Practical Criterion for Estimation of Notch Fatigue Strength: Fatigue Diagrams and Material-Dependency of Notch Effects

2005 ◽  
Author(s):  
Hiroshi Matsuno ◽  
Yoshihiko Mukai
Keyword(s):  
Fatigue Strength ◽  
Stress Concentration ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Stress Ratio ◽  
Notch Root ◽  
Equivalent Stress ◽  
Reduction Factor ◽  
Strength Reduction Factor ◽  
Fatigue Criterion

In the previous paper, authors considered a notch fatigue criterion on the basis of an equivalent stress ratio which was newly proposed as the parameter for the correspondence between cyclic stress conditions of a notched and unnotched specimen. The equivalent stress ratio is represented as a function of a nominal stress ratio and a theoretical stress concentration factor of a notched specimen. It could be derived without difficulty from a hypothesis of plastic adaptation which was newly proposed by the authors and the mechanical models which reflected the hypothesis. In the present paper, in order to confirm the applicability of the equivalent stress ratio, a wide range of published fatigue test data is rearranged on the diagram where the abscissa represents the equivalent stress ratio and the ordinate does the notch-root-concentrated stress range. As a result, the consistent relation proper to material is obtained in spite of the difference of a notch stress concentration factor, a specimen type (a plate or a round-bar) and a loading type (axial, bending, torsional or their combined loading). The relation is formulated in a simple form as an empirical equation. Such a result leads to a notch fatigue criterion that the notch-root-concentrated stress range at the fatigue strength of the notched specimen for any nominal stress ratio is identical with the fatigue strength of the unnotched specimen for the equivalent stress ratio. Moreover, the equation for estimation of a fatigue strength reduction factor can be derived by relating its definition with the notch fatigue criterion. As a result, it is shown that a usually defined fatigue strength reduction factor is represented by multiplying the theoretical stress concentration factor by the unnotched specimen’s fatigue strength ratio which is dependent upon the mean stress. Accordingly, it is clear that the material-dependency of notch effects can be characterized by the steepness of slope of the unnotched specimen’s fatigue strength diagram.

Pipe Joint Management for Risers and Pipelines

2021 ◽  
Author(s):  
Ghiath (Guy) Mansour
Keyword(s):  
Fatigue Strength ◽  
Stress Concentration ◽  
Software Development ◽  
Wall Thickness ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Joint Management ◽  
Pipe Joints ◽  
Best Fit ◽  
Pipe Joint

Abstract Minimizing the stress concentration factor (SCF) in pipe joint welding subjected to fatigue is a major concern. Machining the joint ends is one way to achieve this. However, this adds cost, time, risk of potential crack starters, and loss of wall thickness which is detrimental for fatigue, strength, and engineering criticality assessment (ECA) in particular. Pipe joint sorting (certain joints in sequence) and end matching (rotating the pipe joints for best fit) are other ways. However, this adds time, costly logistics, risk of errors, and does not guarantee the minimum possible SCF is achieved. In a typical project, more pipe joints are procured than required in order to mitigate contingencies. For pipelines, this overage is typically a percentage of the required number of joints or pipeline length. For risers, typically double the required number of joints is procured where half of the joints is sent offshore for installation and the remaining half is kept onshore for a spare riser. Then, it becomes very important to send for installation the best pipe joints that produce the best (lowest) SCFs out of the entire batch of pipe joints. This requires calculating the SCF for every potential match of any random joints to be welded together, and then choosing the best joints. Performing such calculations by spreadsheet is not feasible considering the tremendous number of required iterations and calculations. A pipe joint management software development is presented herein which accomplishes this task and examples provided to illustrate the benefits. Note: Selecting pipe joints with the best end measurements, whether ID, OD, OOR, or thickness does not guarantee that the minimum possible SCFs will be achieved since the SCF is a function of all those measurements.

