Uncertainties and Margins in Environmental Fatigue Analysis Rules

2014 ◽  
Author(s):  
C. Faidy
Keyword(s):  
Strain Amplitude ◽  
Material Properties ◽  
Cyclic Strain ◽  
Fatigue Curve ◽  
Fatigue Analysis ◽  
Fatigue Tests ◽  
Pressure Vessels ◽  
Life Assessment ◽  
Combination Rules ◽  
Reduction Factors

During the past 30 years the main rules for fatigue analysis of pressure vessels were based on elastic approaches in order to evaluate cyclic strain amplitude and compare with an S-N fatigue curve for the corresponding material. After review of some rules in different Nuclear and Non-Nuclear Codes, like ASME Boiler & Pressure Vessel Code Section III, French RCC-M and RCC-MRx, European Standards EN 13445, the major conservatisms and uncertainties of different rules are discussed. All these Codes propose simple rules to evaluate the strain amplitude based on elastic approaches and simplified correction factors (Ke and Kν), transient combination rules and damage cumulating procedure. In the other hand, the material properties are based on standard fatigue tests done on the material associated to reduction factors to consider some particular effects like scatter, scale, surface roughness, mean stress or environmental effects to transfer them from small specimen to real structures. Concerned components in this paper are mainly piping systems. No existing Code covers all the aspects of fatigue with similar conservatisms that can affect the in-service inspection programs and the remaining life assessment of the corresponding components. After the review of different rules, key factors that affect the results and predictions will be identified. Some proposals will be issued to progress in the near future. Finally, a first set of recommendation on fatigue analysis will be presented to improve existing codes on harmonized way, associated to material properties needed, as fatigue curves associated to reduction factors.

Fatigue Design Rules in Pressure Vessel Codes Uncertainties and Remaining Open Points

2014 ◽  
Author(s):  
Claude Faidy
Keyword(s):  
Strain Amplitude ◽  
Pressure Vessel ◽  
Material Properties ◽  
Cyclic Strain ◽  
Fatigue Curve ◽  
Fatigue Analysis ◽  
Pressure Vessels ◽  
Life Assessment ◽  
Combination Rules ◽  
Reduction Factors

During the past 30 years the main rules for fatigue analysis of pressure vessels were based on elastic approaches in order to evaluate a cyclic strain amplitude and compare with an S-N fatigue curve for the corresponding material. After review of some rules in different Nuclear and Non Nuclear Codes, like ASME Boiler & Pressure Vessel Code Section III, French RCC-M and RCC-MRx, European Standards EN 13445, the major conservatisms and uncertainties of different rules are discussed. All these Codes propose simple rules to evaluate the strain amplitude based on elastic approaches and simplified correction factors (Ke and Kv), transient combination rules and damage cumulating procedure. In the other hand, the material properties are based on standard fatigue tests done on the material associated to reduction factors to consider some particular effects like scatter, scale, surface roughness, mean stress or environmental effects to transfer them from small specimen to real structures. Concerned components are mainly piping systems. No existing Code covers all the aspects of fatigue with similar conservatisms that can affect the in-service inspection programs and the remaining life assessment of the corresponding components. After the review of different rules, key factors that affect the results and predictions will be identified. Some proposals will be issued to progress in the near future. Finally, a first set of recommendation on fatigue analysis will be presented to improve existing codes on an harmonized way, associated to material properties needed, as fatigue curves associated to reduction factors.

scholarly journals Flat-Cladding Fiber Bragg Grating Sensors for Large Strain Amplitude Fatigue Tests

2021 ◽  
Author(s):  
Aihen Feng ◽  
Daolun Chen ◽  
Cheng Li ◽  
Xijia Gu
Keyword(s):  
Fiber Bragg Grating ◽  
Strain Amplitude ◽  
Large Strain ◽  
Hysteresis Loops ◽  
Cyclic Strain ◽  
Fatigue Tests ◽  
Bragg Grating ◽  
Large Strain Amplitude ◽  
Bragg Grating Sensor ◽  
Cyclic Strain Amplitude

