Time-domain calculation method for structural fatigue life under multi-axis correlation random excitation

: Prediction of the dynamic response and the fatigue life of structures, i.e. panels, subjected to both high temperatures/ thermal gradients and high level acoustic excitations. The determination of reliable approximations of the statistics of the panel response to the random acoustic excitation will be accomplished by relying on two separate equivalent linearization strategies. The first method uses a Gaussian process. The second method relies on a mean equal to the buckled states and the panel response is obtained as the sum of zero mean fluctuations around these positions.

Welding defects are known to cause crack propagation and reduce structural fatigue performance. Based on the Paris theory of fracture mechanics, research is conducted on evaluation methods for analyzing fatigue crack propagation by adopting random loads with long-term distribution that follows the Weibull distribution for the stress ranges of fatigue loads. This approach is combined with the corrective stress intensity factor (SIF) equation and the method for calculating the reference stress and failure criterion. A large container ship is selected for a simulation, and fatigue crack propagation analysis is conducted on typical critical locations. A detailed comparison of the forecasted fatigue life is carried out between fracture mechanics theory and the S-N curve. The results indicate that the fatigue life values obtained using the two methods are of the same magnitude. In general, for the welded structure, the fatigue life value obtained via the fracture mechanics method is shorter than that obtained via the S-N curve method, while, for the free edge of the structure and the unwelded structure, the predicted fatigue life value is closer than that predicted via the S-N curve method. Moreover, the influence of initial crack defects on the fatigue life is investigated, and the results show that the depth of the initial crack will greatly affect the fatigue life of the target ship in typical locations, but the influence of the shape ratio on the fatigue life is limited. Therefore, in the actual ship construction process, controlling the initial crack depth of components is effective for limiting crack propagation and improving fatigue life. The above conclusions and suggestions can serve as a reference for the structural design and fatigue life evaluation of large container ships.

In this study, an efficient fatigue life estimation approach is proposed based on an equivalent solid model, along with two stress concentration factors, instead of a lattice model to simplify simulations for the strain-life method. One of the stress concentration factors accounts for the stress concentrations at the strut joints in lattice structures without any manufacturing effect, while the other accounts for the stress concentrations due to surface defects on the struts introduced by the additive manufacturing process. A simple solid cell, for which the dimensions are the same as those of the lattice cell, is first used to estimate the fatigue life of the bulk material used in the additive manufacturing process using the Brown–Miller strain-life method. Combinations of two stress concentration factors were then utilized to predict the fatigue life of the lattice structure from the equivalent solid model. Using this approach, computationally expensive lattice structure simulations were eliminated, and the fatigue life of the lattice structure was predicted directly from the bulk material fatigue life estimation. The proposed approach was applied to different lattice types made of Ti-6Al-4V alloy using the laser-powder bed fusion additive manufacturing process. The fatigue life results estimated using the proposed approach were in good agreement with the experimental fatigue life results reported in the literature.

Frequency-based structural modification for the case of base excitation

The Life of Type (LOT) of a naval High Speed Light Craft (HSLC) can be limited by its structural fatigue life. The fatigue life of a ship is influenced by many factors, such as geometry, fabrication quality, the long-term load distribution, and analytical techniques. The complex dependencies between these factors and the fatigue life cause uncertainty in predictions. A lack of understanding of uncertainty can adversely affect the management of LOT risks, resulting in the reduction of availability of the ship and costly repairs. Therefore, improved understanding of the benefits and limitations of different fatigue life evaluation approaches informs the management of risks relating to the ship’s LOT. This paper presents the first phase of work in a comparative analysis of different fatigue life evaluation approaches for naval HSLC. The present work involves a holistic data review, codification of the data to reveal key themes, and individual expert comparative analysis of the different approaches. The next phase of the study is also described.

