Materials and Design 142 (2018) 297–307

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Materials and Design

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Fatigue degradation after salt spray ageing of electromagnetically rivetedjoints for CFRP/Al hybrid structure

Hao Jiang a, Yanjun Cong a, Xu Zhang b,a, Guangyao Li a,c, Junjia Cui a,c,⁎a State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, Chinab College of Automotive and Mechanical Engineering, Changsha University of Science and Technology, Changsha 410114, Chinac Joint Center for Intelligent New Energy Vehicle, Shanghai 200092, China

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• The shear properties after salt sprayageing of CFRP/Al electromagneticriveted joints were investigated.

• The relationship between fatigue be-haviors and ageing time for CFRP/Alelectromagnetic riveted joints wereestablished.

• The fatigue properties degradation lawsafter salt spray ageing of CFRP/Al elec-tromagnetic riveted joints were ob-tained.

• The fatigue fracture appearances aftershort-time and long-time ageing wererespectively analyzed.

⁎ Corresponding author.E-mail address: cuijunjia@hnu.edu.cn (J. Cui).

https://doi.org/10.1016/j.matdes.2018.01.0470264-1275/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 December 2017Received in revised form 22 January 2018Accepted 23 January 2018Available online 3 February 2018

The durability of hybrid carbon fiber reinforced plastics (CFRP)/aluminum alloy (Al) structures is receiving in-creasing attentions in engineering applications. In this paper, the mechanical performance of CFRP/Al electro-magnetically riveted lap joints after exposure in neutral salt spray environment for various ageing time wasstudied. The shear and fatigue tests were conducted. The mechanical property degradation laws and failuremode evolution were obtained. The results showed that the shear and fatigue properties of the CFRP/Al electro-magnetically riveted lap joints linearly decreasedwith the increase of ageing time. In addition, the fatigue test re-sults showed that threeweekswas the critical ageing time for failuremode changing. Fatigue crack initiated fromthe riveted hole (below 3weeks) and the edge of the overlapping region (over 3 weeks) of Al sheet, respectively.Meanwhile, the fracture analysis showed that the cracks of specimens after long-time ageing (over 3 weeks)were initiated from the several corrosion pits simultaneously.

© 2018 Elsevier Ltd. All rights reserved.

Keywords:Electromagnetic rivetingCarbon fiber reinforced plasticsSalt-spray ageingFatigue behavior

1. Introduction

Hybrid carbon fiber reinforced plastics (CFRP)/aluminum alloy (Al)structures are increasingly used in several engineering applications(e.g. construction, aircraft and automotive fields) attributable to the

advantages of lightweight, fuel saving and excellent performance. How-ever, such dissimilar materials (composites and metal) can be onlyjoined by riveting [1], bolting [2] and adhesive bonding techniques [3].As concerns the automatic assembly in manufacturing field, rivetingtechniques have more efficient and simple process comparing withbolting and adhesive bonding [4]. Furthermore, according to the variousriveting power source, the riveting methods could be mainly dividedinto pneumatic, squeeze and electromagnetic riveting [5]. Among

298 H. Jiang et al. / Materials and Design 142 (2018) 297–307

them, electromagnetic riveting (EMR) technique as an advanced joiningmethod is more suitable for riveting composites because of little holeexpansion damage [6], which has broad application prospect in auto-motive and aerospace industries.

In particular, exhaustive researches have been conducted onmacro-scopic performance and microscopic characteristics of electromagneti-cally riveted joints [7–11]. For instance, macroscopic performance (e.g.shear and fatigue properties) of electromagnetically riveted joints wasstudied by Li et al. [7]. The results showed that joining performance ofelectromagnetically riveted joints was obviously better than conven-tional pressure riveted joints. The effect of process parameters forEMR technique on fatigue behavior was investigated by Jiang et al. [8].Correspondingly, the optimum process parameters and three typical fa-tigue failure mechanisms were obtained. In addition, the micro-characteristic researches mainly focused on adiabatic shear band(ASB) in rivet driven head. It was produced during EMR processregarded as high strain rate forming (over 102 s−1). Microstructure inthis zone usually generates severe plastic deformation which makes iteasier to crack [9]. Thus, a new rivet die which could prevent crack for-mation along the ASB direction was proposed by Reinhal et al. [10].Moreover, Deng et al. [11] found that ASBsweremore andmore obviousas discharge voltage increased and the main microstructure evolutionmechanism inside ASBs was twining deformation for Ti Grade 1 rivet.The microstructure deformation mechanism of 2A10 aluminum alloyrivet under electromagnetic impacting was studied by Zhang et al.[12], it was found that the most deformations were focused on theASBs, and rotated sub-grains inside ASBs would translate into dynamicrecrystallization grains under high strain rate.

