Low Cycle Fatigue Loading Conditions Research Articles

Fracture mechanics analysis method for cracked components under very low cycle fatigue loading

In this study, fracture mechanics analysis was performed to calculate crack growth under very low cycle fatigue loading conditions. The calculation results are compared with 11 experimental datasets concerning through-wall cracked pipes made from three different materials under fully reversed large-amplitude cyclic loading. In the analysis, it was assumed that the crack growth rate under large-amplitude cyclic loading is the linear summation of the fatigue crack growth rate and ductile tearing rate. Relevant fracture mechanics parameters were determined using the reference stress approach. It was found that the calculated failure cycles are around half of those using the experimental data regardless of the material and load amplitude. The most important factors affecting the calculation results were the accuracy of estimating J and ΔJ, and the crack closure coefficient when calculating ΔJ. Their effects on the analysis results are discussed herein.

Life assessment of a 42CrMo4 steel under low-cycle fatigue and thermo-mechanical fatigue loading conditions

In this paper, a 42CrMo4 steel is subjected to Low-Cycle Fatigue (LCF) loading and Thermo-Mechanical Fatigue (TMF) loading in the temperature range between 300 °C and 600 °C. The experimental results show that the cyclic softening of the material is more significant with increasing temperature, and that both in-phase and out-of-phase TMF lead to a shorter lifetime than for LCF. Finally, a modification of the fatigue criterion from the literature is proposed in order to take into account the main damage mechanisms involved in LCF and TMF. A good correlation was achieved between the predicted results and the experimental results.

Numerical analysis for the thermal ageing effect on fracture behaviors of CF8A pipes and piping systems under monotonic and very low cycle fatigue loading conditions

In this research, the effect of thermal ageing for CF8A is studied under displacement-controlled low cycle fatigue. Virtual tests of pipe and pipe system are considered and they have the same shape and loading conditions as the pipe and pipe systems tested in Battelle. Only material was changed from A106 Gr. B to CF8A. The pipe tests are four-point bending tests with three different load ratio (R= -1, 0, 1) having through-wall crack. The pipe system test also has through wall crack with seismic loading condition (PGA=1g). To simulate the cyclic loading and thermal ageing effect, multi-axial fracture strain energy based FE damage model and thermal ageing constant “C” was applied. From the simulation, it can be known from the virtual pipe test that crack growth rates of unaged and aged CF8A become similar when loading type is changed from monotonic to cyclic. Also, it is can be known from the virtual pipe system test that the crack growths of unaged and aged CF8A are almost same when the size of through-wall crack is 43.2o. However, the crack growth of unaged CF8A is almost 25% smaller that aged CF8A when the size of through-wall crack is 90o.

Ductile Crack Growth Modeling under Low Cycle Fatigue Loading Condition

This paper presents a numerical method using FE damage analysis to simulate ductile tearing in CT specimen under low cycle fatigue loading conditions. To define a cyclic material properties, the nonlinear kinematic hardening model is adopted. The damage model is defined based on the ductility exhaustion concept using the multi-axial fracture strain energy concept. The proposed model is then applied to simulate two cyclic fracture toughness tests with different R-ratios. Simulated results show overall good agreement with experimental results.

On effect of pre-bending process on low cycle fatigue behaviour of high strength steel using lock-in thermography

