NAT'L INST. OF STAND & TECH

AlllOb DaMMfiT 4-2808 (FDA)

Mechanical Behavior of Ultra

Molecular Weight Polyethylene

Fourth Annual Report for the Period

October 1, 1S82 - September 30, 1983

Task 80-01, NBS-Bureau of Medical Devices lnte raQency Agreement

January 1384

“QC

100

,1156

34-2303

1984

o.' •',* )- VA

* ^

c

* l"g|7 > \X pw] v I / $

^r,s ot *

U.S. DEPARTMENT OF COMMERCE. Maicoim Baidnge, Secretary

NATIONAL BUREAU OF STANDARDS, Ernast Arr.bier, Director

issued February 1984

NBSIR 84-2808 (FDA)

Nahohai nuRnr. Or STAMJAflijs

LliiMAKT uc

Mechanical Behavior of Ultra High Ibo

' U’f S ) Molecular Weight Polyethylene

J.M. Crissman

Principal Investigator

Fourth Annual Report for the Period

October 1, 1982 - September 30, 1983

Task 80-01, NBS-BMD Interagency Agreement

NBS-Bureau of Medical Devices Interagency Agreement

January 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued February 1984

MECHANICAL BEHAVIOR OF ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE

By

J. M. Crissman

Polymer Science & Standards Division

National Bureau of Standards

Washington, D. C. 20234

Abstract

This report describes work done during FY 1983 under task 80-01, NBS-FDA/BMD

(Bureau of Medical Devices) Interagency Agreement. The report covers the fourth

year a a four year project concerned with the study of the morphology and mechanical

properties of ultra high molecular weight polyethylene (UHMWPE). The work done

during FY 1983 dealt principally with the following aspects of the mechanical

behavior of UHMWPE, (1) the temperature dependence of the creep and recovery

behavior in uniaxial extension and compression at small deformations, (2) longer

term (>1 day) creep and recovery behavior in uniaxial extension and compression

at small deformations, (3) the effect of y-irradiation on the creep and recovery

behavior, and (4) the effect of y-irradiation on the environmental stress-crack

resistance of UHMWPE.

TABLE OF CONTENTS Pa^e

1. Introduction 1

2. Comparison of the Short Time Creep Behavior of UHMWPE in Extension

and Compression 3

3. Temperature Dependence of the Short Time Creep and Recovery at

Small Deformations in Uniaxial Extension 3

4. Long Time Creep and Recovery Behavior at Small Deformations ... 11

5. Effect of y-radiation on the Performance of UHMWPE 15

5.1 Density and Crystallinity 19

5.2 Melting Behavior 19

5.3 Tensile Yield Stress 22

5.4 Tensile Yield Point Elongation 22

5.5 Tensile Modulus 22

5.6 Impact Strength 22

5.7 Surface Hardness 22

5.8 Creep 27

5.9 Fatigue 27

5.10 Effect of y-i rradi ation on Environmental Stress-crack

Resistance 32

6. Summary 35

7. References 38

i

i

1.

Introduction

This report describes work done during FY 1983 under task 80-01, NBS-BMD

(Bureau of Medical Devices) Interagency Agreement. A summary of prior work

done during fiscal years 1980 through 1982 is contained in references [1-3]*.

Task 80-01 is concerned with a study of the morphology and time dependent

mechanical properties of ultra high molecular weight polyethylene (UHMWPE),

the polymer used in a variety of orthopedic implant devices.

During FY 1983 the principal areas of investigation were:

1. the temperature dependence of the creep and recovery behavior at small

deformations in uniaxial extension and compression.

2. the long time creep and recovery behavior at small deformations in

uniaxial extension and compression.

3. the effect of y-irradiation on the creep and recovery behavior.

4. the effect of y irradiation on the environmental stress crack resistance of

UHMWPE.

Before proceeding with the discussion of the work done during FY 1983 it

will be useful to first review briefly certain aspects of the work contained in

reference 3. Figures 1 and 2 (Figures 3 and 4 in reference 3) present creep and

recovery data for specimens of the slowly cooled UH-1 UHMWPE (M £ 4.5x10^,

w

3 density = 0.935 g/cm ) in uniaxial extension. Figure 1 shows the creep strain

and strain during recovery for experiments carried out at constant t-j (duration

of the creep step) but varied applied stress. Figure 2 presents the strain

during recovery from experiments done at constant applied stress (8 MPa), but

varied t-j . In both figures the dashed lines represent the recovery predicted

assuming a superposition principle of the form

e R

(t) = e c (t+t

1 )-Ec (t), (1)

^Numbers in brackets indicate references found at the end of this report.

1
where e R (t) is the strain during recovery at a time t after a creep experiment

of duration t-j, and e c

( t+t-j ) and e c (t) are the strains at times t+t-j and t during

the creep experiment. It is clear from both sets of data that the observed

recovery and the predicted recovery deviate significantly at the longer

recovery times, and the actual recovery tends to plateau to a finite value.