These aforementioned works have mainly addressed electromag-netically riveted structure under normal circumstances. However, asfor long-termapplication, the joint usually employed in complicated en-vironmental conditions and would bear the corrosion attack. Especiallyfor the dissimilar material joining joints, there were potential differ-ences between them, which would aggravate ageing effect [13–16].Therefore, it has great significance to evaluate the endurance of thejoints in a severe environment. Recently, Teixeira de Freitas et al. [17]studied peeling resistance of composite/metal bonded joints after expo-sure in salt spray conditions and analyzed the typical fracture appear-ances after peeling tests. Calabrese et al. [18] and Fiore et al. [19]respectively researched the durability of aluminum/steel clinched jointsand composite/Al riveted joints in a salt spray environment and ob-tained their shear properties degradation laws. Calabrese et al. [20]also investigated the shear performance of steel/Al self-pierced jointsin salt spray conditions and obtained the failure mechanisms. Notethat the above-mentioned works mainly focused on the joining

Table 1Properties of the Al 2A10 rivet and Al 5182 T6 sheet materials.

Properties 2A10 aluminum alloy rivet 5182 aluminum alloy sheet

Density(g/cm3)

2.7 2.8

Yield strength(MPa)

250 150

Tensilestrength(MPa)

400 225

Tensilemodulus(GPa)

73.8 70

Poisson ratio 0.33 0.33Hardness (HV) 102 75Chemicalcomposition(%)

Si = 0.25, Fe = 0.2, Cu =3.9–4.5, Mn = 0.3–0.5, Mg =0.15–0.30, Zn = 0.1, Ti = 0.15,Al = balance

Zn = 0.25, Cr = 0.10, Si =0.20, Fe = 0–0.35, Mn =0.20–0.50, Mg = 4.0–5.0, Ti =0.1, Cu = 0.15, Al = balance

methods such as adhesive bonding [17,21–23], clinching [18], conven-tional riveting [19] and self-pieced riveting [20,24] other than EMR. Inaddition, previous works primarily studied the peel and shear propertydegradation in a salt spray environment. While fatigue performancewas more concerned in the automotive field and fatigue degradationbehavior of CFRP/Al riveted joints has not been investigated so far, espe-cially produced by electromagnetic riveting. The resultswere desired bythe engineering applications.

In thiswork, the purpose is to explore themechanical properties (in-cluding shear and fatigue) degradation behavior of CFRP/Al electromag-netically riveted lap joints after salt spray test. Firstly, according to theASTM B117 standard, the CFRP/Al electromagnetic riveted specimenswere exposed in neutral salt spray environment for 7 weeks. Subse-quently, shear and fatigue tests were carried out to obtain the mechan-ical properties of the specimens after various ageing time (1, 3, 5 and 7weeks). Finally, typical fatigue failure fractures were observed.

2. Experimental materials and methods

2.1. Sample preparation

In this paper, single lap riveted specimensweremade by Al 5182 T6/CFRP sheets and Al 2A10 rivets. The CFRP sheets were made of TorayT300 unidirectional carbon fiber (thickness of 0.15 mm per layer) andepoxy resin in an autoclave. The curing cycle was composed of threestages. The first stage heated up to 60° and then kept for half an hour.The second stage heated up to 90° and then kept for half an hour. Thethird stage heated up to 130° and then kept for about 2 h. The resin vol-ume fraction was about 40%. The fiber composite laminate was covered17 plies (ply orientation of 0°/90°) with a thickness of 2.5 mm. Tables 1and 2 present the properties of Al alloy and CFRPmaterials, respectively.

