Drilling-induced Delamination Research Articles

Progressive damage modeling in open hole composite laminates with ultrasound-informed drilling-induced delamination

Insertion of fasteners, often used in aerospace and automotive industries, requires drilling that induces non-uniform delamination between lamina of a carbon fiber reinforced plastic (CFRP). Understanding the effect of drilling-induced delamination on the mechanical performance and associated damage mechanisms (progressive damage and failure) is critical to ensure joining integrity. The present work develops an ultrasound testing (UT) method to quantify the drilling-induced delamination at each individual ply interface for CFRP laminates. We then develop a mesoscale finite element (FE) model of an open hole tension specimen by incorporating the UT-obtained drilling-induced delamination at each interface. This delamination is modeled using cohesive zone elements with a bilinear traction-separation law with progressive damage in each ply modeled using the 3D Hashin along with a progressive damage model. Our FE model, with UT-informed delamination, accurately predicts experimental observations of the stress concentration around the hole, damage progression, and open hole tension strength.

An adaptive modelling approach using a novel modified AOA/SVR for prediction of drilling-induced delamination in CFRP/Ti stacks

Delamination is the most critical damage in drilling the CFRP/Ti stacks under the impact of drilling parameters and tool structure, which makes the traditional theoretical or empirical models not have enough accuracy and be time-consuming due to the multi variables, while the machine learning model would suffer the unsuitable hyper-parameters and have a bad accuracy and generalization ability. This paper proposed an adaptive modelling approach to predict the delamination while drilling the CFRP/Ti stacks. This approach adapted the original arithmetic optimization algorithm (AOA) by adding a random disturbance phase to update the penalty coefficient C and the kernel coefficient γ of the support vector regression (SVR) automatically. In the meanwhile, the approach made use of the energy of the 5 stages in drilling the CFRP/Ti stacks and predicted the delamination damage both at the entrance and exit. The modified AOA optimized the training mean squared error(MSE) in predicting the entrance and exit delamination by 10.27 % and 33.63 %, while the accuracy of the proposed model can reach 96.7 % and 97.17 % respectively. The model got validated, and had a comprehensive ability containing the accuracy and generalization ability.

Experimental and numerical studies on the tensile behaviors of thin-ply and thick-ply open-hole laminates

Laminated composites with thin-ply unidirectional prepregs are popular due to their higher in-situ strength as compared with thick-ply laminates due to different failure mechanisms. Thin-ply and thick-ply open-hole laminates were fabricated and the tensile failure characteristics were studied by combining experimental and numerical simulation approaches. Charge-coupled device camera and de-ply technology were used to obtain failure morphologies of open-hole specimens under tensile loading. A finite element model considering progressive damage and bilinear cohesive model was built based on ABAQUS to study the damage mechanism of open-hole laminates. In thin-ply laminates, the damage plane was relatively neat while in thick-ply laminate broom-shaped damage morphologies were observed. Based on de-ply experiments and numerical prediction, it was found that thin-ply laminates had delamination suppression effect and could restrain the propagation of delamination along the loading direction, which explains the distinct damage patterns existed in thin-ply and thick-ply laminates. Based on numerical prediction, it was found that drilling-induced delamination did not affect the tensile strength of open-hole laminates. Propagation of drilling-induced delamination was also restrained in thin-ply laminates. The results of this research suggested that thin-ply prepregs should be considered when delamination poses critical threat to the structural integrity of laminated composite structures.

Vision-Based Defect Measurement of Drilled CFRP Composites Using Double-Light Imaging

Drilling-induced defects in carbon fiber-reinforced polymers (CFRP) composite laminate can lead to the risk of aircraft structural failure and even aviation disaster. Measuring these defects is, then, inevitable for aerospace vehicles assembly. Tear is a representative drilling-induced delamination defect. But the most widely used measurement method is still based on human vision and vernier calipers. With low measurement efficiency and large measurement error, this is no longer suitable for industrialized large-scale production. Machine vision provides a new technology to address this problem. However, the semi-specular and textured surface around the drilled holes in CFRP laminates makes machine vision-based tear measurement difficult. To address these issues, we propose a novel automatic tear measuring system to obtain the tear point with maximum radial length (TPMRL). The method consists of three parts: (1) a double-light imaging frame is designed to illuminate the object hole under two different lighting conditions and acquire images; (2) a series of image processing procedures are proposed to accurately obtain inner hole information and tear edge information; (3) combined processing of inner hole information and tear edge information is performed to obtain the maximum radial length of the tear area (MRLTA). The experimental results show that the proposed system is able to measure the MRLTA and is superior to some existing relative approaches.

