Ultrasonic Fatigue Endurance Investigation on Plastic Material Nylon 6

doi:10.4236/msa.2011.29174

Paper Menu >>

Journal Menu >>

Materials Sciences and Applicatio ns, 2011, 2, 1293-1297

doi:10.4236/msa.2011.29174 Published Online September 2011 (http://www.SciRP.org/journal/msa)

1293

Ultrasonic Fatigue Endurance Investigation on

Plastic Material Nylon 6

Gonzalo M. Domínguez Almaraz, Erasmo Correa Gómez

Universidad Michoacana de San Nicolás de Hidalgo, Facultad de Ingeniería Mecánica, Santiago Tapia, Morelia, Michoacán, México.

Email: [email protected]

Received May 18th, 2011; revised May 27th, 2011; accepted June 9th, 2011.

ABSTRACT

Ultrasonic fatigue tests were carried out on the plastic material Nylon 6. Special attention was devoted to the tempera-

ture control in order to avoid physic-chemical transformation of this low melting point material. Under ultrasonic fa-

tigue tests, important heat dissipation takes place at the narrow section of hourglass shape specimen leading to high

temperature at this zone. The specimen was calculated to meet the resonance condition with the smallest dimensions at

its narrow section, with aim to reduce the temperature gradient at this zone of this non heat conducting material. Tem-

perature at narrow section was maintained lower than 45˚C using a cooling system with cooling air; under this condi-

tion the ultrasonic fatigue tests were performed. Experimental tests were carried out at low loading range (9% - 12.5%

of the elastic limit of material) in order to control the highest temperature and to avoid that specimen was out of reso-

nance condition. Experimental results are analyzed together with the fracture surfaces and conclusions are presented

concerning the ultrasonic fatigue endurance of this polymeric material.

Keywords: Ultrasonic Fatigue Tests, Polymeric Material, Numerical Simulation, Temperature Control, Fatigue Life,

Fracture Surface

1. Introduction

Industrial applications of plastic materials have been in-

creasing exponentially in the last 30 years: Semiconduc-

tor manufacturing, Medical sector, Food processing,

Electrical power and electronics, Oil drilling and explo-

ration, Oil refinery and transportation, Underwater seis-

mology, Automotive, Aerospace and flight, Chemical

manufacturing, Logging and forestry, Water and waste

treatment, Materials handling 1-3. Plastic material Ny-

lon 6 is a cast nylon polyamide with good wear resis-

tance, coupled with high tensile strength and modulus of

elasticity. It also has high impact resistance, a high heat

distortion temperature and resists wear, abrasion and

vibration 4,5. In addition, nylon polyamides can with-

stand sustained contact with a wide variety of chemicals,

alkalis, dilute acids or oxidizing agents. Another impor-

tant factor, both economically and mechanically, is the

relative light weight of polyamide—approximately 1/8

the weight of bronze, 1/7 the weight of cast iron, and 1/2

the weight of aluminum—which reduces both the inertial

and static loads and eases the handling of large compo-

nents during maintenance or replacement procedures.

Industrial applications of this plastic material, among

others, include: food contact parts, wheels, gears, custom

parts, textile fibers, electric parts, industrial cords, car-

pets, bushings, ropes, slippers, pulleys, etc. 6,7. In most

of its industrial application, this plastic material undergo

oscillating mechanical loading that leads to fatigue con-

dition 8; then, it is of principal interest to investigate

the fatigue endurance of this material in the high and

very high cycle regime.

2. Specimen, Material and Testing

2.1. Specimen

The specimen profile for the ultrasonic fatigue tests of

plastic material Nylon 6 was obtained by numerical

simulation fitting the resonance condition and taking into

account the physical properties, particularly its low heat

dissipation coefficient. Figure 1(a) shows the dimension

of testing specimen; R1 is small in order to reduce the

temperature gradient at this section, generated by ultra-

sonic fatigue tests in this low heat dissipation material.

The Figure 1(b) presents modal numerical result ob-

tained by Ansys software corresponding to longitudinal

Ultrasonic Fatigue Endurance Investigation on Plastic Material Nylon 6

1294

L = 13.5, L1 = 4, L2 = 9.5, 2R1 = 5, 2R2 = 10, R0 = 19.3

(a)

(b)

Figure 1. Specimen dimensions (mm) (a), natural frequency

obtained by Finite Element Method (b).

natural frequency of specimen (20.146 KHz). This fre-

quency was close to excitation frequency of the system

(20 KHz) in order to fit the resonance condition.

2.2. Material

Nylon 6 begins as pure caprolactam with 6 carbon atoms;

this is the origin of name Nylon-6.

When caprolactam is heated at about 533˚ K in an inert

atmosphere of nitrogen for about 4 - 5 hours, the ring

breaks and undergoes polymerization, Figure 2. Then, the

molten mass is passed through spinnerets to form fibres of

Nylon 6.

This thermoplastic material is manufactured by two

types of monomers containing 6 or 12 carbons in the

chain; the commercial names are respectively: nylon 6

and nylon 6/6. Testing material in this study corresponds

to nylon 6 with the principal mechanical and physical

properties shown in Table 1.

