Evaluation of tensile properties of a compositemetal joint with a novel metal insert design by experimental and numerical methods

Xioqun CHENG,Xioqi LI,Jie ZHANG,Qin ZHANG,Yuji CHENG,Benyin ZENG

aSchool of Aeronautic Science and Engineering,Beihang University,Beijing 100083,China

bAVIC China Helicopter Research and Development Institute,Jingdezhen 333001,China

Evaluation of tensile properties of a compositemetal joint with a novel metal insert design by experimental and numerical methods

Xiaoquan CHENGa,*,Xiaoqi LIa,Jie ZHANGa,Qian ZHANGa,Yujia CHENGa,Benyin ZENGb

aSchool of Aeronautic Science and Engineering,Beihang University,Beijing 100083,China

bAVIC China Helicopter Research and Development Institute,Jingdezhen 333001,China

Available online 8 May 2017

*Corresponding author.

E-mail address:xiaoquan_cheng@buaa.edu.cn(X.CHENG).Peer review under responsibility of Editorial Committee of CJA.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.cja.2017.03.016

1000-9361©2017 Chinese Society of Aeronautics and Astronautics.Production and hosting by Elsevier Ltd.

This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Composite-metal joints with a metal insert are one kind of connecting structure.In this paper,tensile experimental tests were carried out to investigate tensile properties of a compositemetal joint with a novel metal insert design.Finite element models of the joint were established,and strain distribution and tensile strength were analyzed.The numerical results are in good agreement with the experimental results.Results show that the joint failure is dominated by shear properties of the resin layer.Increasing the resin layer thickness in a certain range will improve the tensile strength of the joint,while increasing the radius of the fillet on the ending side of the metal insert will decrease the joint strength.Increasing the resin layer plasticity will improve the joint strength.The effect of the embedded depth of the metal insert can be ignored.

©2017 Chinese Society of Aeronautics and Astronautics.Production and hosting by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Composite structure;

Failure mode;

Joints;

Metal insert;

Tensile properties

1.Introduction

Composite structure joints are mainly divided into two types,namely mechanical joints and bonded joints.Mechanical joints can transfer heavy load and are suitable for concentrated load transmission1–4,while bonded jointsare applicable for distributed load transfer without any hole in adherends.5–9Bonded joints have been widely used in engineering structure connection,including connection of composite structures and connection between composite and metal structures.8–11For general bonded joints,there is only one adhesive interface,and thus the load transmission efficiency is low.Therefore,a novel composite-metal bonded joint is developed to improve the load transmission efficiency by increasing the bonding area between a composite and a metal insert.

Investigations on unconventional joints between a metal plate and a composite laminate have been performed.Dvorak et al.5investigated adhesive tongue-and-groove joints for thick composite laminates and concluded that the joints provide superior strength in bonding a steel plate and a laminate under a tensile load.Melogranaa et al.6investigated adhesive toughand-groove joints between thin composite laminates and steel plates,and found that structural failure is consistent with adhesive failure.The joint strength is 40%higher than that of a conventional single-lap joint with an equivalent overlap length.Riberholt11investigated glued bolts in glulam and found glulam glued-in rods can efficiently improve strength and stiffness of a joint.Llopart et al.12investigated the influence of imperfect bonding on the strength of specific doublelap joints.They indicated that imperfect bonding has a significant negative effect on structure strength but hardly influences stiffness.Camanho et al.13investigated bonded metallic inserts for bolted joints in composite laminates and presented that the introduction of metallic inserts releases the stress concentration around the hole.Cheng et al.14studied thermal behaviors and tensile properties of composite joints with a bonded embedded metal plate.

Although they all investigated unconventional joints between a metal plate and a composite laminate,there has been little research on composite-metal joints with a metal insert.Composite-metal joints with a metal insert have been widely used in aircraft,such as the connecting part between rudders and stabilizers,as well as in marine propellers.A joint can not only efficiently transfer a load but also keep a structure contour;thus investigation on this kind of joints is valuable.

In this paper,tensile properties of a composite-metal joint with a novel metal insert design were investigated by experimental and numerical methods.Failure mechanism and its tensile behavior were analyzed,and three parameters(i.e.,adhesive thickness,geometric dimensions of the metal insert,and structure configuration)were discussed.

2.Experimental test and results

2.1.Materials and specimen manufacturing

Composite laminates were made from T700 unidirectional fabric and Epoxy 6808.They were consisted of 72 plies with layup [-45/0/45/90]9S,and the nominal ply thickness was 0.125 mm.The metal inserts were 45#steel.Three specimens were manufactured,and they were named A1,A2,and A3,respectively.The specimen sketch is shown in Fig.1.

