Characterisation of friction and lubrication regimes in premium tubular connections

A friction test rig has been developed to carry out repeated sliding friction tests for premium tubular connections. The test rig enables accurate measurement of friction in various contact regimes which are relevant to the threaded connections between tubular components. Higher load tests can simulate the contact in metal-to-metal seals under very high contact pressures by using perpendicular pin-on-pin tests. The contact in the thread loading flank under intermediate pressures can be simulated by using larger radius coupon-on-coupon tests. The measured coefficient of friction is well correlated with a lubrication parameter combining lubricant film thickness and initial surface roughness. © 2012 Elsevier Ltd. All rights reserved

VALIDATION OF A MODEL MATERIAL TO STUDY FLAW CHARACTERISATION IN CLEAVAGE

ABSTRACT Multiple or complex flaws are typically assessed using flaw assessment procedures, such as BS7910 The objective of the current work is to validate the technique and the model material by performing fracture tests on configurations with twin and complex flaws and corresponding bounding semielliptical flaws using Perspex as a model material. The results show a similar non-conservatism in the characterisation procedure applied to the model material to that reported by Bezensek and Hancock [4] and Wilkes [5] on a mild steel at low temperatures. It is concluded that the above technique and model material accurately represent flaw characterisation under simulated cleavage conditions

Flaw alignment criteria based on limit load solutions for non-aligned through-wall flaws

Multiple flaws are frequently replaced with simpler flaws according to the flaw alignment rules for the purpose of engineering assessment. Such rules typically compare the flaw separation distance with the characteristic dimension, such as flaw length or flaw depth. For configurations involving flaws on parallel offset planes, current ASME Boiler code, Section XI, Article IWA-3300 suggests a fixed distance of 12.4 mm between the offset flaw planes as a flaw alignment rule. Recent experimental work of Hasegawa et al [4] showed this criteria is not always conservative for ductile fracture. In this paper the analytical and computational methods are used to derive plastic collapse load for plates with twin non-aligned through-wall flaws. Results show that in a multiple flaw geometry the global plastic collapse load is independent of the flaw separation when horizontal and vertical separations between flaw tips are equal. Such geometries are of most interest, since experimental work shows that flaws in such geometries interact over distances several times larger than the characteristic distance (flaw length or thickness). A criteria for flaw alignment based on the ratio of a local to global limit load is derived. A local limit load is defined as the load to cause collapse of the ligament between the flaw tips. It is shown that the separation between parallel flaws should be more than three times the flaw length to maintain interaction below 15 per cent and ensure a conservative assessment

Validation of a model material to study flaw characterisation in cleavage

Multiple or complex flaws are typically assessed using flaw assessment procedures, such as BS7910 [1] or ASME Boiler code, Section XI [2]. Such procedures may involve replacing real flaws with simplified flaws in a process of flaw characterisation. Recent work by Bezensek and Hancock [3,4] and Wilkes [5] demonstrated potential lack of conservatism in flaw characterisation procedure when applied to brittle fracture from complex flaws forming a re-entrant sector. In this paper a novel technique is discussed to study flaw characterisation in cleavage. The technique consists of exposing a transparent model material to a controlled thermal shocks produced by a high power laser. The shock locally cracks the material and produces an embedded crack. The objective of the current work is to validate the technique and the model material by performing fracture tests on configurations with twin and complex flaws and corresponding bounding semielliptical flaws using Perspex as a model material. The results show a similar non-conservatism in the characterisation procedure applied to the model material to that reported by Bezensek and Hancock [4] and Wilkes [5] on a mild steel at low temperatures. It is concluded that the above technique and model material accurately represent flaw characterisation under simulated cleavage conditions

The Development Of A Surface Crack In A Thick Vessel Under Ductile Tearing

The development of the shape and size of a flaw in a pressure vessel is important in fitness-for-service evaluations such as leak-before-break. In this work finite element modelling is used to evaluate the mean stresses and the J-integral around a front of a surface-breaking flaw. These results show non-uniform constraint levels and crack driving forces around the crack front at large deformation levels, which contrast those at low deformation levels. A new procedure is developed to estimate the amount of ductile crack extension around a surface-breaking crack on the basis of ductile tearing resistance curves of deep and shallow cracked fracture mechanics samples. The procedure is applied to surface flaws to simulate ductile crack extension under ductile tearing and show the evolution of the initial flaw shape. Results show that both, initially semi-circular and initially semi-elliptical flaws develop towards the same shape in bendin

Brittle fracture from interacting surface breaking defects

Pressure Vessel codes, such as ASME Section XI [8], define characterisationprocedures for complex defects. The present work examines these approaches by studying the interaction of two coplanar surface breaking defects numerically andexperimentally. In the numerical study, the line-spring concept of Rice and Levy [12] was used to model a series of evolving crack profiles which develop in fatigue from two adjacent semi-elliptical starter notches. The defects coalesced in fatigue to form asingle complex defect which initially has a marked re-entrant sector. Enhanced stressintensity factors were determined in the re-entrant sector of the coalesced defects and these were confirmed by experimental observations of rapid fatigue crack growth. The enhanced stress intensity factors were associated with a loss of crack tip constraint asparameterised by T [11]. Experiments designed to measure the lower shelf toughness of defect configurations with re-entrant sectors were performed on a plain Carbon-Manganese steel. Three representative geometries with re-entrant sectors were considered. The low fracture resistance of profiles with pronounced re-entrant sectors leads to non-conservatism in the codified recharacterisation procedures of complex defects failing on the lower shelf

Increased temperature margins due to constraint loss

No abstract available