SCIENCE CHINA Physics, Mechanics & Astronomy 2014, Volume 57, Issue 1

Special Topic: Airworthiness and Fatigue

http://phys.scichina.com:8083/sciGe/EN/volumn/volumn_6444.shtml

http://link.springer.com/journal/11433/57/1/page/1

Aviation safety is the theme given to the International Conference on Airworthiness and Fatigue: 7^th ICSAELS Series Conference, Beijing, China, held on 25–27 March, 2013. This event was initiated by the International Center of Sustainability, Accountability, and Eco-Affordability for Large and Small (ICSAELS) and the Institute of Mechanics, Chinese Academy of Sciences.

Energy efficiency of aviation transport is vital to the conservation of natural resource that requires for a common understanding to address the mechanical, electrical and chemical effects, as well as the multiscale material constituents. The Industrial Revolution has already paved the way from the large scale systems to the very small. Miniaturization and nanotechnology have been the new challenge. Cutting edge scientific and technological endeavors will not mature without paying dues to learning. The interaction of technology and financial measure is an acute challenge of the 21st century. The outsourcing of core technology and to use financial measure as a gauge of net profits should be constrained by technical performance.

In this respect, what was established for mechanical effects can be scaled to balance with chemical effects, though the sustainability and stability of the systems can vary. Spatial-temporal scaling of the mechanical, material structural and chemical entities is necessary in design. This area of research is known as “Multiscaling and Mesomechanics”. More specifically, the three familiar quantities of specific energy density, volumetric energy density and specific power density have been used commercially for specifying the performance of the lithium-ion battery. In other words, the power dissipation behavior of the lithium-ion battery is, in principle, the same as that for the fatigue of metals, except for the difference in space-time scale range. “Power Dissipation Scaling” cannot be over emphasized in order to link the chemical to the mechanical effects that are less distanced from the sensory perception. Hopefully, the aforementioned spirit will be reflected by the theme of “Airworthiness and Fatigue” of this conference. Again, safety, defect, strength and fatigue cannot be separated from overall technical considerations.

The original concept and idea to hold this conference was materialized by the members of the Scientific Advisory Committee. Their contributions can be clearly evidenced from the technical presentations and have laid the foundation to carry the torch to the destination of aviation safety. This is only the first leg of the relay.

This special issue of Science China Physics, Mechanics & Astronomy contains twelve papers selected from the Proceedings of International Conference on Airworthiness & Fatigue, with authors from Australia, Russia, Greece, Italy, United States and China. The original conference manuscripts have been extended to the journal style papers by the authors and have been subjected to normal peer review processing. We hope that this collection of the papers with the current progresses in Airworthiness and Fatigue is of benefit to the relevant research and application community.

SIH George C.

Lehigh University

HONG YouShi

Institute of Mechanics, Chinese Academy of Sciences

Foreword   PDF (249KB)

Cite |  SIH George C., HONG YouShi. Foreword. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 1-1.

Adhesive bonding of composite aircraft structures：Challenges and recent developments   PDF (1080KB)

Cite |  PANTELAKIS Sp., TSERPES K. I.. Adhesive bonding of composite aircraft structures：Challenges and recent developments. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 2-11.

Abstract:    In this review paper, the challenges and some recent developments of adhesive bonding technology in composite aircraft structures are discussed. The durability of bonded joints is defined and presented for parameters that may influence bonding quality. Presented is also, a numerical design approach for composite joining profiles used to realize adhesive bonding. It is shown that environmental ageing and pre-bond contamination of bonding surfaces may degrade significantly fracture toughness of bonded joints. Moreover, it is obvious that additional research is needed in order to design joining profiles that will enable load transfer through shearing of the bondline. These findings, together with the limited capabilities of existing non-destructive testing techniques, can partially explain the confined use of adhesive bonding in primary composite aircraft structural parts.

Application of supersonic particle deposition to enhance the structural integrity of aircraft structures   PDF (946KB)

Cite |  Matthews N., Jones R., Sih G.C. Application of supersonic particle deposition to enhance the structural integrity of aircraft structures. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 12-18.

Abstract:    Aircraft metal components and structures are susceptible to environmental degradation throughout their original design life and in many cases their extended lives. This paper summarizes the results of an experimental program to evaluate the ability of Supersonic Particle Deposition (SPD), also known as cold spray, to extend the limit of validity (LOV) of aircraft structural components and to restore the structural integrity of corroded panels. In this study the potential for the SPD to seal the mechanically fastened joints and for this seal to remain intact even in the presence of multi-site damage (MSD) has been evaluated. By sealing the joint the onset of corrosion damage in the joint can be significantly retarded, possibly even eliminated, thereby dramatically extending the LOV of mechanically fastened joints. The study also shows that SPD can dramatically increase the damage tolerance of badly corroded wing skins.

