Water in Polymers - American Chemical Society

32 Effect of Moisture on Fatigue Crack Propagation in Nylon 66 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 20, 2017 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch032

P. E. BRETZ, R. W. HERTZBERG, J. A. MANSON, and A. RAMIREZ Materials Research Center, Lehigh University, Bethlehem, P A 18015

The fatigue phenomenon in polymers is of increasing fundamental and technological interest. From the standpoint of polymer science, the kinetics and energetics of failure under cyclic loading reflect an interesting balance between energy input due to the applied stress and energy dissipated by dynamic viscoelastic processes. In turn, the position of this balance reflects not only the loading conditions and environment but also polymer structure and morphology. From the technological standpoint, fatigue is important because many applications of engineering plastics involve repetitive or cyclic loads. While the overall fatigue process in a smooth specimen containing no significant flaws includes both the initiation of an active flaw and its growth, many (if not most) real specimens contain pre-existent flaws that can, under appropriate loading conditions, develop into catastrophic cracks (1,2) (often at inconvenient or dangerous times and places). Thus the proper selection of materials and design of parts requires particular attention to the ability of a polymer to resist fatigue crack propagation (FCP). For these reasons, an extensive program on engineering plastics has been conducted in this laboratory to characterize the FCP rates as a function of loading conditions, to identify the micromechanisms of failure, and to elucidate the role of the polymer chemistry and morphology (3-10). Interestingly, as a group, crystalline polymers [notably nylon 66, polyacetal, and poly(vinylidene fluoride)] have exhibited higher values of fracture toughness and lower values of FCP rates than amorphous or poorly crystalline polymers. Following detailed studies of the behavior of amorphous polymers, it was decided to explore the effects of structural and morphological parameters on FCP in nylon 66, polyacetal, and poly(vinylidene fluoride). This paper updates earlier reports on nylon 66. (For other studies on crystalline polymers see references 3, 7-12). When relating polymer chemistry to mechanical behavior the environment is, unfortunately, often neglected. However, the effects of sorbed moisture on the general mechanical behavior of 0-8412-0559-0/ 80/47-127-531 $05.75/ 0 © 1980 A m e r i c a n Chemical Society

Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

WATER IN P O L Y M E R S

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 20, 2017 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch032

532

polyamides have long been documented. Such e f f e c t s have been reviewed by McCrum e t a l . (13), P a p i r e t a l . (14), and Kohan (15), and described i n the trade l i t e r a t u r e (16). G e n e r a l l y Young's modulus and y i e l d s t r e n g t h are s a i d to decrease, and impact toughness to i n c r e a s e , with i n c r e a s i n g water content. Dynamic mechanical spectroscopy and other s t u d i e s provide evidence that water e x i s t s i n both t i g h t l y bound and r e l a t i v e l y f r e e s t a t e s (13,17), and i t has been shown that some p r o p e r t i e s e x h i b i t a t r a n s i t i o n when the water content exceeds the r a t i o of one water molecule per two amide groups (corresponding to t i g h t l y bound water). [Use of the term "bound" does not imply the permanent b i n d i n g of a p a r t i c u l a r molecule to a p a r t i c u l a r s i t e ; as with adhesion, the p o s s i b i l i t y of a dynamic interchange i s assumed.] Much l e s s has been reported about f a t i g u e i n polyamides. The c o n d i t i o n i n g o f nylon 66 a t 50% r e l a t i v e humidity (RH) was s a i d to reduce f a t i g u e l i f e by 30% (16), and long-term soaking i n water was reported to i n c r e a s e FCP r a t e s a t a constant s t r e s s i n t e n s i t y - f a c t o r range ( 3 ) . The aim o f the present program i s to e s t a b l i s h the b a s e - l i n e behavior of FCP i n nylon 66 as a f u n c t i o n of water content, and to deduce the micromechanisms of f a i l u r e . In the f i r s t progress r e p o r t , s i g n i f i c a n t e f f e c t s of moisture content on both FCP r a t e s and f r a c t u r e s u r f a c e s were observed. In t h i s c o n t i n u a t i o n study, using a d i f f e r e n t s e r i e s of nylon 66, some p r e v i o u s l y reported e f f e c t s are confirmed, but some are not. P o s s i b l e reasons are d i s c u s s e d , and f u t u r e d i r e c t i o n s o u t l i n e d . Experimental Two s e r i e s of injection-molded plaques o f n y l o n 66 were obtained through the courtesy of Dr. E. Flexman, Ε. I . duPont de Nemours and Co. The grade concerned, Z y t e l 101, had a nominal number-average molecular weight of 17,000. In the f i r s t study (A), the plaques were 8.6 mm t h i c k , and were r e c e i v e d i n the dry, as-molded c o n d i t i o n ; these plaques had been sealed i n p l a s t i c bags immediately a f t e r molding to prevent moisture pick-up and were s t o r e d i n a d e s i c c a t o r f o l l o w i n g the opening o f the bags. In the second study (Β), the plaques were 6.4 mm t h i c k , and were r e c e i v e d c o n t a i n i n g approximately 2.2 wt % water. Equilibration to v a r i o u s moisture contents was performed according to the methods o u t l i n e d i n Table I . Number-average molecular weights (M ) were measured a f t e r v a r i o u s times o f water immersion i n order to check whether or not the water e q u i l i b r a t i o n procedures induced a s i g n i f i c a n t degree of h y d r o l y s i s . Thus values o f M were determined v i s c o m e t r i c a l l y using s o l u t i o n s i n formic a c i d and the f o l l o w i n g equation (18): n

