A Mathematical Model for Competing Failure Modes in Strain Cycle Fatigue

2006 ◽  
Vol 129 (2) ◽  
pp. 293-303 ◽  
Author(s):  
Gerald T. Cashman
Keyword(s):  
Failure Modes ◽  
High Stress ◽  
Local Strain ◽  
Initiation Site ◽  
Weakest Link ◽  
Strain Relationship ◽  
Powder Metallurgy Alloy ◽  
Link Model ◽  
Cyclic Plastic Strain ◽  
Selection Of

Elevated temperature data for powder metallurgy alloy René 95 generated in vacuum are presented to demonstrate that the life differences observed between surface and internally initiated failures are due to an environmental effect. The transition in behavior from a mode at low stress dominated by internal initiations to a surface dominated mode at high stress is quantitatively described in terms of both a weakest-link model and a local strain relationship. A fatigue failure mechanism is provided that explains that the natural selection of initiation site is based upon the concept that the site displaying the highest local cyclic plastic strain is the location where fatigue initiates.

scholarly journals Fatigue Resistance and Failure Behavior of Penetration and Non-Penetration Laser Welded Lap Joints

2021 ◽  
Vol 34 (1) ◽  
Author(s):  
Xiangzhong Guo ◽  
Wei Liu ◽  
Xiqing Li ◽  
Haowen Shi ◽  
Zhikun Song
Keyword(s):  
Fatigue Resistance ◽  
Failure Modes ◽  
Plate Thickness ◽  
High Stress ◽  
Lap Joints ◽  
Failure Behavior ◽  
Passenger Cars ◽  
Plate Fracture ◽  
The Difference ◽  
Railway Passenger

AbstractPenetration and non-penetration lap laser welding is the joining method for assembling side facade panels of railway passenger cars, while their fatigue performances and the difference between them are not completely understood. In this study, the fatigue resistance and failure behavior of penetration 1.5+0.8-P and non-penetration 0.8+1.5-N laser welded lap joints prepared with 0.8 mm and 1.5 mm cold-rolled 301L plates were investigated. The weld beads showed a solidification microstructure of primary ferrite with good thermal cracking resistance, and their hardness was lower than that of the plates. The 1.5+0.8-P joint exhibited better fatigue resistance to low stress amplitudes, whereas the 0.8+1.5-N joint showed greater resistance to high stress amplitudes. The failure modes of 0.8+1.5-N and 1.5+0.8-P joints were 1.5 mm and 0.8 mm lower lap plate fracture, respectively, and the primary cracks were initiated at welding fusion lines on the lap surface. There were long plastic ribs on the penetration plate fracture, but not on the non-penetration plate fracture. The fatigue resistance stresses in the crack initiation area of the penetration and non-penetration plates calculated based on the mean fatigue limits are 408 MPa and 326 MPa, respectively, which can be used as reference stress for the fatigue design of the laser welded structures. The main reason for the difference in fatigue performance between the two laser welded joints was that the asymmetrical heating in the non-penetration plate thickness resulted in higher residual stress near the welding fusion line.

scholarly journals Stress–strain relationship model of glulam bamboo under axial loading

2020 ◽  
Vol 29 ◽  
pp. 2633366X2095872
Author(s):  
Yang Wei ◽  
Mengqian Zhou ◽  
Kunpeng Zhao ◽  
Kang Zhao ◽  
Guofen Li
Keyword(s):  
Tensile Stress ◽  
Compressive Stress ◽  
Failure Modes ◽  
Axial Loading ◽  
Tensile Failure ◽  
Stress Strain ◽  
Laminated Bamboo ◽  
Bamboo Scrimber ◽  
Strain Relationship ◽  
Strain Model

