Elasto-plastic behavior of thermoplastic composite laminates under cyclic loading

An experimental and numerical study of the elasto-plastic behavior of thermoplastic matrix composite laminates under static and cyclic loads is presented. Off-axis and angle ply specimens cut from laminates of poly(ether ether ketone) (PEEK) reinforced with continuous carbon fibers have been tested under cyclic sinusoidal tensile loads and the hysteresis loops have been monitored. A micro mechanical model, which includes a parabolic criteria based on the plastic behavior of the matrix, has been adopted to study the composite non-linear behavior and a correlation between plastic deformation and a strong rise of damping and temperature at high stresses is outlined. Good agreement is shown between theory and experimental results. The mathematical mdoel presented here can be used to predict the visco-elastic-plastic response of the material at high stresses and its influence in the fatigue damage.     

Mechanical behavior and failure modes of aluminum/bamboo sandwich plates under quasi-static loading

Experimental investigations have been made on the quasi-static mechanical behavior and failure modes of aluminum/bamboo sandwich plates. Thermosetting epoxy resin and thermoplastic Polybond resin were used to bond the aluminum sheets and the bamboo. Tensile, compressive and flexural properties were evaluated. The effects of bond conditions on the mechanical behavior and failure modes were examined. The thermoplastic Polybond resin resulted in a stronger interface bond than the thermosetting epoxy resin. The improvement of the interface bond led to significant increases in compressive and flexural properties. The tensile properties were found to be insensitive to the interface bond. The dominant failure mechanisms affected by the interface bond dictated the mechanical properties of the sandwich plates in individual loading conditions.     

Intersonic delamination in curved thick composite laminates under quasi-static loading

Dynamic delamination in curved composite laminates is investigated experimentally and numerically. The laminate is 12-ply graphite/epoxy woven fabric L-shaped laminate subject to quasi-static loading perpendicular to one arm. Delamination initiation and propagation are observed using high speed camera and load–displacement data is recorded. The quasi-static shear loading initiates delamination at the curved region which propagates faster than the shear wave speed of the material, leading to intersonic delamination in the arms. In the numerical part, the experiments are simulated with finite element analysis and a bilinear cohesive zone model. Cohesive interface elements are used between all plies with the interface properties obtained from tests. The simulations predict a single delamination initiating at the corner under pure mode-I stress field propagating to the arms under pure mode-II stress field. The crack tip speeds transition from sub-Rayleigh to intersonic in conjunction with mode change. In addition to intersonic mode-II delamination, shear Mach waves emanating from the crack tips in the arms are observed. The simulations and experiments are found to be in good agreement at the macro-scale, in terms of load-displacement behavior and failure load, and at the meso-scale, in terms of delamination initiation location and crack propagation speeds. Finally, a mode dependent crack tip definition is proposed and observation of vibrations during delamination is presented. This paper presents the first conclusive evidence of intersonic delamination in composite laminates triggered under quasi-static loading.     

Modeling behavior of cross-ply ceramic matrix composites under quasi-static loading

This paper presents a simplified analysis (model and failure criteria) for predicting the stress-strain responce of cross-ply fiber-reinforced ceramic composite laminates under quasi-static loading and unloading conditions. The model formulation is an extension of the modified shear-lag theory previously introduced by the authors for analyzing unidirectional laminates for the same loading conditions. The present formulation considers a general damage state consisting of matrix cracking in both the transverse and longitudinal plies, as well as fiber failure. These damage modes are modeled by a set of failure criteria with the minimum reliance on empirical data, and can be easily employed in a variety of numerical or analytical methods. The criteria used to estimate the extent of matrix cracking and interfacial debonding are closed-form and require the basic material properties. The failure criterion for fiber failure requires a priori knowledge of a single empirical constant. This parameter, however, may be determined without microscopic investigation of the laminate microstructure. The results from the present simplified analysis match well with the experimental data.The U.S. Government right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged.     

Crack resistance of structural steels under static and cyclic loading

The article deals with the investigation of static and fatigue crack resistance of normalized steels 40Kh and 30KhGSA, and also of steel 40Kh after hardening and tempering at 500°C with the object of working out recommendations for the use of normalized steel 40Kh for making axles of tractor trailers produced at the Chelyabinsk Engineering Works. The article presents the results of static tests of cylindrical specimens with annular cracks under conditions of axial tension which showed that the crack resistance K_1C of heat-treated steel 40Kh is up to 20% higher than after normalization. The crack resistance K_1C of normalized steel 30KhGSA has intermediate values. We carried out fatigue tests of the above-mentioned steels by concentrated bending, and from the results we plotted kinetic diagrams of fatigue failure and determined the threshold values of the characteristics of fatigue crack resistance K_th and K_fc which confirmed the data of the static tests. It was established that normalized steel 40Kh can be recommended as material of axles for tractor trailers with a load capacity of up to 10 tons for which the safety factor K_s is at least 2–2.5.Translated from Problemy Prochnosti, No. 7, pp. 28–31, July, 1990.     

