Constant Stress Amplitude Research Articles

Experimental and theoretical analysis of heat flux at fatigue crack tip under mixed mode loading

The work is devoted to experimental and theoretical study of heat dissipation at the fatigue crack tip during mixed mode loading. The plane samples of stainless steel (AISI 304) were weakened by notch to initiate fatigue crack. There were two types of samples for uniaxial loading and biaxial loading. Infrared thermography and the contact heat flux sensor based on the Seebeck effect were used to measure the dissipated thermal energy. The samples were subject to cyclic loading with constant stress amplitude and different biaxial coefficients. The experimental results confirmed the hypothesis about two stages of heat dissipation at fatigue crack tip under Paris regime. At the first stage, the power of heat flux is proportional to the product of the crack rate by the crack length. The second stage is characterized by a traditional linear relationship between the crack rate and the heat flux. The theoretical approach to calculate dissipation energy at fatigue crack tip was obtained. This technique based on link between the elastic-plastic and elastic strain fields at crack tip using the coupling between Young’s modulus and the secant plasticity modulus.

Fatigue and Fracture Behaviours of FSW and FSP Joints of AA5083-H111 Aluminium Alloy

This research study’s the fatigue behaviour of friction stir welded (FSW) and friction stir processed (FSP) joints for joining AA5083-H111 aluminium alloy. The Joining has been implemented on 3mm thickness. Fatigue properties of the welded joints were evaluated based on the superior tensile properties for FSW at 1500 rpm and a feeding speed of 20 mm/min. the FSP was at 1500 rpm and a feeding speed of 40 mm/min. Fatigue alternating bending test was performed at a constant stress amplitude cantilever with fully reversed (R=-1). The fracture surface topography was also analyzed by the aid of an electron microscope. The experimental fatigue properties for base, FSW & FSP welds of AA 5083-H111 aluminium alloy that were obtained from the experimental work showed that all the tested samples of FSP displayed good fatigue behaviour and better fatigue properties near to base material than what the FSW were. The endurance limits were 81 MPa and 71 MPa for FSP and FSW joints respectively and 87 MPa for base metal.

Analisa Kelesuan Komposit Hibrid daripada Komposit Serat Kenaf/Kevlar yang Diperkukuhkan dengan Epoksi

This study is aimed to investigate the fatigue limit of the hybrid composites of kenaf and kevlar reinforced epoxy. Most of the engineering components used in the military appliance and in high technology competition like Formula One Racing car, uses synthetic fibre as their main component in their structural design due to their high mechanical properties. However, by using this material in a small portion of quantity is extremely expensive, uneconomical and becomes over-designed. In this study, kevlar-29 and a woven kenaf being hybrid together using technique of vacuum resin infusion. The fatigue test was conducted at a constant stress amplitude with frequency of 3 Hz, in accordance to ASTM D3479. It was found that the transition region has significant influence on the fatigue results since there are significant reductions on Young’s Modulus. In addition, the endurance limit for the developed composites was almost similar with the Aluminium Alloy with known knee of shape determining the exact fatigue limit. The fatigue limit was read at a million cycles (1005498 cycles). The SEM study has showed the damage mechanism of this developed composites which is started by matrix cracking and followed by fibre breakage and delamination between kenaf fibre and kevlar fibre once the develop composite reach the failure point. Thus, this development of hybrid composite shows good agreement findings and has high potential to replace or reduce the usage of synthetics fibre.

