Fatigue Life Of Structural Components Research Articles

Predicting the fatigue life of T800 carbon fiber composite structural component based on fatigue experiments of unidirectional laminates

This study is based on the fatigue experiment results of unidirectional laminates and investigates the fatigue performance of T800 carbon fiber/epoxy resin composite structural components through experimental and numerical analysis. Fatigue experiments were performed on [0]16, [90]16, and [±45]8 laminates individually, obtaining the fundamental fatigue parameters necessary for modeling. The fatigue life model for T800 carbon fiber/epoxy resin unidirectional laminates was refined, and fatigue degradation rules were provided for the fatigue progressive damage model. A fatigue progressive damage analysis model for T800 laminates was established based on the 3D Hashin criterion, predicting the fatigue life and fatigue damage failure process of the laminates. A fatigue life prediction model was developed for T800 carbon fiber composite I-beam structural components, which can predict the primary types of fatigue failure, fatigue life and the location of fatigue failure occurrence. The predicted fatigue life of structural components shows good consistency with the experimental results. This method can calculate structural components including ply angles of 0°, 45°, and 90° and obtain the fatigue life contour.

Equivalent characterization of initial crack size of blunt notched components based on test data and physical model

The precise definition of initial crack size applicable to fracture mechanics is crucial for predicting the total fatigue life of structural components. Given that the demarcation between crack formation and crack propagation depends not only on the material but also on the stress level, theoretical and experimental procedures for defining the crack boundary point are developed. Underlying the procedure is that the designed notched members have the same local stress distribution as the structure. Consequently, the fatigue life and initial crack size of S355J2 steel notched components with different scales but similar stress fields were predicted and compared with experimental data. Results show that fatigue life predictions fall within ±2 factor band, and the crack initiation size obtained by AM is closest to the experimental results, with a difference of less than 30.2%, which proves that the approach of treating the initial crack size as a function related to fatigue life and fatigue notch factor is reasonable.

Digital twin-driven framework for fatigue life prediction of welded structures considering residual stress

The welding of steel generates substantial welding residual stress (WRS), which exerts a significant impact on the fatigue life of steel bridges. In this study, a physical model for calculating the fatigue crack growth (FCG) life of welded specimens in the WRS field is established based on the weight function method. Experimental data validates the reliability and precision of the proposed physical model. The impact of WRS on the fatigue life of structural components is scrutinized and analyzed. On this basis, a digital twin (DT) framework driven by a physical-data model is proposed to consider the inherent parameter uncertainty in FCG behavior within the WRS domain. A dynamic Bayesian network (DBN) is used to characterize the evolution characteristics of fatigue crack states over time in the digital space. Particle filter algorithm is used as DBN inference method. The results show that the calculation of the physical model is in good agreement with the experimental values. Neglecting the influence of WRS distribution may lead to an overestimation of fatigue life in welded structures. The DT framework can update uncertain parameters online and realize the accurate prediction of the FCG life in the WRS field.

Improvement of the Fatigue Resistance of Super Duplex Stainless-Steel (SDSS) Components Fabricated by Wire Arc Additive Manufacturing (WAAM)

This study aimed to improve the overall fatigue properties of WAAM-produced SDSS by changing the interpass temperatures. Micro-computed tomography was used to quantitatively characterise the internal defects, such as porosity, in large-volume WAAM-fabricated SDSS materials. An increase in the interpass temperature led to a reduction in the ferrite phase balance by up to 20%. The fatigue anisotropy was still evident, but the fatigue limit in the weakest (transverse) direction was increased to 250 MPa or by approximately 40%. Meanwhile, the increased interpass temperature had no significant effect on fatigue resistance in the longitudinal direction. This study suggests that the interpass temperature can be critical for both achieving isotropic mechanical properties and increasing fatigue life of structural components fabricated with the WAAM method.

