Toughness and fracture mechanisms of glass fiber/aluminum hybrid laminates under tensile loading

Tensile properties and fracture toughness of monolithic aluminum (Al), glass fiber reinforced plastics (GFRPs) and glass fiber/aluminum hybrid laminates (GFMLs) were examined in relation to the fracture processes of plain coupon and single-edge-notched specimens. Elastic modulus and ultimate tensile strength of GFMLs showed characteristic dependences on the kind of Al, fiber orientation and the Al/fiber layer composition ratio. Fracture toughnesses K_C and G_C of A-GFML-UD were comparable to those of GFRP-UD and were much superior to monolithic Al. However, GFML with a transverse crack parallel to the fiber layer deteriorated largely in toughness. Microscopic observation of the fracture zone in the vicinity of the crack tip revealed various modes of micro-cracks in the respective layers as well as fiber fractures and delamination between fiber/Al layers. Such damage advances in GFMLs dependent on the orientation of the fiber layer and the Al/fiber composition ratio strongly influenced the strength and toughness of GFMLs.     

Impact damages and residual bending strength of CFRP composite laminates subjected to impact loading

The purpose of this study is to confirm the decreasing in residual bending strength, and the failure mechanisms experimentally when CFRP composite laminates are subjected to foreign object damage (FOD). Composite laminates used in this test are CFRP orthotropic laminated plates, which are stacked with two-interfaces [0^o ₆/90^o ₆] _sym and four-interfaces [0^o ₃/90^o ₆/0^o ₃] _sym . When the specimen was subjected to transverse impact by a steel ball, the delamination area generated by the impact damage was observed by using the SAM (scanning acoustic microscope). Also, the fracture surfaces obtained by three-point bending test were observed by using the SEM (scanning electron microscope). Further, failure mechanisms were investigated based on the observed delamination areas and fracture surfaces.     

Optimization of functionally graded foam-filled conical tubes under axial impact loading

Metallic foams as a filler in thin-walled structures can improve their crashworthiness characteristics. In this article, nonlinear parametric finite element simulations of FGF foam-filled conical tube are developed and the effect of various design parameters such as density grading, number of grading layers and the total mass of FGF tube on resulting mode shapes, specific energy absorption and initial peak load is investigated. Multi design optimization (MDO) technique and the geometrical average method, both are based on FE model are applied to maximize the specific energy absorption and minimize the impact peak force by estimating the best wall thickness and gradient exponential parameter “m” that controls the variation of foam density. The results obtained from the optimizations indicated that functionally graded foam material, with graded density, is a suitable candidate for enhancing the crashworthiness characteristics of the structure compared to uniform density foam.     

一种新型吸能装置的轴向耐撞击性能分析

基于传统吸能元件薄壁圆管的结构形态及其功能型要求,给出了一种端部带翻转模的新型吸能装置。在薄壁圆管直径和长度给定的前提下,通过改变翻转模诱导半径和管壁厚度,诱发了圆管在轴向冲击荷载下的四种变形模式,并对其中翻转变形模式进行了深入分析,考察了应变率、惯性效应和几何参数对其耐撞击性能的影响规律。有限元计算结果表明,发生翻转变形模式的薄壁圆管荷载-位移历程较平稳,总效率较高,具有较优异的轴向耐撞击性能;材料的应变率效应、翻转模半径与管壁厚度之比(r/t)、管壁厚度与圆管直径之比(t/D)对撞击荷载有较大的影响,而圆管自身的惯性效应对撞击荷载的影响较小。数值模拟结果为耐撞击结构部件的设计提供了参考依据。     

Low velocity impact denting of HSSA lightweight sandwich panel

Slow speed impact by a small mass can cause residual denting without perforation of a fibrous core sandwich panel that has thin facesheets. Denting depends on the kinetic energy, compliance and nose shape of the colliding body as well as the compliance and mass density of the sandwich panel. Collision experiments were carried out with fibrous core sandwich panels of different sizes struck by colliding spheres at small velocities. Analytical models based on either quasi-static or dynamic deformation of plates were developed to calculate the impact force during low speed impact on circular sandwich panels. Finite element analysis using ABAQUS was performed to calculate impact damage on sandwich panels. Results of the analytical and numerical models and the experimental measurements were compared. The dependence of damage on both structural parameters and impact variables was investigated.     

