Paper 19 Vibrations

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Fatigue properties of vibration-welded nylon 6 and nylon 66reinforced with glass fibres

K.Y. Tsang a, D.L. DuQuesnay a,*, P.J. Bates b

a Department of Mechanical Engineering, Royal Military College of Canada, Kingston, Ont., Canada K7K 7B4b Department of Chemical Engineering, Royal Military College of Canada, Kingston, Ont., Canada K7K 7B4

Received 7 March 2006; accepted 14 January 2007Available online 22 February 2007

Abstract

The fatigue properties of vibration-welded butt joints of nylon 6 and 66 with 30 wt% glass fibres made under low and high weld pres-sures were examined. Fatigue testing was conducted at a stress ratio of R = 0.1 in load control at frequencies ranging from 3 to 10 Hz.Specimens tested at both high and low stress levels failed at the weld with no thermal damage. It was found that although weld pressureaffected the fatigue strength, the welded specimens all had similar fatigue behaviour. The reinforced nylon 6 welded in low pressure hadslightly longer fatigue life than the others.Crown Copyright Ó 2007 Published by Elsevier Ltd. All rights reserved.

Keywords: A. Polymer-matrix composites (PMCs); B. Fatigue; C. Damage mechanics; D. Mechanical testing

1. Introduction

Engineering plastics and composites are easy to fabri-cate, low cost, recyclable and exhibit good chemical resis-tance and low densities compared to metals. The demandfor lower density material is high in the aerospace andautomotive industries. In the automotive industry, theyhave replaced metals in parts such as air intake manifolds,air filter housings, resonators, timing gears, radiator fansand radiator tanks.

Similar to metal component fabrication, complex plasticcomponents often consist of assemblies of smaller parts.

There are three major methods to join thermoplasticsparts: mechanical fastening, chemical bonding (adhesives)and physical bonding. The method employed depends onproduction rate, service performance, type of material, partsize, geometry, cost and aesthetics [1]. Vibration weldingbelongs to the physical bonding category, and is a widelyused process for joining thermoplastic parts.

Vibration welding involves bringing two parts togetherunder pressure with one part vibrating at a pre-set ampli-tude and frequency. Vibration dissipates frictional and vis-cous energy, which causes melting to occur at the contactsurface. As molten polymer is produced, it flows out of the weld interface under the applied weld pressure and flashis created. The applied pressure causes the two parts tomerge and is referred to as meltdown. At a target melt-down, the vibration stops, and the interface cools and thensolidifies [2]. Research has found that the strength of vibra-tion-welded joints can depend on amplitude, meltdown,and weld pressure [1–6]. MacDonald [6] studied the effects

of amplitude, meltdown and weld pressure on the tensilestrength of unreinforced and glass fibre reinforced nylon6 and 66 at a weld frequency of 212 Hz. It was found thatthe most influential factor on weld strength was the weldpressure. In addition, the tensile strength of nylon weldedat a relatively low pressure (e.g. 0.8 MPa) was higher thanthat obtained at a relatively high pressure (e.g., 4.0 MPa).

Much research on vibration welding e.g., [1–6] has gen-erally focused on the short-term mechanical properties of thermoplastic materials. Glass reinforced nylon is beingused increasingly in structural applications such as railways

1359-8368/$ - see front matter Crown Copyright Ó 2007 Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2007.01.012

* Corresponding author. Tel.: +1 613 541 6000x6483; fax: +1 613 5428612.

E-mail address: [email protected] (D.L. DuQuesnay).

www.elsevier.com/locate/compositesb

Available online at www.sciencedirect.com

Composites: Part B 39 (2008) 396–404

mailto:[email protected]

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[7], and automotive under hood and exterior components.Hence, further study of the fatigue properties of weldedthermoplastics is necessary particularly since they are usedincreasingly in applications, where they bear considerableweight and cyclic loads.

