Fatigue Life Of Components Research Articles

A Novel Approach for Real-Time Quality Monitoring in Machining of Aerospace Alloy through Acoustic Emission Signal Transformation for DNN

Gamma titanium aluminide (γ-TiAl) is considered a high-performance, low-density replacement for nickel-based superalloys in the aerospace industry due to its high specific strength, which is retained at temperatures above 800 °C. However, low damage tolerance, i.e., brittle material behavior with a propensity to rapid crack propagation, has limited the application of γ-TiAl. Any cracks introduced during manufacturing would dramatically lower the useful (fatigue) life of γ-TiAl components, making the workpiece surface’s quality from finish machining a critical component to product quality and performance. To address this issue and enable more widespread use of γ-TiAl, this research aims to develop a real-time non-destructive evaluation (NDE) quality monitoring technique based on acoustic emission (AE) signals, wavelet transform, and deep neural networks (DNN). Previous efforts have opted for traditional approaches to AE signal analysis, using statistical feature extraction and classification, which face challenges such as the extraction of good/relevant features and low classification accuracy. Hence, this work proposes a novel AI-enabled method that uses a convolutional neural network (CNN) to extract rich and relevant features from a two-dimensional image representation of 1D time-domain AE signals (known as scalograms), subsequently classifying the AE signature based on pedigreed experimental data and finally predicting the process-induced surface quality. The results of the present work show good classification accuracy of 80.83% using scalogram images, in-situ experimental data, and a VGG-19 pre-trained neural network, establishing the significant potential for real-time quality monitoring in manufacturing processes.

Vibration Fatigue of FDM 3D Printed Structures: The Use of Frequency Domain Approach.

Additive manufactured structures are replacing the corresponding ones realized with classical manufacturing technique. As for metallic structures, 3D printed components are generally subjected to dynamic loading conditions which can lead to fatigue failure. In this context, it is useful, and sometimes mandatory, to determine the fatigue life of such components through numerical simulation. The methods currently available in literature for the estimation of fatigue life were originally developed for metallic structures and, therefore, it is now necessary to verify their applicability also for components fabricated with different materials. To this end, in the current activity three of the most used spectral methods for the estimation of fatigue life were used to determine the fatigue life of a 3D printed Y-shaped specimen realized in polylactic acid subjected to random loads with the aim of determining their adaptability also for this kind of materials. To certify the accuracy of the numerical prediction, a set of experimental tests were conducted in order to obtain the real fatigue life of the component and to compare the experimental results with those numerically obtained. The obtained outcomes showed there is an excellent match between the numerical and the experimental data, thus certifying the possibility of using the investigated spectral methods to predict the fatigue life of additive manufactured components.

Autofrettage of high-pressure components made of ultra-high-strength-steel

This work is primarily concerned with the fatigue life of high-pressure bearing components with intersecting holes, typically used in Diesel engine fuel injection systems. The investigation focuses on specimens with orthogonally intersecting holes that have undergone the process of Autofrettage (single mechanical overload), which is typically used to extend the fatigue life of components loaded by cyclic internal pressure. The Autofrettage process induces advantageous, life-time prolonging residual compressive stresses in the highly stressed areas of the components. The resulting residual stress distribution thus influences the fatigue failure and especially the crack propagation behaviour of the components.In previous works, fracture mechanics based approaches were used to describe the crack propagation behaviour for autofrettaged specimens made of the quenched and tempered steel 42CrMo4. Results showed that crack arrest has to be taken into account when calculating fatigue lives of autofrettaged specimens as the endurance limit is otherwise underestimated. As efforts are made to increase the injection pressures of fuel injection systems, in this work, the benefit of using ultra high strength steel for the application described is investigated. In order to achieve reliable results, material testing with samples made of the ultra-high-strength steel W360 was performed. The resulting test data were used to describe the initial loading and cyclic loading behaviour of the material with a suitable material model. Finite element analysis was then performed to simulate the Autofrettage process and subsequent cyclic loading. Based on the simulation results, possible crack initiation was determined. For predicted crack initiation, the simulated residual stress distribution was used to investigate the crack propagation behaviour with fracture mechanics based approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison to the test results showed an overestimation of the predicted fatigue lives. The modelled material behaviour and consequently the residual stress distribution from the simulation models was identified as the decisive factor for the deviation. Still, the comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably. Further investigation regarding the modelling of the material behaviour with focus on the Autofrettage process is still required.

