Low Cycle Fatigue Loading Research Articles

STRUCTURAL FEATURES OF DAMAGES ACCUMULATION IN CONDITIONS OF COMBINED CYCLIC LOADING

A significant part of the elements of machines and structures along with stationary fatigue is subjected to combined impacts of low-cycle and multi-cycle fatigue loading in operation. The physical nature of their fracture in these conditions depends on the ratio of the mode parameters and entails the necessity of advanced research. The predominance of this or that process determines the nature of the damage accumulation which leads to the destruction. Under such conditions, i.e., preliminary cyclic elastoplastic deformation followed by subsequent fatigue loading, the material subjected to preliminary loading at the first stage can be considered the "new material" with the new properties acquired upon cyclic elastoplastic loading which then undergoes further fatigue loading at the second stage. Hence, at the second stage, the new properties of the material are determined by the level of structural changes and damages earlier accumulated in the material. In this case, the damage to the material is considered on the basis of the well-known statement about the staging character of plastic flow, two main processes, i.e., shear, caused by the interaction of dislocations, and destructuring, attributed to violation of the continuity or integrity of the metal. Experimental studies of changes in the durability of cyclically hardened and cyclically softened steel specimens at different levels of preliminary elastoplastic deformation with varying number of cycles and amplitudes of preliminary elastoplastic strain showed the occurrence of an additional damage to the material when combination of loading modes leads to change in the fatigue durability at the subsequent stage of the basic multi-cycle loading. It is shown that correlation between changes in the fatigue durability and structural state of the material, caused by accumulated damage upon preliminary overloads, and, moreover, those changes can be characterized by the ratio of plastic and destructive strains as a Q-factor of the material.

Modeling of the thermomechanical behavior of rubbers during fatigue tests from infrared measurements

This paper presents the establishment of an equation fitting the evolution of the self-heating of a polychloroprene rubber (CR) and a natural rubber (NR) during uniaxial fatigue tests. For the CR, dumbbell shaped samples underwent a wide range of loadings was investigated in order to cover low and high cycle fatigue behavior for several load ratios taken between −0.3 and 0.5. The study consists in firstly measuring the evolution of the self-heating during a fatigue test. Then, after computing the derivative, the resulting curve is fit with an equation composed of three components: two of the form of a negative exponential and a constant. It is then integrated to obtain an equation that would fit the evolution of the self-heating for all the loadings investigated. It is observed that the parameters are correlated with the intensity of the stress and the load ratio applied during the test. The same observations were made for the NR on slim samples. Consequently, it is proposed that one of the parameter is linked to the viscosity, the parameter linked to the first negative exponential is related with the viscosity, the parameter linked to the second negative exponential is related to the damage induced by the low cycle fatigue loading and the parameter integrated from the constant is related to the influence of the presence of a sizable crack on the temperature field.

Crack Growth Behavior of Full-Scale Turbine Attachment Under Combined High and Low Cycle Fatigue

Turbine attachments in the aero-engine are generally subjected to combined high and low cycle fatigue (CCF) loadings, i.e., low cycle fatigue (LCF) loading due to centrifugal and thermal loading stresses superimposed to the aerodynamically induced high cycle fatigue (HCF) loading. The primary focus of this study is to predict the crack growth life for the actual full-scale turbine attachment through experimentally examining the crack growth behavior under CCF loading at elevated temperature. The crack closure effect was first investigated by using the corner-notched (CN) specimen cut from the turbine attachment since the stress state of CN specimen is more similar to turbine attachment than compact tension (CT) specimen. Employing digital image correlation (DIC) technique, the level of crack closure of CN specimen was clarified under different stress ratios (R) for LCF loading. Afterward, a CCF crack growth model for the full-scale turbine attachment was proposed, which takes the crack closure effect, time-independent crack increment, and transient vibrational analysis into account. In order to verify the proposed method, a Ferris wheel system was established to conduct CCF test on the full-scale turbine attachment at elevated temperature. This study provides an effective methodology to predict the fatigue crack growth (FCG) life of full-scale turbine attachment under CCF loading.