The application of High Strength Steel (HSS) on different structural components is becoming more attractive. They have a fine grain structure, low carbon content for improved weldability, and controlled internal purity. These steel grades are frequently used for structural applications. There are a lot of steel components of the considered applications that are subjected to bending and fatigue loading conditions, respectively. It is well known that under critical loading conditions, cyclic stress, which exceeds the material yield stress, can occur at some critical locations such as the inner side of bent components. Combining these two aspects, i.e. the bending process followed by fatigue loading, causes multiple micro-crack initiation inside the inner surface of the bending area, which is followed by propagation of the cracks up to final rupture of material.The main objective of the present study is to investigate the effect of pre-bending process of HSS subjected to low cycle fatigue loading conditions, since so far only very limited amount of research has been focused in this direction. For this purpose, in the first step, a new test set-up was designed to take into account the effect of pre-bending process when the fatigue load has been applied. Lock-in thermography technique was used to monitor the incremental temperature variation during fatigue cycling at the bending root. Using the temperature evolution, the crack initiation and propagation lifetimes were separated from total lifetime. Fractography and Scanning Electron Microscopy (SEM) analyses were performed to study the fracture surface of specimens after bending and fatigue testing. Furthermore, numerical technique approach was used to model the bending and spring back processes along with the fatigue loading in order to understand the effect of bending process on fatigue behaviour of tested material. The developed finite element model provides more information about the multiaxial strain and stress states at and near the bending root after bending process and applied axial fatigue load.

Combined Cycle Fatigue Investigation Based on Energy Principle

We present a modified energy-principle based model of fatigue damage accumulation in high temperature alloys usually used in gas turbine engine under combined high cycle fatigue and low cycle fatigue (LCF/HCF) loading conditions. Our model is based on the energy principle which includes a modified approximation formula that describes fatigue crack origin depending on the relative amplitude of stress intensity in the ranges of both high- and low-cycle fatigue under non-isothermal loading. Functional dependence that presents the influence of HCF mechanisms on a fatigue life of our structural material is gradual and it has not breaks of the curve that yields a possibility to rewrite the equation of the S-N curve with taking into account combined cycle fatigue loading. We used the same number of parameters as the initial model. Note, that new parameter interpretation gives clear physical picture. The proposed model is verified by comparing the computed results with the experimental data for one high temperature alloy GH4133.

Experimental investigation of fatigue crack growth behavior of GH2036 under combined high and low cycle fatigue

Fatigue crack growth rates have been experimentally determined for the superalloy GH2036 (in Chinese series) at an elevated temperature of 550°C under pure low cycle fatigue (LCF) and combined high and low cycle fatigue (CCF) loading conditions by establishing a CCF test rig and using corner-notched specimens. These studies reveal decelerated crack growth rates under CCF loading compared to pure LCF loading, and crack propagation accelerates as the dwell time prolongs. Then the mechanism of fatigue crack growth at different loadings has been discussed by using scanning electron microscope (SEM) analyses of the fracture surface.

Influence of process-induced microstructure on hardness of two Al–Si alloys

This paper analyses the influence of thermal-induced aging on the strength of cast A319 and A356 aluminum alloys. This phenomena is of primary importance especially in the automotive industry, where on one hand the alloys experience Thermo-Mechanical and Low Cycle Fatigue loading conditions and on the other hand their microstructure is induced both by the manufacturing process and the thermal histories. The alloys studied here were produced using an industrial Lost Foam Casting process which affects significantly the microstructure and the precipitation compared to standard Die Casting process. The paper compares mechanical properties of these peculiar alloys in terms of microhardness at the macroscopic level as a function of thermal overaging time and at the microscopic level in terms of properties of the different phases (intermetallics, eutectics) present in the microstructure. Important differences can be observed between these different phases and the results are consistent with similar studies from the literature on different aluminum alloys and processes.

Analysis of Hysteresis Loops of 316L(N) Stainless Steel under Low Cycle Fatigue Loading Conditions

Abstract Low cycle fatigue tests were carried out on 316 L(N) stainless steel at room temperature employing strain amplitudes ranging from ± 0.3 to ±1.0% and a strain rate of 3×10−3s−1. The material showed initial hardening for a few cycles followed by prolonged softening, saturation and final failure. The fatigue life was found to decrease with increase in strain amplitude. The analysis of stable hysteresis loops showed non-Masing behaviour for this material. The elasto-plastic response of the material under cyclic loading was characterized taking into account isotropic and kinematic hardening occurring during cycling. The material parameters were obtained from the experimental hysteresis loops and cyclic stress response of the material. Finite element analysis of elasto-plastic deformation was carried out to obtain the stabilized hysteresis loop and cyclic stress response of the material. The predicted hysteresis loops showed good agreement with experimental results. The low cycle fatigue life prediction was carried out based on plastic strain energy dissipation with cycling.