The specimens were manufactured by the Resin Transfer Molding(RTM)process.In the manufacturing procedure,the carbon fiber T700 unidirectional fabric was made into a preform as designed.Then,a steel plate was inserted into a specific location in the preform to construct a new hybrid composite-metal joint.After this,the joint was put into a mold as a whole.Then liquid epoxy resin was injected into the mold through a bump until the resin infiltrated all the carbon fibers.No other adhesive was used between the laminate and the metal insert.In the end,the whole mold was put into an oven,the temperature of which was increased to 200°C in 30 min and then kept for 2 h.After the whole mold naturally cooled down to room temperature,the structure was taken out from the mold,and some modification work was done,such as deburring,grinding,and cleaning.Detailed material properties are shown in Tables 1 and 2.

2.2.Experimental tests

Considering the width of the specimen laminate end was 100 mm,all experimental tests were carried out on an Instron 8803 500-kN uni-axial universal dynamic servohydraulic material testing machine.A load was applied at a constant displacement rate of 0.5 mm/min until the specimens failed at room temperature.The set-up for a specimen is shown in Fig.2.Strain gauges were used to monitor the axial strain values at six points on specimen surfaces,as shown in Fig.3.

2.3.Experimental results and analysis

Experimental results of the three specimens are shown in Table 3,where the initial damage load is the load corresponding to the first sound from the specimens.Following the first sound,sporadic sound was heard and gradually became louder.Hence,the load corresponding to the first sound is considered to be the initial damage load.Fig.4 shows the tensile strain-load curves of all 6 points(Fig.3).Strains at the symmetric points are uniform,so the experimental results are valid.As shown in Fig.4,the strains at Points 1–5 and 1–6 are greater than those at Points 1–2 and 1–3,that is because the stiffness at Points 1–5 and 1–6 is lower than that at Points 1–2 and 1–3,and stress concentration exists at the edge of the metal insert.It should be noted that Points 1–1 and 1–4 possess lower strains,which may be because the load in the resin layer between the laminate and the metal insert is dominated by shear stress.

Table 1 Mechanical properties of T700/6808.

Table 2 Mechanical properties of Epoxy 6808 and 45#steel.

Table 3 Experimental and numerical failure loads of specimens.

From Table 3,the ultimate loads of A1 and A2 are close,and they are obviously higher than that of A3.However,the initial damage loads of all three specimens are almost the same.For each specimen,there is a big gap between the initial damage load and the ultimate load.As can be seen in Fig.4,the slopes of the strain-load curves almost keep constant.That means damage has no effect on local stiffness at these points.

No change was observed on the outsides of the specimens before specimen failure.Obvious cracks and delamination were found at the laminate end close to the metal when the specimens failed,as shown in Fig.5.From the view of the metal insert,the delamination damages on two sides of the laminate end around the insert are almost symmetrical.The metal inserts were pulled out for a short distance and no damage was found in them.

A big gap between the initial damage loads and the ultimate loads exists,so the damage expanding process may be influenced by some factors which can result in differences of the ultimate loads among different specimens.

The inner damage expanding process couldn’t be observed during the experimental tests.Thus,a Finite Element(FE)model was developed to investigate the failure mechanism and behavior.

3.FE model of joint

3.1.Modeling of joint

FE analysis of the composite-metal joint with novel metal insert design was carried out by using commercial FE code ANSYS.Analysis was performed by APD Lcommand streams.Benefited from the symmetric geometry,only a quarter of the joint was modeled in order to save CPU time and obtain a more accurate result.In addition,the gripping part was removed in the FE model for the same reason.The FE model is shown in Fig.6,in which the composite laminate and the metal insert were connected by resin layer.

All parts were modeled using structural solid SOLID45 elements defined by eight nodes with three degrees of freedom per node.Each composite ply and the resin layer were set as one element through thickness.A fine mesh was adopted in the resin layer region in order to increase the accuracy of stress computation and failure simulation;on the contrary,a coarser mesh was used in the areas away from the resin layer in order to reduce the total number of elements.

Since only one-fourth of the joint was modeled,symmetric boundary conditions were applied on the symmetric planes including two symmetric planes perpendicular toz-axis andy-axis,respectively.Fixed constraint was applied on the end of the laminate.A displacement load was applied on the end of the metal insert through setting load steps.

Meshes,loading,and boundary conditions are shown in Fig.6.

