变角度纤维复合材料层合斜板的颤振分析

假设纤维方向角沿层合板的长度方向线性变化，研究了变角度纤维复合材料层合斜板的颤振．通过坐标变换将斜板变换为正方形板，采用层合板表面连续变化的速度环量来模拟空气对其的作用，速度环量分布利用Cauchy积分公式计算．建立了系统的Lagrange方程并采用Ritz法得到了层合板的自振频率和颤振/不稳定性分离临界速度．通过数值算例验证了本文模型和方法的正确性和收敛性，分析了各个铺层内纤维方向角的变化对自振频率和颤振/不稳定性分离临界速度的影响．研究结果表明，通过纤维的变角度铺设，可有效地提高层合板的基频和颤振/不稳定性分离临界速度．经合理设计的变角度复合材料层合板具有抑制颤振的作用．     

Scaling effect on the tensile properties of [±45/0/±45/0/±45] Polypropylene/Twaron laminates

The effects of scaling on the mechanical response under tension of balanced nonsymmetrical laminates were investigated for a thermoplastic composite: Polypropylene reinforced with Twaron^® fibers. The composite baseline was an 8-ply laminate which consisted of unidirectional plies arranged in the sequence [±45/0/±45/0/±45]. The influence of specimen size on the tensile properties was studied for one (thickness), two (in-plane) and three (volume) dimensional scaling. The stress-strain curves suggested some variation in laminate behavior owing to the dimensional scaling; nevertheless, a further analysis with the classical lamination theory demonstrated that the observed effect was due to small variations in the fiber volume fraction of the laminates. It was concluded that the mechanical properties of these thermoplastic laminates do not exhibit scaling effects. The failure mechanism of the laminates was studied at macroscopic level; a scale effect of the fracture mechanism was observed.     

Laser Machined Plastic Laminates: Towards Portable Diagnostic Devices for Use in Low Resource Environments

Despite significant progress in development of bioanalytical devices cost, complexity, access to reagents and lack of infrastructure have prevented use of these technologies in resource‐limited regions. To provide a sustainable tool in the global effort to combat infectious diseases the diagnostic device must be low cost, simple to operate and read, robust, and have sensitivity and specificity comparable to laboratory analysis. In this mini‐review we describe recent work using laser machined plastic laminates to produce diagnostic devices that are capable of a wide variety of bioanalytical measurements and show great promise towards future use in low‐resource environments.     

Characterization of piezo-film sensors for direct vibration and impact measurements

Piezo-film sensors were employed in determining the dynamic response of [(0 deg/90 deg)₄]_s s-glass/epoxy laminates and 2024 aluminum specimens. Simple beam- and plate-type sensor equations were derived based on classical plate theory incorporating the linear piezoelastic constitutive relationship. A series of vibration and impact tests were conducted for the determination of structural dynamic response. Piezo-film sensors, with a thickness and area of 110 μm and 1×1 cm², respectively, were connected directly to a voltage measurement device in these tests. The first three bending frequencies of the glass-fiber-reinforced plastic (GFRP) cantilever specimen were examined. Experimental results and those simulated by the MARC finite-element code were found to be in good agreement, with the difference between the two being less than five percent. At frequencies above 3 kHz, piezo-film transducers are capable of closely detecting structural dynamic response in the absence of charge amplification. At frequencies lower than approximately 3 kHz, however, the voltage measured directly from a piezo-film sensor underestimates structural response. A modified piezo-film sensor equation is thus proposed for lower frequency measurements. Effect of frequency and piezo-film's size on lower frequency attenuation is explicitly formulated based on a simple RC circuit analogy. Drop tests were also performed on clamped [(0 deg/90 deg)₄]_s GFRP laminates and aluminum targets, with nine piezo-film sensors being glued to the specimen's distal surface in order to determine the low-speed impact response. Specimen transient central deflection subjected to impact loading was identified based on test findings and the plate-type piezo-film sensor equation. The results were found to be in good agreement with the numerical solution obtained from the MARC finite-element code.     