Very-High-Cycle-Fatigue of in-service air-engine blades, compressor and turbine   PDF (1100KB)

Cite |  SHANYAVSKIY A A. Very-High-Cycle-Fatigue of in-service air-engine blades, compressor and turbine. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 19-29.

Abstract:    In-service Very-High-Cycle-Fatigue (VHCF) regime of compressor vane and turbine rotor blades of the Al-based alloy VD-17 and superalloy GS6K, respectively, was considered. Surface crack origination occurred at the lifetime more than 1500 hours for vanes and after 550 hours for turbine blades. Performed fractographic investigations have shown that subsurface crack origination in vanes took place inspite of corrosion pittings on the blade surface. This material behavior reflected lifetime limit that was reached by the criterion VHCF. In superalloy GS6K subsurface fatigue cracking took place with the appearance of flat facet. This phenomenon was discussed and compared with specimens cracking of the same superalloy but prepared by the powder technology. In turbine blades VHCF regime appeared because of resonance of blades under the influenced gas stream. Both cases of compressor-vanes and turbine blades in-service cracking were discussed with crack growth period and stress equivalent estimations. Recommendations to continue aircrafts airworthiness were made for in-service blades.

Fatigue strength of Al7075 notched plates based on the local SED averaged over a control volume   PDF (902KB)

Cite |  BERTO Filippo, LAZZARIN Paolo. Fatigue strength of Al7075 notched plates based on the local SED averaged over a control volume. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 30-38.

Abstract:    When pointed V-notches weaken structural components, local stresses are singular and their intensities are expressed in terms of the notch stress intensity factors (NSIFs). These parameters have been widely used for fatigue assessments of welded structures under high cycle fatigue and sharp notches in plates made of brittle materials subjected to static loading. Fine meshes are required to capture the asymptotic stress distributions ahead of the notch tip and evaluate the relevant NSIFs. On the other hand, when the aim is to determine the local Strain Energy Density (SED) averaged in a control volume embracing the point of stress singularity, refined meshes are, not at all, necessary. The SED can be evaluated from nodal displacements and regular coarse meshes provide accurate values for the averaged local SED. In the present contribution, the link between the SED and the NSIFs is discussed by considering some typical welded joints and sharp V-notches. The procedure based on the SED has been also proofed to be useful for determining theoretical stress concentration factors of blunt notches and holes. In the second part of this work an application of the strain energy density to the fatigue assessment of Al7075 notched plates is presented. The experimental data are taken from the recent literature and refer to notched specimens subjected to different shot peening treatments aimed to increase the notch fatigue strength with respect to the parent material.

From monoscale to multiscale modeling of fatigue crack growth：Stress and energy density factor   PDF (1340KB)

Cite |  SIH G.C. From monoscale to multiscale modeling of fatigue crack growth：Stress and energy density factor. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 39-50.

Abstract:    The formalism of the earlier fatigue crack growth models is retained to account for multiscaling of the fatigue process that involves the creation of macrocracks from the accumulation of micro damage. The effects of at least two scales, say micro to macro, must be accounted for. The same data can thus be reinterpreted by the invariancy of the transitional stress intensity factors such that the microcracking and macrocracking data would lie on a straight line. The threshold associated with the sigmoid curve disappears. Scale segmentation is shown to be a necessity for addressing multiscale energy dissipative processes such as fatigue and creep. Path independency and energy release rate are monoscale criteria that can lead to unphysical results, violating the first principles. Application of monoscale failure or fracture criteria to nanomaterials is taking toll at the expense of manufacturing super strength and light materials and structural components. This brief view is offered in the spirit of much needed additional research for the reinforcement of materials by creating nanoscale interfaces with sustainable time in service. The step by step consideraton at the different scales may offer a better understanding of the test data and their limitations with reference to space and time.

Multiscale approach to micro/macro fatigue crack growth in 2024-T3 aluminum panel   PDF (732KB)

Cite |  SIH G.C. Multiscale approach to micro/macro fatigue crack growth in 2024-T3 aluminum panel. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 51-58.

