Relationship of Coating Failure to Deformation in the Deep Drawn Cup

Deformation of the metal substrate in the deep drawing process was characterized by macroscopic appearance and strain distribution. The results of the analysis were correlated with the modes of coating failure. The characteristic ears and valleys at the top of the cup and buckling of the metal in the valley regions resulted from instabilities raised during the cupping process. Earing was a plastic instability that originated from the combined factors of the crystallographic unit cell and the anisotropy of the rolled metal substrate. Buckling was an elastic instability that occurred when the valleys lost constraint before the ears. Stopping the forming process at intermediate positions before the cup was completely formed revealed four stages in the cupping process. Stage I was the initial indentation with formation of six ears. Stage II was conversion from 6 ears to 4 ears separated by alternating deep and shallow valleys. Stage III, loss of constraint in the deep valleys, led to buckling. Stage IV was total loss of constraint. Relationships between the deep drawing process as described by the 4 stages and coating failure were explored with two coating formulations: one that did not fail during deep drawing and one that failed. Two failure modes were observed on the deep drawn cup. Mode I, coating delamination at the top of the cup, was related to the bucking instability. Mode II, loss of adhesion along the cup wall, was caused by elastic retraction of the unconstrained cup in stage IV.     

Effect of formulation variables on coating properties and performance in the deep draw process

This study demonstrated correspondences between certain coating properties and observed failure modes in the deep draw process. Mode I failure, coating delamination at the cup edge, correlated with residual compressive stresses in the drawn coating. Neither crosslinker concentration nor prepolymer molecular weight in the ranges studied significantly affected residual stresses. However, residual stresses were strongly affected by the amount of wax in the coating formulation to the extent that coatings with less than 1.5% wax exhibited mode I failure, whereas coatings with 1.5% or more did not fail. Some of the coatings that exhibited mode I failure also exhibited mode II failure. Loss of adhesion on the cup wall, mode II failure, correlated with adhesive strength of the coating to the metal substrate, which in the present study was most strongly affected by crosslinker concentration. Coatings with 15 and 20% crosslinker exhibited extended areas of mode II failure, whereas coatings with 10% crosslinker showed no mode II failure. Increasing the molecular weight of the prepolymer improved adhesion somewhat, but not enough to affect the failure mode in the coatings tested, and wax had no measurable effect on the adhesion. Dept. of Macromolecular Science and Center for Applied Polymer Research, Cleveland, OH, 44106-7202. 26567 Bayfair Dr., Olmsted Falls, OH 44138.     

A numerical study on warm deep drawing of polypropylene

Warm deep drawing of polypropylene, a semi-crystalline thermoplastic polymer, is studied using finite element analysis. In this process, a circular polypropylene blank is preheated to a temperature much below its melting temperature and deep drawn into the shape of a flat-bottom cylindrical cup using a punch-die combination, both initially at 25°C. The material model used for the analysis considers the effects of varying temperature and strain rate during the deep drawing process on the depth of draw. The effects of blank holder force, initial blank temperature, blank diameter, and die and punch corner radii on the depth of draw are determined. Thickness, temperature, and strain variations in the drawn cups, punch forces, and failure modes are also determined.     

Torsional elastic instability of double-walled carbon nanotubes

In this paper, a theoretical analysis of the torsional buckling instability of double-walled carbon nanotubes (DWCNTs) and the DWCNTs embedded in an elastic medium is presented based on the continuum elastic shell model and Winkler spring model. Using the proposed theoretical approach, the influences of the aspect ratio, the buckling modes and the surrounding medium on the torsional stability are examined in detail. The simulation results show that the torsional instability of DWCNTs can occur in different buckling modes according to the aspect ratio. The van der Waals (vdW) interaction force between nanotubes reinforces the stiffness of nanoshells. Thus, the DWCNTs possess higher buckling stability than the SWCNTs without considering vdW interaction force.     

Through thickness changes to stiffness and thermal conductivity in thermal barrier coatings subjected to gradient exposure

Sintering-induced increases in the elastic modulus and thermal conductivity of thermally sprayed Thermal Barrier Coatings (TBCs) under constraint by a bond-coated substrate due to thermal gradient exposure have been measured. The results suggest the degree of sintering due to this constrained thermal exposure to be significantly lower when compared to free standing exposed specimens. Overall, a ∼30% increase in elastic modulus and ∼12% increase in thermal conductivity have been measured in a ∼ 450 µm thick TBC after 12 h of gradient exposure. Here, the top surface of the TBC was kept at 1250 °C, and the gradient was determined based on a constant backside cooling air flow rate. The elastic modulus, measured across the thickness of coating, reveals that in the region near the top surface, the elastic modulus increased by ∼ 50%. Contrastingly, no significant increase in elastic modulus was observed near the metal/coating interface. The dynamic evolution of thermal conductivity and sintering under gradient exposure and constraint was calculated using an analytical model which was then used to rationalize the experimental observations.     