Management of Airframe In-Service Pitting Corrosion by Decoupling Fatigue and Environment

Corrosion-induced maintenance is a significant cost driver and availability degrader for aircraft structures. Although well-established analyses enable assessing the corrosion impact on structural integrity, this is not the case for fatigue nucleation and crack growth. This forces fleet managers to directly address detected corrosion to maintain flight safety. Corrosion damage occurs despite protection systems, which inevitably degrade. In particular, pitting corrosion is a common potential source of fatigue. Corrosion pits are discontinuities whose metrics can be used to predict the impact on the fatigue lives of structural components. However, a damage tolerance (DT) approach would be more useful and flexible. A potential hindrance to DT has been the assumption that corrosion-induced fatigue nucleation transitions to corrosion fatigue, about which little is known for service environments. Fortunately, several sources indicate that corrosion fatigue is rare for aircraft, and corrosion is largely confined to ground situations because aircraft generally fly at altitudes with low temperature and humidity Thus, it is reasonable to propose the decoupling of corrosion from the in-flight dynamic (fatigue) loading. This paper presents information to support this proposition, and provides an example of how a DT approach can allow deferring corrosion maintenance to a more opportune time.

Computational analysis of a surface corner crack under cyclic loading

The deterioration due to surface cracks-like flaws is an important phenomenon for the safety point of view. Thus, the main aim of the present research work is to investigate the plate failure caused by a quarter-elliptical corner crack under cyclic loading. A developed analytical algorithm integrates fracture-mechanics-based formulae, and allows to relate the crack growth rate together with the stress intensity at the crack tip and to estimate the fatigue life of structural components. Experimentally verified outcomes show that the proposed computational algorithm represents a powerful tool in the fatigue failure design.

Estimation of Minimum Sonic Fatigue Life of Steel Beam and Plate Structures

Abstract Structures such as those of engine test cells or wind tunnels may be exposed to intense sound for extended periods and should be designed to ensure they are not prone to premature fatigue failure. This paper presents an approach to determining a lower bound to the fatigue life of structural components exposed to complex sound fields, enabling relatively simple evaluation of candidate structural designs. The presented approach is based on established characterizations of the fatigue behaviors of typical steels and relies on classical analytical results that relate the greatest stresses induced in simple structural elements to oscillatory pressure distributions acting on these elements. Practical application of this approach is discussed, as well as the limitations of the approach.

Designing and modeling U-notch fatigue sensor to predict the fatigue life of structural components

Fatigue sensors are used as Structural Health Monitoring (SHM) mechanisms to detect damage on structural components before any failures occur. In this study, a U-notched fatigue sensor was designed and analyzed to predict the fatigue life before catastrophic failure occurs. The ANSYS workbench software and 7075-T6 aluminum alloy were used to model and analyze our U-notch type fatigue sensor. The sensor model consisted of four beams of 305 × 76.2 × 1.6 mm (length × width × thickness) dimensions with different fatigue U-notch radii. It was observed that fatigue life increased with an increase in the U-notch radius. However, the value of the maximum equivalent stress decreased with an increase in the notch radius. In addition, fatigue damage occurred around the U-notch radius when the U-notch radius was equal to or less than 6.4 mm.

Stress Intensity Factor Calculations For Semi-Elliptical Cracked Joints Using Finite Element AnalysisIn 3D

Safety assessment tools are essential for components like pressure vessels and common pipes to minimize the risks during the service of components. All the real or practical materials contain defects or certain uncertainties and understanding the influence of these defects on strength or degradation in properties is necessary to predict the safe life of the components. In this work, material joining defects are considered as a crack according to their shapes and analyse how the crack initiated for the cyclic load conditions and how long the crack will propagate under fatigue loading conditions before final fracture (crack growth rate) was determined using Finite Element Analysis (FEA) which helps to determine the safe service life of the cracked surface components. StressIntensity Factor (SIF) at the crack tips are most important parameter for estimating fatigue life of components. Stress intensity factors values arepredicted from the parameters like CTOD, G, J-integral, and K. In this paper, J-integral approach is used to predict the Stress Intensity Factor values experimentally and the same values are compared with numerically obtained values for semi elliptical cracked surface componentsunder uniaxial tensile loading for constant R-ratio with different a/c (crack aspect) ratio. Crack front is assumed to have an elliptical arc shape during propagation of crack. In this work, conformal mapping technique is used for modeling the crack in 3D; because this model provides an idealmethod to use with relatively small number of degree of freedom without need explicitly mesh cracks. Numerical and experimental results, shows that the results are comparable and obtained values are within the acceptable deviations. From this research it is inferred that, this numerical method will be better tool to find the crack growth rates and predict the fatigue life of structural components and also for establishing inspection intervals for welded pipe joints.