Stokes [8] has studied the fatigue life of vibration-welded

unreinforced polycarbonate (PC), polyetherimide (PEI),modified polyphenylene oxide resin (M-PPO) andpoly(butylenes terephthalate) (PBT), under tension–tensionloading at R = 0.1. The first three polymers are amorphousand PBT is semi-crystalline. Stokes found that the ratio of endurance limit stress to the tensile strength was 0.29 forPC, 0.34 for PEI, 0.22 for M-PPO and 0.31 for PBT.Recently, the authors [9] studied the fatigue behaviour of unreinforced semi-crystalline nylon 6 and nylon 66 vibra-tion-welded under high and low pressure conditions. Itwas observed that the ratio of endurance limit stress tothe tensile strength was approximately 0.3 for fatiguecycling at R = 0.1. In a study by the authors [10], and a

study by Wyzgoski et al. [11] on the fatigue properties of unwelded short glass fibre-reinforced nylon 66, the ratioof endurance limit stress to the tensile strength was approx-imately 0.5 for fatigue cycling at R = 0.1. Jia and Kagan[12] have reported on the effects of testing temperature onthe fatigue S–N behaviour of unwelded glass-reinforcednylon 6 and nylon 66. They showed that at room tempera-ture (23 °C) nylon 66 had slightly better fatigue strengththan nylon 6, but the trend was reversed at lower andhigher temperatures.

Reinforced plastics generally provide higher strengthand stiffness than their unreinforced counterparts. Glass

reinforced plastics are economical to produce and the addi-tion of glass fibres in moderate quantities does not greatlyincrease the density of the material. This study examinesthe fatigue properties of vibration-welded nylon 6 andnylon 66 reinforced with 30 wt% short glass fibres (nylon6 GF and nylon 66 GF), under tension–tension loadingat R = 0.1. Non-welded material data are also includedfor reference. The results are analyzed using stress-numberof cycles to failure (S–N) curves, and the fatigue mecha-nisms and the specimen temperature profiles during testingare examined.

2. Experimental procedures

Nylon 6 (ZytelÒ 73G301 NC010 from DuPont) andnylon 66 (ZytelÒ 70G30HSL NC010 from DuPont), eachreinforced with 30 wt% short glass fibres, were used in thisresearch. Prior to moulding, the nylon pellets were dried inan oven at 80 °C for approximately 24 h. An Engel 55 toninjection moulding machine was used to mould plaqueswith dimensions of 100 mm · 100 mm · 3.2 mm.

The glass fibre orientation within injection-mouldedplaques can significantly affect a reinforced material’smechanical properties. The fibre orientation is influencedby the flow of the material during injection moulding

[5,11,13,14]. It is accepted that the preferential fibre orien-

tation in thin edge-gated plaques is parallel to the melt flowdirection during cavity filling as shown in Fig. 1 [13]. Aschematic is used to illustrate the flow direction as thefibres are physically short and only show up clearly in pho-tographs at high magnification. The details of the fibre ori-entation can be seen is some detail in the fractographs

presented later.The moulded plaques were cut into 60 mm · 100 mm ·

3.2 mm plates parallel to the flow direction (Fig. 2). Thepreferred fibre orientation is therefore along the length of the plate; typical of what is often encountered in industrialcomponents. The cut edges were then milled to create a flatwelding surface, thereby increasing alignment accuracy inthe thickness dimension during welding. Two plates withthe milled edges in contact were then clamped in a fixturemounted on a Branson Mini II Linear Vibration Welder(details of the fixture can be found in [6]). The top platewas then oscillated with respect to the stationary bottomplate by a spring-mass system at a 212 Hz frequency with

a peak-to-peak amplitude of 1.8 mm. When the meltdownreached 1.5 mm, the vibration stopped and the weld solid-ified under constant pressure. Throughout the welding pro-cess, a constant clamping pressure (within ±10% of thetarget value) was applied to the plates by a pneumatic cyl-inder and air pressure regulator. An electronic pressuretransducer linked to a data acquisition system monitoredthe pressure. Similar to prior work on the static tensileproperties of vibration-welded nylon by MacDonald [6]and Mah [15], weld pressures of 0.8 (low) and 4.0 MPa(high) were chosen for this study.