Mode of collapse and energy absorption characteristics of constrained frusta under axial impact loading

The energy absorption performance of right circular frusta subjected to dynamic axial load is studied and compared with the results of quasi-static tests. Frusta of different geometric ratios and end constraints were axially crushed using a drop hammer at initial velocities in the range of 2–5 m/s. The effect of heat treatment on the collapse behaviour and energy absorption is also investigated. The experimental observations indicate that the effects of the end constraints and heat treatment on the energy absorption were qualitatively similar to those observed under quasi-static testing. Due to inertia effects, the absolute values of the energy absorbed by similar frusta were higher under dynamic loads than under quasi-static loads. It has been established that constraining the frusta enhances their energy absorption capacity under static and dynamic loading particularly at the top (smaller diameter). The optimum geometric parameters for maximum energy absorption performance are identified when residual stresses and strain hardening characteristics, arising from spinning the frusta, were removed.     

On impact damage detection and quantification for CFRP laminates using structural response data only

The overall purpose of the research is to detect and attempt to quantify impact damage in structures made from composite materials. A study that uses simplified coupon specimens made from a Carbon Fibre-Reinforced Polymer (CFRP) prepreg with 11, 12 and 13 plies is presented. PZT sensors were placed at three separate locations in each test specimen to record the responses from impact events. To perform damaging impact tests, an instrumented drop-test machine was used and the impact energy was set to cover a range of 0.37–41.72 J. The response signals captured from each sensor were recorded by a data acquisition system for subsequent evaluation. The impacted specimens were examined with an X-ray technique to determine the extent of the damaged areas and it was found that the apparent damaged area grew monotonically with impact energy. A number of simple univariate and multivariate features were extracted from the sensor signals recorded during impact by computing their spectra and calculating frequency centroids. The concept of discordancy from the statistical discipline of outlier analysis is employed in order to separate the responses from non-damaging and damaging impacts. The results show that the potential damage indices introduced here provide a means of identifying damaging impacts from the response data alone.     

The response of honeycomb sandwich panels under low-velocity impact loading

This paper describes the results of an experimental investigation and a numerical simulation on the impact damage on a range of sandwich panels. The test panels are representative of the composite sandwich structure of the engine nacelle Fan Cowl Doors of a large commercial aircraft. The low-velocity impact response of the composites sandwich panels is studied at five energy levels, ranging from 5 to 20 J, with the intention of investigating damage initiation, damage propagation, and failure mechanisms. These impact energy levels are typically causing barely visible impact damage (BVID) in the impacted composite facesheet.A numerical simulation was performed using LS-DYNA3D transient dynamic finite element analysis code for calculating contact forces during impact along with a failure analysis for predicting the threshold of impact damage and initiation of delaminations. Good agreement was obtained between numerical and experimental results. In particular, the numerical simulation was able to predict the extent of impact damage and impact energy absorbed by the structure. The results of this study is proving that a correct numerical model can yield significant information for the designer to understand the mechanism involved in the low-velocity impact event, prior to conducting tests, and therefore to design a more efficient impact-resistant aircraft structure.     

冲击载荷下液压缸内压的流固耦合仿真与振动分析

该文为了研究液压缸受到冲击荷载时冲击瞬间内部流场的变化情况,用有限元分析软件Ansys Workbench对受冲击时液压缸内部流场压力进行了流固耦合分析,同时用动力学波动方程对相同模型的内部压力进行了分析、求解,并把两种方法计算出的轴向中心线压力分布进行了对比.对比结果表明两种方法对最大压力数值的预估误差为3.67％,...     