Drilling-induced delamination measurement using a novel digital image processing algorithm

Drilling-induced delamination measurement using a novel digital image processing algorithm

GMDH-Kalman Filter prediction of high-cycle fatigue life of drilled industrial composites: A hybrid machine learning with limited data

In industrial composites applications, drilling is one of the most common operations and complex processes during the final assembly, which can generate undesirable damage to the manufactured part. Data collection from a given composite’s fatigue life is often costly and time-consuming. To address this challenge, the current case study aims to adapt a hybrid machine learning framework to predict the fatigue life of the drilled Glass Fiber Reinforced Polymer composite laminates (with both unidirectional and woven lay-ups) under a limited and noisy data assumption. Composite specimens were drilled at various cutting speeds and feed rates. The size of the delamination around the hole was scanned by a microscopic camera. Cyclic three-point bending tests were conducted, and results indicated that the drilling-induced delamination size and the composite lay-up affect the specimens’ fatigue lives. The latter were then modeled in two steps. In the first step, an offline deterministic model was established using the group method of data handling along with a singular value decomposition. Pareto multi-objective optimization was applied to prevent overfitting. In the second step, the Kalman filter was employed to update the polynomial of the deterministic model based on minimizing the mean and variance of error between the actual and modeled data. Results showed an excellent learning reliability, with a correlation coefficient of 97.6% and 96.5% in predicting the fatigue life of unidirectional and woven composite laminates, respectively. A sensitivity analysis was performed and indicated that the fatigue life of the samples has been more affected by the drilling feed rate, compared to the cutting speed.

Machining GLARE fibre metal laminates: a comparative study on drilling effect between conventional and ultrasonic-assisted drilling

GLARE laminates are multi-layered metal-composite materials created from bonding sheets of metallic alloys with carbon or glass fibre layers. The application of hybrid-conventional machining processes such as ultrasonic-assisted drilling (UAD) is becoming of great interest to the aerospace industry due to its capability in reducing the cutting forces and tool wear which are directly responsible for drilling-induced delamination. There is rich literature on the conventional drilling (CD) of GLARE, but no work reported using UAD process. This study will fill this gap and investigate the UAD of GLARE laminates using an indigenously developed UAD system. The influence of spindle speed and feed rate on thrust force and surface roughness metrics (Ra and Rz) were investigated under CD and UAD regimes. The quality of the borehole and damage mechanisms in the laminate constituents was examined using scanning electron microscopy (SEM). The contribution of the drilling parameters on the measured outputs was further evaluated using the analysis of variance (ANOVA) statistical analysis. It was found that UAD resulted in a significant reduction in thrust force by up to 65% while surface roughness metrics Ra and Rz were unaffected by the type of drilling process used. SEM analysis showed irregular and fuzzier surfaces in glass fibre layers in holes machined using UAD due to the longitudinal vibration of the tool.

Delamination measurement in glass fibre reinforced polymer (GFRP) composite based on image differencing

Drilling-induced delamination is the most severe material defect in fibrous polymeric composites. Although many delamination measurement methods are available, the outputs of recent digital image processing (DIP) based delamination measurement methods are significantly affected by the non-drilling induced material defects and anomalies like cavities or dirty textured surfaces. Therefore, the main aim of the present research was to develop a method capable of drilling-induced delamination measurement in fibrous polymeric composites based on the image differentiation principle. In an attempt to implement this principle, images of the undrilled composites were referenced/compared with the images of the drilled composites. The proposed delamination measurement method is tested through mechanical drilling experiments in glass fibre reinforced polymer (GFRP) composites and compared with conventional DIP-supported manual measurements. The experimental results show a good agreement between the delamination detected by the naked eye, the conventional and the proposed method. Although the proposed method slightly increases the drilling operation time, the comparative differentiating makes it a good solution to detect drilling-induced material deformations in textured and semi-specular surfaces. The proposed method can separate drilling-induced delamination from other non-drilling-induced material discontinuities or anomalies. Considering the automated, reproducible and complex nature of the novel method, it may replace current qualification operations in high-end industries.

