Propagation Of Circumferential Crack Research Articles

Minor aortic injury may be at risk of progression from uncontrolled shear stress: An in-vitro model demonstrates aortic lesion expansion

Background Non-operative management is considered appropriate treatment for minor aortic injury, while blood pressure and anti-impulse therapy are routinely utilized to prevent higher grade aortic injury progression. However, a universal medical regimen for low grade intimal injuries has not been adopted and risks of low-grade injury progression not well described. The purpose of this study is to determine the fracture response of minimally damaged aortic tissue to the various applied forces. Our hypothesis is that internal circumferential shear within the aortic wall is a primary fracture mode. This knowledge may help guide clinical management to minimize risk of injury progression, including instituting standard medical regimens with anti-impulse therapy and β-blockade for such minor injuries. Methods Human ascending aortic tissue was obtained after aneurysm repair or heart transplant, stored at 4°C and tested within 48 hours. Minor injury was modeled with a small radial notch on the luminal aspect of aortic rings, circumferentially expanded under video acquisition and analyzed to determine lesion propagation. Results 15 rings were obtained from 8 aneurysmal and 4 healthy aortas. All specimens demonstrated circumferential crack propagation. Propagation was longer (8.02 ± 5.92 mm vs 2.70 ± 1.23 mm) and initiation of crack propagation earlier in aneurysmal tissue (1.54 ± 0.17 versus 1.90 ± 0.17 times initial diameter). Conclusions Dilation of minimally injured aortic rings is associated with lesion expansion and injury progression in all specimens including healthy and aneurysmal tissue. This propagation illustrates the mechanical response to increased levels of internal shear, compromising structural integrity and increasing risk of aortic rupture in all injured aortas. Shear forces are routinely generated through normal circumferential aortic expansion with each pulsation, the magnitude of these forces determined by pulse and blood pressure. This suggests minor aortic injuries are not trivial and strategies to reduce shear stress be implemented in all such patients without contraindications to β- blockers.

Simulation of the Propagation of Circumferential Crack in the Cylindrical Element Structure Exposed to Cyclic Loadings

The purpose of these studies was to work out the methods for the computation of crack development in the plate and axisymmetric elements of structures at cyclic loading for the plasto-elastic arrangement using the conception of the accumulation of scattered damages in the material. In the domain in question, we simulated the processes of alternating-sign plasto-elastic deformation and the material crack resistance was estimated using the fatigue test data for smooth specimens. Thermal and elastically-deformed states of the structure were determined for different loading modes using the finite elements method. The material damageability was assessed for the curves of low-cycle fatigue and for the range of plasto-elastic deformations on the route of probable crack propagation using the hypothesis of linear summation. As soon as a critical value of the damageability is achieved the crack is propagated stepwise for the specified step. As an example, consideration was given to the kinetics of circumferential crack in the cylindrical element exposed to the axial-symmetric cyclic loading. The contact of crack edges was taken into consideration for compressing loading semicycles. The obtained data agree with the data obtained using the Newman method. The suggested technique can be used for the estimation of survivability of the elements of steam turbines (heat rotor grooves at start-up-stop modes, blade and disk interlocks, etc.).

Fracture mechanics of shear crack propagation and dissection in the healthy bovine descending aortic media

An intimal tear of an apparently healthy aortic wall near the aortic arch is life-threatening because it may lead to full rupture or to wall dissection in which delamination of the medial layer extends around most of the aortic circumference. The mechanical events underlying dissection are not definitively established. This experimental fracture mechanics study provides evidence that shear rupture is the main mechanical process underlying aortic dissection. The commonly performed tensile strength tests of aortic tissue are not clinically useful to predict or describe aortic dissection. One implication of the study is that shear tests might produce more fruitful simple assessments of the aortic wall strength. A clinical implication is that when presented with an intimal tear, those who guide care might recommend steps to reduce the shear load on the aorta.

Study of Circumferential Crack Propagation under Combined Tension and Torsion Loading at Elevated Temperature

This paper describes the circmferential crack propagation in multiaxial low cycle fatigue loading. Push-pull, reversed torsion and combined push-pull and reversed torsion low cycle fatigue tests were carried out using circumferential notched specimens of type 304 stainless steel at 873K and crack propagation rates were measured by a D. C. potential method. Elastic-plastic finite element analysis was also carried out to evaluate the crack and notch opening displacements and to examine the effect of frictional between cracked faces on Mode III crack. The friction decreased the crack opening displacement and plastic zone size ahead of crack tip. Crack propagation rates in push-pull, reversed torsion and combined push-pull and reversed torsion tests were not correlated with J integral range ΔJE calculated by conventional method and which was discussed in relation to the friction between the fracture faces. The crack opening displacement taking account of the friction effect correlated crack propagation rates in all loading modes within a factor of four band.

Further Numerical Studies on Dynamic Circumferential Crack Propagation in a Large Pipe

Dynamic ductile crack propagation analyses of pre-existing circumferential through cracks in a large pipe subjected to uniaxial tension were conducted by a finite difference dynamic shell code. These dynamic results together with the results of previous dynamic ductile circumferential crack propagation suggest that crack propagation in the presence of large-scale yielding is controlled mainly by the rotary inertia of the two fracturing pipe segments.

Measurement of Fatigue Crack Propagation by Eddy Current Method

The eddy current method was applied to the detection of fatigue crack propagation of notched specimens under rotating bending. The practical device to apply the eddy current method and the test results are described. The depth of fatigue cracks can be measured continuously by the eddy current method, and the accurate value of crack depth is obtained without breaking the test pieces. The method can be applied to ferromagnetic substance as well as to paramagnetic substance, though the detecting probe must be varied so as to suit the materials. A linear relation is obtained between the crack depth and the response of the eddy current circuit irrespective of the environment in which fatigue tests are carried out. The circumferential crack propagation can also be measured by rotating the test piece once or by reciprocating the detecting probe over the notch at several circumferential positions, and the latter method shows better agreement with the crack depth measured by temper coloring. The sensibility of the crack detection is 0.1∼0.05mm. The maximum detectable crack depth is more than 2.5mm in the case of carbon steel (ferromagnetic substance) and is more than 4.0mm in the case of Al alloy (paramagnetic substance).