TMF–LCF life assessment of a Lost Foam Casting A319 aluminum alloy

The LFC (Lost Foam Casting) process affects the microstructure, the mechanical properties, the damage mechanisms and the fatigue failure of the materials. The first purpose of this paper is to study the cyclic mechanical behaviors, damage and lifetime of the A319 aluminum alloy manufactured by the LFC process used in the automotive industry under TMF (Thermo-Mechanical Fatigue) and LCF (Low Cycle Fatigue) conditions. A second objective is to select an effective fatigue criterion which should be easy to apply for the design of structures submitted to complex multiaxial thermo-mechanical loadings. In this way, several energy-based criteria are used to predict fatigue failure. Good agreement between predicted fatigue lifetimes and experimental results was obtained for different TMF and LCF loading conditions.

炭素繊維強化積層複合材の細径ピンによる穿孔と細径ボルトの密集配置による接合

In this paper, a mechanical joint that consists of densely arranged thin bolts is proposed for joining CFRP laminates. Tiny bolts have only low load transfer capacity, but their dense assemblage can provide the same total load transfer capability as a smaller number of large bolts. Bolt holes are opened by piercing sharp pointed pins in a B-stage prepreg and the inserted pins are replaced with release coated tubes, which are kept inserted during the prepreg cure. At the completion of cure, removing the tubes, smooth bolt holes are formed in the CFRP laminate. Ultrasonic vibration assisted pin insertion test was also conducted, leading to a drastic reduction of the required insertion force, but it did not reduce the fiber breakage in the pierced hole. An experimental study was also done to explore the effects of the bolt size and the hole forming method i.e., drilling and piercing, on the joint strength. Finally, a series of tensile shear tests of the proposed joints under both quasi-static and low-cycle fatigue loading conditions was carried out to investigate their possibility and feasibility.

Effect of reinforced fiber length on the joint performance of thermoplastic leaf spring

Joint strength plays a significant role in the performance of leaf spring suspension system. Current work reported the influence of reinforced fiber length on the performance of injection molded thermoplastic leaf spring joint. Leaf springs were molded using 20% short, long glass fiber reinforced polypropylene as well as unreinforced polypropylene and evaluated for the joint strength. Servo hydraulic test facility with suitable fixture is utilized to evaluate the leaf spring joint performance under static and dynamic conditions. Test joints were subjected to completely reversed fatigue loads, wherein long fiber reinforced leaf spring joint exhibited superior performance at high cycle fatigue conditions than that of short fiber reinforced and unreinforced polypropylene leaf spring joints. However, at low cycle fatigue loading conditions, unreinforced and short glass fiber reinforced leaf spring exhibited superior performance than that of long glass fiber reinforced leaf spring joint. High notch sensitivity characteristics of the long glass fiber reinforced polypropylene material contributed to this inferior performance. Load–deflection hysteresis plot of the long glass fiber reinforced leaf spring joint under fatigue loading conditions exhibited a lesser amount of hole elongation compared to that of short glass fiber and unreinforced leaf spring joint. Failure morphology of tested joint under fatigue condition exhibited net-tension and shear-out failures besides bearing damages.