Assessment of Notches in Ceramic Components

1995 ◽  
Vol 117 (3) ◽  
pp. 413-416 ◽  
Author(s):  
A. Bru¨ckner-Foit ◽  
A. Heger ◽  
D. Munz
Keyword(s):  
Stress Concentration ◽  
Failure Probability ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Notch Root ◽  
Stress Gradient ◽  
Ceramic Components ◽  
Weibull Theory

The failure probability of notched tensile bars is calculated using the multi-axial Weibull theory. The influence exerted by the stress concentration factor, the stress gradient in the notch root, and the Weibull exponent is analyzed.

Fatigue Behavior of the Low-Strength Steel Welded Joints Treated by TIG Dressing and Ultrasonic Peening Combined Method

2011 ◽  
Vol 295-297 ◽  
pp. 1885-1889
Author(s):  
Sen Li ◽  
Dong Po Wang ◽  
Hai Zhang ◽  
Bo Tan
Keyword(s):  
Fatigue Life ◽  
Fatigue Strength ◽  
Stress Concentration ◽  
Compressive Stress ◽  
Stress Level ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Combined Method ◽  
Ultrasonic Peening ◽  
Weld Toe

Butt-joint specimens of Q235B low-strength steel were treated by TIG dressing and ultrasonic peening combined method. The paper presents comparative fatigue test for welded specimens in the as-welded condition and specimens treated by TIG dressing, ultrasonic peening treatment (UPT) and the combined method. When the ratio of stress R=0.1, contrasted with the specimens in as welded condition, the fatigue strength of the specimens treated by TIG dressing is increased by 36%. The fatigue strength of the specimens treated by the combined method and UPT are almost the same, which are increased by 57% and 56% respectively. In the high stress level, weld toe treated by the combined method has smaller stress concentration factor than that of UPT, resulting in less release of residual compressive stress. So it's more effective to improve the fatigue life by the combined method. While in the low stress level, the residual compressive stress of weld toe treated by the combined method and UPT are nearly the same. Besides, the effect of stress concentration factor is smaller, thus the fatigue life of the two methods have little difference.

Practical Criterion for Estimation of Notch Fatigue Strength

2004 ◽  
Author(s):  
Hiroshi Matsuno ◽  
Yoshihiko Mukai
Keyword(s):  
Surface Layer ◽  
Fatigue Strength ◽  
Stress Ratio ◽  
Notch Root ◽  
Equivalent Stress ◽  
Cyclic Stress ◽  
Biaxial Stress ◽  
Combined Loading ◽  
Stress Range ◽  
Stress Cycling

In the present study, a practical criterion for the estimation of the fatigue strength of notched specimens is discussed from a practical standpoint of design and maintenance of machines and structures. First of all, a hypothesis of “Fatigue Plastic Adaption” is proposed as one idea that is available to combine microscopic and macroscopic approaches to fatigue plasticity. The hypothesis expresses that, at a surface layer and at a notch root, elastic deformation arising at the cyclic maximum stress is transformed into local and inhomogeneous plastic deformation. Based on the hypothesis, mechanical models are constructed in order to simulate cyclic stress behavior at the surface layer and at the notch root. As a result, “Equivalent Stress Ratio” is formulated as a parameter for correspondence of cyclic stress conditions between notched and unnotched specimens. Moreover, on the basis of the hypothesis of the plastic adaptation, the equation of the equivalent stress ratio is also derived for the case of biaxial stress cycling in torsion, and it is finally expanded for the general case of proportional multiaxial stress cycling. The published fatigue data concerning tension-compression, bending, torsion and their combined loading are rearranged on the diagram where an abscissa indicates the equivalent stress ratio and an ordinate indicates the stress range at the notch root. As the result, it is recognized that the relation between the equivalent stress ratio and the notch-root-concentrated stress range is shown by a certain curve proper to material in spite of difference of stress concentration factors, loading types and mean stresses. Consequently, a criterion for notch fatigue strength is described on the basis of the equivalent stress ratio, i.e., the notch-root-concentrated stress range at the fatigue strength of the notched specimen for any nominal stress ratio is identical with the fatigue strength of the unnotched specimen for the equivalent stress ratio.

