Delamination failure in ceramic matrix composites: Numerical predictions and experiments

The cohesive-zone finite element approach is used to predict initiation and propagation of delamination in relatively complex ceramic matrix composite sub-elements. Two different generic attachment sub-elements are analyzed and tested under applied uniaxial load. Pre-test analyses predict that delamination initiation and growth are the predominant failure mechanism for both of the sub-elements. Experimental results confirm the finite element predictions, and a good qualitative and quantitative agreement is found between the two.     

Machining parameters for an intelligent machining system for composite laminates

Composite laminates exhibit very high in-plane strengths but are plagued by delamination damage when subjected to machining. This is due to their poor transverse strengths and low delamination fracture toughness. In drilling, delamination is initiated when the thrust force exceeds a threshold value, particularly at the critical entry and exit locations of the drill bit. To minimize damage, therefore, it is important to monitor process variables such as the machining forces and the position of the tool relative to the workpiece. The availability of a suitable model coupled with an intelligent control scheme would be a large advancement in the machining of composite laminates. This paper explores the development of such models for machining of composites and for coupling the models to intelligent control strategies. Using a machining center, a series of drilling experiments were conducted on carbon fiber-reinforced composite laminates to determine key process parameters for various cutting conditions. An intelligent machining scheme is proposed as the basis for the design of a new machine tool.     

Estimation of impact-induced damage in CFRR laminates through ultrasonic imaging

Ultrasonic C-scans are used frequently to detect defects in composite components caused during fabrication and damage resulting from service conditions. Evaluation and characterization of defects and damage require experience and good understanding of the composites. Under low-velocity impact loading delamination is observed to be a major failure mode. In this paper, the results of extensive experimental work on damage of carbon fiber reinforced plastic (CFRP) laminates due to low-velocity impact are presented. The study was carried out on 10 different layups over four different thickness. Both drop-weight and projectile impacts simulating practical conditions like tool-drop and runway debris, respectively, were carried out at energy levels varying from 3 to 30 J. The ensuing delamination damage was determined by ultrasonic: C-scans using the pulse-echo immersion method for both projected and layer-wise distribution. While projected delamination was obtained by placing a gate over the backwall echo, layerwise distribution was obtained by successive time delay from the frontwall to the backwall echo covering each interface. Delamination areas were quantified accurately by processing the raw image data using a digital image processing technique. Based on the data obtained an empirical relation was established between the delamination area and the impact energy.     

制孔分层损伤对CF/PEEK复合材料拉伸性能和表面应变分布的影响

目的 研究钻削制孔表面分层损伤与拉伸载荷下开孔碳纤维增强聚醚醚酮（CF/PEEK）复合材料表面应变分布的相关性。方法 通过对CF/PEEK复合材料层合板进行钻削制孔实验,分析不同进给速度对钻削温度、钻削轴向力、制孔出口表面分层和孔壁表面损伤的影响。采用数字图像相关技术（DIC）和力学实验相结合的方法,研究分层损伤程度对开孔CF/PEEK复合材料层合板拉伸性能和表面应变分布的影响。使用扫描电镜观测开孔试件的断裂形貌,分析开孔试件受拉伸载荷时的破坏模式。结果 随着进给速度的增加,钻削温度降低,钻削轴向力提高,出口表面分层和孔壁损伤程度加剧。随着分层损伤程度的增加,层合板的拉伸强度呈现出降低的趋势,试件的拉伸强度从558.4 MPa降低到525.63 MPa,降低了5.87%。在中应力和高应力状态下,试件x方向的最大负应变随着分层损伤程度的增加而增加。在高应力状态下,试件y方向的最大正应变随着分层损伤程度的增加而增加。试件的断裂方式主要是基体开裂、分层和纤维撕裂,断口有纤维脱落和纤维拔出,垂直于载荷方向的纤维破坏模式为剥离破坏,与载荷方向一致的纤维破坏模式为拉伸破坏。结论 钻削制孔表面分层损...     

