Analysis of drilling of composite laminates

This dissertation deals with the characterization, modeling, and monitoring of drilling process of composite materials through various experimental and analytical investigations. Analytical models were developed which predicts critical thrust force and feed rate above which the delamination crack begins to propagate in the drilling of multi-directional laminated composites. The delamination zone was modeled as a circular plate, with clamped edge and subjected to different load profiles. Based on fracture mechanics, classical laminate theory and orthogonal cutting mechanics, expressions were obtained for critical thrusts and feed rates at different ply locations. The proposed models have been verified by experiments and compared with the existing models. It was found that the new developed models provide more accurate and rigorous results than the formers. Quality of holes and drilling-induced damage when drilling fiber reinforced composite laminates were experimentally studied. Several quality responses were measured as indices of drilling performance, including thrust force, delamination size, residual compression strength, and flexural strength. Effects of key drilling parameters on these responses were statistically analyzed, and optimal drilling conditions for high performance and free-damage drilling were identified. Experimental results revealed that the choice of drilling conditions are critical to hole performance especially when these materials are subjected to structural loads. An experimental study of acoustic emission as a tool for in-process monitoring and nondestructive evaluation of drilling of composites was conducted. Acoustic emission was used to examine the relationship between signal response and drilling induced damages. A procedure for discrimination and identification of different damage mechanisms was presented utilizing different signal analysis tools. Based on the results, frequency distribution and energy percentage of most important damage mechanisms occurring during drilling were determined. It was concluded that acoustic emission has a great potential for the application of online monitoring and damage characterization in the drilling of composite structures

THE EFFECT OF SUPPORT PLATE ON DRILLING-INDUCED DELAMINATION

Delamination is considered as a major problem in drilling of composite materials, which degrades the mechanical properties of these materials. The thrust force exerted by the drill is considered as the major cause of delamination; and one practical approach to reduce delamination is to use a back-up plate under the specimen. In this paper, the effect of exit support plate on delamination in twist drilling of glass fiber reinforced composites is studied. Firstly, two analytical models based on linear fracture mechanics and elastic bending theory of plates are described to find critical thrust forces at the beginning of crack growth for drilling with and without back-up plate. Secondly, two series of experiments are carried out on glass fiber reinforced composites to determine quantitatively the effect of drilling parameters on the amount of delamination. Experimental findings verify a large reduction in the amount of delaminated area when a back-up plate is placed under the specimen

Experimental analysis of GFRP laminates subjected to compression after drilling

This paper presents an experimental study of drilling-induced delamination on the compressive properties of woven glass fiber-reinforced epoxy composites. In the drilling of laminated composites, interlaminar cracking or delamination has a detrimental effect on compressive properties. The onset of delamination and the extent of the damage are governed by the cutting forces developed during the drilling process. High cutting forces, in turn, result from the use of improper drilling parameters. This study investigates the effects of feed rate and spindle speed on delamination and residual compressive strength. The composite laminates were cut into the standard dimensions of compression after impact specimens. The drilling of composite specimens was conducted at three different levels of spindle speed and feed rate based on general full factorial design. Analysis of variance was used to find the percentage contribution of the drilling parameters and it was found that feed rate has the most significant influence on the residual compressive strength. A polynomial regression model was also developed to express the residual compressive strength as a function of the selected process parameter

Critical thrust and feed prediction models in drilling of composite laminates

Drilling induced delamination has been recognized as a major problem during drilling of composite materials. The size of the delamination zone has been shown to be related to the thrust force. However, thrust force strongly depends on drilling parameters and it is not possible to control it directly. Thrust force can be correlated with feed rate, the most important parameter affecting thrust force. This paper presents analytical models to predict critical thrust force and feed rate at the onset of delamination. The model proposed is based on elastic fracture mechanics, classical plate bending theory and the mechanics of oblique cutting. An advantage of this model over other proposed models is that to avoid delamination via thrust monitoring, the thrust force will need to be sensed and used in adaptive control, while optimal feed rate can be used directly in CNC command generation to maximize productivity

Analysis of damage mechanisms in drilling of composite materials by acoustic emission