Fig. 1 shows the geometry of the riveted specimen. The thicknessesof riveted CFRP and Al sheets were 2.5 mm and 1.8 mm, respectively.The length of the overlap area was 40mm. The CFRP sheet was put onthe side of manufactured head to prevent crushing damage. All samplesassembled gaskets which had the same thickness of the sheets to avoidbending of specimen in tensile loading. The diameter and length of rivetshaft were 5.0 mm and 11.0 mm, respectively. The hole diameter ofsheets was fixed to 5.1 mm.

To obtain an accurate process dimension, the Al sheets were cuttingby Wire Cut Electrical Discharge Machining (WCEDM) and the CFRPsheets were drilled on a drilling machine with feed rate of 1.5 mm/min and spindle speed of 1500 RPM. The machined specimens andrivets were cleaned by ultrasonic cleaning machine using the alcoholbefore riveting.

2.2. Electromagnetic riveting

The electromagnetic riveting (EMR) experiments were conductedby an electromagnetic forming (EMF)machine (PS 48-16) and a rivetedmould. This EMF equipment has amaximumdischarge energy (E) of 48kJ, a maximum discharge voltage of 16 kV and a maximum capacitanceof 408 μF. The schematic and equipment of electromagnetic riveting arepresented in Fig. 2 (a) and (b). At the beginning, the pre-storage energy(E) stores in the capacitors. When closing the switch, a high-amplitudealternating pulse current is instantaneously generated through the cop-per coil. This induces a powerful alternating magnetic field which fur-ther results in a reversed eddy current in the driver plate (copper with

Table 2Properties of CFRP sheets.

Fiberdensity(g/cm3)

Fibermodulus(GPa)

Fiberstrength(MPa)

Tensilestrength(MPa)

Tensilemodulus(GPa)

Plythickness(mm)

Resincontent(%)

1.76 230 3530 911 76 0.15 40

Fig. 1. Specimen geometry for shear and fatigue tests.

299H. Jiang et al. / Materials and Design 142 (2018) 297–307

high conductivity). And then the induced eddy current produces a re-versed electromagnetic field. Consequently, a powerful repulsive forceis generated by the two magnetic fields and drives the punch to strikethe rivet. It can thus be seen that EMR technique has advantages ofhigh-speed loading, larger impact force and high efficiency.

The rivet forming process is depicted in Fig. 2 (c). The rivet shaft isupset as an oblate driven head. So the sheets can be locked by the drivenhead andmanufacturedhead. Driven head dimension is a significant pa-rameter formechanical properties of riveted joints. Particularly, accord-ing to the previous studies [8,25], the joint with dimensionless

Fig. 2. Schematic and equipment of the electromagnetic riveting: (a) the EMR

parameter D/D0 (D and D0 are the diameter of rivet driven head andshaft, respectively) of 1.5 had a relatively superior fatigue performance.Therefore, the discharge energy (E) was correspondingly set as 5.0 kJ inthis work.

2.3. Salt spray test

The CFRP/Al electromagnetically riveted lap joints were exposedunder neutral salt spray environment condition (5% NaCl solutions, pHof the solution maintaining between 6.5 and 7.2) at a temperature of

schematic, (b) the experiment equipment, and (c) the forming process.

Fig. 3. The shear failure processes of the specimens: (a) unaged specimen, (b) aged specimen.

300 H. Jiang et al. / Materials and Design 142 (2018) 297–307

35 °C in a salt spray chamber (Q-FOG/CCT1100). According to the ASTMB117 standard, the specimens placed with an inclination of 15°–30° tothe horizontal in the salt spray chamber. Fifteen samples were periodi-cally removed from the salt spray chamber at the ageing cycles of 1, 3, 5,

0 2 4 6 8 10 12 140

1

2

3

4

5

6

7

Plastic zone

)N

k(da

olrae

hS

Displacement (mm)

0 weeks

1 week

3 weeks

5 weeks

7 weeks

Elastic zone

Fig. 4. Typical shear load-displacement curves of the specimenswith different ageing time.