Modeling crack growth of an aircraft engine high pressure compressor blade under combined HCF and LCF loading

This paper presents a 3D crack growth simulation of an aircraft engine high pressure compressor blade which fractured in service. Using FRANC3D, an initial crack is inserted and grown using the max tensile stress theory. A numerical method was developed to capture the stress field under low cycle fatigue (LCF) and high cycle fatigue (HCF) loading by superimposing test measured dynamic loading with steady stresses. In doing so, the predicted crack trajectory, aspect ratio, and shape more closely agreed with the fractured airfoils compared to performing the simulation under LCF loads alone. This supports findings from fractographic investigation that the crack exhibits HCF rather than LCF characteristics.

Effects of aluminizing and combined strengthening on the fatigue property of K403 superalloy component under combined high and low cycle loading

Fatigue properties of K403 superalloy blades were tested using a combined high cycle fatigue (HCF) and low cycle fatigue (LCF) loading. Samples were tested in the as-cast, as-aluminized, and as-strengthened conditions. The strengthening treatment was wet sand blasting the blade root, followed by shot peening, and vibratory polishing. Metallographic and scanning electron microscope (SEM) fractographic examinations were conducted on the blades, including detailed characterization of the layered structures formed by aluminizing using energy dispersive spectroscopy (EDS). It has found that aluminizing created a surface which was brittle at the test temperature. Fatigue cracks initiated in the aluminized coating and propagated into the substrate, which reduced the fatigue life of the blades. However, the strengthening treatment improved the fatigue life by creating a compressive residual stress and removing a portion of the brittle outer layer of the aluminized coating.

The fatigue fracture characteristics of the bond zone of aluminum matrix composites (Al-12Si/ABOw) with Al-12Si alloys

The strain controlled low-cycle fatigue (LCF) test of the bond zone of aluminum matrix composites (Al-12Si/ABOw) with Al-12Si alloys was carried out with the test temperature range from 25 °C to 350 °C. The cyclic mechanical response characteristics of material at different test temperatures were analyzed. The microstructural evolution process and fracture mechanism of three characteristic fracture models were studied through SEM observation. The influence of temperatures and load magnitudes on fatigue behaviors were also discussed. The results showed that, the cyclic stress responses of the material exhibited cyclic hardening under low-cycle fatigue loading at 25 °C, and the crack occurred at the casting defects in the composite part first, resulting in a flat fracture surface with both composite part and Al-12Si alloy part. When the temperature raised to 200 °C, the cyclic stress response of the material would show out slight cyclic hardening at beginning stage and distinct cyclic softening during later stage. The casting defects in the composite would no longer be the prior crack source, and the crack would initiate at the large-sized primary Si particles nearby the bond zone which led to a fracture surface with the bond zone shape under low-cycle fatigue loading at 200 °C. With the continued increasing of temperature to 350 °C, the cyclic stress responses of the material showed the characteristics of continuous softening, and the fatigue crack generated at the primary Si phases in the Al-12Si alloy part, which caused the specimens fractured at the unreinforced Al-12Si alloy parts with a fracture surface like common aluminum alloy fracture under high temperature. There was one point had to be mentioned that when the magnitude of cyclic strain loadings (△ε/2) was larger than 0.003, the failure mode would be the fracture of the unreinforced Ai-12Si alloy part at any temperature due to the prior fracture of Al-12Si matrix alloy under relatively high loadings. Microstructure observation indicated that the fatigue fracture features of the material transformed from hard-brittle to soft-ductile gradually with the change of test temperatures from 25 °C to 350 °C.

