Low Cycle Fatigue Damage of Mod.9Cr-1Mo Steel under Non-Proportional Multiaxial Loading

AbstractThis study discusses multiaxial low cycle fatigue damage of Mod.9Cr-1Mo steel under proportional and non-proportional loadings at room and high (823K) temperatures. Strain-controlled multiaxial low cycle fatigue tests were carried out using a hollow cylinder specimen including interruption tests. Strain paths employed were a push-pull straining, a reversed torsion straining and a circle straining. Behaviors of cyclic deformations and failure lives and an evaluation of the failure life are discussed. Surface cracks and microstructures in fatigued specimens are also observed by a digital microscope and a transmission electron microscope (TEM) in order to investigate mechanisms of cyclic deformation, failure and reduction in failure life due to non-proportional loading

Cracking directions in multiaxial low cycle fatigue at high and room temperatures

Cracking direction in multiaxial low cycle fatigue is an important research subject because crack initiation and propagation behavior is a physical background for developing an estimation method of multiaxial low cycle fatigue lives. However, there are a few open questions on cracking direction in multiaxial low cycle fatigue because cracking direction in multiaxial low cycle fatigue is complex and changes depending on stress multiaxiality, strain range, notch and material. This paper overviews cracking directions in tension-torsion low cycle fatigue of low alloy steels and nickel base superalloys. Two types of cracking directions in these materials, maximum shear direction and maximum principal direction, are discussed in relation with strain multiaxiality and an existence of notch and precrack. The two cracking directions in torsion low cycle fatigue of SUS 304 stainless steel are also discussed in relation with strain range. Detailed micro crack observations are finally presented to discuss the two cracking directions in torsion low cycle fatigue of a SUS 304 unnotched specimen

Microstructural study of multiaxial low cycle fatigue

This paper discusses the relationship between the stress response and the microstructure under tension-torsion multiaxial proportional and nonproportional loadings. Firstly, this paper discusses the material dependency of additional hardening of FCC materials in relation with the stacking fault energy of the materials. The FCC materials studied were Type 304 stainless steel, pure copper, pure nickel, pure aluminum and 6061 aluminum alloy. The material with lower stacking fault energy showed stronger additional hardening, which was discussed in relation with slip morphology and dislocation structures. This paper, next, discusses dislocation structures of Type 304 stainless steel under proportional and nonproportional loadings at high temperature. The relationship between the microstructure and the hardening behavior whether isotropic or anisotropic was discussed. The re-arrangeability of dislocation structure was discussed in loading mode change tests. Microstructures of the steel was discussed in more extensively programmed multiaxial low cycle fatigue tests at room temperature, where three microstructures, dislocation bundle, stacking fault and cells, which were discussed in relation with the stress response. Finally, temperature dependence of the microstructure was discussed under proportional and nonproportional loadings, by comparing the microstructures observed at room and high temperatures

Multiaxial fatigue property of type 316 stainless steel using hollow cylinder specimen under combined pull loading and inner pressure

Stress controlled multiaxial fatigue test was carried out using a hollow cylinder specimen of type 316 stainless steel. A newly developed fatigue testing machine which can apply push-pull loading and reversed torsion loading and inner pressure to the hollow cylinder specimen was employed. 5 types of cyclic loading paths were employed by combining zero to pull axial and hoop stresses: a Pull (only axial stress), an Inner-pressure (only hoop stress), an Equi-biaxial (equi-biaxial stress by axial and hoop stresses), a Square-shape (trapezoidal waveforms of axial and hoop stresses with 90-degree phase difference) and a L-shape (alternately axial stress and hoop stress) loading paths. Since directions of principal stresses are fixed in all the tests, all of the loading paths are classified into ‘proportional loading’. In the Pull, the Inner-pressure and the Equi-biaxial tests, fatigue lives can be correlated on a unique line by a maximum equivalent stress based on von Mises. On the other hand, fatigue lives in the Square-shape and the L-shape tests were reduced comparing with that in the other tests, which was caused by yielding of larger plastic deformation

