Analysis and Control of Surface Delamination Defects During Milling of Orthogonal Aramid Fiber?Reinforced Composites Laminates

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1.Key Laboratory of High Efficiency and Clean Mechanical Manufacture，Ministry of Education，Shandong University，Jinan 250061，P.R.China；

2.School of Mechanical Engineering，Shandong University，Jinan 250061，P.R.China

（Received 18 January 2020；revised 20 February 2020；accepted 5 May 2020）

Abstract: The aramid fiber-reinforced composites（AFRC）can increase the durability of corresponding applications such as aerospace，automobile and other large structural parts，due to the improvement in hardness，heat build-up，wear properties and green environmental protection. However，because of its complex multiphase structure and unique heterogeneity and anisotropy，the poor compression fatigue resistance and the incident surface fibrillation are inevitable. To improve the assembly precision of AFRC，mechanical processing is necessary to meet the dimensional accuracy. This paper focuses on the influence of contour milling parameters on delamination defects during milling of AFRC laminates. A series of milling experiments are conducted and two different kinds of delamination defects including tearing delamination and uncut-off delamination are investigated. A computing method and model based on brittle fracture for the two different types of delamination are established. The results can be used for explaining the mechanism and regularity of delamination defects. The control strategy of delamination defects and evaluation method of finished surface integrity are further discussed. The results are meaningful to optimize cutting parameters，and provide a clear understanding of surface defects control.

Key words：aramid fiber-reinforced composites（AFRC）；milling process；delamination defects；surface control；prediction model

0 Introduction

The aramid fiber-reinforced composites（AFRC）can increase the durability of corresponding applica?tions due to the improvement of hardness，heat build-up，wear properties and green environmental protection. However，because of its complex multi?phase structure and unique heterogeneity and anisot?ropy，the poor compression fatigue resistance and the incident surface fibrillation are inevitable.

To obtain the prescribed shape，mechanical processing is needed to meet the requirements of di?mensional accuracy and surface roughness. The common mechanical processing methods of AFRC include turning，drilling，grinding，and milling. Due to complicated interactions between the matrix and the reinforcement during machining，there are often surface defects such as burrs，tearing，layering and thermal damage on processed materials surface after mechanical processing. Delamination defects mainly occur in the process of milling and drilling. Hence，this paper mainly discusses the delamination defects.

Several research works have been undertaken to investigate the phenomena and induced mecha?nism of different machining defects［1-5］. Bunsell［6］found that the fibre showed obvious yield phenome?non under tensile force and ductile fracture during tensile process，which was the main reason of burr defects. Shi et al.［7］pointed out that the interface phase formed between the phase and the matrix will affect the macroscopic mechanical characteristics of the materials. A damage zone is developed in the co?hesive layer at the crack front of which properties de?grade with deformation due to material damage or plastic softening. A stress limit is set for the cohe?sive zone based on the material strength，which serves as a criterion for the damage initiation. Kim et al.［8-9］investigated the inter laminar fracture toughness through experiments. It is found that the interfacial bonding properties of FRC were poor and the delamination defects were more likely to be pro?duced in mechanical processing. Wollbrett-Blitz et al.［10］conducted interlaminar fracture properties test?ing，and found that the lamination defects were formed because of the poor interfacial cohesiveness of materials for AFRC.

Works show that the most common defects on the machined surface generated by conventional ma?chining process are delamination defects which are initiated by stresses applied during engagement of the cutting edge［11］. Delamination defects are strong?ly influenced by fiber content and manufacturing pro?cess of composite part. Therefore，it is necessary to conduct further study on the causes and affect fac?tors of delamination defects during milling of AFRC.

Hocheng et al.［12-13］studied the cutting forces and tool wear on the machinability of composite ma?terials. The effects of the fiber orientation on the quality of machined surface were also investigated.Results showed that the delaminating defects tended to increase with an increase of cutting forces. Be?sides，the propagation of the delamination with in?creasing tool wear is commonly observed during ma?chining composite materials［14-15］. This leads some researchers［16-18］to predict cutting forces and tool wear for studying machining defects.