We have successfully developed a flat-cladding fiber Bragg grating sensor for large cyclic strain amplitude tests of up to ±8,000 με. The increased contact area between the flat-cladding fiber and substrate, together with the application of a new bonding process, has significantly increased the bonding strength. In the push-pull fatigue tests of an aluminum alloy, the plastic strain amplitudes measured by three optical fiber sensors differ only by 0.43% at a cyclic strain amplitude of ±7,000 με and 1.9% at a cyclic strain amplitude of ±8,000 με. We also applied the sensor on an extruded magnesium alloy for evaluating the peculiar asymmetric hysteresis loops. The results obtained were in good agreement with those measured from the extensometer, a further validation of the sensor.

Unifying Monotonic and Hysteresis Material Properties for Notch Plasticity Analysis under Variable Amplitude Loads

2014 ◽  
Vol 891-892 ◽  
pp. 500-505 ◽  
Author(s):  
Xiao Bo Yu ◽  
Qian Chu Liu ◽  
Chris Wallbrink
Keyword(s):  
Strain Amplitude ◽  
Material Properties ◽  
Present Finding ◽  
High Strain ◽  
Cyclic Strain ◽  
Recovery Process ◽  
Material Behaviour ◽  
Variable Amplitude ◽  
Dog Bone ◽  
Conceptual Consideration

This study was performed in support of accurate notch plasticity analysis under variable amplitude loads. Monotonic and cyclic strain-controlled tests were performed on flat dog-bone coupons machined from 6.35mm thick 7075-T651 aluminium alloy plates. The tests with low-high-low strain amplitude transitions revealed both an instant softening following the high strain amplitude cycle and a gradual recovery during the subsequent low strain amplitude cycles. The transition from monotonic-like to hysteresis-like material behaviour was found to be consistent with an overload induced softening and recovery process. A conceptual consideration is thus proposed to unify the characterisation of monotonic and hysteresis material properties. The implications of the present finding on notch plasticity analysis are also discussed.

scholarly journals Damaged Response In Natural Fibre Reinforced Composites: Characterisation and Modelling Under Quasi-Static and Fatigue Conditions

2021 ◽  
Author(s):  
Ziauddin Mahboob
Keyword(s):  
Strain Rate ◽  
Strain Amplitude ◽  
Mechanical Behaviour ◽  
Material Properties ◽  
Inelastic Strain ◽  
Natural Fibre ◽  
Fatigue Tests ◽  
Stiffness Degradation ◽  
Progressive Damage ◽  
Damage Mechanisms

‘Natural’ fibrous material are subjects of accelerated research on account of the non-renewability and environmental costs of traditional ‘synthetic’ engineering fibres like Carbon and Glass. Of all candidates, Flax plant fibres have been found to offer composite reinforcement similar, or even superior, to Glass fibres in specific mechanical properties. Despite repeated evidence of its potential from independent studies, industry adoption of natural fibre reinforcement for load-bearing applications is still negligible, owing to their relatively immature body of research that discourages confidence in their long-term strength, durability, and predictability. This work contributes original findings on the complex damaged-condition response of natural fibre composites (NFC), and proposes modelling approaches to simulate the same. Material properties and mechanical behaviour of several Flax-epoxy composites are determined under tensile and compressive static loading, and correlated to internal damage mechanisms observed by micrography. Stiffness degradation and accumulated permanent strain are quantified along principal in-plane orthrotropic directions, which are used to develop a Continuum Damage Mechanics-based mesoscale model wherein constitutive laws are specifically formulated to reproduce NFC quasi-static response, including their highly nonlinear fibre-direction stiffness loss and inelasticity progression. Current progress of fatigue research is critically and extensively reviewed. Reported fatigue endurance and progressive damage behaviour of several NFC laminates are analysed. Existing knowledge on NFC fatigue damage is found to be insufficient and ambiguous, therefore inadequate for engineering design consideration. The unique fatigue-stiffening phenomenon reported for Flax-epoxy specimens is argued to be a misleading consequence of increasing strain-rate under constant stress-amplitude cycling. To minimise the influence of a varying strain-rate, original constant strain-amplitude fatigue tests are conducted on Flax-epoxy laminates, where no evidence of stiffening is observed. Considering this sensitivity to strain-rate, strain-amplitude controlled fatigue tests may be better suited for NFC investigation. Strain-controlled fatigue lives of Flax-epoxy can be modelled by a linearised strain/log-life relationship. Evolution of several material properties and dissipation phenomena (inelastic strain, peak stress, stiffness, hysteresis energy, superficial temperature) are measured, and correlated with SEM-observed damage mechanisms in the microstructure. An evolution/growth model is proposed to simulate laminate-scale stiffness degradation and cumulative inelastic strain as a function of applied peak strain and fatigue cycles, and is found to well-capture experimental trends for Flax-epoxy. The combined contribution of this work provides much-needed original data on the damaged-condition mechanical behaviour of Flax-epoxy and other NFCs under a variety of loading conditions, clarifies contradictory aspects of critical NFC behaviour, and proposes numerical methods to replicate observed progressive damage and failure in NFCs.

