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Takamoto Itoh - One of the best experts on this subject based on the ideXlab platform.
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influence of notch sensitivity and crack initiation site on low cycle fatigue life of notched components under multiaxial non Proportional Loading
Frattura ed Integrità Strutturale, 2018Co-Authors: Stefano Bressan, Takamoto Itoh, Fumio Ogawa, Filippo BertoAbstract:A series of strain-controlled multiaxial low cycle fatigue (LCF) tests under Proportional and non-Proportional Loading conditions have been conducted on notched specimens. Cylindrical bars of Al 6061 aluminum alloy and AISI 316L stainless steel with four values of stress concentration factors referred to the net section Kt,n were employed. The experimental results evidenced a reduction of fatigue life due to non-Proportional Loading. Furthermore, the crack initiation site has been detected to be moved from the notch tip in the case of steel for high values of notch radius under non-Proportional Loading. Stress concentration factor evaluated in the elastic field Kt,n has been included in the Itoh-Sakane parameter to evaluate the fatigue life, returning a general underestimation of fatigue life especially for high values of Kt,n. Material notch sensitivity and crack initiation position have been taken into account to further modify the model, improving the original results and showing a better assessment.
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evaluation and visualization of multiaxial fatigue behavior under random non Proportional Loading condition
Fracture and Structural Integrity, 2017Co-Authors: Takahiro Morishita, Fumio Ogawa, Takamoto ItohAbstract:In cyclic multiaxial stress/strain condition under nonProportional Loading in which principal direction of stress/strain are changed in a cycle, it becomes difficult to analyze stress/strain ranges because of complexity of multiaxial stress/strain states depending on time in cycles. In order to evaluate stress/strain simply and suitably under non-Proportional Loading, Itoh and Sakane have proposed a method called as IS-method and a strain parameter for life evaluation under non-Proportional Loading ??NP. In the method, 6-components of stress/strain are converted to an equivalent stress/strain indicating the amplitude and the direction of principal stress/strain as a function of time as well as an intensity of Loading nonProportionality fNP. Based on IS-method, the authors also have developed a tool which enables to analyze multiaxial stress/strain condition with the nonProportionality of Loading history and evaluate failure life under nonProportional multiaxial Loading. The tool indicates the analyzed results on monitor and users can understand visually not only variation of the stress/strain conditions but also non-Proportionality during the cycle, which helps the design of material strength.
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analysis of multiaxial low cycle fatigue of notched specimens for type 316l stainless steel under non Proportional Loading
Theoretical and Applied Fracture Mechanics, 2017Co-Authors: Pasquale Gallo, Takahiro Morishita, Takamoto Itoh, Stefano Bressan, Filippo BertoAbstract:Abstract The paper analyzes multiaxial low cycle fatigue tests of notched specimens under Proportional and non-Proportional Loading conditions. Strain controlled multiaxial low cycle fatigue tests were carried out using circumferentially notched round-bar specimens of type 316L stainless steel, with different stress concentration factors. The experimental results show that the crack initiation site is shifted from the notch tip. Based on this finding, a new model for life evaluation is proposed by taking into account the strain gradient in the proximity of the notch tip and the effective maximum strain range. The new model allows evaluating the fatigue life in a narrow scatter band.
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evaluation of multiaxial low cycle fatigue life for type 316l stainless steel notched specimen under non Proportional Loading
Theoretical and Applied Fracture Mechanics, 2016Co-Authors: Takahiro Morishita, Takamoto ItohAbstract:Abstract This study discusses a multiaxial low cycle fatigue (LCF) life of notched specimen under Proportional and non-Proportional Loading conditions at room temperature. Multiaxial LCF tests controlled by strain were carried out using a smooth and a circumferentially notched round-bar specimens of type 316L stainless steel (316LSS). Four kinds of notched specimens of which elastic stress concentration factors K t are 1.5, 2.5, 4.2 and 6.0 were employed. The strain paths employed were Proportional Loading and non-Proportional Loading. The former is push–pull Loading and reversed torsion Loading and the latter is circular Loading achieved by the strain path of which axial and shear strains are loaded by 90° sinusoidal out-of-phase. Total axial strain and total shear strain ranges were the same ranges based on von Mises. An evaluation of the fatigue life for the notched specimen is discussed based on the experimental results and also by employing an inelastic finite element analysis (FEA). The fatigue life is decreased with increase in K t at each strain path and is underestimated by a maximum local strain range evaluated by FEA result. A proposed strain range defined by a local mean strain value near the notch calculated from FEA by taking into account the non-Proportional effect is an appropriate parameter for life evaluation of 316LSS notched specimen under non-Proportional Loading.