Multiaxial fatigue life estimation in low-cycle fatigue regime including the mean stress effect

The goal of the present paper is to discuss the reliability of a strain-based multiaxial Low-Cycle Fatigue (LCF) criterion in estimating the fatigue lifetime of metallic structural components subjected to multiaxial sinusoidal loading with zero and non-zero mean value. Since it is well-known that a tensile mean normal stress reduces the fatigue life of structural components, three different models available in the literature are implemented in the present criterion in order to take into account the above mean stress effect. In particular, such a criterion is formulated in terms of strains by employing the displacement components acting on the critical plane and, then, by defining an equivalent strain related to such a plane. The Morrow model, the Smith-Watson-Topper model and the Manson-Halford model are applied to define such an equivalent strain. The effectiveness of the new formulations is evaluated through comparison with some experimental data reported in the literature, related to biaxial fatigue tests performed on metallic specimens under in-and out-of-phase loadings characterised by non-zero mean stress values.

Residual Stress From Cold Expansion of Fastener Holes: Measurement, Eigenstrain, and Process Finite Element Modeling

Cold expansion (CX) is a material processing technique that has been widely used in the aircraft industry to enhance fatigue life of structural components containing holes. CX introduces compressive hoop residual stresses that slow crack growth near the hole edge. The objective of this paper is to predict residual stresses arising from cold expansion using two different finite element (FE) approaches, and compare the results to measurement data obtained by the contour method. The paper considers single-hole, double-hole, and triple-hole configurations with three different edge margins. The first FE approach considers process modeling, and includes elastic–plastic behavior, while the second approach is based on the eigenstrain method, and includes only elastic behavior. The results obtained from the FE models are in good agreement with one another, and with measurement data, especially close to the holes, and with respect to the effect of edge margin on the residual stress distributions. The distribution of the residual stress and equivalent plastic strain around the holes is also explored, and the results are discussed in detail. The eigenstrain method was found to be very useful, providing generally accurate predictions of residual stress.

Fatigue testing of composite patches for ship plating fracture repair

ABSTRACTThe objective of this study is to examine, through physical testing, the use of composite patches for preventing crack growth and extending the service life of ship plating. Fatigue tests were conducted on steel plates to experimentally validate the effectiveness of using composite patches as a means to prevent crack growth and extend the fatigue life of structural components. Specimens were tested with and without the use of composite reinforcements. Previous results of numerical analysis studies had indicated that a one- to two-order of magnitude increase in service life can be expected for the conditions tested. The experimental results presented here show increases closer to a single order of magnitude. The difference is attributed to two factors: failure mechanisms associated with debonding of the patch and cracking of the patch itself. Of critical importance, therefore, are the implementation of a quality-controlled bonding procedure and optimisation of the geometry and properties of the patch systems based upon the parent plate's properties and fracture conditions.

Fatigue damage and life evaluation of SS304 and Al 7050-T7541 alloys under various multiaxial strain paths by means of energy-based Fatigue damage models

Three energy-based models of Macha, Ellyin and Varvani, respectively, are employed to evaluate fatigue life of SS304 and Al 7050-T7451 alloys undergoing variable multiaxial loading spectra. The Macha damage approach assesses the lives based on the equivalent strain energy densities over counted reversals on the critical plane. The Ellyin model evaluates fatigue life data based on the total strain energy density per cycle, consisting of two parts (elastic and plastic strain energies). These energies are quantified from areas within the axial and shear stress-strain hysteresis loops. The Varvani damage approach assesses fatigue lives through summation of the normal and shear energy ranges calculated on the critical plane where the largest stress and strain Mohr's circles over the counted reversals are determined. The reversals over variable multiaxial loading spectra are counted based on the Wang-Brown multiaxial cycle counting method. Damage values are calculated over reversals and then accumulated over a block loading spectrum. The total damage over block histories is then related to fatigue life Nf in the right-hand side of the damage model equations proposed by Macha, Ellyin, and Varvani, respectively. The choice of damage assessment models and how to determine the fatigue life of structural components under variable loading spectra are discussed.