M n

,-1/a Κ

1/a [η]

(1)

where Κ and a = 1.1x10"^ and 0.72, r e s p e c t i v e l y , and [η] i s the


32.

B R E T Z ET A L .

Table I .


Series

Fatigue Crack

Moisture %RH

a

Propagation

533

C o n d i t i o n i n g Procedure f o r Nylon 66 wt % H 0 2 o

b

Method

A-l

0

A-2

23

0.8-0.9

B-0

44

2.2

A-3

50

2.6

b

B o i l e d f o r 100 hr i n a saturated aqueous^ potassium acetate

B-l

51

2.7

C

B o i l e d f o r 100 hr i n saturated aqueous sodium n i t r i t e

B-2

52

2.8°

B o i l e d f o r 100 hr i n saturated aqueous sodium acetate

B-3

66

4.0°

B o i l e d f o r 100 hr i n saturated aqueous sodium phosphate

B-4

72

4.5

B-5

80

5.7°

A-4,B-6

100

2.2 wt %. Reference 11 d e s c r i b e s a procedure by which such n o n - l i n e a r i t y i n the crack f r o n t was t r e a t e d . F r a c t o g r a p h i c s t u d i e s were conducted on an ETEC Autoscan scanning e l e c t r o n microscope (SEM) a t an a c c e l e r a t i n g p o t e n t i a l of 20 kV. Each f r a c t u r e s u r f a c e was coated with vacuum deposited l a y e r s of gold and carbon p r i o r to examination to prevent s p e c i men charging and minimize the degradation of the f r a c t u r e s u r f a c e under the e l e c t r o n beam.


32.

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Results and D i s c u s s i o n 4 V i s c o m e t r i c determination of M y i e l d e d a value of 1.78x10 , confirming the nominal v a l u e . On b o i l i n g f o r 100 hr, the value of M was reduced by ^20%. Since such a r e d u c t i o n would i n c r e a s e the absolute values of FCP r a t e by an amount not greater than the maximum experimental e r r o r , and would not a f f e c t the general shape of the curve of FCP r a t e vs water content, a c o r r e c t i o n f o r degradation was not a p p l i e d to the f a t i g u e data. Values estimated f o r the percent c r y s t a l l i n i t y v a r i e d not only with thermal treatment, but a l s o with the measurement techniques. As shown i n Table I I , the DSC technique gave c o n s i s t e n t l y lower values of c r y s t a l l i n i t y than the d e n s i t y - g r a d i e n t column. In g e n e r a l , more r e p r o d u c i b l e r e s u l t s were found by using the DSC than was the case with the d e n s i t y method. n


n

Table I I . C r y s t a l l i n i t y i n Nylon 66 by DSC History

C r y s t . by DSC,

and Density Methods

a

C r y s t . by Density,

%

%

Quenched i n o i l

24

37 .5

A i r cooled

27

38 .3

Slow cooled i n o i l

30

44 .9

35

43 .1

Annealed

at 235°C (50

min.)

