The delamination of polymeric coatings from steel Part 3: Effect of the oxygen partial pressure on the delamination reaction and current distribution at the metal/polymer interface

In the third part of this paper, it is shown that oxygen reduction plays the central role in the delamination process. The oxygen partial pressure controls the potential of the local cathode at the metal/polymer interface and potential changes, which are measured after change of the oxygen activity, allow us to determine the local diffusion coefficient of oxygen in the polymer. The delamination is caused by reaction products, like radicals, which are formed during the oxygen reduction at the metal/polymer interface. However, the reaction rate of the oxygen reduction for the simple non-pigmented polymer under investigation is determined by the ohmic potential drop between the defect and the delamination frontier. This potential drop is given by the difference of the open circuit potential at the defect and the intact interface (which results from the surface treatment, such as phosphating) and the ionic conductivity between both sites. Both are confirmed by a direct measurement of the distribution of the galvanic current along the metal/polymer interface.     

Prediction of the location of delamination in the drilling of composite laminates

An analytical approach for the determination of the position of the onset of delamination during the drilling of composite laminates based on linear elastic fracture mechanics is presented in this paper. The critical thrust force calculated from the predicted position of the onset of delamination has a reasonable agreement with experimental results obtained in the drilling of T300/5208 graphite epoxy composite laminates reported in the literature. Thus, the analytical method developed in the present study may be applied to the drilling of composite laminates drilling with the avoidance of delamination.     

Properties and determination of the interface stiffness

The chemical potential of a curved interface contains a term that is proportional to the product of the interface curvature and the interface stiffness. In crystalline materials, the interface stiffness is a tensor. This paper examines several basic issues related to the properties of the interface stiffness, especially the determination of the interface stiffness in particular directions (i.e. the commonly used scalar form of the interface stiffness). Of the five parameters that describe an arbitrary grain boundary, only those describing the inclination are crucial for the scalar stiffness. We also examine the influence of crystal symmetry on the stiffness tensor for both free surfaces and grain boundaries. This results in substantial simplifications for cases in which interfaces possess mirror or rotational symmetries. An efficient method for determining the interface stiffness tensor using atomistic simulations is proposed.     

A neural network thrust force controller to minimize delamination during drilling of graphite-epoxy laminates

Delamination is a well-recognized problem associated with drilling fiber-reinforced composite materials (FRCMs). The most noted problems occur as the drill enters and exits the FRCM. Since drilling is often a final operation during assembly, any defects introduced in parts through the drilling process that result in the part being rejected represent an expensive loss. Studies based on linear-elastic fracture mechanics theory have proposed critical cutting and thrust forces in the various drilling regions that can be used as a guide in preventing crack growth or delamination. Using these critical force curves as a guide, a thrust force controller was developed to minimize the delamination while drilling a graphite-epoxy laminate. A neural network control scheme was implemented which required a neural network identifier to model the drilling dynamics and a neural network controller to learn the relationship between feed rate and the desired thrust force. Experimental results verifying the validity of this control approach as well as the robustness of the design are presented. Visual measurements of the delamination zones were used to quantify the benefits of the thrust force controlled drilling process versus the conventional constant feed rate drilling process.     

Analyzing the methods and merits of recycling fiber metal laminates

It is technically possible to recycle fiber metal laminates (FMLs) by retrieving the aluminum fraction from FML scrap; this reprocessing becomes economically feasible at a throughput of 50 tonnes or more per year. Until then, FML scrap can be dealt with in conventional ways.     

Cathodic delamination of polyurethane films on oxide covered steel - Combined adhesion and interface electrochemical studies

The stability of polyurethane/iron oxide/steel interfaces was evaluated for different chemically modified passive films in humid and corrosive atmosphere. Iron oxide layers were formed on polished steel by thermal annealing in oxygen rich atmosphere, water plasma modification, immersion in alkaline solution and by anodic polarisation in borate buffer solution. Scanning Kelvin Probe studies of the interface stability were performed under the conditions of cathodic delamination. The corresponding progress rates specifically depended on the iron oxide. Complementary measured peel-off forces of the respective coating/substrate systems showed the same trend as detected for the corrosive delamination process. Additional contact angle measurements and investigation of the interfacial ion mobility on the uncoated oxide surfaces confirmed a correlation between high peel forces, increasing oxide surface energy and decelerated cathodic delamination kinetics.     

