Failure Limit State Investigation of Misaligned Welded Plates by Using a Damage Model-Based Upon Triaxiality and Lode Parameters

2020 ◽  
Author(s):  
Iago S. Santos ◽  
Diego F. B. Sarzosa
Keyword(s):  
Numerical Study ◽  
Limit State ◽  
Damage Model ◽  
Stress Triaxiality ◽  
Failure Behavior ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Strain Capacity ◽  
Combined Loads ◽  
Welded Structure

Abstract This paper presents a numerical study using the finite element method to assess the structural integrity of welded plates. Different levels of weld misalignment were introduced on the FEM models to investigate the influence of this welding imperfection parameter on the limit state of the structure. The models were loaded under displacement-controlled condition to introduce traction and torsion loads seeking to understand the effects of combined loads on the strain capacity of the misaligned welded structure. Surface elliptical cracks having different crack-size ratios were modeled to study the crack growth behavior by taking into account the misalignment of the weld and combined loads. The damage model is based on a failure surface and post-initiation behavior to model the ductile crack initiation and propagation steps, respectively. The models provide useful information to track the evolution of damage on the hot spot point of the welded structure. The model used is dependent on stress triaxiality and a Lode-based parameter and the damage level is driven by the plastic strain. The evolution of stress triaxiality and Lode parameter with loading are presented, and the influence of misalignment on them are shown. An exponential softening law was adopted to predict post-initiation failure behavior. The calibration steps of the parameters required for damage model application are shown for a A285 pressure vessel steel. Overall, the numerical models reveal the deleterious effects of weld misalignment and combined torsional and tensile loads on the strain capacity of the weld.

A gradient-enhanced damage model motivated by engineering approaches to ductile failure of steels

2019 ◽  
Vol 28 (8) ◽  
pp. 1261-1296 ◽  
Author(s):  
Andreas Seupel ◽  
Meinhard Kuna
Keyword(s):  
Damage Evolution ◽  
Damage Model ◽  
High Stress ◽  
Stress Triaxiality ◽  
Material Behavior ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Damage Initiation ◽  
Gradient Enhancement ◽  
Von Mises Plasticity

Material models for ductile damage, crack initiation, and crack growth are of high interest, e.g. for metal forming simulations. Empirical engineering approaches are often applied, but the numerical results are sensitive to the discretization if no method is utilized to prevent ill-posedness of the underlying boundary value problem due to strain softening. In order to face this issue, an empirical damage model is equipped with a gradient-enhancement which introduces an additional length scale parameter. Until the initiation of damage, the material is modeled with standard von Mises plasticity. Damage initiation is taken into account by an uncoupled failure indicator. After damage initiation, material degradation is assumed to be driven by a non-local quantity, which depends on plastic deformation and stress triaxiality. During damage evolution, the macroscopic material behavior becomes dependent on hydrostatic stress, which is motivated by well known void growth and coalescence mechanisms. A calibration strategy is developed to determine the parameters of strain hardening, damage initiation, and damage evolution as well as the internal length step-by-step. The proposed model is calibrated to experimental data of a pressure vessel steel. Reasonable predictions of smooth and notched tensile tests as well as a small punch test show the validity of the model for loadings from moderate to high stress triaxialities.

Numerical Investigation of Ductile Fracture in Pipelines Under Complex Loading Using a Phenomenological Damage Model

2021 ◽  
Author(s):  
Iago S. Santos ◽  
Diego F. B. Sarzosa
Keyword(s):  
Ductile Fracture ◽  
Mechanical Response ◽  
Numerical Study ◽  
Damage Model ◽  
Stress Triaxiality ◽  
Numerical Procedure ◽  
Complex Loading ◽  
Experimental Tests ◽  
Compact Tension ◽  
Axial Tensile

Abstract This paper presents a numerical study on pipes ductile fracture mechanical response using a phenomenological computational damage model. The damage is controlled by an initiation criterion dependent on the stress triaxiality and the Lode angle parameter, and a post-initiation damage law to eliminate each finite element from the mesh. Experimental tests were carried out to calibrate the elastoplastic response, damage parameters and validate the FEM models. The tested geometries were round bars having smooth and notched cross-section, flat notched specimens under axial tensile loads, and fracture toughness tests in deeply cracked bending specimens SE(B) and compact tension samples C(T). The calibrated numerical procedure was applied to execute a parametric study in pipes with circumferential surface cracks subjected to tensile and internal pressure loads simultaneously. The effects of the variation of geometric parameters and the load applications on the pipes strain capacity were investigated. The influence of longitudinal misalignment between adjacent pipes was also investigated.

