Fatigue Life Estimation of the Critical Wing Structure in a New-Generation Stol Aircraft

Fatigue crack growth retardation, delayed retardation, initial acceleration, and optimum fatigue crack growth life under single, single periodic, and multiple periodic tensile overloads were investigated in cyclic strain softening AISI 1020 cold-rolled steel using compact type specimens. Macroscopic delayed crack growth retardation was observed following both single and multiple periodic overloads and was best observed at high overload ratios and larger crack lengths. Initial crack growth acceleration at the beginning of an overload interval was not quantitatively evident, however initial acceleration could have occurred before the crack extended to the least measurable scale division. Under constant load range testing, multiple periodic overloads of about 10 to 30 gave optimum or maximum fatigue life in most cases, however values of 1 gave maximum life in some cases. The largest increase in fatigue life was 453 percent. Maximum or optimum fatigue life obtained from multiple periodic overloads was always greater than that obtained from a single overload. In general, the results suggested maximum fatigue life can be obtained by applying a few multiple tensile overloads at high overload ratios only when accelerated crack growth begins following low load crack retardation.

Resistance spot welding (RSW) is now the primary joining process used in the automobile and aerospace sectors. Mechanical parts, when put into service, often undergo cyclic stress. As a result, avoiding fatigue failure should be the top priority when designing these parts. Given that spot welds are a type of localised joining that results in intrinsic circumferential notches, they increase the likelihood of stress concentrations and subsequent fatigue failures of the structure. Most of the fatigue failures in automotive parts originate around a spot weld. To that end, this study seeks to examine the mechanical properties and fatigue behaviour RSW joints made of titanium (Ti) grade 2 alloy and AISI 304 austenitic stainless steel (ASS) with equal and unequal thicknesses of 0.5 and 1 mm. Based on the mechanical properties and fatigue life results, the maximum tensile shear strength and fatigue life for the RSW titanium joint were 613 MPa and 7.37 × 105 cycles for the 0.5–0.5 mm case, 374.7 MPa and 1.39 × 106 cycles for the 1–1 mm case, and 333.5 MPa and 7.69 × 105 cycles for the 1–0.5 mm case, respectively. The maximum shear strength and fatigue life of ASS welded joints were 526.8 MPa and 4.56 × 106 cycles for the 1–1 mm case, 515.2 MPa and 3.35 × 106 cycles for the 0.5–0.5 mm case, and 369.5 MPa and 7.39 × 105 cycles for the 1–0.5 mm case, respectively. The assessment of the shear strength and fatigue life of the dissimilar joints revealed that the maximum shear strength and fatigue life recorded were 183.9 MPa and 6.47 × 105 cycles for the 1 mm Ti–0.5 mm ASS case, 115 MPa and 3.7 × 105 cycles for the 1 mm Ti–1 mm ASS case, 156 MPa and 4.11 × 105 cycles for the 0.5 mm Ti–0.5 mm ASS case, and 129 MPa and 4.11 × 105 cycles for the 0.5 mm Ti–1 mm ASS case. The fatigue life of titanium and stainless steel welded joints is significantly affected by the thickness, particularly at maximum applied stress (0.9% UTS), meaning that similar thicknesses achieve a greater fatigue life than unequal thicknesses. Conversely, the fatigue life of the dissimilar joint reached the greatest extent when an unequal thickness combination was used. The ductile failure of similar Ti and ASS welded joints was demonstrated by the scanning electron microscopy (SEM) examination of fatigue-fractured surfaces under the high-cycle fatigue (HCF) regime, in contrast to the brittle failure noticed in the low-cycle fatigue (LCF) regime. Brittle failure was confirmed by the SEM fatigue of dissimilar joint fractured surfaces due to interfacial failure. The Ti and ASS fractured surfaces presented river-like cleavage facets. On the Ti side, tiny elongated dimples suggest ductile failure before fracture. The topography results showed that the roughness topography parameters of similar and dissimilar fractured specimens made from Ti grade 2 and AISI 304 for the HCF regime were lower than those of the fractured specimens with LCF. The current study is expected to have practical benefits for the aerospace and automotive industries, particularly the manufacturing of body components with an improved strength-to-weight ratio.

Fatigue failure of steel member connections is a well-known failure mechanism for railway steel bridges, which sustain heavy cyclic loads. Their structural members and connections are prone to damage caused by repeated loading of trains, moving at varying speeds. Fatigue damage due to cyclic loads leads to crack formation in members and failure of joints. The designs in Indian Railway Standard (IRS) Bridge Rules are according to the Working Stress Method (WSM). In the present study, fatigue analysis of standard RDSO Heavy Mineral Loading steel plate girder of 24.4 m span is carried out using (i) provisions of IS 800 (2007): Limit State Method, and (ii) software ANSYS. The fatigue life has been estimated for the three cases. For the case study, data of an experimentally tested I-girder available in the literature (Gang et al, ASCE J Compos Construct 16(2), 2000) [1] is considered. The maximum equivalent stresses, deformations, and fatigue life in terms of the number of cycles, are evaluated. In LSM the governing factor for capacity estimation is elastic Lateral Torsional Buckling Moment. Correction for stress range given in IS 800 (2007) estimates lower fatigue life compared to working stress method (IRS bridge rules); as the effect of lateral torsional buckling is not considered in IRS bridge rules. Fatigue life estimated in terms of the number of cycles, using ANSYS is less than that obtained from the provisions of IS 800. It is concluded that the governing criteria for designing long span plate girder bridges are the lateral torsional buckling moment.