(See for example the curves corresponding to t-j

1 s of 1 and 10 seconds in Figure 2).

This result led to the introduction of a new one-dimensional constitutive equation

of the form.

t

e(t) = / J*(a(5),t-£)de +

0

J(a(t) ,0) +<D(/g(aU))d£)

0

( 2 )

In equation 2, e(t) is the strain observed at time t, a(s) is the applied stress

t „ at time C , <f> ( / g (ct(c)) ds) is a functional which depends on the stress history, and

o

J (a , t) is* in the manner of a non-linear compliance multiplied by a. J*(.,.)

denotes the derivative of J(.,.) with respect to the second argument, and

J(0,t) = J*(0 , t)=g(0)=Q. For a two step creep and recovery experiment in which

the duration of both the creep step and the recovery time is t-j , it was shown [3]

that

e R (y,ti) e c

(cr;2t-| ) -

£ ^ ( cr , t-j ) + Ae^^t-j 5 ) 5

where ( t-| ; t-j ) is the strain during recovery at a time t-j after

time t-j, and Ae^ ( t-j ; ti ) is given by

^Rc^VV = 2^(g(a)t

] )-<i)(g(a)2t

1 ).

In Eqn. (4), As ( t-. ; t*. ) represents the deviation of the actual

Kc

that calculated using eqn. (1) (at a time t-j after creep for a

It was then assumed that in its simplest form d>(g(a)t-|) could be

<|)(g(a)t

1

) = g(a)t

1 a

,

where g(a) = (g (a))

a

, then from equation 4 it followed that

Ae

Rc(

t

l

; t

l

) =

9 (ct) (2-2^ ) t^r

( 3 )

creep for a

(4)

recovery from

duration t-j).

represented by

(5)

( 6 )

2
For constant a a plot of log Ae

RC

(tj ;t-j ) versus log t-j should then yield a

straight line with slope a. From such a plot it was found [3] that the data

shown in Figure 2 could be represented to a very good approximation by a

straight line having a slope of 1/3, or

Ae

RC

(

l

l »ti ) = °- 74 9(<*) ( ? )

2. Comparison of the Short Time Creep Behavior of UHMWPE in Extension and

Compression

Figure 3 shows a comparison of the short time creep behavior of the slowly

cooled UH-1 polymer in uniaxial extension and uniaxial compression at four

different levels of applied stress. With the exception of the curves obtained at the

highest stress of 15.6 MPa, there is no significant difference in behavior between

extension and compression over the range of stress and time examined. At 15.6

3

MPa, where the strain approaches ten percent at 10 seconds, the true stress

is significantly larger in extension than in compression and deviations in the

creep behavior are observed.

3. Temperature Dependence of the Short Time Creep and Recovery at Small

Deformation in Uniaxial Extension

Figures 4-6 present short time creep and recovery data obtained on specimens

of the slowly cooled UH-1 polymer in uniaxial extension at temperatures of

3

23 +0.5 °C, 38 +0.5 °C, and 47 +0.5 °C. Only the 10 second creep data

are shown in each figure. In each experiment the applied stress was 4 MPa.

3

A comparison of the 10 second creep data at four different temperatures is

shown in Figure 7. Comparison of the creep data obtained at constant stress

but different temperatures with those obtained at constant temperature but

varied stress indicate that the creep curve obtained at 23 °C and a stress of

4 MPa is roughly equivalent to that obtained at 38 °C and 2.5 MPa.

3
'o

eg'o

roto

NlVdlS

4

Figure

1.

Creep

strain

(-0-)

and

strain

during

recovery

(-I-)

versus

time

(log-log)

coordinates)

from

experiments

carried

out

at

constant

t-j

but

varied

applied

stress

a«.

The

dashed

lines

represent

the

recovery

predicted

using

equation
STRESS

=8

M

Pa

CM'oCO'o

NlVdlS

5

Figure

2.

Strain

during

recovery

(log-log

coordinates)

from

experiments

done

at

constant

applied

stress

but

varied

t^

.

The

dashed

lines

represent

the

recovery

predicted

using

equation

1

.
6

Figure

3.

Comparison

of

creep

behavior

of

UHMWPE

in

uniaxial

extension

(open

symbols)

and

uniaxial

compression

(filled

symbols). M

^4xlO G ,

p=0.935

g/cm 3 .
7

Figure

4.

Creep

strain

in

uniaxial

extension

(filled

circles)

and

strain

during

recovery

(open

circles)

versus

time

for

UHMWPE

(M

%4xl0 6,

p=0.935

g/cm 3)

at

constant

applied

stress

(4

MPa)

but

varied

creep

time

t-,.

Temperature

23

°C.
m

o

CMo

o

CM*o 'o

NlVdlS

8

TIME

(s)

Figure

5.

