Effect of H2-injection on the thermodynamic and transportation properties of natural gas

Condensation of hydrocarbons due to temperature and pressure changes in the pipelines plays an important role in the transportation of natural gas. Injection of hydrogen might change this condensation behavior considerably. The influence of hydrogen on the thermodynamics upon injection, on the Joule–Thomson effect at the pressure reduction stations, on the energy density, on the Wobbe index, and on the pressure drop in the pipelines has been calculated. It has been shown that injection of 25% hydrogen may lead to a temperature drop of several degrees, the temperature drop at the pressure reduction stations reduces by $\text{1}\text{3}$, and the pressure drop in the transport lines increases only slightly. Moreover, at $\text{40}\text{bar}$ and $\text{258}\text{K}$ the amount of liquid condensate is slightly less in the case of hydrogen if the same amount of energy is transported.     

Finite Element Analysis of the Competition Between Crack Deflection and Penetration of Fiber-Reinforced Composites Using Virtual Crack Closure Technique

Crack deflection along the fiber/matrix interface for fiber-reinforced composites is an important condition upon which the toughening mechanisms depend. Sound control for the interface debonding of composites contributes to improving the fracture toughness of composites. Combined with the virtual crack closure technique, a finite element model of composites is proposed to predict the competition between the matrix crack deflection along the interface and the matrix crack penetration into the fibers under the thermomechanical coupling fields. For C/C composites, the effects of the geometry size, fiber volume fraction, fiber coating materials and thermal mismatch on the energy release rate and the crack deflection mechanisms are studied. Results show the fiber coating increases the ability to deflect at large thermal mismatch and small crack sizes, and the TaC coating shows larger effect than the SiC coating. The research provides fundamental method for promoting the toughening design of C/C composites.     

Failure analysis of natural gas buried X65 steel pipeline under deflection load using finite element method

A 3D parametric finite element model of the pipeline and soil is established using finite element method to perform the failure analysis of natural gas buried X65 steel pipeline under deflection load. The pipeline is assumed to be loaded in a parabolic deflection displacement along the axial direction. Based on the true stress–strain constitutive relationship of X65 steel, the elastic–plastic finite element analysis employs the arc-length algorithm and non-linear stabilization algorithm respectively to simulate the strain softening properties of pipeline after plastic collapse. Besides, effects of the soil types and model sizes on the maximum deflection displacement of pipeline are investigated. The proposed finite element method serves as a base available for the safety design and evaluation as well as engineering acceptance criterion for the failure of pipeline due to deflection.     

7050-T7451铝合金低周疲劳平均应力松弛规律

为了研究平均应变对7050-T7451铝合金低周疲劳力学行为的影响,开展了不同应变比(R=-1、-0.06、0.06和0.5)下的室温恒幅低周疲劳试验。结果表明:在对称循环应变下,材料总体表现为循环软化特征;而在非对称循环应变下,材料表现为初始硬化后的循环稳定行为。非对称循环应变导致了材料出现与应变幅相关的平均应力松弛现象。采用Landgraf模型和非线性Maxwell模型分别研究了7050-T7451铝合金的平均应力松弛规律。结果表明:Maxwell模型能够较准确地描述材料的平均应力循环松弛特征,而Landgraf模型更适用于低应变幅下的平均应力松弛描述。     

Finite element model for compression after impact behaviour of stitched composites

A finite element (FE) model using coupling continuum shell elements and cohesive elements is proposed to simulate the compression after impact (CAI) behaviour and predict the CAI strength of stitched composites. Continuum shell elements with Hashin failure criterion exhibit the composite laminate damage behaviour; whilst cohesive elements using traction-separation law characterise the laminate interfaces. Impact-induced delamination is explicitly modelled by reducing material properties of damaged cohesive elements. Computational results have demonstrated the trend of increasing CAI strength with decreasing impact-induced delamination area. Spring elements are introduced into the model to represent through-thickness stitch thread in the composite laminates. Results in this study validate experimental finding that CAI strength is improved when stitching is incorporated into the composite structure. The proposed FE model reveals good CAI strength predictions and indicates good agreement with experimental results, making it a valuable tool for CAI strength prediction of stitched composites.     

Characterization of the interlaminar fracture toughness of a laminated carbon/epoxy composite

Delamination between layers is an important problem in applications of fiber reinforced composite laminates. Tests were carried out to determine the interlaminar fracture toughness of AS4/3501-6 (carbon/epoxy) composite laminates using mixed-mode bending tests. Analysis of the test specimens in terms of mode I and mode II energy release rates showed good agreement between methods based on beam equations, compliance measurements, and detailed finite element analyses. The results showed that the critical mode I energy release rate for delamination decreased monotonically with increasing mode II loading. This is in contrast to some results in the literature. Various analytic representations of the mode interaction from the literature were compared, and shown to fit the data with reasonable accuracy.     