Seismic loading tests of full-scale two-story steel building frames with self-centering braces and buckling-restrained braces

A new steel dual-core self-centering brace (DC-SCB) has both re-centering and energy dissipation properties in seismic loading, reducing residual deformation of a frame structure with a buckling-restrained brace (BRB). Previous works showed that both DC-SCBs and BRBs perform satisfactorily in seismic loading, but study of a full-scale, multi-story steel frame with these two different braces in one test program is very limited. Three full-scale two-story, one-bay steel frame specimens, named as DC-SCBF, SBRBF and SMBF which had DC-SCBs, BRBs and a combination of DC-SCBs and BRBs, respectively, were cyclically tested. Multiple tests of the 9 m-wide, 7.54 m-high braced frame specimens were focused on: (1) validating the system response with DC-SCBs, BRBs and a combination of these two different braces, (2) studying the variation of the lateral stiffness, maximum force and energy distribution of the frames, and (3) discovering the failure and repair characteristics of the frames under seismic loading. Nonlinear time history analyses were conducted on the three prototype braced frames to obtain seismic demands for the test protocol. Theses two-story frame specimens performed satisfactorily up to an interstory drift of 2%, which was close to the mean value of the maximum interstory drift of the prototype frames under 20 Maximum Considered Earthquake (MCE) motions. No damage of the DC-SCB or BRB was observed, but a fracture of the steel beam flange near the joint of the beam web stiffener and gusset plate toe was observed due to low-cycle fatigue loading. In direct contradiction to brace component tests, the energy dissipation of the two-story frame specimen with DC-SCBs is higher than that with BRBs due to higher post-yield stiffness of the DC-SCB. The post-yield lateral stiffness of Specimen DC-SCBF is about 20% of its elastic lateral stiffness and 1.6–1.8 times that of Specimens SMBF and SBRBF. In addition to the energy dissipation, the higher post-yield stiffness is also a better way to reduce the maximum drift of the frame in earthquakes. Moreover, Specimen SMBF that adopts a combination of DC-SCB and BRB along the building height has intermediate lateral stiffness, residual deformation and energy dissipation among all frame specimens, verified as a potential seismic-resisting frame system.

Isothermal and thermomechanical fatigue interaction in fatigue crack propagation behavior of a low-carbon nitrogen-controlled 316 stainless steel

In this work, the effect of superimposing of isothermal low cycle fatigue loading to the thermomechanical fatigue loading on the crack propagation behavior of the naturally initiated short crack in Low-carbon nitrogen-controlled 316 stainless steel was investigated. The experimental results indicated that the crack propagation path depends on the loading condition; the cracks appear to be initiated and propagated at grain boundary perpendicular to the loading axis which might be a relatively week region at elevated temperature under the in-phase type TMF loading and the LCF loading at high temperature, on the other hand, the cracks initiated and propagated by the transgranular mode under the out-of-phase type TMF loading and the LCF loading at middle temperature. The short crack growth rate has also affected the microstructure, i.e., the intergranular crack exhibits higher growth rate compared with the transgranular crack. In addition, the crack growth rate was accelerated by superimposing of the isothermal low cycle fatigue loading to the main thermo-mechanical fatigue loading. The short crack growth rate could be predicted according to summation law of crack growth behavior based on the J-integral approach considering with crack propagation path.

Superimposed Effect of High Cycle Fatigue and Low Cycle Fatigue Loadings on Small Crack Propagation Around Cooling Hole in A Directionally Solidified Ni-base Superalloy

Superimposed Effect of High Cycle Fatigue and Low Cycle Fatigue Loadings on Small Crack Propagation Around Cooling Hole in A Directionally Solidified Ni-base Superalloy

Stress-induced formation of TCP phases during high temperature low cycle fatigue loading of the single-crystal Ni-base superalloy ERBO/1

The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states.

A New Empirical Life Prediction Model for 9–12%Cr Steels under Low Cycle Fatigue and Creep Fatigue Interaction Loadings

Low cycle fatigue (LCF) and creep fatigue interaction (CFI) loadings are the main factors resulting in the failure of many critical components in the infrastructure of power plants and aeronautics. Accurate prediction of life spans under specified loading conditions is significant for the design and maintenance of components. In the present study, various LCF and CFI tests are conducted to investigate the effects of temperature, strain amplitude, hold time and hold direction on the fatigue life of P92 steel. To predict fatigue life under different experimental conditions, various conventional life prediction models are evaluated and discussed. Moreover, a new empirical life prediction model is proposed based on the conventional Manson-Coffin-Basquin (MCB) model. The newly proposed model is able to simultaneously consider the effects of temperature, strain amplitude, hold time and hold direction on predicted life. The main advantage is that only the known input experimental parameters are required to perform the prediction. In addition to the validation made through the experimental data of P92 steel conducted in the present paper, the model is also verified through numerous experimental data reported in the literature for various 9–12% Cr steels.

