Strain-based Design Research Articles

Theoretical analysis of mode I fracture of adhesively bonded bi-material DCB joints

There has been a disputation about how to achieve a pure mode I fracture and thus measure its toughness for an adhesively bonded bi-material double cantilever beam (DCB) specimen. This paper therefore develops a theoretical methodology to calculate the normal and shear stress distributions in the adhesive layer and to perform data reduction and mode partitioning for generic bi-material DCB specimens. The theoretical model is validated using experimental data. The adhesive layer in a generic bi-material DCB joint is loaded both in normal and shear based on the predicted stress distributions in the adhesive layer, resulting in a mixed mode fracture behavior. The prediction results substantiate that a strain-based design principle eliminates the shear stresses and thus leads to pure mode I fracture in bi-material DCB specimens. This paper justifies that the established data reduction method is applicable for measuring mode I fracture toughness when a bi-material DCB is designed according to the strain-based design criterion.

Study on applicability of mathematical model of pipe-soil interaction

Abstract The applicability of the soil spring calculation model given in China’s standard SY/T 7403 “Code for strain-based design of oil & gas transmission pipelines” and GB/T 50470 “Seismic technical code for oil and gas transmission pipeline engineering” to large-diameter pipelines is not clear. In this paper, the soil spring calculation formulas recommended in the two standards are analyzed, and the corresponding finite element numerical simulation models are established. The applicability of the lateral and vertical soil spring models in the two standards is discussed. The results show that the calculation accuracy of the two standards needs to be improved, and the applicability is different for different pipe diameters and soil properties.

Linking the microstructure with strain-life curves for improved utilization of the lightweight potential of thick-walled nodular cast iron

The assessment of the fatigue life of critical, highly stressed component areas is an important aspect in the design of thick-walled components for wind turbines made of nodular cast iron. With the demand for resource efficiency through lightweight design combined with a certain capability to withstand extreme loads and special events, this issue is becoming increasingly important. However, there are still gaps in the description of local strain-based material behaviour. For the design and local strain-based fatigue assessment of large cast components, possibilities to determine the fatigue life as accurately as possible must be provided by standards and guidelines. The paper aims to provide a wide range of material and fatigue data from one common source for general design interests. During research projects at Fraunhofer LBF, material properties for the PRAM-based fatigue life assessment and the elastic-plastic material behaviour have been derived from 9 nodular cast iron grades with 44 microstructural variations. To describe the influence of the technological size effect on cyclic stress-strain and PRAM-N curves the quasi-static parameters (tensile strength and elongation at break) and the microstructure (nodularity and graphite particle density) are considered. In addition, suggestions for scatter bands in the strain-based design approach are given to the designers of thick-walled GJS components.

Quantification of the local mechanical behavior in dissimilar metal welds using digital image correlation instrumented cross-weld tensile testing

The local yielding behavior in base metal, heat-affected zone, fusion boundary region, and weld metal of low-alloy steel/Alloy 625 filler metal welds was quantified using digital image correlation instrumented cross-weld tensile test. The tested welds exhibited undermatching, matching, or overmatching weld metal yield strength with significant gradients in the local yielding behavior. An undermatching weld yielded at 69 MPa below the base metal yield stress, accumulating to 0.72% total strain. The base metal in an overmatching weld had 110 MPa lower yield strength than the weld metal. The strong strain hardening response in the Alloy 625 weld metal, within the uniform elongation range, and its constraining effect on the fusion boundary region and heat affected zone, led to extensive strain accumulation, necking, and final failure in the base metal of all tested welds. The yielding behavior of the tested welds was compared to stress-based criteria, utilizing minimum specified and as-delivered yield and ultimate tensile strength, and to strain-based criteria. The capability of digital image correlation instrumented cross-weld tensile testing to quantify local yielding and strain accumulation demonstrates potential application in proving conformity to stress-based and strain-based design criteria of dissimilar and matching filler metal welds.

Reliability index of a pipe transporting hydrogen submitted to seismic displacement

The reliability index was determined for a pipe presenting a corrosion defect, subjected to an internal hydrogen pressure, buried in a soil with a reaction and subjected to displacement due to seismic activity. The results are obtained by calculating the failure condition: strain demand greater than strain resistance, which is usually the basis of local strain-based design (LSBD). From a probabilistic point of view, this condition results from the product of the two probability distributions, Demand and Resistance. The Resistance is assumed to follow a normal distribution. The strain Demand is calculated assuming that the earthquake probability density follows a Gutenberg–Richter distribution. The reliability index is calculated by analytical method. This method is used to predict the reliability index of three different reference periods or seismic zones located in France.