Experimental study on cold press workability of aluminum-polyethylene sandwich laminates

The cold press workability of aluminum-polyethylene sandwich laminates is experimentally investigated by a deep drawing process with a conical die. “Planium” of 2 mm thickness, in which the core layer, high density polyethylene, is sandwich-laminated between two aluminum sheets, is used as a test specimen. Soybean oil is employed as a lubricant. The dimensions of the drawing die set are determined for a circular, blank so that the tensile fracture of the drawn.cup occurs at the top corner of the punch. The limiting draw ratio (LDR) is experimentally explored by using many blanks of various initial diameters. In general the maximum punch load in successful drawing increases linearly with increasing draw ratio. Because of the sandwich structure, the fracture of the drawn cup occurs twice. The initial fracture (LDR_s) corresponds to a fine bending fracture at the aluminum surface. The second fracture (LDR_B) is the complete fracture of the whole laminate. The effect of punch corner radius, working velocity and thickness fraction of aluminum and polyethylene on LDR_S and LDR_B are studied. The strain distributions of deep drawn cup in three orthogonal directions are analyzed experimentally.     

Effect of segmented cracks on TGO growth and life of thick thermal barrier coating under isothermal oxidation conditions

This paper is devoted to a comparative study on the isothermal oxidation of thick thermal barrier coating (TTBC) with and without segmented cracks produced by atmospheric plasma spray (APS) process. Accordingly, the growth of thermally grown oxide (TGO) and its effect on the degradation of the coating were investigated. Thick top coat in both segmented crack and conventional thick TBC reduced the double layered TGO growth rate slightly. The segmented crack thick TBC demonstrated longer isothermal oxidation life in comparison with that of the conventional thick TBC at 1100 °C. The dominant failure mechanism was spallation due to lateral cracking within the TGO and/or within TBC near the TGO layer, called mixed failure. Stress, and consequently strain, induced on the TTBC due to progressive TGO growth, seems to be primarily responsible for the crack initiation and propagation leading to the coating failure. Increment of elastic energy stored within the top coat due to the increasing of TGO thickness, finally causes thick thermal barrier coating failure in high temperature isothermal oxidation.     

Synthesis and characterization of acrylic-grafted polyester coatings for automotive pre-coated metal

An automotive pre-coated metal (PCM) system has been investigated to remove the wet coating process, such as pre-treatment, dip coating and spray coating for environmental regulations. However, automotive pre-coated metal sheets must have high flexibility and stiffness to overcome the harsh conditions such as encountered in cutting, press and the stamping process. For these reasons, a series of acrylic-grafted polyester coatings (i.e., AGP-0, AGP-10, AGP-20, AGP-30) were designed to satisfy both the surface hardness and the formability for an automotive PCM. The characteristics of the resins were measured by GPC, FT-IR and ¹H NMR. The viscoelastic behavior and flexibility was evaluated using DMA and UTM. The physical properties such as pendulum hardness, pencil hardness and adhesion were measured to define the effect of incorporating acrylates. A cylindrical deep drawing tester was used to evaluate the formability of coatings.     

BFRP筋轴心受压力学性能试验研究

为详细考察玄武岩纤维增强复合筋(BFRP)的轴心受压力学性能,设计制作了36个受压试件,测试BFRP筋的受压破坏模式、抗压强度、压缩弹性模量、压缩变形率,研究直径、长细比、破坏模式对BFRP筋受压力学性能的影响.试验结果表明,BFRP筋的受压破坏模式分为剪切、胀裂、失稳三种,以剪切破坏为主;受压应力—应变关系为线弹性,...     

