A review of mechanical drilling for composite laminates

Composite laminates (CFRP, GFRP, and fiber metal composite laminates) are attractive for many applications (such as aerospace and aircraft structural components) due to their superior properties. Usually, mechanical drilling operation is an important final machining process for components made of composite laminates. However, composite laminates are regarded as hard-to-machine materials, which results in low drilling efficiency and undesirable drilling-induced delamination. Therefore, it is desirable to improve the cost-effectiveness of currently-available drilling processes and to develop more advanced drilling processes for composite laminates. Such improvement and development will benefit from a comprehensive literature review on drilling of composite laminates. This review paper summarizes an up-to-date progress in mechanical drilling of composite laminates reported in the literature. It covers drilling operations (including conventional drilling, grinding drilling, vibration-assisted twist drilling, and high speed drilling), drill bit geometry and materials, drilling-induced delamination and its suppressing approaches, thrust force, and tool wear. It is intended to help readers to obtain a comprehensive view on mechanical drilling of composite laminates.     

Piezoresistivity of unidirectional carbon/epoxy composites for multiaxial loading

The applied strain of carbon fibre reinforced plastics (CFRPs) is measurable by their electrical resistance changes. For damage monitoring of laminated CFRPs, piezoresistivity strongly affects the measured electrical resistance change through residual strain relief attributable to delamination cracks. Although several studies of CFRP laminates’ piezoresistivity have been published, this study uses single-ply CFRP for specific piezoresistivity measurements in four directions. A review of the theory of in-plane piezoresistivity reveals orthotropic properties of CFRP piezoresistivity. In the present study, piezoresistivity of multiaxial loading is derived, and the unsymmetrical piezoresistivity matrix is calculated using the measured piezoresistivity here. Effects of multiaxial loading in a misaligned unidirectional laminate are also discussed here. The misaligned laminate causes large shrink in the transverse direction during tensile tests; poor electrical contacts at electrodes increases the electric current in the transverse direction; these two effects cause decrease of electrical resistance for the poor electrical contact specimen with large fibre misalignment.     

Three-Dimensional Analysis of the Effect of Material Randomness on the Damage Behaviour of CFRP Laminates with Stochastic Cohesive-Zone Elements

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre de-bonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance in terms of the global properties and design methods for such materials. This paper presents a numerical modelling based analysis of the effect of material randomness on delamination damage in CFRP materials by the implementation of a stochastic cohesive-zone model (CZM) within the framework of the finite-element (FE) method. The initiation and propagation of delamination in a unidirectional CFRP double-cantilever beam (DCB) specimen loaded under mode-I was analyzed, accounting for the inherent microstructural stochasticity exhibited by such laminates via the stochastic CZM. Various statistical realizations for a half-scatter of 50 % of fracture energy were performed, with a probability distribution based on Weibull’s two-parameter probability density function. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. This indicates that deterministic analysis of composites using average properties may be non-conservative and a method based on probability may be more appropriate.     

Occurrence and propagation of delamination during the machining of carbon fibre reinforced plastics (CFRPs) - An experimental study

The machining of carbon fibre reinforced plastics (CFRPs) is often accompanied by delamination of the top layers of the machined edges. Such damage necessitates time-consuming and costly post-machining and in some cases leads to rejection of components. The work described in this paper systematically investigates the occurrence of delamination of the top layers during the machining of CFRP tape, with the focus being on the process of contour milling. The occurrence and propagation of delamination were studied by milling slots in unidirectional CFRP specimens having different fibre orientations and mainly analysing the slot tip. This allowed the key mechanisms to be clarified. The results show that delamination is highly dependent on the fibre orientation and the tool sharpness. The experiments allow derivation of a novel system for describing the occurrence and propagation of delamination during milling. Furthermore, the principles also apply for drilling. The results allow customisation of the machining procedure to reduce and in some cases totally avoid delamination, leading to a significant increase in the quality of components.     

Numerical Modeling of Exit-Ply Delamination During Drilling of CFRPs Considering Thermal Effects

Abstract

A finite element (FE) model for exit-ply delamination during drilling carbon fiber reinforced polymers (CFRPs) laminates is presented. The current FE model is developed to predict critical thrust force at the onset of delamination for 1 and 2 plies under the twist drill for various cutting temperatures. The interface behavior for delamination onset is modeled using surface based cohesive zone model (CZM). The numerical predictions for critical thrust force are compared with experimental thrust forces for various number of plies under the twist drill over a range of cutting temperature. Thrust force predictions were found to match with experimental data.     

