Stiffness Degradation Curves Research Articles

Fatigue performance characterization of a composite butt joint configuration

Fatigue performance characterization of a composite butt joint configuration was studied. The study focused on failure mode, damage evolution, and tensile stiffness degradation analyses. Effects of surface ply orientation, doubler thickness, joint attachment, and defect on the joint fatigue performance were evaluated. The bonded-bolted joints with two-row fasteners installed in each overlap section had an extremely high fatigue performance. For such a bonded-bolted joint configuration with thick doubler, a minor disbond defect improved, rather than reduced, the joint fatigue performance. This outcome was examined using the corresponding joint tensile stiffness degradation curves, and a dominant failure mechanism was identified based on damage characteristics. The study showed that the butt joint made using appropriate elements, such as a thick doubler with two-row fasteners installed in each bonded overlap section, could have very good fatigue performance regardless of the presence of a minor disbond defect.

Seismic Behavior of Two-Story Brick Masonry Building

In order to investigate the seismic behavior include hysteretic curve, skeleton curve and stiffness degradation, four masonry structures with different details are tested under pseudo-dynamic test and low cycle lateral load test. One kind of full-scale building is confined masonry with tie-column and ring-beam. Other is common brick masonry. Test results indicate that the tie-column and ring-beam play an important role for developing higher resistance and better deformability. And tie-column and ring-beam can effectively confined brick wall to improve the seismic performance of masonry structure. The stiffness degradation curves are similar. The stiffness degradation is modeled as a function of the effective stiffness and lateral displacement.

Micro-mechanical fatigue modelling of unidirectional glass fibre reinforced polymer composites

A multi-scale numerical fatigue modelling methodology is proposed for the characterization of axial tension–tension fatigue behaviour of unidirectional Glass Fibre Reinforced Polymer (GFRP) composites. The methodology is developed to link the dominant fibre failure mechanism to composite fatigue properties, by a three-step geometrical up-scaling process. This study aims to provide composite fatigue predictions by only using the material properties of fibres and matrix. Compared to the experimental results, the fatigue model over-predicts fatigue lives and gives reasonable estimations of axial stiffness degradation curves of the unidirectional GFRP composite dog-bone specimens.

Numerical simulation of three-point bending fatigue of four-step 3-D braided rectangular composite under different stress levels from unit-cell approach

This paper reports the three-point bending fatigue behavior of 3-D carbon/epoxy braided composite in experimental and finite element analyses (FEAs) approaches. In the experimental approach, the stiffness degradation and maximum deflection curves were obtained to illustrate the relationship between applied stress levels and number of cycles to failure. In FEA approach, a user-defined material subroutine UMAT which characterizes the stiffness matrix and fatigue damage evolution of the 3-D braided composite was developed. It was incorporated with a finite element code ABAQUS/Standard to calculate the stiffness degradation and maximum deflection of the 3-D braided composite during each loading cycle. From the comparisons between experimental and FEA results, good agreements prove the validity of the unit cell model and the subroutine UMAT.

Settlement analysis of shallow foundations on sand

The evaluation of ground movements is one of the most important issues in geotechnical engineering applications. In this context, the paper is aimed at prediction of the load–settlement performance of shallow foundations by taking soil non-linearity into account. The approach illustrated is based on the operational stiffness concept, characterising field behaviour with reference to relative foundation settlements, indicating pseudo-strain levels. A strain correction factor and a modulus correction factor, relating laboratory and field stiffness degradation curves, are introduced. Values of these factors are supplied on the basis of well-documented tests performed on Ticino sand, and can be adopted in elastic relationships for the prediction of load–settlement curves. The procedure, developed in order to supply the designer with a practical tool, is described, and some applications are presented.

Simple Shear Behavior of Calcareous and Quartz Sands

The monotonic and cyclic simple shear behavior of several loose skeletal calcareous sands is investigated and compared to that of common Nevada and Ottawa quartz sands in order to draw contrasts between these types of materials and to develop stiffness and modulus degradation curves for quasi-nonlinear ground response analysis. Mobilized frictional resistance and cyclic strength are generally higher for the calcareous sands, whereas shear modulus and damping are larger for quartz sand at strain levels between 0.05 and 1%. Degradation properties do not appear to be very sensitive to the initial cyclic stress ratio. Differences in behavior between the calcareous and quartz sands are presumably due to contrasts in grain geometry, hardness, gradation and the amount of intraparticle voids.

Free-Field Measurements to Disclose Lateral Reaction Mechanism of Piles Subjected to Soil Movements

Soil-pile interaction remains to be the most ambiguous yet one of the most crucial aspects in the design of laterally loaded soil-pile systems subjected to embankment-induced movements. This paper proposes a new method that is capable of producing soil stiffness degradation curves, which are the outcome of real field behavior through free-field measurements. Soil-pile interaction mechanism can be solved with the proposed method for any possible case either the piles are constructed before the embankment construction or during and after. For any time considered, the method enables the computation of resultant stress effects on the pile cross section and the accompanying deflections. To provide a basis of comparison, an example problem has been solved with the proposed method and with two well-known commercial finite-element softwares. Obtained results indicated the capability of the proposed method to disclose real field behavior, which can be attributed to its inherent property of being also an observational method.