Assuming a heat of f u s i o n of 46.8

cal/g

(196

J/g) (23).

In the case of slow c o o l i n g the water content corresponding to complete b r i d g i n g of amide groups by water molecules (Ξ 1 water molecule per 2 amide groups) would range between 4.4 wt % and 5.6 wt % ( f o r % c r y s t a l l i n i t i e s of 30% and 45%, r e s p e c t i v e l y ) . Fatigue Crack Propagation. R e s u l t s obtained w i t h S e r i e s Β specimens confirm the e a r l i e r observations that water content i n nylon 66 not only s i g n i f i c a n t l y a f f e c t s FCP r a t e s but a l s o may decrease or i n c r e a s e FCP r a t e s , depending on the percent water (3,11). Figures 1-3 show the FCP r e s u l t s f o r one specimen of each of the moisture contents i n v e s t i g a t e d . For those moisture contents f o r which d u p l i c a t e t e s t s were performed, the maximum d i f f e r e n c e i n growth r a t e between the two specimens was a f a c t o r of 2; g e n e r a l l y , much b e t t e r agreement was observed. As noted p r e v i o u s l y (11), the range of response from "best" to "worst" i s broad; at AK^MPav^m, the FCP r a t e f o r the worst specimen i s about 25 times that of the best specimen. When FCP r a t e s (at constant ΔΚ) are p l o t t e d against water content (Figure 4 ) , a pronounced




9 10 Δκ.

MPfl

νίΤ

Figure 1. Typical logarithmic plots of FCP rate per cycle (da/dN) vs. the stress intensity factor range (AK) for nylon 66 (Series A) containing various levels of moisture


BRETZ

ET AL.

Fatigue

Crack

Propagation

537


32.

Figure 2.

Crack growth rate vs. stress intensity factor range for nylon 66 (Series B) containing various levels of moisture




538

Figure 3.

Crack growth rate vs. stress intensity factor range for the remaining Series Β nylon 66 specimens



32.

BRETZ

ET AL.

