Investigation on delamination morphology during drilling composite laminates

Drilling-induced delamination of composite materials is a key factor that affects the quality of subsequent machining. To investigate the developing process of delamination, experiments with different drilling depth are conducted. In order to observe the delamination of different cross-sections in radial direction of the hole, the grinding method is adopted. Three-dimensional morphology of delamination at the exit of hole is obtained. The regularity of delamination with the change of drilling depth is analyzed, and the existence of “hidden delamination zone” is obtained finally. Due to the rebound effect of hole diameter and the inverted cone of drill guide section, the “hidden delamination zone” will be generated under the condition that the edge of delamination area is compressed tightly again. The critical thrust force of delamination is also studied, and it is proved to be correct.     

Terahertz spectroscopy approach of the fiber orientation influence on CFRP composite solid laminates

Terahertz ray (T-ray) imaging applications have provided one of the most promising new powerful nondestructive evaluation techniques, and new application systems are under process development for area applications. Detecting flaws and defects in fiber reinforced plastic (FRP) composite laminates due to flaws in FRP composite laminate that affect laminate properties, including stiffness, strength, and thermal behavior, is very important. In this study, a new time-domain spectroscopy system was utilized for detecting and evaluating the flaws in FRP solid composite laminates. Extensive experimental measurements in reflection mode were made to map out T-ray images. In particular, electromagnetic properties, such as refractive index, were estimated in this characterization procedure. The estimates of properties were in good agreement with known data. Using these characteristic material properties, we successfully demonstrated the characteristics of the T-ray behavior propagating through FRP composites. Furthermore, layup effect and flaws of FRP composite laminates were observed in reflection mode, and limitations were discussed in the T-ray processing.     

加载速度对板材压弯成形性能影响的研究

利用有限元分析软件ABAQUS建立了压弯成形过程的三维有限元模型,研究了不同加载速度对板材压弯成形性能的影响。模拟时,在许可范围内分别取了5个不同的加载速度(27、10.8、2.7、1.54、1.06)×102mm/s,分析了它们对成形板材内部应力场、应变场的分布以及板材关键尺寸的影响。结果表明:模拟时,加载速度对几何形状不规则板材应力集中区域的等效应力、应变分布影响较大。成形板材两直边区域夹角则在不同加载速度下产生了小幅度的波动。结合模拟结果以及YF32-400压弯设备,选择1.06×102mm/s作为实际加载速度,由现场实验证明这一选择的合理性。     

A genetic algorithm for the optimization of fiber angles in composite laminates

A genetic algorithm for the optimization of composite laminates is proposed in this work. The well-known roulette selection criterion, one-point crossover operator, and uniform mutation operator are used in this genetic algorithm to create the next population. To improve the hill-climbing capability of the algorithm, adaptive mechanisms designed to adjust the probabilities of the crossover and mutation operators are included, and the elite strategy is enforced to ensure the quality of the optimum solution. The proposed algorithm includes a new operator called the elite comparison, which compares and uses the differences in the design variables of the two best solutions to find possible combinations. This genetic algorithm is tested in four optimization problems of composite laminates. Specifically, the effect of the elite comparison operator is evaluated. Results indicate that the elite comparison operator significantly accelerates the convergence of the algorithm, which thus becomes a good candidate for the optimization of composite laminates.     

Drilling of uni-directional glass fiber reinforced plastic (UD-GFRP) composite laminates

Drilling of fiber reinforced plastic (FRP) composite materials is a field open to a plethora of questions. Drilling-induced damage is a research area that has not been explored exhaustively. The present research endeavor is an effort to correlate drilling-induced damage with drilling parameters. Tool point geometry is considered a major factor that influences drilling-induced damage. Experiments were conducted and drilling-induced damage was quantified using the digital image processing technique. The results also reestablished the cutting speed to feed ratio as an important variable that influences drilling-induced damage. Mathematical models for thrust, torque, and damage are proposed that agree well with the experiments.     

Small mechanical press with double-axis servo system for forming of small metal products

Multi-axis servo systems have been applied to many machine tools in recent decades, but they are not commonly used for press machines. They are particularly uncommon for small presses because it is still believed that the precision of a small press is dependent on the precision of its components and parts. However, high-precision components and parts are difficult to manufacture and assemble, so the costs of small precision presses are roughly equal to or higher than those of large press machines. Moreover, the accuracy of conventional small presses worsens over long operation times, particularly when dealing with off-center loading frequency. Therefore, we propose a small mechanical press with an adjustable driving system, using a slide and double-axis servo system, for manufacturing small products. Through this proposed design, we expect higher-quality products to be produced for lesser cost than conventional small presses.     