Rheology and roll coating dynamics of metallic basecoats for automotive pre-coated metal sheet

Rheological behavior and coating dynamics of solvent-based metallic basecoats using 3-roll coaters were investigated for the application process based on pre-coated metal (PCM) sheet technology that can promisingly diminish the environmental pollutants in automotive OEM line. Rheological properties of the metallic basecoats related to optical appearance and color variations were measured by altering the content of aluminum (Al) flake and temperature of the coating solution. The results showed that the basecoats exhibited a very weak shear-thinning behavior regardless of Al flake contents in this study. However, their elastic behaviors were intensified with increasing the Al flake content. From the 3-roll coating experiments with forward and reverse coating modes, the operability windows in the coating process, demarcating the uniform coating flow from defects, were established. In particular, it was possible to elucidate the relationship between the capillary number and speed ratio conditions for the desired coating thickness in roll coating systems accompanied with reverse coating mode between the pick-up and metering rolls. It was verified from the cross-sectional images of coating layers that most of Al flakes inside the basecoat layer were oriented parallel to the coating surface under the continuous roll coating flows.     

Sintering‐induced delamination of thermal barrier coatings by gradient thermal cyclic test

Lifetime is crucial to the application of advanced thermal barrier coatings (TBCs), and proper lifetime evaluation methods should be developed to predict the service lifetime of TBCs precisely and efficiently. In this study, plasma‐sprayed YSZ TBCs were subjected to gradient thermal cyclic tests under different surface temperatures, with the aim of elucidating the correlation between the coating surface temperature and the thermal cyclic lifetime. Results showed that the thermal cyclic lifetime of TBCs decreased with the increasing of surface temperatures. However, the failure modes of these TBCs subjected to thermal cyclic tests were irrespective of different surface/BC temperatures, that is, sintering‐induced delamination of the top coat. The thickness of thermally grown oxide (TGO) was significantly less than the critical TGO thickness to result in the failure of TBCs through the delamination of top coat. There was no phase transformation of the top coat after failure. In contrast, in the case concerning the top coat surface of the failure specimens, the elastic modulus and microhardness increased to a comparable level due to sintering despite of the various thermal cyclic conditions. Consequently, it is conclusive that the failure of TBCs subjected to gradient thermal cyclic test was primarily induced by sintering during high‐temperature exposure. A delamination model with multilayer splats was developed to assist in understanding the failure mechanism of TBCs through sintering‐induced delamination of the top coat. Based on the above‐described results, this study should aid in facilitating the lifetime evaluation of the TBCs, which are on active service at relatively lower temperatures, by an accelerated thermal cyclic test at higher temperatures in laboratory conditions.     

不同加速试验对环氧/聚氨酯涂层失效机制的影响

研究了模拟南海海洋大气环境下铝合金表面环氧锌黄/丙烯酸聚氨酯涂层体系的失效过程,通过循环加速、紫外/冷凝和中性盐雾的对比试验,探讨了在循环加速试验中紫外-冷凝试验和盐雾试验分别对复合涂层失效过程以及对聚氨酯面漆和环氧底漆失效机制的影响。结果表明,紫外-冷凝过程对丙烯酸聚氨酯面漆有较强的破坏作用,但对复合涂层体系整体的阻抗变化影响较小;紫外-冷凝过程中,位于面漆玻璃化转变温度范围内的试验温度,以及紫外光对氨基甲酸酯基团的破坏作用,是导致面漆发生破坏的主要原因。盐雾试验对面漆的失光率和色差变化影响不大,但连续的盐雾渗透对涂层体系的阻抗下降具有明显的加速作用,同时导致涂层与基材的附着力显著降低。相比单独的紫外-冷凝试验或盐雾试验,循环加速试验综合考虑了南海海洋大气中强太阳辐射、高温高湿、高盐分和温差等环境因素,能更准确反映南海大气环境中复合涂层的失效过程。     

Torsional buckling of double-walled carbon nanotubes

The mechanical instability of doubled-walled carbon nanotubes subject to torsion motion is investigated through molecular dynamics. A newly revealed buckling mode with one or three thin, local rims on the outer tube was discovered while the inner tube shows a helically aligned buckling mode in three dimensions. The distinct buckling modes of the two tubes imply the inapplicability of continuum mechanics modeling in which it is postulated that the buckling modes of the constituent tubes have the same shape. In view of this problem, a new concept of the equivalent thickness of double-walled carbon nanotubes is introduced, which enables the Kromm shell model to be applied to the derivation of the torsional buckling angle without the restraint of the two tubes having identical shapes.     