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Crack

Propagation

539

minimum i s noted at about 2.5% water. Thus, as water i s imbibed by an e s s e n t i a l l y dry polymer, the FCP r a t e f i r s t decreases to a minimum value and then r a p i d l y i n c r e a s e s , reaching a maximum v a l u e on s a t u r a t i o n . This behavior c e r t a i n l y suggests the existence of competitive e f f e c t s (and hence mechanisms), the balance depending on the amount of water present. While a complete r a t i o n a l i z a t i o n of t h i s i n t e r e s t i n g behavior must await completion of dynamic mechanical c h a r a c t e r i z a t i o n , an e x p l a n a t i o n must s u r e l y take i n t o account the hydrogenbonding and p l a s t i c i z i n g nature of water i n polyamides (12,13,14, 15). The s o r p t i o n of i n c r e a s i n g p r o p o r t i o n s of water has long been known to s h i f t both the g l a s s - t o - r u b b e r and secondary t r a n s i t i o n s to i n c r e a s i n g l y lower temperatures (13); f o r example, Tg decreases from 80°C to -15°C f o r t h i s nylon as the moisture content of nylon 66 i n c r e a s e s from ^ 0 wt % to 8.5 wt % water (15). I t i s important to note, however, that the r a t e of decrease of Tg i s g r e a t e s t below about 2 wt % of water (14,24,26). This decrease i n Tg has been a t t r i b u t e d to the breaking of hydrogen bonds between amide groups and the b r i d g i n g of amide groups by water (13,14). P a r a d o x i c a l l y w h i l e Tg i s being decreased by the s o r p t i o n of water, water molecules are apparently a b l e to pack w e l l i n t o the f r e e volume, as shown by a decrease i n the s p e c i f i c volume of the t i g h t l y bound water (26). At the same time, the s m a l l - s t r a i n modulus does not decrease (and may even i n c r e a s e s l i g h t l y ) when water i s added to a c o n c e n t r a t i o n of up to ^2.6% (15,25). I t i s i n t e r e s t i n g to c o n s i d e r the water/amide group s t o i c h i o metry a s s o c i a t e d with water contents of about 2.6 wt %. Assuming a degree of c r y s t a l l i n i t y of 40%, and that the water remains i n the amorphous phase, 2.6 wt % water would correspond to one water molecule per 3.7 amide groups. [This value i s lower than that reported e a r l i e r (11) due to c o r r e c t i o n of the degree of crystallinity.] I t i s a l s o i n t e r e s t i n g that a s l i g h t DSC peak begins to occur at vL00°C when the water content reaches ^2.6%; t h i s suggests that some water begins to behave i n a r e l a t i v e l y " f r e e " manner a t about t h i s c o n c e n t r a t i o n . Returning to the q u e s t i o n of FCP r a t e s , i t seems l i k e l y that an i n c r e a s e d segmental m o b i l i t y must be a s s o c i a t e d with the presence of s m a l l (2.6 wt % water w i l l increase the extent of p l a s t i c s t r a i n s experienced by the bulk m a t e r i a l ahead of the crack t i p , and thus tend to i n c r e a s e the FCP r a t e a t a given a p p l i e d l o a d range. In other words, the s t r a i n per l o a d i n g c y c l e , Δε, must then i n c r e a s e , s i n c e the t e s t i s performed under constant load range Δσ, and Δε = Δσ/Ε. Thus, the nylon that contains l o o s e l y bound water accumulates more damage per l o a d i n g c y c l e than would be the case with the d r i e r specimens; a higher FCP r a t e w i l l then be expected at a given value of Δσ (and hence Δ Κ ) . This behavior i n d i c a t e s that the e f f e c t of a decreasing modulus overwhelms the b e n e f i c i a l e f f e c t of high l o c a l i z e d segmental m o b i l i t y (a question of the r e l a t i v e s c a l e of motions). Second, t h i s weakening e f f e c t w i l l be a c c e l e r a t e d by any h y s t e r e t i c heating that occurs (see d i s c u s s i o n below). I f we accept the c o n c l u s i o n that a minimum i n the FCP r a t e occurs when the water content i s between 2 wt % and 3 wt %, i t i s i n t e r e s t i n g to see i f any other p r o p e r t i e s e x h i b i t d i s c o n t i n u i t i e s i n a s i m i l a r range of water content. Let us consider i n t u r n , f r a c t u r e toughness, the development of h y s t e r e t i c h e a t i n g , and the nature of the deformation as revealed by m i c r o s c o p i c examination of the f r a c t u r e s u r f a c e ( f r a c t o g r a p h y ) . F r a c t u r e Toughness. With t y p i c a l g l a s s y m a t e r i a l s subjected to an FCP t e s t of the kind performed i n t h i s study, crack growth proceeds at an e v e r - i n c r e a s i n g r a t e as c y c l i n g proceeds, u n t i l the specimen f r a c t u r e s c a t a s t r o p h i c a l l y . A l s o , FCP r e s i s t a n c e tends to be p o s i t i v e l y c o r r e l a t e d with s t a t i c f r a c t u r e toughness (_3). In such cases, i t i s o f t e n p o s s i b l e to estimate a c r i t i c a l value of ΔΚ f o r f a s t f r a c t u r e from the l a s t value of ΔΚ p r i o r to f r a c t u r e , ΔΚ . While t h i s v a l u e does not correspond to a true max value of f r a c t u r e toughness (or energy) i t does g i v e a r e l a t i v e ranking of toughness ( 6 ) . With more d u c t i l e m a t e r i a l s , c a t a s t r o phic f r a c t u r e may not occur, and d e f i n i t i o n of an u l t i m a t e f a i l u r e p o i n t i n terms of ΔΚ becomes impossible. As summarized i n Table I I I , t e r m i n a l f a s t f r a c t u r e was obtained only a t water contents lower than 4.0 wt %. Thus


Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980. YES

NO

NO

YES

TERMINAL F A S T FRACTURE

STRIATIONS

0 G BANDS

SPHERULITIC FAILURE

0 % NO

LEVEL

YES

NO

NO

YES

NO

0.8

2.6

NO

NO

YES

YES

NO

NO

YES

YES

SOME SOME

2.2

T E S T S - 10 H Z

FRACTURE S U R F A C E WHITENING

MOISTURE

ALL

MW = 1 7 , 0 0 0

NO

NO

YES

YES

YES

2.8

NYLON

OBSERVATIONS

MOISTURE-BEARING

FRACTOGRAPHIC

TABLE III

NO

NO

YES

YES

YES

4.0

66

OF

NO

NO

YES

NO

YES

4.5

NO

NO

YES

NO

YES

5.7

NO

NO

NO

NO

YES

8.5


32.