Simultaneous measurement of humidity and temperature using a polyvinyl alcohol tapered fiber bragg grating

An optical fiber sensor based on a multimode tapered fiber cascading fiber Bragg grating has been proposed and experimentally demonstrated for the simultaneous measurement of humidity and temperature. The sensor was constructed using a tapered fiber that was coated with polyvinyl alcohol and a fiber Bragg grating with high reflectivity. The measurement of humidity and temperature was achieved by monitoring changes in reflective optical power and spectral shift, respectively. Due to the different measurement methods, the effect of temperature on humidity measurement may be ignored. The theoretical analysis and experimental results show that the highest sensitivities of 0.33 µW/%RH and 10.9?pm/°C were achieved when the diameter of the taper waist was 26?µm and the thickness of coating was 3.3?µm. Due to the advantages of good linearity, low cost of fabrication and convenient operation, the proposed sensor is promising for simultaneously measuring humidity and temperature.     

Some observations on the interlaminar strength of composite laminates

Definition of the influence of interlaminar stresses on the failure characteristics of composite laminates may require the development of novel experimental characterization procedures. The so-called free edge problem in laminate elasticity offers a cnovenient mechanism to accomplish this purpose because of the high interlaminar stresses in the neighbourhood of a free boundary. The detailed design of a laminate specimen which can exhibit catastrophic delamination induced by interlaminar tension is presented, along with a preliminary failure hypothesis to characterize this mode of rupture and the associated experimental demonstration of the phenomena.     

Research on the forming process of three-dimensional metal parts fabricated by laser direct metal forming

To improve the forming quality of parts fabricated by laser direct metal forming (LDMF), the forming process of three-dimensional parts under open-loop LDMF system was studied in this paper. The influencing rule of scanning space on the forming quality was studied, and the optimal scanning space was derived. The software of control system about scanning style is also optimized, and thus, the forming quality of parts is improved. During fabricating three-dimensional parts, uneven heat distribution caused by multi-overlapping is one of the main factors affecting the sidewall forming quality. To solve the uneven heat distribution, the strategy of changing scanning speed was put forward. The influence of the standoff distance on the height of single-trace cladding layer was studied, and it was suggested that uneven surface of parts was caused by instability of process parameters, which could be compensated automatically in the condition of suitable standoff distance in the process of LDMF. Thus a so-called self-regulation effect is reached. Typical multi-overlapped parts with good forming quality are fabricated using the above-mentioned methods.     

Analysis of the tribological behaviour of polymer composite tool materials for sheet metal forming

Today's automotive industry shifts its focus on customised production, facing an increasing demand for medium and small batch production, where cost-effective manufacturability of sheet metal forming dies comes into the foreground. Filled polymers offer possibilities to fulfil such requirements in the ambit of prototype tools manufacturing or in small batch production of sheet metal components. This paper presents investigations dealing with tribological and tool design aspects of using polymeric materials for sheet metal forming purposes. Friction and wear behaviour of two polymer composites on sheet metal counterface materials have been investigated. A new testing method for wear evaluation of polymeric materials for sheet metal forming using a Strip Drawing Test facility is presented and discussed. A method to predict lifetime of polymeric stamping dies using the linear wear–distance relation W_l/s measured with the new testing method is also proposed. Significant improvements in friction and wear performance of polymer composites have been observed using sheet materials with structured surfaces. A theoretical model for abrasive friction and wear of polymer composites on sheet metal counterface material pairs has been developed, based on the results obtained by measurement of friction and wear.     

Effects of cutting edge radius and fiber cutting angle on the cutting-induced surface damage in machining of unidirectional CFRP composite laminates

Carbon fiber reinforced polymer composite laminates are anisotropic, inhomogeneous, and mostly prepared in laminate form before undergoing the finishing operations. The edge trimming process is considered as one of the most common finishing operations in the industrial applications. However, the laminate surface is especially prone to damage in the chip formation process, and the most common damage mode is burrs. Burrs may increase cost and production time because of additional machining; they can also damage the surface integrity. Many studies have been done to address this problem, and techniques for reducing burr size in material removal process has been the focus of the research. Nonetheless, the combined effects of the cutting edge radius and the fiber cutting angle on the burr formation have seldom been conducted, which in turn restricts to find out the mechanism of burr formation. The purpose of the present paper is to study the particular mechanism that leads to burr formation in edge trimming of CFRP laminates and investigate the effects of fiber cutting angle and cutting edge radius on burr formation. The results indicate that the burrs are prone to form in the fiber cutting angle range of 0° < χ < 90° when a large cutting edge radius of the tool is used for both milling and drilling of CFRP composites.     