Analysis on Stop-hole Parameters for Fatigue Cracks at Arc Notch in Steel Bridge Deck

Two types of fatigue cracks at arc notch in steel bridge deck were repaired by drilling stop-holes. The effect of stop-holes with different diameters and positions was considered. Based on finite element models, the variation laws of stress distribution and the effects of stress concentration were compared for different stop-hole diameters and positions. Analysis results indicated that stop-hole can effectively improve the stress concentration at crack tip and the fatigue life of components can be considerably increased. The crack-stopping performance enhances with the increase of stop-hole diameter, but large stop-hole cannot effectively retard crack growth. The stop-hole performs well with the location at -0.5D∼0.5D. The maximum stress point still appears at crack tip when the stop-hole is outside or inside the crack. The stop-hole diameter has no effect on the stop-hole location.

Numerical modeling and experimental verification of residual stress distribution evolution of 12Cr2Ni4A steel generated by shot peening

Shot peening is a widely used surface strengthening technique which can improve the fatigue life of metal components by introducing reasonably distributed compressive residual stress. The accurate prediction of residual stress distribution of parts is a tough challenge in the simulation of shot peening. In this paper, the numerical calculation of the residual stress of shot peening is investigated with 12Cr2Ni4A steel as the target. A two-dimensional Gaussian distribution model is proposed to calculate the number of impact shots. A method is proposed to calculate shot peening coverage using displacement and displacement gradient, which increases the identification accuracy of peened surface topography and provides a scientific method for coverage calculation. A random multi-shot finite element model for high strength steel is established considering the elastoplastic of shot. The effects of the process parameters, initial surface morphology and initial residual stress on the residual stress distribution after shot peening are investigated. The accuracy of the proposed method is verified by comparing the measured and simulated results. • A two-dimensional Gaussian distribution model is proposed to calculate the number of impact shots. • An improved model for predicting the residual stress distribution in shot peening is developed. • A method to calculate the shot peening coverage using displacement and displacement gradient is proposed. • The effects of treatment parameters and initial state on residual stress are studied. • The experiment is used to verify the proposed model.

A quantitative assessment of the impact of corrosion on fatigue life of aircraft components

Management of corrosion, and repair and restoration of corrosion‐affected structures pose a daunting challenge to the aircraft industry. This paper presents analytical studies to examine and quantify the effects of corrosion of Al-alloys on the fatigue life of aircraft structures. Two scenarios are considered: (i) fatigue of a pre-corroded material and (ii) fatigue in a corrosive environment. The corrosion damage is idealized using standard crack models, and cycle-by-cycle fatigue crack growth analysis is carried out for an aircraft horizontal stabilizer under spectrum loading conditions. A few aircraft operational scenarios are considered, and fatigue life calculations presented. The zone which experiences 18% lower stress than the maximum stress zone is shown to turn fatigue critical if vulnerable to corrosion. The results point to the risk posed by corroded zones if missed while focusing attention on high stress zones. The computations indicate that an exposure of even 25% of operational life to corrosive environment can reduce life by 40–55%. The study quantifies the significance of corrosion while arriving at design safe life, remaining useful life and maintenance intervals.

