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.
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.
H
N
H
N
n
O
O
533˚K
N
2
n
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)
ρ
(g/cm3)
σy
(MPa)
Comp. σ
(MPa)
E
(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
C
ooling ai
r
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.
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