Thermomechanical Manipulation of Crack-Tip Stress Field for Resistance to Stress Corrosion Crack Propagation

Abstract

Corrosion-assisted propagation of an existing crack is profoundly influenced by the stress intensity at the crack tip. This article presents the first results of thermomechanical conditioning (TMC) for local manipulation of material at and ahead of the crack tip, in an attempt to retard/stop crack propagation. Prenotched round tensile specimens of mild steel were subjected to rotating bending to generate a fatigue precrack, and then to apply localized thermomechanical conditioning. The threshold stress intensity factor (K _ISCC ) for stress corrosion cracking (SCC) of precracked specimens with and without TMC was determined in a caustic environment. Results suggest that TMC can increase K _ISCC . Finite element analysis of the specimens suggests development of compressive stresses at and around the crack tip, which is expected to improve the resistance to stress corrosion crack propagation (since stress corrosion cracks can propagate only under tensile loading).

Appendix

In order to account for the eccentricity, the specific method used for the determination of the centroid of the ligament was as follows.

The fatigue crack depth (a _f ) was measured at twelve 30 deg intervals, as shown in Figure 12, by using a profile projector. The area of the ligament that associates with each 30 deg wedge is

$$ A_{{W_{i} }} = \frac{\pi } {{12}}{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{2} $$

(a)

where a _m is the depth of the machined notch (m).

The total area of the ligament is

$$ A = {\sum\limits_{i = 1}^{i = 12} {\frac{\pi } {{12}}} }{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{2} $$

(b)

Therefore, the equivalent ligament diameter is

$$ d = {\sqrt {\frac{{4A}} {\pi }} } $$

(c)

The first moments of area of a 30 deg wedge about the X and Y axes, respectively, are

$$ M_{{X_{i} }} = \frac{\pi } {{18}}{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{3} \sin \theta $$

(d)

$$ M_{{Y_{i} }} = \frac{\pi } {{18}}{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{3} \cos \theta $$

(e)

The total moments of area of the 12 wedges about the X and Y axes are

$$ M_{X} = {\sum\limits_{i = 1}^{i = 12} {{\left( {\frac{\pi } {{18}}{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{3} \sin \theta } \right)}} } $$

(f)

$$ M_{Y} = {\sum\limits_{i = 1}^{i = 12} {{\left( {\frac{\pi } {{18}}{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{3} \cos \theta } \right)}} } $$

(g)

Then, dividing these moments of area (Eqs. [f] and [g]) by the ligament area (Eq. [b]) gives the coordinates of the centroid of the ligament (Figure 5).

$$ \overline{X} = \frac{{{\sum\limits_{i = 1}^{i = 12} {{\left( {\frac{\pi } {{18}}{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{3} \sin \theta } \right)}} }}} {{{\sum\limits_{i = 1}^{i = 12} {\frac{\pi } {{12}}} }{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{2} }} $$

(h)

$$ \overline{Y} = \frac{{{\sum\limits_{i = 1}^{i = 12} {{\left( {\frac{\pi } {{18}}{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{3} \cos \theta } \right)}} }}} {{{\sum\limits_{i = 1}^{i = 12} {\frac{\pi } {{12}}} }{\left( {0.5D - a_{{f_{i} }} - a_{m} } \right)}^{2} }} $$

(i)

Therefore, the eccentricity (ε) is

$$ \varepsilon = {\sqrt {\overline{X} ^{2} + \overline{Y} ^{2} } } $$

(j)

Fig. 12

Eccentricity (ε) of the final ligament