Influence of the Selected Fatigue Characteristics of the Material on Calculated Fatigue Life under Variable Amplitude Loading

2011 ◽  
Vol 104 ◽  
pp. 197-205 ◽  
Author(s):  
Adam Niesłony ◽  
Andrzej Kurek
Keyword(s):  
Fatigue Life ◽  
Energy Parameter ◽  
Correct Selection ◽  
Constant Amplitude ◽  
Random Loading ◽  
Data Set ◽  
Variable Amplitude ◽  
Comparison Results ◽  
Machine Elements ◽  
Fatigue Characteristics

The algorithm of fatigue life determination for machine elements subjected to random loading uses fatigue characteristics of the material determined under constant-amplitude loading. They are usually stress or strain characteristics as well as characteristics using the energy parameter. Their correct selection influences correctness of the obtained results related to the experimental data. The paper presents analysis of convergence of the calculated fatigue lives of some constructional materials subjected to random loading under uniaxial loading state. For calculations concerning one material the same loading state was assumed and fatigue characteristics were determined on the basis of one data set obtained under constant strain amplitude tests. Calculated fatigue lives based on different fatigue characteristics were compared and their convergences were tested. It has been proved that convergences are different depending on the material. The comparison results were presented in form of graphs.

The Effect of Crack Growth Retardation when Comparing Constant Amplitude to Variable Amplitude Loading in an Aluminium Alloy

2014 ◽  
Vol 891-892 ◽  
pp. 948-954 ◽  
Author(s):  
Madeleine Burchill ◽  
Simon A. Barter ◽  
Michael Jones
Keyword(s):  
Growth Rate ◽  
Fatigue Life ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Growth Retardation ◽  
Crack Front ◽  
Constant Amplitude ◽  
Variable Amplitude ◽  
Growth Increments ◽  
Crack Growth Retardation

It has often been observed that the growth of short fatigue cracks under variable amplitude (VA) cyclic loading is not well predicted when utilising standard constant amplitude (CA) crack growth rate/stress intensity data (da/dN v DK). This paper outlines a coupon fatigue test program and analyses, investigating a possible cause of crack growth retardation from CA-only testing. Various test loading spectra were developed with sub-blocks of VA and CA cycles, then using quantitative fractography (QF) the sub-block crack growth increments were measured. Comparison of these results found that, after establishing a consistent uniform crack front using a VA load sequence, the average crack growth rate then progressively slowed down with the number of subsequent CA load cycles applied. Further fractographic investigation of the fracture surface at the end of each CA and VA sub-block crack growth, identified significant crack front morphology differences. Thus it is postulated that a variation or deviation from an efficient crack path is a driver of local retardation in short crack growth during CA loading. This may be a source of error in analytical predictions of crack growth under VA spectra loading that may need to be considered in addition to other potential effects such asless closure whilst cracks are small. For aircraft designers, using solely CA data for fatigue life predictions this may result in non-conservative estimates of total crack fatigue life, producing unexpected failures or an increased maintenance burden.

Applicability of Equivalent Constant Amplitude Loading for Assessing the Fatigue Life of Pipelines and Risers and the Influence of Compressive Stress Cycles

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
Mohammad Iranpour ◽  
Farid Taheri
Keyword(s):  
Experimental Investigation ◽  
Fatigue Life ◽  
Compressive Stress ◽  
Time History ◽  
Life Assessment ◽  
Constant Amplitude ◽  
Safety Factors ◽  
Variable Amplitude ◽  
Fatigue Life Assessment ◽  
Stress Cycles

Fatigue life assessment of pipelines and risers is a complex process, involving various uncertainties. The selection of an appropriate fatigue model is important for establishing the inspection intervals and maintenance criteria. In offshore structures, the vortex-induced vibration (VIV) could cause severe fatigue damage in risers and pipelines, resulting in leakage or even catastrophic failure. The industry has customarily used simple fatigue models for fatigue life assessment of pipelines and risers (such as the Paris or Walker models); however, these models were developed based on constant amplitude loading scenarios. In contrast, VIV-induced stress-time history has a variable amplitude nature. The use of the simplified approach (which is inherently non conservative), has necessitated the implementation of large safety factors for fatigue design of pipelines and risers. Moreover, most of the experimental investigations conducted to date with the aim of characterizing the fatigue response of pipelines and risers have been done based on incorporation of constant amplitude loading (CAL) scenarios (which is unrealistic), or converting the variable amplitude loading (VAL) scenarios to an equivalent CAL. This study demonstrates that the use of such approaches would not be lead to accurate assessment of the fatigue response of risers subject to VIV-induced VAL. The experimental investigation performed in this study will also clarify the underlying reasons for the use of large safety factors by the industry when assessing the fatigue life of pipelines and risers. In addition, an experimental investigation was also conducted to highlight the influence of the compressive portion of VIV stress-time history on the fatigue life of such components. It is shown that the compressive stress cycles significantly influence the fatigue crack growth response of risers, and their presence should not be ignored.