复合材料层合板的低速冲击损伤及其剩余压缩强度研究

本文采用理论和实验方法研究了复合材料层合板的低速冲地及其剩余压缩强度。文中利用有限元方法和能量转换原理计算了层合板受到低速冲击的受载最危险状态，以及此时的应力分布；并用Ｔｓａｉ－Ｗｕ张量准则判断损伤情况，对产生损伤的单元进行相应的刚度折减，且作重复计算直至不产生新的损伤为止；最后，对受冲击的层合板还进行剩余压缩强度计算。在实验中，采用激光全息无损检测法测量了层合板的冲击损伤，并对受冲击的层合板进行     

层板复合材料的疲劳剩余刚度统计分布模型

在分析现有关于层板复合材料疲劳剩余刚度衰退模型的基础上，根据层板复合材料在疲劳载荷作用下刚度衰退变化的现象，研究了疲劳载荷对层板复合材料刚度衰退的影响，建立了一个新的、更为合理的用于描述层板复合材料在常幅疲劳载荷作用下的刚度衰退模型，导出了剩余刚度统计分布的表达式，给出了确定模型参数的方法。为验证该模型，设计了几组测试实验，并利用试验结果对模型参数进行了估计。利用该统计分布模型，可以预报层板复合材料在给定应力水平的疲劳载荷作用下循环指定周次时剩余刚度的统计分布。实验数据表明，理论预报和实验结果符合得是很好的。     

Effect of transverse cracks on the effective thermal expansion coefficient of aged angle-ply composites laminates

A modified shear-lag analysis, taking into account the concept of stress perturbation function, is developed and applied to evaluate the effect of transverse cracks on the effective thermal expansion coefficient of aged angle-ply composites laminates. Effects of number of 90° layers and number of θ° layers in the outer angle-ply laminates on the reduction of the effective axial coefficient of thermal expansion have also been studied. The results of this paper represent well the dependence of the reduction of the effective axial coefficient of thermal expansion on the hygrothermal conditions, the fibre orientation angle of the outer layers, the number of cracked cross-ply layers and the number of un-cracked outer θ° layers in laminate.     

Investigation on interlaminar delamination tendency of multidirectional carbon fiber composites

In this investigation carbon fiber reinforced laminates with different orientation layups are prepared and studied under tensile loading condition. Multiple strain measurement techniques, namely, resistive strain gauges, embedded optical sensors and digital image correlation are used to analyze stress-strain behavior simultaneously through the thickness of composite materials, and to determine the sequence of failure in different plies. Inconsistencies of strains measured through different methods is correlated with the tendency for interlaminar delamination, therefore demonstrating the ability of multi-instrument approach to describe damage progress through the thickness of multidirectional laminates. Complementary analysis through acoustic emission methods reveals that the angle of off-axis surface plies can influence the sequence of failure under tensile loading condition, and damage monitoring capabilities of acoustic emission system is directly affected by delamination tendency of surface plies. Remarkably, the delayed failure of off-axis plies is shown to be related to reorientation of these layer towards loading direction using infrared thermography method.     

Ply-by-Ply Failure Analysis of Laminates Under Dynamic Loading

Ply-by-ply failure analysis of symmetric and anti-symmetric laminates under uniform sinusoidal transverse dynamic loading is performed for a specified duration. The study investigates the first ply failure load, followed by the detection of successive ply failures along with their failure modes using various failure theories. Some of the well-established failure theories, mostly used by the researchers, are considered for the failure prediction in laminates. The finite element computational model based on higher order shear deformation displacement field is used for the failure analysis and the complete methodology is computer coded using FORTRAN. The ply-discount stiffness reduction scheme is employed to modify the material properties of the failed lamina. The failure theories used in the analysis are compared according to their ability to predict failure load, failed ply, failure mode and progression of failure. The failure analysis is performed for both the cross-ply and angle-ply laminates with all edges simply supported and clamped. The significance of fibre orientation and stacking sequence in terms of the strength of a laminate and failure progression is also highlighted.     

The onset of mixed-mode intralaminar cracking in a cross-ply composite laminate

The intralaminar fracture toughness of a unidirectionally reinforced glass/epoxy composite is determined experimentally at several mode I and mode II loading ratios. The crack propagation criterion, expressed as a quadratic form in terms of single-mode stress intensity factors (alternatively, linear in terms of energy release rates), approximates the test results reasonably well. The mixed-mode cracking criterion obtained is used to predict the intralaminar crack on set in a cross-ply glass/epoxy composite under off-axis tensile loading. Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 6, pp. 785–794, November–December, 2008.