Gustavo H. B. Donato

Constraint Effects and Crack Driving Forces in Surface Cracked Pipes Subjected to Reeling

This study reports an extensive set of non-linear analyses of circumferentially cracked pipe subjected to reeling. To describe the evolving levels of stress triaxiality ahead of crack front with increased loading, the present work employs the J-Q methodology to characterize fracture response for different crack configurations. The numerical simulations demonstrate that large values of crack-tip driving forces arise in cracked pipes under reeling which, at the same time, are accompanied by strong constraint loss at the crack-tip. Moreover, plane-strain analyses of clamped SE(T) fracture specimens reveal crack-tip conditions which are more similar to cracked pipes subjected to reeling with identical (relative) crack depths, which contrasts sharply to conditions displayed by standard fracture specimens such as the C(T) specimen. Overall, the results reported here encourage further investigations in the development and utilization of constraint-designed SE(T) specimens under clamp conditions in fracture assessments of cracked pipes subjected to reeling.Copyright

Influence of Turbine Inlet Temperature on the Efficiency of Externally Fired Gas Turbines

Many researchers have considered externally fired gas turbines (EFGT) as an option for the implementation of biomass-fueled power plants. The EFGT cycle with regeneration or the gas-vapor combined cycle using one EFGT, also known as externally fired combined cycle (EFCC), could lead to significant efficiency improvements if compared to current technology used for power generation from biomass. This work presents one improved numerical model used for the simulation of EFGT cycle. The results were obtained with a numerical model for the EFGT cycle coupled with a model for the high temperature heat exchanger (HTHE) that is necessary for the cycle implementation. The model of the heat exchanger is based in correlations for the Colburn and friction factors, obtained with CFD simulations. In previous work, the model included only laminar regime for the heat exchanger. The present work extends the correlations that describe the behavior of the heat exchanger to turbulent and transitional regimes. The updated model of the EFGT cycle is used to investigate the influence of the turbine inlet temperature over the cycle efficiency. The results obtained confirm that the pressure drop caused by the heat exchanger is one important parameter that influences the cycle efficiency. The feasibility of the EFGT cycle is discussed taking into consideration that the highest temperature in EFGT cycle is not in the turbine inlet, but in the high temperature heat exchanger.

Crack Growth Testing of an X80 Pipeline Girth Weld Using SE(T) and SE(B) Fracture Specimens

Accurate measurements of fracture resistance properties, including crack growth resistance curves for pipeline girth welds, become essential in defect assessment procedures of the weldment region and the heat affected zone, where undetected cracklike defects (such as lack of penetration, deep undercuts, root cracks, etc.) may further extend due to to high tension stresses and strains. This work presents an investigation of the ductile tearing properties for a girth weld made of an API 5L X80 pipeline steel using experimentally measured crack growth resistance curves ((J-Δa curves). Use of these materials is motivated by the increasing demand in the number of applications for manufacturing high strength pipes for the oil and gas industry including marine applications and steel catenary risers. Testing of the pipeline girth welds utilized side-grooved, clamped single edge notched tensiles (SE (T)) specimens and three-point (3P) bend single edge bend (SE(B)) specimens with a weld centerline notch to determine the crack growth resistance curves based upon the unloading compliance (UC) method using a single specimen technique. Recently developed compliance functions and η-factors applicable for SE (T) and SE(B) fracture specimens with homogeneous material and overmatch welds are introduced to determine crack growth resistance data from laboratory measurements of load-displacement records. This experimental characterization provides additional toughness data which serve to evaluate crack growth resistance properties of pipeline girth welds using SE (T) and SE(B) specimens with weld centerline cracks.Copyright

Applicability of SE(T) and SE(B) Fracture Specimens in Crack Growth Measurements of Pipeline Girth Welds

This work presents an investigation of the ductile tearing properties for a girth weld made of an API 5L X80 pipeline steel using experimentally measured crack growth resistance curves (J-Δa curves). Use of these materials is motivated by the increasing demand in the number of applications for manufacturing high strength pipes for the oil and gas industry including marine applications and steel catenary risers. Testing of the pipeline girth welds utilized side-grooved, clamped SE(T) specimens and 3P bend SE(B) specimens with a weld centerline notch to determine the crack growth resistance curves based upon the unloading compliance (UC) method using a single specimen technique. The shallow-crack SE(B) specimen provides an R-curve which, albeit slightly more conservative, exhibits levels of J-values which are relatively comparable to the levels of J corresponding to the deeply-cracked SE(T) specimen at a fixed amount of crack growth, Δa. This experimental characterization provides additional toughness data which serve to evaluate crack growth resistance properties of pipeline girth welds using SE(T) and SE(B) specimens with weld centerline cracks.Copyright

Fatigue Life Estimation of Welded Joints Including the Effects of Crack Closure Phenomena

The integrity of mechanical components, particularly when they experience considerable fatigue damage during its operating life, can be strongly influenced by the presence of residual stress fields and mechanical heterogeneity. Premature closure of crack flanks greatly influences fatigue crack growth rate. Extensive elastic-plastic finite element analyses have been carried out to investigate detailed crack closure behavior in center cracked welded compact tension (CT) specimens with one level of weld strength mismatch. The finite element results show that homogeneous, soft material has higher crack opening loads than heterogeneous material with 50% overmatch conditions. Fracture testing conducted on C(T) specimens to measure fatigue crack growth rates for an ASTM A516 Gr. 70 steel weldment provide the experimental data to support such behavior. The fatigue life can be reduced by more than 100% for a condition of 50% overmatch when compared with the evenmatch condition. It was verified that most of time spent in fatigue propagation life is consumed at the beginning of the propagation life.Copyright