Damage tolerance of a hybrid lower wing bonded stiffened panel with a Fiber Metal Laminate skin

Effects of rivet hole arrangement on fatigue performance of thin sheets for fuselage: DIC and numerical calculation

An optimal design approach for calibrated rolls with respect to fatigue life

Chapter 40 - Case History: Composite Patch Reinforcement of T-38 Lower Wing Skin

The lower wing skin is one of the primary structures of an aircraft. To further improve the fatigue and damage tolerance (F&DT) performance of the lower wing, fibre metal laminates (FML) are proposed as a new material solution. FML consist of thin metal layers bonded with layers of fibre composites. This concept has potentially a large design freedom and its layups could be tailored for specific applications by varying the number, thickness, orientation, and material type of the metal and fibre constituents. This study has been performed to explore the possibilities of lay-up optimisation for FML and to apply the concept of FML to a wing structure. This research aimed to develop a design optimisation methodology for FML that satisfies F&DT criteria. The optimisation methodology should reveal the contribution of individual criteria to the obtained solutions. Furthermore, the method will be used to design a lower wing skin consisting of FML where F&DT and additional compatibility criteria are met. As a result, an analytical model is developed that comprises all the functionality to design a wing structure consisting of FML lower panels and aluminium upper panels. The lay-up solutions are obtained by evaluating the laminates for fatigue crack initiation (FCI), fatigue crack propagation (FCP) and residual strength (RS). These properties are obtained by means of prediction methods, which are implemented into a genetic algorithm optimisation environment. The scientific contribution is delivered by developing a method to reverse the prediction methods to find the lay-ups that satisfy the required property instead of determining the properties of a given lay-up. The lay-up solutions are defined by three parameters: thickness of metal layers, number of metal layers and the grade of a laminate. The amount and orientation of the fibre plies are defined in this grade. With the optimisation method, the lowest weight solution in the design space is determined by generating solutions based on crossover and mutation operators and ranking the satisfying solutions based on their weight. The method considers the optimisation of flat-plates and wing cross-sections. For cross-section optimisation, only the numbers of metal layers along the cross-section are optimised to comply with manufacturing constraints. Additionally, a thickness step constraint is introduced to prevent stress concentrations between elements and to force a distributed thickness along the cross-section. As a final step, the cross-section optimisation is linked to a wing design module that is now capable of dimensioning an aircraft wing structure with the lower panel consisting of FML and the upper panel of aluminium. The thickness of the aluminium skin is defined as variable for the upper skin and evaluated using buckling criteria. As output, a complete optimised cross-section is obtained. To further improve the computation time and to simply the optimisation routine, a regression analysis is performed on the prediction methods for FCI, FCP and RS to obtain approximations for these methods. These approximate functions replace the prediction methods with high correlation and with assurance that the same solutions are obtained as output. The functions are replaceable with other functions representing different criteria to have a generic design method. The influence of optimisation settings, approximations and different design criteria are extensively investigated. The output of the design method depends on the accuracy of the prediction methods and the accuracy of the performed regression, because a small difference in prediction influences the obtained optimal and near optimal design solutions. Further, genetic algorithm proved to be a robust method when optimising single elements or flat-plates when the settings are well-defined. In case of cross-section optimisation, due to the increasing size of the design space the method proved to be inefficient sometimes with the case that once in a while satisfying solutions were not obtained or were stuck at local minima. This problem is solved by defining an initial input to the procedure and increasing or decreasing the upper and lower boundary of the design space. A convergence loop for genetic algorithm is implemented to automate this process in this design method. In conclusion, the method has the ability to compare and evaluate material configurations, to investigate the influence of various design criteria on the lay-up solutions and to optimise the wing material for minimised weight for different sets of load cases and wing geometries.

At present, most of the research methods for vibration fatigue of welded structures mainly focus on uniaxial stress, ignoring the influence of shear stress. To this end, by combining the ASME structural stress method with the random and vibration analysis theory outlined in the IEC 61373 standard, a new method for evaluating the fatigue life of multi-axis random vibration problems in the frequency domain has been proposed. This method extends the structural stress method to multi-axis scenarios to accurately extract the local multi-axis structural stress state at the weld toe. Its advantage lies in the fact that it not only accounts for the influence of load frequency distribution and structural modal vibrations on fatigue life, but also incorporates the effect of local multiaxial stress conditions in the weld on fatigue life. Additionally, it includes corrections for non-proportional multiaxial stress conditions, resulting in fatigue assessment results that more closely reflect actual conditions. It was validated by comparing the local multiaxial stress, phase difference between shear and normal stress, and equivalent structural stress power spectrum of 0° and 30° fillet welded specimens with test results. Subsequently, it was applied to a multiaxial random vibration fatigue assessment of a vehicle-mounted electrical cabinet with experimental verification. The results indicate that fatigue life estimates based on a multi-axis stress state are lower than those obtained using a uniaxial method. Compared to traditional uniaxial methods, the multi-axis fatigue life estimates show a significant reduction ranging from 4.20% to 88.35%, effectively accounting for damage caused by shear stress. The fatigue assessment results are more closely aligned with experimental data, thereby validating the effectiveness of the proposed new method. The frequency-domain multiaxial random vibration fatigue assessment method proposed in this article provides a new technology for the design and evaluation of welded structures of vehicle-mounted equipment in rail vehicles. This method reduces costs during the design phase of rail vehicles, offering positive economic implications.