Fig. 1. Flow direction and predominant glass fibre orientation for plaque.

Fig. 2. Pre-welding cut for nylon plaques used in butt welds. Dashed line

represents axis of cut and shaded surface represents moulded edges.

K.Y. Tsang et al. / Composites: Part B 39 (2008) 396–404 397



The welded specimens were subjected to static tensileand tension-tension fatigue tests. For the static tensile test,the non-welded and welded plaques were machined intorectangular specimens with a gauge cross-section of 3.2 mm · 28 mm, as illustrated in Fig. 3. As can be seenin Fig. 3, the axis for the tensile specimens is perpendicular

to the polymer flow direction and fibre orientation. For thewelded specimens, the flash is maintained intact (Fig. 3).To minimize stress concentrations on the edge of machinedspecimens, a numerically controlled milling machine wasused to produce the specimens with a smooth finish onall edges. Following the ASTM D638 standard, the speci-mens were tested at a constant displacement rate of 5 mm/minute until fracture. At least four trials were con-ducted for each material and weld condition to determinethe standard deviation of the stress at fracture.

Similar procedures were used to manufacture the dog-bone fatigue specimens. The only difference was that thefatigue specimens had a gauge length of 50.8 mm and a

cross-section of 19.0 mm by 3.2 mm, as shown in Fig. 4.Each specimen was examined closely to ensure excellentalignment in the joints because misalignment could causestress-concentrations or ‘‘notch effects’’ that would

adversely affect the fatigue life of the welded joints andcause scatter in the data. The few that were observed toexhibit poor alignment were rejected.

The specimens were subjected to sinusoidal tension–ten-sion fatigue testing at constant amplitude with the ratio of minimum stress to maximum stress, R = 0.1. Only stress

levels below the measured tensile strength were examined.Three trials were conducted at each stress level. The endur-ance limit stress was defined as the maximum stress atwhich two specimens endured at least 5 · 106 cycles with-out failure. Note that the stress-life curves are not flat atthis point and the fatigue limit, which is the stress belowwhich fatigue failure is not expected, is near but has notyet been achieved. The test frequencies used for the speci-mens were based on trial and error. For example, at stresslevels that were close to the tensile strength of the material,3 Hz was used. If the specimen did not undergo thermalfatigue, then a higher frequency was used. On the otherhand, if the specimen did show signs of thermal fatigue,

then the data were discarded and the test was repeated ata lower frequency. The same frequency was used for allspecimens at each stress level to eliminate additional vari-ables. For welded specimens, the test frequency rangedfrom 5 to 10 Hz, while for non-welded specimens the fre-quency ranged from 3 to 10 Hz. The maximum cyclic loadfrequency of 10 Hz was chosen to avoid thermal fatigue.

Nylons are known to be hygroscopic polymers. Allproperties are affected by moisture level. In this study,the fatigue tests were performed in the dry as moulded(DAM) condition. To maintain DAM condition, all speci-mens were stored in aluminum-lined bags prior to use.

During fatigue testing, each specimen was bagged withdry desiccant and sealed in plastic wrap surrounding thegauge section. As an added control, the temperature of the weld was monitored during testing by placing a thermo-couple near the weld zone inside the desiccant wrap. Sharpincreases in temperature would indicate heating by mois-ture-induced viscous heating of the nylon materials.Finally, a scanning electron microscope (SEM) was usedto examine the fatigue fracture surfaces to help identifythe fatigue mechanisms operative in these glass reinforcedpolymers.