Investigations of the effect of heat pipes on the temperature reduction and fatigue life of the components in the turning of high carbon high chromium steels

In the present work the twisted Mild steel pipes of lesser and little higher twists are employed for the passive cooling purposes to reduce the temperature at the chip tool interface. It is found that, the pipe of higher twist has resulted in the greater reduction of temperature as compared to that of the lower twist. Further in this work, the twisted mild steel heat pipes of diameters 4 mm, 6 mm, 8 mm and 10 mm were accommodated in HSS carbide tipped tool and the turning of HCHCR work pieces were carried out on a conventional lathe. It is noticed that, the temperature near the cutting zone reduced with the increase in the diameter of the heat pipe. In this work, the effect of diameter of the heat pipe on the fatigue life of the machined parts is also studied. The results shows that, the increase in the diameter of the heat pipe increases the fatigue life of the machined component. Finally in this work, the effect of mass flow rate of the cutting fluid on the fatigue life of the turned HCHCR component using a heat pipe of diameter 6 mm is studied. It is observed that, the increase in the mass flow rate increases the fatigue life of the machined HCHCR component.

Design against Fatigue of Super Duplex Stainless Steel Structures Fabricated by Wire Arc Additive Manufacturing Process

Additive manufacturing (AM) is increasingly used to make complex components for a wide spectrum of applications in engineering, medicine and dentistry. Wire arc additive manufacturing (WAAM), as one of AM processes, utilises electric arc and metal wire to fabricate fully dense and heavy metal parts at relatively low costs and high-energy efficiencies. WAAM was successfully applied in the production of several welding-based metal structures. Recently, there was a growing interest in WAAM processing of super duplex stainless steels (SDSS) due to their high strength and excellent corrosion resistance, which make them the prime choice for load-bearing structures in marine applications. Although a number of studies investigated the microstructural and mechanical properties of WAAM-processed SDSS components, little is known regarding their fatigue performance, which is critical in engineering design. This study reports on the outcomes of fatigue tests and fracture surface fractography of WAAM-processed SDSS. The results obtained indicate a significant anisotropy of fatigue properties and fatigue crack initiations resulting from internal defects rather than surface flaws. Based on these experimental results, we suggest an effective design methodology to improve the fatigue life of the WAAM-fabricated SDSS components. We also indicate that post-manufacturing surface treatments should not be underlined for the enhanced fatigue resistance of WAAM-processed SDSS structures.

A FCG Model and the Graphical User Interface Under Matlab for Predicting Fatigue Life: Parametric Studies

Abstract The focus of this research work was predicting the fatigue life of mechanical components used for industrial and transport systems. To understand how the phenomenon of fatigue occurs in a material, the fatigue crack growth is studied. The purpose of this work was to create a graphical user interface (GUI) under Matlab to allow researchers to conduct the parametric studies of fatigue crack propagation to predict fatigue life. In this work, three models for fatigue crack propagation were used: those of Paris, Walker and Forman in order to study the three parameters: the Paris exponent m, load ratio R and hardness K IC , respectively. In addition, a novel model FCG was developed to study the influence of the hardening parameters (K′, n′) on fatigue crack propagation. The comparison of the simulation results with those in the literature shows good agreement.

Set-up of a Generalized Dataset for Crack-Closure-Mechanisms of Cast Steel

In components, crack propagation is subjected to crack-closure-mechanisms which affect the build-up of the relevant threshold stress intensity factor range during cyclic loading. As structural parts are exposed to service loads incorporating a variety of load ratios, a significant change of the long-crack threshold value occurs, leading to a severe stress ratio dependency of crack-closure-mechanisms. Thus, an extensive number of crack propagation experiments is required to gain statistically proven fracture mechanical parameters describing the build-up of closure effects as crack growth resistance curves.The article presents a generalized dataset to assess the formation of crack-closure-mechanisms of cast steel G21Mn5+N. Numerous crack propagation experiments utilizing single edge notched bending (SENB) sample geometries are conducted, incorporating alternate to tumescent stress ratios. The statistically derived, generalized crack growth resistance curve features the impact of closure effects on the crack propagation rate in a uniform manner. To extend the dataset to arbitrary load ratios, the long-crack threshold approach according to Newman is invoked. The generalized dataset for the cast steel G21Mn5+N is validated by analytical fracture mechanical calculations for the utilized SENB-sample geometries. Incorporating a modified NASGRO equation, a sound correlation of analytical and experimental crack propagation rates is observed. Moreover, the derived master crack propagation resistance curve is implemented as a user-defined script into a numerical crack growth calculation tool and supports a local, node--based numerical crack propagation study as demonstrated for a representative SENB-sample. Concluding, the derived dataset facilitates the calculation of fatigue life of crack-affected cast steel components subjected to arbitrary stress ratios.