Quasi-static axial compression of thin-walled circular aluminium tubes

This paper presents further experimental investigations into axial compression of thin-walled circular tubes, a classical problem studied for several decades. A total of 70 quasi-static tests were conducted on circular 6060 aluminium tubes in the T5, as-received condition. The range of D/t considered was expanded over previous studies to D/t=10–450. Collapse modes were observed for L/D10 and a mode classification chart developed. The average crush force, F_AV, was non-dimensionalised and an empirical formula established as F_AV/M_P=72.3(D/t)^0.32. It was found that test results for both axi-symmetric and non-symmetric modes lie on a single curve. Comprehensive comparisons have been made between existing theories and our test results for F_AV. This has revealed some shortcomings, suggesting that further theoretical work may be required. It was found that the ratio of F_MAX/F_AV increased substantially with an increase in the D/t ratio. The effect of filling aluminium tubes with different density polyurethane foam was also briefly examined.     

Drilling of hybrid aluminium matrix composites

This paper presents the influence of cutting parameters on thrust force, surface finish, and burr formation in drilling Al2219/15SiCp and Al2219/15SiCp-3Gr composites. The composites were fabricated using the liquid metallurgy method. The tools used were commercially available carbide and coated carbide drills. The results revealed that feed rate had a major influence on thrust force, surface roughness, and exit burr formation. Graphitic composites exhibit lesser thrust force, burr height, and higher surface roughness when compared to the other material. The reduced thrust force and burr height is attributed to the solid lubricating property of the graphite particles. The higher surface roughness value for Al2219/15SiCp-3Gr composite is due to the pullout of graphite from the surface. The chips formed when machining graphitic composites are more discontinuous when compared to SiCp reinforced composites and hence advantageous.     

Low velocity impact resistance of ceramic/polyurea composite plates: experimental study

Journal of Mechanical Science and Technology - Laminated and segmented ceramic/polyurea composite plates were fabricated by coating bulk ceramic plate and bonding ceramic tablets with polyurea,...     

The effect of axial loading and axial restraint on the thermal ratchetting of thin tubes

The finite element method has been used to investigate the thermal ratchetting behaviour of thin tubes subjected to steady, internal pressure and cyclic, linear, through-thickness temperature distributions. An elastic-perfectly plastic material behaviour model was used and uniaxial behaviour was related to multiaxial behaviour via the von Mises yield criterion and the Prandtl-Reuss flow rule. Results for a tube without axial stress, a tube with axial stress (corresponding to a tube with an end closure) and a tube constrained to have no axial strain, are presented. Correlations with a simplified analytical solution were attempted. Good correlations were obtained for the tubes without axial restraint. The correlation for the axially constrained tube was poor because the thermal loading is essentially different. In the case of the axially constrained tube, if the thermal load is high enough, yielding occurs through the whole of the wall thickness simultaneously in each half cycle. This is not possible in the other two cases considered.     

Flexibility of axially symmetric bellows under axial loading

A simple energy method is used to find an expression for the flexibility of axially symmetric elastic bellows under axial loading. Results are expressed in terms of a single geometrical parameter, and are shown to be in good agreement with existing exact solutions for a range of values of the geometrical parameter from zero to a value large enough to include the most flexible designs.A simple analysis is also made of conditions under which the yield condition is reached.The results are useful for design purposes.     

The role of shape memory alloy on impact response of glass/epoxy laminates under low temperature

The paper aims to evaluate the impact response of glass/epoxy laminates with embedded shape memory alloy (SMA) subject to low velocity impact at various temperatures. For the goal, the impact tests were performed by using an instrumented impact-testing machine at three temperatures: 293K, 263K and 233K for the baseline (laminates without SMA wires) and SMA laminates (laminates with embedded SMA wires). And the resultant damages were inspected through the scanning acoustic microscope (SAM). Also, based on the impact force history and the damage configuration, the impact resistance parameters were employed to evaluate damage resistance of laminates with embedded SMA wires. As a result, it was observed that the damage resistance of glass/epoxy laminates is influenced by embedded SMA wires and embedding SMA wires into laminates does not compromise the structure any differently to laminates without wires. In fact, it has been shown that under lower temperature, the SMA laminates have a little superior damage resistance compared with the baseline laminates.     