Paradox of fatigue of perfect soft metals in terms of micro plasticity and damage

Since the production of engineering parts is under rigid control, which does not permit any cracks, the fatigue problem of high frequency stressing is modeled for perfect soft metals in macro, in terms of energy spent on damage. According to the last research, irreversible strains of instant relaxation, (relief), created at any cycle, can be calculated even for very small durations of a single pulse. They are the origin of irreversible energy spent on micro plasticity of single grains, with consequent damage by microcracking.In soft metals fatigue appears through the state of elasticity, below the yielding limit. It is found that only a little part of the irreversible energy, (defined as a μ-quant of fatigue), is responsible on the fatigue process. Operator of integrity, complementary to damage, is introduced to describe the preservation of the solids under damage, while their elastic modulus remains constant. The definition of reactive stress is given and it is used to express the alteration of stresses by declining integrity.It is shown that damage shifts the yielding point down. Equation of this shift is obtained and used to predict the passive elastic energy spent on damage under state of constant maximum stress amplitude. μ-quant of fatigue reflects the dislocation’s pressure vs the resistance of lattice. It is a material parameter in macro, found specific for any metal. The value of μ-quant were estimated for steel, aluminium, titanium and copper, demonstrating its wide range and its central place in the life time of soft metals.Two distinct constitutive equations of the life numbers of pulses are obtained for constant stress and for constant strain amplitude resp., as a function of the mu-quant, stressing and of the pulse factors, without free parameters. The equations show that fatigue life under constant strain amplitude is prolonged vs constant amplitude stress state, against lower residual strength. An equation linking the mu-quant with temperature is proposed, The way to express the nonlinearity of the μ-quant vs stressing is shown. The differential of the equation of fatigue process is given. Method of evaluation the remaining life-time of equipment under fatigue is presented. Paradox of fatigue at low stressing is solved as a tunneling effect, cased by micro plasticity. It explains the transformation of soft metal into brittle solid under low consumption of energy.

Fatigue behavior of direct laser deposited Ti-6.5Al-2Zr-1Mo-1V titanium alloy and its life distribution model

The present work aims to investigate the fatigue behavior of Direct Laser Deposition (DLD) Ti-6.5Al-2Zr-1Mo-1V titanium alloy under constant amplitude stress. 22 pieces of DLD Ti-6.5Al-2Zr-1Mo-1V titanium alloy standard cylinder specimens were tested under a stress level of 800 MPa with a stress ratio of 0.06. Fatigue fractography and fatigue life data were obtained. Through the fracture surface analysis, the specimens were divided into two categories in accordance with the location of crack initiation and defect types. Comparison of fatigue life and behavior between two specimen types was given, which was followed by a discussion about the impact of defect type, size and position on the fatigue life of the specimen. The fatigue test results also show a large variation of fatigue life. To illustrate the statistical characteristics of the fatigue life, probabilistic analysis was performed, and a novel bimodal lognormal model was established. The model has a good fit with the experimental data and can reduce the scatter of the fatigue life significantly.

Machine Learning Based Prognostics of Fatigue Crack Growth in Notch Pre-cracked Aluminum 7075-T6 Rivet Hole

Constant stress amplitude fatigue tests were conducted on the notch pre-cracked Aluminum 7075-T6 rivet hole dog-bone coupons. Monitoring of visible surface crack length by special surface engraving using digital microscope images and by ultrasonic sensors signals was carried out to yield fatigue crack length measurements in relation to number of fatigue cycles applied. The experimental results provide ultrasonic sensor validation for fatigue crack length measurements. Fracto-graphic examination of failed fatigue surfaces has provided further confirmation of notch pre-crack length, crack initiation process, and crack growth marker bands. These experimental inputs were used in NASGRO and AFGROW software fatigue crack growth simulations. The simulation results did not match the crack initiation fatigue life measured by experiments. However, there was good agreement with crack growth simulations of larger cracks. Hence, we plan to develop a machine learning application that will learn the fatigue crack initiation and crack growth processes from data obtained from our own experiments and other fatigue data available from AFGROW databases. Nonlinear AutoRegressive models with eXogenous input (NARX) artificial neural network were used to predict crack growth longer than 5.0-mm. Particle filtering modeling with Bayesian updating was applied to these experimental data for prognostics of fatigue crack growth. A concept design and preliminary implementation results will be presented.

