Modeling of Crack Propagation Behavior of High Density Polyethylene by Using Crack Layer Theory: Parametric Study

Creep and fatigue slow crack propagation of engineering thermoplastics display continuous or discontinuous manner depending on the test condition. It could be simulated accurately by use of crack layer theory with theoretical backgrounds. But many input parameters complexify the use of CL theory. Thus the investigation on the effect of material parameters on the CL growth is necessary for the comprehensive understanding. In this paper, a parametric study of CL growth simulation of single edge notched tension specimen in creep condition was performed. Several material parameters were varied so that the effect of input parameters on the CL growth behavior could be understood. Total lifetime is used to figure out the effect of the parameters quantitatively. This study would be beneficial to understand the effect of material parameters on the slow crack growth behavior of high density polyethylene.

Development of a Novel Hybrid Unified Viscoplastic Constitutive Model

Hastelloy X and stainless steel 304 are alloys widely used in industrial gas turbines components, petrochemical industry and energy generation applications; In the Pressure Vessel and Piping (PVP) industries they are used in nuclear and chemical reactors, pipes and valves applications. Hastelloy X and stainless steel 304 are favored for these types of applications where elevated temperatures are preferred for better systems’ efficiencies; they are favored due to its high strength and corrosion resistance at high temperature levels. A common characteristic of these alloys, is its rate-dependent mechanical behavior which difficult the prediction of the material response for design and simulation purposes. Therefore, a precise unified viscoplastic model capable to describe Hastelloy X and stainless steel 304 behaviors under a variety of loading conditions at high temperatures is needed to allow a better and less conservative design of components. Numerous classical unified viscoplastic models have been proposed in literature, to predict the inelastic behavior of metals under extreme environments. Based on Miller and Walker classical unified constitutive models a novel hybrid unified viscoplastic constitutive model is introduced in the present work, to describe the inelastic behavior caused by creep and fatigue effects at high temperature. The presented hybrid model consists of the combination of the best aspects of Miller and Walker model constitutive equations, with the addition of a damage rate equation which provides a description of the damage evolution and rupture prediction capabilities for Hastelloy X and stainless steel 304. A detailed explanation on the meaning of each material constant is provided, along with its impact on the hybrid model behavior. Material constants were calculated using the recently developed Material Constant Heuristic Optimizer (MACHO) software, to ensure the use of the optimal material constants values. This software uses the simulated annealing algorithm to determine the optimal material constants in a global surface, by comparing numerical simulations to an extensive database of experimental data. To validate the capabilities of the proposed hybrid model, numerical simulation results are compared to a broad range of experimental data at different stress levels and strain amplitudes; besides the consideration of two alloys in the present work, would demonstrate the model’s capabilities and flexibility to model multiple alloys behavior. Finally a quantitative analysis is provided to determine the percentage error and coefficient of determination between the experimental data and numerical simulation results to estimate the efficiency of the proposed hybrid model.

A Revisit of Material and Structural Failures

In this paper, we revisit the issues related to material and structural failures. In particular, we employ a similar bridging function between the typical structural failure, the so-called column buckling, and the typical material failure under compression, to link the low stress high cycle and the high stress low cycle fatigue. A part of the intention of this paper is to come up with simple formulas as guidelines in engineering practice for both material and structural failures in both static and dynamic situations.

Stiffness Update Procedure in Iterative Global-Local Analysis of Columns

The purpose of this study is to develop a stiffness update technique to be used in a computationally efficient finite element solution for the analysis of columns undergoing local deformations, within the procedure of iterative global-local analysis. The computational problem that arises is that the stiffness matrix is formulated according to the global model, and as a result, considerably large number of iterations is required when the local deformations are significant. To overcome this difficulty, a stiffness update technique is presented in which the displacement field of the global model is altered at each step to consider the locally induced softening behaviour in order to accelerate the convergence. This goal is achieved by introducing embedded discontinuities in the beam element.