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Creep-fatigue life evaluation of high chromium ferritic steel under non-Proportional Loading
Gruppo Italiano Frattura, 2016Co-Authors: Takahiro Morishita, Takamoto Itoh, Y. Murakami, H. TanigawaAbstract:T Multiaxial creep-fatigue tests under non-Proportional Loading conditions with various strain rates were carried out using a hollow cylinder specimen of a high chromium ferritic steel at 823K in air to discuss the influence of non-Proportional Loading on failure life. Strain paths employed were a push-pull Loading and a circle Loading. The push-pull Loading test is Proportional strain path test. The circle Loading test is non-Proportional strain path test in which sinusoidal waveforms of axial and shear strains have 90 degree phase difference. The failure life is affected largely by the strain rate and the non-Proportional Loading. This paper presents a modified strain range for life evaluation considering the strain rate based on a non-Proportional strain parameter proposed by authors. The strain range is a suitable parameter for life evaluation of tested material under non-Proportional Loading at high temperatur
Jeong Whan Yoon - One of the best experts on this subject based on the ideXlab platform.
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a user friendly anisotropic ductile fracture criterion for sheet metal under Proportional Loading
International Journal of Solids and Structures, 2021Co-Authors: Yanshan Lou, Jeong Whan YoonAbstract:Abstract Anisotropic ductile fracture is of special importance for accurate modeling of failure in plastic forming of lightweight sheet metals. This research introduces a user-friendly approach to model Loading direction effect on fracture limits of sheet metals. The approach is combined with a newly developed fracture criterion (DF2016) to illustrate anisotropic fracture in shear, uniaxial tension and plane strain tension as well as fracture under balanced biaxial tension. The anisotropic DF2016 criterion is applied to describe Loading direction effect on the fracture limits of two sheet metals: 6k21 aluminum alloy and a steel sheet of DP980. The predicted Loading direction effect is observed to agree with the experimental results with high accuracy. The applications demonstrate that there are three advantages of the proposed approach: high accuracy, good flexibility and being very user-friendly in the calibration of anisotropic parameters. Therefore, we recommend the proposed anisotropic DF2016 criterion for numerical failure prediction in stamping of strong anisotropic metals.
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combined anisotropic and distortion hardening to describe directional response with bauschinger effect
International Journal of Plasticity, 2019Co-Authors: Eunho Lee, Thomas B Stoughton, Jeong Whan Yoon, Hyunsung ChoiAbstract:Abstract Directional anisotropic hardening under non-Proportional Loading is modeled. A recently proposed coupled quadratic-nonquadratic (CQN) yield function (Lee et al., 2017b) successfully captured the directional hardening behavior under the Proportional Loadings. This paper shows a constitutive equation to capture both directional hardening response and Bauschinger effect through an extended model of the CQN yield function with the homogeneous anisotropic hardening (HAH) model (Barlat et al., 2011). In the present study, the role of the CQN yield model is to capture the directional hardening behavior, and the HAH model is to follow non-Proportional Loading including the Bauschinger effect. The validation, with some material data, shows that the present model can follow the directional hardening under non-Proportional Loading in the stress-strain data. It is also shown that the present model can describe the yield surface distortion with Bauschinger and anisotropic hardening response. Finally, it is compared with other pre-existing models in order to analyze the utility of the proposed model.