3. Results and discussion

3.1. Tensile results

Static tensile tests were performed on both welded andnon-welded specimens to characterize the strength andconsistency of the welds. A large variation in tensilestrengths would imply inconsistency in the plaque mould-ing and vibration welding processes, which would likelyresult in scatter in the fatigue data. The tensile test resultsfor reinforced nylon 6 and 66 are presented in Fig. 5.The standard deviations are relatively small, suggestingthat there was consistency in the moulding and welding

processes.

Fig. 3. Schematic diagram of tensile specimen. Arrows indicated thedirection of polymer flow.

Fig. 4. Geometry of non-welded and welded fatigue specimens.

398 K.Y. Tsang et al. / Composites: Part B 39 (2008) 396–404



The non-welded specimens of reinforced nylon 66 hadstatistically higher tensile strength than the welded speci-mens. This result was expected as the glass fibres at theweld are generally oriented in the weld plane and thus offerlittle reinforcement when stress is applied normal to thisinterface [5,14,16]. Among welded reinforced nylon 66specimens, low weld pressure yields superior tensile weldstrengths than high weld pressure. This is again consistentwith the literature data [6,15].

For reinforced nylon 6, at 95% confidence interval, thetensile strength of non-welded specimens was not signifi-cantly different than that of specimens welded at low pres-

sure. This was unexpected and may be due to theunexpectedly high standard deviation in the non-weldedtensile strength. However, the average tensile strength of non-welded specimens was higher than those weldedat high pressure. Similar to nylon 66 results, specimenswelded at low pressure had a higher tensile weld strengththan those welded at high pressure. All welded specimensfailed at the weld during these tensile tests. This is believedto be due to the less-favourable fibre orientation at theweld and also due to small notches created by vibrationwelding.

4. Fatigue results

The fatigue data are plotted as S–N curves (stress vs.number of cycles to failure) for short glass fibre reinforcednylon 6 and 66 welded at low (0.8 MPa) and high(4.0 MPa) pressure in Figs. 6 and 7. Non-welded specimendata are also displayed on the graphs for reference. To dis-tinguish high cycle fatigue from low cycle fatigue regions, atransition region is shaded on the graphs. The curves areextrapolated to their static tensile strength (as dashed lines)to complete the fatigue profiles. The lack of data in theextrapolated regions for some conditions is due to sev-eral factors. There was a limited number of specimens

machined from the batch of nylon resin available; at cyclic

stress levels just below the average static strength relativelylong fatigue lives were typically observed; and at high stresslevels it was observed that thermomechanical fatigue orductile quasi-static failure was possible. Since the primaryinterest of this research was to assess the viability of vibra-tion welding for applications requiring fatigue durability inthe long life region, the low-cycle and transition regionsreceived less attention.

The S–N curves are fitted with a Basquin equation of theform

S max ¼ AN b

f ð1Þ

where N f is the number of cycle to failure and A and b arecoefficients obtained from the experimental data. The coef-ficients, endurance limits, transition ranges, and correlationcoefficients (R2) are listed in Table 1. The coefficients listedin Table 1 are strictly fitting parameters valid within thehigh cycle testing range and do not necessarily have a phys-

ical meaning.

Fig. 5. Tensile strength for butt welds of nylon 6 and 66 reinforced with30 wt% glass fibres welded at 0.8 MPa (LP) and 4.0 MPa (HP) and non-welded (NW). Shown with error bars representing ± one standarddeviation.

Fig. 6. S–N curves at R = 0.1 for glass fibre reinforced nylon 6 with weldpressure of 0.8 MPa (LP), 4.0 MPa (HP) and non-welded (arrows indicateunbroken specimens). The shaded area represents the transition range.

Fig. 7. S–N curves at R = 0.1 for glass fibre reinforced nylon 66 with weldpressure of 0.8 MPa (LP), 4.0 MPa (HP) and non-welded (arrows indicateunbroken specimens). The shaded area represents the transition range.