A review of the mechanics of metal spinning

This review presents a thorough survey of academic work on the analysis and application of the mechanics of spinning. It surveys most literature published in English and the most important publications in German and Japanese languages. The review aims to provide insight into the mechanics of the process and act as a guide for researchers working on both metal spinning and other modern flexible forming processes.The review of existing work has revealed several gaps in current knowledge of spinning mechanics: the evolution of the stress state and the strain history of the workpiece in both conventional and shear spinning is not well understood, mainly due to the very long solution times that would occur in modelling the process throughout its duration with a sufficiently fine mesh to capture detailed behaviour through the workpiece thickness; the evolution of microstructure, residual stress and hence springback, has not been examined—either numerically or by experiment; the failure mechanisms of spinning – fracture and wrinkling – are only partially understood, through analogy with other processes, and as yet models of the process have not made use of contemporary damage mechanics; the design of toolpaths required to make particular parts without failure remains an art, and cannot currently be performed automatically with confidence. Studies on novel process configurations in spinning have shown that great potential for innovation in spinning exists. The process has the potential to be more flexible, to produce a wider range of shapes, and to form more challenging materials.     

Cr涂层对Zr-1Nb合金包壳微动磨损行为的影响

采用三维白光干涉仪、扫描电子显微镜、能谱仪等表征技术对比研究Cr涂层Zr-1Nb合金包壳和Zr-1Nb合金包壳与格架在模拟压水堆一回路水环境下的微动磨损行为及损伤机制。结果表明,Cr涂层显著提高Zr-1Nb合金包壳的抗微动磨损性能。此外,对磨副为刚凸时,Zr-1Nb合金包壳微动磨损机制以磨粒磨损和剥层磨损为主,而Cr涂层Zr-1Nb合金包壳由于表面硬度较高,且表面形成具有保护作用的三体层,其损伤机制以黏着磨损和材料单向转移为主。对磨副为弹簧时,Zr-1Nb合金包壳微动磨损机制主要为剥层和黏着磨损,Cr涂层Zr-1Nb合金包壳主要为磨粒磨损。     

A failure analysis of water jet drilling in composite laminates

Delamination is a major concern in the manufacturing processes of composite materials. It reduces not only the structural integrity of the laminate but also the long-term reliability of the assembly. Water jet drilling, in spite of its advantages of no tool wear and thermal damage, often creates delamination composite laminate at bottom. The current paper presents an analytical approach to study the delamination during drilling by water jet piercing. The analysis uses fracture mechanics with plate theory to describe the mechanism of delamination. This model predicts an optimal water jet pressure for no delamination as a function of hole depth and material parameters (opening-mode delamination fracture toughness and modulus of elasticity). Good agreement is achieved with data obtained from water jet drilling of graphite epoxy laminate. The predicted optimal water jet pressure can be applied in a control scheme for maximizing the productivity of water jet drilling of composite laminates.     

Study on arall failure behaviour under tensile loading

This paper studies the failure behaviour of ARALL (Aramid Aluminum Laminate) under tensile loading by means of acoustic emission (AE), optical metallography (OM) and scanning electronic microscope (SEM), and analyzes the fracture appearance of ARALL and its fracture characteristics. The damage models of ARALL are concluded in this paper. The results show that ARALL will yield under tensile loading, and its strength will decrease as the content of resin increases. During fracture process, AE amplitude distribution curves show that there are three obvious peaks, which respectively correspond to separation of the interface between fibers and resin, local delamination damage and fracture of a small quantity of fibers, and delamination damage of large area and final fracture of a large numbers of fibers. Dynamic damage and fracture process of ARALL can be detected by AE.     

Comparison of Lamb waves and pulse echo in detection of near-surface defects in laminate plates

One of the problems faced in ultrasonic nondestructive testing (NDT) of composite laminates is near-surface delaminations which may be due to impact damage. The normal incidence pulse echo technique has difficulty in resolving echoes from this type of defect since they often lie within the length of the transmitted ultrasonic pulse. Although a high frequency probe may be used, it has its limitations and could be problematic in composite materials in which ply reflections may interfere with defect reflections. The use of an acoustic delay, together with a high frequency probe, may improve the detection. In this paper, a different approach has been developed using the fundamental Lamb wave (S₀ mode) to detect delaminations in unidirectional fibre composite materials. It has been found that the Lamb wave amplitude decreases significantly over a delamination region. The decrease in amplitude is strongly dependent upon the depth of the delamination and is most sensitive to the delaminations near to the surface of the laminate. By scanning the transducer over the surface, it is possible to measure the size and depth of this kind of delamination. This technique is comparable to the delayed pulse echo technique and can be used to complement other techniques.     