Analysis of delamination in drilling glass fiber reinforced polyester composites

Glass fiber reinforced plastic (GFRP) composite materials are finding increased application in aeronautical, automobile and structural applications. Drilling is a complex process, owing to their tendency to delaminate is used to join composite structures. In the present work, an attempt has been made to develop empirical relationships between the drilling parameters such as fiber orientation angle, tool feed rate, rotational speed and tool diameter with respect to delamination in drilling of GFR–polyester composites. The empirical relationship has been developed by using response surface methodology. The developed model can be effectively used to predict the delamination in drilling of GFRP composites within the factors and their limits are studied. The result indicated that the increase in feed rate and drill diameter increases the delamination size whereas there is no clear effect is observed for fiber orientation angle. The spindle speed shows only little effect on delamination in drilling of GFR–Polyester composites.     

Machining damage in FRPs: Laser versus conventional drilling

Comparison of the performance of conventional drilling (CD) and fiber laser machining (FLM) of unidirectional (UD) and multidirectional (MD) glass (G) and carbon (C) fiber reinforced plastics (FRPs) has been presented with an emphasis on machining damage characterization. FLM induced damages are heat affected zone (HAZ), matrix recession, kerf width, and tapered cut whereas CD induced damage include fiber pull-outs, fuzzing, spalling, matrix cracking and delamination. Scanning Acoustic Microscopy (SAM) proved a useful technique to quantify machining damage by providing a ply-by-ply damage analysis. In addition, Scanning Electron Microscopy (SEM) provided detailed views of inherent damage mechanisms during FLM and CD. Machining damage investigation reveals a strong dependence on anisotropy of the composite laminate with greatest matrix recession occurring for UD-CFRP laminates. FLM was found to be efficient with decreased machining times and less set-up requirements. However, surface characteristics of CD specimens were found to be superior to FLM specimens.     

Effect of damage levels and curing stresses on delamination growth behaviour emanating from circular holes in laminated FRP composites

This paper deals with the study of the effect of drilling induced delamination damage levels and residual thermal stresses (developed during manufacturing process of cooling the laminate form curing temperature to room temperature) on delamination growth behaviour emanating form circular holes in graphite/epoxy laminated FRP composites. Two sets of full three dimensional finite element analyses (one with the residual thermal stresses developed while curing the laminate and the other without residual thermal stresses i.e. with mechanical loading only) have been performed to calculate the displacements and interlaminar stresses along the delaminated interfaces responsible for the delamination onset and propagation. Modified crack closure integral (MCCI) techniques based on the concepts of linear elastic fracture mechanics (LEFM) have been used to calculate the distribution of individual modes of strain energy release rates (SERR) to investigate the interlaminar delamination initiation and propagation characteristics. Asymmetric variations of SERR obtained along the delamination front are caused by the overlapping stress fields due to the coupling effect of thermal and mechanical loadings. It is found that parameters such as ply orientation, drilling induced damage levels and material heterogeneity at the delaminated interface dictate the interlaminar fracture behaviour of laminated FRP composites.     

复合材料低速冲击损伤研究及等效模型的应用

复合材料低速冲击损伤的特殊性及危害性使得对航空复合材料冲击损伤的评估尤为重要。该文通过建立数值计算模型并结合实验数据解决了4个方面的应用问题:1)在ABAQUS子程序VUMAT中引入损伤模式及损伤演化,结合层间连接单元对层合板低速冲击损伤进行了模拟;2)损伤容限设计方法要求对含缺陷结构的极限强度做出正确的评估,通过ABAQUS子程序USDFLD引入损伤模式及材料折减方案,得到了含圆孔的层合板极限拉压强度;3)通过ABAQUS子程序UMAT引入损伤模式及刚度折减方案,结合层间连接单元,模拟了含预制分层的层合板压缩失效问题;4)针对共用铺层结构的工程有限元计算问题,提出了力学等效模型,将该模型应用到结构级的静力实验模拟并拓展至结构冲击模拟。     