It can be noticed that investigation on the de?lamination defects of fiber-reinforced plastics，are re?lated to the defects control and cutting forces，as a function of cutting conditions，the distribution of sta?ple fibers in the polymeric matrix，and the angle of inclination of staple fibers［19-20］. The available litera?ture on AFRC is limited even though the demand for AFRC is increasing. Milling is the machining opera?tion most frequently used in manufacturing parts of fiber-reinforced plastics，because components made of composite materials are commonly produced in a net-shape manner，which often requires the removal of excess materials to control tolerances.

Machining of composite materials is based on brittle fracture mechanics theory. Machined surface defects are the most direct parameters to evaluate the cutting process.In the estimation of delamination de?fects，the modeling is critical to the accuracy of as?sessment. It is postulated that the cutting of compos?ites materials should be based on fracture mechanics theory，as chip separation occurs due to fracture rath?er than plastic deformation. However，currently few analytical models are proposed for composite machin?ing and many proposed models for these materials are either empirical or using the same shear plane the?ory as for metals. Among different predictive model?ing techniques used，mechanistic modeling method is the most robust，simple and efficient one.

In this paper，a series of milling experiments are conducted for analyzing the mechanisms of delamina?tion defects.The effects of cutting conditions and the fiber cutting angle are considered.The control strate?gy of different types of delamination defects and eval?uation method of finished surface integrity are dis?cussed. A computing method and model based on brittle fracture for delamination defects are estab?lished. The control strategy of delamination defects and evaluation method of finished surface integrity are further discussed. The established results could contribute to control the defects and improve machin?ing process during milling of AFRC laminates.

1 Experiment

1.1 Experimental procedure

The milling experiments are conducted on a three-axis CNC machine center with a maximum spindle speed of 28 000 r/min. The composite mate?rial used in the tests（epoxy matrix reinforced with 55% of aramid fiber），supplied by Beijing space?craft factory，is produced by autoclave with a fiber orientation of 0/90° as shown in Fig.1（a）. The fixa?tion of the composite material（plate）is made as ob?served in Fig.1（b），to eliminate the vibrations and displacement during experimental procedure. Ma?chined surface is observed by scanning electron mi?croscope（SEM）to clarify the surface defect caused by the cutting process.

Fig.1 Workpiece and experimental setup for the test

A 6 mm four-flute cemented carbide end mill is used for milling tests，and the diameter is manufac?tured according to ISO. The helix angle is 30° and the rake angle is 10° with the clearance angle of 9°and flute length of 10 mm. Down milling process is adopted in this paper. The spindle speed is 8 000 r/min，and the feed speeds are selected to be 0.5，0.75 and 1 mm/r.The axial depth-of-cut is the thick?ness of the workpiece. The radial depth-of-cut is set to be 1 mm.

1.2 Experimental results

Fig.2 shows the AFRC machined surface of side and top planes for machining directions at 0°with different feed speeds.

Fig.2 Delamination defects for AFRC for machining direc?tions at 0°

Fig.3 shows the AFRC machined surface of side and top planes for machining directions at 45°with different feed speeds.

Fig.3 Delamination defects for AFRC for machining direc?tions at 45°

2 Results Analyses and Discussion

The delamination defects can be recognized as the chipping and protruding of fibers. Fig.4 shows the development and propagation of delamination during milling process.

Fig.4 Schematic of the development of delamination

By comparing the experimental results，the presence of delamination defects on the machined edge of the composites materials is categorized.Two types of delamination are observed and sche?matically shown in Fig.5. Type I delamination de?scribes areas where the surface fibers have been bro?ken and removed some distance inward from the ma?chined edge，which is called tearing delamination.Type II delamination consists of uncut fibers that protrude outward from the machined edge，which is recognized as uncut-off delamination.

Fig.5 Types of delamination for machined surface

According to the experimental results，the best surface quality in terms of surface roughness and lowest damage is at the 0° machining direction.While for the side surface of the machined work?piece，the surface in the 45° fiber direction shows good surface quality.

It can be seen that when the cutting direction is in 0°，the fiber directions are in 90° and 0°（perpen?dicular to each other）. The delamination defects are both in the machined surface and in the side surface，as shown in Fig.2. While for the cutting direction in 45°，the fiber directions are in 45° and 135°（perpen?dicular to each other）. The delamination defects are all in machined surface according to Fig.3.

