scholarly journals Crack formation during solid pyrolysis: evolution, pattern formation and statistical behaviour

2019 ◽  
Vol 475 (2229) ◽  
pp. 20190211 ◽  
Author(s):  
Yen T. Nguyen ◽  
Thomas J. Pence ◽  
Indrek S. Wichman
Keyword(s):  
Crack Initiation ◽  
Crack Length ◽  
Principal Stress ◽  
Crack Formation ◽  
Structural Integrity ◽  
Degradation Process ◽  
Maximum Principal Stress ◽  
Evolution Pattern ◽  
Average Crack ◽  
Forming Defects

As solids pyrolyse during combustion, they lose chemical and structural integrity by gradually degrading into residual char and forming defects such as voids, fissures and cracks. The material degradation process, which is coupled to the crack formation process, is described using a theoretical model and is numerically simulated using the finite-element method for a generic, charring, rubber-like material. In this model, a slab of material is subjected to an external, localized heat flux and, as the material degrades, cracks form when the local principal stress exceeds a defined cracking threshold. The magnitude of the cracking threshold σ c is systematically varied in order to examine its influences on crack initiation, evolution, distribution and behaviour over time. When σ c exceeds the maximum principal stress for the entire process, σ m , then no cracks are generated. We quantify how the average crack spacing, total crack length and crack initiation time depend upon the ratio σ c / σ m . Two characteristic domains of crack formation behaviour are identified from the crack initiation behaviour. Correlations are produced for the crack length evolution and final crack length values as functions of σ c / σ m . Crack intersection patterns and behaviour are described and characterized.

Crack Initiation Predictions of the AFM-Specimen under Bend and Torsion Loading

2011 ◽  
Vol 488-489 ◽  
pp. 266-269
Author(s):  
Shi Fan Zhu ◽  
Li Zhu ◽  
Qing Fen Li
Keyword(s):  
Crack Initiation ◽  
Principal Stress ◽  
Crack Front ◽  
Complex Loading ◽  
Strain Energy Release ◽  
Maximum Principal Stress ◽  
Loading Conditions ◽  
Energy Release Rates ◽  
Strain Energy Release Rates ◽  
Computational Predictions

In this paper, the computational predictions of crack initial breakpoint and deflection angles under bend and torsion loading conditions are investigated in conjunction with the modified virtual crack closure integral (MVCCI)-method by using the all fracture modes (AFM) specimen and commercial software ANSYS. The separated strain energy release rates (SERRs) along the crack front are calculated and subsequently converted to the stress intensity factors (SIFs) by using Irwin´s equations. Based on the SIFs results, the crack initiation predictions are presented by the maximum principal stress σ1'-criterion. Results show that when the AFM-model under a series of combined proportional bend and torsion loading conditions, asymmetrical stress fields are produced along the crack front. The presented investigation also shows that the maximum principal stress σ1'-criterion in conjunction with the MVCCI-method provides a powerful numerical tool for general computational approach to the fracture analysis of complex loading conditions.

The Influence of Notches on Mechanical Behavior at Elevated Temperatures: Some Metallographic Observations

1968 ◽  
Vol 90 (1) ◽  
pp. 37-44 ◽  
Author(s):  
R. M. Goldhoff ◽  
A. J. Brothers
Keyword(s):  
Crack Initiation ◽  
Principal Stress ◽  
Elevated Temperatures ◽  
Maximum Principal Stress ◽  
Crack Initiation And Propagation ◽  
Metallographic Study ◽  
Initiation And Propagation ◽  
Section Size ◽  
Notch Geometry ◽  
Stress Components

A metallographic study of ruptured and unruptured notch bars is reported in this paper. The alloys are Cr-Mo-V steels tested in the range 1000–1050 deg F. Crack initiation and propagation in these alloys are studied as a function of the steel’s ductility with test bar section size and notch geometry as variables. It is shown that these factors interact in a complex way to control crack initiation and propagation. The mode and rate of crack initiation and propagation in ductile alloys depend mainly on shear processes although principal stress components are important under certain conditions. Alternatively, the maximum principal stress dominates the behavior in low ductility steels. In the former case, notched bar life is limited by crack propagation while in the latter case it is controlled by crack initiation. The influence of geometrical variables and metallurgical factors are discussed.