Creep

strain

in

uniaxial

extension

(filled

circles)

and

strain

during

recovery

(open

circles)

versus

time

for

UHMWPE

(M

%4xlO G

,

p=0.935

g/cni J

)

at

constant

applied

stress

(4

MPa)

but

varied

creep

time

t-,

. w

Temperature

38

°C.
*

NlVdlS

1 9

figure

6.

Creep

strain

in

uniaxial

extension

(tilled

circles)

and

strain

during

recovery

(open

circles)

versus

time

for

UHMWPE

(M

£4xl0

6

,p

=

0.935

g/cm 3)

at

constant

appl i

ed

stress

(4

MPa)

but

varied

creep

time

t-,

.

Temperature

47

°C.
4->

53

<u

<+- i+- •f- TJrZ3o • M— O0

-t-> <TJ CM

CO r—1 >

zc

ro -ct

-o 03 cu r“ •* o C_3 O o u

r—2 ooi

in c_> 0

<u1 CO

+J CO

S-o

4-

n

<_> c 0 o• r— CO

to CM c

0) < +-> X

<v • 03 r— a.

03 21

*1

X

03r- li C

13 <<

t> srrto to

O) a) > is_ CS 3 4-> a 03

S- Q- O) ai a.

O)

s_ a> o +j

r~*

0)

s-CD

•r— L

I!

10
Following the same procedures outlined earlier, Ae

RC

( t-j ; t-j ) was calculated

for each of the recovery curves shown in Figures 4-6 and these results are

presented in Figure 8. As was the case for an applied stress of 8 MPa

(Figure 2) the room temperature data can still be represented by a straight

line having a slope of 1/3. At the two higher temperatures the data can also

be represented by straight lines, however the slope decreases with increased

temperature, decreasing from 1/3 at 23 °C, to 0.271 at 38 °C, to 0.225 at

47 °C. This means that the exponent a and the term (2-2

a

) in equation 6 are

dependent on the temperature.

4. Long Time Creep and Recovery Behavior at Small Deformations

Creep and recovery data are shown in Figure 9 for specimens of the

slowly cooled UH-1 polymer which were under load for times up to 405 hours

(approximately 17 days). The creep data were obtained in uniaxial extension

at an applied engineering stress of 4 MPa and at room temperature (23 +0.5 °C).

The one creep curve shown corresponds to the longest time under load (405 hours).

It can be seen that after about 10 seconds under load (approximately 1 day) a

"break point" occurs beyond which the creep proceeds more slowly, and on log-log

coordinates the creep can be represented quite well by a straight line. In

earlier work by Turner and Moore [4,5] on the creep behavior of lower molecular

weight polyethylenes and polypropylene, it was found that the breakpoint or

"discontinuity" depended on the processing conditions in the following ways.

1. The break point moves to longer times as the rate of cooling from above

the melting point decreases. This movement corresponds to a horizontal shift

only of the break point.

2. The break point moves from one strain to a smaller one in a vertical

fashion as the storage time prior to testing increases. The vertical shift is

large for the quenched polymer and undetectable for the fully annealed material.

11
COO

C\JO

12

Figure

8.

Values

of

Ae^c

(t^;t^)

versus

t-j

on

log-log

coordinates

obtained

at

three

different

temperatures.

A

23

°C, 0

38

°C,

and

47

°C.
NlVdiS

c_? oco

C\J

03co

to

c:

a> -Mx

<u

03X

03

sc

13c

-O

aioou

>)o

to

O) +-> C3 > O o -a jc o c

CD 03 LO

s- o 03 -a c_ c r ii 03 C r CL +-> a> ii O) s- S— CO O O 4-

03

a)

S-

=3CD

13

duration

of

the

creep

step.

Creep

data

are

shown

only
3. When plotted on log-log coordinates the creep curve maintains the same

shape for a range of material states and therefore a range of coordinates of

the break point.

4. Beyond the break point the slope of the creep curve is independent of the

stress level and temperature for strains below about 3% in uniaxial extension

and for temperatures up to at least 60 °C.

5. The break point decreases in time with increased temperature or applied stress.

Also shown in Figure 9 are the recovery data obtained from experiments in

which the creep time, t-, , was varied from 0.28 to 405 hours. For each of the

recovery curves shown in Figure 9 Ae

R

£(t-| ; t-j ) was calculated, following the

procedures outlined earlier, and these values are shown plotted versus t-j

in Figure 10. The two points corresponding to t-j's of 100 and 300 seconds are

taken from Figure 4. Out to times of nearly 10 seconds (approximately 1 day)

the data can be represented by the same function given earlier in equation (7).

5 However, at times approaching 10 seconds a "breakpoint" also occurs in

Ae

rc

( t-j ; ti) . From the results shown in Figure 10 it would appear that at the

longer times the contribution of the plasticity term in equation (2) becomes

essentially constant. Therefore, in its present form, either equation 5 or 7

is not applicable to situations in which creep occurs for times longer than about

one day. It is also interesting that the break point in the Ae

RC

(t^;t

i

- ) versus

t-| plot occurs at essentially the same time as it does in the creep curve

(Figure 9).