A study on the failure mechanisms of carbon fiber/epoxy composite laminates using acoustic emission

The complex failure mechanisms that are commonly considered as the distinctive characteristic of composites are being amenable to nondestructive test advance. This research adopts the acoustic emission technique to study the failure mechanisms and damage evolution of carbon fiber/epoxy composite laminates. Effects of different lay-up patterns and hole sizes on the acoustic emission response are studied to set up the mapping between the failure properties and the acoustic signal features such as the energy, counting and amplitude. Moreover, the microscopic properties of different composite specimens after fracture are watched and analyzed by scanning electron microscope (SEM). Based on the mapping conception, the controlling microscopic failure mechanisms of composites including the splitting matrix cracking, fiber/matrix interface debonding, fiber pull-out and breakage as well as delamination are identified. It is expected the influence of complex lay-up patterns and sizes on the damage and failure properties of composites is represented by creating true mapping based on the acoustic emission technique.     

Finite Element Analysis of the Void Growth and Interface Failure of Ductile Adhesive Joints

Progressive failure of ductile porous adhesive joint generally includes two competitive failure modes: the void growth of ductile adhesive layer and the interface debonding between the adhesive layer and bonding plates. The damage evolution behavior and ultimate strength of ductile adhesive joint are largely dominated by their evolving interactions. However, most of the existing research failed to predict the damage evolution of these two failure modes simultaneously. After the variational weak form of dynamic equilibrium for two adhesive solids with a finite-thickness adhesive layer and two discontinuous cohesive interfaces is given, this paper studies theoretically the competition between these two failure modes using explicit finite element analysis (FEA). The finite-deformation Gurson–Tvergaard–Needleman (GTN) model is used to predict the void growth of adhesive layer, and the bilinear cohesive model as a ABAQUS module is used to simulate the interface debonding. For single-lap joint under tensile loads, effects of the cohesive strengths, the initial void volume fraction, and the thickness of adhesive layer on their interactions are explored. Besides, the ultimate strengths by FEA are also compared with analytical solutions. Numerical results show that dominating failure mode changes from the interface debonding to the failure of adhesive layer at about the cohesive strength 40 MPa and the thickness of adhesive layer 0.5 mm for FM-73 ductile adhesive joint.     

Finite Element Analysis of Hydrogen Transport in Steel Pressure Vessel at Room Temperature

Hydrogen embrittlement is commonly considered as an important failure mechanism for steel pressure vessels and pipes made of such as Cr–Mo and 4130X steels under high-pressure hydrogen environments, which means hydrogen atom can easily penetrate and diffuse into the metal, leading to the distortion of microscopic lattice and the degradation of macroscopic strength and fracture toughness. Under the support of the National Key Fundamental Research and Development Project of China (2015.1-2019.12), we aim to launch a series of theoretical, experimental, and numerical research on the macroscopic damage evolution and microscopic fracture of steel structures under high-pressure hydrogen environment, which ultimately commits to gaining deep insight into the hydrogen embrittlement mechanisms. This work studies the hydrogen transport mechanisms in Cr–Mo steel pressure vessels under different hydrogen environments using finite element analysis (FEA), which is fundamental to subsequent research on the hydrogen-induced damage evolution and crack behaviors. The purpose of this paper is to explore the effects of the initial hydrogen concentrations and structural sizes on the hydrogen transport mechanisms in 2.25Cr-1Mo pressure vessels with a nozzle at room temperature. Numerical results by comparing different hydrogen concentration distributions show that structural discontinuities tend to accelerate the hydrogen embrittlement sensitivity.     

Recent developments on damage modeling and finite element analysis for composite laminates: A review

Under complex environments such as continuous or cyclic loads, the stiffness degradation for the laminated composites such as the carbon fiber reinforced polymer matrix composites is an important physical and mechanical response to the damage and failure evolution. It is essential to simulate the initial and subsequent evolution process of this kind of damage phenomenon accurately in order to explore the mechanical properties of composite laminates. This paper gives a comprehensive review on the general methodologies on the damage constitutive modeling by continuum damage mechanics (CDM), the various failure criteria, the damage evolution law simulating the stiffness degradation, and the finite element implementation of progressive failure analysis in terms of the mechanical response for the variable-stiffness composite laminates arising from the continuous failure. The damage constitutive modeling is discussed by describing the evolvement of damage tensors and conjugate forces in the CDM theory. The failure criteria which interpret the failure modes and their interaction are compared and some advanced methods such as the cohesive theory which are used to predict the damage evolution properties of composites are also discussed. In addition, the solution algorithm using finite element analysis which implements progressive failure analysis is summarized and several applicable methods which deal with the numerical convergence problem due to singular finite element stiffness matrices are also compared in order to explore the whole failure process and ultimate load-bearing ability of composite laminates. Finally, the multiscale progressive failure analysis as a popular topic which associates the macroscopic with microscopic damage and failure mechanisms is discussed and the extended finite element method as a new finite element technique is expected to accelerate its practical application to the progressive failure analysis of composite laminates.