2.3. Testing

Tests were carried out at room temperature and with no

control of environmental humidity. The loading ratio was

R = −1 for all ultrasonic fatigue tests; under this condi-

tion the highest temperature is located at the narrow sec-

tion of hourglass shape specimen. In order to constrain

the highest temperature in the specimen to 45˚C, a cool-

ing system with cool air was implemented. Figure 3

shows the plastic specimen on the ultrasonic fatigue ma-

chine and the two cooling system injectors.

533˚K

Figure 2. Chemical transformation to obtain plastic mate-

rial Nylon 6.

Table 1. Principal mechanical (a) and physical (b) proper-

ties of plastic material Nylon 6.

(a)

(g/cm3)

σy

(MPa)

Comp. σ

(MPa)

(GPa) HR Elongation

(%)

1.15 82 103 2.75 R115 20

(b)

Melting T. (˚C)Glass T. (˚C) Thermal Conduc. (W/m-˚K)

215 50 0.25

Crack path

Figure 3. Specimen attached to ultrasonic fatigue machine

and cooling system injectors.

Ultrasonic Fatigue Endurance Investigation on Plastic Material Nylon 6

1295

Stresses at the specimen narrow section were evalu- ated

by numerical simulation applying displacements at the

extremes of specimen, Figure 4. Calibration of dis-

placements for the experimental testing was determined

by a high resolution inductive sensor: a linear relation

was obtained between the applied voltage on the ultra-

sonic machine and the displacement at the specimen free

extreme. Results show that for 11 volts applied on the

ultrasonic fatigue machine, the displacement registered at

the free extreme of specimen was 18 μm, reduced to 15

μm because 3 μm of electronic noise, Figure 5. Then, the

ratio displacement/voltage was: 15/11  1.35 μm/volt.

Figure 4. Numerical simulation to determine the stress distribution along the specimen.

Figure 5. Registration of ratio displacement/voltage for the ultrasonic fatigue tests.

Ultrasonic Fatigue Endurance Investigation on Plastic Material Nylon 6

1296

All tests were loaded between 9.5% and 13% of the ma-

terial elastic limit in order to keep the temperature at the

narrow section of specimen below 45˚C.

3. Results

In Figure 6 are plotted the experimental results obtained

on the polymeric material Nylon 6. Vertical axis repre-

sents the ratio in percent between the nominal applied

stress σn and the yield stress σY of this material. The low

values for this ratio (between 9.5% and 13%), corre-

sponding to the temperature control at the specimen nar-

row section: in increasing the load this temperature in-

creases and may surpass the glass transition temperature

of the material, inducing high deformation and, in most

of the cases, the specimen is carried out of resonance

condition. Crack propagation was observed at the narrow

section of specimen in a perpendicular plane to the speci-

men longitudinal direction; crack origin was not clearly

indentified: in some fracture surface it seems to be local-

ized inside the specimen, associated with the high tempe-

rature at this zone.

Crack path at the specimen narrow section

Figure 6. Fatigue endurance and crack path for polymeric

material Nylon 6.

4. Discussion

Ultrasonic fatigue endurance has been obtained for the

polymeric material Nylon 6. A critical parameter to carry

out experimental tests was the temperature on specimen

9-12; in order to control this parameter three actions

were undertaken: 1) the diameter at specimen narrow

section was small (5 mm) with aim to reduce the tem-

perature gradient at this zone; 2) the applying load was

low (9.5% to 13% of material yield stress) in order to

limit the heat dissipation associated with the increase on

temperature; and 3) cool air was injected at the specimen

narrow section. The Figure 7 shows the fracture surface

for a specimen tested close to12% of its elastic limit.

Some testing specimens present a visible polymeric

degradation localized at the crack propagation path, as

shown in Figure 8.

These specimens were loaded at highest level: close to

12% of elastic limit of material. This should be a limit

under the described conditions in order to carry out ul-

trasonic fatigue testing without an important thermal ef-

fect. Polymeric degradation induced by heat dissipation

may hide or overlapping the mechanical fatigue endu-

rance of this plastic material.

Temperature effect is observed in the Figure 7: at the

cooling air sites no important degradation of polymeric

chains was observed; then, crack is expected to initiate at

the high plastic deformation, associated with the high

temperature zones. Ultrasonic fatigue tests were carried

out on this polymeric material under temperature control;

nevertheless, no internal parameters such as: polymer

structure, molecular weight, crosslinking, and filler or

diluent type and content 13,14, were analyzed at this

stage of work.

Cooling air

ooling ai

Figure 7. Fracture surface and cooling air localization for

the plastic specimen.

Ultrasonic Fatigue Endurance Investigation on Plastic Material Nylon 61297

Figure 8. Polymeric degradation at crack propagation path.

5. Conclusions

1) Ultrasonic fatigue tests on polymeric materials were

carried out under temperature specimen control.