Numerical modeling for thermographic inspection of fiber metal laminates

This study combines experiments and finite element analysis to assess the capabilities and limitations of both pulsed and lock-in thermography for the detection of delaminations in fiber metal laminate (FML) panels. The assessment is based on the minimum detectable defect size and the maximum depth at which a defect can be detected. Based on experimental validation, a finite element model is created and used to predict the detectability of defects and to explain the heat transfer mechanism associated with the thermographic inspection. Both modeling and experimental results show that thermography is suitable for the inspection of FMLs. However, delaminations smaller than 5×5 mm² or those located deeper than 0.6 mm from the inspected surface are more challenging to detect experimentally. Numerical analyses showed that it is necessary to double the energy input used to be able to detect defects as small as 3×3 mm² located at 0.6 mm depth and those as small as 10×10 mm² located at 1.65 mm depth.     

Effect of bonding interface on delamination behavior of drawn Cu/Al bar clad material

Cu/Al bar clad material was fabricated by a drawing process and a subsequent heat treatment.During these processes,intermetallic compounds have been formed at the interface of Cu/Al and have affected its bonding property.Microstructures of Cu/Al interfaces were observed by OM,SEM and EDX Analyser in order to investigate the bonding properties of the material.According to the microstructure a series of diffusion layers were observed at the interface and the thicknesses of diffusion layers have increased with aging time as a result of the diffusion bonding.The interfaces were composed of 3-ply diffusion layers and their compositions were changed with aging time at 400 °C.These compositional compounds were revealed to be η2,(θ+η2),(α+θ) intermetallic phases.It is evident from V-notch impact tests that the growth of the brittle diffusion layers with the increasing aging time directly influenced delamination distance between the Cu sleeve and the Al core.It is suggested that the proper holding time at 400 °C for aging as post heat treatment of a drawn Cu/Al bar clad material would be within 1 h.     

Lamb wave based ultrasonic imaging of interface delamination in a composite T-joint

In this article, the complete characterization of the interface delamination in a composite T-joint using ultrasonic guided Lamb waves is discussed. Correlation between the numerically simulated and experimentally obtained B-scans was established. In the B-scan images, different Zones were identified and could be utilized for differentiating the healthy and delaminated T-joints. The length and the width of delamination were computed from traversing the transducers in two orthogonal directions (i.e. called the B-scans, and the D-scans). For the first time, backward propagating mode (called “Turning Lamb Modes”) that are generated at corner of the T-joint was observed and verfied. Using a single linear line scan of the “Turning Lamb Modes”, both the width and length of interface delamination could be computed. In the present study, the experiments were carried out employing air-coupled ultrasonic transducers.     

Study on interface between titanium-coated diamond and metal matrices

Study on interface between titanium-coated diamond and metal matrices@朱永伟 @谢光灼 @张新明 @周卓平~~     

Strength of fiber reinforced metal laminates with a circular hole

The effect of notching on fiber-reinforced metal laminates (FML) is studied 3-2 lay up carbon fiber aluminum laminates specimens containing a circular hole were employed in notch tests. The notch effect for a composite laminate was investigated and compared with carbon/epoxy composites and aluminum sheets. The degradation of notch strength for carbon fiber aluminum laminates was smaller than for carbon/epoxy composites. The point and average stress criteria were used in the notch strength analysis. A finite width correction factor was also used. The strength degradation models involve a single parameter that is a function of the hole size and the characteristic length. These models were compared with experimental data and good agreement was found.     

Effects of special drill bits on drilling-induced delamination of composite materials

Drilling is the most frequently employed operation of secondary machining for fiber-reinforced materials owing to the need for joining structures. Delamination is among the serious concerns during drilling. Practical experience proves the advantage of using such special drills as saw drill, candle stick drill, core drill and step drill. The experimental investigation described in this paper examines the theoretical predictions of critical thrust force at the onset of delamination, and compares the effects of these different drill bits. The results confirm the analytical findings and are consistent with the industrial experience. Ultrasonic scanning is used to evaluate the extent of drilling-induced delamination. The advantage of these special drills is illustrated mathematically as well as experimentally, that their thrust force is distributed toward the drill periphery instead of being concentrated at the center. The allowable feed rate without causing delamination is also increased. The analysis can be extended to examine the effects of other future innovative drill bits.     

On the prediction of delamination during deep-drawing of polymer coated metal sheet

This paper describes the development and application of a numerical model that can predict the delamination of the polymer coating from the steel substrate during deep-drawing. Experimentally characterized cohesive zones are used to describe the interface between the polymer and the steel and are capable of modelling delamination or a prior partial loss of adhesion during an axisymmetric deep-drawing simulation. A parameter is proposed that quantifies the interfacial integrity and is used to assess the influence of the tooling radii, the clearance between the punch and the die and the coating thickness on the interfacial integrity.     