The Effect of Diamondlike Carbon Overcoat on the Tribological Performance of the Dimple/Gimbal Interface in Hard Disk Drives1

2016 ◽  
Vol 138 (4) ◽  
Author(s):  
Youyi Fu ◽  
Vlado A. Lubarda ◽  
Frank E. Talke
Keyword(s):  
Stainless Steel ◽  
Principal Stress ◽  
Elastic Moduli ◽  
Crack Formation ◽  
Hard Disk ◽  
Disk Drive ◽  
Maximum Principal Stress ◽  
Fretting Wear ◽  
Carbon Overcoat ◽  
Diamondlike Carbon

Fretting wear at the dimple/gimbal interface of a hard disk drive suspension was investigated for stainless steel dimples in contact with stainless steel gimbals coated with diamondlike carbon (DLC) of different thicknesses and different elastic moduli. Scanning electron microscopy (SEM) was used to evaluate the size and characteristics of the wear scar of both the dimple and the gimbal. Fretting wear and fatigue-type cracks were found predominantly on the dimple. For different dimple/gimbal combinations tested in this study, the least amount of wear was obtained for the case of a 690 nm thick DLC overcoat. Numerical simulations were performed to calculate the maximum principal stress in the dimple and the gimbal with the goal of correlating wear and the maximum principal stress. The maximum principal stress in both the dimple and the gimbal was found to increase with an increase of the elastic modulus of the DLC overcoat on the gimbal. On comparing the experimental and simulation results, we conclude that wear and fatigue crack formation can be explained by the different level of the maximum principal stress in both the dimple and the gimbal.

scholarly journals Elastic Simulation of Joints with Particle-Based Fluid

2021 ◽  
Vol 11 (15) ◽  
pp. 6900
Author(s):  
Su-Kyung Sung ◽  
Sang-Won Han ◽  
Byeong-Seok Shin
Keyword(s):  
Principal Stress ◽  
Human Body ◽  
Yield Condition ◽  
Maximum Principal Stress ◽  
Human Motion ◽  
The Difference ◽  
The Moment ◽  
External Forces ◽  
Physical Changes ◽  
Elastic Simulation

Skinning, which is used in skeletal simulations to express the human body, has been weighted between bones to enable muscle-like motions. Weighting is not a form of calculating the pressure and density of muscle fibers in the human body. Therefore, it is not possible to express physical changes when external forces are applied. To express a similar behavior, an animator arbitrarily customizes the weight values. In this study, we apply the kernel and pressure-dependent density variations used in particle-based fluid simulations to skinning simulations. As a result, surface tension and elasticity between particles are applied to muscles, indicating realistic human motion. We also propose a tension yield condition that reflects Tresca’s yield condition, which can be easily approximated using the difference between the maximum and minimum values of the principal stress to simulate the tension limit of the muscle fiber. The density received by particles in the kernel is assumed to be the principal stress. The difference is calculated by approximating the moment of greatest force to the maximum principal stress and the moment of least force to the minimum principal stress. When the density of a particle increases beyond the yield condition, the object is no longer subjected to force. As a result, one can express realistic muscles.

FEM Stress Analysis and Strength of Scarf Adhesive Joints of Similar Adherend Subjected to Impact Tensile Loadings

2007 ◽  
Author(s):  
Toshiyuki Sawa ◽  
Yuya Hirayama ◽  
He Dan
Keyword(s):  
Young’S Modulus ◽  
Principal Stress ◽  
Stress Wave ◽  
Young's Modulus ◽  
Three Dimensional ◽  
Maximum Principal Stress ◽  
Adhesive Joints ◽  
Stress Wave Propagation ◽  
Adhesive Thickness ◽  
Three Dimensional Finite Element

The stress wave propagation and stress distribution in scarf adhesive joints have been analyzed using three-dimensional finite element method (FEM). The FEM code employed was LS-DYNA. An impact tensile loading was applied to the joint by dropping a weight. The effect of the scarf angle, Young’s modulus of the adhesive and adhesive thickness on the stress wave propagations and stress distributions at the interfaces have been examined. As the results, it was found that the point where the maximum principal stress becomes maximum changes between 52 degree and 60 degree under impact tensile loadings. The maximum value of the maximum principal stress increases as scarf angle decreases, Young’s modulus of the adhesive increases and adhesive thickness increases. In addition, Experiments to measure the strains and joint strengths were compared with the calculated results. The calculated results were in fairly good agreements with the experimental results.

A Statistical-Based Fatigue Crack Initiation Model for Axial Flaws in Zr-Nb Pressure Tubes

2014 ◽  
Author(s):  
Cheng Liu ◽  
Douglas Scarth ◽  
Alain Douchant
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Rise Time ◽  
Structural Integrity ◽  
Fatigue Crack Initiation ◽  
Mechanical Damage ◽  
Fuel Bundle ◽  
Test Parameters ◽  
Pressure Tubes ◽  
Criteria For Evaluation

Flaws found during in-service inspection of CANDU Zr-2.5Nb pressure tubes include fuel bundle scratches, debris fretting flaws, fuel bundle bearing pad fretting flaws, mechanical damage flaws and crevice corrosion marks. The CSA Standard N285.8 contains procedures and acceptance criteria for evaluation of the structural integrity of CANDU Zr-2.5Nb pressure tubes containing flaws. One of the requirements is to evaluate the flaws for fatigue crack initiation. There was a need to develop a statistical-based model of fatigue crack initiation at flaws for use in deterministic and probabilistic assessments of Zr-2.5Nb pressure tubes. A number of fatigue crack initiation experiments have been performed on notched specimens from irradiated and unirradiated Zr-2.5Nb pressure tube material with a range of hydrogen equivalent concentrations. These experiments were performed in an air environment and included temperature and load rise time as test parameters. The test data has been used to develop a statistical-based model of fatigue crack initiation at flaws that covers the effects of flaw root radius, load rise time and irradiation. This paper describes the development of the statistical-based model.