Abstract:    When two contacting solid surfaces are tightly closed and invisible to the naked eye, the discontinuity is said to be microscopic regardless of whether its length is short or long. By this definition, it is not sufficient to distinguish the difference between a micro-and macro-crack by using the length parameter. Microcracks in high strength metal alloys have been known to be several centimeters or longer. Considered in this work is a dual scale fatigue crack growth model where the main crack can be micro or macro but there prevails an inherent microscopic tip region that is damaged depending on the irregularities of the microstructure. This region is referred to as the "micro-tip" and can be simulated by a sharp wedge with different angles in addition to mixed boundary conditions. The combination is sufficient to model microscopic entities in the form of voids, inclusions, precipitations, interfaces, in addition to subgrain imperfections, or cluster of dislocations. This is accomplished by using the method of "singularity representation" such that closed form asymptotic solutions can be obtained for the development of fatigue crack growth rate relations with three parameters. They include: (1) the crack surface tightness δ^* represented by δ_o/δ_∞ = 0.3-0.5 for short cracks in region Ⅰ, and 0.1-0.2 for long cracks in region Ⅱ, (2) the micro/macro material properties reflected by the shear modulus ratio μ^* (=μ_micro/μ_macro varying between 2 and 5) and (3) the most sensitive parameter d^* being the micro-tip characteristic length d^* (=d/d_o) whose magnitude decreases in the direction of region Ⅰ→Ⅱ. The existing fatigue crack growth data for 2024-T3 and 7075-T6 aluminum sheets are used to reinterpret the two-parameter da/dN=C(ΔK)ⁿrelation where ΔK has now been re-derived for a microcrack with surfaces tightly in contact. The contact force will depend on the mean stress sm or mean stress ratio R as the primary parameter and on the stress amplitude σ_a as the secondary parameter.

Application of aluminum foil for “strain sensing” at fatigue damage evaluation of carbon fiber composite   PDF (722KB)

Cite |  PANIN Sergey, BURKOV Mikhail, LYUBUTIN Pavel, ALTUKHOV Yurii. Application of aluminum foil for “strain sensing” at fatigue damage evaluation of carbon fiber composite. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 59-64.

Abstract:    Surface layer of a loaded solid is an individual structural level of deformation that was shown numerously within concept of physical mesomechanics. This gives rise to advance in its deformation development under loading as well as allows using this phenomenon to sense the strain induced structure changes. It is of specific importance for composite materials since they are highly heterogeneous while estimating their mechanical state is a topical applied problem. Fatigue tests of carbon fiber composite specimens were carried out for cyclic deformation estimation with the use of strain sensors made of thin (80 μm) aluminum foil glued to the specimen's surface. The surface images were captured by DSLR camera mounted onto an optical microscope. Strain relief to form during cyclic loading was numerically estimated using different parameters: dispersion, mean square error, universal image quality index, fractal dimension and energy of Fourier spectrum. The results are discussed in view of deformation mismatch in thin foil and bulk specimen and are offered to be applied for the development of Structural Health Monitoring (SHM) approach.

Study of methods for evaluating the probability of multiple site damage occurrences   PDF (836KB)

Cite |  ZHANG JianPing, ZHANG JianYu, BAO Rui, ZHENG XiaoLing. Study of methods for evaluating the probability of multiple site damage occurrences. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 65-73.

Abstract:    Two methods currently available for evaluating the probability of Multiple Site Damage (MSD) occurrence were studied in this paper. One of the methods is a probabilistic analysis approach based on the statistical theory and fatigue characteristics of each structural detail, and the other is an approach which defines the initial damage scenario by means of Monte-Carlo simulation, and multiple initial crack scenarios are randomly generated. A modified method based on the Monte-Carlo simulation was proposed in this paper, in which the random fluctuation of the stress was considered to give more accurate evaluation results. In the presented method, the probability of MSD occurrence in a structural element containing multiple details was calculated based on the Monte-Carlo simulation and the p-S-N curve of a single structural detail. Fatigue tests were accomplished using specimens containing 21-similar-details to obtain the fatigue life corresponding to MSD occurrence. Tests on single-detail specimens and static calibration tests were also conducted to get the basic fatigue properties of the material and the degree of stress fluctuation. The aforementioned three methods were compared and validated via the test results. The influence of the stress random fluctuation degree on the probability of MSD occurrence and influence of the distribution types on evaluating the MSD occurrence probability were discussed.

