Steven Cooreman

Finite element simulation of dynamic brittle fracture in pipeline steel: A XFEM-based cohesive zone approach

Leaking in a CO2 pipeline could escalate to sudden crack propagation, due to a large temperature drop. The resulting drop in fracture toughness together with the pressure stresses at the defect plane leads to pipeline brittle fracture. The main objective of this study is to monitor and predict brittle fracture behaviour of API X70 pipeline steel by means of experimental and numerical approaches, respectively. Dynamic fracture properties of CO2 line pipe steels are generally assessed using the Charpy impact test. To this end, Charpy V-notch tests are performed at different temperatures in order to study the resistance of materials subjected to impact loading conditions. The Charpy test provides valuable indications on the impact properties of materials. Using the experimental results the ductile to brittle transition temperature curve is presented. The extended finite element method based cohesive zone approach is introduced to model the brittle fracture at low temperature. After validation of the developed model against experimental observation significant results from the simulation are graphically presented and discussed.

Parameter identification for anisotropic plasticity model using digital image correlation: Comparison between uni-axial and bi-axial tensile testing

The basic principle of the described procedure for plastic material identification is the generation of a complex and heterogeneous deformation field, which is measured by digital image correlation (DIC) and compared to Finite Element (FE) simulations. In this paper two tests for the identification of the hardening behaviour and the yield locus of DC06 steel are compared: a uni-axial test on a perforated rectangular specimen and a bi axial tensile test on a cruciform specimen. The work hardening of the material is assumed to be isotropic and the yield locus is modelled by the anisotropic Hill48 criterion. The identification results for the different material parameters, based on both the uni- and the bi-axial test, are discussed and show a significant agreement.

Advances in post-necking flow curve identification of sheet metal through standard tensile testing

The standard tensile test is still the most common material test to identify the hardening behavior of sheet metal. When using standard equipment and well-known analytical formulas, however, the hardening behavior can only be identified up to the point of maximum uniform elongation. Several methods which deal with the problem of extended flow curve identification of sheet metal through a tensile test have been proposed in the past. This paper gives an overview of the four classes of methods to identify post-necking hardening behavior of sheet metal through tensile testing. In addition, identification methods from the first (average values across the neck), second (Bridgeman correction, modified Siebel and Schwaigerer correction) and third class (special case of the VFM) are used to identify the post-necking hardening behavior of DC05. Finally, these results are used to assess the validity of the different methods.

Computation of Stress Concentration Factors for Tubular Joints

Stress concentration factors for tubular joints were computed using solid quadratic elements. The results of the computations are compared with experiments reported in the literature and with expressions reported in the literature and in design codes. An influence of element size and element type was observed, which leads to recommendations regarding element size of four quadratic elements in thickness, which is finer than in different published recommendations. A parametric study was performed, showing that stress concentration factors from the literature are not always conservative, particularly at the crown toe of the chord, while they tend to be overconservative at the chord saddle. The stress concentration factor for the inside of the member was also computed; it is found that it can be close to the stress concentration factor at the weld toe for both the in plane or out of plane bending modes.Copyright

Understanding thermal warping and sagging in enamelled steel parts through an integrated FE simulation

When a steel part is enamelled, residual stresses are induced due to the difference in thermal dilatation between steel and enamel. Those stresses can give rise to buckling and warping. Slender designs, such as baking trays and architectural panels, are especially prone to these defects. The present paper proposes a finite element procedure which allows simulation of the complete cycle of forming, springback and enamelling and, as a result, allows prediction of warping and buckling in enamelled steel parts. To that end, the phenomenological models describing the thermomechanical behaviour of porcelain enamels have been implemented in Abaqus. These models allow for both stress and structural relaxation and, as a result, allow the simulation of the complete enamelling process, i.e. cooling down from around 850°C (liquid phase) to room temperature (solid phase). The finite element (FE) methodology is validated based on two case studies: simulation of the Klotz test and prediction of warping in enamelled baki...

Quantification and microstructural origin of the anisotropic nature of the sensitivity to brittle cleavage fracture propagation for hot-rolled pipeline steels

This work proposes a quantitative relationship between the resistance of hot-rolled steels to brittle cleavage fracture and typical microstructural features, such as microtexture. More specifically, two hot-rolled ferritic pipeline steels were studied using impact toughness and specific quasistatic tensile tests. In drop weight tear tests, both steels exhibited brittle out-of-plane fracture by delamination and by so-called “abnormal” slant fracture, here denoted as “brittle tilted fracture” (BTF). Their sensitivity to cleavage cracking was thoroughly determined in the fully brittle temperature range using round notched bars, according to the local approach to fracture, taking anisotropic plastic flow into account. Despite limited anisotropy in global texture and grain morphology, a strong anisotropy in critical cleavage fracture stress was evidenced for the two steels, and related through a Griffith-inspired approach to the size distribution of clusters of unfavorably oriented ferrite grains (so-called “potential cleavage facets”). It was quantitatively demonstrated that the occurrence of BTF, as well as the sensitivity to delamination by cleavage fracture, is primarily related to an intrinsically high sensitivity of the corresponding planes to cleavage crack propagation across potential cleavage facets.