From Figs. 6 and 7, the S–N curves are linear and par-allel to each other in the low cycle fatigue region (1–100cycles). Beyond 1000 cycles, in the high cycle fatigueregion, the maximum stress decreases with number of cycles for all conditions. The transition point between highand low cycles lies within the range of 100–1000 cycles forboth materials. The transition range (1000–10000 cycles)for unreinforced nylon observed in a prior study [9] is an

order of magnitude larger than for the reinforced nylons.This may be due to the formation of microcracks at theends of glass fibres that are often observed in the matrixof reinforced plastics [17,18]. The occurrence of thesemicrocracks accelerates the crack initiation process, andmay cause a shift in the transition range to a lower numberof cycles.

5. Effect of vibration welding

Important trends were observed in the data presented inthe S–N curves of Figs. 6 and 7. As expected, they show

that the fatigue lives of the non-welded specimens werelonger compared with the welded specimens, i.e., weldingreduces fatigue life. For reinforced nylon 6, at a stress levelof 60 MPa, the fatigue life of the non-welded specimenswas at least one order of magnitude longer than weldedones. Similarly for nylon 66 at a stress level of 70 MPa,the fatigue life of the non-welded specimens is three ordersof magnitude longer than the welded specimens. Moreover,the endurance limit for non-welded reinforced nylon 66(60 MPa) was comparable to the tensile strength of thereinforced nylon 66 welded at high pressure (61 MPa). Thisis consistent with the short–term properties of nylon wherethe non-welded specimens have higher static tensilestrength than the welded ones. The ‘‘strength-life equalrank assumption’’ [19,20], which proposes that a materialwith greater tensile strength will have longer fatigue life,appears to be valid. This trend is consistent with the fatiguebehaviour of unreinforced nylons observed by the authors[9].

The reduced fatigue lives of the welded specimens wereprobably caused in part by the glass fibre orientation atthe weld as mentioned earlier in the tensile test results,where the fibres were aligned parallel to the weld planeand offered little reinforcement, when stress was appliednormal to this interface [6,15]. Another cause could be

the stress concentrations, or notches, created at the weld

line during vibration welding which are ideal sites forcrack-initiation. With the presence of notches, the numberof loading cycles needed for crack initiation is decreasedand the total fatigue life is dominated by crack propagation[8].

Weld details introduce stress concentrations into speci-mens and components and are typically the weak spotsespecially in fatigue loading. The reduction in fatigue

strength due to a stress concentration is determined bycomparing the fatigue strength of the unwelded materialto that of the welded one. The fatigue notch factor, K f , isthe ratio of the fatigue strength of the unwelded materialto that of a welded specimen at a given fatigue life. It isa combination of material and geometry effects and gener-ally increases for most materials as a function of life up tothe fatigue limit. In this study, fatigue notch factors werecalculated for the vibration welded specimens in the longlife region (105 cycles) and at the endurance limit (5 · 106

cycles). The magnitude of K f was 1.2 and 1.6 for reinforcednylon 6 welded at low and high pressures, respectively, at

105

cycles and 1.5 for both weld pressure at 5 · 106

cycles.For nylon 66 reinforced with glass fibres, K f was 1.7 and1.9 for low and high pressures, respectively, at 105 cyclesand 2.0 for both weld pressures at 5 · 106 cycles. Twotrends were seen. As the stress decreased, K f increased;and as the static strength of the material increases, the K f value also increased.

6. Effect of weld pressure

From Fig. 6 for the welded reinforced nylon 6, it wasobserved that weld pressure had an effect on fatigue lifeespecially in the low cycle region. At stress levels of 60and 50 MPa, the fatigue life of reinforced nylon 6 weldedat low pressure was an order of magnitude greater thanfor nylon 6 welded at high pressure. At the stress level of 40 MPa, the fatigue life of specimens welded at low pres-sure was 50% greater than that of specimens welded at highpressure. This is also consistent with the findings on thetensile strength of the welded specimens in this study andwith the ‘‘strength-life equal rank assumption’’.

