOI: https://doi.org/10.1007/BF02324422