7 weeks. These removed samples were immediately cleaned, dried andpreserve in sealed dry storage box with silica-gel desiccant to avoid fur-ther corrosion during storage. In particular, three of themwere used forshear testing, and the remaining twelve were used for fatigue testing.

2.4. Property tests and fracture observations

The shear testswere conductedwith Instron 5985 universal testma-chine equippedwith a 150 kN load cell. The cross-head ratewas set to 2mm/min. Three quasi-static shear specimenswere tested at each ageingtime. The fatigue tests were conducted with Instron 8801 Electro-hydraulic servo fatigue machine. The specimens were tested under aload of sinusoidal wave shape. The stress ratio (minimum stress/maxi-mum stress) was set as 0.1 and the frequencywas set as 20 Hz. Four dif-ferent maximal cyclic stresses (185.5 MPa, 172.2 MPa, 159.0 MPa and145.7 MPa) were employed. In addition, three specimens at each stresslevels were tested for better comparisons of fatigue properties. Ruptureappearance after fatigue tests was observed by a scanning electron mi-croscope (JSM-6360LV).

3. Results and discussion

3.1. Shear load evolution

Fig. 3 shows the typical shear failure processes of unaged and agedspecimens, respectively. At displacement of 0–3 mm, the rivets weretilted under shear load, and the Al sheet of aged specimens bent. The

0 1 2 3 4 5 6 7

4.5

5.0

5.5

6.0

6.5 Maximum shear load

Shear load reduction

)N

k(da

olrae

hsm

umi

xaM

Ageing time (weeks)

0.0

0.1

0.2

0.3

SLRaged=-0.254T+6.308

noi

tcu

der

daol

rae

hS

MSLaged=-0.0385T+0.0458

Fig. 5.Maximum loads and load reduction evolution of the specimens at increasing ageing time.

301H. Jiang et al. / Materials and Design 142 (2018) 297–307

bending phenomenon was caused by the asymmetric construction ofthe sheets in the lapping zone. At displacement of 3–6 mm, CFRPsheet occurred progressive compressive collapse around the hole. Thedominating failure mode of unaged specimens was fiber broken, whilethat of aged specimens was delamination. This demonstrated that thecarbon fiber/epoxy matrix interface bonding strength decreased in saltfog environment. The reasonwas that the chloride ions (Cl−1)would in-vade into CFRP with the water molecules, and then injure the carbonfiber/epoxy matrix interface in the salt fog environment [26]. Mean-while, Al sheet of unaged specimen was bent. At displacement of6–11 mm, the damage degree of CFRP sheet gradually became severe

Fig. 6. The shear failure appearance of the ri

as the test process continued, until the rivet separated from the Alsheet. In addition, it could be seen that the Al sheet of aged specimensbent early. This indicated that the Al sheet of aged specimens deformedunder the smaller shear force, which further implied that the stiffness ofAl sheets decreased markedly after long time ageing.

Fig. 4 shows the shear load-displacement curves of the specimens atincreasing ageing time (ageing time of 0 weeks represents unaged spec-imens). It could be observed that all the curves could be divided intotwo significant zones: the elastic zone and plastic zone. The elasticzone was characterized by a linear increase of the curves. The displace-ment rangewas around 0–0.8 mm. In this zone, the shear load increased

veted joints with different ageing time.

105

106

140

150

160

170

180

190

200

Ageing time Fitting formula

0 weeks SM=865.6Nf -0.1263

1 week SM=887.1Nf -0.1306

3 weeks SM=669.3Nf -0.1112

5 weeks SM=612.2Nf -0.1060

7 weeks SM=670.7Nf -0.1171

)aP

M(sserts

cilcyc

mu

mixa

M

Fatigue life (cycles)

Fig. 7. The fatigue results and related fitting curves of the joints with different ageing time.

302 H. Jiang et al. / Materials and Design 142 (2018) 297–307

linearly with the displacement. The shear load was contributed by twofactors: the former was the shear resistance caused by the extrusionpressure between hole wall and rivet; the latter was the friction resis-tance due to the contact pressure between CFRP and Al sheets.