Prediction of Ductile Fracture as a Process Controlled by Cavitation Instability

2009 ◽  
Author(s):  
Jiri Novak
Keyword(s):  
Ductile Fracture ◽  
Deformation Theory ◽  
Fracture Strain ◽  
High Stress ◽  
Stress Triaxiality ◽  
Plastic Behavior ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Theory Of Plasticity ◽  
Cavitation Instability

Recently, we applied criterion of initiation of deformation bands based on bifurcation analysis as a criterion of ductile fracture. Experience shows that this procedure yields realistic results if plastic behavior is described by deformation theory of plasticity, with corresponding stress-strain dependence — especially with transition between strain hardening stages III and IV. But it is generally known that under high stress triaxilities, fracture strain depends strongly on stress triaxiality. If deformation theory of plasticity is suitable for modeling of constitutive properties of polycrystalline metals, it should lead to good results in prediction of cavitation instability as a criterion of ductile fracture under high triaxialities as well. We present prediction of fracture strains for reactor pressure vessel steel, in comparison with experimental results. Criterion of cavitation instability based on deformation theory of plasticity predicts similar dependence of fracture strain on stress triaxiality as the classical Rice-Tracey void growth model does, but, moreover, in contrast to the Rice-Tracey model, it predicts absolute values of critical strains. Finally, important role of deformation theory of plasticity in other areas of material engineering and structural integrity analysis is shortly remembered.

Residual Stress and Constraint Effects on Fracture in the Transition Temperature Regime

2008 ◽  
Author(s):  
K. S. Lee ◽  
A. H. Sherry ◽  
M. R. Goldthorpe
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Residual Stresses ◽  
Numerical Study ◽  
Notch Root ◽  
Crack Tip ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Low Constraint ◽  
Crack Tip Constraint

This paper presents the results of a combined experimental and numerical study aimed at quantifying the influence of self-balancing residual stresses on the fracture toughness constraint benefit of a ferritic pressure vessel steel tested in the cleavage fracture regime. Tests were performed on standard and pre-compressed, high constraint, compact-tension (CT) and low constraint, single-edge-notched tension (SENT) specimens at a temperature close to the Master Curve reference temperature T0. Pre-compression is undertaken prior to pre-cracking to establish a residual stress across the uncracked ligament, which is highly tensile at the pre-crack notch root and balanced by compressive stresses further ahead of the notch. The pre-crack is subsequently introduced into material ahead of the notch, within the tensile residual stress region, specimen by electro-discharge machining and fatigue. The tests demonstrate an influence of tensile residual stresses on the apparent fracture toughness properties for both CT and SENT specimens. The tests on low constraint specimens illustrate the constraint benefit on cleavage toughness for this material, and the influence of residual stresses in reducing this benefit. The paper shows how the observed behaviour can be quantified through using two parameter fracture mechanics. Here, the J-integral is determined by taking full account of the influence of preloading on the crack driving force. Both the elastic-T-stress and the elastic-plastic Q-stress are calculated and demonstrated as constraint indexing parameters. The results demonstrate a reduction in constraint benefit for cracks located within highly bending residual stress fields. Thus, when exploring any possible benefit in fracture toughness due to crack tip constraint, it is critical that the combined influence of the primary and secondary stresses on crack tip constraint be taken fully into account.

Prediction of Cleavage Fracture in the Ductile-to-Brittle Transition Region of Pressure Vessel Steels: A Probabilistic Model

2006 ◽  
Vol 324-325 ◽  
pp. 283-286
Author(s):  
Xiao Sheng Gao ◽  
Gui Hua Zhang ◽  
T.S. Srivatsan
Keyword(s):  
Fracture Toughness ◽  
Cleavage Fracture ◽  
Pressure Vessel ◽  
Stress Triaxiality ◽  
Weibull Model ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Proposed Model ◽  
Modified Weibull ◽  
Weibull Stress

This paper presents a modified Weibull stress model, which accounts for the effects of plastic strain and stress triaxiality at the crack tip region. The proposed model is applied to predict cleavage fracture in a modified A508 pressure vessel steel. It is demonstrated that the Weibull modulus (m) remains constant in the temperature range considered, while the threshold Weibull stress (σw-min) decreases with an increase in temperature due to reduction of the yield stress and the scale parameter of the Weibull model (σu) increases with temperature reflecting the influences of temperature on both material flow properties and toughness. The proposed model accurately predicts the scatter of the measured fracture toughness data and the strong effects of constraint and temperature on cleavage fracture toughness.

Dislocation structures in 16MND5 pressure vessel steel strained in uniaxial tension at −196°C

2005 ◽  
Vol 96 (8) ◽  
pp. 909-912
Author(s):  
Karel Obrtlík ◽  
Christian Robertson ◽  
Bernard Marini
Keyword(s):  
Uniaxial Tension ◽  
Pressure Vessel ◽  
Pressure Vessel Steel ◽  
Vessel Steel
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