Experiments were also performed to determine the extent to which the

break point depends upon variables such as the magnitude of the applied stress

and the temperature (points 4 and 5 cited earlier from the work of Turner and

Moore [4,5]). For this purpose 0.75 cm diameter cylinders of the slowly cooled UH-1

polymer similar to those described in reference [2] were tested in uniaxial

14
compression. Results from experiments done at room temperatures (23 +0.5 °C)

in which the applied engineering stress was varied are shown in Figure 11. It

can be seen that the break point does move toward earlier times as the stress is

increased, although the dependence is not a strong one. Beyond the break point the

creep curves form a set of parallel straight lines (on log-log coordinates). In

compression even the specimen loaded to the 16 MPa shows the same behavior as

that found at the smaller loads. At 10

6

second the strain in the specimen loaded

to 16 MPa approaches 10 percent (x=0.9), whereas in uniaxial extension at a stress

6

of 16 MPa and a time of 10 seconds the material has already elongated to a

stretch ratio of 5 (see Figure 3 in reference [2]).

The influence that temperature has on the position of the break point is

demonstrated in Figure 12 where creep curves obtained at four different

temperatures are shown. For these experiments the specimens were held at the

operating temperature for one hour prior to loading in order to approach

temperature equilibrium. With temperature, there is quite a strong shift in

the position of the break point, more than two decades in time for an increase

in temperature from 23 to 64 °C. To the right of the break point the creep

curves have the same constant slope (on log-log coordinates) as those shown in

figure 11. For the purpose of predicting the very long time creep behavior

of UHMWPE (30-50 years), what is not known at this point is whether the creep

curves continue indefinitely as straight lines on a log-log plot, or at yet

longer times than those investigated whether the shape of the curves changes.

5. Effect of y-Radiation on the Performance of UHMWPE

Sterilization of UHMWPE components in orthopedic prostheses is done using

high energy radiation, most commonly gamma irradiation. The typical dose is

in the 2-4 Mrad range. In addition to sterilization, high energy radiation

also causes chemical crosslinking and/or degradation of the polymer [6-8].

In the absence of oxygen, radiation is said to produce a net crosslinking effect

15
/ l“l ( U \

'yCj

^N \ 7 ( V I; 3v

16

Figure

10.

Values

of

Ae^

t-j

;t-|

)

versus

on

log-log

coordinates

For

slowly

cooled
17 i I

Figure

11.

Creep

of

slowly

cooled

UH-1

in

uniaxial

compression

at4

levels

of

applied

stress

at

23

°C. A

indicates

the

approximate

location

of

the

break

point.
N IVU 13

18

Figure

12.

Creep

of

slowly

cooled

UH-1

in

uniaxial

compression

at

four

different

temperatures

A

indicates

the

approximate

location

of

the

break

point.
which occurs preferential ly in the amorphous regions [9], There is additional

evidence that some crosslinking occurs within the crystals, possibly limited

to the fold surfaces [10,11]. Crosslinking and/or degradation can alter the

mechanical behavior from that of the unirradiated polymer. Section 5 describes

various aspects of the changes in physical and mechanical behavior of UHMWPE

brought about by high energy radiation.

5.1 Density and Crystallinity

One effect of high energy radiation is to increase the density, or percent

crystallinity, of the UHMWPE. Figures 13 and 14 indicates how the percent

crystallinity and density increase with increased irradiation dose.

Both sets of data indicate that the density increases with dose for doses

up to at least 10 Mrad. Two explanations for the increase in density have been

given; (1) highly constrained tie molecules are preferential ly severed by the

irradiation, thus removing hinderance to further crystal 1 ization [9]; and (2)

greater localized chain mobility occurs due to energy absorption during

irradiation, and the material tends to reorganize in the direction of higher

crystal 1 inity [12]. There is also evidence that subsequent to the irradiation

aging of UHMWPE occurs, and the density continues to increase over a period of

many months [13].

5.2 Melting Behavior

The effect of high energy radiation on the melting behavior of UHMWPE is

less well defined than in the case of the density. Bhateja et. al [9] report

that for an experimental sample of UHMWPE, which was subjected to 3 radiation,

an increase in the temperature of the DSC melting peak of about 5-7°C occurred

over the first 16 Mrad of radiation. These results are shown in Figure 15.

On the other hand. Roe et al . [12] compared the DSC curves for two specimens

of a commercial grade UHMWPE, which were a irradiated to 0.0 and 10.0 Mrad,

19
Figure 13. The degree of crystal 1 ini ty, calculated from the density versus irradiation dose (gamma irradiation) for UHMWPE from two different

commercial sources. (From reference 12).

Dose (MRAD)

Figure 14. Density as a function of irradiation dose (3 irradiation) for

two different experimental samples of UHMWPE ( 0 ,#) and normal

molecular weight polyethylene (Q) . (From reference 9).

