Effect of low temperature tensile properties on crack driving force for Arctic applications

Abstract

Many petroleum companies expand their activities further north towards the Arctic region, resulting in design temperatures down to −60• C, which is much lower than what is usual for most current petroleum installations. As properties of steels are temperature dependent, it is of great interest to evaluate the effects of low temperature on the crack driving force in steels. The present work investigates these effects numerically using finite element (FE) models of single-edge-notchedtension (SENT) specimens with crack depths 0.1 ≤ a/W ≤ 0.5. The effects of Lüders strain, yield strength and crack depth on the crack tip opening displacement (CTOD) and the relation between CTOD and crack mouth opening displacement (CMOD) are studied, and it is shown that an increase in yield strength and Lüders strain, as a result of Arctic temperature, intensifies the crack driving force. It is also shown that the crack depth has very little influence on the effect of Lüders strain on the CTOD. An approximate model that can be used to estimate the CTOD based on yield strength, Lüders strain and loading is proposed for gross stress levels σ G /σ y ≤ 0.5 and a crack depth a/W = 0.5. It is finally shown that the tensile properties have a more significant effect on the CTOD-CMOD relation than the crack depth for a SENT specimen.Mange petroleumsselskaper utvider sine aktiviteter nordover mot arktiske områder. Dette fører til designtemperaturer ned til −60• C, noe som er mye lavere enn det som er vanlig for de fleste nåvaerende petroleumsinstallasjoner. Ettersom egenskapene til stål er temperaturavhengige, er det av stor interesseå evaluere effektene av lav temperatur på den drivende kraften for sprekkvekst i stål. Dette arbeidet undersøker disse effektene numerisk ved hjelp av elementmetoden med modeller av single-edge-notched-tension (SENT)-bruddprøver med sprekkdybder 0.1 ≤ a/W ≤ 0.5. Effektene av Lüders tøyning, flytespenning og sprekkdybde på sprekkspissutvidelsen (CTOD) og sammenhengen mellom CTOD og sprekkmunningsutvidelsen (CMOD) er studert, og det er påvist at økt flytespenning og Lüders tøyning, som et resultat av arktisk temperatur, fører til økt drivende kraft for sprekkvekst. Det er også vist at sprekkdybden har liten innflytelse på effekten av Lüders tøyning på CTOD. Det er foreslått en omtrentelig modell som kan bli brukt tilå estimere CTOD basert på flytespenning, Lüders tøyning og last for bruttospenningsnivåer σ G /σ y ≤ 0.5 og sprekkdybde a/W = 0.5. Det er til slutt påvist at materialets strekkegenskaper har en større virkning på CTOD-CMOD-forholdet enn sprekkdybden for en SENT-prøve.i ii PrefaceThe work presented here is a Master's thesis written at Department of Engineering Design and Materials at Norwegian University of Science and Technology (NTNU). The thesis work is carried out as a part of SMACC (Studies of materials behavior for future cold climate applications), which is a research project with several industry partners, and with SINTEF and NTNU as research partners.The author wishes to thank SINTEF fo...

Study of low‐temperature effect on the fracture locus of a 420‐MPa structural steel with the edge tracing method

“…Ren 2 carried out tensile tests of a 420-MPa steel with temperature ranging from 0°C down to −90°C and found that the Lüders strain [3][4][5] increased as the temperature decreased. It has been demonstrated that there are considerable oil and gas resources in the Arctic region 1 ; the low-temperature effect should be considered in the selection of structural steels.…”

“…Previous research has shown that decreasing temperature increases the yield strength of most steels. Ren 2 carried out tensile tests of a 420-MPa steel with temperature ranging from 0°C down to −90°C and found that the Lüders strain [3][4][5] increased as the temperature decreased. For most structural steels, as the temperature decreases continuously, the fracture behaviour will transform from ductile to brittle (DBT), [6][7][8][9][10][11][12] reducing the steels' ductility and fracture toughness.…”

Numerical study on the effect of the Lüders plateau on the ductile crack growth resistance of SENT specimens

“…Investigation on the Lüders plateau effect on crack driving force by Dahl have demonstrated that the existence of Lüders plateau intensified the crack driving force. Larger Lüders plateau corresponds to higher crack driving force (Dahl et al, 2018). Ductile crack growth lies in the competition of crack driving force and crack resistance.…”

“…It is well-known that most structural steels show obvious temperature dependent mechanical properties, such as yield stress, ductile-brittle fracture transition behavior. Previous investigations demonstrate that for some steels, the so-called Lüders plateau which is influenced by loading rate, ferrite grain size, yield stress, et al may occur in uniaxial tension test (Beardsmore et al, 2013;Dahl et al, 2018;Hallai and Kyriakides, 2013;Han et al, 2017;Liu et al, 2015;Mazière et al, 2017;Ren et al, 2015;Tsuchida et al, 2006). It has been reported that lowering testing temperature will increase the Lüders plateau length, see Fig.…”

“…Ren et al performed tensile tests of a 420MPa structural steel with temperatures varying from 0 to -90 ℃ and created a relation between temperature and the Lüders plateau length (Ren et al, 2015). Dahl numerically investigated the effect of the yield stress and Lüders plateau length on crack driving force with single edge notched tensile (SENT) specimens and found that the increasing yield stress and Lüders plateau length will intensifies the crack driving force (Dahl et al, 2018). In Dahl's work, a horizontal plateau was used to simplify the Lüders behavior.…”