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anisotropic hardening and non associated flow in Proportional Loading of sheet metals
International Journal of Plasticity, 2009Co-Authors: Thomas B Stoughton, Jeong Whan YoonAbstract:Abstract Conventional isotropic hardening models constrain the shape of the yield function to remain fixed throughout plastic deformation. However, experiments show that hardening is only approximately isotropic under conditions of Proportional Loading, giving rise to systematic errors in calculation of stresses based on models that impose the constraint. Five different material data for aluminum and stainless steel alloys are used to calibrate and evaluate five material models, ranging in complexity from a von Mises’ model based on isotropic hardening to a non- associated flow rule (AFR) model based on anisotropic hardening. A new model is described in which four stress–strain functions are explicitly integrated into the yield criterion in closed form definition of the yield condition. The model is based on a non-AFR so that this integration does not affect the accuracy of the plastic strain components defined by the gradient of a separate plastic potential function. The model not only enables the elimination of systematic errors for Loading along the four Loading conditions, but also leads to a significant reduction of systematic errors in other Loading conditions to no higher than 1.5% of the magnitude of the predicted stresses, far less that errors obtained under isotropic hardening, and at a level comparable to experimental uncertainty in the stress measurement. The model is expected to lead to a significant improvement in stress prediction under conditions dominated by Proportional Loading, and this is expected to directly improve the accuracy of springback, tearing, and earing predictions for these processes. In addition, it is shown that there is no consequence on MK necking localization due to the saturation of the yield surface in pure shear that occurs with the aluminum alloys using the present model.
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sheet metal formability analysis for anisotropic materials under non Proportional Loading
International Journal of Mechanical Sciences, 2005Co-Authors: Thomas B Stoughton, Jeong Whan YoonAbstract:Abstract Sheet metal formability is conventionally assessed in a two-dimensional plot of principal strains or stresses in comparison to a forming limit curve. This method of assessment implicitly assumes that the forming limit is isotropic in the plane of the sheet. While the assumption of isotropy in the forming limit is perhaps a good engineering approximation, it is intrinsically inconsistent with the use of material models that are anisotropic. Since the trend today is to utilize models with full anisotropy in order to more accurately capture the physics of material behavior, the issue of anisotropy of forming limits must also be addressed. The challenge is that the forming limit is no longer defined by a curve but requires the definition of a surface in strain or stress space, and therefore it is no longer appropriate to view these limits with the convenience of two-dimensional diagrams. Furthermore, recent developments in the characterization of sheet forming limits under non-Proportional Loading suggest that is advantageous to view forming limit behavior in terms of stresses rather than strains, a view that is adopted in this paper. A solution to the challenge of assessing formability for an anisotropic material is proposed that rescales the stresses by a factor so that the scaled stresses have the same relationship to a single forming limit curve in a 2D plot in stress-space, as the actual stresses have to the true anisotropic forming limit in 3D space. The rescaling enables engineers to accurately view the formability of all the elements at the same time for a given finite element analysis of an application. This paper also discusses other challenges of using stresses in the assessment of formability, focusing on an analysis of the 2-Stage Forming Benchmark highlighted in the Numisheet ’99 Conference. Stresses are found in this application to unload to non-critical values after reaching critical levels earlier in a forming process, which suggests that a full integration of the stress-based forming limit criterion with FE simulation is required to detect critical states that may temporarily occur during the forming process.
Thomas B Stoughton - One of the best experts on this subject based on the ideXlab platform.
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combined anisotropic and distortion hardening to describe directional response with bauschinger effect
International Journal of Plasticity, 2019Co-Authors: Eunho Lee, Thomas B Stoughton, Jeong Whan Yoon, Hyunsung ChoiAbstract:Abstract Directional anisotropic hardening under non-Proportional Loading is modeled. A recently proposed coupled quadratic-nonquadratic (CQN) yield function (Lee et al., 2017b) successfully captured the directional hardening behavior under the Proportional Loadings. This paper shows a constitutive equation to capture both directional hardening response and Bauschinger effect through an extended model of the CQN yield function with the homogeneous anisotropic hardening (HAH) model (Barlat et al., 2011). In the present study, the role of the CQN yield model is to capture the directional hardening behavior, and the HAH model is to follow non-Proportional Loading including the Bauschinger effect. The validation, with some material data, shows that the present model can follow the directional hardening under non-Proportional Loading in the stress-strain data. It is also shown that the present model can describe the yield surface distortion with Bauschinger and anisotropic hardening response. Finally, it is compared with other pre-existing models in order to analyze the utility of the proposed model.