Leaf springs are designed to bear loads as well as shocks in automotive vehicles. Two leaves of glass fiber-reinforced composites (GFRCs) of various shapes sandwiched between steel plates were analyzed for application in a minitruck. Computer-aided engineering analysis was performed for five different types of GFRC material leaf springs: flat leaf, flat and parabolic leaf, both parabolic leaf, both parabolic leaf springs with aluminium alloy bushes at the eye-end and spring steel multi-leaf springs. A silencer pad was used in the parabolic leaf spring to reduce delamination and vibration at contact points of the mating leaf. The various shapes and combinations of leaves provided varying parameters, namely, the deformation, maximum equivalent strain, maximum equivalent stress and fatigue life. The CAE results showed that compared with the other combinations, the flat leaf and parabolic leaf combinations provided the maximum equivalent strain, maximum equivalent stress and fatigue life.

This article introduces a design procedure to find the optimum fiber orientations of carbon/epoxy composite laminates for fatigue life advancement. The approach incorporates a fatigue failure tensor polynomial model and employs a hybrid algorithm, combining particle swarm optimization and sequential quadratic programming. Firstly, material properties of quasi-static and fatigue of the carbon/epoxy composites, fabricated by the vacuum-assisted resin transfer molding method, were determined to be used in the model. Various design problems involving two optimization scenarios were then solved using the hybrid algorithm. The algorithm’s performance was also evaluated by specific test problems, confirming its speed and robustness. The optimally fiber-oriented carbon/epoxy composite laminates having maximum fatigue lives were obtained for many critical in-plane cyclic loading cases. To validate the proposed design procedure, two optimum designs were experimentally verified under uniaxial loading conditions. The results indicated a good correlation between the estimated fatigue life of the optimally designed laminates and experimental data. This methodology offers a promising approach for the design of carbon/epoxy composite laminates with superior fatigue strength, particularly significant in specific industrial applications.

Warm mix asphalts (WMA), because of their low production and compact temperatures, may have different behaviors in long term. In the present work, the energy-based criteria along with the 50% reduction in initial stiffness (Nf50%) using four-point bending test under controlled-strain conditions of 1000 microstrain were applied to compare the prepared two warm mix and HMA samples. All these criteria illustrate properly the effect of mix asphalt properties (additive type) on its fatigue performance. A noteworthy point in this regard is the difference between Nf50% values of the studied samples with the real failure point. For HMA and zycotherm WMA (ZWMA), loading cyclic number at the failure moment occurs almost 80% higher than the fatigue life estimated using Nf50% while for Sasobit WMA (SWMA) this value is declined to 28%. The RDEC method, compared to other methods, indicated the maximum fatigue life and consistency with the failure point. Comparing the energy-based methods with Nf50% method revealed that ERR, ERR&B, and ERP have the maximum consistency with fatigue life in terms of 50% reduction in initial stiffness. For SWMA, the fatigue life at Nf50% was larger than that of various energy-based methods but almost equal to that of the RDEC method. However, for two WMA mixes prepared using ZWMA and HMA, all energy methods revealed a fatigue life longer than that of Nf50%.

Fatigue performance of hydraulic asphalt concrete under uniaxial constant-amplitude tensile cycle loading

Effects of micron-sized Si particles on the static and dynamic mechanical properties and fracture behavior of Al-xSi alloy sheets

The present work describes an experimental investigation of the fatigue durability of AISI 304 and AISI 316L austenitic stainless steels, which have regular reliefs (RR) of the IV-th type, formed by ball burnishing (BB) on flat surfaces, using a computer numerical control (CNC) milling center. The methodology and the equipment used for obtaining regular reliefs, along with a vibration-induced fatigue test setup, are presented and described. The results from the BB process and the fatigue life experiments of the tested austenitic stainless steels are gathered, using the approach of factorial design experiments. It was found that the presence of RR of the IV-th type do not worsen the fatigue strength of the studied steels. The Pareto, t-test and Bayesian rule techniques are used to determine the main effects and the interactions of significance between ball burnishing regime parameters. A stochastic model is derived and is used to find when the probability of obtaining the maximum fatigue life of parts made of AISI 304 or 316L reaches its maximum value. It was found that when the deforming force, the amplitude of the sinewaves and their wavenumber are set at high values, and the feed rate is set at its low value, the probability to reach maximum fatigue life for the parts made of AISI 304 or 316L is equal to 97%.