Measurement of Mechanical Properties on Line Pipe: Comparison of Different Methodologies

Line pipe manufacturers always have to verify the mechanical properties on pipe to make sure that the pipe meets the requirements specified by the standard and/or customer. This involves measurement of mechanical properties along the hoop direction. The most accurate way to do so is by performing a ring expansion test, which, however, requires dedicated tools. The two other methodologies consist of standard tensile tests on either non-flattened round bar samples or so called ‘flattened tensile samples’. Round bar samples have the disadvantage that only part of the pipe’s wall thickness is considered. Furthermore they can only be used in case of larger OD/t ratios. Tests on flattened samples, on the other hand, require a flattening operation, which induces additional plastic deformation. However, this flattening operation is not standardized. Moreover, it was observed that the mechanical properties — especially the yield strength — resulting from tensile tests on flattened samples largely depend on test parameters such as residual deflection, extensometer position, flattening procedure, etc. Most manufacturers prefer to test flattened samples, because sample preparation is straightforward and cheap. Moreover it only requires a standard tensile bench.An extensive FEA (Finite Element Analysis) study was launched to investigate the influence of those parameters on the measured yield strength. The applied FEA methodology consists of three steps. First the complete pipe forming process is modeled (in a simplified way). Next a pipe sample is flattened. Finally a tensile sample is cut from the flattened pipe sample and loaded in tension. The mechanical material behaviour is described by a combined kinematic-isotropic hardening model, which allows taking into account the Bauschinger effect. The results are also compared to simulations of ring expansion tests and tests on round bar samples.Next a dedicated experimental test campaign was performed to verify the results of FEA. Results of ring expansion tests are compared to results obtained on round bar samples and flattened tensile samples.The results of this study have shown that the applied methodology significantly affects the measured yield strength. Moreover tests on insufficiently flattened samples could considerably underestimate the actual yield strength on pipe. Finally some guidelines are provided to improve the reproducibility of the measured yield strength when using flattened samples.Copyright

Simulation of a Thick Plate Forming Benchmark Using a Multi Scale Texture Evolution and Anisotropic Plasticity Model

Thick plate and sheet materials are often characterised by an inhomogeneous distribution of properties such as yield strength and anisotropy throughout their thickness. Forming of these materials involves further heterogeneous evolution of these properties. A recently developed computational framework [1, now allows these heterogeneities to be modelled via a hierarchical multi-scale material modelling scheme: the evolution of texture and plastic anisotropy can be tracked and individually updated at every integration point in a finite element model, in a computationally efficient manner. In this paper we present the application of this multi-scale model to a benchmark forming simulation, the three point bending test of thick plate steels. A number of hot rolled high strength low alloy steels were considered, two of which are presented here. The results of the simulations are validated against experimental results. Comparison is made between computed and experimental deformed shapes and strain fields, using data acquired by digital image correlation. Predictions of heterogeneously evolved textures are compared with experimental macro-textures, acquired by XRD, at key locations in the final deformed samples. Such models for plate steel forming simulations that are able to provide accurate predictions of deformation textures and derived quantities in the entire volume of the material can be crucial to study further processing steps and properties of the final product.

Estimation of Weld Induced Residual Stresses of Thick Welded Sections Using Numerical Welding Simulation

Welding processes are quite complex in nature as they involve the interaction of multiple physical phenomena. The thermal history developed during the process causes the material to undergo complex phase transformations. Non-uniform contraction and expansion of heat affected, softer, areas constrained by cooler, harder, areas of the material induce residual stresses and the weldment undergoes permanent deformation. When thick sections are welded, the formation of phases and the distribution of residual stresses are even more complex. These could lead to failure of the welded components during service. For the design of pressure vessel applications understanding the phase formation and residual stress distribution in thick weldments is critical. Numerical simulation was used to understand the influence of multiple variables on the residual stresses induced by a welding process The challenges involved in performing welding simulation of thick sections are described. Furthermore numerical and experimental results are compared and discussed.Copyright