含低速冲击损伤复合材料层合板的压缩失效

通过含低速冲击损伤的两种平面编织复合材料层合板（G803／5224、G827／5224层合板）的压缩试验,研究了低速冲击损伤对复合材料层合板失效行为的影响。试验后采用超声c扫描检测、外观检查与断口侧面宏观观察方法对损伤与失效特征进行了对比分析。结果表明,两种含低速冲击损伤层合板被压缩时,G803／5224层合板冲击背面首先发生子层微屈曲,微屈曲沿着垂直于压缩方向扩展,最后剪切分层失效;G827／5224层合板冲击背面首先发生子层屈曲分层,屈曲分层也是沿着垂直于压缩方向扩展,最后剪切屈曲失效。两种层合板低速冲击后压缩的失效模式与光滑板压缩失效模式基本相同。     

High-temperature mechanical behaviour of flaw tolerant alumina–zirconia multilayered ceramics

The mechanical behaviour of alumina–zirconia multilayered ceramics designed with thin internal compressive layers has been investigated under flexural loading at room and high temperature. Young’s modulus and the sintering evolution of each layer have been experimentally determined up to 1200 °C, to account for the residual stress distribution in the layered composite. The fracture behaviour has been assessed by indentation – strength experiments at different temperatures and by a fracture mechanics analysis. Experimental findings showed that improvement in mechanical properties of the laminate at high temperatures in comparison to the alumina-based monolithic material was essentially related to the distinct modes of failure observed as a function of the temperature, in the presence of energy release mechanisms such as crack bifurcation and/or delamination that may be used as a tool for designing tolerant materials at high temperatures.     

Detailed modelling of delamination buckling of thin films under global tension

Tensile specimens of metal films on compliant substrates are widely used for determining interfacial properties. These properties are identified by the comparison of experimentally observed delamination buckling and a mathematical model which contains the interface properties as parameters. The current two-dimensional models for delamination buckling are not able to capture the complex stress and deformation states arising in the considered uniaxial tension test in a satisfying way. Therefore, three-dimensional models are developed in a multi-scale approach. It is shown that, for the considered uniaxial tension test, the buckling and associated delamination process are initiated and driven by interfacial shear in addition to compressive stresses in the film. The proposed model is able to reproduce all important experimentally observed phenomena, like cracking stress of the film, film strip curvature and formation of triangular buckles. Combined with experimental data, the developed computational model is found to be effective in determining interface strength properties.     

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

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Mikrobielle Werkstoffzerstörung – Grundlagen: Grundvorgänge der Korrosion

Microbial deterioration of materials – fundamentals: Basic corrosion processes Microbial growth on metal surfaces induces a number of corrosion reactions which are due to changes in the conditions on the boundary layer. The corrosion systems in question are numerous and damage mechanisms correspondingly manifold. Metallic materials corrode according to known electrochemical mechanisms, whereby microorganisms act indirectly. They produce biofilms which are the cause of concentration cells (oxygen, pH, metal salts) and finally lead to local corrosion effects. Another widespread mechanism is based on the formation of sulphide by sulphide-reducing bacteria which stimulate electrochemical partial reactions. A third large group is attributed to the acid producing microorganisms which attack both metallic and inorganic materials. There is no indication that microorganisms are directly involved in the basic reactions of metal corrosion. The corrosion mechanism of inorganic materials, such as the concrete/sulphuric acid system, is of a purely chemical nature and involves processes, such as binding rupture by ion exchange, solvation, hydrolytic cleavage and chemical conversion. Contrary to this, microorganisms participate directly in the deterioration process of organic materials. For each natural polymer a microorganism exists which is capable of complete or partial decomposition of the polymer, whereby it, or additives (e.g. plasticizers). act as C and/or N sources. Explanations of mechanism are, therefore, transferred to the field of microbiology and physico-chemical interpretation is only partially possible.     

The extent of laser-induced thermal damage of UD and crossply composite laminates

Composite materials exhibit poor quality cut surfaces due to spalled fibres, fuzzing, and delamination when routed by conventional tools. Laser beam cutting offers an ideal means for the cutting of fibre composite materials, being a non-contact and virtually force-free manufacturing method. However, in the shaping operation of composite materials after curing, thermal damage associated with laser energy can be produced. It leads to poor assembly tolerance and long-term performance deterioration. In the current investigations, 3-dimensional anisotropic heat conduction models based on moving point heat source for thermal analysis are presented to predict heat affected zone (HAZ). Also, the conductivity models allow to consider the anisotropic heat conductivity for unidirectional (UD) and [0/90] laminates. Taking the immersing heat source and Mirror Image Method into account would further improve the prediction of HAZ. Extensive experiments were conducted on composite materials to examine HAZ, and compared with experimental results. The analytical results show a good agreement with experiments.     