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evolution of subsequent yield surfaces and elastic constants with finite plastic deformation part ii a very high work hardening aluminum alloy annealed 1100 al
International Journal of Plasticity, 2010Co-Authors: Akhtar S Khan, Amit V Pandey, Thomas B StoughtonAbstract:Abstract Results are presented on the evolution of subsequent yield surfaces with finite deformation in a very high work hardening annealed 1100 aluminum alloy. In Part I [Khan, A.S., Kazmi, R., Stoughton, T., Pandey, A., 2009a. Evolution of subsequent yield surfaces and elastic constants with finite plastic deformation. Part 1: a very low work hardening aluminum alloy (Al-6061–T6511) 25, 1611–1625.] of this paper, similar results are presented for a very low work hardening aluminum alloy. Those results were very different from the present ones, and all the results were for Proportional Loading paths. The subsequent yield surfaces are determined in tension, free end torsion and combined tension–torsion Proportional and non-Proportional Loading paths, using 10 μe deviation from linearity definition of yield. Yield surfaces are also determined after linear, bi-linear, and non-linear unLoading paths after finite deformation under tension, free end torsion, and combined tension–torsion Loading. The initial yield surface is closer to the von-Mises surface and the subsequent yield surfaces show distortion, expansion, positive cross-effect, and “nose” in the Loading direction. Additionally, the subsequent yield surfaces after non-Proportional Loading paths show shrinkage and compounded distortion. The yield surfaces after unLoading depict strong anisotropy, positive cross-effect and exhibits different proportion of distortion in each Loading conditions. The Young’s and shear modulus decrease with plastic deformation and this decrease is much less than those reported in the published literature.
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anisotropic hardening and non associated flow in Proportional Loading of sheet metals
International Journal of Plasticity, 2009Co-Authors: Thomas B Stoughton, Jeong Whan YoonAbstract:Abstract Conventional isotropic hardening models constrain the shape of the yield function to remain fixed throughout plastic deformation. However, experiments show that hardening is only approximately isotropic under conditions of Proportional Loading, giving rise to systematic errors in calculation of stresses based on models that impose the constraint. Five different material data for aluminum and stainless steel alloys are used to calibrate and evaluate five material models, ranging in complexity from a von Mises’ model based on isotropic hardening to a non- associated flow rule (AFR) model based on anisotropic hardening. A new model is described in which four stress–strain functions are explicitly integrated into the yield criterion in closed form definition of the yield condition. The model is based on a non-AFR so that this integration does not affect the accuracy of the plastic strain components defined by the gradient of a separate plastic potential function. The model not only enables the elimination of systematic errors for Loading along the four Loading conditions, but also leads to a significant reduction of systematic errors in other Loading conditions to no higher than 1.5% of the magnitude of the predicted stresses, far less that errors obtained under isotropic hardening, and at a level comparable to experimental uncertainty in the stress measurement. The model is expected to lead to a significant improvement in stress prediction under conditions dominated by Proportional Loading, and this is expected to directly improve the accuracy of springback, tearing, and earing predictions for these processes. In addition, it is shown that there is no consequence on MK necking localization due to the saturation of the yield surface in pure shear that occurs with the aluminum alloys using the present model.