Crack growth under static and cyclic loading in hot-stretched PMMA

Crack growth behaviour under static and cyclic loading was investigated using anisotropic plates of PMMA oriented by hot-stretching. Both tests were performed at room temperature for samples with different degrees of orthotropy. A slight increase in the degree of orthotropy considerably improves the resistance to both static and cyclic crack growth in the case where the crack propagates perpendicularly to the hot-stretched direction. A power law relationship between crack growth rate and stress intensity factor may hold for both types of crack growth in the ranges of orthotropy tested. The experimental data for static crack growth were compared with a viscoelastic criterion based on the crack opening displacement theory for fracture. The criterion discussed here explains comparatively not only the beginning of cracking from a pre-introduced crack, but also the crack growth rate in oriented PMMA.     

Behavior of particle-filled polymer composite under static and dynamic loading

Experimental results of quasi-static and dynamic fracture of particle-filled polymer composite (PFPCM) “ALTUGLAS EI CH25” with a matrix of polymethylmethacrylate (PMMA) are reported in this paper. PMMA matrix is filled with rubber particles, as result a shock-resistant transparent composite is produced. The main task was to investigate experimentally and theoretically the fracture toughness of this composite under static and dynamic loading. A high-rate loading has been created by impulse magnetic field. Analysis of fracture process and its relation with the load parameters and material microstructure have been established. Application of the original testing method enabled determination of fracture toughness at very short loading times and comparison of the results with material dynamic properties. Theoretical analyses are based in general on the notion of delayed fracture. More specifically, the theoretical analysis is based on experimental results and on the hypothesis of fracture incubation time, or delay time.     

Modeling of ratcheting behavior under multiaxial cyclic loading

Summary.  A two-surface plasticity theory is used to predict ratcheting strain under multiaxial loading. A kinematic hardening rule that combines the Mroz and Ziegler hardening rules is employed along with the plastic modulus given as an exponential function of the distance between the yield surface and the bounding surface. Model results are compared with the experimental data obtained on medium carbon steel under proportional and nonproportional axial-torsional loading. The model predicts reasonably well the experimental ratcheting behavior at relatively low cycles. Predictions overshoot the actual ratcheting strains at high cycles, yet the results look favorable compared with other data found in the literature. Received July 29, 2002; revised January 15, 2003 Published online: May 20, 2003 The authors gratefully acknowledge financial support for this work, in part from Brain Korea 21 Program at Pohang University of Science and Technology, and in part from National Natural Science Foundation of China and TRAPOYT.     

Mechanical behavior of glass/epoxy tubes under combined static loading. Part I: Experimental

A series of biaxial static tests of E-glass/epoxy tubular specimens [±45]₂, subjected to combined torsion and tension/compression were performed to simulate complex stress states encountered in a wind turbine rotor blade. The failure locus in the effective axial-shear stress plane was derived experimentally while in-plane strain tensor components were measured in the tube outer surface. By means of shell theory and strain measurements in the surface of the specimen, the in-plane shear response of the outer ply was obtained, revealing dependence each time to the combined tube loading. The correlation established between the ratio of transverse normal and in-plane shear stress in the principal coordinate ply system and the elastic shear modulus, suggested a strong dependence, warning on the implications for design and certification procedures.     

Design and development of hybrid composite bistable structures for energy absorption under quasi-static tensile loading

The composite bistable structures discussed in this paper consist of a material-based mechanism with a saw-tooth strain/stress curve. They have a more damage-tolerant behavior, and considerably higher strain-to-failure than the corresponding monolithic material.     

Lateral behavior of concrete under uniaxial compressive cyclic loading

In reinforced concrete structures under seismic loading, concrete is subjected to compressive cyclic stress. Although cyclic stress–strain response has been described before, the cyclic behavior of strains in the direction orthogonal to loading has not been characterized yet. Such behavior can be of great importance for evaluating the efficiency of the confinement under cyclic loading. For this purpose an experimental program on cylindrical specimens of concrete strength from 35 to 80 MPa subjected to uniaxial cyclic compression was carried out. Stress versus longitudinal and lateral strains curves have been obtained both for the hardening and softening branches under monotonic and cyclic loading. Governing parameters of the lateral behavior are identified and correlated to describe the response of the lateral strain. Additionally, an analytical model to obtain the lateral deformations of concrete under cyclic uniaxial compression has been formulated and verified experimentally. Finally, some examples are presented in order to illustrate the applicability of the proposed model and its possible incorporation into a 3D constitutive cyclic model.     