BRETZ ET AL.

Fatigue Crack

Propagation

543

r e l a t i v e values of Δ Κ ^ χ cannot be obtained at higher water contents. However, while the d i f f e r e n c e s i n A K f o r specimens c o n t a i n i n g 3.7 MPaviï) . These a r r e s t l i n e s , which were a s s o c i a t e d with the p e r i o d i c i n t e r r u p t i o n of the FCP t e s t to read the c r a c k - t i p p o s i t i o n , imply the occurrence of creep. ( I f creep i s c l o s e l y confined to the c r a c k - t i p , i t may w e l l be a s s o c i a t e d with crack b l u n t i n g . ) Some f i n e r l i n e s were observed as w e l l ( f o r d i s c u s s i o n see the f o l l o w i n g s e c t i o n ) . As mentioned above, i t was observed that specimens (Series A and B) c o n t a i n i n g Î4.0 wt % water f a i l e d by r a p i d , unstable ( c a t a s t r o p h i c ) crack propagation i n the f i n a l load c y c l e (see Figures 5a and 5b). At higher water contents, however, the specimens f a i l e d by s t a b l e but very r a p i d crack propagation (da/dN > 1 mm/cycle) at values of ΔΚ > 4MPav^m (see F i g u r e 5 c ) . The f a s t - f r a c t u r e surfaces (Figures 5a and 5b) were c h a r a c t e r i z e d by a grouping of c r i s p , curved l i n e s that emanated from some c e n t r a l p o i n t along the boundary of the crack f r o n t j u s t p r i o r to i n s t a b i l i t y . Interestingly, this fast fracture region e x h i b i t e d no evidence of s t r e s s - w h i t e n i n g (though such whitening was o f t e n seen i n the s t a b l e crack growth r e g i o n ) ; t h i s was so r e g a r d l e s s of the water l e v e l i n any specimen. The d i s t i n g u i s h i n g f e a t u r e noted on the t e r m i n a l f r a c t u r e s u r f a c e s i n the specimens c o n t a i n i n g 8.5% water was the e x i s t e n c e of widely separated a r r e s t l i n e s corresponding to the extent of t e a r i n g i n each load c y c l e (Figure 5 c ) . (These a r r e s t l i n e s w i l l be discussed below.) Other f r a c t u r e markings were observed and are d e s c r i b e d i n the p r e l i m i n a r y r e p o r t (11). M i c r o s c o p i c Appearance of F r a c t u r e Markings. As was the case with macroscopic o b s e r v a t i o n s , major d i f f e r e n c e s were noted i n the m i c r o s c o p i c (380X) f r a c t u r e s u r f a c e micromorphology of specimens e q u i l i b r a t e d to d i f f e r e n t moisture l e v e l s . At t h i s higher m a g n i f i c a t i o n ( c f . F i g u r e 5), the "dry specimens revealed c r i s p , f a c e t - l i k e markings over the e n t i r e f a t i g u e f r a c t u r e surface [see F i g u r e 7a]. The average s i z e of these s m a l l f a c e t s i s approximately 10 ym, c l o s e to the s p h e r u l i t e s i z e of 6.5 ym (determined by e t c h i n g i n x y l e n e ) . Such f a c e t i n g suggests that l i m i t e d crack t i p p l a s t i c deformation had occurred, an o b s e r v a t i o n which i s c o n s i s t e n t with the f a c t that the s u r f a c e s observed at low m a g n i f i c a t i o n were f l a t . A g e n e r a l l y s i m i l a r appearance was noted 11




546

Figure 5. Appearance of fatigue fracture surfaces of nylon 66 (Series A) as a function of moisture content: (a) — 0 wt %, (b) 2.6 wt %, (c) 8.5 wt %. Series Β specimens behaved in a similar fashion, with evidence for ductility at water contents >2.7%. Terminal fracture regions also visible. Arrow shows direction of fatigue crack growth.