含损伤复合材料层合板振动特性研究

本文旨在找出损伤对复合材料层合板振动特性的影响。复合材料在直升机桨叶上的应用,实现了桨叶优化设计,改善了旋翼气动性能,使桨叶的寿命增加到上万小时,甚至达到无限寿命。因此,使用复合材料已成为现代直升机桨叶的发展趋势。对G827／3234、G803／3234以及G814／3234等三种铺层材料的复合材料层合板进行了振动试验研究与理论分析,得到了振动特性与材料、铺层方式的关系。进一步对这些层合板在含有穿孔、分层损伤情况下的振动特性进行了研究。结果表明,理论分析结果与试验结果吻合,证明了所建模型的有效性。该研究结果对直升机复合材料桨叶结构损伤容限分析与设计具有一定的参考价值。     

Delamination during drilling in polyurethane foam composite sandwich structures

The objective of this paper is to study the influence of drilling velocity, feed rate, and flank length on the delamination of polyurethane foam sandwich structures. A Taguchi-based design of experiments was used to assess the importance of the drilling parameters, and scanning electron microscopy (SEM) was used to assess the damage from drilling. The drilling of sandwich structures results in significant damage caused by delamination and surface roughness around the drilled holes. The drilling process was evaluated based on a factor called the delamination factor, which is defined as the ratio of the maximum diameter of the damage zone, measured using SEM, to the standard hole diameter (drill diameter). Analysis of variance of the experimental results showed that cutting speed was the most significant parameter among the controllable parameters during drilling of sandwich specimens followed by flank length and feed rate. Finally, confirmation tests were performed to make a comparison between the experimental results and the correlation results. The damage mechanisms are explained using SEM.     

Prediction of damage in small curvature bending processes of high strength steels using continuum damage mechanics model in 3D simulation

Sheet metal bending of modern lightweight materials like high-strength low-alloyed steels (HSLA) is one major challenge in metal forming, because conventional methods of predicting failure in numerical simulation, like the forming limit diagram (FLD), can generally not be applied to bending processes. Furthermore, the damage and failure behaviour of HSLA steels are changing as the fracture mechanisms are mainly depending on the microstructure, which is very fine-grained in HSLA steels composed with different alloying elements compared to established mild steels. Especially for high gradients of strain and stress over the sheet thickness, as they occur in small curvature bending processes, other damage models than the FLD have to be utilised. Within this paper a finite element (FE) 3D model of small curvature bending processes is created. The model includes continuum damage mechanics model in order to predict and study occurring failure by means of ductile coherence loss of the material and crack formation with respect to influencing process parameters. Damage parameters are determined by inverse numerical identification method. The FE-model is strain based validated considering the deformation field at the outer bending edge of the specimen by using an optical strain measurement system. The Lemaitre based damage model is calibrated against the experimental results within metallographic analysis adapting the identified damage parameters to the bending process und thus adjusting the crack occurrence in experiment and simulation. Using this model the bendability of common HSLA steel, used for structural components, is evaluated with respect to occurring damage and failure by numerical analysis.     

Model Investigation on Composite Failure Prediction of π-Joint Structures

This paper was concerned with the tensile mechanics behavior of the composite π-joint under static tensile loading. The numerical strength analysis methodology was presented containing the basis assumption for the analysis, the material modeling, and the selected element type. It was assumed that the composite ply had transverse isotropic material properties and the adhesive had linear elastic properties. With the goal of the strength analysis to determine the onset of the damage initiation and the ultimate...     

The influence of coating compliance on the delamination of thermal barrier coatings

Low relative density yttria stabilized zirconia (YSZ) thermal barrier coatings have been deposited on NiCoCrAlY over-lay bond-coated Hastelloy-X substrates by an electron beam-directed vapor deposition (EB-DVD) method. The coatings have been applied to both roughened and smooth bond coat surfaces. During thermal cycling, those deposited on roughened surfaces spalled by delamination with the failure located in the YSZ layer close to the thermally-grown oxide (TGO). The cracks were initiated at “corn kernel” defects in the YSZ layer by a bond coat rumpling mechanism. Coatings applied to smooth bond coat surfaces had much longer (factor of two) spallation lifetimes and delamination occurred at the bond coat/TGO interface by the nucleation and coalescence of interfacial voids. In both cases, the delamination lifetime decreased with coating density and significantly exceeded the lifetimes of higher density coatings made by conventional electron beam deposition methods. The enhanced life of the more porous coatings is consistent with an increased compliance. This reduces the YSZ layers stored strain energy contribution to the driving force for interface delamination.