External Axial Cracks Research Articles

Using J-integral analysis to predict the failure of HDPE pipes under crack face load and internal pressure

Cracks in pipes used for water transfer have been identified as a significant contributor to water loss and potential failures. To assess fracture behavior, the J-integral was computed for 3D finite element HDPE pipes (DN 355) with axial internal and external cracks (with a/ c and a/ t ratios of 0.5) subjected to a range of internal pressure (16–30 bar) and crack face load condition. Four ratios of a/ c ranging from 0.3 to 0.9 (with an increment of 0.2) were examined with the internal crack. Results indicate that under face crack loading, the external axial crack exhibits a higher J-integral intensity than the internal axial crack. J-integral values greater than 8.4 kJ/m2 were found to lead to pipe failure, with higher internal pressures (25–30 bar for external crack, and 30 bar for internal pressure). Moreover, the a/ c ratio plays a significant role in influencing the distribution of J-integral along internal crack arc length. For pipes of DN 355 size with axial cracks, internal pressure exceeding 20 bar is not recommended. These findings underscore the importance of considering J-integral values in pipe design to prevent failure.

Comparisons of creep constraint and fracture parameter C* of different types of surface cracks in pressurized pipes

The surface cracks in pressurized pipes include internal axial crack (IAC), internal circumferential crack (ICC), external axial crack (EAC) and external circumferential crack (ECC). The unified creep constraint parameter Ac and fracture parameter C* for ICCs, EACs and ECCs have been calculated by three-dimensional finite element analyses. The Ac solutions for ICCs with high aspect ratio (1.2 ≤ a/c ≤ 2) have been obtained, and the estimation method of Ac for EACs and ECCs are given. The Ac and C* of the four types of pipe cracks have been comparatively investigated. The results show that the four types of cracks have similar distributions of Ac and C*, and the order of the constraint level and C* from high to low is IAC > EAC > ICC > ECC. Among all pipe cracks, the IACs and EACs with larger depth a/t and lower aspect ratio a/c have the highest constraint and C* levels, thus they are the most dangerous cracks. The ICCs and ECCs with smaller a/t and higher aspect ratio a/c have the lowest constraint and C* levels, hence they are relatively safer cracks. The IACs and EACs should be avoided as far as possible in manufacturing and service of high-temperature pipes.

Bursting pressure of autofrettaged cylinders with inclined external cracks

Autofrettaging a pressure vessel improves its pressure capacity. This is reliable if there isn’t any crack or other type of flaws. In this paper, the effects of external surface cracks on bursting pressure of autofrettaged cylinders are studied. It is observed that bursting pressure decreases considerably (up to 30%) due to external cracks in the cylinders without autofrettage. This reduction increases for high levels of the applied autofrettage. External axial cracks have more effects than inclined cracks. Comparing experimental and numerical results show that the numerical methods can acceptably predict the bursting pressure of the autofrettaged cracked cylinders. These predictions are valid when the fracture parameter (J-Integral) is calculated from the modified equation that takes into account the effects of residual stresses.

Different fracture performance of all-steel and all-aluminium cylinders with axial cracks

The linear elastic and the nonlinear elasto-plastic fracture mechanics analysis on all-metal (all-steel and all-aluminum) cylinder with different axially oriented cracks were carried out using the three-dimensional finite element method and the experimental method. The crack mouth opening displacement CMOD and the crack driving forces ( K I for elastic deformation state and J I for elasto-plastic deformation state) of the all-steel cylinder and the all-aluminum cylinder containing axial deep cracks, were obtained. Through analysis of the calculated CMOD and crack driving forces for the all-steel cylinder and the all-aluminum cylinder with cracks, whose sizes are often met, respectively, in the engineering applications, the fracture behaviors of the two kinds of all-metal cylinders are compared. The CMOD for the two kinds of all-metal cylinders with external axial cracks were measured by an experimental method and good agreements between the calculated CMOD and tested CMOD were reached. Some CMOD and crack driving force expressions about the crack sizes, internal pressure and location along the crack front are obtained.

Stress Intensity Factors for C-Shaped Specimens. Application of Statically Indeterminate Fracture Mechanics to Displacement-Controlled C-Shaped Specimens.

Stress intensity factors for load-controlled C-shaped specimens (LC-CSS) are discussed in some literatures, but those for displacement-controlled C-shaped specimens (DC-CSS), which are often used for environmental fracture toughness testing, are not. In this paper, an analytical procedure is shown by which to estimate mixed-mode stress intensity factors for DC-CSS with some internal and external axial cracks. The results obtained are summarized as follows : (1) Analytical results for LC-CSS show good agreement with the numerical data in the literature. (2) Mode II stress intensity factors are always negligible as compared to mode I stress intensity factors for commonly used thin-walled rings. (3) Explicit formulation of the stress intensity factors obtained in this paper is effective for both practical use and the discussion of influential parameters. (4) Several parameter studies for DC-CSS show that it is important for the analysis of such statically indeterminate structures to consider the variation of the system's compliance due to the presence of cracks.

A fracture behaviour investigation of lower carbon seamless tube with longitudinal crack

The transverse true stress-strain curve of DIN 1629 BI.3 St45 seamless tube has been determined by cylinder expansion tests. The J value for internally pressurised open ended tubes with external axial crack has been determined by compliance tests. The results have been compared with the EPRI solutions for an externally axially cracked cylinder and used to improve a J- estimation procedure. The comparisons suggest that the EPRI solutions could be conditionally used to estimate J for material stress-strain curves of straight-line-and-parabola type and that the improved J estimation procedure could be used to make good estimates of J from the relations of internal pressures and changes in external volume of a unit length cylinder of a single specimen. Finally, the externally axially cracked cylinder (EACC) is treated as one kind of fracture specimen which has an advantage of service simulation. The J resistance curve has been got from the EACC specimen for St45 seamless tube.

Stress intensity factors for axial cracks in hollow cylinders subjected to thermal shock

Stress intensity factors are determined for one and two, internal and external axial cracks in hollow cylinders subjected to stress gradients arising from a thermal shock. A closedform weight function formula was used in the calculation. Results covering a wide range of cylinder geometries are presented in graphical form.