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sheet metal formability analysis for anisotropic materials under non Proportional Loading
International Journal of Mechanical Sciences, 2005Co-Authors: Thomas B Stoughton, Jeong Whan YoonAbstract:Abstract Sheet metal formability is conventionally assessed in a two-dimensional plot of principal strains or stresses in comparison to a forming limit curve. This method of assessment implicitly assumes that the forming limit is isotropic in the plane of the sheet. While the assumption of isotropy in the forming limit is perhaps a good engineering approximation, it is intrinsically inconsistent with the use of material models that are anisotropic. Since the trend today is to utilize models with full anisotropy in order to more accurately capture the physics of material behavior, the issue of anisotropy of forming limits must also be addressed. The challenge is that the forming limit is no longer defined by a curve but requires the definition of a surface in strain or stress space, and therefore it is no longer appropriate to view these limits with the convenience of two-dimensional diagrams. Furthermore, recent developments in the characterization of sheet forming limits under non-Proportional Loading suggest that is advantageous to view forming limit behavior in terms of stresses rather than strains, a view that is adopted in this paper. A solution to the challenge of assessing formability for an anisotropic material is proposed that rescales the stresses by a factor so that the scaled stresses have the same relationship to a single forming limit curve in a 2D plot in stress-space, as the actual stresses have to the true anisotropic forming limit in 3D space. The rescaling enables engineers to accurately view the formability of all the elements at the same time for a given finite element analysis of an application. This paper also discusses other challenges of using stresses in the assessment of formability, focusing on an analysis of the 2-Stage Forming Benchmark highlighted in the Numisheet ’99 Conference. Stresses are found in this application to unload to non-critical values after reaching critical levels earlier in a forming process, which suggests that a full integration of the stress-based forming limit criterion with FE simulation is required to detect critical states that may temporarily occur during the forming process.
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a general forming limit criterion for sheet metal forming
International Journal of Mechanical Sciences, 2000Co-Authors: Thomas B StoughtonAbstract:The forming limit of sheet metal is defined to be the state at which a localized thinning of the sheet initiates during forming, ultimately leading to a split in the sheet. The forming limit is conventionally described as a curve in a plot of major strain vs. minor strain. This curve was originally proposed to characterize the general forming limit of sheet metal, but it has been subsequently observed that this criterion is valid only for the case of Proportional Loading. Nevertheless, due to the convenience of measuring strain and the lack of a better criterion, the strain- based forming limit curve continues to play a primary role in judging forming severity. In this paper it is shown that the forming limit for both Proportional Loading and non-Proportional Loading can be explained from a single criterion which is based on the state of stress rather than the state of strain. This proposed criteria is validated using data from several non-Proportional Loading paths previously reported in the literature for both aluminum and steel alloys. In addition to significantly improving the gauging of forming severity, the new stress-based criterion is as easy to use as the strain-based criterion in the validation of die designs by the finite element method. However, it presents a challenge to the experimentalist and the stamping plant because the state of stress cannot be directly measured. This paper will also discuss several methods to deal with this challenge so that the more general measure of forming severity, as determined by the state of stress, can be determined in the stamping plant.
Masahiro Takanashi - One of the best experts on this subject based on the ideXlab platform.
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low cycle fatigue life of ti 6al 4v alloy under non Proportional Loading
International Journal of Fatigue, 2012Co-Authors: Takamoto Itoh, Yuuta Shimizu, Hiroshi Nakamura, Masahiro TakanashiAbstract:Abstract Multiaxial low cycle fatigue life of Ti–6Al–4V under non-Proportional Loading was studied. Strain-controlled multiaxial fatigue tests at room temperature were carried out using tubular specimens. The strain paths employed were push–pull Loading, reversed torsion Loading, and two kinds of 90° out-of-phase Loadings. The former two Loadings are Proportional Loading tests where the principal directions of stress and strain are fixed in the cycle. The latter two are non-Proportional Loading tests where there is a 90° phase difference between axial and shear Loadings, and the principal directions are cyclically rotated continuously. Failure lives are reduced obviously by non-Proportional Loadings in comparison with those in Proportional Loading tests. This paper focuses on determining a suitable fatigue model for evaluating the failure lives of Ti–6Al–4V under multiaxial Loading.
