Theoretical and experimental study of foam-filled lattice composite panels under quasi-static compression loading

In this paper, a simple and innovative foam-filled lattice composite panel is proposed to upgrade the peak load and energy absorption capacity. Unlike other foam core sandwich panels, this kind of panels is manufactured through vacuum assisted resin infusion process rather than adhesive bonding. An experimental study was conducted to validate the effectiveness of this panel for increasing the peak strength. The effects of lattice web thickness, lattice web spacing and foam density on initial stiffness, deformability and energy absorbing capacity were also investigated. Test results show that compared to the foam-core composite panels, a maximum of an approximately 1600% increase in the peak strength can be achieved due to the use of lattice webs. Meanwhile, the energy absorption can be enhanced by increasing lattice web thickness and foam density. Furthermore, by using lattice webs, the specimens had higher initial stiffness. A theoretical model was also developed to predict the ultimate peak strength of panels.     

纤维缠绕复合材料夹芯圆柱体准静态压缩吸能机制

提出并设计了一种新型纤维缠绕复合材料夹芯圆柱体吸能结构单元。为探讨其在准静态压缩载荷作用下初始损伤的产生、扩展及演变规律,基于ABAQUS建立该单元数值分析模型,并开展了准静态压缩试验。数值模拟与试验现象综合分析表明,准静态压缩载荷作用下单元结构的响应具有三阶段特征,包括初始线弹性压缩阶段、渐进损伤阶段和结构破坏阶段。固体浮力芯材在压缩载荷作用下产生塑性损伤变形和剪切断裂破坏,纤维缠绕复合材料表层在芯材横向膨胀效应引起的环向应力作用下发生环向纤维的拉伸断裂破坏,导致单元结构稳态吸能过程的终止。研究结果表明,该单元比吸能效率远高于传统的复合材料圆柱壳结构。     

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.     

Mechanical properties of ceramic fiber-reinforced concrete under quasi-static and dynamic compression

The research herein is made on the quasi-static and dynamic mechanical properties of ceramic fiber reinforced concrete (CRFRC for short) through the adoption of a hydraulically-driven testing system as well as a 100-mm-diameter split Hopkinson pressure bar (SHPB) system. As test results have turned out, such quasi-static properties as compressive strength, splitting tensile strength and flexural strength of CRFRC increase with the rise in the volume fraction of fiber. Within the strain range of 20–120 s⁻¹, the effect of the axial strain acceleration on the dynamic strength of CRFRC could be ignored. Therefore, the dynamic increase ratio (DIF) derived from SHPB tests can truly reflect the dynamic enhancement of CRFRC. The dynamic strength, critical strain and specific energy absorption (SEA) of CRFRC are sensitive to the strain rate. The addition of ceramic fiber to plain concrete can significantly improve its properties—dynamic strength, critical strain and energy absorption. And also, an analysis is conducted of the mechanism for strengthening and toughening the concrete.     

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.     

Fracture mechanisms of wood/polyester laminates under quasi-static compression and shear loading

There is increasing use of natural fiber/polymer composites as alternatives to traditional structural materials like concrete and metals and to the inorganic fibers like carbon. While the fracture mechanisms during crushing of synthetic fiber/polymer composites have been thoroughly studied, limited information is available on post-fracture investigation and identification of the dominant fracture mechanisms of wood/polyester composites. In this study laminates of Douglas-fir veneer were fabricated using a catalyzed polyester resin and their potentials as energy absorbers have been investigated and discussed. Factors for this study were (i) laminates symmetry (face layers of 0° or 90°), (ii) lay-up balance (balanced and unbalanced) and (iii) number of lamina (8, 11, and 12). Samples were tested under quasi-static Combined Loading Compression (CLC) and their compressive performances were compared to control specimens using glass fiber as reinforcement. Results indicated that the effect of symmetry on compressive properties of wood veneer/polyester laminates was significant with laminates with face layers of 90° and core layers of 0° had the highest deflection to failure. Increasing the wood/polyester laminate thickness enhanced their energy absorbing ability by bringing more fracture mechanisms into play but it noticeably reduced the laminates compressive modulus. Despite the brittle failure of glass fiber composites wood laminates exhibited a progressive fracture mechanisms with shear buckling as the dominant mode of failure in symmetric samples. This progressive failure with high energy absorbing ability make wood/polyester laminates a good candidate to be used as an energy absorber structure where high deflection to failure and longer failure time are required.     

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.     