Asymmetry in interface and bending property of Al/Cu/Al bimetallic laminates

This work was aimed to study the interfacial microstructures and three-point bending properties of Al/Cu/Al bimetallic laminates produced by the asymmetrical roll bonding and annealing. It is found that the microstructure and bonding strength of the Al/Cu interface are different with those of the Cu/Al interface. The interfacial microstructure of Cu/Al interface is improved due to the large interfacial plastic deformation caused by the different rotation speeds of roll in the asymmetrical roll bonding process. The bonding strength between Al and Cu layer can be enhanced by the moderate atomic diffusion, but is dramatically depressed by the formation of intermetallic compounds in the interface.The bending strength of bimetallic laminates is enhanced when the Cu/Al interface is loaded in tension because of the improvement of stress transition and damping by the Cu/Al interface during the three-point bending deformation. The bending fracture reveals that the interfacial cracks can be inhibited from the restricted stress concentration in the improved Cu/Al interface.     

固态金属剪切连接组织及界面演变研究

组织状态及界面形成过程对固态金属剪切连接后的连接强度及性能有重要影响,采用实验研究的方法,结合理论分析实施了不同压下量、空冷和水冷Q345钢剪切连接实验。通过微观组织分析、界面结合过程探究和连接强度测定,分析了组织演变和界面形成机制,得到了剪切连接过程的组织及界面演变规律。结果表明:随压下量增加,界面晶粒细化程度提高,组织得到明显细化,界面孔洞逐渐消失,形成共有晶粒界面;晶粒细化驱动为动态再结晶,界面形成为三阶段过程;压下量为125%空冷后,晶粒尺寸由原态40μm细化到10μm左右,连接界面消失,连接强度为500.56 MPa。     

一种提高汽车板件刚度的工艺设计

针对汽车板件拉深成形后刚度不足的问题,在修边工序中利用压料芯在刚度不足区域成形加强筋,然后经整形复原其设计形状,提高了板件的塑性变形量,从而达到提高板件刚度的目的,为类似板件的工艺设计提供了一种思路。     

Ag-Cu-Ti钎料钎焊金刚石的界面微观组织分析

在真空炉中采用Ag-Cu-Ti钎料对金刚石磨粒进行了真空钎焊试验,实现了金刚石与钢基体的高强度连接.采用SEM对金刚石与钎料界面、金刚石表面碳化物形貌进行了观察分析,采用EDS分析了金刚石与钎料界面的成分变化,采用Raman对焊后的金刚石结构进行了分析.结果表明,Ag-Cu-Ti钎料中的Ti元素在界面处发生偏析,并在金刚石表面生成尺寸小于1 μm块状TiC,金刚石在焊接过程的高温中没有发生石墨化,最后在界面上形成了金刚石/TiC/钎料/钢基体的梯度结合层.     

钛/铝/钛三层板的一次爆炸复合制备及界面结合性能评价

为评价一次爆炸复合工艺制备的钛/铝/钛三层复合板的界面结合性能,利用SEM、EDS对钛/铝/钛复合板的双层界面组织形貌以及界面元素分布进行了表征;对钛/铝/钛三层复合板进行了拉伸实验和弯曲变形实验。研究结果表明：复合板界面主要由波状界面和平直状界面构成;铝元素与钛元素在界面上发生了互扩散;拉伸和弯曲变形结果表明,一次爆炸复合工艺制备的钛/铝/钛三层复合板具有较大的抗拉强度和优良的界面结合性能,可以承受后续较大的二次塑性变形。     

Effects of surface roughness,texture and polymer degradation on cathodic delamination of epoxy coated steel samples

The Scanning Kelvin Probe (SKP) technique was used to investigate the effects of surface roughness, texture and polymer degradation on cathodic delamination of epoxy coated steel. The cathodic delamination rate of the epoxy coatings dramatically decreased with increased surface roughness of the underlying steel substrate. The surface texture of the steel substrates also had a significant effect in that samples with parallel abrasion lines exhibiting faster cathodic delamination in the direction of the lines compared to the direction perpendicular to the lines. The cathodic delamination kinetics of epoxy coatings previously exposed to weathering conditions increased with prolonged exposure due to pronounced polymer degradation. SEM observation confirmed that the cyclic exposure to UV radiation and water condensation caused severe deterioration in the polymer structures with surface cracking and erosion. The SKP results clearly showed that the cathodic delamination of the epoxy coatings was significantly influenced by the surface features of the underlying steel substrates and the degradation of the coatings.