20
Figure 15. Effect of 3 radiation dose on the DSC (differential scanning

calorimetry) peak melting temperature. 9,1 - UHMWPE,

- normal molecular weight polyethylene. (From reference 9).

21
respectively, and found little or no change in position of the primary melting

peak centered around 137 °C. A small secondary melting peak was observed at

around 120 °C for the specimen irradiated to 10 Mrad.

5.3 Tensile Yield Stress

The tensile yield stress is defined as the peak engineering stress occurring

at tensile yield. Several studies have been made of the effect of high energy

radiation on the tensile yield stress of UHMWPE. Data showing tensile yield

stress as a function of radiation dose, for doses up to 200 Mrad, are shown in

Figure 16. The yield stress is found to increase with increased dose.

5.4 Tensile Yield Point Elongation

The effect of y irradiation on UHMWPE is to decrease the elongation at

yield [12]. Most of the reduction occurs during the first 5 Mrad of irradiation

(Figure 17).

5.5 Tensile Modulus

The effect of high energy irradiation on UHMWPE is to increase the tensile

modulus with increased dose. Data are shown in Figure 18 for polymer subjected

to 3-i rradiation [9] and y-i rradi ati on [12].

5.6 Impact Strength

DuPlessis at al . [14] report that the impact strength of irradiated UHMWPE

decreases significantly with an increase in irradiation dose. Their data are

shown in Figure 19 for specimens irradiated in the presence of three different

envi ronments.

5 . 7 Surface Hardness

DuPlessis et al. [14] report that the surface hardness (Type D-Shore) of

UHMWPE increases when the polymer is irradiated (y rays) in the presence of

crosslinking agents, but not so if irradiated in a nitrogen atmosphere (Figure 20).

22
(a) y irradiated UHMWPE from two different commercial sources. Aging

time 6-24 months, (from reference 12).

(b) 3 irradiated experimental UHMWPE. © ,• - UHMWPE, 0- normal molecular

weight PE (From reference 9).

Figure 16. The effect of irradiation dose on the tensile yield stress

of UHMWPE.

23
IRRADIATION COSE (M ran )

Figure 17. Elongation at the yield point stress as a function of

irradiation dose for two different commercial UHMWPE

polymers, y

- irradiation (From reference 12).

24
(a) 3 - irradiation, 0.5% secant modulus,® ,# - Two different

experimental samples of UHMWPE, 0 - normal molecular weight

PE. (from reference 9).

(b) y

- irradiation, commercial UHMWPE (from reference 12).

Figure 18. Effect of radiation dose on the tensile modulus of UHMWPE.

25
40

20

\\

Figure 19. Izod impact energy as a function of dose for UHMWPE radiation

crossl inked (yrays) in the presence of crosslinking agents or nitrogen. 0-N 2 , X-CHeCH, and *-CHeCH + CTFE (chlorotri fl uoro- ethylene). (From reference 14).

Figure 20. Surface hardness of UHMWPE as a function of radiation dose for

UHMWPE radiation crossl inked (a rays) in the presence of cross- linking agents or nitrogen. 0-N 2 , X-CHeCH, and •-CHeCK + CTFE.

(From reference 14).

26
5.8 Creep

Exposure to high energy radiation improves the resistance of UHMWPE to

creep, even at relatively low irradiation doses. An example of the short time

creep and recovery behavior at small deformations in uniaxial extension is

shown in Figure 21 for specimens of the slowly cooled UH-1 polymer (Mw £ 4x10°

and c =0.935 g/cm prior to irradiation) subjected to relatively low doses of y3 irradiation. The creep strain after 10 seconds decreases with increased dose.

Figure 22 shows similar data for specimens irradiated with the same dosages as

given in Figure 21, but tested at 38 °C.

A comparison of the uniaxial creep behavior over a period of two weeks is

shown in Figure 23 for a commercial UHMWPE polymer both unirradiated and irradiated at three different doses. At the longer times the creep strain is

significantly less in more highly irradiated samples.

Bhateja and Andrews [15] report a similar behavior for samples of UHMWPE

irradiated to doses of 3-irradiation as high as 64 Mrad. Their results are shown

in Figure 24.

5.9 Fatigue

Only limited data are available on the effect of high energy radiation

on the fatigue behavior of UHMWPE. Table 5.9.1 shows some of the fatigue

results of Nusbaum and Rose [16]. The samples were cycled for the number

of cycles indicated and at the peak stress indicated. After TO'

7 cycles at

4

1.7 x 10' kPa several of the irradiated specimens showed a pronounced neck,

whereas unirradiated samples did not. They concluded that the irradiation

rendered the UHMWPE slightly weaker than the as-received polymer, and where

cyclic loading is concerned, irradiation may aggravate creep under some

condi tions.

27
NlVdJLS

28

Figure

21.

Effect

of

Y

irradiation

on

the

short

time

(10

seconds)

creep

and

recovery

behavior

in

uniaxial

extension

of

the

slowly

cooled

UH-1

polymer

at

23

±5

°C.