Sensor-based strategies for accurate prediction of drilled hole surface integrity of CFRP/Al7075 hybrid stack

The airplane structure is often made up using carbon fiber reinforced plastic (CFRP) stacking with a metallic sheet to improve excessive load carrying and shock-absorbing capability with reduced fuel economy. It is a challenging task to drill dissimilar CFRP-Al metallic stack because the hot continuous metallic chip mutilates the hole surface in addition to drilling-induced delamination. The present work addresses the parametric effect on delamination, roughness, and circularity error of the drilled hole along with chip morphology using different drill geometry on the fabricated CFRP [0°/-45°/90°/45°]2s stack over an aluminum 7075 sheet. A drill tool dynamometer was used to capture axial thrust with associated torque variations during drilling which were further used in process monitoring. The most critical unlike interface drilling has been scrutinized using instantaneous force-torque slopes along with scanning electron micrographs. The machining time loss during CFRP→Al interface drilling has also been processed. Finally, a comparative assessment has been made on the predictability of hole quality features among various sensor-based strategies using response surface methodology. The drilled-hole surface quality was significantly improved at high spindle speed (2250 rpm), and low feed rate (0.025 mm/rev) using a low drill point angle (110°). The thrust force was found to be a prime indicator of drilled hole integrity than torque whereas peak torque and its slope in the initial stages of the drilling cycle strongly correlated with surface delamination. The top surface delamination was somewhat severe (≥ 1.6) at high drill thrust (≥ 40 N) with high torque (≥ 0.45 Nm).

An Experimental Comprehensive Analysis of Drilling Carbon Fiber Reinforced Plastic Tubes and Comparison With Carbon Fiber Reinforced Plastic Plate

Today, carbon fiber reinforced plastic (CFRP) composites, which are the main areas of use such as aerospace and automotive, can be produced by different methods. Especially in the production of tubular composites, filament winding and roll wrapping methods are used generally, while vacuum-assisted resin transfer molding method or vacuum bagging method comes to the fore in the production of composite plates. These materials produced by different methods may exhibit different behavior in terms of machinability. In this study, the drilling machinability characteristics of CFRP tubes produced by filament winding and roll wrapping methods were investigated and compared with the CFRP plate produced with vacuum bagging and the results were presented comparatively. Experimental results showed that the composite plate generates more force and damage when drilled compared to the tubes, so its machinability is more difficult. Higher thrust force and damage occur in filament wound tube compared to roll wrapped tube. In addition, although the thrust force does not increase with the use of support in composite tubes, a significant improvement in drilling-induced delamination damage behavior is obtained. Compared to the tubes, a better borehole surface quality is obtained in the plate, while more dust chip formation is observed.

Effect of seawater immersion on the drill induced delamination failure in glass fiber reinforced polymer composites – Experimental and statistical analysis

Composite materials, particularly glass fiber reinforced polymer (GFRP) composite, have established a strong presence in the marine industry. Its marine applications range from constructing massive naval vessels to constructing simple fishing boat hulls. Previous research has established that seawater aging has a detrimental effect on the physical and mechanical properties of GFRPs. Additionally, drilling composites are known to cause delamination damage, resulting in a high failure rate. Since composite components are assembled with fasteners, drilling operations are unavoidable. Thus, the current study examines the effect of seawater aging at a macroscopic level while considering the input parameters spindle speed and feed and material thickness and the response variables as peel-up and push-down delamination damages. The result indicated higher failure rates in the seawater-treated composites than in untreated composites. The maximum damages were seen in the 60 days of immersed composites, wherein the peel-up delamination damage increased by 290.22, 305.86, and 250.81 %, and the push-down delamination damage increased by 69.48, 76.38, 64.28 % in 6, 8 and 10 mm composites, respectively. The combinational effect of seawater degradation of GFRP composites and the damages caused by the cutting forces (function of speed and feed) attribute to the failure rate increase concerning the drilling-induced delamination damages in the composites. In addition to investigating the effect of seawater immersion on damages, the study also provides the regression analysis-based mathematical model having high goodness of fit and predicting capability to determine the damages for the given values of input variables.

Selection of an appropriate non-destructive testing method for evaluating drilling-induced delamination in natural fiber composites

Drilling is a principal machining process that is generally operated to make holes in composite parts to join and assemble into a complicated structures. Recently, the use of natural fiber composites (NFCs) has increased in engineering and industrial applications. Choosing an appropriate non-destructive technique to evaluate the drilling-induced delamination in NFCs is a vital and essential task. This paper investigates three different non-destructive techniques for evaluating and determining the size and shape of drilling-induced delamination in NFCs. According to the results from this research, it can be inferred that the image processing technique using a flatbed scanner can be utilized as an accessible, economical, and efficient method for assessment of drilling-induced delamination in NFCs.

