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Impact damage on annealed and on tempered flat glass
Journal of Non-Crystalline Solids 38 & 39 (1980) 413-418 Q No~th-Holland Publishing Company
I M P A C T D A M A G E ON A N N E A L E D A N D ON T E M P E R E D F L A T G L A S S J.
R. V a r n e r ,
W.
Hallwig,
and
A. W a l t e r
Materials Department S c h o o l of E n g i n e e r i n g and A p p l i e d S c i e n c e U n i v e r s i t y of C a l i f o r n i a , Los A n g e l e s Los A n g e l e s , C a l i f o r n i a U.S.A.
I m p a c t d a m a g e was p r o d u c e d on a n n e a l e d a n d on tempered flat glass specimens using a variety of q u a r t z p a r t i c l e s and s t e e l b a l l b e a r i n g s . I m p a c t s w i t h q u a r t z p a r t i c l e s r e s u l t e d in d a m a g e w h i c h is t y p i c a l for s h a r p i n d e n t e r s , a l t h o u g h c o n s i d e r a b l e s c a t t e r in the s i z e of s i t e s was present. Ball b e a r i n g s c a u s e d H e r t z i a n f r a c t u r e . It w a s p o s s i b l e to c o r r e l a t e the r e s u l t i n g s t r e n g t h d e g r a d a t i o n w i t h i m p a c t e n e r g y and d a m a g e m o r p h o l o g y . T e m p e r i n g was e f f e c t i v e in r e d u c i n g d a m a g e and loss of s t r e n g t h . The r e q u i r e d f r a c t u r e s u r f a c e e n e r g y to p r o d u c e the H e r t z i a n d e f e c t s was e s t i m a t e d to be o n l y a small f r a c t i o n of the a v a i l a b l e k i n e t i c (impact) e n e r g y . INTRODUCTION It has b e e n k n o w n for s o m e t i m e t h a t f l a w s on g l a s s s u r f a c e s are r e s p o n s i b l e for the low p r a c t i c a l s t r e n g t h s of g l a s s p r o d u c t s ; h o w ever, the s y s t e m a t i c i n v e s t i g a t i o n of i m p a c t d a m a g e and the r e s u l t ing s t r e n g t h d e g r a d a t i o n is a r e l a t i v e l y r e c e n t d e v e l o p m e n t (1,5). T e c h n i q u e s h a d b e e n d e v e l o p e d to c h a r a c t e r i z e the c o m p l e x i m p a c t s i t e s and the r e s u l t i n g f r a c t u r e o r i g i n s (4,5). T h e s e e a r l i e r exp e r i m e n t s h a d u s e d SiC p a r t i c l e s to p r o d u c e the damage, and the imp a c t m e t h o d h a d not a l l o w e d c a l c u l a t i o n of the i m p a c t e n e r g y . Acc o r d i n g l y , it w a s d e c i d e d to e x t e n d this w o r k by u s i n g p a r t i c l e s t h a t w o u l d be m o r e like t h o s e t h a t are e n c o u n t e r e d in s e r v i c e c o n d i t i o n s and to use an i m p a c t m e t h o d t h a t was w e l l d e f i n e d . Spherical p a r t i c l e s w e r e a l s o i n c l u d e d , b e c a u s e t h e y are g e o m e t r i c a l l y r e g u l a D and w o u l d aid in the s t u d y of the e f f e c t of t e m p e r i n g on d a m a g e r e s i s t a n c e and s t r e n g t h d e g r a d a t i o n . EXPERIMENTAL
PROCEDURE
S p e c i m e n s w e r e p r e p a r e d f r o m a c o m m e r c i a l s o d a - l i m e - s i l i c a flat glas~ d r a w n in the F o u r c a u l t p r o c e s s ( s u p p l i e d by F l a c h g l a s AG D e l o g - D e t a g Weiden, West Germany). A n n e a l e d s p e c i m e n s w e r e 5 0 x 5 0 x 2 0 mm, the t e m p e r e d s p e c i m e n s w e r e t h i c k e ~ 3.75 mm, to a s s i s t the d e v e l o p m e n t of an a p p r o p r i a t e t e m p e r a t u r e g r a d i e n t d u r i n g q u e n c h i n g . Temp e r i n g w a s c a r r i e d out by air q u e n c h i n g the s p e c i m e n s f r o m 6 0 0 ° C (Tg of this g l a s s is 540 o C), a p r o c e d u r e t h a t r e s u l t e d in c o m p r e s sive s t r e s s e s of a b o u t 21 M N / m 2 at the s u r f a c e . I m p a c t i n g was d o n e u s i n g a c e n t r i f u g e t h a t had b e e n d e s i g n e d and c o n s t r u c t e d for t h i s p r o j e c t . The s p e c i m e n s w e r e m o u n t e d on the arms of the c e n t r i f u g e , and the p a r t i c l e s w e r e d r o p p e d i n t o the p a t h of