Glulam bamboo has been preliminarily explored for use as a structural building material, and its stress–strain model under axial loading has a fundamental role in the analysis of bamboo components. To study the tension and compression behaviour of glulam bamboo, the bamboo scrimber and laminated bamboo as two kinds of typical glulam bamboo materials were tested under axial loading. Their mechanical behaviour and failure modes were investigated. The results showed that the bamboo scrimber and laminated bamboo have similar failure modes. For tensile failure, bamboo fibres were ruptured with sawtooth failure surfaces shown as brittle failure; for compression failure, the two modes of compression are buckling and compression shear failure. The stress–strain relationship curves of the bamboo scrimber and laminated bamboo are also similar. The tensile stress–strain curves showed a linear relationship, and the compressive stress–strain curves can be divided into three stages: elastic, elastoplastic and post-yield. Based on the test results, the stress–strain model was proposed for glulam bamboo, in which a linear equation was used to describe the tensile stress–strain relationship and the Richard–Abbott model was employed to model the compressive stress–strain relationship. A comparison with the experimental results shows that the predicted results are in good agreement with the experimental curves.

Integrity Assessment and On-Site Repair of a Floating Production Platform

2005 ◽  
Author(s):  
Mauro G. Marinho ◽  
Alexandre M. Pope ◽  
Luiz Claudio Meniconi ◽  
Luiz Henrique M. Alves ◽  
Cesar Del Vecchio
Keyword(s):  
High Stress ◽  
Local Strain ◽  
Business Unit ◽  
Gusset Plates ◽  
Strain Measurements ◽  
Integrity Assessment ◽  
High Stress Concentration ◽  
Production Platform ◽  
Welding Defects ◽  
Classification Society

Following the warning of a flooded bow horizontal brace of a semi-submersible production platform, an inspection diving team was mobilized and cracks were found at both bow and aft K-joints. Analysis of the service life of the platform, together with the results of structural analysis and local strain measurements, concluded that cracking was caused by fatigue initiated at high stress concentration points on the gusset plates inserted in the tubular joints. As a consequence of the fractured plates other cracks were nucleated close to the intersection lines of the braces that compose the K-joints. Based on this analysis different repair possibilities were proposed. To comply with the production goals of the Business Unit it was decided to repair the platform on-site and in production in agreement with the Classification Society. The proposed repair contemplated the installation of two flanges on the gusset plates between the diagonal braces by underwater wet (UWW) welding. Cracks at the gusset plates were also removed by grinding and wet welding. Defects located at the braces are being monitored and repaired by the installation of backing bars, by wet welding, followed by grinding and welding from the inside. To carry out the job two weld procedures and ten welder-divers were qualified.

scholarly journals Strain Accommodations among Twin Variants in Ti and Mg

Crystals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 453
Author(s):  
Ping Zhou ◽  
Guo-Zhen Zhu
Keyword(s):  
Mechanical Property ◽  
Great Influence ◽  
Local Strain ◽  
Deformation Texture ◽  
Variant Selection ◽  
Slip Systems ◽  
Twinning Shear ◽  
Active Variant ◽  
Shed Light ◽  
Selection Of

The selection of twin variants has a great influence on deformation texture and mechanical property in hcp metals where slip systems are limited and twinning types are abundant during deformation. Local strain accommodations among twin variants are considered to shed light on variant selection rules in Ti and Mg alloys. Five kinds of strain accommodations are discussed in terms of different regions that are affected by the twinning shear of primary twin. These regions contain (I) the whole sample, (II) neighboring grain, (III) adjacent primary twin in neighboring grain, (IV) adjoining primary twin within the same parent grain, and (V) multi-generation of twinning inside the primary twin. For a potentially active variant, its operation needs not only relatively higher resolved shear stress but also easily accommodated strain by immediate vicinity. Many of the non-Schmid behaviors could be elucidated by local strain accommodations that variants with relatively higher SFs hard to be accommodated are absent, while those with relatively lower SFs but could be easily accommodated are present.