2) Experimental fatigue life registered was from 5.9 ×

108 cycles to 2.9 × 109 cycles, between the loading range:

9.5% - 13% of the material elastic limit.

3) Scattering experimental points was not plotted on

Figure 6, only the tendency points. Crack initiation and

propagation were not clearly indentified; nevertheless,

crack initiation should be localized inside the specimen

as shown in Figure 7: high temperatures in this zone

induce polymeric degradation and, therefore, should in-

duce the crack initiation.

4) Concerning the crack propagation, results show that

fracture surface was always perpendicular to the longitu-

dinal axis of specimen and localized close the specimen

narrow section.

5) The fracture surfaces present differentiate zones: at

the centre a visible polymeric degradation caused by

mechanical loading and high temperature of these non

conducting material, as shown in Figure 7; at peripheral

zones, reduction of polymeric degradation, particularly

for the cooling air zones.

6) Further ultrasonic fatigue investigations on these

materials are necessary in order to improve the under-

standing of: temperature limits and the internal and ex-

ternal parameters controlling the fatigue endurance of

polymeric materials.

6. Acknowledgments

The authors are grateful to the University of Michoacan

(UMSNH) in Mexico for the facilities received during

this work. Special mention of gratitude for the CONA-

CYT (National Counsel for Science and Technology) in

Mexico City, for the financial support destined to this

project.

REFERENCES

[1] E. Lokensgard, “Industrial Plastics: Theory and Applica-

tions,” Fifth Edition, Delmar-Cengage-Learning Edition,

Albany, New York, 2010, 560 Pages.

[2] D. Perić, M. Vaz Jr. and D. R. J. Owen, “On Adaptive

Strategies for Large Deformations of Elasto-Plastic Solids

at Finite Strains,” Computational Issues and Industrial

Applications, Computer Methods in Applied Mechanics

and Engineering, Vol. 176, No. 1-4, 1999, pp. 279-312.

[3] H. Becker and L. E. Locascio, “Polymer Microfluidic

Devices” Talanta, Vol. 56, No. 2, 2002, pp. 267-287.

[4] J. J. Huang and D. R. Paul, “Comparison of Fracture Be-

havior of Nylon 6 Versus an Amorphous Polyamide

Toughened with Maleated Poly (ethylene-1-octene) Elas-

tomers,” Polymer, Vol. 47, No. 10, 2006, pp. 3505-3519.

[5] T. Liu, I.Y. Pang, L. Shen, S. Y. Chow and W.-D. Zhang,

“Morphology and Mechanical Properties of Multiwalled

Carbon Nanotubes Reinforced Nylon-6 Composites,”

Macromolecules, Vol. 37, No. 19, 2004, pp. 7214-7222.

doi:10.1021/ma049132t

[6] L. Huang, E. Allen and A. E. Tonelli, “Inclusion Com-

pounds Formed between Cyclodextrins and Nylon 6,”

Polymer, Vol. 40, No. 11, 1999, pp. 3211-3221.

[7] Y. Li, Z. Huang and Y. Lu, “Electrospinning of Nylon-6,

66,” Terpolymer European Polymer Journal, Vol. 42, No.

7, 2006, pp. 1696-1704.

[8] M. G. Wyzgoski and G. E. Novak, “Fatigue-Resistant

Nylon Alloys,” Journal of Applied Polymer Science, Vol.

51, No. 5, 1994, pp. 873-885.

[9] Y. Miyano, M. Nakada and R. Muki, “Prediction of Fa-

tigue Life of a Conical Shaped Joint System for Fiber

Reinforced Plastics under Arbitrary Frequency, Load Ra-

tio and Temperature,” Mechanics of Time-Dependent

Materials, Vol. 1, No. 2, 1997, pp. 143-159.

[10] W. Chen and F. L. Cheng, “Tension and Compression

Test of Two Polymers under Quasi-Static and Dynamic

Loading,” Polymer Testing, Vol. 21, No. 2, 2002, pp.

113-121. doi:10.1016/S0142-9418(01)00055-1

[11] N. Jia and N. V. A. Kagan, “Effects of Time and Tem-

perature on the Tension-Tension Fatigue Behavior of

Short Fiber Reinforced Polyamides,” Polymer Compos-

ites, Vol. 19, No. 4, 1998, pp. 408-414.

[12] L. W. McKeen, “Fatigue and Tribological Properties of

Plastics and Elastomers,” 2nd Edition, William Andrew

Publishing, Amsterdam, 2010, 312 Pages, ISBN: 978-0-

08-096450-8.

[13] H. Van Melick and H. K. Van Dijl, “High-Temperature

Testing of Stanyl Plastic Gear: A Comparison with Ten-

sile Fatigue Data,” Gear Technology, 2010, pp. 59-65.

[14] M. Jenkins, J. Snodgrass, A. Chesterman, R. H.

Dauskardt and J. C. Bravman, “Atomic Force Microscopy

Studies of Fracture Surfaces From Oxide/Polymer Inter-

faces,” Materials Research Society Symposium, Vol. 654,

2001, pp. AA271-AA275.