Modeling of surface roughness effect on dry contact friction in metal forming

Interfacial conditions such as friction and roughness substantially affect the process characteristics of metal forming. This study developed a dry friction model that accounted for the adhesion and interference effects of surface roughness. A sliding friction coefficient was suggested to provide fundamental information about the interfacial conditions of the contact surface. The proposed model was easily verified by published experiments and predicted values agreed with experimental results. Accordingly, friction coefficient ?? clearly increased as relative roughness R _m (=?roughness of tool $ R_a^T $ /roughness of workpiece $ R_a^M $ , measured as interference effect) increased. Simulations confirmed that the friction coefficient ?? decreased as dimensionless stress S _m (=?contact pressure p _m /tensile strength $ \sigma_u^0 $ ) increased at small strain hardening exponent n-values. Under the conditions of large n and small R _m values, the friction coefficient ?? initially decreased and then increased. It then slightly decreased as dimensionless stress S _m increased. However, this trend became less apparent as relative roughness R _m increased since friction coefficient ?? simply decreased.     

Spring-back characteristics of fiber metal laminate (GLARE) in brake forming process

The brake forming process has been considered as a feasible method for producing fiber metal laminate (GLARE) stringer. However, the spring-back developed during brake forming leads to serious problems in the final dimensional tolerance of the stringer. A series of experiments were performed to examine the effect of tool design and process parameters on the spring-back of GLARE. The parameters studied include punch radius, punch speed, forming load, and forming temperature. This paper shows that both design and process parameters can significantly affect the amount of spring-back. Scanning electron microscopy (SEM) was also carried out for the observation of delamination or cracking in the bent zone.     

一种U形金属波纹管成形工艺的研究

介绍了成形质量对U形金属波纹管疲劳寿命的影响,分析了各种成形工艺的特点及其成形质量,提出一种综合了几种成形工艺优点的新工艺,并通过简单的试验验证了新工艺对成形质量的改善.试验结果表明:新工艺能保证波形几何尺寸、改善波纹表面应力状态和粗糙度,可为波纹管的工艺设计提供参考.     

Investigation on the forming force and surface quality during ultrasonic-assisted incremental sheet forming process

The integration of ultrasonic vibration into the incremental sheet forming (ISF) process can significantly reduce the forming force and bring other benefit     

Effects of fiber distribution on elastic-viscoplastic behavior of long fiber-reinforced laminates

In this paper, effects of transverse fiber distribution on the elastic-viscoplastic behavior of long fiber-reinforced laminates subjected to in-plane tensile loading are studied using a homogenization theory. To this end, a unit cell with random fiber distribution is arranged point-symmetrically with respect to cell facet centers and also Y-periodically. By discussing these two fiber arrangements in terms of standard deviations and a radial distribution function, it is demonstrated that the point-symmetric cell arrangement can enhance the randomness of fiber distribution in comparison with the Y-periodic arrangement. Then, on the assumption that the random fiber distribution generated by the point-symmetric cell arrangement prevails on the transverse section in each lamina, the in-plane elastic-viscoplastic deformation of carbon fiber/epoxy laminates is analyzed using a homogenization theory of nonlinear time-dependent composites. The analysis based on the perfectly periodic hexagonal fiber distribution in laminae is additionally performed for comparison. It is thus shown that the transverse randomness of fiber distribution in laminae has negligible influence on the macroscopic elastic-viscoplastic behavior of laminates, though it markedly affects the microscopic distribution of stress and strain. It is also shown that the analysis predicts very well the macroscopic behavior observed in the corresponding experiments.     

Effects of friction laws on metal forming processes

In this paper, the Coulomb friction law and the constant shear friction law were compared and investigated in detail using a rigid-plastic finite element method with emphasis on their application in bulk metal forming. The ring compression test for two different materials was used to evaluate the two friction laws, and then their effects on metal flow lines and forming loads for various friction-sensitive metal forming processes including strip rolling, ring gear forging, multistep extrusion, and pipe shrinkage and expansion were investigated. It was shown that considerable differences exist between the two friction laws, especially in friction-sensitive metal forming processes.