Papers by Keyword: Stiffness Degradation

Abstract: The fatigue behaviour of [0°/±45°] triaxial braided composites is investigated. Tension-tension and compression-compression loading is chosen to study main damage mechanisms and their effect on the stiffness degradation. In both cyclic loading cases a high stiffness degradation can be observed shortly after the start of cyclic loading. Inter-fibre fractures in the braid yarns with subsequent delaminations are responsible for this behaviour during tension-tension loading. Compression-compression loading leads to kink band formation in the axial yarns favoured by yarn undulations. In consequence of the observed material degradation, S-N curves based on a defined level of stiffness degradation are proposed for fatigue dimensioning instead of load cycles to fracture.

Abstract: Based on the fatigue damage mechanism, fatigue life, stiffness degradation, crack width change, bending, shear fatigue properties and other aspects, this paper introduces the research progress of the fatigue properties of ordinary reinforced concrete beams. And the existing reinforced concrete beam flexural, shear fatigue properties of research ideas, methods and results are summarized, providing the basis for further study on the fatigue performance of reinforced concrete beams. At present, the research results show that the fatigue damage of reinforced concrete beam is basically in accordance with the law of the three stages. In the early stage of fatigue, the tensile concrete cracks and exits, and the damage develops rapidly. In the middle of fatigue crack growth, fatigue damage is developed into a more moderate linear growth. In the late stage of fatigue, fatigue fracture occurs in the steel bar, and the bearing capacity of the beam is quickly lost.

Abstract: Carbon fibre composites used in many safety-critical applications experience damage due to operating environment such as bird strike, hailstorm etc., In some cases, the damage is barely visible and its presence in the structure can cause accelerated damage leading to catastrophic failure. In this paper, the results of fatigue damage progression in woven CFRP laminate subjected to constant amplitude loading for un-impacted specimens as well as impacted specimens are presented. The stiffness of the specimens was periodically monitored during fatigue testing to arrive at the rate of damage progression. Special experiments were carried out under a programmed version of an equivalent fighter aircraft loading – FALSTAFF (Fighter Aircraft Loading Standard for Fatigue and Fracture) and it was observed that the damage progression is slower in case of spectrum loading compared to constant amplitude loading. The effect of load sequencing on damage is investigated in this paper.

Abstract: Constructions of pressure vessels like rocket launch vehicles, missiles, rocket propellant tanks, and filament wound pipes for civil and military applications are made out of high strength, high stiffness and light weight composites filaments. Filament winding techniques are used for fabrication of such cylinders and pipes. Many materials like glass fibers, carbon fibers and Kevlar fibers are used due to their good strength when it is subjected to internal pressure as well as external pressure. Basalt fibers are new materials that are fabricated from hard dense basalt rocks. Basalt fibers can be used in the place glass fibers due to their good mechanical behavior when subjected to internal pressure. Plates and beams generally resists bending loads and pipes and tube structures resists internal forces developed through internal and external pressure. This work concentrates the fabrication of filament wound pipes using filament wound techniques and the burst pressure test is carried out. In fuel tanks of rockets, If any one of the layer fails due to internal pressure, there will be mild leakage. For this reason it is mandatory to find out the ply by ply failure. The first ply failure of basalt filament wound pipes subjected to internal pressure is calculated using Finite element analysis. Then the stress and progressive failure analysis was carried out. Maximum stress failure criterion is used for the finite element analysis.

Abstract: Bilinear hysteretic models are an attractive choice in modeling the elasto-plastic behavior of various materials to alternate loads, due to their simplicity and ease of interpreting the results. However, choosing the parameters of the model without taking into account some restrictions can lead to highly unrealistic results. These restrictions are analyzed, and the allowable ranges for the model parameters are determined. The conclusions can be used for a wide range of plastic and composite materials. The significant errors that can occur in the case of the inadequate selection of model parameters are illustrated by a practical application.

Abstract: The concrete dam under dead load, water pressure load and wind load is in high stress state for a long time, whose safety is in a dangerous station to a certain extent. The static calculation of the dam was done by ANSYS. In order to know the static characteristics of the dam, and to provide a theoretical basis for dam health diagnosis and routine maintenance, the variation of the dam stress and displacement caused by the stiffness degradation and mass loss was studied. The influence degree of the dam mass loss and stiffness degradation on stress and displacement were discussed. The results show that stress along the dam height (Y direction) is higher than it along the horizontal direction (X direction), the maximum stress difference is to about 4 times. The influence degree of dam stress and displacement caused by stiffness degradation is more serious than those by mass loss.

Abstract: The paper deals with progressive and fatigue damage of long fiber E-glass epoxy composite, its residual stiffness degradation and corresponding transverse matrix crack density induced by load-controlled tension. Constant-amplitude fatigue tests in repeated tension of plain [±60]_S; [±30]_S; [0]₈ and [0/90₂/±45/90]_S samples were performed. Sudden onset of transverse matrix cracking and consequent gradual increase of its density has been observed in off-axis plies. The crack density increases with increasing number of cycles or load. Consequently, residual stiffness of the laminate decreases. It has been concluded that progressive/fatigue damage of the laminate is not a continuous homogenous process but the series of discrete sudden events emerging at ply level.

Abstract: A seismic performance of full-scale precast reinforced concrete beam-column corner joint with corbel was examined in the laboratory. The precast beam-column joint designed using BS8110:1:1997 was tested under lateral cyclic loading up to +1.5% drift. Displacement ductility of precast beam-column joint with corbel was determined. The visual observation showed that the damage occurred at the corbel of beam-column joint. Major cracks were also observed at the cast-in-place area above the joint area. In this paper, the damage state of the specimen is categorized in accordance to HAZUS®99 and the vulnerability of the specimen was assessed using fragility curve.

Abstract: Steel-Basalt FRP Composite Bar (S-BFCB) is a new kind of substitute material for longitudinal reinforcement, with high elastic modulus, stable post-yield stiffness and excellent corrosive resistance. It is made up of steel wrapped by basalt FRP in longitudinal direction. Based on mechanical properties of S-BFCB and the plane cross-section assumption, the moment-curvature relationship and stiffness on flexural members at different stages have been analyzed and verified by experiment. Flexural member reinforced by S-BFCB can make full use of the strength of FRP. By the principle of equivalent bar stiffness, the results show that the curvature and stiffness are almost the same results at pre-yield stage. While after yield, flexural member reinforced by S-BFCB has stable secondary stiffness and high bearing capacity. With the increase of fiber, the ultimate bearing capacity is improved.

Abstract: Experiment and Finite element analysis were used to study the strengths of composite laminates with and without holes. Finite element progressive damage analysis with Tsai-Wu criteria was applied to calculate the compressive residual strength of composite laminate. Abaqus and its user subroutine USDFLD were used for the finite element analysis. Tsai-Wu criteria as the failure criteria was realized in USDFLD as a Fortran program segment, and the stiffness degradation for the damaged elements was also realized by USDFLD. Analytic results gotten by progressive damage method were very close to the experimental results.