Fatigue Life Calculation in Conditions of Wide Spectrum Random Loadings – The Experimental Verification of a Calculation Algorithm on the Example of 41Cr4 Steel

2012 ◽  
Vol 726 ◽  
pp. 17-26
Author(s):  
Bogdan Ligaj ◽  
Grzegorz Szala
Keyword(s):  
Fatigue Life ◽  
Wide Spectrum ◽  
Loading Conditions ◽  
Test Results ◽  
Structural Elements ◽  
Random Loading ◽  
Calculation Algorithm ◽  
Fatigue Characteristic ◽  
Life Tests ◽  
Fatigue Characteristics

Precision of fatigue life calculations of structural elements in programmed loading conditions is connected with proper elaboration of loading spectrum and assumption of a proper fatigue characteristic. On the base of literature data and own research there has been elaborated an algorithm for fatigue life calculations in random loading conditions with wide spectrum. Calculations were performed with the usage of chosen mathematical models of two-parametric fatigue characteristics. Results calculated with accordance to the described procedure were validated with experimental test results of specimens made of 41Cr4 steel with a method of programmed fatigue life tests.

Microstructural Behaviour Study and FEA-Based Fatigue Simulation on Parabolic Leaf Spring

2011 ◽  
Vol 462-463 ◽  
pp. 419-424
Author(s):  
Fayyadh Nakhaie Ahmad Refngah ◽  
Shahrum Abdullah ◽  
Azman Jalar ◽  
L.B. Chua
Keyword(s):  
Fatigue Life ◽  
Fatigue Test ◽  
Leaf Spring ◽  
Constant Amplitude ◽  
Variable Amplitude ◽  
Fea Model ◽  
Fracture Area ◽  
Tension And Compression ◽  
Behaviour Study ◽  
Parabolic Leaf Spring

It is compulsory to have a good fatigue life to a component that is heavily subjected to cyclic loading. One of the good examples is parabolic spring, which is one of the components in suspension system for large vehicles. It serves to absorb, store and release back the damping energy due to road irregularity, bump and holes. These activities involve a lot of camber deflection that caused by the tension and compression loads. In reality, the loading that subjected to parabolic spring is variable amplitude loading, but most of the manufacturer used constant amplitude (CA) loading for the fatigue test. The objective of this paper is to relate the simulation result with the microstructure behaviour of the leaf spring that failed due to fatigue. A full scale fatigue test was carried out until that parabolic spring meet failure. In order to investigate the fatigue life, CA signal was generated based on an actual fatigue test on the parabolic spring, and it was then analysed using the FEA-based fatigue simulation. A microstructure study was then performed for both fracture and non-fracture area. From the FEA-based simulation, it gave the prediction on damage that occurred at the critical area and also the prediction on the lowest cycle with respect to the FEA model. In the actual fatigue test, the failure was occurred at the centre part of the spring, which is at bolt join of assembly hole. The microstructure analysis showed that the grain at the fracture area indicated some different from the non-fracture area in term of size, phase and precipitation of carbon.

Corrosion Fatigue of AZ91E-T6 Cast Magnesium Alloy in a 3.5 Percent NaCl Aqueous Environment

1995 ◽  
Vol 117 (3) ◽  
pp. 293-298 ◽  
Author(s):  
R. I. Stephens ◽  
C. D. Schrader ◽  
K. B. Lease
Keyword(s):  
Fatigue Life ◽  
Magnesium Alloy ◽  
Fatigue Behavior ◽  
Fatigue Tests ◽  
Mean Stress ◽  
Constant Amplitude ◽  
Variable Amplitude ◽  
Cast Magnesium Alloy ◽  
Cast Magnesium ◽  
Variable Amplitude Fatigue

The objective of this research was to obtain and compare constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5 percent NaCl aqueous corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environment. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0, −1 and −2, Region II constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. Mean strains influenced fatigue life only if accompanied by significant mean stress. The Morrow and Smith, Watson, and Topper mean stress models provided both accurate and inaccurate fatigue life calculations. Likewise, variable amplitude fatigue life calculations using the local strain approach and based upon the formation ofal mm crack at the keyhole notch were both accurate and fairly inaccurate depending on the specific model used.

scholarly journals Experimental Study on Variable-Amplitude Fatigue of Welded Cross Plate-Hollow Sphere Joints in Grid Structures

2018 ◽  
Vol 2018 ◽  
pp. 1-15 ◽  
Author(s):  
Jin-Feng Jiao ◽  
Hong-Gang Lei ◽  
Y. Frank Chen
Keyword(s):  
Fatigue Life ◽  
Hollow Sphere ◽  
Fatigue Behavior ◽  
Fatigue Tests ◽  
Metallographic Analysis ◽  
Variable Load ◽  
Random Loading ◽  
Variable Amplitude ◽  
Different Types ◽  
Variable Amplitude Fatigue

The fatigue stress amplitude of the welded cross plate-hollow sphere joint (WCPHSJ) in a grid structure varies due to the random loading produced by suspending cranes. A total of 14 specimens considering three different types of WCPHSJs were prepared and tested using a specially designed test rig. Four typical loading conditions, “low-high,” “high-low,” “low-high-low,” and “high-low-high,” were first considered in the tests to investigate the fatigue behavior under variable load amplitudes, followed by metallographic analyses. The experimental and metallographic analysis results provide a fundamental understanding on the fatigue fracture form and fatigue mechanism of WCPHSJs. Based on the available data from constant-amplitude fatigue tests, the variable-amplitude fatigue life of the three types of WCPHSJs was estimated using the Miner rule and Corten-Dolan theory. Since both accumulative damage theories yield virtually same damaging results, the Miner rule is hence suggested to estimate the fatigue life of WCPHSJs.

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