Strain-based design, direct macrocyclization, and metal complexation of thiazole-containing calix[3]pyrrole analogues

AFIR- and StrainViz-based evaluation of macrocyclic ring strain allowed rational design and high-yield synthesis of calix[3]pyrrole analogues. Among them, calix[1]pyrrole[2]thiazole afforded various metal complexes including water-stable organozinc.

Numerical Simulation Study on the Performance of Buried Pipelines under the Action of Faults

The present paper investigates the mechanical behavior of buried steel pipelines crossing an active fault. Permanent ground deformation induced by an earthquake will cause serious damage to buried steel pipelines, resulting in buckling failure or even cracking damage to pipelines. Based on ABAQUS software, version 6.13., the model of an interacting soil–pipeline system is established, accounting for large strains and displacements and nonlinear material behavior, as well as contact and friction at the soil–pipeline interface. Numerical analysis is conducted through the incremental application of fault displacement. Combined with the force and deformation characteristics of buried pipelines, a strain-based design criterion is chosen to study the vertical displacement, axial compressive, and tensile strain of buried pipelines, etc. This paper focuses on the effects of horizontal fault displacement, fault type, and fault angle on the structural response of the pipe. The failure of the pipeline, such as wall wrinkling, local buckling, or rupture is identified. Furthermore, the effects of the pipeline internal pressure and pipe wall thickness are investigated. The results show that, when the pipeline depth is 1.5 m under the action of the fault, the buried pipeline will not be subject to beam buckling damage, and both tensile damage and shell buckling damage will occur. In this case, the critical displacement of the tensile failure is more than three times that of the shell buckling failure, which indicates that shell buckling damage is a greater threat to the pipeline. The pipeline is most susceptible to damage under the action of a strike-slip reverse fault. When the fault angle is equal to 45 degrees, the pipeline is more likely to be damaged, while it is relatively safe at a fault angle with 90 degrees. The results of this investigation can determine the fault displacement during pipeline failure and provide some reference for pipeline design.

Method of strain analysis of pipelines under landslides: An improved semi-analytical method for softening bending moment-curvature

The accurate description of the elastic‒plastic stress‒strain of a pipeline is used to evaluate the risk that the pipeline will undergo failure during a landslide. This paper proposes a complete set of refined semi-analytical methods (SAMs) for pipeline elastic-plastic strain to accurately evaluate the safety of pipelines. First, a concise model for the calculation of landslide thrust is established based on the pipe-soil interaction mechanism as load boundaries of the SAMs. Secondly, the plastic softening bending moment-curvature relationship (BMCR) is derived to establish a difference equation for the deformation of the pipeline during the plastic stage based on the principle of internal force balance of the pipe section, and then the bending plastic strain is calculated accurately. Finally, the axial strain is fully calculated considering the interaction between axial force and bending of the pipeline. The comparison of the results of small-scale landslide tests, large-scale landslide tests, and SAM calculations shows that the improved SAM has good applicability, with 20% better accuracy than traditional methods. In conclusion, the SAM proposed in this paper can effectively guide the design and evaluation of the strain-based design pipeline.

ApOL-Application Oriented Workload Model for Digital Human Models for the Development of Human-Machine Systems

Since musculoskeletal disorders are one of the most common work-related diseases for assemblers and machine operators, it is crucial to find new ways to alleviate the physical load on workers. Support systems such as exoskeletons or handheld power tools are promising technology to reduce the physical load on the humans. The development of such systems requires consideration of the interactions between human and technical systems. The physical relief effect of the exoskeleton can be demonstrated in experimental studies or by simulation with the digital human model (DHM). For the digital development of these support systems, an application-oriented representation of the workload is necessary. To facilitate digital development, an application-oriented workload model (ApOL model) of an overhead working task is presented. The ApOL model determines the load (forces, torques) onto the DHM during an overhead screw-in task using a cordless screwdriver, based on experimental data. The ApOL model is verified by comparing the simulated results to the calculated values from a mathematical model, using experimental data from three participants. The comparison demonstrates successful verification, with a maximum relative mean-absolute-error (rMAE) of the relevant load components at 11.4%. The presented ApOL model can be utilized to assess the impact of cordless screwdriver design on the human workload and facilitate a strain-based design approach for support systems e.g., exoskeletons.