The Effects of Grain Size, Dendritic Structure and Crystallographic Orientation on Fatigue Crack Propagation in IN713C Nickel-Based Superalloy

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendrite structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting porosities and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendrite areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendrite areas. Here, we propose a concept of 'Crack Propagation Unit (CPU)' for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendrite structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pile-up at the grain boundary or in-between the γ/ γ' channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters.

Low cycle fatigue of alloys in hot section components: progress in life assessment

Hot section components are exposed to high temperature operation and thermal transients during service life. The research of improved performances leads to consider the risk of crack initiation due to isothermal (LCF) or thermal-mechanical (TMF) low cycle fatigue loading in component design. This article gives a short survey of progress in LCF and TMF life estimation procedures due to the description of metallic alloys behavior (superalloys, cast iron, stainless steels). Assessing the lifetime under cyclic loading conditions requires constitutive models accounting for viscoplasticity at service temperatures or during transients. These models can be validated at the laboratory scale using thermal mechanical fatigue tests. Crack initiation life prediction models can be quite simple or need to account for creep-fatigue interactions or environmental effects. Examples illustrate advances in materials studies and models over the years in aero-engine and automotive applications.

Numerical simulation of cyclic plasticity in mechanical components under low cycle fatigue loading: accelerated material models

Numerical simulations of components subjected to low-cycle fatigue loading require an accurate modeling of the material cyclic plasticity behavior until complete stabilization. In some circumstances, especially in case of small plastic strains, it may happen that the material model needs a huge number of cycles to reach complete stabilization, which results into an unfeasible simulation time. An acceleration technique, based on a fictitious increase of the parameter that controls the speed of stabilization in the combined (kinematic and isotropic) model, may be used. To check the efficiency and the correctness of the acceleration technique, the case of a welded cruciform joint under low cycle fatigue, taken from the literature, is here considered. The joint can be analyzed with a two-dimensional finite element model, which permits a relatively fast simulation to be completed until stabilization even with a combined kinematic-isotropic plasticity model (reference case). A comparison of this reference case with accelerated models is performed. Results in term of equivalent total strain range show that the acceleration procedure does not alter the welded joint cyclic behavior at stabilization, whereas it drastically reduces the computational time.

Microstructure-based RVE modeling of the failure behavior and LCF resistance of ductile cast iron

In this work the failure behavior of ductile cast iron microstructure subjected to tensile and low-cycle fatigue loadings is simulated by a 3-D, FE Reference Volume Element approach. A fully ferritic matrix is considered as representative of the low-hardness, high-ductility material class of nodular cast irons. Plastic flow potential rule, ductile and low cycle fatigue damage models are implemented at the micro-scale for the matrix constituent in conjunction with nonlinear cyclic hardening laws, and periodic boundary conditions are imposed over the RVE at the meso-scale. Different values of triaxiality are imposed. Numerical results confirm experimental findings of the behavior at the meso-scale and correctly predict the LCF lifetime, driving the interpretation of inner strain distribution, voids interaction and triaxiality effects on failure mechanisms.