For glass fibre-reinforced nylon 66 (at the 95% confi-dence level), weld pressure did not have a significant effecton fatigue life especially in the high cycle region. This trend

was also observed in unreinforced nylon 66 [9]. As shown

Table 1Fatigue S–N curve data for glass fibres reinforced nylon 6 and 66 welded at low and high welding pressure

Material Welding pressure(MPa)

Coefficient A

(MPa)Exponentb

Endurance limit (MPa)based on 5 · 106 cycles

Transition range(cycles)

R2

Nylon 6 w GF 0.8 (low) 198 À0.118 30 100–1000 0.87Nylon 6 w GF 4.0 (high) 98 À0.071 30 100–1000 0.93Nylon 6 w GF Non-welded 163 À0.081 50 N/A 0.96

Nylon 66 w GF 0.8 (low) 98 À0.071 30 100–1000 0.93Nylon 66 w GF 4.0 (high) 98 À0.071 30 100–1000 0.93Nylon 66 w GF Non-welded 162 À0.060 60 N/A 0.97




in Fig. 7, beyond 104 cycles the S–N curves for low andhigh pressure welds converged.

According to Stokes [2], there is an inverse relationshipbetween the weld pressure and the melt film thickness. It isbelieved that a thick melt film increases the probability of the glass fibres reorienting themselves towards being per-

pendicular to the weld line [6,21–23]. As a consequence,the tensile strength is increased, and according to the‘‘strength-life equal rank assumption’’, the fatigue lives of the specimens are improved, when the weld pressure isdecreased. This theory is supported by evidence observedon the fatigue fracture surface under the SEM as shownin Fig. 8a and b.

Fig. 8 shows the fracture surfaces of reinforced nylon 6specimens welded at low and high pressures. The low weldpressure condition shows more fibres orientated perpendic-ular to the weld plane, which may have increased the weldstrength and resulted in a longer fatigue life. This is consis-tent with the fracture surface observed in previous studies

[6,15].Fig. 9a and b show the fatigue fracture surfaces of rein-

forced nylon 66 specimens welded at low and high pres-sures. The glass fibres are positioned parallel to the weldplane with the same (transverse to the load) orientation

for both specimens welded at low as well as high pressures.This is consistent with their fatigue data presented earlier,where they have similar fatigue stress-life profiles.

7. Effect of glass fibres

Fig. 10 shows the effect of glass fibres on nylon 6 weldedat low pressure. As can be seen in Fig. 10, there is a signif-icant difference between the curves of nylon 6 with andwithout glass fibres welded at low pressure throughoutthe low and high cycle fatigue region. The fatigue lives of the reinforced nylon 6 are longer than unreinforced nylon6 by an order of magnitude at stress levels of 40 and50 MPa. The endurance limit stresses are 30 MPa and20 MPa, for reinforced and unreinforced nylon 6, respec-tively. The nylon 6 specimens with and without glass fibreswelded at high pressure followed a similar pattern. This isconsistent with the ‘‘strength-life equal rank assumption’’mentioned earlier.

Fig. 11 shows the effect of glass fibres on nylon 66welded at low pressure. The S–N data for reinforcedand unreinforced nylon 66 are indistinguishable in thelow cycle fatigue and transition regions. In the high cycleregion, the S–N curves begin to diverge from each other.At a stress level of 40 MPa, the fatigue life of the rein-

forced nylon is longer than that of the unreinforced nylon

Fig. 8. (a) Nylon 6 w GF welded at low pressure. Notice the randomdirection and vertical positioning of the fibres. Some of the fibres that arepositioned perpendicular to the weld surface are marked with circles and(b) nylon 6 w GF welded at high pressure; the arrow indicates the

preferred fibre orientation.

