Local buckling of stitched composite laminate

Due to relatively low interlaminar strength, delamination is a common failure mode of composite laminates. Through-thickness stitching is shown to improve the delamination resistance of laminated composites. Under in-plane compressive loading, significant strength reduction occurs due to coupling between delamination and delamination buckling. In this paper, an energy-based model was developed to predict the effect of critical stitching parameters on the delamination buckling strength of stitched laminates. Excellent agreement was found between the model results and a corresponding finite element analysis.     

Effect of Delamination on Mechanical Properties of Al-Li Alloy Modified by Rare Earth Element Ce

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详谈PCB钻孔孔壁镀铜开裂问题及解决方案

PCB作为重要的电子部件,其钻孔质量的优劣直接关系到产品性能的稳定程度,尤其在航空、航天、军事等重要领域,钻孔质量在电气连接特性及使用可靠性方面起着至关重要的作用。文章针对PCB钻孔过程中出现的孔壁镀层开裂问题,结合金属切削理论基础,从钻孔参数、刀具选择、板材结构与叠层、分段钻孔、材料对比等方面进行了详尽的分析与交叉实验比较,下面对具体解决过程与结果进行阐述。     

2.25Cr-1Mo-0.25V钢148 mm厚板尾部分层缺陷分析及控制工艺

采用扫描电镜及电子探针对55 t钢锭成材148 mm厚2.25Cr-1Mo-0.25V钢板尾部分层缺陷进行观察分析,结果表明：分层缺陷是由于模铸保护渣卷入钢液造成的。为减少钢板尾部分层缺陷,对冶炼工艺进行优化,精炼过程中采用碳粉、电石及1.2～2.0 kg/t_钢Al线复合脱氧工艺,真空后软吹12～14 min,采用高粘度、低熔点、高熔速及膨胀系数较大的模铸保护渣。优化工艺后,钢板尾部分层缺陷出现概率分别为11.3%、9.6%、6.9%,分层缺陷导致钢板废品率由原8.1%降至1.7%。     

Investigation of organic coatings and coating defects with the help of time-of-flight-secondary ion mass spectrometry (TOF-SIMS)

Among the commercially available surface analytical methods time-of-flight-secondary ion mass spectrometry (TOF-SIMS) plays a prominent role. This is due to its outstanding surface sensitivity combined with the detailed molecular information obtained. Therefore, the method is extremely well suited for the chemical characterization of the uppermost layers of the material under study. The detailed knowledge about the composition of these layers can be favourably used to support the development of coatings and to elucidate coating defects. This article describes the principles of TOF-SIMS and illustrates its ability by a number of examples from daily work. The samples analysed include typical coating defects, such as cratering and delamination, as well as undisturbed coating layers where we were interested in the chemical composition of the uppermost layers.     

复合材料桨叶蒙皮层间裂纹的应力强度因子求解方法研究

蒙皮分层是复合材料桨叶疲劳破坏的主要形式.文中以应力强度因子为蒙皮分层扩展参数,应用准三维方法及复合材料桨叶结构模型,推导带预扭角的直升机桨叶蒙皮的三维应力-应变关系;同时依据各向异性体界面断裂力学,分析蒙皮存在初始裂纹时裂尖的振荡性,并采用相互积分法给出应力强度因子的求解方法,最后通过数值算例论证文中的方法是可行的.     

LSM–YSZ interactions and anode delamination in solid oxide electrolysis cells

Symmetric cells of the configuration air/LSM//YSZ//LSM/air have been fabricated and electrically tested under impressed voltage conditions to understand the anode delamination behavior commonly observed during the operation of solid oxide electrolysis cells (SOEC). Electrical performance degradation has been measured with time at various applied voltages ranging from 0 to 0.8 V with respect to OCV, and cell component microstructural and chemical changes have been examined. Post-test observations indicate the development of a weak anode–electrolyte interface leading to the delamination of the anode from the electrolyte surface. Microstructural analysis of the anode–electrolyte interface revealed extensive morphological and chemical changes including the formation of lanthanum zirconate, an uneven porous interface, and localized grain boundary porosity in the electrolyte. An anode delamination mechanism based on morphological change and compound formation at the anode–electrolyte interface is proposed.     

Influence of electrospun Nylon 6,6 nanofibrous mats on the interlaminar properties of Gr-epoxy composite laminates

The present work aims at investigating the influence of electrospun Nylon 6,6 nanofibrous mat to interlaminar strength. Mode I and the Mode II fracture mechanics of virgin and nanomodified laminates are investigated. Nanomodified laminates are fabricated by interleaving an electrospun nanofibrous mats in laminate mid-plane. Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests are performed on both virgin and nanomodified configuration. Results show that electrospun nanofibrous mat is able to increase by 23.2% the mechanical energy absorbing capability and by about 5% the G_IC. ENF tests reveal that the nanofibrous mats contribute to improve the maximum stress before the material crisis (6.5% of increment) and a measurable increment of (8.1%) the maximum mechanical energy that can be absorbed by the material during the crack propagation is registered. The acoustic emission (AE) technique is used to monitor both DCB and ENF tests. The AE information highlight that the nanofibrous mats mitigate the interlaminar matrix failure on both the fracture modes.     

Low energy impact damage monitoring of composites using dynamic strain signals from FBG sensors - Part II: Damage identification

In part I of this two-paper series, a method for the localization of an impact using dynamic strain signals from fiber Bragg grating (FBG) sensors is presented. In this paper, an inverse numerical-experimental method allowing to identify the damage based on experimentally measured eigenfrequency changes is developed and validated. The damage identification is limited to a region in the vicinity of the impact position predicted by the localization method. The eigenfrequency changes are determined experimentally from dynamic strain signals obtained with embedded FBG sensors and the parameters of a homogenized damage model are adjusted to fit the numerical results to the experimental data.     

Identification of material parameters for modelling delamination in the presence of fibre bridging

Delamination processes often exhibit an increase in delamination resistance, or R-curve, with crack extension. It is shown that cohesive laws can represent the R-curves due to large-scale fibre bridging and that the shape of the cohesive laws can be derived from conventional experimental results. Two approaches are investigated for determining the shape parameters of cohesive laws. The first approach consists of extracting the cohesive parameters from experimental R-curves through the use of a new semi-analytical equation. The second approach consists of a numerical optimization procedure that identifies material parameters by reducing the error between a finite element model and the experimental load–deflection results. The second approach is advantageous when fibre bridging introduces inaccuracies in the experimental energy release rate measurements. In addition, the second approach can be extended to allow more complex approximations of cohesive laws.