Prediction of residual stress fields after shot-peening of TRIP780 steel with second-order and artificial neural network models based on multi-impact finite element simulations

Shot-peening is a mechanical surface treatment widely employed to enhance the fatigue life of metallic components by generating compressive residual stress fields below the surface. These fields are mainly impacted by the selection of the process parameters. The aim of this work is to propose a hybrid approach to conduct two predictive models: second-order model and feed-forward artificial neural network model. For this purpose, a 3D multiple-impact finite element model coupled to a central composite design of experiments was employed. A parametric analysis was also conducted to investigate the effect of the shot diameter, the shot velocity, the coverage, and the impact angle on the induced residual stress profile within a TRIP780 steel. It was found that both models predict with good agreement, the residual stress profile as a function of the process parameters and can be used in shot-peening optimization due to their responsiveness.

Evaluation method of fatigue life for weld joints with defects in the horizontal restraint components of main steam pipelines

Horizontal restraint components are mainly used in nuclear power plants to limit the horizontal displacement of the “super pipelines”, such as the main steam pipelines and main water supply pipelines. Horizontal restraint components are very important for ensuring the safety of the second loop of nuclear power plants under unexpected conditions. However, because the size of the horizontal restraint components are quite large, and the pipelines are complicated, defects or cracks easily occur in the weld joints connected with the components and pipelines. The fatigue crack growth in the weld joints is one of the important reasons of the fatigue failure for the horizontal restraint components. Therefore, investigating the fatigue crack growth of the weld joints is significant to accurately predict the service life of the horizontal restraint components. In this paper, multiple cracks were found in the weld joints of horizontal restraint components in a nuclear power plant. In order to evaluate the fatigue life of the weld joints, fatigue crack growth tests were carried out under different temperature. The material parameters, C and n, of the Paris Equation for the weld joints were obtained and compared with those for the base metal (ferrite steel) in the ASME code. Results showed that the results show that the fatigue crack growth rate of the weld joints in the room temperature is lower first and then higher than that of the material in the high temperature weld joints and the base metal at room temperature, but the fatigue crack growth rate of the weld joints in the high temperature is always higher than that of the base metal at room temperature. The fatigue life of the horizontal restraint components calculated by the Paris Equation from the experiments is quite different compared with the fatigue life calculated from the base metal (ferrite steel) in the ASME code at room temperature.

3D numerical modelling of turning-induced residual stresses – A two-scale approach based on equivalent thermo-mechanical loadings

Nowadays, fatigue life of engineering components is a major topic. In the literature, extensive works have been reported in order to assess the machining-induced surface integrity which is known to be a first-order parameter for the fatigue resistance. This paper lies in the framework of turning-induced residual stress prediction by presenting an up-dated method based on a two-scale approach. Previous works aimed at building a 3D hybrid method consist of modelling the turning consequences through equivalent thermo-mechanical loadings combining a finite element model with experimental tests. The new proposed global hybrid strategy keeps the same concept but has the advantage of defining the thermo-mechanical shapes more accurately thanks to local 2D orthogonal cutting models. Moreover, the calibration of the equivalent thermo-mechanical loadings is easier as only turning forces have to be measured, which makes it more accessible to end-users. Finally, the model provides results considering the effective machined surface topography instead of considering a basic flat surface. After carefully detailing each step of the numerical method, a validation case study concerning a longitudinal turning operation on a 15-5PH martensitic stainless steel is proposed. By comparing numerical results with experimental residual stresses coming from X-Ray diffraction, the new numerical method shows its superiority to predict, within few hours, much more accuratly the residual stress state induced by a real longitudinal turning operation.

