Frequency-based structural modification for the case of base excitation

Based on the typical aerospace frame structure, the fatigue life law of the structure under multi-axis correlation time-domain random excitation is studied. By constructing multi-axis time-domain random excitation with different correlation coefficients, multi-axial fatigue theory and structural vibration fatigue life time-domain method are used to calculate the fatigue life of the structure under different excitations; by constructing the structural finite element model in MSC. Pastran and nCode-DesignLife, and Co-simulation calculates the structural fatigue life. The results show that there is a negative correlation between the correlation coefficient of the applied multi-axis time-domain random excitation and the structural fatigue life, that is, the larger the correlation coefficient, the shorter the fatigue life.

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.

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.

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.

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.

: 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.

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.

The presented study experimentally and theoretically researches vibrational fatigue of an aluminum-alloy specimen for harmonic and random loads. The main aim of this study is to determine the influence of modal parameter changes that occur during the experimental fatigue test, on the correctness of the numerical fatigue life prediction. Firstly, the material’s fatigue parameters were obtained with harmonic base excitation of the specimen near its natural frequency. During the harmonic fatigue testing the changes in the specimen’s natural frequency and damping loss factor were monitored as the fatigue damage was accumulated at the fatigue zone. Secondly, with a validated numerical model of the specimen the stress transmissibility was obtained for the case of the random-vibration base excitation. Finally, by respecting the stress response along with the experimentally obtained material fatigue parameters the vibration fatigue life was estimated for the case of the random vibration load. The numerically predicted fatigue life was compared to the experimental results, obtained with the electro-dynamic shaker. From this comparison the influence of the damping-loss-factor changes on the calculated fatigue life was clearly shown. For the case of the observed specimen, the damping loss factor included in the fatigue life estimation should be increased by more than 100 percent to give reliable prediction of the fatigue life. The presented research shows new possibilities and critical aspects in the area of accurate high-cycle vibration fatigue life-estimation of dynamic structures.

Assessing tunnel basement structure damage under train excitation: Impacts of surrounding rock deterioration

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.

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

Airborne structural parts are designed for critical operating flight and landing load conditions following the certification guidelines for enough strength and rigidity. But in the development test environments they will be subjected to loads differently than expected in the operating life unexpectedly leading to premature fatigue failures. Instances like fastener loosening due to loss of positive lock in primary attachments will trigger vibratory dynamic loads and fail the structural attachments. It is essential to avoid these unsafe situations by considering such events in the design. This paper aims at dealing one such instance practically observed during flight testing of helicopter. Redesign and analysis is carried out that show simulation of critical stress levels for dynamic vibration loads for fastener loosing effects and design accomplishment to avoid such premature structural failures and maintain original damage-tolerant behavior with better strength and fatigue life levels to meet operation life requirements.

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.

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

Shape memory alloy strands as cross-ties: Fatigue behavior and model-cable net tests