Design and analysis of a novel mechanical loading machine for dynamic in vivo axial loading

This paper describes the construction of a loading machine for performing in vivo, dynamic mechanical loading of the rodent forearm. The loading machine utilizes a unique type of electromagnetic actuator with no mechanically resistive components (servotube), allowing highly accurate loads to be created. A regression analysis of the force created by the actuator with respect to the input voltage demonstrates high linear correlation (R(2) = 1). When the linear correlation is used to create dynamic loading waveforms in the frequency (0.5-10 Hz) and load (1-50 N) range used for in vivo loading, less than 1% normalized root mean square error (NRMSE) is computed. Larger NRMSE is found at increased frequencies, with 5%-8% occurring at 40 Hz, and reasons are discussed. Amplifiers (strain gauge, linear voltage displacement transducer (LVDT), and load cell) are constructed, calibrated, and integrated, to allow well-resolved dynamic measurements to be recorded at each program cycle. Each of the amplifiers uses an active filter with cutoff frequency at the maximum in vivo loading frequencies (50 Hz) so that electronic noise generated by the servo drive and actuator are reduced. The LVDT and load cell amplifiers allow evaluation of stress-strain relationships to determine if in vivo bone damage is occurring. The strain gauge amplifier allows dynamic force to strain calibrations to occur for animals of different sex, age, and strain. Unique features are integrated into the loading system, including a weightless mode, which allows the limbs of anesthetized animals to be quickly positioned and removed. Although the device is constructed for in vivo axial bone loading, it can be used within constraints, as a general measurement instrument in a laboratory setting.     

Deformation and failure mechanisms of lattice cylindrical shells under axial loading

The lattice cylindrical shells wound from the planar lattice plates, which have significant applications in aerospace engineering, exhibit different deformation modes with their planar counterparts because of the curvature of the cell wall. In this paper, deformation mechanisms are systematically investigated and failure analyses are conducted for the lattice cylindrical shells with various core topologies. Analytical models are proposed to predict the axial stiffness, critical elastic buckling load or effective yield strength of these shells. Finite element simulations are carried out to identify the validity of the models. The models can be employed for the optimal design. As an example, we construct the failure map for the Kagome lattice cylindrical shell made from an elastic ideally-plastic material. Various failure mechanisms, including yielding, global elastic buckling and local elastic buckling are taken into account. Moreover, optimizations are performed to minimize the weight for a given stiffness or load-carrying capacity for three types of lattice cylindrical shells. It is found that the Kagome and triangular lattice cylindrical shells have almost equivalent load-bearing capacity and both significantly outperform the hexagonal one under axial compression.     

圆锥滚子轴承轴向载荷与预紧力矩的关系

研究圆锥滚子轴承在预紧状态下轴向载荷与预紧力矩及最小预紧力的关系.首先,对受载的圆锥滚子轴承的滚子进行受力分析,推导轴向载荷和滚子的内、外滚道,内圈挡边法向接触载荷的关系;其次,通过对滚动体的内、外滚道,内圈挡边的法向接触载荷与预紧力矩的分析,推导轴向载荷与预紧力矩的关系;并且,针对一对圆锥滚子轴承,讨论其轴向载荷与最小预紧力的关系.通过以上分析,最后得出圆锥滚子轴承在预紧状态下轴向载荷与预紧力矩的线性关系和最小预紧力的表达式.     

Modes of deformation in aluminium rings subjected to static axial compression

Aluminium rings of varying height/outside diameter/inside diameter ratios machined from extruded solid bars, were subjected to static axial compression. Etched diametral planes of deformed cylinders revealed the existence of shear bands, the configuration of which were found to change with initial specimen geometry and deformation.     

Experimental studies on mechanical behavior of carbon fiber laminates under static and cyclic loading

An experimental investigation of the mechanical behavior of carbon fiber-reinforced plastic laminates under quasistatic and cyclic loading is performed. A uniform nonlinear elastic deformation of carbon fiber reinforced plastic laminates with uniform increment of the tangent modulus is studied. A condition for the initiation of the microscopic damage process was identified. The analysis of discreet continuous stages of the damage process followed by energy release was conducted. A diagram of deformation up to full loss of carrying capacity of the material is plotted. The proposed approach can be useful for diagnostics of elements of composite structures in operation.