A novel optimised nacelle to alleviate wind turbine unsteady loads

A comparative study of the effectiveness of an innovative optimised nacelle on fatigue loads alleviation and performance improvement in both upwind and downwind configurations has been conducted. The idea is to optimise wind turbine nacelles using flaps to push the flow towards outer spans, which leads to improved turbine performance, as well as mitigates unsteady loads, therefore, extending the fatigue life of wind turbine's components. Theoretical explanations indicate that in the case of upwind configuration, increasing downstream velocity leads to a reduction in turbine power. However, lower wind speeds seen by inner spans lead to mitigate thrust load generated by inner spans. In the case of downwind configuration, increasing upstream velocity leads to improve turbine power based on the turbine Euler equation, and pushing flow towards outer spans with higher lift/drag ratios leads to mitigate unsteady loads generated by inner spans. Experimental results show that using flaps to optimise the nacelle shape not only mitigates unsteady loads and therefore extends the fatigue lifetime of blades but also improves turbine power and yaw stability. Furthermore, using flaps in downwind configuration can compensate for the adverse effect of wind turbine yaw misalignment on the wind turbine performance and unsteady loads.

Concentrated Chloride Effects on Chromate Inhibition of Al Alloy-316 Stainless Steel Galvanic Couple-Induced Corrosion

Concentrated Chloride Effects on Chromate Inhibition of Al Alloy-316 Stainless Steel Galvanic Couple-Induced Corrosion

Surface integrity evolution of a Ni-based single crystal superalloy by laser shock peening

The single crystal superalloy SRR99 experimental samples were single point treated by laser shock peening (LSP) with laser pulse energy of 5–7 J. The surface morphology, micro-hardness, surface residual stress and surface microstructure of samples prior and after LSP were determined by corresponding characterization instrument. From the surface morphology, a circular pit was formed in surface, which due to the severe plastic deformation induced by LSP, and the diameter & maximum relative height of circular pit were increase with laser pulse energy. From the micro-hardness and residual stress test results of samples, these were increased after LSP treatment. In detail, the initial microhardness and residual stress were 398 HV and -498 MPa, with laser pulse energy of 5 J, the microhardness and residual stress were increased to 455 HV and -843 MPa, when the laser pulse energy was increased to 7 J, the microhardness and residual stress were increased to 512 HV and -942 MPa. In addition, the surface roughness of samples was increased too, which reflect the severe plastic deformation. By observing the microscopic morphology of the bottom of the pit, it is found that there are convex structure and crater shape defect structures on the bottom surface, and the formation of these structures may affect the surface roughness of the shock area. Although LSP caused certain degree of twist deformation of the shape of the γ' phase, the original strengthening method characteristics of the matrix phase and strengthening phase was not destroyed, which still maintained the creep strength and thermal fatigue properties of superalloy. The experiment show that LSP can cause significant work hardening effect on the surface of superalloy, and the generated high residual compressive stress can effectively delay the initiation and propagation of cracks. Therefore, LSP technology can effectively improve the fatigue life of single crystal blade components.

Assessing the magnetic flux leakage contraction parameters for the fatigue life prediction of SAE1045 steel specimens

The aim of this paper is to assess the stress concentration zone (SCZ) of a circular smooth specimen at an early stage. Following the detection of the stress concentration zone, the specimen residual life was calculated and compared to the experimental result. Uniaxial loading on the servo-hydraulic testing machine was used for the laboratory testing. The stress concentration tester was used to detect the SCZ on the specimen, based on spontaneous surface micro magnetic signals on the ferromagnetic surface. In addition, cyclic testing for fatigue characteristics was conducted at 70% and 75% of ultimate tensile stress (UTS). The results indicated that the estimated residual life for the 70% UTS was nearly identical to the experimental value, based on two magnetic parameters, i.e, Kmax and the contraction value. Meanwhile, the estimated residual life for the 75% UTS was equivalent to the experimental findings. The minimum and maximum error values for life estimation using both parameters were 0.47% and 24.35%, respectively, for the contraction value. The correlation value, R2, between the estimated life using MMM and experimental life was 0.95, showing good estimation. All the estimated life data using this method was distributed within the range of a 95% to 90% confidence interval. Hence, the proposed method can be used to predict the fatigue life of ferromagnetic components through magnetic flux leakage signals, especially in the stress concentration region.