According to Figs.2，3，it also can be seen that，when cutting direction is fixed，feed rate is the key parameter which influences the surface quality in milling of AFRC composite materials. Fig.6 shows the explanation of how feed rate affects the delamination defects.

When feed rate is low，the volume of materials involved in machining per tooth is less，and the fi?bers can be broken easily. Hence，there is minor lamination defects when feed rate is low in the direc?tions of 0°and 45°. The value of feed rate should be selected as low as possible.

Fig.6 Explanation of feed rate effects on delamination defect

3 Prediction Model for Delamina?tion Defects

3.1 Analysis of type I defects

According to experimental results，type I de?fects cause bending fracture in plane，and the fibers yield in feed direction. The type I defects can be ana?lyzed by adopting minimum bending radius theory.The minimal fiber curvature radiusrminis decisive to attain the transverse rupture strain. It depends on the ultimate tensile strainεBand the fiber diameterdfiberas shown in Eq.（1）.

The values ofεBanddfiberare listed in Table 1 for the materials used in this paper.

Table 1 Materials properties for AFRC

When the workpiece is placed in 45°，by using the minimal curvature radius，swerve mechanisms in two perpendicular directions can be modeled.Fig.7 shows schematically how the fibers avoid the cutting tool in the laminate plane for cutting fiber di?rection in 45°and 135°.

As shown in Fig.7，to ensure the fibers bend around the radiusrmin，the fibers must move freely at a depth ofΔ1andΔ2in laminate plane，i.e.，they must be delaminated.Δ1andΔ2can be calculated ac?cording to Fig.7.

Fig.7 Bending of fibers in laminate plane at different fiber directions

3.2 Analysis of type II defects

Fig.8 schematically shows when the workpiece is placed in 0° during the machining process，and how the fibers avoid the cutting tool in the laminate plane and in the vertical plane for cutting fiber direc?tion in 90°and 0°.

As shown in Fig.8，to ensure the fibers bend around the radiusrmin，the fibers must move freely at a depth ofΔ1in laminate plane and at a depth ofΔ2in vertical plane.Δ1can be calculated according to Fig.8 as

Fig.8 Bending of fibers in laminate and vertical planes

As for the fibers in 0°，the fibers are bended in vertical plane which is perpendicular to the laminate plane. It is assumed that the tool moving along the feed path represents a level obstacle for protruding fibers. To ensure the fibers bend at this angle around the respective radiusrmin，they must move freely at a depth ofΔ2from the component edge. Ac?cording to Fig.8，Δ2can be calculated fromφanddfiberas

The lamination defects depth can be obtained according to Eq.（2）and Eq.（3）for fiber direction in 0° and 90°. According to Eq.（4）and Eq.（5），the lamination defects depth can be determinated for fi?ber directions in 45°and 135°.

Fig.9 shows the prediction model and the ex?perimental results.

Fig.9 Comparison between prediction and experimental results

The prediction results of the model agree well with the experimental results. It implies that the es?tablish model can be used to control the defects of delamination during milling of orthogonal AFRC.

4 Conclusions

Delamination defects lead to a significant reduc?tion in fatigue life for composite laminates. The pro?posed model can be used for predict delamination which can avoid serious accidents in the areas of aeronautical and aerospace engineering，and auto?motive industry.

From the developed model and the cutting ex?periments，the following conclusions can be ob?tained：

（1）The delamination growth behavior，charac?terized by arc shape crack，is well predicted via the numerical method by considering the geometric and mechanical properties of AFRC laminates.

（2）Two deflection mechanisms of fibers pro?trusions are distinguished and the mechanism of dif?ferent delamination defects is explained.

（3）The numerical model can predict fiber de?lamination accurately in the milling process. Com?paring the numerical and experimental results，a good agreement within an error of 5% between the numerical predicted and experimental results has been confirmed.

（4）The feed rate has a negative effect on sur?face integrity.The delamination defects become seri?ous when feed rates increase. Hence，for AFRP composites，low feed rate is preferred to get better surface quality.