A general class of non-linear kinematic models to predict mean stress relaxation and multiaxial ratcheting in fatigue problems – Part I: Ilyushin spaces

Ratcheting is an accumulation of plastic strain that can influence fatigue lives of structural components due to the premature exhaustion of the material ductility, much earlier than predicted by traditional fatigue crack initiation models. Ratcheting is usually associated with a significant mean stress component in either uniaxial or multiaxial stress-controlled histories. The very same process can induce mean stress relaxation in strain-controlled histories, affecting fatigue lives due to consequent mean or maximum stress effects. Such processes are mainly caused by a local distortion of the yield surface, which would require the use of complex yield functions other than von Mises’ to be properly described. The addition of non-linear terms to the kinematic hardening rules compensates for this requirement, rendering it possible to model ratcheting effects using the von Mises yield function without dealing with distortion. In this two-part work, the formulation of the main non-linear kinematic (NLK) models is unified into a generalized equation, represented using engineering notation in a reduced-order five-dimensional (5D) space that may lower in half the associated computational cost. Part I introduces the proposed 5D stress and strain spaces, which are a scaled version of Ilyushin’s 5D spaces. These 5D spaces are then applied to the qualitative study of uniaxial ratcheting, multiaxial ratcheting, and mean stress relaxation. Part II of this work derives all incremental plasticity equations from the NLK approach in the spaces proposed in Part I, and discusses its advantages over the classical 6D formulation. These NLK models are then used in Part II to quantitatively predict uniaxial ratcheting, multiaxial ratcheting, and mean stress relaxation, validated from experiments with 316L steel cylindrical and tubular specimens.

Fatigue Life Prediction Based on Crack Growth Analysis Using an Equivalent Initial Flaw Size Model: Application to a Notched Geometry

Several methods for fatigue life prediction of structural components have been proposed in literature. The fatigue life prediction based on crack growth analysis has been proposed to assess the residual fatigue life of components, which requires the definition of an initial flaw. Alternatively, Fracture Mechanics crack growth-based fatigue predictions may be used to simulate the whole fatigue life of structural components assuming that there are always initial defects on materials, acting as equivalent initial cracks [1,2]. This latter approach is applied to a notched plate made of P355NL1 steel [3]. Fatigue crack growth data of the material is evaluated using CT specimens, covering several stress R-ratios. Also, S-N fatigue data is available for the double notched plate, for a stress R-ratio equal to 0 [3]. An estimate of the equivalent initial flaw size is proposed, using a back-extrapolation calculation [1,2]. The crack propagation model takes into account the elastic–plastic deformations in the crack-tip area within the calculation, based on the cyclic J-integral method. The performances of predictions are analyzed and deviations discussed.

Effect of baseline calibration on assessment of long-term performance of cable-stayed bridges

To enable the long-term health monitoring and condition assessment of a long-span cable-stayed bridge, a reasonable baseline finite element model to reflect the current structural behaviour is indispensable. With proper calibration, the model provides realistic predictions of the static and dynamic behaviour. This paper describes the development of baseline model of Ting Kau Bridge in Hong Kong using the cable forces as the basic parameters, with the measured frequencies and the initial equilibrium configuration of the bridge as the reference. Methods to estimate the fatigue lives of structural components are reviewed with special attention paid to the mean stress effect. The short-term performance and long-term performance of Ting Kau Bridge are studied based on both the initial model without baseline calibration and the baseline model. While the stress amplitudes due to moving loads do not change much, the mean stresses do change at many critical locations. As the mean stress effect is significant in the estimation of fatigue lives, baseline calibration of cable-stayed bridges is considered essential.