Mechanistic-empirical Design of Perpetual Road Pavement Using Strain-based Design Approach

Present paper deals with the development of a Mechanistic-Empirical model of the strain-based design of perpetual road pavement using Odemark's principle. The bituminous pavement which can withstand minimum design traffic of 300 msa has been classified as perpetual pavement in this paper. The pavement has been considered as a three-layered system with a top layer of bituminous mix followed by unbound granular materials which rest on soil subgrade. The constituent bituminous layer thickness in the pavement has been determined by limiting the radial tensile strain at the bottom of the bituminous layer against fatigue and the vertical compressive strain at the top of the subgrade against rutting. The allowable strain against rutting and fatigue has been used in the present analysis from mechanistic-empirical correlations recommended in IRC:37-2018. The pavement section has been transformed into a homogeneous system by Odemark's method for application of Boussinesq's theory. To validate the thickness of the perpetual pavement, the strain at different layer interfaces in the pavement was compared using IITPAVE software, which shows the pavement section using present method is safe against rutting but marginally fails under fatigue. Moreover, conventional pavement thickness obtained using IRC:37-2018 were compared with the present method, which shows reasonably good convergence. It has been found that the bituminous layer thickness in a layered system of pavement seems to be more sensitive to fatigue than rutting. In this backdrop, modified fatigue and rutting strain values have been recommended for the design of perpetual road pavement.

Fracture behavior analysis of X80 pipelines welded joints with unequal wall thickness

Pipeline transportation is one of the main methods for transporting oil and natural gas worldwide. The structural safety of welded joints of large diameter pipes with cracks and unequal wall thickness has become a research focus due to discontinuous geometrical structure and cracks in weld seams. In this paper, the tensile experiment and analysis of specimens with unequal wall thickness across welding seam based on the digital image correlation technology are carried out. Moreover, the influence of transition length and transition form on fracture behavior of welded joints of unequal wall thickness pipeline is obtained. In addition, a three-dimensional finite element model of welded joint of unequal wall thickness pipe with oblique line transition form and circular arc transition form is established. and the accuracy of the full-scale pipeline finite element model modeling method is verified by small-scale experiment data. On this basis, the influence of geometrical parameters of the transition section and wall thickness ratio, pipe diameter, and weld strength matching coefficient on the crack driving force of welded joints with unequal wall thickness is obtained. From the perspective of strain-based design, a method for obtaining the minimum transition length section and avoiding girth weld fracture is proposed. The research results guide the design and safety evaluation of welded joints with unequal wall thickness.

Probability of failure of a pipe exposed to seismic displacement and internal pressure

The failure probability and the reliability index have been determined for a pipe submitted to internal pressure, exhibiting a corrosion defect, embedded in a soil with a ground reaction, and underwent the displacement due to seismic activity. Results are obtained by computing the condition of failure: strain demand higher than strain resistance which is typically the Strain Based Design (SBD) basis. From the probabilistic point of view, this condition results in the overlay of the two probability distributions, namely, demand and resistance. An analytical method is proposed to compute the common area between the strain demand and resistance distribution and then to get the probability of failure. The strain demand is assumed to follow a power-law distribution and the strain resistance is a Normal one. The strain demand is computed assuming that the probability density of seismic waves follows a Gutenberg - Richter distribution law. This simple method is also used to predict the failure probability of different reference periods or seismic zone. It is also used to examine the influence of the coefficient of variation of the strain resistance distribution when using vintage pipe steels.

Tensile Strain Capacity Prediction Model of an X80 Pipeline with Improper Transitioning and Undermatched Girth Weld.

As an important component of strain-based design, the tensile strain capacity (TSC) concept has been extensively used for pipelines that experience expectable plastic strain for both installation and service. However, some stress-based designed pipelines have experienced unforeseen plastic strain in the past decade that resulted in failure. It seems that the tensile strain capacity has gradually become an important requirement for geohazard risk management and pipeline maintenance of stress-based design pipelines. The tensile strain capacity of an X80 pipeline is investigated. The assessment in this work was based on the fracture initiation–control-based limit state. This limit state corresponds to the onset of stable tearing and generally provides a reasonably conservative estimate. Besides that, factors such as wall thickness, material’s strain hardening capacity, toughness, weld strength mismatch, HAZ (heat-affected zone) softening, pipe wall thickness, high–low misalignment, and internal pressure were also investigated to construct a prediction model of the X80 vintage pipeline.

