Experimental Investigation and Failure Criterion Development for CFR Composite Tubular Specimens Under Combined Axial and Pressure Loading

The use of composite structures in military aircraft has grown significantly in recent years. As an example, the F-15 is approximately 2 percent composite by weight compared to the more recent AV-8B which is approximately 31 percent by weight. Projections for the use of composite materials in future aircraft design indicate that this trend will continue. A significant portion of the increased use of composites involves essential aircraft structural components—wings, stabilators, control surfaces, etc. Consequently, regular inspection of these structures at military installations will become increasingly necessary to help assure structural integrity.KeywordsInspection DataUltrasonic InspectionMilitary AircraftMilitary InstallationSearch UnitThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Turbo generator shafts are manufactured through the extrusion process. This results in formation of weak planes along the extrusion direction. Under service loading (e.g. cyclic torsion due to electrical line faults), large longitudinal cracks often form in these shafts before the appearance of any circumferential cracks. The presence of these cracks could severely compromise the shaft resonance frequencies. Here, we investigated the dynamic response of solid turbo generator shafts with longitudinal and circumferential cracks. The longitudinal cracked section of the shaft section was modeled as a shaft with reduced effective torsional rigidity. The effective torsional rigidity was found to be a function of ratio of crack depth to the shaft radius only. The circumferential cracked section was modeled as a torsional spring, with the torsional spring constant determined using fracture mechanics principles. It was found that the resonance frequency of the shaft may be little affected by the presence of a longitudinal crack. The resonance frequencies of the shaft with the circumferential crack depend on the crack length and its location. The effects of crack surface interactions for both longitudinal and circumferential cracks were also investigated. For circumferential cracked shafts, the sever crack surface interaction results in the peak response frequency approaches to that of un-cracked shafts. However the frequency where the peak response occurs for a longitudinally-cracked shaft generally exceeds that of un-cracked shaft first resonance frequency.

On damage tolerance design of fuselage structure (longitudinal cracks)

Turbo generator shafts are often subjected to cyclic torsion resulting in formation of large longitudinal cracks as well as circumferential cracks. The presence of these cracks could greatly impact the shaft resonance frequencies. In this paper, dynamic response of a shaft with longitudinal and circumferential cracks is investigated through a comprehensive analytical study. The longitudinally cracked section of the shaft was modeled as an uncracked shaft with reduced torsional rigidity. Torsional rigidity correction factor (i.e., the ratio of torsional rigidity of the cracked shaft to that of the uncracked shaft) was obtained from finite element analysis and was shown to be only a function of crack depth to the shaft radius. The resonance frequency and frictional energy loss of a shaft with a longitudinal crack were found little affected by the presence of the crack as long as the crack depth was less than 20% of the shaft radius even if the entire shaft is cracked longitudinally. Moreover, we showed that the longitudinal crack location could be more conveniently identified by monitoring the slope of the torsional response along the shaft. The circumferential crack was modeled as a torsional spring with a torsional damping. The torsion spring and damping constants were obtained using fracture mechanics. For a shaft with both a longitudinal crack and a circumferential crack, the resonance frequency was governed by the longitudinal crack when the circumferential crack depth was less than 30% of the shaft radius.

To study the failure conditions of anisotropic loess in the principal stress space, a series of true triaxial tests were performed on loess sampled in the Bailu Plateau of Xi’an, as loess deposits have a strong cross-anisotropic structure. The cuboid specimens of the prepared samples were cut along the vertical and horizontal directions to enable testing in any sector of the three-dimensional principal stress space. The shear strength laws of the p–q planes with different intermediate principal stress ratios corresponding to different principal stress spaces were revealed. The failure surface was assumed to be symmetric with respect to the vertical axis of the principal stress space octahedral plane, which varies as a function of the Lode angle. On the same octahedral plane, the shear strength of a specimen, as the major stress, is applied along the vertical direction and is larger than that along the horizontal direction. Failure surfaces in all sectors are ellipses, which are symmetric with respect to the vertical axis, and the loess structure is not damaged under the stress conditions of true triaxial compression. Once the structure was damaged, the failure surface of the cross-anisotropic loess on the octahedral planes presented as a circle or a curved triangle as the average principal stress increased.