Fatigue resistance of AL6XN super-austenitic stainless steel welded with electromagnetic interaction of low intensity during GMAW

Plates of AL6XN super-austenitic stainless steel with a single-V groove preparation were gas metal arc welded (GMAW) with and without electromagnetic interaction of low intensity (EMILI) during welding using an ER-NiCrMo3 filler wire and 97% Ar + 3% N2 as shielding gas. The fatigue behavior of the welded joints was evaluated under constant stress amplitude (Δσ/2) between 135 and 170 MPa (R = 0.1) and uniaxial load. The Wohler diagram indicated that for stress amplitude of 170 MPa, 4.19 × 105 and 2.96 × 105 cycles were required for failure without and with EMILI, respectively, whereas for 135, 140, and 145 MPa, 1 × 107 cycles were reached without failure. Welding with EMILI was found to have a positive effect nearby fatigue limit. Observation of the fractures indicates that failures started on the surface of the specimens in the weld metal (WM) due to the stress concentration induced by the abundant presence of precipitates located along the interdendritic spaces in this zone of the welded joint. These particles acted as crack-nucleating agents and then the crack propagated throughout the WM. Fractography revealed brittle fracture associated to cleavage.

A Multi-scale Corrosion Fatigue Damage Model of Aluminum Alloy Considering Multiple Pits and Cracks

A multi-scale model is developed to link the continuum damage variable in macroscale to the number density of multiple pits and cracks in microscale for studying the corrosion fatigue of aluminum alloy from multi-scale viewpoint. The developed model is used to predict the coherent multi-scale corrosion fatigue process of aluminum alloy component in the 3.5 wt% NaCl water solution under constant stress amplitude at a nominal frequency of 5 Hz, and the numerical prediction results are compared with the experimental results. It shows that the model is effective and can be used to study the corrosion fatigue mechanisms of aluminum alloy from both macro- and microscale viewpoints.

Out-of-Phase TMF lifetime calculation of EN-GJS-600 (ASTM 80-55-06) ductile cast iron based on strain increase tests and evaluation of cyclic deformation behavior in isothermal measuring intervals

Components of internal combustion engines not only undergo mechanical high and low-cycle fatigue but also thermally induced loadings during start-stop and load changes. Accordingly, an efficient lifetime calculation method of the ductile cast iron EN-GJS-600 for Out-of-Phase thermomechanical fatigue (OP TMF) loading was developed based on Morrow’s relation between cyclic deformation and fatigue lifetime behavior. As an alternative to established fracture mechanics based approaches, the slope of the Wöhler curve is calculated from data determined in at least one each of a TMF strain increase and TMF constant amplitude test. Based on the Physically Based Lifetime calculation (PhyBaL) method previously published for isothermal low cycle fatigue (LCF) and high cycle fatigue (HCF) loadings, and data taken from only one strain increase and at least one constant amplitude OP TMF test, the Wöhler curve can be described as according well with constant amplitude OP TMF tests performed for validation. In the present work, triangular TMF cycles were applied at 5 mHz (0.005 Hz) with a temperature range from 50 to 350 °C. Cyclic deformation behavior as input data for calculation of the slope of the Wöhler curve was determined by stepwise increases of mechanical strain amplitude in OP TMF tests. By constant amplitude tests at different mechanical strain amplitudes, the lifetime and cyclic deformation behavior at OP TMF loading were determined for validation. Moreover, applicability of measurement cycles at constant temperature and low stress amplitude applied in defined intervals of regular OP TMF cycles proved to match the OP TMF cyclic deformation behavior well. This opens up the prospect of characterizing cyclic deformation behavior in more complex, near service TMF cycles as a baseline for lifetime assessment using the PhyBaL approach outlined above.