Low-Cycled Fatigue Life of S30408 Stainless Steel at Liquid-Nitrogen Temperature

2018 ◽  
Author(s):  
Yingzhe Wu ◽  
Huaijian Xu ◽  
Qunjie Lu ◽  
Jinyang Zheng ◽  
Ping Xu
Keyword(s):  
Stainless Steel ◽  
Fatigue Life ◽  
Liquid Nitrogen ◽  
Strain Amplitude ◽  
Hysteresis Loops ◽  
Cyclic Strain ◽  
Liquid Nitrogen Temperature ◽  
Cyclic Stress ◽  
Cyclic Softening ◽  
Fatigue Tests

This paper is concerned with the low-cycled fatigue life of S30408 austenitic stainless steel at 77 K. Strain-controlled low-cycled fatigue tests were performed in a liquid-nitrogen bath covering a strain-amplitude range of 0.4%–1.0%. The role of the reduced temperature is examined during the low-cycled fatigue tests by comparing the fatigue performance to the one at ambient temperature that was obtained in our previous work. It is found that the cryogenic low-cycled fatigue life is significantly improved by a factor of 5–10 in the low strain-amplitude range of 0.4%–0.5%, resulting from the pronounced hardening effect due to the low temperature. However, the cryogenic improvement gradually reduces with the increasing strain-amplitude. At 77 K, the cyclic stress amplitude increases rapidly at the beginning of the fatigue test, and no cyclic softening is found due to the cryogenically constrained movement of the dislocations. The fatigue hysteresis loops and fatigue stress-strain curves shows that the cyclic plastic strain at cryogenic temperature accounts for a limited proportion in the total cyclic strain, and the damage may occurs explosively at the beginning of the cyclic load at 77 K.

Damaged Response In Natural Fibre Reinforced Composites: Characterisation and Modelling Under Quasi-Static and Fatigue Conditions

2021 ◽  
Author(s):  
Ziauddin Mahboob
Keyword(s):  
Strain Rate ◽  
Strain Amplitude ◽  
Mechanical Behaviour ◽  
Material Properties ◽  
Inelastic Strain ◽  
Natural Fibre ◽  
Fatigue Tests ◽  
Stiffness Degradation ◽  
Progressive Damage ◽  
Damage Mechanisms