Sample selection of prognostics validation test based on multi-stage Wiener process

2018 ◽  
Vol 233 (4) ◽  
pp. 605-614
Author(s):  
Zhiao Zhao ◽  
Yong Zhang ◽  
Guanjun Liu ◽  
Jing Qiu
Keyword(s):  
Wiener Process ◽  
Process Model ◽  
Failure Modes ◽  
Early Stage ◽  
Sample Selection ◽  
Degradation Process ◽  
Time Interval ◽  
Degradation Data ◽  
Multi Stage ◽  
Selection Of

Sample allocation and selection technology is of great significance in the test plan design of prognostics validation. Considering the existing researches, the importance of prognostics samples of different moments is not considered in the degradation process of a single failure. Normally, prognostics samples are generated under the same time interval mechanism. However, a prognostics system may have low prognostics accuracy because of the small quantity of failure degradation and measurement randomness in the early stage of a failure degradation process. Historical degradation data onto equipment failure modes are collected, and the degradation process model based on the multi-stage Wiener process is established. Based on the multi-stage Wiener process model, we choose four parameters to describe different degradation stages in a degradation process. According to four parameters, the sample selection weight of each degradation stage is calculated and the weight of each degradation stage is used to select prognostics samples. Taking a bearing wear fault of a helicopter transmission device as an example, its degradation process is established and sample selection weights are calculated. According to the sample selection weight of each degradation process, we accomplish the prognostics sample selection of the bearing wear fault. The results show that the prognostics sample selection method proposed in this article has good applicability.

Design for Reliability Analysis of Vibration-Induced Failures due to Equipment Assembly Bending Load and Vibration Responses

2017 ◽  
Vol 2017 (1) ◽  
pp. 000517-000525
Author(s):  
Josh Liew ◽  
Otto Fanini
Keyword(s):  
Failure Modes ◽  
High Stress ◽  
Qualification Test ◽  
Reliability Design ◽  
Operational Conditions ◽  
Bending Load ◽  
Stress Levels ◽  
Equipment Failures ◽  
Field Deployment ◽  
Field Failure

Abstract The oil and gas industry subsurface formation evaluation instruments experience significant challenging vibrations and shock levels. Equipment development requirements for these instruments include reliability and durability under these severe operating conditions. The engineering design for this equipment utilizes tools that enable the estimation of service lifetime, maintenance cycles, and related costs. These engineering tools model failure modes and their acceleration factors and how failures interact under certain circumstances. Laboratory test data and operations failure history are used to validate the model results. Incorporating equipment operational failure history into the reliability design after failure analysis, enables determination of failure modes and the length of stress level exposure. Before the equipment is commissioned to field operational service, it is subjected to a batch of environmental qualification tests under objective pass-fail criteria. The environmental qualification test conditions and adopted stress levels are acquired through measurements made with sensors (temperature, acceleration, and shock) in the equipment assembly during field operational conditions in targeted environments. After the equipment passes the qualification tests and final inspection, it is commissioned to field commercial service. This paper studies the development of specific equipment failures during operational field deployment after they were subjected to standard environmental qualifications tests. Various investigative actions focused on determining the cause and circumstances that led to the unexpected field failure. Results helped to introduce corrective updates to the equipment design and manufacturing, durability and reliability design models and procedures, environmental stress levels, and corresponding qualification test conditions. The equipment failures were examined, and comprehensive 3D custom vibration and stress modeling were conducted for the entire equipment assembly and each assembly module. The modeling results pinpointed and confirmed the high stress levels in the failure areas. These high stress levels exceeded the assembly construction strength thresholds, causing failures. The equipment assembly was modified and reinforced to properly support the detected stresses and provide the required lifetime reliability and durability for the operational service. A full 3D model of the equipment assembly was used for the vibration and bending load analysis including all mechanical assembly parts, electronics modules, couplings, and attachments. The 3D model was meshed with Tet and Hex elements in ANSYS application software, failure-prone and critical regions were meshed with finer divisions. In this analysis the electronics modules assembly were considered with all parts, attachments, structural frames, linkages, carriers, and printed circuit board (PCB) modules properly attached and connected to the main chassis structural carrier. Geometries, mass, module and assembly attachments, and material properties were assigned to components in this model. External loads and boundary conditions environmentally imposed to the assembly were applied in the model. Environmental conditions, shock, and vibration (x, y, and z) recorded from similar equipment deployed in subsurface operations in equivalent wells and geological formations were used in the modeling parameters. Displacement modeling data and analysis was performed for all mechanical structural components, PCB electronics module assemblies and assembly components, and module electronics component attachments. A model harmonic analysis under static conditions was performed to detect the oscillatory modes and vibratory resonances and the extent of oscillatory displacements. A structural and main carrier chassis modal analysis was conducted for the entire model, identifying the dominant oscillatory modes and natural structural oscillatory frequencies. The displacement can be used for detection of maximum allowable plastic deformation threshold and cyclic fatigue analysis of attachments, structural support members, and linkages for equipment service lifetime durability and reliability assessment. Past field instrumented operational conditions with documented failures and lab characterization of failure modes along with failure behavior and failure triggering thresholds have provided limits for the mechanical and electronics assembly technology with maximum acceleration level of random vibration and maximum equivalent stress level tolerated by the equipment's structural assembly, standard design techniques, and materials. With these structural stress and displacement limits the 3D modeling results were inspected for the entire assembly, identifying the points in the mesh model where these limits were exceeded. The inspection determined that these recommended limits had been exceeded according to the model results, placing a reduced importance to the adjustment of tolerable maximum stresses and displacements. The mesh points with excessive stress and displacement-induced fatigue coincided with the areas where field failure had been detected in examined field failed units. Because of this modeled assembly performance result and details from the externally imposed operational shocks and vibration, the equipment mechanical and electronics assembly structural design were re-engineered to produce an updated model simulation results that did not exceed the demonstrated cumulative failure threshold stresses in lab tests and field operations. The modified equipment assembly was built and environmentally re-tested in the lab environment with more instrumentation points and scrutiny around the failure critical areas. The test results were successful. After deployment of the new and updated equipment assembly version, its field deployment has not observed similar field failures compared with the previous design version. These modeling and engineering tools, qualification test procedures, and methods can be used to validate a new design or understand the most effective and economical approach to iterate the design before it is launched to field operations or after a field failure.