A novel method and tool which enhance the fatigue life of structural components with fastener holes

The fatigue failure around fastener holes in metal structures is particularly dangerous, since it leads to inevitable accidents. The presence of residual compressive hoop stresses around these holes, imparted by means of mandrel coldworking methods (including split sleeve cold expansion and split mandrel coldworking) closes the existing cracks and impedes the formation of new ones and thereby extends the fatigue life of corresponding component. Although this type of pre-stressing of fastener holes has been of great advantage over recent years it has some disadvantages: significant and nonsymmetrical with respect to the plate middle plane axial gradient of the residual hoop stresses due to axial force flow passing through the plate, considerable surface upset, etc. This article presents a novel patented method and a tool ensuring “pure” radial cold hole expansion – introducing nearly uniform residual hoop stresses around the hole along its axis having a minimized and symmetric gradient with respect to the plate middle plane. Because of this advantage, the method has been called by the authors “symmetric cold expansion”. Another advantage of the method is that it ensures one-sided process like the split sleeve and split mandrel. The carried out finite element (FE) simulations proved that the residual stresses in the novel method are much more uniformly distributed in comparison with the mandrel coldworking methods. The fatigue life improvement of the novel method in comparison with mandrel coldworking methods has been proved experimentally through fatigue cyclic test.

Fracture Behaviour of 20MnMoNi55 Steel in DBT Region under Corrosive Environment

Different mechanical properties like strength, fracture toughness and fatigue life of structural components reduce due to corrosion of the material. In this paper the effect of corrosion on fracture toughness in ductile to brittle transition (DBT) region of 20MnMoNi55 steel is investigated. The corrosion properties of the material are measured by electrochemical impedance spectroscopy (EIS) test. The loss of ductility and fracture toughness due corrosion is measured by measuring master curve reference temperature in DBT region. Both single and multi–temperature method is used to measure the master curve reference temperature for both base material and corroded specimen. A comparative study is made to measure embrittlement that occurs due to corrosion in DBT region. From the EIS study it has been observed that the corrosion resistance properties decrease with time when the material corroded. Also the embrittlement occurring due to corrosion is reflected in the shift in master curve.

Dynamic substructuring and reanalysis methods in a surrogate-based design optimization environment

In light weight structure design, vibration control is necessary to meet strict stability requirements and to improve the fatigue life of structural components. Due to ever-increasing demands on products, it is generally more convenient to include vibration prerequisites in a design process instead of using vibration control devices on fixed designs. One of the main difficulties associated to design optimization of complex and/or large structures is the numerous computationally demanding Finite Element (FE) calculations. The objective of this research is to present a novel strategy for efficient and accurate optimization of vibration characteristics of structures. In the proposed strategy, a sub-structuring method is utilized. The FE model of the complete structure is partitioned, reduced and then reassembled. This increases the computational efficiency of dynamic analyses. Moreover, this method is coupled with a novel reanalysis technique to speed up the repeated structural analyses. These methods are finally embedded in a surrogate-based design optimization procedure. An academic test problem is used for the validation of this novel approach.

Importance of Residual Stresses and Surface Roughness regarding Fatigue of Titanium Forgings

Abstract This paper presents the results of a long-term research program aimed at developing qualitative and quantitative design guidelines for the use of mechanical surface treatments designed to improve the fatigue life of structural components. High cycle fatigue tests were performed on planar four-point bending specimens derived from Ti-6Al-4V pancake forgings with a mill-annealed microstructure. The high cycle fatigue behavior of specimens with different surface conditions (as-forged and machined) in both an unpeened and a shot peened state was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layers were carried out. The as-forged surface state exhibits a stress distribution with significant compressive stresses near the surface, resulting in equilibrium tensile stresses in the depth. When the tensile stresses were exposed by machining a bordering surface, a distinct decrease in the fatigue strength was observed. In such cases, a shot peening treatment was shown to improve the fatigue strength. Square edges lead to a decrease in the fatigue strength, which could be aggravated by shot peening.