The plastic zone was characterized by a reduction of curves slope.The displacement range was around 0.8–13 mm. The curves first in-creased to the peak and then decreased. In this zone, two sub-stepscould be obviously observed. At the first sub-step, the shear load in-creased to the peak. This phenomenon was caused by the progressivecompressive collapse of CFRP sheet around the hole referred as bearingfailure mechanism. Specifically, it could be observed from the Fig. 3(a) that the rivet extruded CFRP sheet gradually and the CFRP sheetaround the hole damaged progressively from the displacement of 3mm to 6 mm. Meanwhile, it could be seen from Fig. 3 that the Al sheetbent during the shear test. This resulted in a gradual decrease of the

Table 3The Weibull parameters results for four stress levels with different ageing time.

Ageingtime

Maximal cyclicstress (MPa)

Shapeparameter β

Scaleparameter α

Mean fatigue life(cycles)

0 weeks 185.5 4.71 213,345 195,209172.2 11.36 401,812 384,248159.0 8.33 765,612 722,410145.7 5.75 1,251,431 1,158,225

1 week 185.5 7.63 164,958 154,986172.2 10.84 317,733 303,282159.0 5.89 626,452 580,574145.7 9.44 929,049 881,654

3 weeks 185.5 4.12 113,486 103,037172.2 6.56 223,364 208,228159.0 7.33 511,918 480,003145.7 9.61 780,517 741,278

5 weeks 185.5 3.62 89,045 80,263172.2 5.07 192,340 176,742159.0 3.11 408,685 365,543145.7 6.58 691,337 644,596

7 weeks 185.5 4.75 66,161 60,566172.2 6.88 115,463 107,910159.0 4.01 271,314 245,954145.7 9.97 427,588 406,734

friction resistance between the sheets. Therefore, the shear load wasmainly provided by the bend resistance offered by Al sheet and theshear resistance offered by the rivet and sheets. The little load dropcould be seen occasionally, due to local broken and collapse of the car-bon fiber around the CFRP sheet hole.

At the second sub-step, the shear load decreased from the peak tothe end. This illustrated that the residual resistance of the joint beganto decrease. Specifically, it could be found from the Fig. 3 (a) that thehole of CFRP sheet further expanded and the rivet gradually pulled outfrom the sheets (a slow unbuttoning phenomenon) at the displacementof 11mm. This implied that the joint was severely damaged and its me-chanical stability was compromised. It was also the reason that theshear load progressively dropped rather than suddenly.

In general, it could be seen fromFig. 4 that the trends of all the curvesvaried consistent, especially overlap ratio was quite high in the elasticzone. This indicated that ageing time had a slight effect on the elasticproperties of the electromagnetically riveted joints. In addition, theslope of the curves showed that the stiffness of the joints had not de-creased with the ageing time increased. However, there was a great dif-ference in the plastic zone, the load capacity decreased with the ageingtime. This was mainly related to the performance degradation of CFRPsheet, due to the compressive collapse of CFRP sheet around the hole be-came weaken, so as to cause shear resistance down with the increasingof ageing time. Specifically, the chloride ions (Cl−1) would damage thecarbon fiber, the epoxy matrix and their interface [28]. In addition, ac-cording to Alessi et al. [29], water being absorbed into CFRP sheetwould cause some side effects, such as swelling and cracking due to os-mosis, plasticization of the epoxy matrix and degumming damage ofcarbon fiber/epoxy matrix interface.

In order to better understand the degradation law of mechanicalproperties of the electromagnetically riveted joints after ageing test,the maximum shear load and reduction proportion evolution were ob-tained as shown in Fig. 5. The shear load reduction (SLR) was calculatedby following Eq. (1):

SLR ¼ P0−PX

P0ð1Þ

where PX and P0 are the maximum shear load of the specimen after

0 1 2 3 4 5 6 70.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

Stress levels Related formula

185.5 MPa MFLaged=-18646T+178482

172.2 MPa MFLaged=-36898T+354156

159.0 MPa MFLaged=-63791T+683030

145.7 MPa MFLaged=-94157T+1067800

)selcyc(

efile

ugitaf

naeM

Ageing time (weeks)

(a)

0 1 2 3 4 5 6 7

0.0

0.2

0.4

0.6

0.8(b) Stress levels Related formula

185.5 MPa FLRaged=-0.0955T+0.0856

172.2 MPa FLRaged=-0.0960T+0.0784

159.0 MPa FLRaged=-0.0882T+0.0546

145.7 MPa FLRaged=-0.0812T+0.0781noitc

uder

efile

ugita

F

Ageing time (weeks)

Fig. 8. Fatigue performance evolution of the joints at increasing ageing time: (a) mean fatigue life, and (b) fatigue life reduction.