Drilling induced delamination assessment of nanoparticles reinforced polymer matrix composites

The knowledge of polymer matrix composites machinability has increased with its rise in multi-field structural applications. Drilling of polymer matrix composites is required to achieve structural integration with greater accuracies. Delamination due to drilling affects the material strength and it occurs at both the entry and hole exit planes. The delamination on the exit plane is critical. The effect of nano-fillers on drilling-induced delamination of reinforced plastics has not been documented extensively and in this article, an attempt is made to explore the application potential of montmorillonite clay and graphene with amine functionalization in minimizing delamination through toughening the epoxy matrix. Experimentation was based on Taguchi’s L16 approach. The delamination factor and circularity ratio were analyzed using an image processing technique in Matlab and Image J. To evaluate the effects of various parameters and parameters interaction effect on the delamination factor, ANOVA was employed and Signal/noise ratios were calculated. The mathematical models were developed for delamination. It was observed that the reduction in the delamination factor was observed in nanoclay and graphene reinforced polymer matrix toughened composites in comparison to the base composites with a reduction in thrust force, specific cutting energy, and increase in circularity ratio.

Assessment of drilling-induced delamination and tool wear in carbon fibre reinforced laminates

A highly accurate X-ray damage characterization technique was used in this study to investigate the correlation between hole damage, tool wear and thrust force in drilling of carbon fibre reinforced laminates. Woven carbon/epoxy panels were drilled with a twist tool for 592 holes and the internal damage induced by drilling was investigated by high-resolution penetrant-enhanced X-radiography for comparison with damage evaluated by visual inspection. The study shows that the dominant damage mode induced by drilling is delamination and that tool wear has a significant effect on the size of the delaminated area. The analyses also show that the delaminated areas estimated by visual inspection are much smaller than those identified by X-radiography. This finding indicates that visual observation techniques may provide dangerously unconservative indications and should be used in association with more reliable damage evaluation methods to assess the hole quality of drilled carbon fibre reinforced composites.

Analytical study and experimental investigation on delamination in drilling of CFRP laminates using twist drills

Delamination in composite laminates is one of the most common and critical defects, it would significantly reduce the stiffness and strength of components. This paper aims at reducing the drilling-induced delamination to improve hole quality. First, drilling tests are carried out to examine the distribution and amplitude of thrust force on drill under different cutting conditions. Empirical thrust force models are developed using experimental data. Then, analytical models of critical thrust force (CTF) are separately established considering two drilling states, considering the actual interactions between the drill and laminate. Anisotropy of composite, tool geometry and load distribution on drill are considered. Comparisons with experimental data and existing models show that the proposed models are capable of predicting CTFs under different loading conditions. In addition, the critical feed rates at different ply locations are determined. This study can provide guidance for the selection of cutting parameters to reduce drilling-induced delamination.

Drilling performance of uncoated brad spur tools for high-strength carbon fiber-reinforced polymer laminates

High-strength carbon fiber-reinforced polymers have been a promising alternative to conventional fibrous composites because of their extremely high properties. Mechanical drilling is a necessary operation to create boreholes for riveting and bolting different composite structures into assemblies. However, the high-strength carbon fiber-reinforced polymers pose much more serious machining issues than the conventional ones. The present work aims to investigate the drilling performance of one type of uncoated carbide brad spur tools when applied in machining of high-strength carbon fiber-reinforced polymer laminates. The wear mechanisms of uncoated carbide brad spur drills were figured out. Additionally, the tool wear influences on the thrust force, hole dimensional accuracy, and drilling-induced delamination were quantified. The acquired results indicate that the tool wear has a significant impact on the drilling process and the quality of machined hole wall surfaces. The dominant wear mode is proved to be abrasion wear. Moreover, the uncoated brad spur drills appear to exhibit high resistance to the abrasion wear in the drilling of high-strength carbon fiber-reinforced polymer laminates.