Effect of state of carbon on fatigue properties and dislocation structure of Fe-0.017mass%C alloy

Carbon plays an important role in controlling the mechanical properties of steels. In this study, the effect of different states of carbon―solute carbon, fine transgranular cementite, or coarse intergranular cementite―on the fatigue properties of Fe-0.017mass%C alloy was investigated. Transmission electron microscopy was used to examine the dislocation structure. Rigorous X-ray line profile analysis was conducted to evaluate the change in dislocation density, dislocation arrangement, and dislocation character as a function of the state of carbon. The fatigue limit ratio (fatigue strength/ultimate tensile strength) under the constant stress amplitude conditions was the highest for the solute carbon specimen, followed by the transgranular cementite and intergranular cementite specimens. The solute carbon specimen exhibited a prominent vein dislocation structure, whereas the transgranular cementite and intergranular cementite specimens showed tangled and cellular dislocation structures, respectively. X-ray line profile analysis revealed that the vein structure presumably consisted of a bunch of edge dislocations, whereas the cell structure mostly consisted of screw dislocations. The fractured surface of the solute carbon specimen showed large asperity and no clear striations, whereas the transgranular and intergranular cementite specimens revealed clear striations. Moreover, large cracks were observed along the striations, presumably owing to the existence of cementite.

Variation of the uniaxial tensile behavior of ultrafine-grained pure aluminum after cyclic pre-deformation

To explore the influence of cyclic pre-deformation on the mechanical behavior of ultrafine-grained (UFG) materials with a high stacking fault energy (SFE), UFG Al processed by equal-channel angular pressing (ECAP) was selected as a target material and its tensile behavior at different pre-cyclic levels D (D = Ni / Nf, where Ni and Nf are the applied cycles and fatigue life at a constant stress amplitude of 50 MPa, respectively) along with the corresponding microstructures and deformation features were systematically studied. The cyclic pre-deformation treatment on the ECAPed UFG Al led to a decrease in flow stress, and a stress quasi-plateau stage was observed after yielding for all of the different-state UFG Al samples. The yield strength σYS, ultimate tensile strength σUTS, and uniform strain ɛ exhibited a strong dependence on D when D ≤ 20%; however, when D was in the range from 20% to 50%, no obvious change in mechanical properties was observed. The micro-mechanism for the effect of cyclic pre-deformation on the tensile properties of the ECAPed UFG Al was revealed and compared with that of ECAPed UFG Cu through the observations of deformation features and microstructures.

A Micro-Mechanism-Based Continuum Corrosion Fatigue Damage Model for Steels

A micro-mechanism-based corrosion fatigue damage model is developed for studying the high-cycle corrosion fatigue of steel from multi-scale viewpoint. The developed physical corrosion fatigue damage model establishes micro-macro relationships between macroscopic continuum damage evolution and collective evolution behavior of microscopic pits and cracks, which can be used to describe the multi-scale corrosion fatigue process of steel. As a case study, the model is used to predict continuum damage evolution and number density of the corrosion pit and short crack of steel component in 5% NaCl water under constant stress amplitude at 20 kHz, and the numerical results are compared with experimental results. It shows that the model is effective and can be used to evaluate the continuum macroscopic corrosion fatigue damage and study microscopic corrosion fatigue mechanisms of steel.

An experimental investigation of the fatigue damage behaviour of adhesively bonded joints under the combined effect of variable amplitude stress and temperature variation

Under working conditions, adhesively bonded structures are exposed to variable cyclic loading and temperature variation. However, there is still limited number of works dealing with their relationship. The present study investigates the combined effect of variable amplitude stress and variation of temperature on the fatigue damage behaviour of adhesively bonded scarf joints. Initially, experiments at constant amplitude stress, which showed reduction of fatigue strength with increasing temperature, were carried out to establish base SN curves for calculations of cumulative damage. Then, tests under variable amplitude fatigue (VAF) were performed with constant temperature and with two, four and six temperature changes. Temperatures for VAF experiments were −10, 23 and 50 °C. VAF at constant temperature demonstrated: (i) damage retardation at −10 and 23 °C and damage acceleration at 50 °C and (ii) no clear effect on the fracture mode. Conversely, VAF with temperature variation showed: (i) high damage acceleration with an increase proportional to the number of temperature changes and (ii) a trend of more adhesive failure. For lifetime predictions made with a linear damage rule, good correlation was obtained at constant temperature. However, under variable temperature conditions caution must be taken since predictions overestimated fatigue lifetime.