炭素繊維強化積層複合材と金属材の細径ピンによる接合

The adhesive layer in adhesive bonded joints can be seen as a dense assemblage of tiny adhesive bonding elements. Each element has low load transfer capacity, but its high density arrangement works smooth stress distribution, resulting in effective load transmission and high joint efficiency. In this paper, a type of mechanical joints between CFRP laminates and metal plates consisting of closely arranged ultra-thin pins is proposed. Each pin has only low shear strength, but its dense arrangement achieves high joint strength. Sharp-pointed, small-diameter pins can be pierced through B-staged CFRP laminate without cutting reinforcing fibers and inserted into pre-drilled holes in the metal plate to be joined. Since excess resin fills the gap between the pins and pin holes, the inserted pins are fixedly secured to the inner face of the hole, once the cocuring process is completed. A series of tensile shear tests of the proposed joint under both quasi-static and low-cycle fatigue loading conditions was carried out to investigate the possibility and feasibility of the present method. The tested joints showed both static and fatigue strength comparable to those of the base metal.

Fatigue crack growth from foreign object damage under combined low and high cycle loading. Part II: A two-parameter predictive approach

An approach based on a two-parameter fatigue theory has been developed to predict FOD-induced small crack growth under combined low and high cycle fatigue loading conditions. In this approach, both ΔK and Kmax are recognized as being important to the fatigue crack growth process and therefore included in a crack growth formulation. An elastic stress intensity factor is not appropriate for the small cracks studied here, particularly due to the influences of residual stresses and stress concentration produced at FOD indentations. The El Haddad approach was used as a correction factor, providing a modified stress intensity range ΔKmod and maximum stress intensity Kmod,max. The levels of stress concentration and residual stresses were calculated by a general finite element code ABAQUS. The results show that small crack growth data compare well with those of long cracks when correlated with a new two-parameter crack driving force (ΔKmod)n′(Kmod,max)p′(1-ΔKmod,th/ΔKmod)0.25, where n′ and p′ are material constants derived from long-crack fatigue tests. This means that FOD-induced small crack behaviour may be predicted using long-crack experimental results.

Fatigue crack growth from foreign object damage under combined low and high cycle loading. Part I: Experimental studies

The study aims to understand small crack growth behaviour in Ti–6Al–4V plate specimens after foreign object damage (FOD), under combined low and high cycle fatigue (LCF/HCF) loading conditions. The loading block studied represents one of the simplest loading sequences experienced by aero-engines and each block includes a single LCF cycle at a load ratio (R) of 0.01 with 1000 HCF cycles at R of 0.7 superimposed at peak load.In Part I of this paper, the experimental studies are described. FOD was simulated by firing a 3mm hardened steel cube onto targeted specimens with an impact velocity of 200m/s, leading to a blunt “V” notch in a flat surface. Microstructural features associated with the FOD damage, e.g. microcracks, were characterized by optical and scanning electronic microscopy. The microcracks were found to be preferred sites for subsequent crack propagation. The growths of these FOD-initiated small cracks under combined LCF/HCF loading were drastically different from the long-crack results obtained from corner-cracked specimens. Stress relief annealing was conducted on selected specimens to remove the residual stresses due to FOD. The results indicated that residual stresses significantly affected both the development of crack front shape and the fatigue life.

Dynamic strain aging under tensile and LCF loading conditions, and their comparison in cold worked 316L stainless steel

Tensile and low-cycle fatigue (LCF) tests were carried out in a wide temperature range from 20 to 750 °C at strain rates of 1 × 10 −4–1 × 10 −2/s for 17% cold worked 316L stainless steel to investigate the conditions for the occurrence of dynamic strain aging (DSA) and its effects on material properties during tensile and LCF deformations. DSA introduced anomalous changes of tensile and LCF properties, and the DSA regimes under tensile and LCF loading conditions coincided with each other. During tensile deformation, DSA can be manifested in the forms of the plateau in the variation of strength with temperature, the minima in the variation of ductility with temperature, the serrated yielding in the stress–strain curves, and the negative strain rate sensitivity (SRS). As for LCF deformation, it can be manifested in the forms of the plateau or the peak in the variation of cyclic peak stress with temperature, the negative temperature dependence of plastic strain amplitude or softening ratio, the negative SRS, and the negative strain rate dependence of plastic strain amplitude or softening ratio.