Fig. 9. (a) Nylon 66 w GF welded at low pressure and (b) nylon 66 w GFwelded at high pressure, the arrows indicate the preferential fibreorientation in the bulk moulded part.




by an order of magnitude and nylon 66 has a higherendurance limit. The glass fibres may not play an effectiverole in the low cycle fatigue region, and only become sig-nificant in the high cycle fatigue region (>50000 cycles)for nylon 66. One possible reason for this phenomenonis the glass fibre orientation at the weld. Compared tonylon 6, the fibres in nylon 66 may be more oriented inthe plane of the weld (this is indeed observed and dis-cussed in the fracture surface section of this paper).Under higher stresses, and in spite of the presence of glass

fibres, the strain at the weld exceeds the elongation atbreak of this poorly oriented composite material and fail-ure quickly occurs. Under lower stresses, the rigidity of the composite material at the weld interface lowers thestrain relative to that experienced in unreinforced mate-rial, and therefore reduces the rate of crack growth. Thefatigue properties of the reinforced material are thusimproved in the lower stress region.

8. Fatigue endurance ratio

Fatigue tests can be extremely time consuming; tensile

tests are much less so. If the endurance limit of the material

can be related to the tensile strength, this can reduce theamount of effort required to estimate the material endur-ance limit. The ratio of endurance limit stress to tensile

strength (re/ru) is called the fatigue endurance ratio. Theresults for the nylon 6 and 66 specimens examined are pre-sented in Table 2.

From Table 2, the lowest observed fatigue enduranceratio for glass fibre reinforced vibration-welded nylon spec-imens is 0.33. The average value for all the welded rein-forced nylons is 0.40. Hence, the following rule of thumbis conservatively suggested: the fatigue limit stress atR = 0.1 can be estimated as 0.33 of the tensile strengthfor the vibration-welded reinforced nylons. Similar to thetensile and fatigue data, the re/ru ratios for the unweldedspecimens are superior to the welded ones.

From a study performed by Stokes [8], the re/ru ratioof unreinforced vibration-welded polycarbonate (PC),poly(butylenes terephtalate) (PBT), polyetherimide (PEI)and modified polyphenylene oxide resin (M-PPO) weldedat 120 Hz and tested at 10 Hz with R = 0.1 is of the orderof 0.3 and they all failed away from the welds. In a priorstudy on unreinforced nylon 6 and nylon 66 [9], the authorsfound the re/ru ratio to be 0.3 under the same loadingconditions. Therefore, the re/ru ratio observed for thevibration-welded reinforced nylons with low and high weldpressures was near the re/ru ratios observed for variousbutt-welded thermoplastics under similar testing conditions.

Rules of thumb for endurance ratios are well known forother fatigue conditions in metals. At R = À1, steel andaluminium typically have a re/ru ratio of 0.5 and 0.3,respectively, [24]. This ratio is known to decrease withincreasing R ratio and the presence of welds. When the R

ratio is increased to 0.1, the re/ru ratio is reduced to about0.33 and 0.2 for steel and aluminum alloys, respectively.Nylon 6 and nylon 66 with 30 wt% glass fibres have are/ru ratio similar to that of unwelded steels.

9. Temperature profiles

Fig. 12 shows the maximum temperatures that were

recorded for the welded reinforced nylon specimens.

Fig. 11. S–N curves at R = 0.1 for nylon 66 compounds with weldpressure of 0.8 MPa (arrows indicate unbroken specimens).

Table 2re/ru Ratio for non-welded and welded unreinforced and 30 wt% glassfibre reinforced nylon 6 and 66 welded at low and high pressure

Material Pressure (MPa) re/ru

Nylon 6 w GF Non-welded 0.48Nylon 6 w GF 0.8 (low) 0.33Nylon 6 w GF 4.0 (high) 0.42

Nylon 66 w GF Non-welded 0.52Nylon 66 w GF 0.8 (low) 0.39Nylon 66 w GF 4.0 (high) 0.49Nylon 6 [9] Non-welded 0.42Nylon 6 [9] 0.8 (low) 0.32Nylon 6 [9] 4.0 (high) 0.29Nylon 66 [9] Non-welded 0.36Nylon 66 [9] 0.8 (low) 0.27Nylon 66 [9] 4.0 (high) 0.27

Fig. 10. S–N curves at R = 0.1 for nylon 6 compounds with weld pressureof 0.8 MPa (arrows indicate unbroken specimens).