Fatigue Life Properties of Circumferentially-Notched Bar of Austenitic Stainless Steel With Various Concentration Factors

2018 ◽  
Author(s):  
Naoaki Nagaishi ◽  
Michio Yoshikawa ◽  
Saburo Okazaki ◽  
Hisao Matsunaga ◽  
Junichiro Yamabe ◽  
...  
Keyword(s):  
Stainless Steel ◽  
Fatigue Life ◽  
Stress Concentration ◽  
Austenitic Stainless Steel ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Stress Ratio ◽  
Ambient Air ◽  
Fatigue Tests ◽  
Concentration Factors

Fatigue tests were performed using three types of round-bar specimens of Type 304, meta-stable, austenitic stainless steel. The specimens had circumferential notch with stress concentration factors, Kt, of 2, 3 or 6.6. Load controlled fatigue tests were conducted at stress ratio, R, of 0.1 and −1 in ambient air at room temperature. At R of 0.1, fatigue life was decreased with an increase in the stress concentration factor. Conversely, at R of −1, the stress concentration factor had little influence on the fatigue life. To understand the mechanism of the stress ratio effect, local deformation behavior at and beneath the notch root during the fatigue test was computed by means of finite element analysis considering that the plastic constitutive model describes the cyclic stress-strain response.

Two- and Three-Dimensional Cases of Stress Concentration, and Comparison With Fatigue Tests

1936 ◽  
Vol 3 (1) ◽  
pp. A15-A22 ◽  
Author(s):  
R. E. Peterson ◽  
A. M. Wahl
Keyword(s):  
Fatigue Strength ◽  
Stress Concentration ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Practical Importance ◽  
Three Dimensional ◽  
Fatigue Tests ◽  
Carbon Steels ◽  
Dimensional Case ◽  
Strain Measurements

Abstract This paper reports the results of a study of some two- and three-dimensional cases of stress distribution with particular reference to shafts having fillets or transverse holes, these being of considerable practical importance. To determine the stress-concentration factor kt in such cases, strain measurements were made, using a specially developed extensometer with a gage length of 0.1 in. The results of these strain measurements indicate that for shaft fillets in bending (three-dimensional case) the stress-concentration factor kt is little different from the values obtained photoelastically on flat specimens having the same r/d ratio (a two-dimensional case). A comparison of these values of kt (both for shafts with fillets and with transverse holes), with data from fatigue tests, leads to the following observations: (1) In some cases fatigue results are quite close to theoretical stress-concentration values. (2) Fatigue results for alloy steels and quenched carbon steels are usually closer to theoretical values than are the corresponding fatigue results for carbon steels not quenched. (3) With decrease in size of specimen, the reduction in fatigue strength due to a fillet or hole becomes somewhat less; and for very small fillets or holes the reduction in fatigue strength is comparatively small. (4) Sensitivity factors determined for small specimens should not be applied to the design of machine parts regardless of size.

Effects of Stress Concentration and Reinforcement on the Fatigue Strength of Rectangular Openings

1985 ◽  
Vol 22 (04) ◽  
pp. 339-350
Author(s):  
Pin Yu Chang
Keyword(s):  
Fatigue Life ◽  
Fatigue Strength ◽  
Stress Concentration ◽  
Maximum Stress ◽  
Design Method ◽  
Reduction Factor ◽  
Strength Reduction Factor ◽  
Approximate Methods ◽  
New Methods ◽  
Current Design

Large rectangular openings on the strength deck are a major concern of the designers of commercial and military ships. Currently used methods are inaccurate for the prediction of stress concentration factor (SCF). There is no commonly accepted criterion for the maximum stress. The current design method cannot be used to explain the behaviors of the structure when the stress at a small area is calculated as greater than the ultimate strength of the metal. Nor can it explain the failures that occur at relatively low nominal stress. All these problems can be better explained and analyzed by the concept of fatigue strength and fatigue life. This paper proposes some new methods for the prediction of SCF, approximate methods for the prediction of fatigue strength reduction factor due to stress concentration, and fatigue life of openings. Validation of the new method is also included.

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