Fracture mechanics analysis of delamination buckling of a porous ceramic foam coating from elastic substrates

Ceramic foams are ideal materials for thermal protection systems such as those used as a thermal shield on the space shuttle. The working temperature difference between the outer and inner surfaces of the ceramic foam is extremely high. Under this adverse condition, the ceramic foam coating could buckle from its underneath structures. Therefore, the general problem of damage due to buckling delamination of a ceramic coating on an elastic substrate is investigated in this paper. The delamination buckling amplitude and the stress intensity factor at the tip of buckling region are derived in closed form in terms of the porosity of the ceramic foam. Based on the maximum amplitude and the critical stress intensity factor criteria, critical temperatures of coating buckling from the substrate are established. A fitting formula of maximum buckling amplitude as a function of outer surface temperature is given. It is found that higher porosity and length-to-thickness ratio of the coating will result in a smaller stress intensity factor. However, it seems that the buckling amplitude is independent of the porosity of ceramic foams.     

Comparison of test methods estimating the stiffness of ultrathin coatings

A key engineering parameter of thin coatings is their stiffness. Stiffness characterization of ultrathin coatings with a nanometer scale thickness is experimentally challenging. In this work, three feasible methods have been used to estimate the Young’s modulus of metal coatings on polymer films. The methods are: (1) nanoindentation, (2) strain-induced elastic buckling and (3) peak-force measurements integrated in atomic force microscopy. The samples were prepared by atomic layer deposition of TiO₂ (6 and 20 nm thick) and mixed oxides of TiO₂ and Al₂O₃ (4 and 20 nm thick). The differences in estimated Young’s modulus are interpreted in terms of the underlying assumptions and test conditions. Their specific advantages and drawbacks are also compared and discussed. In particular, the nanoindentation necessitates a sufficiently sharp indenter tip to make localized measurements dominated by the coating. The strain-induced elastic buckling method is simple in practice, but showed a large scatter due to variation in local coating thickness and irregular deformation patterns. The stiffness characterization using atomic force microscopy gave the most consistent results, due to a sharp tip with a radius comparable to the thinnest coating thickness. All methods gave a higher Young’s modulus for the TiO₂ coating than for the mixed oxide coating, with a variation within one order of magnitude between the methods.     

涂装车间清洗溶剂回收在线利用的研究

调查了涂装生产中清洗溶剂的耗量及所占VOC排放比例，提出了在线利用的思路。阐述了蒸馏回收原理和溶剂回收系统的特点，通过清洗能力、溶解力等试验验证了面涂清洗溶剂可以蒸馏回收用于中涂机器人旋杯、阀门等设备的清洗。     

Buckling instability during the 180°-peeling of brass shim from polychloroprene rubber

The 180°-peel adhesion of brass shim firmly bonded to a steel plate with a rubber interlayer has been studied as a function of rubber thickness, t, and test temperature. At low temperatures, peeling occurs via 'normal' interfacial detachment and the peel force increases to a plateau with rubber thickness. At higher temperatures and with larger values of t, compressive forces during peeling become sufficient to cause buckling of the bonded shim behind the major peel front. This causes a segment of predetachment and results in a unique type of stick-slip peeling pattern, in which alternate bands of cohesive and interfacial failure are formed. The instability is in approximate accord with a model assuming the buckling of a column on an elastic foundation.     

Impact Behavior of Coated Low-Density Bodies

Impact failure of the coating of space shuttle thermal protection tiles was confirmed to occur by a flexure process at low projectile velocities. The critical projectile velocity was measured for a variety of projectile sizes. The same failure process and damage sequence was observed in quasi-static indentation tests. In these tests, the mechanical behavior agreed reasonably well with the elastic model of an infinite plate bonded to a semiinfinite foundation. The model indicates that the critical fracture load could be increased by increasing coating strength, thickness, or foundation stiffness, or by decreasing the Young's modulus of the coating.     

在储油罐罐顶上的试验研究

采用何种材料对立式金属油罐顶进行保温,避免过多的散热损失是设备管理的重要任务之一。本文研究通过对需要保温的储油罐罐顶上保温材料与防腐保温涂料的使用对比,对采用防腐保温涂料的罐顶进行保温效果的测试和散热损失的计算证明,降低了罐顶的表面温度减少了罐顶的散热损失,节能效果明显。     

含分层损伤复合材料层合板的屈曲特性研究

本文利用有限元方法,对压缩载荷作用下含各种形状脱层损伤的复合层合板的屈曲模态和载荷进行了研究。对不同的脱层损伤尺寸,层合板分别有整体,混合和局部三种屈曲模态,穿透脱层和埋藏脱层的层合板其临界载荷的变化规律是相同的。