Fatigue Life Assessment and Static Testing of Structural GFRP Tubes Based on Coupon Tests

Fiber-reinforced polymer (FRP) hollow tubes are used in structural applications, such as utility poles and pipelines. Concrete-filled FRP tubes (CFFTs) are also used as piles and bridge piers. Applications such as poles and marine piles are typically governed by cyclic bending. In this paper, the fatigue behavior of glass-FRP filament-wound tubes is studied using coupons cut from the tubes. Several coupon configurations were first examined in 24 tension and five compression monotonic loading tests. Fatigue tests were then conducted on 81 coupons to examine several parameters; namely, loading frequency as well as maximum-to-ultimate (σmax∕σult) and minimum-to-maximum (σmin∕σmax) stress ratios, including tension tension and tension compression, to simulate reversed bending. The study demonstrated the sensitivity of test results and failure mode to coupon configuration. The presence of compression loads reduced fatigue life, while increasing load frequency increased fatigue life. Stiffness degradation behavior was also established. To achieve at least one million cycles, it is recommended to limit (σmax∕σult) to 0.25. Models were used to simulate stiffness degradation and fatigue life curve of the tube. Fatigue life predictions of large CFFT beams showed good correlation with experimental results.

Stiffness degradation model for biaxial braided composites under fatigue loading

Biaxial braided composites are gaining popularity, in particular for the small business jets, where FAA requires take off weights of 5670 kgf (12,500 lb). In the present research carbon/epoxy biaxial braided composites were manufactured using low cost vacuum assisted resin transfer molding (VARTM). Extensive tension–tension fatigue tests were performed on biaxial braided carbon/epoxy composites for various braid angles. Experimental data clearly indicated that S– N diagram could be represented by Sigmoidal function. This paper specifically addresses stiffness degradation of braided composites with braid angle of 25°. In addition to Sigmoidal function, it was observed that the three stages of stiffness degradation curve can be represented by Bradley function. Sigmoidal function in conjunction with Bradley function is explored to predict the fatigue life and the residual fatigue modulus.

Deformation characteristics of two cemented calcareous soils

The effect of cementation on the deformation characteristics of two cemented calcareous soils was investigated through a series of undrained triaxial tests performed under both monotonic and cyclic loading conditions. Increasing the level of cementation significantly increased the initial stiffness, resulting in the stiffness being more independent of the confining pressure. The curves of stiffness degradation with strain obtained from both cemented and uncemented calcareous soils compared with those of other noncalcareous soils revealed that calcareous soil attains a faster rate of modulus reduction with a higher strain threshold. It was also observed that the pattern of stiffness degradation is very similar in both cemented and uncemented samples. The stiffness degradation curves obtained from cyclic tests were found to lie within the range defined by the corresponding monotonic tests. The effect of number of cycles on the stiffness during cyclic loading was also examined and is found to depend on whether the postyield behaviour is controlled by the cohesive or the frictional response. Examination of the variation of damping ratio with strain revealed that the observed difference in the stiffness degradation curves between calcareous and noncalcareous soils was also reflected in the damping ratio, with the damping ratio of calcareous soils being below the range defined for noncalcareous soils.Key words: calcareous soils, triaxial test, shear modulus, damping ratio, repeated loading.

Research on the seismic behavior of HPC shear walls after fire

Four shear wall specimens made of high-performance concrete with blast-furnace-slag (BFS-HPC) are tested and studied under low frequency cyclic loading. Prior to the cyclic test, three specimens are subjected to elevated temperatures (400°C, 600°C) and two of them are mixed with polypropylene fiber. Based on experimental results, different curves are drawn and analyzed to study the seismic behavior of BFS-HPC shear walls after fire. They include load vs. lateral displacement hysteretic curves, load vs. lateral displacement skeleton curves, energy dissipation capacity curves, and stiffness degradation curves etc. Besides, the effect of polypropylene fiber on the seismic behavior of BFS-HPC shear wall after fire is also examined. This investigation demonstrates that elevated temperature exposure dramatically decreases the seismic performance of BFS-HPC shear walls, whereas polypropylene fiber can remarkably improve the seismic behavior of BFS-HPC shear walls after fire.

The presence of edge contact and its influence on the debonding of patched panels

A formulation is presented which accounts for configurations of debonding patched cylindrical panels for which the edge of the debonded segment of the patch maintains sliding contact with the corresponding base structure. The formulation is incorporated with that which accounts for configurations for which a contact zone is present and for which no contact between the debonded segments of the patch and base structure occurs during debonding. Analyses are performed for two types of loading conditions: Applied circumferential tension and applied internal pressure. Results of numerical simulations based on analytical solutions of the problems of interest are presented in the form of threshold and stiffness degradation curves for specific structures with various geometries, material properties and support conditions, and reveal characteristic behavior of the evolving composite structure. In particular, the effects of edge contact on the debonding scenarios of the evolving structure are elucidated.

Imperfect Columns with Biaxial Partial Restraints

An inelastic theoretical study of the effect of biaxial partial end restraints on the response of hollow rectangular steel nonsway columns with or without biaxial crookedness and residual stresses is presented. The partial end restraints considered have linear, elastic‐plastic, or trilinear moment‐rotation characteristics. The fundamental total equilibrium equations for the problem are derived. An algorithm is presented based on a coupling between an iterative tangent stiffness approach and a finite‐difference scheme for evaluating the spatial response of the materially nonlinear columns. The effect of partial end restraints, as well as the individual and combined influence of crookedness and residual stresses is also explained by means of load‐deflection, and bending stiffness degradation curves. Several interesting conclusions are drawn regarding the behavior of columns as affected by end restraints, imperfections and slenderness.

End Restraint Effect on Steel Column Strength

Nonsway columns with initial crookedness and residual stresses, and partially restrained by linear, elastic‐plastic, or nonlinear end restraints are studied in the inelastic range. An algorithm based on an iterative tangent stiffness approach coupled with a finite‐difference equilibrium method is developed relative to the minor axis of the column cross section. The effect of end restraint on the response of imperfect columns is explained. The influence of imperfections on the behavior of partially restrained columns is identified by means of stiffness degradation curves. Finally, new design expressions are proposed for evaluating steel column strength which make use of a “plastic effective length factor” and the use of the proposed procedure illustrated by means of a design example.