Figure 6. Coarse and fine crack arrest lines found on the fatigue fracture surface of nylon 66 equilibrated to contain 5.7 wt % water. Arrow shows direction of fa tigue crack growth.


ET AL.

Fatigue

Crack

Propagation


BRETZ

Figure 7. Fracture surface appearance of nylon 66 associated with different moisture contents: (a) —0 wt % water, (b) 5.7 wt % water, (c) 8.5% water. ΔΚ = 2.6 MPa\/m. Arrow shows direction of fatigue crack growth.

American Chemical Society Library 1155 16th St. N. W. Rowland; Water in Polymers Washington, D. C.Society: 20036 ACS Symposium Series; American Chemical Washington, DC, 1980.



548

Figure 8. Fatigue striations in nylon 66 equilibrated to contain 5.7 wt % water. Arrow shows direction of fatigue crack growth.

Figure 9. Correlation between macro scopic growth increments per loading cycle and striation spacings as a function of Δ Κ in nylon 66 equilibrated to several moisture levels

ΔΚ, MPa>/m



32.

BRETZ ET A L .

Fatigue Crack

Propagation

549

with specimen A-2 (0.8 wt % water). However, a t higher moisture contents [see f o r example, Figures 7b and 7 c ] , s e v e r e l y perturbed f r a c t u r e s u r f a c e s were observed, c o n s i s t e n t with the occurrence of extensive p l a s t i c deformation and drawing a t the crack t i p . The use of a higher m a g n i f i c a t i o n a l s o permits a more de t a i l e d assessment of other s u r f a c e markings. Recent s t u d i e s (5, 29) of the fractography of polymers subjected to FCP t e s t s have i d e n t i f i e d two types of l i n e a r f r a c t u r e markings o r i e n t e d p a r a l l e l to the advancing crack f r o n t : discontinuous growth bands (DGB) and c l a s s i c a l f a t i g u e s t r i a t i o n s . The DGB markings, u s u a l l y found at low values of ΔΚ, represent d i s c r e t e increments of crack growth corresponding to the dimension of the p l a s t i c zone a s s o c i a t e d with the crack t i p . In t h i s r e g i o n , the macroscopic crack advances i n a discontinuous f a s h i o n , w i t h the crack remaining p e r i o d i c a l l y dormant f o r as many as 105 c y c l e s . True f a t i g u e s t r i a t i o n s are found a t higher ΔΚ l e v e l s , and r e f l e c t the extent of crack advance i n a s i n g l e load c y c l e . Regardless of the water content o r ΔΚ l e v e l , no DGB s have yet been observed i n any nylon specimens ( e i t h e r S e r i e s A or Series B ) . By c o n t r a s t , the discontinuous growth of f a t i g u e cracks has been described i n s t u d i e s of both c r y s t a l l i n e and amorphous polymers [ f o r example, i n p o l y a c e t a l (29,30), p o l y ethylene (_31) , and a v a r i e t y of p o o r l y c r y s t a l l i n e or amorphous polymers ( 5 ) ] . While the process of DGB formation i n p o o r l y c r y s t a l l i n e or amorphous polymers i s f a i r l y w e l l understood (_5, 12,29,32) the r o l e of w e l l developed c r y s t a l l i n i t y i s not c l e a r . Hence the reason why DGB bands are not observed i n nylon 66 i s as yet unknown. Markings b e l i e v e d to be c l a s s i c a l f a t i g u e s t r i a t i o n s (Figure 8) were, however, observed i n specimens having water contents i n the range between 2.2 and 5.7 wt %, i n c l u s i v e . S i m i l a r markings have been noted by others f o r s e v e r a l c r y s t a l l i n e polymers (31 ,_33,34_,j^, 36.>.37) , and i n t e r p r e t e d as f a t i g u e s t r i a t i o n s (though without f i r m evidence). The c r i t i c a l t e s t of whether or not such markings are true f a t i g u e s t r i a t i o n s i s to determine by measurement whether the spacings of the markings agree with the corresponding increments of macroscopic crack advance. In f a c t , examination of our specimens suggests that the markings are i n f a c t true f a t i g u e s t r i a t i o n s , f o r the spacings do correspond to the increments of crack advance (see, f o r example, F i g u r e 9 ) . The f i n d i n g s confirm the data obtained i n the p r e l i m i n a r y study (11), which c o n s t i t u t e the f i r s t unequivocal evidence f o r the formation of true f a t i g u e s t r i a t i o n s i n c r y s t a l l i n e polymers. [As p r e v i o u s l y noted, the measurements of crack length r e c e i v e d a s p e c i a l averaging technique because of the para b o l i c nature of the crack f r o n t (11)]. 1