Effects of loading condition on very-high-cycle fatigue behaviour and dominant variable analysis   PDF (923KB)

Cite |  LEI ZhengQiang, XIE JiJia, SUN ChengQi, HONG YouShi. Effects of loading condition on very-high-cycle fatigue behaviour and dominant variable analysis. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 74-82.

Abstract:    The specimens of a high carbon chromium steel were quenched and tempered at 150℃, 180℃ and 300℃. Such specimens were tested via rotating bending and a push-pull type of axial loading to investigate the influences of loading condition on the behaviour of very-high-cycle fatigue (VHCF). Experimental results show the different influences of inclusion size on the fatigue life for the two loading conditions. Predominant factors and mechanism for the fine-granular-area (FGA) of crack origin were discussed. In addition, a reliability analysis based on a modified Tanaka-Mura model was carried out to evaluate the sensitivity of inclusion size, stress, and ΔK_FGA to the life of VHCF crack initiation.

Fatigue crack growth in fiber-metal laminates   PDF (827KB)

Cite |  MA YuE, XIA ZhongChun, XIONG XiaoFeng. Fatigue crack growth in fiber-metal laminates. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 83-89.

Abstract:    Fiber-metal laminates (FMLs) consist of three layers of aluminum alloy 2024-T3 and two layers of glass/epoxy prepreg, and it (it means FMLs) is laminated by Al alloy and fiber alternatively. Fatigue crack growth rates in notched fiber-metal laminates under constant amplitude fatigue loading were studied experimentally and numerically and were compared with them in monolithic 2024-T3 Al alloy plates. It is shown that the fatigue life of FMLs is about 17 times longer than monolithic 2024-T3 Al alloy plate; and crack growth rates in FMLs panels remain constant mostly even when the crack is long, unlike in the monolithic 2024-T3 Al alloy plates. The formula to calculate bridge stress profiles of FMLs was derived based on the fracture theory. A program by Matlab was developed to calculate the distribution of bridge stress in FMLs, and then fatigue growth lives were obtained. Finite element models of FMLs were built and meshed finely to analyze the stress distributions. Both results were compared with the experimental results. They agree well with each other.

Scatter of fatigue data owing to material microscopic effects   PDF (814KB)

Cite |  TANG XueSong. Scatter of fatigue data owing to material microscopic effects. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 90-97.

Abstract:    A common phenomenon of fatigue test data reported in the open literature such as S-N curves exhibits the scatter of points for a group of same specimens under the same loading condition. The reason is well known that the microstructure is different from specimen to specimen even in the same group. Specifically, a fatigue failure process is a multi-scale problem so that a fatigue failure model should have the ability to take the microscopic effect into account. A physically-based trans-scale crack model is established and the analytical solution is obtained by coupling the micro-and macro-scale. Obtained is the trans-scale stress intensity factor as well as the trans-scale strain energy density (SED) factor. By taking this trans-scale SEDF as a key controlling parameter for the fatigue crack propagation from micro-to macro-scale, a trans-scale fatigue crack growth model is proposed in this work which can reflect the microscopic effect and scale transition in a fatigue process. The fatigue test data of aluminum alloy LY12 plate specimens is chosen to check the model. Two S-N experimental curves for cyclic stress ratio R=0.02 and R=0.6 are selected. The scattering test data points and two S-N curves for both R=0.02 and R=0.6 are exactly re-produced by application of the proposed model. It is demonstrated that the proposed model is able to reflect the multiscaling effect in a fatigue process. The result also shows that the microscopic effect has a pronounced influence on the fatigue life of specimens.

Fatigue damage rule of LY12CZ aluminium alloy under sequential biaxial loading   PDF (585KB)

Cite |  WANG YingYu, ZHANG DaChuan, YAO WeiXing. Fatigue damage rule of LY12CZ aluminium alloy under sequential biaxial loading. SCIENCE CHINA Physics, Mechanics & Astronomy, 2014, 57 (1): 98-103.

Abstract:    A series of biaxial two-level variable amplitude loading tests are conducted on smooth tubular specimens of LY12CZ aluminium alloy. The loading paths of 90° out-of-phase, 45° out-of-phase and 45° in-phase are utilized. The fatigue damage cumulative rules under two-level step loading of three loading paths are analyzed. By introducing a parameter a which is a function of the phase lag angle between the axial and the torsional loading, a new multiaxial nonlinear fatigue damage cumulative model is proposed. The proposed model is evaluated by the experimental data for two-level loading, multi-level loading of LY12CZ aluminium alloy, and multi-level loading of 45 steel. Fatigue lives predicted are within a factor of 2 scatter band.