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the r o t a t i n g g l a s s p l a t e s . The i m p a c t v e l o c i t i e s t h a t w e r e a v a i l a b l e w i t h t h i s m a c h i n e w e r e 60, 76, 92, 117, 150, and 181 k m / h (17, 21, 25, 32, 45, a n d 50 m/s). T h e n u m b e r of i m p a c t s i t e s on e a c h s p e c i m e n w a s k e p t s m a l l by u s i n g a l i m i t e d n u m b e r of p a r t i c l e s for e a c h test. Also, the s p e c i m e n s w e r e m a s k e d , l e a v i n g o n l y a small c i r c u lar a r e a e x p o s e d . Quartz particles were sands from three sources. O n e of t h e s e was m i l l e d p r i o r to c l a s s i f i c a t i o n in o r d e r to p r o d u c e s h a r p p a r t i c l e s . E a c h of the s a n d s was c l a s s i f i e d into v a r i o u s p a r t i c l e - s i z e f r a c t i o n s by u s i n g s i e v e s . The s a n d s w e r e s e l e c t e d b e c a u s e t h e y p r o v i d e d v a r ious p a r t i c l e m o r p h o l o g i e s (very s h a r p to r o u n d e d ) . The f i n e s t s i z e f r a c t i o n was 0 . 1 - 0 . 2 5 mm, and the c o a r s e s t was 0 . 6 3 - 1 . 0 mm, w i t h s e v e r a l o t h e r f r a c t i o n s in b e t w e e n t h e s e two. The s p h e r i c a l parti c l e s w e r e h a r d e n e d s t e e l b a l l b e a r i n g s w i t h a d i a m e t e r of 1.04 mm. C h a r a c t e r i z a t i o n of the i m p a c t s i t e s w a s b e g u n u s i n g an o p t i c a l microscope. T h i s was f o l l o w e d by s c a n n i n g e l e c t r o n m i c r o s c o p y of PVC r e p l i c a s of the s u r f a c e s . A f t e r f r a c t u r e , the o r i g i n s w e r e d i r e c t l y o b s e r v e d in the s c a n n i n g e l e c t r o n m i c r o s c o p e , and m e a s u r e m e n t s of H e r t z i a n c o n e s and s u r f a c e c o n t a c t c r a c k s w e r e p e r f o r m e d on these specimens. F r a c t u r e s t r e n g t h s w e r e d e t e r m i n e d u s i n g the c o n c e n t r i c - r i n g test. The m a s k i n g w h i c h w a s d e s c r i b e d e a r l i e r e n s u r e d t h a t f r a c t u r e o c c u r red w i t h i n the s m a l l e r of the two rings, i.e. t h e r e was no p r o b l e m in a p p l y i n g the s t r e n g t h f o r m u l a . T a p e w a s a p p l i e d to the t e n s i l e s u r f a c e p r i o r to l o a d i n g . T h i s h a d no e f f e c t on the s t r e n g t h , but it did h o l d the b r o k e n p i e c e s t o g e t h e r so t h a t the f r a c t u r e o r i g i n c o u l d be r e c o v e r e d for e x a m i n a t i o n . RESULTS
AND DISCUSSION
All t h r e e s a n d s p r o d u c e d d e f e c t s w h i c h are t y p i c a l for s h a r p i n d e n ters, i.e. m e d i a n c r a c k s w h i c h are p e r p e n d i c u l a r to the s u r f a c e and e x t e n d i n t o the glass, and l a t e r a l v e n t s w h i c h o f t e n r e a c h the surface, c a u s i n g s p a l l i n g . F i g u r e 1 s h o w s an i m p a c t s i t e w h i c h i l l u s trates these features. In this case, the l a t e r a l v e n t s are not f u l l y d e v e l o p e d and do not r e a c h the s u r f a c e . In Fig. 2 s a n d p a r ticles of the s a m e t y p e t h a t c a u s e d the d e f e c t seen in Fig. 1 are shown. V e r y f e w s h a r p p o i n t s are to be seen, yet this s a n d p r o d u c e s s h a r p - p a r t i c l e i m p a c t sites. A c o m p a r i s o n of the s i t e s p r o d u c e d by the t h r e e s a n d s shows t h a t the s a n d s h a v i n g the m o r e i r r e g u l a r , s h a r p e r g r a i n s w e r e m o r e e f f e c t i v e in p r o d u c i n g f u l l y d e v e l o p e d l a t e r a l v e n t s . Strength measurements to be d i s c u s s e d l a t e r i n d i c a t e t h a t the r o u n d e d g r a i n s w e r e less e f f e c t i v e in p r o d u c i n g m e d i a n v e n t s as well. The steel ball b e a r i n g s p r o d u c e d H e r t z i a n d a m a g e w h i c h is t y p i c a l of blunt indenters. O n l y the c o n t a c t c r a c k on the s u r f a c e was f o r m e d at the l o w e s t v e l o c i t y used. At h i g h e r v e l o c i t i e s , the H e r t z i a n cone developed, then multiple surface cracking occurred. At v e r y h i g h v e l o c i t i e s , m a t e r i a l s p a l l e d off a r o u n d the c o n t a c t c r a c k and radial cracks developed. Figure 3 shows a specimen after fracture. The l o w - v e l o c i t y i m p a c t (60 km/h) p r o d u c e d a s i m p l e c o n t a c t c r a c k and f u l l y d e v e l o p e d subs u r f a c e cone. N o t i c e t h a t the f r a c t u r e m a r k i n g s c l e a r l y i n d i c a t e t h a t f r a c t u r e s t a r t e d at the l o w e r e d g e of the cone, e x t e n d e d into the g l a s s and t u r n e d b a c k to the s u r f a c e on b o t h sides of the e x p o s e d
J.R. Yarner et al. / Tempered Flat Glass
cone
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surface.