Engineering

stress

2.5

and

4.0

MPa.

ED;

o-0

Mrad,

a-3.74

Mrad,

and

B^i-6.28

Mrad

dose
CDc

s_

cd >> ai e e

aj -r- > CT> o e U LxJ cde

-a oe o

fO CO • CL CO aj

a> CO

at +-> 0 e to -o a

r—. •a .

^ 1 (O

t/>

— E

TO ID 2; Eo -a co a a> C\J cd r— • </> o co o 1 COo o 1

co -a r— c: s fd

a; o

£: r— r\ • r— LO a +J tO

a) E

+->e -t-j o

r~ 4— r^.

IS)

O • co

CD e

JE o 1 +-> •r— <

LO E E A O a; "o

+-> to e X E o CD 21

•1—

-E> 1—

O to tO 1 i

• r“ O

-aX

<o to e 1— e E tO

*1“ Q.

21

>- E

•r— LO

4- • O E CM O

-E> •r— V) u > LO

a> tO O) 4- -E E

4- CD +J

UJ JO LO

CMCM

a>e

cn

NIVU1S

29
STRAIN

Figure 23. Room temperature uniaxial creep behavior of UHMWPE(the

slowly cooled UH-1 polymer) irradiated with various dosejf

of y-irradiation. Engineering stress 4 MPa. a-0 Mrad,

0-1.18 Mrad, v-3.74 Mrad, andQ-6.28 Mrad.

30
50

i

c

o

in

01—

ICT1 IO°

l

[O'

_IlO2 lO3 Time (h)

Figure 24. Creep strain (uniaxial extension)

at an engineering stress of 10.3

C-16Mrad, and D-64 Mrad. (3 radi

versus time for UHMWPE

MPa. A-OMrad, 3-4Mrad,

ation) (From reference 1 5).

31
Table 5.9.1. Fatigue in Uniaxial Tension of UHMWPE at 23 °C*

6 7 2.5x10 cycles in water at a 10 cycles in serum at a 4 4

peak stress of 1.03x10 kPa peak stress of 1.72x10 kPa

Specimen As received Irradiated** As received Irradiated*

Yield Stress 2.36 2.47

(xl0

4

kPa)

Strain 0.15 0.15

Lower Yield 2.22 2.1 2.19 2.05

Stress (xlO

4

kPa)

Strain 0.53 0.67 2.8 3.3

*From reference

**2.5 MPa dose

16.

(0-radiation)

5.10 Effect of y-Irradiation on Environmental Stress-Crack Resistance

The results of tests on the environmental stress-crack resistance (ESCR)

of unirradiated UHMWPE at 90 °C and 75 °C were reported in reference [2]. The

test method used has been described in references [2] and [17]. During FY 1983

the effect of -/-irradiation on the ESCR of the slowly cooled UH-1 polymer

was investigated at 75 °C. The results from these latest tests are presented

in Table 5.10.1. The test conditions and specimen preparation (prior to

irradiation) were the same as in the previous work where the specimens were tested

fully submerged in the stress-cracking agent. The effect of y-irradiation is in

each case one of decreasing the failure time in the presence of the stress-cracki ng

agent. The surprising result is that the specimens irradiated with a dose of

3.74 Mrad failed much sooner than those irradiated with the higher dose of

6.28 Mrad. Two factors may have contributed to the large decrease in failure time

observed for the three specimens irradiated with a dose of 3.74 Mrad. Firstly,

due to a limited availability of specimens irradiated with the 3.74 Mrad dose, all

three specimens had been subjected to prior deformation before the ESCR test,

32
whereas the specimens irradiated with the higher dose had not. However, the

maximum strain to which they were subjected in uniaxial extension was only about

1%. Secondly, the attachment of the extensometer during the creep measurements did

cause a slight "nicking" of the surface of the specimen. In order to minimize

this effect during the ESCR test the nicked side of the specimen was turned inward

to form the inner surface of the bent strip. Examination of the failed specimens

under an optical microscope did not reveal any apparent correlation between the

position of the failure and the location of the nick. In each case the failure

originated on the outer surface and at the center of the strip rather than at

an edge. A third possibility is that the ESCR exhibits and minimum at a dose

between 0 and 6.28 Mrad.

In reference [2] it was demonstrated that the static fatigue lifetime (lifetime

under constant load) represented a lower bound to the cyclic fatigue lifetime. As

.noted earlier, Nusbaum and Rose [16] concluded that, there cyclic loading is

concerned, irradiation may aggravate creep under certain conditions. In view

of the ESCR results observed here it would appear to be important to investigate

more closely the influence of small doses of y-irradiation on the fatigue life

of UHMWPE

.

Table 5.10.1

Stress-Crack Resistance of v-Irradiated UHMWPE^ a; at 75°C

Dose

(Mrad)

Mean

Failure time

(Hrs.