Texture Analysis for Crack Detection in Fracture Mechanics

In the context of the metal-mechanic industry, there is a strong demand for material behavior measuring methods in the presence of cracks under stress conditions. This behavior is usually quantified by means of fracture toughness tests using parameters such as the stress intensity factor K, the CTOD, or the J integral. Regardless of the parameters used to quantify fracture toughness, all tests require knowledge of the correct length of the existing fatigue pre-crack in the sample that generated the failure. Hence, most laboratories have adopted visual measurement methods using a stereoscopic magnifying glass or a profile projector. Additional techniques, such as the use of heat tinting, help users and researchers to visualize the stable crack growth front. With the improvement in image processing and computer vision techniques, the present paper proposes the application of a new method for border detection by texture analysis, in order to get the corresponding contour of the crack front in postmortem analyses of SE(B) samples with high precision. The results suggest that the proposed method could be applied with high precision to images of fracture toughness tests for crack length measurement, having achieved a discrimination rate of 98%. The results also suggest that the method can be applied to samples that have not undergone heat tinting.

Effects of Plastic Deformation on Fatigue Life of Superduplex Steel Tube Umbilical

This work presents an experimental investigation of the effects of plastic strain on the fatigue behavior of superduplex steel tubes. Fatigue tests using conventional axial loading and a resonant bending setup conducted on 15mm OD tubes made of SAF2507 superduplex steel provides S × N data upon which effects of different levels of plastic strain can be assessed. Despite the inherent scatter in the measured fatigue data, the experiments reveal consistent trends and relatively small effects of plastic strain on fatigue behavior of superduplex steel tubes.© 2012 ASME

Effects of Weld Strength Mismatch on Estimation Procedures for J Fracture Parameter Using Clamped SE(T) Specimens

This work presents an exploratory development of J estimation procedures for deep and shallow cracked tension SE(T) specimens based upon plastic eta factors which incorporate the effects of weld strength mismatch. The considered technique includes estimation of J from plastic work. The primary objective is to derive estimation procedures which are applicable to determine J fracture parameters for a wide range of a/W-ratios, material flow properties and different levels of weld strength mismatch using clamped SE(T) specimens of varying geometries. Very detailed non-linear finite element analyses for plane-strain models provide the evolution of load with increased load-line displacement and crack mouth opening displacement, which are needed to determine the estimation procedures. Laboratory testing of a welded structural steel using SE(T) specimens provide the data needed to evaluate J evolution for the welded joints based upon the proposed methodology. The crack driving force evolution is compared against the results obtained using plastic eta factors for homogeneous specimens and demonstrates the importance of considering mismatch effects on crack-tip stress fields and fracture parameters. The present analyses, when taken together with previous studies, extend the body of results which serve to determine J integral using tension SE(T) specimens with varying geometries and mismatch levels.Copyright

A FAD-Based Procedure for Defect Assessments of Welded Structures Including Effects of Weld Strength Mismatch: Micromechanics Approach and Application to Pipeline Girth Welds

ECA procedures of crack-like defects based upon the FAD philosophy have undergone extensive developments in the past decade to form the basis for industrial codes and guidelines for structural integrity assessments. However, the application of these procedures in welded structural components with mismatch in tensile properties between the weld and base metal remains a potential open issue. Weld strength mismatch may significantly alter the crack-tip driving forces, such as J and CTOD, thereby producing crack-tip stresses quite different than the fields that arise in corresponding homogeneous material. Weld strength mismatch also affects the plastic collapse load for the structural component which further complicates the interplay between fracture and plastic instability before gross yield section takes place. This work describes the development of a microme-chanics-based FAD methodology building upon a local fracture parameter, characterized by the Weibull stress (σw ), to incorporate the effects of weld strength mismatch on crack-tip driving forces. As a further refinement, the study also addresses an exploratory application of a limit load analysis including effects of weld strength mismatch to correct the loading trajectory incorporated into the FAD procedure. Fracture testing of girth welds obtained from an API X80 pipeline steel provide the data needed to validate the proposed modified FAD procedure in failure predictions. Such an application serves as a prototype for a wide class of integrity assessment problems involving the effects of weld strength mismatch.Copyright

Effects of Weld Strength Mismatch on Estimation Procedures for J and CTOD Fracture Parameters Using SE(B) Specimens

This work presents an exploratory development of J and CTOD estimation procedures for welded fracture specimens under bending based upon plastic eta factors and plastic rotation factors. The techniques considered include: i) estimating J and CTOD from plastic work and ii) estimating CTOD from the plastic rotational factor. The primary objective is to gain additional understanding on the effect of weld strength mismatch on estimation techniques to determine J and CTOD fracture parameters for a wide range of a/W-ratios and mismatch levels. Very detailed non-linear finite element analyses for plane-strain models of SE(B) fracture specimens with center cracked, square groove welds provide the evolution of load with increased load-line displacement and crack mouth opening displacement which are required for the estimation procedure. The results show that levels of weld strength mismatch within the range ±20% mismatch do not affect significantly J and CTOD estimation expressions applicable to homogeneous materials, particularly for deeply cracked fracture specimens. The present analyses, when taken together with previous studies, provide a fairly extensive body of results which serve to determine parameters J and CTOD for different materials using bend specimens with varying geometries and mismatch levels.Copyright