Experimental thrust forces and delamination analysis of GFRP laminates using candlestick drills

Drilling is usually performed using twist drills while assembling the composite components. However, it is necessary to adopt appropriate tool as the risk of delamination damage is high. Many researchers have performed experimental and theoretical study concerning drilling-induced delamination damage in composites utilizing special drills. The article performed drilling experiments of glass fiber reinforced plastic laminates utilizing candlestick drills. Ten candlestick drills with different drill tip geometries were compared in thrust forces and push down delamination. The results revealed the drilling behavior of the outer cutting edges and the outer drill tips for candlestick drill, and also indicated that the push down delamination depended on exit thrust force instead of maximum thrust force. What is more, thrust forces and push down delamination were significantly reduced when holes were produced using reasonable candlestick drill tip geometry. Finally, the analysis was useful for selecting appropriate candlestick drill tip geometry and conducting candlestick drill tip geometry optimization.     

Study of delamination in drilling carbon fiber reinforced plastics (CFRP) using design experiments

In this paper is presented a new comprehensive approach to select cutting parameters for damage-free drilling in carbon fiber reinforced epoxy composite material. The approach is based on a combination of Taguchi’s techniques and on the analysis of variance (ANOVA). A plan of experiments, based on the techniques of Taguchi, was performed drilling with cutting parameters prefixed in an autoclave carbon fiber reinforced plastic (CFRP) laminate. The ANOVA is employed to investigate the cutting characteristics of CFRP’s using high speed steel (HSS) and Cemented Carbide (K10) drills. The objective was to establish a correlation between cutting velocity and feed rate with the delamination in a CFRP laminate. The correlation was obtained by multiple linear regression. Finally, confirmation tests were performed to make a comparison between the results foreseen from the mentioned correlation.     

复合材料层合板低速冲击损伤容限的改进方法和影响因素

依据笔者在这方面的研究和前人的工作，以及现有各种改进炭纤维增强树脂基复合材料冲击性能的方法，分析和总结了复合材料层合结构冲击损伤以及损伤容限，其中主要是冲击后压缩强度的重要影响因素，并且讨论了这些因素的作用。     

Circular cutting strategy for drilling of carbon fiber-reinforced plastics (CFRPs)

The drilling process of carbon fiber-reinforced plastics (CFRPs) is the most commonly employed machining operation due to the necessity of joining these materials. However, these materials are prone to delaminate during the process, and the presence of this defect is the most cause of rejection for CFRP products, especially those produced for the aeronautic industry. Therefore, this article aims to study a drilling strategy (named circular drilling strategy) by using dedicated tools with different diameters, in order to reduce the extension of delaminations. Holes with different diameters (6, 8, and 10 mm) were obtained both with the conventional and with the proposed drilling strategy under distinct cutting conditions that mainly differ in the feed rates (62, 125, and 250 mm/min) and cutting velocities (50, 75, and 100 m/min). The effect of the cutting parameters and tool diameter on the cutting forces and delamination factor was studied for both the conventional and circular drilling process. The results proved that the proposed technique produces better hole quality and lower thrust forces than the conventional one under the same cutting conditions.     

The fatigue damage mechanics of a carbon fibre composite laminate: I—development of the model

The mechanics of fatigue damage of a carbon fibre composite laminate is developed. In this system, damage consists of a delamination front, with associated matrix cracking, which propagates inwards from the sample edges. The elastic stiffness of the laminate is related to the current level of damage, and is used to measure it. The damage growth rate is a power function of the stress amplitude and of the mean stress, and is independent of damage when cycling is at constant stress amplitude. Failure occurs when the damage reaches a critical level which depends on the maximum stress seen in the loading cycle. The results are applied to life prediction in Part II of this work.     