A numerical and experimental investigation on the evolution of three-dimensional surface topography of 12Cr2Ni4A steel in shot peening

Shot peening is widely used as a surface strengthening technology as it can enhance fatigue life of mechanical components by introducing reasonably distributed compressive residual stress. However, improper selection of processing parameters would inevitably lead to deterioration of the surface quality, and negatively affect on the fatigue performance. In this paper, a random multiple-shot model was proposed to accurately calculate the evolution of 3D surface topography 12Cr2Ni4A steels during shot peening. The simulated results are in good agreement with the experimental results, with an error of less than 10%. It is found that surface roughness parameter Sa first increases significantly and then decreases slowly with the increase of peening time. Moreover, the surface height after shot peening obeys a normal distribution and the 3D surface roughness is positively correlated with the initial roughness.

Novel Fatigue-Damage Sensor for Prediction of Remaining Fatigue Lifetime of Mechanical Components and Structures

In this paper, a novel Internet of Things (IoT) fatigue damage sensor is presented to predict the fatigue damage accumulation level and residual fatigue life of critical mechanical and structural components. The intelligent radio-frequency identification (RFID)-integrated fatigue sensor system has special designed breakable mini beams with multiple parallel oriented geometry and symmetric U-type stress concentrated notches. The fatigue sensing beams with different stress and fatigue lifetime levels are designed to estimate the fatigue damage accumulation and remaining fatigue life of unidirectional and multidirectional structural or mechanical elements, including composite structures. The fatigue damage sensor attached on a real structure goes through the same fatigue life experience of critical structural elements or mechanical components from the beginning of service life or any point in the service life to the end of service life. A fatigue sensor with five parallel symmetric U notched mini beams with different fatigue lifetimes (10% N, 25% N, 50% N, 70% N, 85%–90% N) normalized to the total life (N) of a structure are considered to predict the remaining fatigue lifetime of a real structure using the cumulative damage Miner’s rule. The mini fatigue sensor beams attached to the surface of a structure are designed to fail, depending on their fatigue life, earlier than the total fatigue lifetime of the structure, like a fatigue fuse system. The fatigue damage sensing beams fail gradually while passing through the same fatigue cycles as a structural member. When the service lifetime of a real monitored structure is very close to the total fatigue life, the critical maximum lifetime (80%–90% N) notched mini sensor beam breaks and issues an alarm that the service life of the real structural part is approaching its fatigue strength limit and the end of its service life. The smart fatigue damage sensor is wireless-enabled, which makes it possible to check the health status of the sensor over a sensor network as often as one likes. Since the wireless distributed fatigue sensor network system monitors the fatigue health conditions of structures periodically or on demand, the data collected on the sensor network system can be used not only for condition-based fatigue life prediction but also for sensor-based predictive fatigue maintenance and the development of new fatigue design tools for fatigue-sensitive complex and large engineering structures or mechanical systems.

Determining the endurance limit of AISI 4340 steels in terms of different statistical approaches

In engineering applications, fatigue phenomenon is a key issue and needs to be analyzed in the beginning of design phase in case of any component exposed to alternating loading on operation otherwise catastrophic fatigue failure may cause. Component can be designed with safe-life, fail-safe, and damage tolerant approach based on whether redundant load path and damage sensitive. Before starting analyzing the structure, material allowable data needs to be presented in a reliable way to predict fatigue life of components. SN curves with presented confidence levels are the robust approach to make a prediction on safe life of a structure in terms of fatigue. In this point, there are so many approaches to determine fatigue limit of materials and issue shall be handled by statistical manner. In literature, different staircase and curve fitting methods were presented to estimate endurance limit of materials and some reliability manuscript published. In this paper, fatigue limit of AISI 4340 steel will be investigated through most convinced staircase and curve fitting approaches and their reliability will be queried.