scholarly journals An analytically formulated structural strain method for fatigue evaluation of welded components incorporating nonlinear hardening effects

2018 ◽  
Vol 42 (1) ◽  
pp. 239-255 ◽  
Author(s):  
Xianjun Pei ◽  
Pingsha Dong
Keyword(s):  
Fatigue Evaluation ◽  
Nonlinear Hardening ◽  
Structural Strain ◽  
Strain Method

An Analytical-Based Structural Strain Method for Low Cycle Fatigue Evaluation of Girth-Welded Pipes

2017 ◽  
Author(s):  
Xianjun Pei ◽  
Wei Wang ◽  
Pingsha Dong
Keyword(s):  
Test Data ◽  
Low Cycle Fatigue ◽  
Deformation Condition ◽  
Structural Stress ◽  
Pipe Section ◽  
Fatigue Evaluation ◽  
Welded Pipe ◽  
Structural Strain ◽  
Outer Fiber ◽  
Strain Method

As a further extension to the structural stress based master S-N curve method adopted by ASME Div 2 since 2007, this paper presents an analytical-based structural strain method for girth-welded piping components. Here, structural strain is defined as outer and inner fiber strains calculated corresponding to a deformation condition in which a pipe section plane before deformation remains as a plane after deformation. The analytical formation takes into account all possible plastic deformation conditions a pipe section subjected to a combined remote cyclic bending and axial tension. A simple numerical procedure is used for solving both outer fiber and inner fiber strains, as well as the corresponding elastic core size. For fatigue evaluation purpose, the outer fiber strain can be used to calculate the corresponding pseudo elastic structural stress range so that the structural stress based master S-N curve can be directly used. Under linear elastic deformation conditions, the structural strain definition becomes exactly the same as that calculated by the structural stress method which is the basis on the ASME Div 2 master S-N was developed. A set of a recent full scale girth-welded pipe component test data in low-cycle regime was analyzed using the structural strain method. The results showed that all these new test data fall well within the ASME Div 2 master S-N curve scatter band defined by mean+-standard deviations. In addition to its demonstrated effectiveness, the key advantage of this structural strain method is its simplicity for dealing with girth-welded pipe sections, since finite element stress analysis is no longer needed.

A Comprehensive Structural Strain Method Incorporating Strain-Hardening Effects: From LCF to Ratcheting Evaluations

2018 ◽  
Author(s):  
Xianjun Pei ◽  
Pingsha Dong ◽  
Shaopin Song ◽  
David Osage
Keyword(s):  
Low Cycle Fatigue ◽  
Numerical Algorithms ◽  
Complex Stress ◽  
Structural Effect ◽  
Complex Stress State ◽  
Material Nonlinearity ◽  
Strain Behavior ◽  
Nonlinear Stress ◽  
Structural Strain ◽  
Strain Method

As a further extension to the structural strain method first introduced by Dong et al [1], this paper presents an enhanced structural strain method which incorporates material nonlinearity and for two typical weld structures, i.e. weldment with plate sections (e.g. gusset weld or cruciform weld etc.) and weldment with beam sections. (e.g. pipe structures). A modified Ramberg-Osgood is introduced to capture nonlinear stress strain behavior of the material. A set of numerical algorithms is used to deal with complex stress state induced by structural effect such as beam section and plane strain condition. The proposed structural strain method is then applied to analysis of fatigue data of weldment made from different materials including steel, aluminum and titanium. It is shown that the enhanced structural strain method provides a unified way to correlate fatigue life of weldment in both high cycle and low cycle fatigue regime. The method is also used to study ratcheting problem raised up by Bree. A modified Bree diagram is given by considering material nonlinearity.

A Structural Strain Method for Fatigue Evaluation of Welded Components

2014 ◽  
Author(s):  
Pingsha Dong ◽  
Xianjun Pei ◽  
Shizhu Xing
Keyword(s):  
Fatigue Test ◽  
High Cycle Fatigue ◽  
Low Cycle Fatigue ◽  
Structural Stress ◽  
Cycle Fatigue ◽  
Equilibrium Conditions ◽  
Curve Method ◽  
Structural Stresses ◽  
Structural Strain ◽  
Strain Method

In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to low cycle fatigue regime in which plastic deformation can be significant while an elastic core is still present. The method is formulated by taking advantage of elastically calculated mesh-insensitive structural stresses based on nodal forces available from finite element solutions. The structural strain definition is consistent with classical plate and shell theory in which a linear through-thickness deformation field is assumed a priori in both elastic and elastic-plastic regimes. With considerations of both yield and equilibrium conditions, the resulting structural strains are analytically solved if assuming elastic and perfectly plastic material behavior. The formulation can be readily extended to strain-hardening materials for which structural strains can be numerically calculated with ease. The method is shown effective in correlating low-cycle fatigue test data of various sources documented in the literature into a single narrow scatter band which is remarkable consistent with the scatter band of the existing master S-N curve adopted ASME B&PV Code since 2007. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure in 2007 ASME Div 2 Code can now be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. More importantly, both low cycle and high cycle fatigue behaviors can now be treated in a unified manner. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hook’s law. In low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. The use of a pseudo-elastic structural stress definition is not fundamental, but merely a means to put low-cycle and high-cycle fatigue test data in a conventional stress-based S-N data representation which is typically preferred in engineering practice, than a strain-based representation.

