Pipe-defect assessment based on the limit analysis, failure-assessment diagram, and subcritical crack growth

We analyze the serviceability of a pipe containing a defect by the following three methods: limit analysis, failure-assessment diagram, and subcritical crack growth. The use of the proposed complex approach is explained by the fact that, despite the plasticity of pipe steels, they can suffer quasibrittle fracture (under the conditions of plane deformation) or fail as a result of plastic collapse. Moreover, the final fracture of a pipe with defect is preceded by the stage of stable crack growth. Therefore, in analyzing the serviceability of the pipe, one should also take into account the “leak-before-break” criterion. To take into account all these circumstances, it is necessary to use the indicated three methods, as in the case of evaluation of the serviceability of a pipe of API X52 steel containing a surface semielliptic notch. Defects of this sort reflect the influence of the surface gouges and dents responsible for the failures of pipes under the action of internal pressure. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 1, pp. 119–127, January–February, 2006.     

A strain-energy-density-based lifetime prediction model for notched specimens: Polycarbonate under thermal fatigue

Polycarbonate is more and more extensively used in engineering because of its good mechanical properties. Pieces of polycarbonate are used in environments with variable temperature, especially in electronic devices. Thermal stresses could become important and, for this reason, the effects of thermal stresses must be taken into account in designing these pieces. We propose a method for the prediction of the life of notched specimens based on the density of dissipated strain energy. The laws of behavior of polycarbonate at various temperatures are determined, and the fatigue tests performed on smooth specimens give the laws of thermal fatigue of the material. The fatigue tests on notched specimens and finite-element-method computations enable us to establish the relationship between the stress concentration factor, the density of strain energy dissipated at the notch roots, and the density of nominal strain energy. A life-prediction model is proposed and discussed. Laboratory of Mechanical Reliability, Metz University, Metz, France. Published in Problemy Prochnosti, No. 6, pp. 32–42, November–December, 1998.     

On Corrosion Fatigue Emanating from Notches: Stress Field and Electrochemistry

We analyze the processes of initiation and growth of corrosion fatigue cracks on the surface of a semicircular notch depending on the stress field and local electrochemical processes running inside the notch. For this purpose, we consider a typical corrosion-active system formed by low-strength carbon steel placed in a 3% NaCl solution. In this system, the electrochemical dissolution of the metal is predominant. A semicircular notch is regarded both as a stress concentrator and as a local corrosion electrolyzer. The field of elastoplastic stresses near the notch is computed by the finite-element method with the help of the CASTEM-2000 software package. The early stages of corrosion fatigue are investigated and the existence of a critical density of cracks q _* on the surface of the notch is demonstrated (above this density, the cracks undergo intense coalescence leading to the formation of the macrocrack). We propose a model of this process and an analytic relation for the prediction of the number of cycles N _* corresponding to the critical density q _*. This relation takes into account the constants of electrochemical dissolution of the metal, the density of corrosion current i _cor, and the effective stress _eff, which can be regarded as the mean level of stresses in a segment 0 x x _eff from the notch tip.     

A review of fracture toughness transferability with constraint and stress gradient

In this review paper, only constraint and stress gradient approaches to transferability of fracture toughness are examined. The different constraint parameters are defined and discussed, and one example is given in each case. Factors that influence the constraint are studied. Special attention is given to the actual trends to use the plastic constraint in the material failure master curve and the material transition temperature master curve. The paper also deals on the influence of T stress on the crack path and out‐of‐plane constraint and on the influence of thickness on fracture toughness. The uses of plasticity with gradient and the relative stress gradient in local fracture approaches are also examined.     

Analysis of Failure Emanating from a Notch

Materials Science - Notch-like defect assessment cannot be done by using classical fracture mechanics (mechanics of cracks). In order to prevent overconservatism, the concepts of notch fracture...     

Hydrogen effect on local fracture emanating from notches in pipeline from steel API X52

The assessment of local strength at notches in pipeline steel API X52 has been done for conditions of cathodic hydrogen charging. The relationship between hydrogen concentration and critical (failure) loading has been found. The existence of some critical hydrogen concentration which causes the significant loss of local fracture resistance of material was also shown.     

Dynamic fracture toughness at crack initiation, propagation and arrest for two pipe-line steels

The influences of temperature and loading rate on fracture toughness of two pipe-line steels at initiation, K_1d, and at arrest, K_1a, and on stretch zone height were measured using specific techniques. Particular attention was given to the mechanism of delamination, typical for the gas internal pressure pipe fracture.     

Low-cycle biaxial fatigue of a polycrystalline magnesium-lithium alloy

We analyze the life duration of a magnesium-lithium alloy subjected to biaxial low-cycle fatigue under out-of-phase combined tension-compression and torsion. It was discovered that the life duration depends on the angle of the phase shift between the two perpendicular cyclic loads. The maximum life duration is attained in the case where the phase shift is absent, i.e., =0. It was also shown that deformation always remains plastic if exceeds a critical angle _c, i.e., no unloading of the specimen may occur. Fatigue properties are studied using the von Mises equivalent stresses and strains. We also present data on the evolution of the density of deformation twinning. It is assumed that the decrease in life duration is explained by permanent changes in the direction of loading in the presence of a phase shift, which inhibits the process of stabilization of plastic deformation in the constituent grains of polycrystals.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 19–30, January – February, 1995.     

Sensitivity of pipelines with steel API X52 to hydrogen embrittlement

The following cases of hydrogen influence on pipeline metal were considered: gaseous hydrogen under internal pressure in notched pipes and electrochemically generated hydrogen on external pipe surface from soil aqueous environment. The burst tests of externally notched pipes under pressure of hydrogen and natural gas (methane) were carried out after the pipe has been exposed to a constant “holding” pressure. It has been shown that even for relatively “soft” test conditions (holding pressure p = 20 bar and ambient temperature) the gaseous hydrogen is able to penetrate into near surface layers of metal and to change the mechanism of local fracture at notch. The sensitivity to hydrogenating of given steel in deoxygenated, near-neutral pH NS4 solution under soft cathodic polarisation was studied and the assessment local strength at notches in pipeline has been made for this conditions. Here, the relationship between hydrogen concentration and failure loading has been found. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown.     

Fracture toughness of high strength steel—using the notch stress intensity factor and volumetric approach

In the present paper, the crack initiation has been studied using notched circular ring specimens. A new criteria in brittle mode I fracture, based on normal stress at tip notch and the volumetric approach has been developed. The critical value of the notch intensity factor has been considered as fracture toughness.