New approach to failure of pre-cracked brittle materials based on regularized solutions of strain gradient elasticity

A high geo-stress environment severely damages tunnel structures owing to the large deformation of the surrounding rock, thereby threatening their safety. In this study, the Wushaoling highway tunnel group, which passes through a high geo-stress environment, is investigated as a case study. The spatial distribution of different types of tunnel cracks is analyzed based on the site observations. The lining crack variations, such as circumferential and longitudinal cracks, with the factors including grade of the surrounding rock, location of buried depth, and design type of the tunnel lining are investigated. Various structural reinforcement technologies are presented based on the damage degrees of the tunnel linings. Several finite element models of supporting structures are established to reveal the mechanism of tunnel crack control technology. The results show that longitudinal and circumferential cracks are the main characteristics of tunnel lining damage in high geo-stress environments, accounting for approximately 29.4 and 53.2% of the total cracks, respectively. SIVb-, SVc-, and SVd-type linings show numerous longitudinal cracks per kilometer. The longitudinal cracks appear primarily on the tunnel crown and hance. In terms of the lining support types in Grade V surrounding rocks, the number of longitudinal cracks per kilometer increases with the lining grade. The number of tunnel cracks per kilometer tends to increase with the buried depth. Four tunnel structure reinforcement treatment measurements for lining cracks in high geo-stress condition were innovatively proposed, which were proved effective in deformation controlling and strengthening the tunnel lining using numerical investigation. The key contribution of this research is to reveal the characteristics and evolution mechanism of tunnel lining cracks in high geo-stress condition, and provide effective treatments for the tunnel lining cracks. In addition, the findings from the study on the tunnel lining cracks also provide industry practitioners with a comprehensive guide regarding the characteristics and control techniques of the tunnel lining cracks, which can serve as a steppingstone to facilitate the construction technology development of the transportation industry.

Stress paths and mechanical behavior of a sandstone in hollow cylinder tests

We have been developing a Structural Health Monitoring (SHM) System for Carbon Fiber Reinforced Plastic (CFRP) structure of aircraft. This system can diagnose structural integrity of the composite and metal aircraft structure by analyzing a Lamb wave which is a type of ultrasonic wave. Our SHM system consists of Macro Fiber Composite (MFC) piezoelectric actuators, Fiber Bragg Grating (FBG) optical fiber sensors, interrogation unit and control/analysis software. The MFC actuators and the FBG sensors are worked as source and detector of the Lamb wave, respectively. The Lamb wave propagates into aircraft structures and if there are some damages and defects in the structures, measured waveform will be changed. The analysis software of the SHM system can calculate degree of the damages and the defects by evaluation of the waveform change. The SHM system has been developed as an inspection and maintenance support system for the composite aircraft structure and it will reduce operation cost and improve flight safety. In the previous study, we had shown that the SHM system could diagnose debonding length quantitatively in the edge of the skin/stringer bonding region, generally called Hot-Spot. And Probability of Detection (PoD) of the system was also evaluated. In this study, the SHM system is enhanced to be able to diagnose more widely region and its damage detection ability is evaluated. Test results show that the SHM system can diagnose structural integrity precisely not only in the HotSpot region but also entire bond-line of the composite aircraft structure. doi: 10.12783/SHM2015/25

On damage tolerance design of fuselage structure—circumferential cracks

This paper describes the development of an instrumented tap test and imaging system for the inspection of defects and damages in composite aircraft structures, especially honeycomb sandwich constructions. The resulting tap test images based on the impact duration and displayed in a C- scan format can readily reveal the shape and extent of honeycomb core damages. The images also reveal considerable detail of internal substructures such as core splices, ply build-up, and changes of foam core density. Based on a grounded-spring mechanical model, the tap scan images can be converted into images that show the quantitative changes of the local stiffness. A manual tap scan imaging system was assembled for laboratory experiments. Using actual composites parts for airplanes, impact damages and substructures were imaged. In addition, effects of the tapper mass, impactor radius, tap velocity and operator dependence were studied. A compact fieldable system is being built and an automated tapping and imaging system is also under development. The technique was compared to other similar devices currently available.

The shear strength failure surface of soil is the base of analyzing the characteristics of deformation and strength. In this paper, the tests of remold loess with different moisture contents under complicated stress condition was carried out by a true tri-axial apparatuses with a new structural cell which was developed by Xi'an University of Technology. The stress and strain curves of remolded loess were obtained under different consolidation pressures and different stress paths. The variation laws of strength failure lines under different stress paths were analyzed here. At the same middle principal stress parameter, the shear strength q f increases with mean normal stress p. The q f - p strength lines of remold loess with different water content pass through the same point on the p coordinate axial. In other words, the strength failure surfaces of the remolded loess with different moisture content are cone in the principal stress space. On the π plane, the points of strength failure of remold loess with same water content on the π plane changes regularly, and the strength failure surfaces of the remolded loess with different water content are similar. With increasing of moisture content, the strength curve on the π plane shrinks gradually, and the strength failure lines are similar under the different water content condition. Comparing the measured results with the strength criterion, the strength failure surface can be described by Xing Yichuan surface located at the outside of Mohr-Coulomb surface and the Matsouka-Nakai surface, and the inside of generalized Misses surface.