Modelling of residual stress field in spherical mandrelling process

Jack-up platforms are designed to work in three conditions: Transit, Preloading and Operating. The fatigue life of the jack-up platforms in operating condition will be determined to be similar to that of offshore fixed steel structures. In preloading conditions, fatigue damage is usually ignored. Up to now, the fatigue damage of the jack-up leg structure in transit conditions has been calculated at approximately 20% of the total fatigue damage of the jack-up leg structure in two conditions (transit and operating). The approximate calculation method is usually accepted by consultants and register agencies. If the approximation is used, the fatigue life of the jack-up leg structure will be calculated only as for the jacket structure of the fixed offshore structure, with 80% of the allowable fatigue life based on standards. The approximation proved to have many disadvantages: the different travel times of each jack-up rig cannot be mentioned; hot spots that need to be maintained during the transit condition have not been pointed out; it is difficult to guarantee the safety of the jack-up leg structures in the transit condition. In order to overcome the limitations of the approximation method, this paper will propose a method to predict the overall fatigue life of the jack-up leg structure in three main problems. Firstly, we use the analysis method of fatigue of fixed steel offshore structures for jack-up leg structures in operating conditions. Secondly, we suggest a method to analyze the fatigue of the structures in transit conditions. Herein, motion analysis and determination of inertia forces on the leg structure are performed by the Boundary Element Model (BEM) in SACS software. Then the inertia forces are assigned to a Finite Element Model (FEM) in SACS to decide the internal forces of the structures. Hotspot stresses are determined by combining nominal stress from FE analysis results with a concentrated stress factor from the analysis of joint local models in the ANSYS program. Then, fatigue damage and fatigue life of hotspots of the structure are determined in the transit condition. Finally, a formula is suggested to determine total fatigue damage in operating conditions and transit conditions with different cases in relation to different fraction factors. These results are used to predict fatigue life corresponding to the most dangerous cases of structural joints. These new suggestions are applied to fatigue analysis for jack-up Tam Dao 05. Currently, the Tam Dao 05 platform has been operating in the Vietnam East Sea. Doi: 10.28991/CEJ-2022-08-03-06 Full Text: PDF

Prediction of fatigue life of a flexible foldable origami antenna with Kresling pattern

In order to delve into the fatigue life distribution of solid rocket motor charge structures under vibration loads, equivalent vibration loading conditions for numerical analysis were hereby established based on measured transportation vibration load spectra. The fatigue life of hydroxyl-terminated polybutadiene (HTPB) charge under various working conditions was measured through fatigue cyclic loading tests, and a fatigue life model was correspondingly constructed. Subsequently, numerical simulations were conducted using the UMAT secondary development technology and a three-dimensional finite element model of the solid rocket motor. Building on these simulation results, the fatigue life distribution characteristics of the charge were then analyzed. As indicated by the study findings, under vibration loads, the overall fatigue life of the motor charge gradually decreases with increasing loading cycles and vibration amplitude. The regions of significant fatigue life decline initially emerge at the root of the charge’s star-shaped holes, subsequently propagating to the inner walls of the charge, and eventually to the outer surfaces of the charge. From the perspective of the fatigue life of the charge structure as a whole, the parts with small fatigue life, i.e., the danger zones, are mainly distributed in the star-shaped holes of the charge and in the part of the area where the outer surface of the charge is bonded to the liner. These areas necessitate special attention.

Background: Fatigue damage is one of the main failure modes in ship structures. This type of damage usually starts from weak point of the structure as welded joints, sites of stress concentrations and cracks, whose propagation can lead to the failure of the ship structures. Cyclic loadings that ships encounter during their service life are one of the main causes that can produce fatigue damage, especially loads due to wave, mainly analyzed in this work. Once the crack is started, also lower stress cycles, which would have negligible effects on intact components, can propagate the crack. Objective: This paper wants to resume the most used fatigue strength assessment approaches, in order to analyze drawbacks and advantages and to provide the necessary background knowledge for the development of a future and reliable theoretical/numerical models for predicting the fatigue life of ship structures subjected to collision events and different sea states. Methods: This scientific work will thus consider the main theoretical approaches in time and frequency domain by using energy spectral methods. Collision rules are also assessed to check the hull girder ultimate bending capacity in the damaged state. Results: Cyclic stresses determination in specific structural details of the hull girder and welded joints is discussed, in order to evaluate the relevant maximum stress range useful for the subsequent fatigue studies, performed by finite element analysis. In a collision scenario, also in minor damages, fatigue cracks may appear and propagate, reducing structural strength and structures fatigue life. Conclusion: Structural loads determination can be categorized in different methods, but the frequency-domain method is the most extensively used methods in fatigue analysis procedure, because it requires significantly less computational efforts than the time-domain method. Once determined the stress distribution, in most cases fatigue damage is calculated using the Palmgren–Miner cumulative damage rule in combination with the S–N curves of materials, structural details and welded joints. In a scenario where a damaged ship encounters large wave amplitudes, the damage accumulation could lead to low-cycle fatigue.

Fatigue damage accumulation theory is one of the core contents in structure fatigue strength design and life prediction. Among them, the nonlinear damage model can overcome the shortcomings of the linear damage model, which takes the loading sequence effect into account. Besides, the loading interaction cannot be ignored for its profound influence in damage accumulation behavior. In the paper, some commonly-used methods of the linear and nonlinear fatigue damage accumulation theory are investigated. In particular, a modified nonlinear fatigue damage accumulation model which considers the effects of loading sequences as well as loading interactions on fatigue life is developed, and a load interaction parameter is obtained by analyzing damage models which assumes that the load logarithm ratio between adjacent stress levels can characterize this phenomenon. Finally, the modified model is employed to predict the fatigue life of high pressure turbine disc. Moreover, comparison is made between the experimental data as well as the predicted lives using the Miner’s rule, the Ye’s model, and the modified model.

