Three-dimensional stress and progressive failure analysis of ultra thick laminates and experimental validation

Test methods and analysis capabilities for fibre reinforced composites are generally limited to thin laminates. However, extending the range of application of composite materials to thick laminates is essential for a multitude of possible composite structures. This paper presents an adapted three-point bending test for a new quasi isotropic stacking sequence for non crimped fabrics for the application in ultra thick laminates (UTL). In addition, numerical simulation capabilities for thick laminates using a multiscale analysis are shown. The three-point bending test setup is developed to examine the failure behaviour of 30–60 mm thick coupons.     

Multiscale analysis of stiffness and fracture of nanoparticle-reinforced composites using micromechanics and global–local finite element models

A combined micromechanics analysis and global–local finite element method is proposed to study the interaction of particles and matrix at the nano-scale near a crack tip. An analytical model is used to obtain the effective elastic modulus of nanoparticle-reinforced composites, then a global–local multi-scale finite element model with effective homogeneous material properties is used to study the fracture of a compact tension sample. For SiO₂ particle-reinforced epoxy composites with various volume fractions, the simulation results for effective elastic modulus, fracture toughness, and critical strain energy release rate show good agreement with previously published experimental data. It is demonstrated that the proposed parametric multi-scale model can be used to efficiently study the toughness mechanisms at both the macro and nano-scale.     

On the influence of laminate stacking on buckling of composite cylindrical shells subjected to axial compression

The buckling loads of laminated cylinders can strongly depend on the position of the differently oriented layers within the shell. This paper deals with two different laminated orthotropic cylinders with opposite stacking sequence of the laminate layers. Cylinders of this construction had been thoroughly tested within a BRITE EURAM project. Analytical and semi-analytical methods have been used to predict the buckling loads, and the results are reported in this paper as well as test results for comparison. An explanation of the striking influence of stacking sequence is given. With some more examples the findings are verified. It is suggested that the presented results can be used for benchmarking purpose.     

Limits of asymptotic solutions in low-velocity impact of composite plates

This paper presents a study of the low velocity impact response of composite plates by using dimensional analysis and lumped-parameter models. Simple analytical expressions are obtained and used to construct a characterization diagram that shows the relationship between the maximum normalized impact force and two non-dimensional parameters. For a given impact event, it is shown that the non-dimensional parameters can be obtained by analytical, computational or experimental methods. Once these parameters are determined, the characterization diagram can be used to estimate the type of response, and maximum impact force. Furthermore, the diagram can be used to select adequate simple models for a given impact situation. It is shown that these simple models provide very good approximations of the impact response for a pre-determined wide range of impact parameters.     

Multiscale modeling for mechanical properties of carbon nanotube reinforced nanocomposites subjected to different types of loading

Multiscale modeling was presented for the nonlinear properties of polymer/single wall carbon nanotube (SWNT) nanocomposite under tensile, bending and torsional loading conditions. To predict the mechanical properties of both armchair and zigzag SWNTs, a finite element (FE) model based on the theory of molecular mechanics was used. For reducing the computational efforts, an equivalent cylindrical beam element was proposed, which has the unique advantage of describing the mechanical properties of SWNTs considering the nonlinearity of SWNT behavior. For a direct evaluation of the rigidities of the proposed equivalent beam, the data obtained through atomistic FE analyses of SWNT were fitted to six different equations, covering the three types of loading for both armchair and zigzag configurations. The proposed equivalent beam element was then used to build a cylindrical representative volume element (RVE) using which the effects of the interphase between SWNT and the polymer on the mechanical properties of RVE could be studied. It was found that while the interphase has a small effect on the nanocomposite stiffness, the ratio of (SWNT length)/(RVE length) dramatically affects the nanocomposite stiffness.     

Multiscale fracture simulation of three-point bending asphalt mixture beam considering material heterogeneity

A multiscale modeling frame combining macroscale and mesoscale is proposed in basis of the aggregate generation and packing algorithm so that the heterogeneity feature of asphalt mixture can be involved in the computational model. In this frame, the mesoscopic scale is used in some concerned local regions and the macroscopic scale is used in the other regions. As examples, a series of two-dimensional multiscale models of cracked three-point bending asphalt mixture beam are created and then the effects of crack location and aggregate distribution near the crack on cracking behaviors are evaluated. Finally, some important conclusions are concluded.     

改进的Curvelet变换图像降噪方法

与小波变换相比,Curvelet变换等多尺度几何分析方法,可以更好地逼近含线奇异的高维函数。基于Curvelet变换的图像去噪方法,即Window Shrink算法,考虑到Curvelet变换系数之间的相关性,利用软阈值方法降噪。通过窗口邻域操作,对变换后的每一个Curvelet系数自适应地进行萎缩处理,降低噪声系数权重以提高信噪比。实验表明,该方法一定程度上改进了传统Curvelet去噪方法“过扼杀”Curvelet变换系数的缺点,可以较好地保持图像边缘。在噪声方差σ=25时,小波,Curvelet以及Window Shrink去噪算法的峰值信噪比（PSNR）分别为28．59、29．25和29．93,后者明显优于前二者。     

Multiscale analysis of carbon nanotube-reinforced nanofiber scaffolds

Biopolymers play a significant role in tissue engineering, as they simulate the physiological environment required for the development of tissue cultures. Use of carbon nanotube polymeric scaffolds for tissue engineering applications has gained attention recently due to the enhanced mechanical properties of carbon nanotubes. In this paper, a hierarchical approach by studying the atomistic properties of carbon nanotube based polymers using molecular dynamics and coupling the scales through complex multi-scale nonlinear hyperelastic material-based mathematical homogenization models are developed. These homogenization methods offer a systematic and rigorous treatment of up-scaling the properties from the micro or nanoscale to the macroscales. The material constitutive properties estimated using the developed methods for nanotube polymeric scaffold materials show excellent comparison with experimental studies.     