The tensile and low-cycle fatigue behavior of cold worked 316L stainless steel: influence of dynamic strain aging

Tensile and low-cycle fatigue (LCF) tests were carried out in air in a wide temperature range from 20 to 750 °C and strain rates of 1×10 −4–1×10 −2/s to investigate the influence of strain rate on tensile and LCF properties of cold worked 316L stainless steel, especially in the dynamic strain aging (DSA) regime. Dynamic strain aging caused a change in the tensile properties such as strength and ductility in the temperature range from 250 to 600 °C. This temperature range coincided well with the regime of negative strain rate sensitivity. Cyclic stress response at all test conditions was characterized by an initial hardening during the few cycles, followed by gradual softening until failure. Temperature and strain rate dependence on cyclic softening appears to come from a change of the cyclic plastic deformation mechanism and the DSA effect. The regimes of DSA between tensile and LCF loading conditions in terms of the negative strain rate sensitivity were consistent with each other. At the elevated temperature, a drastic reduction in fatigue resistance was observed, and this is attributed to the effects of oxidation, creep, and dynamic strain aging and interactions among these factors. The regime of DSA accelerated a reduction in fatigue resistance by enhancing crack initiation and propagation.

냉간가공된 316L 스테인리스 강의 인장 및 저주기 피로 물성치에 미치는 동적변형시효의 영향

Tensile and LCF(low cycle fatigue) tests were carried out in air at wide temperature range 20-750 and strain rates of 110-4//s-110-2/ to ascertain the influence of strain rate on tensile and LCF properties of prior cold worked 316L stainless steel, especially focused on the DSA(dynamic strain aging) regime. Dynamic strain aging induced the change of tensile properties such as strength and ductility in the temperature region 250-600 and this temperature region well coincided with the negative strain rate sensitivity regime. Cyclic stress response at all test conditions was characterized by the initial hardening during a few cycles, followed by gradual softening until final failure. Temperature and strain rate dependence on cyclic softening behavior appears to result from the change of the cyclic plastic deformation mechanism and DSA effect. The DSA regimes between tensile and LCF loading conditions in terms of the negative strain rate sensitivity were well consistent with each other. The drastic reduction in fatigue resistance at elevated temperature was observed, and it was attributed to the effects of oxidation, creep and dynamic strain aging or interactions among them. Especially, in the DSA regime, dynamic strain aging accelerated the reduction of fatigue resistance by enhancing crack initiation and propagation.

Low-cycle fatigue life prediction for Waspaloy

Methodologies of correlating and predicting life to failure in low cycle fatigue are investigated for Waspaloy at various temperatures. The viscoplastic material model proposed by Chaboche was initially adopted in order to predict the inelastic behaviour observed in low cycle fatigue. Life predictions under low-cycle fatigue loading conditions for the disc material Waspaloy were investigated with two models, the strain range partitioning method (SRP) and the linear combination of creep and fatigue damage laws. In the SRP rule, decomposition method using a computer modeling for determining the model parameters is proposed. The capabilities of the two models were assessed using strain-controlled LCF test results on Waspaloy.

The performance of a bonded composite fastener under static and fatigue loading conditions

The behaviour of a proprietary composite fastener bonded to a chopped strand mat/polyester resin laminate has been examined. Fasteners adhesively bonded with a two-part epoxy and a one-part cyanoacrylate adhesive were evaluated under both static and fatigue loading conditions and the results compared with similar data obtained with fasteners directly bonded into laminates. Under static loading and low cycle fatigue loading conditions, the bonded-in system gave a superior performance. At approaching 10 6 cycles the fatigue performance of each system was similar. The results for the adhesively bonded fasteners were shown to be sensitive to surface preparation techniques, whilst the performance of the bonded-in fasteners was improved by design modification of the fastener.