A series of water damage phenomena of asphalt pavement during service show that water is very important. To comprehensively analyze the influence of water vapor factors on the response of pavement structure, this article first tests the water vapor diffusion coefficient of asphalt pavement materials at different temperatures establishes the water vapor concentration field model of the pavement structure, and analyzes the actual water vapor distribution of the pavement structure; then, the relationship between viscoelastic parameters of the mixture and water vapor concentration is established. Based on this, the finite element model of the pavement structure considering the water vapor factor is established, and the influence of the water vapor factor on the pavement structure response and fatigue life is quantified. It is found that water vapor has an important influence on the mechanical response. On the hottest days in summer, the attenuation of surface modulus caused by water vapor has the greatest impact on the upper layer of the pavement structure. Under the influence of water vapor, the position just below the wheel of the upper layer at the bottom of the horizontal tension strain increases by 108.26%, and the horizontal tension strain of the other layers increases by about 5%, increasing the risk of upper layer cracking. At the same time, water vapor reduces the stress that can be borne by the surface layer, while the rest of the stress is borne by the base. Compared with the fatigue life calculation method in the specification, the actual non-uniform water vapor concentration field reduces the fatigue life of the surface layer by 12.246%. This shows that the current structural calculation does not consider the water vapor factor, which makes the pavement structure more dangerous, and the influence of the water vapor factor should be fully considered.

Fatigue is a phenomenon of material damage due to loading that occurs repeatedly. If the material is repeatedly loaded, the material will experience fatigue at low loading compared to the loading applied to the material to cause fracture with static loads. In this thesis, the prediction of fatigue life that occurs in wind turbines with repeated loading caused by wind conditions is analyzed. The cyclic loads that occur are accumulated from the lowest to the highest loads in a period of one year. Rain flow cycle calculations are used to estimate the cumulative damage of the final beam under the in-service loading conditions. The fatigue life of a wind turbine with loading caused by certain wind conditions is calculated by the RFCA calculation method, SN-Curve, and Miner’s rule. The contribution of wind load to the fatigue life of the wind turbine structure in this study shows that the largest is 15,905 MPa, the smallest load is 0.270 MPa. The fatigue life of this wind turbine structure has yet to be determined because the motor load that occurs is very small.

The polyethylene terephthalate (PET) foam sandwich structure is widely used for ships and for studying the influence of the seawater environment on its mechanical properties as an important way to evaluate its safety performance. To study the effect of the seawater environment on the mechanical properties of the PET foam sandwich structure, this paper mainly studies the static properties, interlayer cracking properties, and fatigue failure properties of the PET foam sandwich structure under the simulated seawater environment at a constant temperature of 40°C. The compressive properties test, tensile properties test, three-point bending properties test, double cantilever beam test, and four-point test were carried out on PET foam specimens, glass fiber reinforced plastic (GFRP) panel specimens, and PET foam sandwich structure specimens with different immersion times. The multiphysics software COMSOL was used to model and analyze the mode I interlayer cracking properties of the PET foam sandwich structure. The comparative analysis of the experimental results shows that the seawater environment has an effect on the compression properties of PET foam, the tensile and flexural properties of GFRP panels, and the interlayer cracking properties of the PET foam sandwich structure, and as the immersion time is extended, the effect is more obvious. Seawater immersion can also lead to a reduction in the fatigue life and ultimate fatigue of the PET foam sandwich structure.

Thin-walled structures exhibit complex nonlinear response under thermo-acoustic loadings. Complex stress-strain states decrease the fatigue life of structures seriously. Based on the thermo-acoustic response obtained, the rain flow cycle counting scheme is used to calculate the number of fatigue cycles. Then the Miner accumulative damage model is employed to predict high cycle fatigue life, combined with various nonzero mean stress models, including Morrow TFS, SWT. The nonlinear response and fatigue life of 2024-T3 aluminum plate are obtained under different combinations of thermo-acoustic loadings. Results show that the fatigue life of pre-buckled plate decreases with the increase of temperature. For post-buckled plate, as the temperature increases, the fatigue life of plate undergoing persistent snap-through keeps going down till the lowest, and then increases after entering intermittent snap-through regime. At high temperatures, the influence of high temperature on the S-N curve must be considered, the results may be erroneous otherwise.