3.2.Failure criteria and material property degradation

No damage of the metal insert was observed in the experimental tests;therefore,only the damage and failures of the composite laminates and the resin layer were considered in this paper.

3.2.1.Composite material

The Hashin failure criteria are suitable for composite materials.15–23The failure criteria listed below were employed to predict the damage of the composite laminates.They were proposed by Shokrieh based on the Hashin failure criteria.18,22

Fiber tensile failure(σxx＞ 0):

where σxx, σyy,and σzzare the normal stresses components alongx,y,andzaxes,respectively. σxy, σxz,and σyzare the shear stresses components.XTandXCare the longitudinal tensile and compressive strengths.YTandYCare the transverse tensile and compressive strengths.ZTandZCare the tensile and compressive strengths along thickness.Sxy,Sxz,andSyzare the shear strengths.

As soon as failure occurs in an element of the composite laminate,its stiffness is degraded to a certain value according to the following degradation rule proposed by Tan.22,23The material property degradation rule of the composite is shown in Table 4.

3.2.2.Resin layer

The failures of the resin layer is predicted using the following criterion:

where σr3is the equivalent stress of the resin layer, σ1and σ3are the maximum and minimum principle stresses,respectively,and[σ]is the allowable stress.

In addition,for the resin layer,its stiffness is degraded to zero as soon as failure is predicted to simulate debonding.Considering the limitation of ANSYS,the stiffness after failure was obtained by the original stiffness multiplying a small coefficients:

Table 4 Composite stiffness degradation rule.

By this way,the element of the resin layer cannot carry any load as soon as it fails.Moreover,plasticity of the resin has not been taken into account.

4.Numerical results

4.1.Model validation

Numerical strain-load curves of the points are plotted in Fig.4.As can be seen,the errors of the strain-load curves are very small and acceptable.Numerical results of the failure load are listed in Table 3 along with the corresponding experimental results.The numerical results agree with the experimental test data.Fig.5(b)shows the damage of the composite laminate under the ultimate load.There are three main failure modes,namely fiber tensile failure,matrix tensile failure,and delamination.The cracks and delamination that are externally visible mainly occurred in the composite laminate on the starting side of the embedded part of the metal insert(Fig.1).That is consistent with the experimental results as shown in Fig.5(a).Compared with the experimental results,the FE model is considered to be effective.

4.2.Failure mechanism

Based on the experimental test and Fig.5(b),the laminate fails in only a small-area region under the ultimate load,and structural failure is mainly dominated by resin properties.

Fig.7 shows the von Mises stress distribution of the resin layer under a small load.As can be seen,there is stress concentration in the resin layer on the starting side and Fillet A of the metal insert.

Fig.8 shows the damage expanding process of the resin layer.When the tensile load is applied up to 14.8 kN,initial damage is predicted in the resin layer around the starting side of the embedded part.This load is close to the experimental initial damage loads.This behavior is because there is a sudden change of the specimen’s cross-sectional area around the starting side,so a serious stress concentration occurs in this region.

The stress in this region is released after damage,so the damage doesn’t lead to a serious expansion in the laminate.When the tensile load reaches 36.6 kN,damage occurs in the resin layer at Fillet A of the metal insert.Then,the damage keeps expanding from the two regions along the resin layer.When the load increases to about 44 kN,the damage quickly expands across the whole resin layer and then the joint fails.

According to the stress nephogram,peeling and shear stresses are found in the resin layer around the starting side when the initial damage occurs.As the load increases,the resin layer damage is mainly resulted from the shear stressSxz.Fig.9 shows the shear stressSxznephogram of the resin layer when the damage quickly expands to the whole resin layer.

5.Discussion

As the tensile behavior of the joint is mainly dominated by adhesive properties,only the effects of resin layer parameters were discussed,including resin layer thickness,location,and shape.The resin layer location was characterized by the embedded depth,and the shape was characterized by the fillet radius of the metal insert.

5.1.Effect of resin layer thickness

Five thicknesses ranging from 0.1 to 0.25 mm were considered,and this thickness range is usually used in bonded composite structures.24,25Except the resin layer thickness,the other parameters were kept the same as those in numerical modeling before.The ultimate tensile loads are shown in Table 5.From the table,the ultimate load increases with the thickness increasing.Failures start at the same locations and the damage expanding process is the same as well.

Fig.10 shows the load-displacement curves with different thicknesses.With the thickness increasing,structural stiffness and ultimate tensile loads increase.It is because a thick resin layer can release stress concentration,and thus the initial damage occurs later,the expanding of the damage is slower,and the structural strength increases as well.Therefore,the thicknesses should be designed as large as possible in this range,which can increase the structure strength.