303H. Jiang et al. / Materials and Design 142 (2018) 297–307

ageingX and0 weeks. It could be observed that after ageing 1, 3, 5 and 7weeks lost a maximum load of about 13.5%, 15.6%, 25.6% and 29.7%, re-spectively. The maximum load of the joints nearly decreased linearlywith the increasing of ageing time. Consequently, the maximum shearload (MSL) and load reduction evolution were fitted by Eq. (2) andEq. (3), respectively:

MSLaged ¼ c1T þ d1 ð2Þ

SLRaged ¼ c2T þ d2 ð3Þ

where T represents the ageing time; c and d are the coefficients. Thefitting results are depicted in Fig. 5, which can provide a design refer-ence for the CFRP/Al electromagnetic riveted joints in the serious envi-ronmental conditions.

3.2. Shear failure behavior evolution

The salt in the surfaces of the sheets was cleaned using the brushafter shear tests. The shear failure appearance of the riveted jointswith different ageing time is shown in Fig. 6. The driven head side re-ferred to the contact surface of CFRP and Al sheets, while themanufactured head side referred to the non-contact surface. It couldbe observed that all the specimens were nearly failed with the samemode: the hole of CFRP occurred severe compressive collapse, Al sheetwas bent and the rivet locked in the CFRP sheet. This was a typical bear-ing failure mode, which was gradual and not sudden catastrophic fail-ure. Thus, bearing was usually the favored failure mode for the jointsendured high loading in order to avoid a sudden fall of the load capabil-ity [30].

In addition, it could be obviously seen that a large amount of whiterust was formed, and the erosion of the driven head side was muchmore serious than that of the manufactured head side for all the ageingtime. Moreover, on the driven head side, corrosion pits were formed onthe surface of Al sheets, especially around the edge of the overlappingregion. This was caused by the galvanic and crevice corrosion effects[31]. Specifically, the CFRP/Al sheets and CFRP sheet/Al rivet formedelectrochemical couple respectively, so as to activate galvanic corrosionphenomenon. Among them, CFRP acted as cathode, while Al acted asanodic. Thus, the Al sheet would induce anodic dissolution. Besides,the rate of galvanic corrosion in salt spray condition was directly influ-enced by the oxygen content [19]. The edge of the overlapping regionusually existed a relatively larger crevice, which was easier to contactwith air. Thus, these regions produced the corrosion pits more quickly.

In addition, no pits were found around the hole of Al sheets, indicatingthat the sealing of electromagnetically riveted joints was excellent.

3.3. Fatigue life evolution

In order to better compare the fatigue lifewith different ageing time,power function denoted as Eq. (4) are employed to fit the fatigue testdata [32,33]:

SM ¼ a N f� �b ð4Þ

where SM represents themaximal cyclic stress, Nf represents fatigue lifeand a and b are coefficients. The fatigue results and related fitting curves(S-N curves) of the joints with different ageing time are depicted inFig. 7. It could be observed that the related curves fitted well with theexperimental results. Specific correlation coefficients were shown inFig. 7, which could provide design references for practical engineeringapplications. In addition, it could be found from the curves that the fa-tigue life decreasedwith the increase of ageing time. But the detailed fa-tigue degradation laws were difficult to obtain from this figure.

The mean fatigue life was the optimal parameter for comparing thefatigue properties with different ageing time and the fatigue life ofriveted structures generally obeyed Weibull distribution. Thus, two-parameter Weibull distribution function was used to handle fatiguedata statistically due to its accurate life analysis and suitable for smallsamples [34–37]. The probability density function f(t) are denoted as:

f tð Þ ¼ βα

tα

� �β−1

exp −tα

� �β" #

ð5Þ

whereα is the scale parameter,β is the shape parameter and t is the ran-dom variable value. After obtaining the scale parameter α and shape pa-rameter β through themethods as described in previous studies [38,39],the mean fatigue life can be obtained using Eq. (6).