Mechanical characteristics of swollen elastomers under cyclic loading

The environmental and economic concerns have raised the popularity of biodiesel as a potential replacement for conventional fuel. However, the incompatibility of engineering rubber components with biodiesel affects significantly the performance of the components. Majority of the compatibility studies focus on evaluating the degradation of mechanical properties of the rubbers due to contamination of different types of biodiesel. Nevertheless, the resulting mechanical responses of swollen rubbers, in particularly under cyclic and fatigue loading conditions, are rarely investigated. In engineering applications where elastomeric components are concurrently subjected to fluctuating mechanical loading and contamination of hostile liquids such as biodiesel, it is crucial to investigate the mechanical responses of these components for durability analysis. In this view, the present study aims to investigate the effect of swelling, due to biodiesel diffusion in the elastomers, on the macroscopic mechanical responses under cyclic loading conditions. Simple stress-free immersion tests are conducted on elastomers and the resulting mechanical responses are evaluated. The focus of the present work is on the effect of biodiesel diffusion on the inelastic responses classically observed in elastomers under cyclic loading conditions, i.e. stress-softening, hysteresis and stress relaxation. The results show that the above inelastic responses decrease significantly when the swelling level increases.     

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.     

Surface flaws behavior under tension,bending and biaxial cyclic loading

Fatigue surface crack growth and in-plane and out-of-plane constraint effects are studied through experiments and computations for the aluminum alloy D16T. A tension/bending central notched plate and cruciform specimens under different biaxial loadings with external semi-elliptical surface cracks are studied. The variation of the fatigue crack growth rate and surface crack paths is studied under cyclic tension, bending and biaxial tension–compression loading. For the experimental surface crack paths in the tested specimens, the T-stress, out-of-plane T_z factor, local triaxiality parameter h and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of the I_n-integral are calculated as a function of the aspect ratio by finite element analysis to characterize the constraint effects along the semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface, as well as at the deepest point of the semi-elliptical surface crack front, of the tested tension/bending plate and cruciform specimens. From the results, characteristics of the fatigue surface crack growth rate as a function of the loading conditions are established.     

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.     

缝合式三维编织C/SiC复合材料拉伸加卸载力学行为

本文通过开展缝合式三维编织C/SiC复合材料的准静态单轴拉伸及循环加卸载试验,研究加卸载行为对复合材料的损伤及材料内部能量耗散的影响。通过对材料的断口分析,探究加卸载对材料破坏强度的影响规律。结果表明,加卸载行为会消耗材料内部的能量,对纤维束与基体之间的界面造成损伤,进而降低材料的承载能力;材料滞回曲线的面积随着卸载点应力的增大而增大;材料的整体失效属于脆性破坏,复合材料断口表现出明显的分层现象,且单轴拉伸时断口相对于循环加卸载更整齐。      

Mechanical and structural behavior of a swelling elastomer under compressive loading

Swelling elastomers are a new breed of advanced polymers, and over the last two decades they have found increasing use in drilling of difficult oil and gas wells, remediation of damaged wells, and rejuvenation of abandoned wells. It is important to know whether an elastomer type or a certain seal design will function properly and reliably under a given set of oil or gas well conditions. This paper reports the results of an experimental and numerical study conducted to analyze how compressive and bulk behavior of an actual oilfield elastomer changes due to swelling. Tests were carried out on ASTM-standard compression and bulk samples (discs) before swelling and after different swelling periods. Elastic and bulk modulii were experimentally determined under different swelling conditions. Shear modulus and Poisson’s ratio were estimated using derived isotropic relations. Cross-link chain density and number average molecular weight were obtained using predictive equations of polymer physics. Mechanical testing was also modeled and simulated using the nonlinear finite element package ABAQUS, material model being Ogden hyperelastic model with second strain energy potential.Values of elastic and shear modulus dropped by more than 90% in the first few days, and then remained almost constant during the rest of the 1-month period. Poisson’s ratio, as expected, showed a mirror behavior of a sharp increase in the first few days. Bulk modulus exhibited a fluctuating pattern; rapid initial decrease, then a slightly slower increase, followed by a much slower decrease. Salinity shows some notable effect in the first 5 or 6 days, but has almost no influence in the later days. As swelling progresses, chain density decreases, much more sharply in the first week and then showing almost a steady-state behavior. In contrast, cross-link average molecular weight increases with swelling (as expected), but in a slightly fluctuating manner. Very interestingly, Poisson’s ratio approaches the limiting value of 0.5 within the first 10 days of swelling, justifying the assumption of incompressibility used in most analytical and numerical models. In general, simulations results are in good agreement with experimental ones. Results presented here can find utility in selection of swelling elastomers suitable for a given set of field conditions, in improvement of elastomer-seal and swell-packer design, and in modeling and simulation of seal performance.