5.2.Effect of embedded depth of metal insert

The embedded depth is shown in Fig.1.Embedded depths from 0 to 25 mm were considered,and the corresponding ultimate tensile loads are shown in Table 6.The ultimate load nearly keeps constant with different depths.The damage expanding process is the same except that the embedded depth is 0 mm.

Fig.11 shows the load-displacement curves with six embedded depths.The initial damage load has no relationship with the embedded depth,but a greater depth causes a longer embedded length on the metal narrow side,a faster resin layer damage expansion around the starting end,and then more metal deformation;thus the slope of the load-displacement curve decreases with an increase of the embedded depth.

Table 5 Ultimate tensile loads of joint with five resin layer thicknesses.

Table 6 Ultimate tensile loads of joint with different embedded depths.

Table 7 Ultimate tensile load of joint with different fillet radii.

5.3.Effect of radius of Fillet A

Fillet radii from 3 to 25 mm were considered,and the corresponding ultimate tensile loads are shown in Table 7.The ultimate load decreases with the fillet radius increasing,and according to the damage expanding process,increasing the fillet radius will lead to an earlier initial damage.The joint strength improves as the fillet radius decreases.

Fig.12 shows the load-displacement curves with five radii of Fillet A.As can be seen,the slopes of the curves are almost the same when the radius is 3,5,and 10 mm,while those when the radius is 15 and 25 mm are almost the same.It means that when the radius is shorter than a certain value,it has no effect on structural stiffness,while the stiffness decreases when the radius is greater than that value.As increasing the fillet radius,the area of the resin layer and the volume of the metal insert decrease(the stiffness of the metal insert is greater than that of other parts),so it will lead to a reduction of structural stiffness,an earlier initial damage,and a lower ultimate load.

5.4.Effect of resin layer properties

The important mechanical parameters of epoxy resin include modulus,strength,plasticity,etc.In engineering practice,however,the difference of modulus is always small among different kinds of epoxy resin(about 3 MPa),and it is obvious that increases of resin strength and plasticity may promote the joint strength.Only the effect of resin plasticity on the joint strength was investigated here.

A horizontal plastic section is added on the linear stressstrain curve of brittle resin as shown in Fig.13.65.6 MPa is selected as the yield stress of the resin,which equals to that of epoxy resin 6808.The modulus and Poisson ratio of the linear segment remain unchanged with epoxy resin 6808.

Table 8 Ultimate loads of joint with different epoxy resin plasticity.

Correspondingly,choose maximum equivalent plastic strain criterion as the resin failure criterion:

The strengths of the joint with different epoxy resin plasticity are listed in Table 8.

As shown in Table 8,with an increase of the resin plasticity,the joint ultimate load increases obviously.However,according to the FEA result,the failure mode of the joint is not changed in the above case,because the position of the stress concentration is not changed while the plasticity reduces the stress concentration.

6.Conclusions

In this paper,tensile properties of a composite-metal joint with a novel metal insert design were investigated by experimental tests.A relevant numerical model was built and validated by experimental results.The failure mechanism of the joint was analyzed and the effects of the resin layer thickness,embedded depth,and fillet radius were discussed.Some conclusions are obtained as follows:

(1)Because of the free boundary effect and a sudden change in stiffness,resin layer failure and laminate local damage occur on the starting side of the embedded part of the metal insert.Next,damage of the resin layer expands inside with large-area delamination,and then resin layer damage occurs on the ending side and expands along the bondline.When resin layer delamination reaches a certain degree,the laminate close to the starting side is damaged and finally the joint fails.There exists a big gap between the initial damage loads and the ultimate loads,which means that the joint can still carry loads after the initial damage occurs.

(2)The joint failure is dominated by shear properties of the resin layer,and thus improvement of the resin layer shear properties can enhance the tensile properties of the joint.

(3)The adhesive thickness between the metal insert and the composite laminate has an effect on the tensile strength of the joint.In the range of 0.10–0.25 mm,a greater thickness will contribute to a higher tensile strength of the joint.The failure strength of the joint should improve with an increase of the resin layer plasticity.

(4)An increase of the fillet radius on the ending side will reduce the tensile strength of the joint.The ultimate load is hardly influenced by the embedded depth,but the slope of the joint load-displacement curve decreases with an increase of the embedded depth.

Acknowledgements

We would like to thank the National Natural Science Foundation of China(No.11472024)for f i nancial support.