‘Natural’ fibrous material are subjects of accelerated research on account of the non-renewability and environmental costs of traditional ‘synthetic’ engineering fibres like Carbon and Glass. Of all candidates, Flax plant fibres have been found to offer composite reinforcement similar, or even superior, to Glass fibres in specific mechanical properties. Despite repeated evidence of its potential from independent studies, industry adoption of natural fibre reinforcement for load-bearing applications is still negligible, owing to their relatively immature body of research that discourages confidence in their long-term strength, durability, and predictability. This work contributes original findings on the complex damaged-condition response of natural fibre composites (NFC), and proposes modelling approaches to simulate the same. Material properties and mechanical behaviour of several Flax-epoxy composites are determined under tensile and compressive static loading, and correlated to internal damage mechanisms observed by micrography. Stiffness degradation and accumulated permanent strain are quantified along principal in-plane orthrotropic directions, which are used to develop a Continuum Damage Mechanics-based mesoscale model wherein constitutive laws are specifically formulated to reproduce NFC quasi-static response, including their highly nonlinear fibre-direction stiffness loss and inelasticity progression. Current progress of fatigue research is critically and extensively reviewed. Reported fatigue endurance and progressive damage behaviour of several NFC laminates are analysed. Existing knowledge on NFC fatigue damage is found to be insufficient and ambiguous, therefore inadequate for engineering design consideration. The unique fatigue-stiffening phenomenon reported for Flax-epoxy specimens is argued to be a misleading consequence of increasing strain-rate under constant stress-amplitude cycling. To minimise the influence of a varying strain-rate, original constant strain-amplitude fatigue tests are conducted on Flax-epoxy laminates, where no evidence of stiffening is observed. Considering this sensitivity to strain-rate, strain-amplitude controlled fatigue tests may be better suited for NFC investigation. Strain-controlled fatigue lives of Flax-epoxy can be modelled by a linearised strain/log-life relationship. Evolution of several material properties and dissipation phenomena (inelastic strain, peak stress, stiffness, hysteresis energy, superficial temperature) are measured, and correlated with SEM-observed damage mechanisms in the microstructure. An evolution/growth model is proposed to simulate laminate-scale stiffness degradation and cumulative inelastic strain as a function of applied peak strain and fatigue cycles, and is found to well-capture experimental trends for Flax-epoxy. The combined contribution of this work provides much-needed original data on the damaged-condition mechanical behaviour of Flax-epoxy and other NFCs under a variety of loading conditions, clarifies contradictory aspects of critical NFC behaviour, and proposes numerical methods to replicate observed progressive damage and failure in NFCs.

scholarly journals Flat-Cladding Fiber Bragg Grating Sensors for Large Strain Amplitude Fatigue Tests

2021 ◽  
Author(s):  
Aihen Feng ◽  
Daolun Chen ◽  
Cheng Li ◽  
Xijia Gu
Keyword(s):  
Fiber Bragg Grating ◽  
Strain Amplitude ◽  
Large Strain ◽  
Hysteresis Loops ◽  
Cyclic Strain ◽  
Fatigue Tests ◽  
Bragg Grating ◽  
Large Strain Amplitude ◽  
Bragg Grating Sensor ◽  
Cyclic Strain Amplitude

We have successfully developed a flat-cladding fiber Bragg grating sensor for large cyclic strain amplitude tests of up to ±8,000 με. The increased contact area between the flat-cladding fiber and substrate, together with the application of a new bonding process, has significantly increased the bonding strength. In the push-pull fatigue tests of an aluminum alloy, the plastic strain amplitudes measured by three optical fiber sensors differ only by 0.43% at a cyclic strain amplitude of ±7,000 με and 1.9% at a cyclic strain amplitude of ±8,000 με. We also applied the sensor on an extruded magnesium alloy for evaluating the peculiar asymmetric hysteresis loops. The results obtained were in good agreement with those measured from the extensometer, a further validation of the sensor.

Low-Cycle Fatigue of Large-Diameter Bolts

1967 ◽  
Vol 89 (1) ◽  
pp. 53-60 ◽  
Author(s):  
A. L. Snow ◽  
B. F. Langer
Keyword(s):  
Fatigue Strength ◽  
Yield Strength ◽  
Low Cycle Fatigue ◽  
Large Diameter ◽  
Fatigue Curve ◽  
Fatigue Tests ◽  
Strength Reduction ◽  
Additional Information ◽  
Thread Root ◽  
Reduction Factors

New constant-deflection amplitude fatigue data on bolting materials are presented. The relationship of cyclic yield strength to static yield strength is examined. Several series of full-size stud fatigue tests are analyzed and compared to the basic failure curve to obtain experimental fatigue strength reduction factors for bolts and studs. The variation of fatigue strength reduction factors with thread root radius, stud size, thread taper, and other variables is noted. A new design fatigue curve for bolts is proposed which reflects the additional information available at the present time.

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