Critical investigation on the influence of welding heat input and welding residual stress on stress intensity factor and fatigue crack propagation

Abstract This paper investigates the influence of the change in welding heat input (HI) and welding residual stress (RS) on the behavior of stress intensity factor (SIF) and fatigue crack propagation (FCP) by means of finite element method. Three welding HI cases were examined using a bead-on-plate FE model. One of the applied welding HI cases represents welding conditions used in experiments. Furthermore, for the sake of comparison, a stress-relieved case that does not consider the influence of welding RS was employed. Experimental data used in the analyses and verification were taken from a Ph.D. dissertation conducted by Kusuba ( http://hdl.handle.net/10069/16925 ). A thermal-elastic-plastic analysis was firstly conducted to simulate welding process and to predict welding RS. A powerful fracture analysis technique named WARP3D interaction integral method that provides crack face traction integral was employed. WARP3D is an open research code for nonlinear FEA of large-scale, 3-D solids and structures subjected to static and dynamic loads. The employed technique was used to precisely calculate SIF with considering the influence of welding RS under different constant stress amplitude loadings. The results revealed that the ranges of welding RS induced by the simulated welding HI cases have a negligible influence on the behavior of SIF and FCP, whilst when increasing the magnitude of those welding RS a larger influence can be observed. The influence of welding RS induced by the simulated welding HI cases on FCP was manifested by comparing FCP obtained by the cases that consider the effect of welding RS with that given by the stress-relieved case. A large reduction of fatigue lives was obtained when welding RS is considered compared with that obtained by the stress-relieved case. A reduction percentage of 35.9–40.7% in fatigue lives was obtained when welding RS is considered compared with that calculated by the stress-relieved case for the applied stress loading conditions. The calculated fatigue lives were verified with experimental data taken from the literature. Facture behaviors of the studied cases were discussed, as well. Furthermore, a basic study was conducted to examine the influence of the change in the magnitude of RS on FCP behavior.

Prestraining Induced Enhancement in the Fatigue Limit Obtained by Load Increasing Thermal Method for Metastable Austenitic Stainless Steel

In the present work, the authors evaluate the fatigue limit of as received 1.4301 (18–10) austenitic stainless steel and its variation due to uniaxial prestraining. A short time load increasing thermal method is adopted to overcome the problems (time consuming and high material cost) involved in conventional S–N curve derivation, particularly in case of high cycle fatigue. Different extent of tensile prestrain imparts different magnitude of pre‐existing martensite to specimens before the start of fatigue test. It is observed that increased amplitude of fatigue limit is proportional to the amount of prestrain. The applicability of the employed fast technique is demonstrated by validating the obtained fatigue limit against classical constant stress amplitude test results for each amount of prestrain.

Effect of UV Ageing on the fatigue life of bulk polyethylene

Polymers operating in various weathering conditions must be assessed for lifetime performance. Particularly, ultraviolet (UV) radiations alters the chemical structure and therefore affect the mechanical and fatigue properties. The UV irradiation alters the polymer chemical structure, which results into a degradation of the mechanical and fatigue behavior of the polymer. The polymer properties degradation due to UV irradiation is the result of a competitive process of chain scission versus post-crosslinking. Although few studied investigated the effect of UV irradiation on the mechanical behaviour of thermoplastics, fewer examined the UV irradiation effect on the fatigue life of polymers. This study focuses on investigating the effect of UV irradiation on the fatigue properties of bulk semi-crystalline polymer; the low density Polyethylene (LDPE). Tensile specimens were exposed to different dose values of UV irradiation then subjected to fatigue loading. The fatigue tests were achieved under constant stress amplitude at a frequency of 1Hz. The results show an important decrease of the fatigue limit with increasing absorbed UV irradiation dose.

