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As an important part of the fatigue load spectrum simplification of aircraft structures, the small-load truncation can significantly save the time and cost of fatigue tests. However, there is no widely accepted standard method for determining the small-load truncation level. In this paper, the existing small-load truncation levels in the literature are collected and divided into two determination methods, which are based on the material fatigue limit and the spectral maximum load. The links between the two determination methods are explored. The relationship model of the small-load truncation levels is established. The results show that the two methods are the same in essence,and the latter is an extension of the former combining the characteristics of aircraft loads and structural materials.The relationship model established between the small-load truncation levels could provide helpful references for the determination of the small-load truncation levels in the engineering fatigue load spectrum simplification.

As composite materials are playing more important role in advanced aircraft structures,the change of mechanical properties of composites during service is of significant importance for the overall safety of the aircraft.In order to achieve the goal of fatigue evaluation and life prediction of composite components of aircraft based on guided wave in-situ detection,firstly,the fatigue evolution law of composite materials is studied from the perspectives of macroscopic phenomenology and microscopic physics.Then,the potential of guided wave phase velocity and mode conversion phenomenon for fatigue characterization is discussed through analyzing the guided wave field.At the same time,a deep learning framework is constructed to extract fatigue evolution features from the guided wave field in a data-driven manner.Finally,a fatigue evolution model based on the Bayesian model averaging method is proposed to predict the residual fatigue life of the composite specimen.The results show that,by extracting and analyzing the guided wave propagating features,the fatigue state of composite materials can be accurately characterized.Combining the Bayesian model averaging method and the confidence interval criterion,the goal of residual life prediction before specimen fatigue failure is achieved.

Among the fatigue life models of honeycomb sandwich structures,the most researched is the S-N curve model.This paper classifies and summarizes the commonly used forms of honeycomb sandwich structures’ S-N curve model. At the same time,the fatigue tests data of honeycomb sandwich structure is collected and normalized according to the relevant mechanical principles,and some general results were obtained. Based on the material properties of the honeycomb core,several influence factors on the fatigue performance of the structure are discussed.

Fiber reinforced resin matrix composites(FRP) have been widely used in aerospace and other advanced structures. The study of low-speed impact damage of FRP is of important engineering application value. The numerical simulation models of composite structural damage under low velocity impact are classified and evaluated. The low-velocity impact model is composed of two kinds of sub-models: the impact process model and the material damage evolution model. The key elements of each type of model and their processing methods are listed. The commonly used combinations are collected and reviewed, and six simulation models appeared constantly in the literatures are listed. The numerical evaluation of two examples with the listed six models is completed. Results show that for orthogonal laminated plates, the damage shapes and areas given by the six models can be predicted accurately, but for the angle-ply laminated plate, the delamination damage morphology achieved by the model of Puck criterion, considering shear nonlinearity and based energy release rate is closer to the experimental results.

Honeycomb sandwich panels have become more and more widely used in the aerospace field, and the fatigue problems that occur during their application have become increasingly prominent. At present, there is little research on the fatigue of honeycomb sandwich panels, and the test methods used in the study are also very different. Under different test methods, the fatigue process of Nomex honeycomb sandwich panels varies greatly. This article mainly reviews the research progress of the fatigue of honeycomb sandwich panels. The purpose is to compare the fatigue damage process of honeycomb sandwich panels under different test methods and summarize the similarities and differences of the fatigue performance of honeycomb sandwich panels. By introducing the fatigue test method, fatigue damage process, fatigue life prediction method and fatigue life curve of honeycomb sandwich panel, the research direction of the fatigue of honeycomb sandwich panel is proposed.

In the engineering application like anti-fatigue design, the accurate understanding of the material performance is the foundation for the application of material. As a kind of aeronautical material, aluminum-lithium alloys are favored for their high specific strength and high specific stiffness. Compared with conventional aluminum alloys, aluminum-lithium alloys exhibit unique characteristics towards various aspects of performance, fatigue and fracture properties included. Significant differences can also be found in the performance of aluminum-lithium alloys developed at different times. The aim of this paper is to review different properties of aluminum-lithium alloys by comparing the performance difference between aluminum-lithium alloys and conventional aluminum alloys, and comparing the performance difference among different aluminum-lithium alloys. Based on the analyses of development history, mechanical properties, fatigue limit and fatigue resistance, and fatigue crack growth resistance, some suggestions for material selection in structural design are given.

The fatigue property data of 2 D plain-woven T300 carbon-fiber/SiC matrix composite material, which was published in literatures since 1990, was collected and categorized. The fitting of the total life S-N curve was conducted on the collected data. According to the shape parameter a of the S-N curve, T300 C/SiC material is classified into 3 categories. It is shown in the curve fitted results that the shape parameter a in the total life S-N curve represents the toughness of the material, in which case the larger parameter a is, the tougher the material is. Meanwhile the dimension parameter b represents the fatigue crack propagation rate, in which case the smaller parameter b is, the more sensitive to stress the fatigue damage is. On the basis of the fitted data, it is concluded that the endurance limit of 2 D plain-woven T300 carbon-fiber/SiC matrix composite material is approximately 70% of its strength.

The layout merits of wing internals can influence the weight of wing structure greatly, so it is necessary to perform the layout optimization design. The concurrent subspace optimization method is used to solve the layout optimization problem of wings, by which the layout optimization problem is divided into three subspaces of spar location, stringer and thickness optimization, and the design variables are optimized in their own subspace separately. After the optimization of each subspace. The design variables are coordinated in the system to maintain the optimization variables of the subspace with lightest weight unchanged. The approximate one-dimensional search method is used to coordinate the design variables of other subspace, and the variables are iterated until convergence. The results show that the method has high optimization efficiency, can obtain better optimization results, and has practical engineering value.