E Tð Þ ¼Z þ∞

0tf tð Þdt ¼ αΓ 1þ 1

β

� �ð6Þ

where Г is the gamma function [40].Table 3 shows the Weibull parameters results for four stress levels

with different ageing time. The fatigue life reduction (FLR) was defined

Table 4The fatigue failure appearance of the joints with different ageing time.

Ageing time Stress level 145.7 MPa Stress level 159.0 MPa Stress level 172.2 MPa Stress level 185.5 MPa

0 weeks

1 week

3 weeks

5 weeks

7 weeks

304 H. Jiang et al. / Materials and Design 142 (2018) 297–307

as:

FLR ¼ N f0−Nfx

N f0ð7Þ

where Nfx and Nf0 are the mean fatigue life at a certain stress level afterageing X and 0 weeks. The mean fatigue life and life reduction propor-tion evolution are shown in Fig. 8 (a) and (b), respectively. Themean fa-tigue life (MFL) and life reduction evolution were fitted by Eq. (8) and

Eq. (9), respectively:

MFLaged ¼ e1T þ f 1 ð8Þ

FLRaged ¼ e2T þ f 2 ð9Þ

where T represents the ageing time; e and f are the coefficients. It couldbe seen that the decline trends of fatigue lifewith four stress levels weresimilar at increasing ageing time. Taken the stress level 145.7 MPa as anexample, the specimens after 1, 3, 5 and 7 weeks respectively reduced

Fig. 9.Macroscopic and microscopic images of the typical fatigue fracture appearance for the specimens after short-time ageing (below 3weeks).

305H. Jiang et al. / Materials and Design 142 (2018) 297–307

fatigue life of 23.9%, 36.0%, 44.3% and 64.9% comparing with unagedspecimens. In general, the decrease amplitudes of fatigue propertiesfor CFRP/Al electromagnetic riveted joints were much higher than thatof shear properties after salt spray ageing tests.

3.4. Fatigue failure behavior evolution

The ageing time affected not only the fatigue life but also the failuremodes. Table 4 shows the fatigue failure appearance of the joints withdifferent ageing time. It could be seen that all the specimens rupturedat the Al sheet under fatigue cyclic loading, but the failure locations inAl sheets were different. Specifically, the specimens without ageingbroke from the riveted hole to the edge of Al sheet under all stress levelsand the fracture direction was perpendicular to the load direction. Thefracture mode of specimens after ageing 1 week was similar with thespecimens without ageing other than the warping of the Al sheets.This was caused by stiffness degradation of Al sheet after salt fog ageing.The specimens after ageing 5 weeks and 7 weeks broke from one side ofthe Al sheet to the other under all stress levels.Moreover, it could be ob-served that some of the fracture paths were ragged. This was caused bythe irregular distribution of corrosion pits around the fracture paths. Be-cause these pits would change the direction of cracks propagation. Thespecimens after ageing 3 weeksmixed of two fracturemodes. This illus-trated that 3 weeks was the critical ageing time for different fatigue fail-ure modes. The phenomenon was caused by the crack initiation siteschanging. Below 3 weeks, fatigue crack started from the hole of Alsheet and this locationwas themost vulnerable part in the CFRP/Al elec-tromagnetic riveted joint. Over 3 weeks, the fatigue crack started fromthe edge of the overlapping region on the Al sheet, demonstrating thatthis part became the most vulnerable part. This was due to severe

corrosion attack in these regions, caused by the crevice corrosion andgalvanic corrosion as discussed above. In addition, this further demon-strated that the corrosion resistance around riveted hole wasmuch bet-ter than that around the edge of the overlapping region. The resultswere consistent with the observation in Fig. 6.