)

Shortest

Failure time

(Hrs.

)

Longest

Failure time

(Hrs .

)

0 733 (b) 625 911

3.74 117 (c) 110 120

6.28 323 (d) 247 473

(a) Slowly cooled UH-1 polymer

(b) Average for five specimens

(c) Average for three specimens

(d) Average for eight specimens

33
6. Summary

During FY 19S3 work done under Task 80-01 dealt principally with the

following aspects of the mechanical behavior of UHMWPE:

1. The temperature dependence of the creep and recovery behavior in uniaxial

extension and uniaxial compression at small deformations.

2. The longer term (> 1 day) creep and recovery behavior in uniaxial extension

and uniaxial compression at small deformations.

3. The effect of y-irradiation on the creep and recovery behavior.

4. The effect of y-irradiation on the environmental stress-crack resistance

of UHMWPE.

All of the experiments described in this report which were carried out in

our own laboratory were done on specimens of the UHMWPE polymer identified in

fi ? the previous reports as slowly cooled UH-1 (M, - 4 x 10 , density = 0.935 g/cm )

3

The short time creep behavior (<. 10 seconds) has been studied in both

uniaxial extension and uniaxial compression for a range of applied engineering

stresses from 2.5 to 15.6MPa. At stress levels of 8MPa and below no

significant difference in behavior is observed between extension and compression.

3

However, at higher levels of stress, where at times approaching 10 seconds

the true stress becomes significantly different in the two cases, deviations

in the creep behavior do occur.

The temperature dependence of the short time creep and recovery has also

been studied at temperatures ranging from 23 to 47°C. Most of the experiments

were done in uniaxial extension at a stress of 4MPa. With respect to

conditions which may pertain to the clinical environment, it is observed that

the creep curve for 23°C and 4MPa is roughly equivalent to that at 38°C

and 2.5MPa. It is found that the nonrecoverable component of the creep, or

34
"plasticity," which was described in detail in reference [3], at the higher

temperatures can still be represented by the relation:

4>

(

9 (ct) t-| ) = g(o)tfj.

However, the exponent a is found to decrease with increasing temperature,

changing from 0.333 at 23°C, to 0.271 at 38°C, to 0.225 at 47°C.

Investigation of the creep behavior at longer times in both uniaxial

extension and uniaxial compression reveals that at times longer than about 1

day (at 23°C) a break occurs in the creep curve beyond which the creep proceeds

more slowly and on log-log coordinates can be represented by a straight line

having a slope of about 0.06. The slope of this portion of the creep curve is

found to be independent of either the temperature or applied stress, at least

within the temperature and stress range for which the long time experiments

were carried out. Further examination of the "break point" indicates that it

moves to shorter times as either the applied stress or temperature is increased.

Study of the creep and recovery behavior for creep times up to 405 hours show

that beyond the break point the plasticity term cj)(g(a)t-|) approaches a constant

value and no longer obeys the relation g(a)t-j .

The effect of y irradiation on the performance of UHMWPE has also been

investigated. In addition to sterilization, high energy radiation also causes

crosslinking and/or degradation which can alter the mechanical behavior and

other properties of the polymer. At doses in the range used for sterilization

(2-4 Mrad) the density is found to increase and there is some evidence that the

melting point may also be increased. Furthermore, both the tensile yield

stress and tensile modulus increase with increased dose, whereas the tensile

35
yield point elongation and impact strength decrease. Exposure to high energy

radiation improves the resistance of UHMWPE to creep, even at relatively small

doses. Both short term and long term creep experiments indicate that the

amount of creep is reduced, even for a dose as small as 1.18 Mrad (a = 4MPa).

At larger stress levels (10.3 MPa) a significant reduction in the creep is

observed for doses of 4 Mrad and greater. There are only very limited data

on the effect of high energy radiation of the fatigue behavior of UHMWPE.

The available data do suggest that where cyclic loading is concerned, radiation

may result in a greater amount of creep under certain conditions.

Finally, the effect of y-irradiation on the environmental stress-crack

resistance of UHMWPE has been investigated. Tests carried out on specimens

irradiated with doses of 0, 3.74, and 6.28 Mrad indicate that a significant

decrease in ESCR occurs as a result of y-irradiation, however, the results

are surprising in that the specimens irradiated with the lesser dose failed

much sooner than those irradiated at the greater dose. It is not clear at this

point whether the observed behavior resulted from the fact that the three

specimens tested which were irradiated at 3.74 Mrad had been subjected to small

deformations in uniaxial extension prior to testing, or whether the ESCR goes

through a minimum in the dose range between 0 and 6.28 Mrad.

It has been demonstrated that the static fatigue lifetime (lifetime under

constant load) represented a lower bound to the cyclic fatigue lifetime. Also,

Nusbaum and Rose [16] concluded that, where cyclic loading is concerned, irradiation

may aggravate creep under certain conditions. In view of the ESCR results observed

here it would appear to be important to investigate in much greater detail the

influence of small doses of y-irradiation on the fatigue life of UHMWPE.