Modelling of fatigue damage progression and life of CFRP laminates

A progressive fatigue damage model has been developed for predicting damage accumulation and life of carbon fibre‐reinforced plastics (CFRP) laminates with arbitrary geometry and stacking sequence subjected to constant amplitude cyclic loading. The model comprises the components of stress analysis, fatigue failure analysis and fatigue material property degradation. Stress analysis of the composite laminate was performed by creating a three‐dimensional finite element model in the ANSYS FE code. Fatigue failure analysis was performed by using a set of Hashin‐type failure criteria and the Ye‐delamination criterion. Two types of material property degradations on the basis of element stiffness and strength were applied: a sudden degradation because of sudden failure detected by the fatigue failure criteria and a gradual degradation because of the nature of cyclic loading, which is driven by the increased number of cycles. The gradual degradation of the composite material was modelled by using functions relating the residual stiffness and residual strength of the laminate to the number of cycles. All model components have been programmed in the ANSYS FE code in order to create a user‐friendly macro‐routine. The model has been applied in two different quasi‐isotropic CFRP laminates subjected to tension–compression (T–C) fatigue and the predictions of fatigue life and damage accumulation as a function of the number of cycles were compared with experimental data available in the literature. A very good agreement was obtained.     

一种改进的内聚力损伤模型在复合材料层合板低速冲击损伤模拟中的应用

针对传统内聚力损伤模型(CZM)无法考虑层内裂纹对界面分层影响的缺点,提出了一种改进的适用于复合材料层合板低速冲击损伤模拟的CZM。通过对界面单元内聚力本构模型中的损伤起始准则进行修正,考虑了界面层相邻铺层内基体、纤维的损伤状态及应力分布对层间强度和分层扩展的影响。基于ABAQUS用户子程序VUMAT,结合本文模型及层合板失效判据,建立了模拟复合材料层合板在低速冲击作用下的渐进损伤过程的有限元模型,计算了不同铺层角度和材料属性的层合板在低速冲击作用下的损伤状态。通过数值模拟与试验结果的对比,验证了本文方法的精度及合理性。     

An engineering model for deformation of CFRP plates during penetration

The response of brittle CFRP laminated plates to penetration by cylindrical indenters has been studied using a combined experimental and analytical investigation. The two major damage modes caused by delamination and plugging are found to divide the plate response into three distinct stages. An analytical model, which accounts for the geometric parameters of the response, is developed to describe the deformation behaviour of the laminate in the post-delamination stage. Using the experimental measurements of strains and displacements at the centre, the model is used to estimate the size of the internal damage (delamination) and its effective bending modulus. The predicted delamination sizes for various laminate thicknesses are successfully correlated with those measured using C-scans. The agreement instills confidence in using the present analytical approach to reduce the number of tests required to characterize the penetration response of laminates.     

Damage-Free Hole Making in Fiber-Reinforced Composites: An Innovative Tool Design Approach

Drilling is an inevitable machining technique that facilitates the assembly of composite parts. Drilling with the traditional tools causes significant damage to the composite parts. The forces that are produced during drilling are mostly accountable for the damage or rejection of the parts. Therefore, the main aim of the present investigation is to reduce the forces and subsequent damage that is induced during drilling operation. Thus, new tool geometry has been conceptualized, designed, developed, and implemented to investigate the drilling behavior of sisal-epoxy and nettle-epoxy laminates. A comparative analysis has been made to compare the performance of the developed tool with the widely used twist drill. The performance has been compared based on the forces and associated drilling-induced damage. It was found that the developed tool geometry gives better results than the traditional twist drill in terms of minimization of forces and damage as well.     

复合材料层板损伤过程的刚度分析

本文对典型铺设的[0₂/±45₂/90₂]_s碳/环氧复合材料层板中的典型损伤状态进行了实验观察,测定出损伤对层板刚度引起的下降率.建立了横向裂纹扩展、分层伴以横向裂纹扩展的三维有限元分析模型,计算出对层板刚度引起的下降率,并与实验值进行了比较.结果表明,横向裂纹和分层是层板的主要损伤型式,分层损伤扩展是一个主导性的稳定的损伤扩展过程,是导致刚度下降的主要因素.     

复合材料层合板静态压痕与损伤行为

发展了层合复合材料面内及分层失效模型,模型考虑了横向压剪残余剪切变形和分层残余相对位移。提出一种块体单元包含若干铺层的模型,以减少复合材料逐层分析时的计算量。对一种碳纤/环氧层合复合材料进行了横向压痕试验,测量了不同压力下凹坑的深度,用超声C扫测得损伤面积。用发展的模型在ABAQUS平台上通过用户材料子程序UMAT模拟了上述试验,分析了试验曲线特征点对应的损伤机制,通过与试验结果的对比,验证了本文模型的合理性。