Effect of Cold Storage on Mechanical Properties of Aorta

Aortic allografts have been widely used in treatments of congenital heart diseases with satisfactory clinical outcomes. They were usually cryopreserved and stored for surgical use. The objective of this work was to investigate the effect of cold storage on mechanical properties of aorta, since the compliance mismatch was one important factor associated with the complication after graft surgery. The segments of porcine descending aorta were divided into two groups: the fresh samples which were tested within 24 hours after harvesting served as control group, and frozen samples which were stored in −20°C for 7 days and then thawed. The uniaxial tension tests along circumferential direction and indentation tests were conducted. The average incremental elastic moduli within each stretch range were obtained from the experimental data obtained during tension tests, and the elastic moduli were also calculated by fitting the force-indentation depth data to Hertz model when the tissue was stretched at 1.0, 1.2, 1.4 and 1.6. In addition, the average incremental elastic moduli of both fresh and frozen aortic tissue along axial direction were also obtained by using uniaxial tension tests. The comparison showed that cold storage definitely increased the average incremental elastic modulus of the aortic tissue along circumferential direction; however, the difference is not significant for the elastic moduli along axial direction.

Research on Residual Strength of Corroded Tubing of Gas Well With Sustained Casing Pressure

Corrosion and sustained casing pressure have serious threats to the integrity of tubing of gas well. Researching the residual strength of corroded tubing has great significance to ensure the safety of gas well. The finite element method was used to study the relationships between residual strength and corrosion defects size, internal pressure, external pressure, axial load. The results show that, for tubing with uniform corrosion, the defect depth, internal pressure and external pressure have greater impacts on the von Mises equivalent stress of tubing, and the defect width and defect length have little effects on it. For tubing with pitting corrosion, the defect depth, internal pressure and external pressure have greater impacts on the von Mises equivalent stress of tubing, while the defect radius has little effect on it. These simulation data were fitted into the functions of residual strength of corroded tubing according to different corrosion morphology types. Both of the verifications of the fitting results show that most of the error between the original calculation data and the fitting calculation data is less than 4%. The fitting formulas can be used conveniently to evaluate the safety of the tubing of gas well with sustained casing pressure.

Analysis of In-Plane Compression and Bending of Honeycombs With Laminated Cell Walls

Using laminated composite cell walls in honeycomb structures can bring potential advantages such as increased specific stiffness, greater options for material selection, and very importantly, improved manufacturing efficiency. In this paper, the in-plane elastic responses of honeycomb structures with laminated composite cell wall are investigated. An analytical model was developed to obtain the effective elastic properties of the honeycombs based on the laminate properties of the cell wall. The derived homogenization properties were then used to predict the in-plane compression and bending behaviors of the structures. The predicted results were compared with those from finite element analysis of the full detailed honeycomb models. The results of the homogenized solid model, with significant computational cost savings, were in good agreement with those of honeycomb models with full geometric details. It was also demonstrated that the proposed model can be effectively and efficiently used for composite cell wall design and material selection.

Modeling of Welding Joint Using Effective Notch Stress Approach for Misalignment Analysis

This research represents the methodology to develop a weld model to assess the structural integrity of welded joints based on stress analysis by finite element method (FEM) and experimental validation. The stress distribution in the welded joints mainly depends on geometry, loading type and material properties. So, it is a great challenge to develop a weld model to predict the behavior of stress distribution and weld stiffness in the joints. In this study, the effective notch stress approach has been used for weld joint modeling. Parameter tuning has been done for the lowest experimental validation error. The effective notch radius is the only tuning parameter in this weld model. The weld model with effective notch radius in between 0.1 to 0.2 mm has shown a good agreement with the experimental results. Through this study, the weld model based on effect notch stress has been validated experimentally for the first time. The validated weld model was then used for misalignment analysis. Both experimental and FE results confirmed that axial misalignment of 20% of specimen’s thickness would have increased maximum principle stresses more than 25–30%.

Isogeometric Analysis of 3D Dynamic Thermo-Mechanical Phase-Field Model for Cubic-to-Tetragonal Phase Transformations in Shape Memory Alloys

In this paper, we study the dynamic thermo-mechanical behaviors of 3D shape memory alloy (SMA) nanostructures using the phase-field (PF) model. The PF model is based on the Ginzburg-Landau theory and requires a non-convex free energy function for an adequate description of the cubic-to-tetragonal martensitic phase transformations. We have developed a model that includes domain walls, treated as a diffuse interface, which leads to a fourth-order differential equation in a strain-based order parameter PF model. Arising numerical challenges have been overcome based on an isogeometric analysis (IGA) framework. Microstructure morphology evolution and consequent thermo-mechanical properties have been studied on SMA nanostructures of different geometries. The numerical results are in agreement with experimental observations. The developed coupled dynamic model has provided a better understanding of underlying microstructures and behaviors, which can be used for development of better SMA-based devices.