Evaluation of Mechanical Properties and Microstructure of X70 Pipeline Steel with Strain-Based Design

The microstructure and mechanical properties of X70 pipeline steel with a ferrite/martensite dual-phase microstructure produced by thermo-mechanical controlled processing were investigated by tensile tests, Charpy V-notched (CVN) impact tests, drop-weight tear tests, guided-bend tests, scanning electron microscopy and transmission electron microscopy combined with thermodynamic simulation analysis. All the mechanical properties met the strength, ductility, toughness and deformability properties requirements of X70 grade pipeline steel with strain-based design. The shear fracture area and absorbed energy of CVN at −10 °C were >97% and >205 J in base metal (BM), weld metal (WM) and heat affected zone (HAZ) with low transition temperature, indicating adequate resistance to propagating fracture. The microstructure of WM was mainly intragranular acicular ferrite that can guarantee high strength, toughness and over matching requirements of the welded joint. Because of being exposed to successive heat inputs, the ferrite plus martensite/bainite microstructure of BM was heated between Ac1 and Ts forming the HAZ. However, a high CVN impact toughness of 345 J at −10 °C in HAZ was obtained, which indicated that the excellent mechanical properties of BM would not be seriously deteriorated during the welding thermal cycles with the reasonable addition of Ti and Nb.

A comparative study on constraint parameters for characterizing fracture toughness of X60 and X100 pipe steels

The fracture toughness resistance curve (e.g., J-R curve and crack tip opening displacement (CTOD) or δ-R curve) is important in facilitating strain-based design and integrity assessment of oil and gas pipelines, where influence of ground movement and presence of planar weld flaws must be accounted for. In the present study, duplicate single edge notched bend (SE(B)) and clamped single edge notched tension (SE(T)) specimens with two different initial crack lengths were tested at room temperature for longitudinally-oriented specimens machined from X60 and X100 pipe steels where the notch is along the through-thickness direction. The results showed the expected trends of higher fracture toughness resistance curves for shallow-cracked over deep-cracked specimen types and tension over bending loading modes. Eleven constraint parameters (i.e., QHRR, QSSY, QLM, QBM, A2, A2BM, h, Tz, Cp, Ap, Vp) were calculated based on 3-D finite element analysis for all tested SE(B) and SE(T) specimens. The results indicated that the shallow-cracked SE(B) specimens exhibit a constraint condition similar to the intrinsically low-constraint deep-cracked SE(T) specimens made of the X60 pipe steel. Among all of the constraint parameters, the Ap parameter based on the equivalent plastic strain isoline was found to correlate the best to the material fracture toughness for currently tested specimens. Furthermore, a linear master curve between various critical fracture toughness values and Ap parameter was obtained for all the tested specimens with different crack lengths, loading modes and pipe steels.

Failure prevention of seafloor composite pipelines using enhanced strain-based design

Abstract Steel pipes are widely used in offshore structures and buried seafloor pipelines. Buried single-wall pipelines or piles of fixed offshore platforms crossing the earthquake fault can experience very large displacement, which could cause considerable damages or failures to the pipes. The elevated strains induced in pipelines from welding connection between the pipes also can drastically reduce the pipe’s performance. Therefore, to prevent these types of damages and failures, this research used an enhanced strain-based design method along with finite element analysis, developed bonded and unbonded double-wall composite pipes, which were in replacement of the single-wall ones, and studied the effects of welding on the double-wall composite pipeline in terms of wrinkles, ovalization, stresses, and strain. Here, the double-wall annulus of the pipelines was grouted with polymer. Extensive experiments were performed with displacement loads being applied to the pipelines in clay and in air and the results were analyzed. It was found that using an enhanced strain-based design method, failures in the pipelines could be significantly reduced or possibly even eliminated. This work would also potentially lead to a new area of research in the oil and gas industries since the elevated strains induced in pipelines due to weld could lead to several modes of failure. In addition, this research found some significant impacts in terms of safety due to the bond between steel and polymer and weld imperfection in the pipelines.

New Methods for Predicting Strain Demand of Arctic Gas Pipelines across Permafrost under Frost Heave Displacement

With increasing gas resource development in the Arctic region, gas pipeline installations in permafrost regions are becoming important. Frost heave of pipeline foundation soils may occur when a chilled gas pipeline passes through unfrozen areas with frost-susceptible soils. The stress and strain behaviors caused by the differential frost heave will directly affect the safety of the pipeline. A nonlinear finite element model (FEM) computing the mechanical responses of the buried gas pipeline subjected to frost heave load was established and successfully validated with the results of a large-scale indoor pipe-soil interaction experiment carried out in Caen in France. Utilizing C# language and object-oriented visual programming techniques, a new customized parametric strain calculation software was developed. The effects of pipe diameter, pipe wall thickness, pipe internal pressure, and peak soil resistance on the longitudinal strain of X60, X70, and X80 steel pipes have been investigated quantitatively. For the first time, a fitting semiempirical equation and trained backpropagation neural network (BPNN) for predicting pipeline strain demand subjected to frost heave load were proposed based on 2688 groups of FEM results. The comparison results have proved their high accuracy and lower running time cost. The proposed new methods can be applied in the strain-based pipeline design and safety evaluation of pipelines in service. It is in the hope of supplementing existing theory and identifying new approaches for arctic gas pipeline installations.