As can be seen in Fig. 12, the temperatures of the spec-

imens were well below the glass transition temperature, T g,of their corresponding resins and no thermal fatigue tookplace.

Figs. 13 and 14 present typical temperature–timeprofiles for the materials tested in this study. For both

reinforced nylons, the temperature profile is reasonablysteady during testing at various stress levels andfrequencies.

10. Fracture surface

For reinforced material, fatigue crack propagation pro-cess is more local and on a smaller scale compared withthat of the unreinforced material due to the presence of the glass fibres [17,18]. Therefore, no crack or other charac-teristics can be seen with unaided visual inspection. How-ever, under SEM with a higher magnification, interestingfracture surfaces are observed. Fig. 15 shows the fracturesurfaces observed for both reinforced nylons welded underlow pressure. Thumbnails along the edge of the fracturesurfaces are seen in the micrographs. Once again, thecracks appear to originate from the edge due to the notcheffect.

The fracture surfaces for reinforced nylons at variousweld pressures, stress levels and materials are similar; there-fore, a similar fracture mechanism is assumed. Horst andSpoormaker [17,18] suggested the following fracture mech-anisms. The fibre ends are the sites with the highest stressand the cracks begin to grow at those locations. Cracks

Fig. 13. Temperature profile for nylon 6 reinforced with glass fibreswelded at low pressure at various stress levels.

Fig. 14. Temperature profile for nylon 66 reinforced with glass fibres

welded at low pressure at various stress levels.

Fig. 15. SEM micrographs showing the thumbnails of: (a) nylon 6 with30% GF welded at low weld pressure and tested at 50 MPa and (b) nylon

66 with 30% GF welded at low weld pressure and tested at 35 MPa.

Fig. 12. Maximum temperature reached by selected specimens for allnylon compounds, weld pressure and stress levels.




continue to grow with the cyclic loading and the fibresde-bond from the matrix. The cracks at the matrix/fibres

interface grow into the matrix and finally, fast fractureoccurs. Fig. 16 shows a typical SEM fractograph illustrat-ing fibre de-bonding and fibre pull-out for a nylon 6 spec-imen tested in the present study. The evidence on all thefractographs for nylon 6 and nylon 66 specimens observedin this study is consistent with the proposed fracturemechanism.

11. Conclusions

The fatigue stress-life curves of glass fibre reinforcednylon 6 welded at 4.0 MPa and nylon 66 welded at

0.8 MPa and 4.0 MPa are all similar at R = 0.1, whilenylon 6 welded at 0.8 MPa has slightly better fatigue prop-erties. In general, low weld pressure appears to give betterfatigue performance than high weld pressure in vibrationwelded nylons. In the absence of hysteretic heating, fatiguecracks initiate in the weld material. Fibre de-bonding andmatrix microcracking lead to fatigue crack propagationand fast fracture under cyclic loading. A rule of thumb issuggested for vibration-welded nylons reinforced with30 wt% short glass fibres: the fatigue limit stress atR = 0.1 is 0.33 of the tensile strength. Vibration-weldedreinforced nylon specimens retain a relatively high fractionof their tensile strengths under cyclic loading. Hence, vibra-

tion welding of short glass fibres reinforced nylons appearsto be a viable process for fatigue sensitive components.

Acknowledgements

Financial support of this research by NSERC Auto21NCE and the Centre for Automotive Materials andManufacturing is gratefully acknowledged. Additional rec-ognition to Alcan International, Kingston Research andDevelopment Centre and DuPont Canada Research andDevelopment for their in-kind support of testing facilitiesand materials.

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Paper 19 Vibrations

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