Influence of cutting conditions in drilling of CFRP/Al stacked composites

Composite laminates are gorgeous for several applications such as aerospace and aircraft structural components due to their excellent properties. Typically, mechanical drilling has been important machining operation for components made of composite laminates. Nevertheless, laminated composites are considered as hard-to-machine material which results in low drilling efficiency and drilling-induced delamination which is undesirable. This paper reviews the experiments during drilling of CFRP/Al stacked and sandwich composites. The machinability facets of these material stacks has been generally used in aerospace applications, it has been studied based on impact of drill material, drill geometries, and drilling process parameters such as speed and feed. Composite material requires high spindle speed and low feed rate, whereas drilling aluminum requires stability between speedsto feed rate. The review reports essential results and gap in the collected literature for CFRP/Al stacked and sandwich composites. A compromise between several parameters is required during drilling of multi-material stacks. The problems and solutions allied to drilling of multi-material stacks are deliberated and the directions in which the research on drilling of multi-materials may be carried out are suggested in this paper. It is intended to assist readers to acquire a thorough view on mechanical drilling of laminated composite.

A review on strengthening, delamination formation and suppression techniques during drilling of CFRP composites

Carbon fiber-reinforced composite laminates are progressively appealing for several applications in various manufacturing sectors such as aeronautical and automobile, because of their amazingly predominant mechanical properties. This has subsequently led to an increase in research of various machinability techniques emphasizing on both conventional and non-conventional methods. Typically, conventional drilling is one of the most significant machining processes for most kinds of fiber composite laminates. The most unsolicited damage termed as delamination during drilling has recently become one of the most influential research subjects as numerous engineers in manufacturing sector have been trying to prevent this problem over decades. This review paper briefly discusses about the incorporation of different types of fillers in carbon fiber-reinforced composites. The paper also summarizes the steady progress in drilling-induced delamination of carbon fiber laminates. It covers the delamination formation mechanism, delamination suppression strategies, and the optimization of various drilling parameters to reduce the delamination damage. The present article provides researchers an opportunity to deepen their knowledge on specific aspects by exploring different methods for reducing delamination in CFRP composites.

Analytical and experimental investigation of the parameters in drilling flax/poly(lactic acid) bio-composite laminates

Understanding the influences of drilling parameters on machinability characteristics of NFRCs is of value to both academia and manufacturing industries. The present work investigates drilling-induced delamination and hole quality of fully biodegradable flax/poly(lactic acid) composites in response to cutting speed, feed rate, drill diameter, and drill material. Among various types of drilling-induced damages, exit-ply delamination is found to be the most destructive damage and cutting thrust force is identified as the main reason. The experimental results revealed that thrust force is highly dependent on drilling parameters where feed rate and drill material were found to be the most influential parameter on it. The choice of drill bit in terms of diameter and material has a considerable effect on the machinability. Drilling with HSS drills resulted in nearly 60% lower thrust force and better hole quality compared to that with carbide drills. While the size of delamination damage in the samples drilled at the lowest feed (0.02 mm/rev) sounds to be unrelated to cutting forces, the overall trend confirms that decreasing the value of thrust force leads to lowering delamination damages. Hence, an effective strategy in avoiding such undesirable defects is calculating the critical thrust force below which delamination is negligible. An analytical model is developed based on the theory of virtual work to determine the critical thrust force at delamination onset. Through validating with experimental data, the proposed model agrees better with a more accurate prediction than existing models.

Research on prediction method of mechanical properties of open-hole laminated plain woven CFRP composites considering drilling-induced delamination damage

Carbon fiber reinforced polymer (CFRP) is widely used in machinery and construction due to their extra-ordinary mechanical properties. Up to now, conventional drilling is the most common machining process to facilitate holes on CFRP laminates for the component assembly. However, drilling-induced delamination is widely considered as the most undesirable damage and challenging failure mode in this process. The present paper aims to propose a convenient and quick method to predict the mechanical properties of open-hole laminated plain woven CFRP composites more accurately. First, the delamination results of nondestructive test by C-scan and simulation were compared after practical experiment and numerical simulation of drilling process. Second, an equivalent parameter was proposed to simplify the delamination regions with complex boundaries, and the mechanical property evaluation model was constructed by introducing the initial delamination damage area, which was obtained from the drilling numerical simulation results. Finally, the effectiveness of the proposed method was demonstrated by analyzing the mechanical properties and failure modes of CFRP with processing delamination damage under loading conditions of tension, compression and bending. Therefore, the proposed method in this paper can help understand the degradation mechanism of composites after drilling and provide guidance for the design of open-hole laminated plain woven CFRP composites.