Biaxial compression-compression, biaxial tension-compression and compression-shear tests were carried out on self-compacting concrete (SCC) using the rock true triaxial machine and compression-shear hydraulic servo machine to explore the biaxial mechanical properties of SCC. The failure modes and stress-strain curves of SCC under different loading conditions were obtained through experiment. Based on the comparison with the biaxial loading test data of ordinary concrete, the following conclusions are drawn: the failure modes and failure mechanisms under biaxial compression-compression and biaxial tension-compression are similar between SCC and ordinary concrete. Under compression-shear loading, the oblique cracks formed on the lateral surface of the specimen parallel to the shear direction gradually increased and the friction marks on the shear failure section were gradually deepened with the increase of axial compression ratio. The development trend of the stress-strain curve in the principal stress direction was not related to the lateral stress. Under the influence of lateral compressive stress, the principal compressive stress of SCC was increased by 55.78% on average; under biaxial tension-compression, the principal tensile stress of SCC had a maximum reduction of 62.79%; and under the compression-shear action, the shear stress of SCC had a maximum increase of 3.35 times. Compared with the biaxial stress test data of ordinary concrete, it can be seen that the lateral compressive stress had a more significant effect on the principal stress of SCC under biaxial loading. Subsequently, the strength criterion equations of SCC under biaxial loading were proposed based on the principal stress space and octahedral space stress respectively, which have shown good applicability in practice.

Biaxial compression-compression, biaxial tension-compression and compression-shear tests were carried out on self-compacting concrete (SCC) using the rock true triaxial machine and compression-shear hydraulic servo machine to explore the biaxial mechanical properties of SCC. The failure modes and stress-strain curves of SCC under different loading conditions were obtained through experiment. Based on the comparison with the biaxial loading test data of ordinary concrete, the following conclusions are drawn: the failure modes and failure mechanisms under biaxial compression-compression and biaxial tension-compression are similar between SCC and ordinary concrete. Under compression-shear loading, the oblique cracks formed on the lateral surface of the specimen parallel to the shear direction gradually increased and the friction marks on the shear failure section were gradually deepened with the increase of axial compression ratio. The development trend of the stress-strain curve in the principal stress direction was not related to the lateral stress. Under the influence of lateral compressive stress, the principal compressive stress of SCC was increased by 55.78% on average; under biaxial tension-compression, the principal tensile stress of SCC had a maximum reduction of 62.79%; and under the compression-shear action, the shear stress of SCC had a maximum increase of 3.35 times. Compared with the biaxial stress test data of ordinary concrete, it can be seen that the lateral compressive stress had a more significant effect on the principal stress of SCC under biaxial loading. Subsequently, the strength criterion equations of SCC under biaxial loading were proposed based on the principal stress space and octahedral space stress respectively, which have shown good applicability in practice.

Biaxial compression-compression, biaxial tension-compression and compression-shear tests were carried out on self-compacting concrete (SCC) using the rock true triaxial machine and compression-shear hydraulic servo machine to explore the biaxial mechanical properties of SCC. The failure modes and stress-strain curves of SCC under different loading conditions were obtained through experiment. Based on the comparison with the biaxial loading test data of ordinary concrete, the following conclusions are drawn: the failure modes and failure mechanisms under biaxial compression-compression and biaxial tension-compression are similar between SCC and ordinary concrete. Under compression-shear loading, the oblique cracks formed on the lateral surface of the specimen parallel to the shear direction gradually increased and the friction marks on the shear failure section were gradually deepened with the increase of axial compression ratio. The development trend of the stress-strain curve in the principal stress direction was not related to the lateral stress. Under the influence of lateral compressive stress, the principal compressive stress of SCC was increased by 55.78% on average; under biaxial tension-compression, the principal tensile stress of SCC had a maximum reduction of 62.79%; and under the compression-shear action, the shear stress of SCC had a maximum increase of 3.35 times. Compared with the biaxial stress test data of ordinary concrete, it can be seen that the lateral compressive stress had a more significant effect on the principal stress of SCC under biaxial loading. Subsequently, the strength criterion equations of SCC under biaxial loading were proposed based on the principal stress space and octahedral space stress respectively, which have shown good applicability in practice.