Reliability of Welded Structures Containing Cracks in Heat-Affected Zones

2000 ◽  
Vol 122 (4) ◽  
pp. 225-232 ◽  
Author(s):  
David B. Lanning ◽  
M.-H. Herman Shen
Keyword(s):  
Fracture Toughness ◽  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Crack Front ◽  
Surface Crack ◽  
Material Properties ◽  
Coarse Grained ◽  
Welded Structures ◽  
Weakest Link ◽  
Link Model

This study investigates the reliability of a plate containing a semi-elliptical surface crack intersecting regions of dissimilar material properties. A weakest-link model is developed to express fracture toughness distributions in terms of effective crack lengths that account for the varying stress intensity factor along the crack front. The model is intended to aid in the development of fracture toughness distributions for cracks encountering local brittle zones (LBZ) in the heat-affected zones (HAZ) of welded joints, where lower-bound fracture toughness values have been measured in the laboratory when a significant portion of the crack front is intersecting the coarse-grained LBZs. An example reliability analysis is presented for a surface crack in a material containing alternating bands of two Weibull-distributed toughnesses. [S0892-7219(00)01203-6]

scholarly journals Spirulina Culture in Bangladesh XI: Selection of a Culture Medium, Suitable for Culturing a Local Strain of Spirulina

1970 ◽  
Vol 41 (3) ◽  
pp. 227-234
Author(s):  
R Khatun ◽  
P Noor ◽  
N Akhter ◽  
MAA Jahan ◽  
M Hossain ◽  
...  
Keyword(s):  
Spirulina Platensis ◽  
Climatic Condition ◽  
Large Scale ◽  
Culture Medium ◽  
Culture Media ◽  
Local Strain ◽  
Condition Effect ◽  
Selection Of

Large scale Spirulina culture is possible in Bangladesh inspite of unpredictable climatic condition. Effect of different culture media on the growth of a local strain of Spirulina (Spirulina platensis) was studied. Bangladesh medium (Bd1) was found to be more favourable for the growth of the alga. All together three culture media were included in this study. Bangladesh J. Sci. Ind. Res. 41(3-4), 227-234, 2006

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