Investigation of the multiaxial fatigue behaviour of 316 stainless steel based on critical plane method.

https://v2.sherpa.ac.uk/id/publication/6807?template=romeoIn this work the multiaxial behaviour of 316 stainless steel is studied under the lens of critical plane approach. A series of experiments were developed on dog bone shaped hollow cylindrical specimens made of type 316 stainless steel. Five different loading conditions were assessed with: (i) only tensile axial stress, (ii) only hoop stress, (iii) combination of axial and hoop stresses with square shape, (iv) combination of tensile axial and hoop stresses with Lshape and (v) combination of compressive axial and hoop stresses with L-shape. The fatigue analysis is performed with four different critical plane theories, namely Wang-Brown, FatemiSocie, Liu I and Liu II. The efficiency of all four theories is studied in terms of the accuracy of their life predictions and crack failure plane angle. The best fatigue life predictions were obtained with Liu II model and the best predictions of the failure plane were obtained with Liu I model

Evaluation and visualization of multiaxial fatigue behavior under random non-proportional loading condition

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

Influence of Notch Sensitivity and Crack Initiation Site on Low Cycle Fatigue Life of Notched Components under Multiaxial Non-proportional Loading

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

Influence of notch sensitivity and crack initiation site on low cycle fatigue life of notched components under multiaxial non-proportional loading

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 nonproportional 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

Optimización de empaques aplicando lineamientos de diseño para el ensamble validado en los contextos de empaque, transporte y desempaque

Experimental tests were conducted on additive manufactured Ti-6Al-4V titanium alloy to investigate the mechanical and crack properties under multiaxial cyclic loading. Selective Laser Sintering technique (SLS) was employed to fabricate four types of cylindrical hollow specimens. The typology of each specimen is defined by the orientation of the layers and by the application of a stress-relieving heat treatment after the production process. Stress-strain cyclic curves of the materials were obtained to investigate the material cyclic plastic behavior, that resulted independent of specimen variety. Strain-controlled multiaxial low cycle fatigue tests under proportional and non-proportional loading paths were carried out on the specimens. Not heat-treated specimens exhibited a higher low cycle fatigue resistance both for proportional and non-proportional loading. Drastic initial softening was detected in the majority of the tests. Additional hardening was detected in part of non-proportional tests, which is atypical for this alloy. The mutual influence of applied load and microstructural characteristics on fatigue life are finally discussed

Critical plane based method for multiaxial fatigue analysis of 316 stainless steel

In this work, the fatigue behaviour of 316 stainless steel is studied with different critical plane models. Seven cylindrical samples were used for the study, being subjected to different complex loading paths, generating combined stresses along the axial and transversal sample directions, these being: individual axial stress, individual hoop stress, alternating axial and hoop stress, a proportional combination of axial and hoop stress, and a non-proportional combination of L-shaped and square-shaped axial and hoop stress. The fatigue analysis is performed using five critical plane models; named Fatemi-Socie, Varvani-Farahani, Gan-Wu-Zhong, Liu I and Liu II. The models were assessed based on their fatigue life and crack angle prediction capacity. The Gan-Wu-Zhong recently proposed critical plane model was examined and provided acceptable results for the multiaxial loads tested on 316 steel. Nevertheless, Fatemi-Socie produced the most accurate results in terms of cracking orientation and Liu II gave the best fatigue life predictions.Financial support of Programa Operativo FEDER from the Junta de Andalucia (Spain) through grant reference UMA18-FEDERJA-250 is greatly acknowledged. Support from the Oceanic Engineering Research Institute from Malaga is also acknowledged. Industrial support from Bettergy and Dr Nicolas Ordo ̃nez is greatly acknowledged, as well as access to different structures and materials in the energy industry. We would also like to acknowledge funding for open access charge: Universidad de Malaga / CBUA