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fatigue crack initiation and growth behavior of ti 6al 4v under non Proportional multiaxial Loading
International Journal of Fatigue, 2011Co-Authors: Hiroshi Nakamura, Takamoto Itoh, Masahiro Takanashi, Yuuta ShimizuAbstract:Abstract This paper deals with multiaxial low cycle fatigue crack behavior of Ti–6Al–4V under non-Proportional Loading. Strain controlled fatigue tests under Proportional Loading and non-Proportional Loading with 90° out-of-phase difference between the axial and shear strains e and γ were carried out on tubular specimens at room temperature. As a result, Mises strain based fatigue lives under non-Proportional Loading were approximately 1/10 of those under Proportional Loading. The specimen surfaces were observed to evaluate life reduction. These results showed that non-Proportional Loading caused 10 times more cracks than those of Proportional Loading. Non-Proportional Loading changes directions of the principal stress and strain axes, and high shear stresses acting on many planes stimulate more slip planes. Consequently, non-Proportional Loading resulted in many cracks. In addition, the high shear stresses also resulted in larger misorientation under non-Proportional Loading. The crack initiation life defined as the length 2 a of 30 μm did not differ significantly between Proportional Loading and non-Proportional Loading. However, the fatigue cracks under non-Proportional Loading propagated faster those under Proportional Loading. This is because the fatigue cracks under non-Proportional Loading are subjected to more severe strain field than those under Proportional Loading, even though the crack size and applied strain are the same. Thus, the significant reduction in fatigue life under non-Proportional Loading is caused by the accelerated crack growth due to a higher strain intensity factor.
Yuuta Shimizu - One of the best experts on this subject based on the ideXlab platform.
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low cycle fatigue life of ti 6al 4v alloy under non Proportional Loading
International Journal of Fatigue, 2012Co-Authors: Takamoto Itoh, Yuuta Shimizu, Hiroshi Nakamura, Masahiro TakanashiAbstract:Abstract Multiaxial low cycle fatigue life of Ti–6Al–4V under non-Proportional Loading was studied. Strain-controlled multiaxial fatigue tests at room temperature were carried out using tubular specimens. The strain paths employed were push–pull Loading, reversed torsion Loading, and two kinds of 90° out-of-phase Loadings. The former two Loadings are Proportional Loading tests where the principal directions of stress and strain are fixed in the cycle. The latter two are non-Proportional Loading tests where there is a 90° phase difference between axial and shear Loadings, and the principal directions are cyclically rotated continuously. Failure lives are reduced obviously by non-Proportional Loadings in comparison with those in Proportional Loading tests. This paper focuses on determining a suitable fatigue model for evaluating the failure lives of Ti–6Al–4V under multiaxial Loading.
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fatigue crack initiation and growth behavior of ti 6al 4v under non Proportional multiaxial Loading
International Journal of Fatigue, 2011Co-Authors: Hiroshi Nakamura, Takamoto Itoh, Masahiro Takanashi, Yuuta ShimizuAbstract:Abstract This paper deals with multiaxial low cycle fatigue crack behavior of Ti–6Al–4V under non-Proportional Loading. Strain controlled fatigue tests under Proportional Loading and non-Proportional Loading with 90° out-of-phase difference between the axial and shear strains e and γ were carried out on tubular specimens at room temperature. As a result, Mises strain based fatigue lives under non-Proportional Loading were approximately 1/10 of those under Proportional Loading. The specimen surfaces were observed to evaluate life reduction. These results showed that non-Proportional Loading caused 10 times more cracks than those of Proportional Loading. Non-Proportional Loading changes directions of the principal stress and strain axes, and high shear stresses acting on many planes stimulate more slip planes. Consequently, non-Proportional Loading resulted in many cracks. In addition, the high shear stresses also resulted in larger misorientation under non-Proportional Loading. The crack initiation life defined as the length 2 a of 30 μm did not differ significantly between Proportional Loading and non-Proportional Loading. However, the fatigue cracks under non-Proportional Loading propagated faster those under Proportional Loading. This is because the fatigue cracks under non-Proportional Loading are subjected to more severe strain field than those under Proportional Loading, even though the crack size and applied strain are the same. Thus, the significant reduction in fatigue life under non-Proportional Loading is caused by the accelerated crack growth due to a higher strain intensity factor.