Influence of crack initiation on short crack propagation and cyclic lifetime of AA 7475-T761

The propagation of short cracks covers the main part of the lifetime of cyclic loaded components. Thus, investigations in this field are necessary to enable reliable lifetime predictions. To investigate the influence of different crack initiation scenarios on the cyclic lifetime, SEN-specimens were fatigued under constant stress amplitude. For additional support of the crack initiation in some specimens a single overload was introduced after 200 cycles, in others the crack initiation was forced by laser cuts in the notch root. In all experiments, the crack length was measured with a high-resolution DC-potential drop method. This investigation approves and explains the relationship between the cyclic lifetime and the presence of steps on the fracture surface, as a reason of a retarded crack coagulation. The results clearly indicate that the scatter of the cyclic lifetime is determined by the differences in the crack initiation scenarios. Further on, a significant influence of a single overload on the cyclic lifetime can be verified.

Study of load history effects on the high cycle fatigue properties of high-strength low-alloy steel from self-heating measurements

In the context of high cycle fatigue (HCF), the experimental characterization of the fatigue properties is often performed by using specimens in a virgin state (i.e., without preliminary loading), and with a constant stress amplitude for each specimen. However, the load history applied to a real structure is more complex and the fatigue life prediction remains a difficult task because of the time dedicated to the classical fatigue tests (i.e., the specimen is loaded until failure) and the dispersion of fatigue lives. The load history effects on the HCF properties is characterized using an alternative method: self-heating measurements under cyclic loadings. This method is based on the observation of the mean steady state temperature evolution of a specimen under a successive series of cyclic loadings with increasing stress amplitude for each loading series. A probabilistic two-scale model was developed from the self-heating method able to predict HCF properties. Some self-heating tests are performed to study the influence of a load history effects. It seems that the plasticity is the most influential factor. So, the evolution of the plasticity is observed at the surface of the material under cyclic loading. There is a significant evolution in function of the plastic pre-strain.

Microscale High-Cycle Fatigue Testing by Dynamic Micropillar Compression Using Continuous Stiffness Measurement

The continuous stiffness method (CSM) is used for fatigue testing FIB-fabricated micropillars up to the high cycle fatigue (HCF) range on a commercial nanoindentation platform. The frequency of ~ 40 Hz allows reaching several million (106) cycles within hours under constant nominal stress amplitude testing conditions. It is shown that both the evolution of the strain and plastic strain amplitudes can be evaluated from the harmonic measurements provided by the lock-in amplifier. Defining a custom threshold on strain amplitude decrease as failure criterion further enables the determination of S-N curves. The new method was validated by tests on ECAP copper, which is shown to exhibit the same cyclic softening during micropillar testing as observed macroscopically. The calculated plastic strain amplitude is also well in line with literature data. Generally, the new method has a great potential for studying the local cyclic behavior of individual layers and phases of complex systems and would also be of great advantage for studying local fatigue effects at interfaces.

Thickness‐Dependent Tensile and Fatigue Behavior of A Single‐Slip‐Oriented Cu Single Crystal

AbstractTo explore the micro‐mechanism for the size effect of the mechanical behavior of metallic crystals, the tensile and fatigue behavior of Cu single crystal with t of 0.1−2.0 mm is investigated. The results show that with the reduction of t, an obvious increase in σYS and a slight decrease in σUTS take place; meanwhile, the δ evidently reduces, especially as t < 0.6 mm, while the Nf first sharply increases and then decreases at a constant stress amplitude of 80 MPa. The activated slip system reduces with t under tensile loading, and the fracture modes are transferred from ductile to slip separation rupture, whereas under cyclic loading, slip separation rupture becomes dominant especially at t = 2.0 and 0.1 mm. Correspondingly, the tensile microstructures are transformed from the cell‐walls to dislocation cells, while dislocation cells are the dominant microstructure in fatigued specimens, and the dislocation density obviously decreases with decreasing t. In a word, a strong dependence of tensile and fatigue behavior on t is exhibited for the Cu single crystal.