Fatigue of bonded steel interfaces under cyclic shear loading and static normal stress

Hybrid joints that combine mechanical fastening and bonding provide potential joining alternatives for high strength steel structures. An experimental and analytical research program has been initiated to assess the static and cyclic Mode II shear strength of epoxy bonded steel interfaces subjected to static normal (Mode I) pre-stress. During cyclic loading, shear stress vs. interface displacement could be divided into elastic and inelastic regions with a power law equation describing the inelastic curve. Hysteresis loop shape varied with normal stress, shear stress and accumulated fatigue cycles. A shear stress amplitude threshold equal to about 50% of the fracture shear stress was observed.     

Fracture toughness of coal under quasi-static and impact loading

The fracture toughness of specimens of Canadian coal, expressed as k_q, was determined under both quasi-static and dynamic loading conditions. In all tests, a wedge loaded compact tension (WLCT) specimen was used. The quasi-static tests were carried out in a servohydraulic testing machine while in the dynamic testing, a Split Hopkinson Bar was used. Also studied was the effect of orientation of the crack with respect to the coal seam. Loading rates ranging from 0.1 MPa √m/sec. to 6 × 10⁴ MPa √m/sec. were investigated. The results show a large increase of over one order of magnitude between k_q in the quasi-static and that obtained at high loading rates.     

Behaviour of composite substructures with full and partial shear connection under quasi-static cyclic and pseudo-dynamic displacements

Six full-scale steel-concrete composite substructures with full and partial shear connection representing half bay within a moment-resisting frame were built and tested both in a, quasi-static cyclic and pseudo-dynamic regime. The specimens embodied two composite beams with full shear connection and four companion beams with two different degrees of partial shear connection all with exterior rigid joints. Tests were conducted in a set-up purposely designed to impose to specimens reversed horizontal displacements in a low-cycle high amplitude regime. The characteristics of the test apparatus ready for different structural members subjected to horizontal loading are illustrated. Moreover, the seismic performances of the specimens in terms of yielding and of maximum strength capacity as well as of displacement ductility are assessed. Finally, the accuracy of a European code calibrated upon monotonic loading is provided and other local characteristics of the composite substructure behaviour with low partial shear connection are commented upon.

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Résumé Six poutres mixtes béton-acier à échelle réelle ayant des connections au cisaillement complètes et partielles, représentant une démi-travée, dans le cadre d’une structure qui résiste aux moments, ont été construites et testées aussi bien dans un régime cyclique quasi-statique que dans un régime pseudo-dynamique. Les spécimens sont constitués de deux poutres composites ayant des connections au cisaillement, complètes et quatre poutres, de même dimension ayant deux différents degrés de connections au cisaillement partielles, toutes ayant des jointures extérieures et rigides. Les essais ont été faits selon une configuration étudiée intentionnellement pour imposer aux spécimens des déplacements horizontaux et opposés dans un régime à bas cycle et à haute amplitude. On expose les caractéristiques de l’appareillage destiné aux essais des différents membres structuraux soumis aux charges horizontales. En outre, les performances sismiques des spécimens ont été évaluées aussi bien du point de vue de l’éervement, et de la capacité maximale de résistance que de la ductilité du déplacement. Enfin, une comparaison d’un code européen calibré sur une charge monotone est vérifiée et d’autres caractéristiques locales de la conduite de la substructure mixte ayant un degré bas des connections au cisaillement, sont commentées.

     

On the quasi-static granular convective flow and sand densification around pile foundations under cyclic lateral loading

The saturated sand surrounding an offshore pile foundation under quasi-static cyclic lateral load can show the physical phenomena of macromechanical densification and convective granular flow. Based on the results from physical model tests at different geometrical scales, this paper provides a certain quantification of such phenomena and discusses their causes and consequences. The progressive sand densification leads to subsidence of the soil surface and a significant stiffening of the pile behaviour. Conversely, the ratcheting convective motion of two closed cells of soil beneath the pile-head is responsible for an endless grain migration at the soil surface, the inverse grading of the convected material and a direct shear of the sand at the distinct boundary of the revolving soil domain. In this respect, and from a macromechanical perspective considering the soil as a continuum, it appears that the convecting material tends to follow gradient lines of shear stress during its ratcheting motion. Concluding the paper, the practical relevance of these phenomena and their extrapolation to other conditions are briefly discussed.