Using Finite Element Method for Stress-Strain Evaluation of Commonly Used Buried Pipelines in Fault

In different kinds of buried pipelines, L245 and L360 are the most used which are chosen by the China Pipeline Design Institute. For studying the stress and deformation characteristics of buried pipelines with different specifications across faults, this paper established a physical model of cross-fault buried pipelines and a finite element model of pipelines crossing the fault zone, which adopts the finite element method and ANSYS software. The models take pipeline material, soil material, grid division, load application method, and other factors into consideration, concentrating on the nonlinear solution of L245 and L360 buried pipelines under the condition of strike-slip fault soil. The results illustrate that pipelines with larger diameters are more conducive to resisting the stress and deformation caused by faults. Moreover, the strain and dislocation amount of the pipeline increases with the increase of the dislocation amount when a fault occurs. Furthermore, the resistance is optimal when the angle of intersection between the fault and the pipe is 60, while further research and analysis are needed for special cases. This work can provide a direction for the optimization of parameters for pipeline design especially strain-based design.

Constraint effect on fracture toughness resistance curves of an X60 pipe steel

The fracture toughness resistance curve (e.g. J-R curve and crack tip opening displacement (CTOD) or δ-R curve) is important in facilitating strain-based design and integrity assessment of oil and gas pipelines, where influence of ground movements and presence of planar weld flaws must be accounted for. In this paper, duplicate single edge bend (SE(B)) and clamped single edge tension (SE(T)) specimens with two different initial crack sizes were prepared from an X60 pipe steel. The longitudinally-oriented specimens were notched in the through-thickness direction and experimentally tested at room temperature. In general, the results showed the expected trends of higher resistance curves for both shallow-cracked versus deeply-cracked specimen types and tension versus bending loading modes. Eleven constraint parameters (i.e. QHRR, QSSY, QLM, QBM, A2, A2BM, h, Tz, Cp, Ap, Vp) were calculated based on three-dimensional (3-D) finite element analyses (FEA) for various tested SE(B) and SE(T) specimens.The results and analysis indicated that the shallow-cracked SE(B) specimens exhibited a constraint condition similar to the intrinsically low-constraint deeply-cracked SE(T) specimens. Among all of the constraint parameters, the Ap parameter based on the normalized equivalent plastic strain contour was found to have the most promising trend in terms of correlating the material’s toughness to constraint level for currently tested specimens. Further on-going investigations are underway to extend the evaluation to resistance curves for specimens made from X100, X80 and X70 pipe steels to allow for the characterization of strain hardening effect, as well as the evaluation of their associated weld metals and heat affected zones (HAZs).

Study on Inelastic Strain-Based Seismic Fragility Analysis for Nuclear Metal Components

The main purpose of this study is to investigate the feasibility of the seismic fragility analysis (FA) with the strain-based failure modes for the nuclear metal components retaining pressure boundary. Through this study, it is expected that we can find analytical ways to enhance the high confidence of low probability of failure (HCLPF) capacity potentially contained in the conservative seismic design criteria required for the nuclear metal components. Another goal is to investigate the feasibility of the seismic FA to be used as an alternative seismic design rule for beyond-design-basis earthquakes. To do this, the general procedures of the seismic FA using the inelastic seismic analysis for the nuclear metal components are investigated. Their procedures are described in detail by the exampled calculations for the surge line nozzles connecting hot leg piping and the pressurizer, known as one of the seismic fragile components in NSSS (Nuclear Steam Supply System). To define the seismic failure modes for the seismic FA, the seismic strain-based design criteria, with two seismic acceptance criteria against the ductile fracture failure mode and fatigue-induced failure mode, are used in order to reduce the conservatism contained in the conventional stress-based seismic design criteria. In the exampled calculation of the inelastic seismic strain response beyond an elastic regime, precise inelastic seismic analyses with Chaboche’s kinematic and Voce isotropic hardening material models are used. From the results of the seismic FA by the probabilistic approach for the exampled target component, it is confirmed that the approach of the strain-based seismic FA can extract the maximum seismic capacity of the nuclear metal components with more accurate inelastic seismic analysis minimizing the number of variables for the components.