An Analytical Structural Strain Method for Steel Umbilical in Low Cycle Fatigue

2018 ◽  
Vol 141 (1) ◽  
Author(s):  
Wei Wang ◽  
Xianjun Pei
Keyword(s):  
Plastic Deformation ◽  
Production System ◽  
Low Cycle Fatigue ◽  
Structural Stress ◽  
Cycle Fatigue ◽  
Additional Processing ◽  
Subsea Production System ◽  
Welded Pipe ◽  
Structural Strain ◽  
Strain Method

Pipes, especially risers, pipelines, and umbilicals, are extensively used in the subsea production system. Umbilical, as a controlling component of subsea production system, as well as other pipes, will resist reeling, unreeling, and additional processing before on-site installation, which might lead to yielding and plastic deformation of the pipe. This plastic deformation often results in low cycle fatigue (LCF) issue of the pipes, and how to effectively estimate the corresponding fatigue life has become a topic of practical engineering interest. In the present paper, a structural strain method is applied to determine the elastic core of the pipe and to calculate the pseudo structural stress. The pseudo structural stress concept has been applied to analyze the pipe in LCF regime. Further, the results obtained have been compared with the experimental and other available data. It can be seen that the results coincide well with the experimental data. In addition to the demonstrated effectiveness, the key advantage of this pseudo structural stress approach is the simplicity in dealing with girth-welded pipe sections, since finite element stress analysis is unnecessary.

Symptom Validity Testing

2018 ◽  
Vol 23 (6) ◽  
pp. 14-15
Author(s):  
Lee H. Ensalada
Keyword(s):  
Memory Deficits ◽  
Symptom Validity ◽  
Sensory Cues ◽  
Validity Testing ◽  
Symptom Validity Testing ◽  
Tunnel Vision ◽  
Blurry Vision ◽  
The Common ◽  
Fatigue Evaluation ◽  
Choice Testing

Abstract Symptom validity testing (SVT), also known as forced-choice testing, is a means of assessing the validity of sensory and memory deficits, including tactile anesthesias, paresthesias, blindness, color blindness, tunnel vision, blurry vision, and deafness. The common feature among these symptoms is a claimed inability to perceive or remember a sensory signal. SVT comprises two elements: a specific ability is assessed by presenting a large number of items in a multiple-choice format, and then the examinee's performance is compared to the statistical likelihood of success based on chance alone. These tests usually present two alternatives; thus the probability of simply guessing the correct response (equivalent to having no ability at all) is 50%. Thus, scores significantly below chance performance indicate that the sensory cues must have been perceived, but the examinee chose not to report the correct answer—alternative explanations are not apparent. SVT also has the capacity to demonstrate that the examinee performed below the probabilities of chance. Scoring below a norm can be explained by fatigue, evaluation anxiety, inattention, or limited intelligence. Scoring below the probabilities of chance alone most likely indicates deliberate deceptions and is evidence of malingering because it provides strong evidence that the examinee received the sensory cues and denied the perception. Even so, malingering must be evaluated from the total clinical context.

Impairment Questions and Answers

1999 ◽  
Vol 4 (4) ◽  
pp. 4-4
Keyword(s):  
Symptom Validity ◽  
Validity Testing ◽  
Symptom Validity Testing ◽  
Negative Results ◽  
Tunnel Vision ◽  
Questions And Answers ◽  
Blurry Vision ◽  
Fatigue Evaluation ◽  
Choice Testing ◽  
Rule Out

Abstract Symptom validity testing, also known as forced-choice testing, is a way to assess the validity of sensory and memory deficits, including tactile anesthesias, paresthesias, blindness, color blindness, tunnel vision, blurry vision, and deafness—the common feature of which is a claimed inability to perceive or remember a sensory signal. Symptom validity testing comprises two elements: A specific ability is assessed by presenting a large number of items in a multiple-choice format, and then the examinee's performance is compared with the statistical likelihood of success based on chance alone. Scoring below a norm can be explained in many different ways (eg, fatigue, evaluation anxiety, limited intelligence, and so on), but scoring below the probabilities of chance alone most likely indicates deliberate deception. The positive predictive value of the symptom validity technique likely is quite high because there is no alternative explanation to deliberate distortion when performance is below the probability of chance. The sensitivity of this technique is not likely to be good because, as with a thermometer, positive findings indicate that a problem is present, but negative results do not rule out a problem. Although a compelling conclusion is that the examinee who scores below probabilities is deliberately motivated to perform poorly, malingering must be concluded from the total clinical context.

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