基于多尺度数值模型的复合材料各向异性热膨胀系数预测

依据复合材料内部纤维在基体内的排布规律及层合板铺层特性,基于多尺度方法,建立单层板和层合板代表性体积单元(RVE)模型,施加相应的边界条件,预测单层板的热膨胀系数和工程常数,进而预测复合材料层合板各向异性的等效热膨胀系数。通过与实验数据对比发现,基于正六边形单层板RVE模型预测的热膨胀系数,相比理论预测值,整体更接近实验值,其中预测的单向T300/5208碳纤维增强环氧树脂基复合材料、P75/934碳纤维增强环氧树脂基复合材料和C6000/Pi碳纤维增强环氧树脂基复合材料的横向热膨胀系数与实验结果的误差分别只有3%、1%和2%;采用单层板RVE预测的单向ECR/Derakane 510C玻璃纤维增强乙烯基酯树脂基复合材料的工程常数与实验值最大相差7.5%;层合板RVE模型预测的正交AS4/8552碳纤维增强环氧树脂基复合材料厚度方向的热膨胀系数与实验结果误差可以忽略,只有0.08%。最后以大型复合结构常用的正交铺层结构为研究对象,基于给出的单层板和层合板RVE模型预测了不同铺层复合材料烟道层合板的等效热膨胀系数,环向铺层比例对厚度方向的热膨胀系数影响较小。     

The performance of unreinforced masonry walls subjected to low-velocity impacts: Finite element analysis

A simplified discrete-crack finite element modelling approach has been developed to model the performance of unreinforced brickwork and blockwork masonry walls subject to out-of-plane impacts. The approach involves the use of linear elastic solid elements for masonry units in conjunction with a specially formulated contact interface model for masonry joints. Key features of the latter include: (i) a Mohr–Coulomb failure criterion; (ii) a cohesive crack model for initial fracture; (iii) inclusion of dilatancy. The contact interface model has been implemented in LS-DYNA, a three-dimensional non-linear explicit finite element program. The modelling approach was used to simulate the behaviour of a series of unreinforced walls tested previously in the laboratory. It was found that the dynamic response of full-scale masonry walls could be predicted with reasonable accuracy. However, parametric studies showed that wall response was highly dependent on small changes in loading impulse, base friction, fracture energy, joint failure stress and angle of dilatancy.     

Grid indentation analysis of composite microstructure and mechanics: Principles and validation

Several composites comprise material phases that cannot be recapitulated ex situ, including calcium silicate hydrates in cementitous materials, hydroxyapatite in bone, and clay agglomerates in geomaterials. This requirement for in situ synthesis and characterization of chemically complex phases obviates conventional mechanical testing of large specimens representative of these material components. Current advances in experimental micro and nanomechanics have afforded new opportunities to explore and understand the effect of thermochemical environments on the microstructural and mechanical characteristics of naturally occurring material composites. Here, we propose a straightforward application of instrumented indentation to extract the in situ elastic properties of individual components and to image the connectivity among these phases in composites. This approach relies on a large array of nano to microscale contact experiments and the statistical analysis of the resulting data. Provided that the maximum indentation depth is chosen carefully, this method has the potential of extracting elastic properties of the indented phase which are minimally affected by the surrounding medium. An estimate of the limiting indentation depth is provided by asssuming a layered, thin film geometry. The proposed methodology is tested on a “model” composite material, a titanium-titanium monoboride (Ti–TiB) of various volumetric proportions. The elastic properties, volume fractions, and morphological arrangement of the two phases are recovered. These results demonstrate the information required for any micromechanical model that would predict composition-based mechanical performance of a given composite material.     

Delamination damage analyses of adhesively bonded lap shear joints in laminated FRP composites

Three-dimensional non-linear finite ele- ment analyses have been carried out to evaluate the out-of-plane stresses in the adhesive layer existing between the lap and the strap adherends of the Lap Shear Joint (LSJ) in laminated FRP composites for varied delamination lengths. The delaminations are presumed to be pre-embedded in the thin resin rich layer existing between the first and second plies of the strap adherend. Sublaminate technique has been used to model the LSJ with the delamination. Contact finite element analyses have been performed in order to avoid interpenetration of delaminated surfaces. The effects of varied delamination lengths on the peel and interlaminar shear stresses and the individual modes of Energy Release Rate (ERR) in the delamination zones are highlighted in this paper. It is seen that three-dimensional effects exist near the free edges of the overlap end of the joint. The delamination propagation significantly affects the stress distributions in the adhesive layer existing between the lap and the strap adherends of the LSJ. The variations of interlaminar stresses and ERRs on both the delamination fronts are found to be significantly different and thus, indicate that the propagation of delamination does not occur at same rate at the two delamination fronts. This may throw some light to the evaluation of structural integrity of the LSJ in the presence of pre-embedded delaminations.