The fatigue fracture appearance of the specimen without ageing hasbeen investigated by our previous study [39]. Fig. 9 shows macroscopicand microscopic images of the typical fatigue fracture appearance forthe specimens after short-time ageing (below 3 weeks). There werethree typical zones. Specifically, the radial pattern fringes in the zone 1revealed that this regionwas the typical crack initiation zone.Moreover,manymicrocrackswere observed in the zone 4 (the holewall). This fur-ther proved that the fatigue cracks initiated from the riveted hole. Thefatigue lines like shelly pattern (in the zone 2) and striations (in thezone 5) were found. This suggested that the region was the typicalcrack propagation zone. Moreover, the cracks always propagatedalong normal direction of the fatigue lines as depicted in zone 2 (thedash line arrow). The dimples in the zone 3 implied that this regionwas the final fracture zone. Therefore, it could be generalized that thecracks of specimenswith short-time ageing under fatigue loading firstlystarted from the hole wall of Al sheet. Subsequently, the cracks propa-gated from that place to both sides of the Al sheet until one side brokeup. This result was nearly the samewith the fatigue fracture of the spec-imens without ageing [39].

Fig. 10 shows macroscopic and microscopic images of the typical fa-tigue fracture appearance for the specimens after long-time ageing(over 3 weeks). In the zone 1 and 2, the corrosion pits were observedat the bottom of Al sheet (the side of contacting CFRP sheet). Thesmall regional dimples were observed at the opposite of the pits. Thedimples usually represented the ductile fracture and located at the

Fig. 10. Macroscopic and microscopic images of the typical fatigue fracture appearance for the specimens after long-time ageing (over 3 weeks).

306 H. Jiang et al. / Materials and Design 142 (2018) 297–307

opposite of the fatigue initiation zone. This illustrated that the cracksinitiated from the pits and there were several fatigue crack initiations.The cracks direction was propagated from the pits to dimples asdepicted in zone 1 and 2 (the dash line arrow). Specifically, took zone1 as an example, the fatigue trench lines were observed in the zone 4,which were usually produced by intersection of crack propagationzones with different crack initiations. This indicated that there weremore than one fatigue crack initiations in the zone 1, which was consis-tent with the above analysis. In addition, a sub-region of striations anddimples were observed in the zone 5. These were local propagationand final fracture zones caused by one of the crack initiations. This dem-onstrated that local fatigue fracture had occurred. In the zone 3, therewere great regional tensile dimples, illustrating that this region wasthe main fatigue final fracture zone of the specimen. This resulted inthe final failure of the specimen. In general, it could be noted that thecracks of specimens with long-time ageing initiated from the severalcorrosion pits simultaneously.

4. Conclusions

This paper investigated themechanical behavior of the CFRP/Al elec-tromagnetically riveted lap joints in neutral salt spray environment. Theshear and fatigue experimentswere conducted to evaluate themechan-ical performance. Based on the experimental results, the conclusionswere drawn as following:

1. The shear properties of the CFRP/Al electromagnetically riveted lapjoints gradually decreased with the increase of ageing time. Espe-cially after ageing 1, 3, 5 and 7 weeks lost a maximum load of about13.5%, 15.6%, 25.6% and 29.7%, respectively. All the shear specimenswith different ageing time were failed with a typical bearing mode.

2. The S-N curves of the specimens with different ageing timewere ob-tained. The results showed that the fatigue life decreasedwith the in-crease of ageing time. Especially after ageing 1, 3, 5 and 7 weeks lostfatigue life of about 23.9%, 36.0%, 44.3% and 64.9%, respectively.

3. Three weeks was the critical ageing time for fatigue failure modeschanging. Below ageing of 3 weeks, fatigue crack initiated from thehole of Al sheet. Over ageing of 3 weeks, the fatigue crack initiatedfrom the edge of the overlapping region on the Al sheet.

4. The fatigue fracture appearance showed that the cracks of specimensafter short-time ageing (below 3 weeks) were initiated from themicrocracks in the hole wall of the Al sheet. The cracks of specimensafter long-time ageing (over 3 weeks) were initiated from severalcorrosion pits of Al sheet simultaneously.

Acknowledgement

This project is supported by The Key Project of Chinese NationalPrograms (No. 2016YFB0101704) and The National Key Research andDevelopment Program of Hunan Province (2017GK2090).

Data availability

The raw/processed data required to reproduce these findings cannotbe shared at this time due to technical or time limitations.

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