Influence of Slope Gradient on Distribution Rule of Geostress Field in River Valleys

2013 ◽  
Vol 404 ◽  
pp. 365-370 ◽  
Author(s):  
Qi Tao Pei ◽  
Hai Bo Li ◽  
Ya Qun Liu ◽  
Jun Gang Jiang
Keyword(s):  
Numerical Simulation ◽  
Stress Concentration ◽  
Principal Stress ◽  
Three Dimensional ◽  
Maximum Principal Stress ◽  
Slope Gradient ◽  
Bank Slope ◽  
Distribution Rule ◽  
River Valleys ◽  
Gradient Change

During the construction of hydropower station, the change of slope gradient in river valleys often takes place. In order to study influence of slope gradient change on distribution rule of geostress field, the three dimensional unloading models under different slope gradients were established by finite difference software (FLAC3D). After numerical simulation, the results were as follows: (1) The phenomenon of stress concentration at the bottom of river valleys was obvious, which appeared the typical stress fold. Both the depth of stress concentration zone and the principal stress values significantly increased with the increment of slope gradient. (2) Maximum principal stress values increased less in shallow part of upper bank slope (low stress zone) but increased more in the nearby slope foot with the increment of slope gradient, causing great difference in geostress field of bank slope. (3) There was some difference in released energy of bank slope due to slope gradient change in river valleys. In order to distinguish the difference, stress relief zone was further divided into stress stably released zone and stress instability released zone. Finally, take Ada dam area of the western route project of South-to-North Water Transfer as an example, the results by numerical simulation were reliable through comparing the distribution rule of geostress field for the dam, which could provide important reference for stability of the design and construction of steep and narrow river valleys.

J and CTOD Based Tensile Strain Capacity Prediction for Pipelines with a Surface Crack in Girth Weld

2021 ◽  
Author(s):  
Youn-Young Jang ◽  
Nam-Su Huh ◽  
Ik-Joong Kim ◽  
Young-Pyo Kim
Keyword(s):  
Crack Initiation ◽  
Internal Pressure ◽  
Surface Crack ◽  
Driving Force ◽  
Tensile Strain ◽  
Structural Integrity ◽  
Accurate Estimation ◽  
Long Distance ◽  
Crack Driving Force ◽  
Girth Welding

Abstract Long-distance pipelines for the transport of oil and natural gas to onshore facilities are mainly fabricated by girth welding, which has been considered as a weak location for cracking. Pipeline rupture due to crack initiation and propagation in girth welding is one of the main issues of structural integrity for a stable supply of energy resources. The crack assessment should be performed by comparing the crack driving force with fracture toughness to determine the critical point of fracture. For this reason, accurate estimation of the crack driving force for pipelines with a crack in girth weld is highly required. This paper gives the newly developed J-integral and crack-tip opening displacement (CTOD) estimation in a strain-based scheme for pipelines with an internal surface crack in girth weld under axial displacement and internal pressure. For this purpose, parametric finite element analyses have been systematically carried out for a set of pipe thicknesses, crack sizes, strain hardening, overmatch and internal pressure conditions. Using the proposed solutions, tensile strain capacities (TSCs) were quantified by performing crack assessment based on crack initiation and ductile instability and compared with TSCs from curved wide plate tests to confirm their validity.

Damage of Reinforced Concrete Structures due to Steel Corrosion

2015 ◽  
Vol 1111 ◽  
pp. 187-192
Author(s):  
Corina Sosdean ◽  
Liviu Marsavina ◽  
Geert de Schutter
Keyword(s):  
Reinforced Concrete ◽  
Building Materials ◽  
Structural Integrity ◽  
Colorimetric Method ◽  
Concrete Structures ◽  
Steel Corrosion ◽  
Degradation Process ◽  
Reinforced Concrete Structures ◽  
Chloride Ingress ◽  
Rc Structures

Reinforced concrete (RC) became one of the most widely used modern building materials. In the last decades a great interest has been shown in studying reinforcement corrosion as it became one of the main factors of degradation and loss of structural integrity of RC structures. The degradation process is accelerated in the case of RC structures situated in aggressive environments like marine environments or subjected to de-icing salts. In this paper it is shown how steel corrosion of the embedded rebars occurs and how this affects the service life of reinforced concrete structures. Also, an experimental study regarding the combined effect of carbonation and chloride ingress was realized. Samples with and without rebars were drilled from a RC slab which was stored in the laboratory for two years. Non-steady state migration tests were realized in order to determine the chloride profile, while the carbonation depth was measured using the colorimetric method based on phenolphthalein spraying. It was concluded that carbonation has a significant effect on chloride ingress, increasing it.

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