1.Luo C,Xiong JJ.Static pull and push bending properties of RTM-made TWF composite Tee-joints.Chinese J Aeronaut2012;25(2):198–207.

2.Cheng X,Xiong JJ,Peng B,Cheng ZL,Li HY.Mechanical properties of RTM-made composite cross-joints.Chinese J Aeronaut2009;22(2):211–7.

3.Lessard LB,Shokrieh MM.Two-dimensional modeling of composite pinned-joint failure.J Compos Mater1995;29(5):671–97.

4.Shokrieh MM,Lessard LB.Effects of material nonlinearity on the three-dimensional stress state of pin-loaded composite laminates.J Compos Mater1996;30(7):839–61.

5.Dvorak GJ,Zhang J,Canyurt O.Adhesive tongue-and-groove joints for thick composite laminates.Compos Sci Technol2001;61(8):1123–42.

6.Melogranaa JD,Grenestedt JL,Maroun WJ.Adhesive tongueand-groove joints between thin carbon fiber laminates and steel.Compos Part A2003;34(2):119–24.

7.Mouritz AP,Gellert E,Burchill P,Challis K.Review of advanced composite structures for naval ships and submarines.Compos Struct2001;53(1):21–41.

8.Liu SF,Cheng XQ,Zhang Q,Zhang J,Bao JW,Guo X.An investigation of hygrothermal effects on adhesive materials and double lap shear joints of CFRP composite laminates.Compos Part B Eng2016;91:431–40.

9.Wang SW,Cheng XQ,Guo X,Li X,Chen G.Influence of lateral displacement of the grip on single lap composite-to-aluminum bolted joints.Exp Mech2016;56(3):407–17.

10.Ko¨erwien T.Principles of the vacuum assisted process and its application for aerospace components.Proceedings of the 3rd international symposium on composites manufacturing;2006 May 17–18;Marknesse.Cologne:German Aerospace Center;2006.

11.Riberholt H.Glued steel bolts for glulam.Build Res Pract1980;8(3):146.

12.Llopart LP,Tserpes KI,Labeas GN.Experimental and numerical in vestigation of the influence of imperfect bonding on the strength of NCF double-lap shear joints.Compos Struct2010;92(7):1673–82.

13.Camanho PP,Matthews FL.Bonded metallic inserts for bolted joints in composite laminates.Proc Mech Inst Eng Part L J Mater Des Appl2000;33–40.

14.Cheng XQ,Yang MM,Zhang Q,Zhang J.Thermal behavior and tensile properties of composite joints with bonded embedded metal plate under thermal circumstance.Compos Part BEng2016;99:340–7.

15.Hashin Z,Rotem A.A fatigue failure criterion for fiber reinforced materials.J Compos Mater1973;7(4):448–64.

16.Chang FK,Chang KY.A progressive damage model for laminated composites containing stress concentrations.J Compos Mater1987;21(9):834–55.

17.Chang KY,Liu S,Chang FK.Damage tolerance of laminated composites containing an open hole and subjected to tensile loadings.J Compos Mater1991;25(3):274–301.

18.Fan JF,Cheng XQ,Wang SW,Guo X,Zhang T.Experimental and numerical investigation of composite bolted π-joint subjected to bending load.Compos Part B Eng2015;78(5):324–30.

19.Chen WH,Lee SS.Numerical and experimental failure analysis of composite laminates with bolted joints under bending loads.J Compos Mater1995;29(1):15–36.

20.Oh JH,Kim YG,Lee DG.Optimum bolted joints for hybrid composite materials.Compos Struct1997;38(1):329–41.

21.Chen WH,Lee SS,Yeh JT.Three-dimensional contact stress analysis of a composite laminate with bolted joint.Compos Struct1995;30(3):287–97.

22.Tserpesa KI,Labeasb G,Papanikosb P,Kermanidis T.Strength prediction of bolted joints in graphite/epoxy composite laminates.Compos Part B Eng2002;33(7):521–9.

23.Tan SC.A progressive failure model for composites laminates containing openings.J Compos Mater1991;25(5):556–77.

24.Cheng XQ,Baig Y,Hu RW,Gao YJ,Zhang JK.Study of tensile failure mechanisms in scarf repaired CFRP laminates.Int J Adhes Adhes2013;41(1):177–85.

25.Yu M,Xu XW.Study of the compression strength of scarf patch repaired composite structures.J China Univ Mining Technol2008;37(5):709–14.

15 April 2016;revised 12 July 2016;accepted 13 December 2016