Effect of additive manufacturing on fatigue crack propagation of a gas turbine superalloy

Additive manufacturing (AM) offers new possibilities in gas turbine technology by, for example, allowing for more complex internal cooling channels. One such application, where AM can improve the function by new designs, is burners. However, the fatigue performance, especially the fatigue crack propagation of AM gas turbine material, is not fully known. In the present study, an AM adopted nickel-based superalloy Hastelloy X was subjected to low-cycle fatigue (LCF) loading at room temperature. The LCF tests were conducted in strain control on additive manufactured smooth bars with two different build orientations (with an angle of 0° and 90° relative to the building platform). During the the manufacturing process, an AM component often solidifies with a dendritic structure. Initial fractography of the ruptured LCF specimens revealed that the dendritic structure was visible on the fracture surface. It was noted that the dendritic structure could easily be mistaken for regular striations although they represent a different fracture mechanism. The fracture surfaces were therefore cross sectioned and possible correlations between fracture surface characteristics and underlying microstructure were studied using electron backscatter diffraction and electron channelling contrast imaging. The outcome was used to discuss the effect of AM microstructure on the LCF crack propagation.

Change in Interlayer Strength and Fracture Toughness of Carbon-Carbon Composite Material under the Impact of Cyclic Loads

The effect of low-cycle fatigue loading on interlayer strength and fracture toughness of discretely reinforced carbon-carbon composite material (CCCM) for friction use is studied. The material demonstrates good resistance to fatigue loads. In the described mechanisms for increasing CCCM fracture toughness after applying fatigue load, the fiber–matrix interface plays a key part. The dependences of CCCM fracture toughness and interlayer strength on the fatigue loading parameters are determined.

Low Cycle Fatigue Failure Analysis of a Ni‐Based Single Crystal Superalloys at 850 °C

This paper presents a low cycle fatigue (LCF) failure analysis on [001], [011], and [111] orientations of a Ni‐based single crystal superalloys at 850 °C. Fracture mechanism and microstructure damage evolution are revealed through crystal plastic theory and fracture morphology analysis. Strain‐controlled LCF tests are conducted with the standard round bar samples; the fracture morphology and microstructure characteristics, such as the fracture mechanism and microstructure damage evolution, are observed via scanning electron microscopy (SEM), optical microscopy (OM). Crystal plastic theory is adopted to establish the crystal slip system framework, considering elastic and plastic strain rate, elastic orientation factors, and the resolved shear stress (Rss). The life prediction model is established based on Levkovitch damage theory, theoretical analysis, and test results has good consistency. The results demonstrate that fatigue life on [001] orientation is the longest, but considering the effects of stress amplitude and Young's modulus, [011] orientation exhibits the best fatigue performance. Under LCF loading, the initial crack starts to occur on surface of specimen if the internal defects are small and negligible; in contrast the initial crack starts to occur in subsurface or internal of specimen if the microstructure defects exist in material. The LCF cross‐section of SC belongs to the quasi‐cleavable type, the crack propagates in the direction of {111} plane and then extends along the vertical stress axis until the specimen fractures.

Stress-Induced Formation of TCP Phases During High Temperature Low Cycle Fatigue Loading of the Single-Crystal Ni-Base Superalloy ERBO/1

The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950°C (load-ratio: 0.6, tensile stress range: 420 to 740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 2000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only µ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary µ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states.

Understanding strain controlled low cycle fatigue response of P91 steel through experiment and cyclic plasticity modeling

In this paper, constitutive models for cyclic plasticity, namely, Chaboche and the Ohno-Wang models, are used to simulate the low cycle fatigue response of tempered ferritic-martensitic P91steel. A series of uniaxial strain-controlled low cycle fatigue tests are conducted at room temperature to evaluate various fatigue parameters. P91 steel shows cyclic softening behavior at all strain amplitudes. The analysis of the shape of hysteresis loops reveals that P91 steel deviates from Masing behavior. Plastic strain energy is used as a parameter to characterize the fatigue damage. The Morrow energy model estimates experimental average plastic strain energy density accurately. Remnant tensile properties are evaluated to quantify the damage during low cycle fatigue loading. The evaluation of back stress components and material Jacobian are implemented through a user defined subroutine UMAT into the commercial finite element package ABAQUS. Ohno-Wang material model predicts the hysteresis loop shape, area, and cyclic softening nature better than the Chaboche model. The predicted fatigue lives by simulation correlates well with the experimental fatigue lives within ± 200 cycles. These results suggest that energy based parameters can provide a reliable alternative to strain based approach in evaluating the fatigue damage of P91 steel.