36
References

[1] G. B. McKenna, F. Khoury, and J. M. Crissman, "Relationship Between

Morphology and Mechanical Properties of Ultra High Molecular Weight

Polyethylene," First Annual Report to the Food and Drug Administration (FDA),

Bureau of Medical Devices (BMD), under Task 30-01, NBS-3MD Interagency

Agreement, October, 1980 (NBSIR-2209 FDA).

[2] J. M. Crissman, F. Khoury, and G. B. McKenna, "Relationship Between

Morphology and Mechanical Properties of Ultra High Molecular Weight

Polyethylene," Second Annual Report to the FDA, Bureau of Medical Devices (BMD),

under Task 80-01, NBS-BMD Interagency Agreement, December, 1981 (NBSIR

82-2493 FDA).

[3] J. M. Crissman, L. J. Zapas, and F. Khoury, "Relationship Between

Morphology and Mechanical Properties of Ultra High Molecular Weight

Polyethylene," Third Annual Report to the FDA, Bureau of Medical Devices

(BMD), under Task 80-01, NBS-BMD Interagency Agreement, March, 1983

(NBSIR 83-2696 FDA).

[4] S. Turner, Poly. Eng. & Sci . , 6_, 306 (1966).

[5] D. R. Moore and S. Turner, Plastics & Polymers, 42_, 41 (1974).

[6] A. Charlesby, Atomic Radiation and Polymers , Pergamon, New York, 1960.

[7] F. A. Makhlis, Radiation Physics and Chemistry of Macromolecules , Wiley,

New York, 1975.

[8] M. Dole, Polym. Plast. Techno! . Eng., 1_3, 41 (1979).

[9] S. K. Bhateja, E. H. Andrews, and R. J. Young, J. Polym. Sci., 21 ,

Polymer Physics Edition, 523 (1983).

[10] E. H. Andrews and I. G. Voigt-Marti n , Proc. R. Soc. London Ser. A, 327 ,

251 (1972).

37
[11] R. Nagaeawa and K. Kobayashi , J. Appl. Phys., 41_, 4276 (1970).

[12] R. J. Roe, E. S. Grood, R. S. Shastri , C. A. Gosselin, and F. R. Noyes,

J. Biomed. Mat. Res., 15., 209 (1981 ).

[13] S. K. Bhateja, Polymer Commun., 23_, 645 (1982).

[14] T. A. du Plessis, C. J. Grobbelaar, and F. Marais, Radiat. Phys. Chem.,

9, 647 (1977).

[15] S. K. Bhateja and E. H. Andrews, Polymer, 24, 160 (1983).

[16] N. J. Nusbaum and R. M. Rose, J. Biomed. Mater. Res., 1_3, 557 (1979).

[17] J. M. Crissman, J. Testing and evaluation, JTEVA, lj_. No. 4, 273 (1983).

38
II

114

E S

• ' 1 4 *

L. .£ ttf! G * C? mm. bibliographic data

SHEET 'See instructions

i

1 . publication of

REPORT NO.

NBSIR 84-2808 (FDA)

2. Performing Orja r

.. Reoor; Nc. 3. Fubi^catior Date January 1984

4 . 1 1 1 .-t and sub i iTle

Mechanical Behavior of Ultra High Molecular Weight Polyethylene

Fourth Annual Report for the Period October 1, 1982-September 30, 1983

Task 80-01, NBS-Bureau of Medical Devices Interagency Agreement

-. AUT rtOR ( Si John M. Crissman

6. PERFORMING ORGANIZATION Ilf io ml or oine' tnon NBS. see instructions,

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l. Type of Report & Period Coverec

Annual Report

10/1/82 - 9/30/83

10 . supplementary notes

__ Document describes a compute' program; SF-185, FlPS Software Summary. , s attached.

' ZlfZi'Wr or fnt °LI"I.'SIT.Z'' H "ocm"! . .i.nilic.,..

This report describes work done during FY 1983 under task 80-01, NBS-FDA/BMD (Bureau

of Medical Devices) Interagency Agreement. The report covers the fourth year of a four year project concerned with the study of the morphology and mechanical

properties of^ultra high molecular weight polyethylene (UHMWPE). The work done during FY 1983 dealt principally with the following aspects of the mechanical behavior of UHMWPE, (1) the temperature dependence of the creep and recovery

behavior of uniaxial extension and compression at small deformations, (2) longer

term (>1 day) creep and recovery behavior of uniaxial extension and compression

at small deformations, (3) the effect of y-i rradiation on the creep and recovery behavior, and (4) the effect of y-i rradiation on the environmental stress-crack

resistance of UHMWPE.

12. KEY WORDS to twelve entries; cfphcoetico / orcer; cop itaiize only proper nomes; end seporote key words Py semicolons,

Creep; gamma irradiation; polyethylene; recovery; ultra high molecular weight.

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