Microporosities in casting alloys cannot be avoided in the casting process, which reduces the fatigue life of structure greatly.The current progress of microprosity characteristics(size, shape and position)in casting alloys and their effects on fatigue life is reviewed and prospected, including the types of porosities, distribution rules of porosity characteristics, and prediction method of maximum porosity size and the fatigue life prediction method of castings containing porosities.Through reviewing the fatigue life prediction model, it is concluded that the current fatigue life prediction method of casting is not yet mature.The future research of this field is prospected.The future research should rely on advanced light sources to perform in-situ fatigue experiments or molecular dynamics simulations to study the microscopic mechanism of fatigue failure, and a quantitative prediction model for fatigue life considering different characteristics of porosities and the interaction between them should be established.

The fatigue tests of Plexiglas Resist45 PMMA at different frequencies were carried out and the S-N curves together with hysteresis loops were obtained. The results show that the fatigue life increases with frequency at low frequency domain. Creep damage and mechanical property rate-dependence are two attributes of frequency effect on fatigue life. And the frequency modified approach was employed to describe and estimate the fatigue life under different frequencies at low frequency domain and the calculated results are consistent with the experimental results.

For critical zone approach, the stress eigenvalues of a certain point, line or area in the vicinity of notch are used as the failure criteria. This paper presents a reasonable explanation for notch effect. Therefore, critical zone approach is considered as a fatigue life predict on method for notched specimens with a promising prospect in engineering application. The stress field intensity approach and the theory of critical distances for three-dimensional structures are first elaborated considering three-dimensional engineering structure characteristics. Several parameters are accurately defined and described. Then the two methods and the nominal stress approach are applied to predicting fatigue life of an aero engine disc. The results indicate that the proposed stress field intensity approach and the theory of critical distances for three-dimensional structures provide an excellent prediction on fatigue life. The two methods have a better prediction effect than the nominal stress approach. [ABSTRACT FROM AUTHOR]

Mixed-mode energy-based delamination growth criteria of laminated composites are summarized after an extensive literature survey. Ten criteria of them are analyzed comprehensively based on Reeder's research using 22 groups of test data from 11 kinds of materials. Evaluation results show that, B-K criterion with three parameters and Power-Law criterion with four parameters are relatively better considering the complexity of the expression and the prediction accuracy. This study contributes for the choice of the proper criteria in the analysis of delamination growth behaviors in future. [ABSTRACT FROM AUTHOR]

The size effect on the strength of fiber reinforced plastic(FRP) composite is investigated. The two main factors, statistical size effect and free edge effect, are responsible for the size effect in width. Statistical size effect originates from material inherent defects and it is described by the traditional Weibull weakest link theory. Free-edge effect comes into being because of stress singularity at edge and "waste width" is proposed to link the final failure strength with free-edge effect. Then a mixed model for the size effect in width is constructed by synthesizing the two effects. The experimental data from D Kujawski are employed to verify the proposed model and it is shown that the mixed model can predict the final failure strength of specimens with different widths. [ABSTRACT FROM AUTHOR]

The actual load spectrum applied on structures is the key of the fatigue life monitoring. This paper presents a method of physical prototype centered assessment for fatigue life of structure based on on-line monitoring data. Firstly the load spectrums of fatigue hot regions are recorded by some sensors, then the load of the structure is inversely computed by physical prototype, finally the fatigue life of the structure is estimated according to the measured stress and strain history. A specimen of lug is tested and the strains of eight points near the hole are recorded, and then the fatigue life is estimated. The results show the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A method for predicting the fatigue life of notched specimen considering the size effect is proposed. In this method, firstly the notched specimen is made equivalent to a reference smooth specimen. The ratio of the stresses on them is dependent on the fatigue notch factor. The size of the reference smooth specimen is determined according to the principle of having the same area of possible crack initiation surface as the notched specimen. Then based on the weakest link theory, the reference smooth specimen is made equivalent to a smooth specimen which has the same size as the smooth specimen whose experimental data is already available (here after to be called the standard smooth specimen). The ratio of stresses on them is dependent on Weibull exponent and the ratio of their sizes. Based on the above equivalences, the relation between the notched specimen and the standard smooth specimen is established, and the parameter of equivalent stress is introduced. Employing this parameter to refer to the test data of standard smooth specimen would lead to the fatigue life of notched specimen. This model reflects the influence of fatigue damage field area on the fatigue life of notched specimen, and can eliminate the phenomenon that referring to the test data of smooth specimens of different sizes using the same value of fatigue damage parameter would result in different values of fatigue life. An example of centre hole notched LY12CZ specimens is accomplished. The predicted result is in good agreement with the test result, which shows that the method is effective and practical. [ABSTRACT FROM AUTHOR]

A stiffness degradation model with two parameters is presented to describe the composite laminate's characteristic stiffness degradation rule under cyclic loading. The normalized stiffness degradation curves processed by this model agrees well with each other for different specimens under the same fatigue loading. Therefore a new fatigue life prediction method is presented considering the specimen's individual historical fatigue information and the stiffness degradation rule. Four kinds of E-glass/Epoxy laminates and five kinds of laminates with central holes are tested to verify the presented method. Results show that the fatigue life of composite specimen can be accurately predicted when considering its individual historical information. The prediction results agree fairly well with the experimental results by adopting the residual stiffness data about the characteristic damage state(CDS) only. [ABSTRACT FROM AUTHOR]