Papers by Keyword: Damage Tolerance

Abstract: The paper highlights key questions in ensuring safe operation of aging civil\transport aircraft in Russia. Presented is the analysis of current requirements to fatigue, fail-safe and damage tolerance for transport aircraft structures stated in FARs, Advisory Circulars (UAS) and Russian Airworthiness Regulations\ AR IAC Aviation Regulations. The paper gives the design goals and actual service life values\ service years of aging aircraft fleet and data on full-scaled fatigue tests, together with methods and approaches to ensure safe operation of aircraft structures in case of multiple site damages, corrosion and materials degradation for Russian aging fleet.

Abstract: Crack-growth-rate tests were conducted on compact, C(T), specimens made of 7075-T7351 aluminum alloy over a wide range of constant-amplitude loading (R = P_min/P_max = 0.1 to 0.9) to establish the baseline crack-growth-rate curve for life-prediction analyses. Both compression precracking and load-reduction methods were used. A crack-closure analysis was used to collapse the ΔK_eff-rate data into a fairly narrow band over many orders of magnitude in rates using an appropriate plane-strain constraint factor. Life predictions were made on C(T) specimens using the FASTRAN Version 5.42 life-prediction code. Some improvements had been made in the code and the predictions were made under cycle-by-cycle simulations. Life predictions under Christmas-Tree-type loading using the rainflow-on-the-fly methodology were very good. And the predicted results on three different aircraft spectrum loading histories (a modified Falstaff, modified Mini-TWIST (Level III), and a modified Wing-Gust-Maneuver spectrum), agreed to within 20% of the test data.

Abstract: Greatly reduced inspection intervals of skin damage repairs pose a significant financial problem for aging commercial aircraft fleets. Such intervals for visual inspections are the result of simplified conservative repair substantiation analyses, based on the same crack propagation scenarios and curves that were established in the initial project development. These neglect the structural role of external repair (“doubler”) and consider only the increase in hidden crack path. A more refined approach to reassessing inspection intervals after a repair may keep maintenance jobs in accordance with common C-check routines in most cases. This approach, based on new crack growth simulations for worst case scenarios that could occur at the region of repair, uses respective kinetic equation and new geometric stress intensity factor functions, obtained in additional FEM (Finite Element Method) analyses. In particular, for standard rectangular repairs, the number of possible geometric configurations is astonishing considering length, width, skin and “doubler” thickness, reinforced panel dimensions, and frame and stringer cross sections. This investigation deals mainly with defining a minimum sufficient number of intermediate crack length values for FEM analyses in each propagation scenario. A conservative but efficient definition of most relevant parameters for a new numerical analysis campaign is another important issue. The results obtained are helpful for the improvement of the operational efficiency and safety of an aging fleet.

Abstract: A review of some of the various fatigue models introduced over the years for both metallic materials, in particular aluminium alloys followed by fatigue and durability concerns associated with composite materials. The move towards light weight and high stiffness structures that have good fatigue durability and corrosion resistance has led to the rapid move from metal structures to composite structures. With this brings the added concern of certifying new components as the damage mechanisms and failure modes in metals differ significantly than composite materials such as carbon fiber reinforced polymers (CFRP). The certification philosophy for composites must meet the same structural integrity, safety and durability requirements as that of metals. Hence this is where the challenge now lies. Substantial work has been conducted in the reparability of composite structures through bonding using various adherend thicknesses and joint types and has been shown to have higher durability than mechanically fastened repairs for thin adherends however these are currently unacceptable repair methods as they cannot be certified. Repairs are designed on the basis that the repair efficiency can be predicted and should be designed conservatively with respect to the various failure modes and include the surrounding structure.

Abstract: Mechanical properties, including fracture toughness and the fatigue crack propagation, have been investigated for two different microstructures in TC21 titanium alloy. The characteristic parameters of lamellar microstructure were measured through quantitative metallography. On the basis of analyzing the influence of microstructure on the fracture toughness of TC21 titanium alloy, the mathematical models between the fracture toughness and microstructure parameters were constructed. Furthermore, the relationship between fatigue crack growth rate and mean length values of α lamellar have been also investigated and a novel predicting model was established.