Summary and Conclusions S e v e r a l major c o n c l u s i o n s ( t e n t a t i v e l y drawn i n the e a r l i e r




550

study) are confirmed (or r e v i s e d ) by t h i s more extensive study of the e f f e c t s of water on f a t i g u e crack propagation i n nylon 66: 1. Fatigue crack growth r a t e s i n nylon 66 are very s e n s i t i v e to moisture content i n the range from -0.2 wt % ("dry") to 8.5 wt % ( s a t u r a t e d ) . A 25-fold v a r i a t i o n e x i s t s between the f a s t e s t and slowest r a t e (at a constant ΔΚ of 3 MPav^n). 2. In c o n t r a s t to the case of f a t i g u e i n un-notched s p e c i mens, the f a t i g u e crack growth r a t e s e x h i b i t a sharp minimum at water contents i n the range of 2 wt % to 3 wt % — a range corresponding to about 1 water molecule per 4 amide groups. 3. The e x i s t e n c e of the minimum must s u r e l y r e f l e c t the presence of competing f a t i g u e mechanisms. I t i s proposed that the decrease of crack growth r a t e s at low water contents r e f l e c t s the a b i l i t y of t i g h t l y bound water to i n c r e a s e f r a c t u r e energy or to i n c r e a s e crack b l u n t i n g due to h y s t e r e t i c heating l o c a l i z e d at the crack t i p . At higher concentrations of water, the r e l a t i v e increase of crack growth r a t e i s a t t r i b u t e d to the r a p i d decrease i n the room temperature modulus of nylon 66, causing a lowering of the r e s i s t a n c e exerted by the bulk m a t e r i a l to the advance of the crack and an i n c r e a s e i n crack growth r a t e . In a d d i t i o n , the greater h y s t e r e t i c heating noted at higher moisture contents may c o n t r i b u t e to a f u r t h e r decrease i n the modulus. 4. These observations are supported by the onset of per c e p t i b l e heating a t water contents above about 2 wt % water, and by the onset of s t r e s s - w h i t e n i n g and a s i g n i f i c a n t degree of p l a s t i c deformation i n the same c o n c e n t r a t i o n range. While p l a s t i c deformation i s o f t e n a s s o c i a t e d with high f r a c t u r e energies, i t i s l i k e l y that the i n c r e a s e i n f r a c t u r e energy i s overwhelmed by a decrease i n modulus due to p l a s t i c i z a t i o n by f r e e water and to h y s t e r e t i c heating (see c o n c l u s i o n number 2). There i s a l s o some evidence that t r a n s - s p h e r u l i t i c f a i l u r e occurs at low water contents and r e o r g a n i z a t i o n of the s p h e r u l i t i c s t r u c t u r e at high water contents. 5. C l a s s i c a l f a t i g u e s t r i a t i o n s are observed at intermediate water contents, confirming e a r l i e r f i n d i n g s . However, no evidence was found (at any water content or ΔΚ l e v e l ) f o r discontinuous growth bands, which are found i n many other polymers. 6. From a t e c h n o l o g i c a l viewpoint, i t i s important to recognize that the f a t i g u e response of nylon 66 as a f u n c t i o n of water content i s s t r o n g l y dependent on the t e s t method — i n p a r t i c u l a r on the presence or absence of notches and, presumably, the frequency. Acknowledgement This work was Research.

supported i n p a r t by the O f f i c e of Naval


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