Composite Structures Volume 131, November 01, 2015, Pages 107-114 Analysis of damage mechanisms in drilling of composite materials by acoustic emission (Article) Zarif Karimi, N.a , Minak, G.a , Kianfar, P.b a Alma Mater Studiorum-Università di Bologna, Department of Industrial Engineering DIN, via Fontanelle 40, Forlì, Italy b Amirkabir University of Technology, Department of Bioengineering, 424 Hafez Ave., Tehran, Iran View references (49) Abstract Conventional methods for analysis of drilling of composite materials usually study the amount of damaged area and effective parameters. However, these methods do not provide investigators with sufficient information regarding drilling-induced damage. In this paper, a procedure for discrimination and identification of different damage mechanisms based on the analysis of acoustical signals emitted during the process is presented. Using principle component analysis for data reduction and unsupervised pattern recognition analysis, the drilling process was divided into three main stages, i.e. entry stage, cutting stage and exit stage. Different methods of signal processing were then used to identify and discriminate the most active damage mechanisms in each stage. As a result, matrix cracking, delamination, fiber pull out and friction were discriminated and the frequency distribution of each one was identifie

Investigation on delamination and flexural properties in drilling of carbon nanotube/polymer composites

The drilling of composite laminates is difficult to control and often leads to delamination that significantly affects the strength of the structure. Of the mechanical properties of composite materials affected by drilling-induced damage, flexural strength has received very little attention. In the present paper, experiments were conducted to analyze the thrust force, delamination factor and residual flexural strength in the drilling of woven E-glass fiber-epoxy composites reinforced with functionalized multi-walled carbon nanotubes. The process parameters considered for the experiments are the feed rate, spindle speed, drill diameter, and the weight percentage of carbon nanotubes present in nanocomposite laminates. Drilling experiments were conducted based on Taguchi design of experiment and three-point bending tests were then done to assess the residual flexural strength of drilled specimens. Analysis of variance and Taguchi S/N ratio analysis were performed to investigate the influence of input parameters on each individual drilling characteristics. In addition, the orthogonal array with grey relational analysis was employed to simultaneously optimize the multiple performance characteristics of the drilling process. According to the results, the feed rate is the factor which has the greatest influence on the thrust force and delamination factor, followed by spindle speed. Residual flexural strength, however, is mostly influenced by nano content, followed by feed rate

Experimental investigation on delamination in nanocomposite drilling

This article addresses the influence of cutting parameters on drillinginduced delamination of woven glass fiber-epoxy composites reinforced with functionalized multi-walled carbon nanotubes (MWCNTs). The input parameters include feed rate, cutting speed, drill size, and wt. % carbon nanotubes present in nanocomposite laminates. Experiments were conducted based on Taguchi L16 orthogonal array and analysis of variance was conducted to determine the significance of each parameter. The results indicate that the main effects of nano content, feed rate, and spindle speed are significant, while the effect of drill diameter is negligible. Furthermore, the optimum drilling conditions for minimum delamination were determined according to the Taguchi's S/N ratio analysis

Inter-laminar Fracture Toughness Measurement of the Mixed-mode Delamination Using Acoustic Emission Technique

No abstract available

Damage analysis for low velocity impacted composite laminates using acoustic emission technique

Characterization of dynamic induced damage is one of the most controversial issues in the application of composite structures. To this aim, Acoustic Emission (AE) technique has been qualified as a robust tool for damage sequence analysis due to its high sensitivity to damage mechanisms. First, AE reference map was created using a couple analysis of Fuzzy C-Means (FCM) clustering and Principal Component Analysis (PCA). The low velocity impact tests were then conducted on composite plates. Finally, Impact AE signals were discriminated and the sequence of damage mechanisms during impact process was discovered. As a result, it is concluded that AE method accompanied with the presented method is a new appropriate approach to discriminate damage mechanism sequences in impacted composite specimens

Damage evaluation of laminated composites under low-velocity impact tests using acoustic emission method

The main goal of this investigation is to characterize the damage in laminated composites under low-velocity impact tests using a new cost-effective approach. To this aim, a quasi-static test was first carried out to obtain initial information about impact tests. Low-velocity impact tests were then applied in unidirectional glass/epoxy composite specimens, and acoustic emission signals were captured during impact events. Next, acoustic emission signals were analyzed using wavelet approach to distinguish released energy related to each distinct damage mechanism. Besides, an approach was provided to estimate threshold impact energy from the quasi-static test, beyond which damage significantly extends. As a final point, the acoustic emission-based procedure using wavelet transform method was proposed to predict the total damage area. Finally, it was found that this acoustic emission methodology can be a capable approach in damage characterization under impact loads in composite structures