Abstract: In this work, the performance of a new methodology, based on the Dual Boundary Element Method (DBEM) and applied to reinforced cracked aeronautic panels, is assessed. Such procedure is mainly based on two-dimensional stress analyses, whereas the three-dimensional modelling, always implemented in conjunction with the sub-modelling approach, is limited to those situations in which the so-called secondary bending effects cannot be neglected. The connection between the different layers (patches and main panel) is realised by rivets: a peculiar original arrangement of the rivet configuration in the two-dimensional DBEM model allows to take into account the real in-plane panel stiffness and the transversal rivet stiffness, even with a two dimensional approach. Different in plane loading configurations are considered, depending on the presence of a biaxial or uniaxial remote load. The nonlinear hole/rivet contact, is simulated by gap elements when needed. The most stressed skin holes are highlighted, and the effect of through the thickness cracks, initiated from the aforementioned holes, is analysed in terms of stress redistribution, SIF evaluation and crack propagation. The two-dimensional approximation for such kind of problems is generally not detrimental to the accuracy level, due the low thickness of involved panels, and is particularly efficient for studying varying reinforcement configurations, where reduced run times and a lean pre-processing phase are prerequisites.The accuracy of the proposed approach is assessed by comparison with Finite Element Method (FEM) results and experimental tests available in literature.This approach aims at providing a general purpose prediction tool useful to improve the understanding of the fatigue resistance of aeronautic panels.KEYWORDSDBEM, full scale aeronautic panel, 2D/3D crack growth, MSD, doubler-skin assembly, damage tolerance

Abstract: Because of its advantage of zero casualties, Unmanned Aerial Vehicle (UAV) plays an important role in battlefield monitoring and information acquiring, thus caught great attention of the world. But with large quantities of UAVs equipped, uncertain life-span has become an important constraint of operating UAV safely and economically. So assessing the service life has important military and economic values. In the entire UAV system, the airframe life severely affects operation of UAV. According to method of damage coefficient, evaluation model on life of UAV airframe was established. In the case of security, its fatigue life was determined.

Abstract: This paper mainly presents a method that can characterize the damage tolerance capability of composite laminates based on the knee-point feature. Based on the experimental study and numerical result, the Knee-point mechanism has been investigated. A new influence coefficient, δ, was introduced, which is applied to calculating the influence of every piles damage to the residual strength of the whole laminates. Finally, a conclusion can be drawn that the damage of 0° piles can represent the damage of the whole laminates, whilst the damage area of 0° piles can be applied to characterizing the damage tolerance capability of composite laminates.

Abstract: In this paper, two carbon fiber/bismaleimide resin composites were prepared via ex-situ toughening technique. The damage resistance and damage tolerance of these two fiber-reinforced polymer composites were then studied in terms of low-velocity impact test and compression strength after impact (CAI). The results indicated that the T300 carbon fiber-reinforced composites had better damage resistance response and higher damage tolerance, and the visual observation of damage was also achieved. However, the T700 carbon fiber-reinforced composites had higher breaking elongation, and therefore the composite was not sensitive to the low-velocity impact. In addition, the CAI strength of this composite was still low.

Abstract: In an effort to improve the reliability and reduce the operating costs of helicopter structure, an increasing emphasis is being placed on the damage tolerant approach for life management of helicopter structure. Corrosion damage of helicopter dynamic components and life reliability prediction approach has been systematically analyzed. Based on the conclusion of fatigue life under normal environment, the model for life reliability analysis of helicopter dynamic components has been established. A hybrid approach based on a mixture of the traditional safe-life and damage tolerance techniques can be used as an optimal strategy for ensuring helicopter structural integrity; the life of a helicopter main rotor blade under corrosion environment has been obtained according to flaw-tolerance method. According to probabilistic fracture mechanics theory, the analytical model of structures with initial corner cracks has been established. Thus the service life and inspection intervals according to the request of reliability can be determined; it is very valuable in engineering for life prediction and monitoring of helicopter dynamic components under corrosion environment.