Fig.
1
Fig.
2
Fig. i. T r a n s m i t t e d l i g h t p h o t o m i c r o g r a p h of i m p a c t s i t e p r o d u c e d u s i n g q u a r t z s a n d (size f r a c t i o n : 0 . 4 0 - 0 . 6 3 mm) at i m p a c t v e l o c i t y of 92 km/h; m e d i a n c r a c k s f o r m the s t a r p a t t e r n ; l a t e r a l v e n t s b e t w e e n p a i r s of m e d i a n c r a c k s are not f u l l y d e v e l o p e d ; 275X. Fig. 2. P h o t o m i c r o g r a p h of s a n d of s a m e t y p e u s e d to m a k e i m p a c t site s h o w n in Fig. i; s i z e f r a c t i o n : 0 2 5 - 0 . 3 1 5 mm; 47X. The s i z e of the d e f e c t s was m e a s u r e d p r i o r to s t r e n g t h t e s t i n g . In the c a s e of i m p a c t sites p r o d u c e d by q u a r t z g r a i n s , this w a s d o n e for all s i z e f r a c t i o n s of all t h r e e sands but at just one i m p a c t v e l o c i t y - - 9 2 km/h. T h e s u r f a c e size of t h e s e i m p a c t s i t e s was t a k e n as the d i s t a n c e f r o m the c e n t e r of i m p a c t to the e d g e of the s p a l l e d off r e g i o n . U s i n g this c r i t e r i o n of size, it was seen t h a t the f r e s h l y m i l l e d s a n d p r o d u c e d the l a r g e s t d e f e c t s and the least s c a t ter w i t h i n a s i z e f r a c t i o n . It was t y p i c a l to h a v e a v a r i a t i o n in b o t h size and s h a p e for s i t e s p r o d u c e d by any p a r t i c u l a r size f r a c tion. The g r a i n s v a r y in s i z e and shape, and any s i n g l e g r a i n m a y s t r i k e the s u r f a c e w i t h a corner, an edge, or a flat s u r f a c e . The t e n d e n c y was to h a v e i n c r e a s e d size of d e f e c t s for i n c r e a s e d s i z e of sand particles. The s u r f a c e m e a s u r e of the size of i m p a c t s i t e s p r o d u c e d by s p h e r e s is the d i a m e t e r of the c o n t a c t crack. For the a n n e a l e d s p e c i m e n s , the d i a m e t e r of the c o n t a c t c r a c k i n c r e a s e d w i t h i n c r e a s i n g i m p a c t v e l o c i t y , but t h e r e s e e m e d to be a m a x i m u m at 150 km/h. Surface s p a l l i n g m e n t i o n e d e a r l i e r m i g h t a c c o u n t for the s m a l l e r a v e r a g e v a l u e o b t a i n e d at 181 km/h. A l t h o u g h t h e r e is c o n s i d e r a b l e of the s p e c i m e n s i m p a c t e d w i t h
s c a t t e r in the d a t a for the s t r e n g t h s s a n d p a r t i c l e s , t h e t r e n d is that the
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Varner et al. / T e m p e r e d Flat Glass
s t r e n g t h d e c r e a s e s as t h e s i z e of t h e d e f e c t s i n c r e a s e s . T h e r e is an i n d i c a t i o n t h a t t h e s h a p e of t h e p a r t i c l e s h a s an i n f l u e n c e in that the sand with rounded grains had the highest strength values for particular defect sizes. This would mean that the median vents a r e n o t as deep, e v e n t h o u g h t h e s u r f a c e s i z e is t h e same. The opp o s i t e t r e n d is s e e n w i t h t h e f r e s h l y m i l l e d , s p l i n t e r y sand.
Fig.
3
Fig.
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Fig. 3. S c a n n i n g e l e c t r o n m i c r o g r a p h of f r a c t u r e o r i g i n ; i m p a c t w a s w i t h 1.04 m m d i a m e t e r s t e e l s p h e r e at 60 k m / h ; s p e c i m e n w a s t i l t e d in m i c r o s c o p e to e n a b l e o r i g i n a l s u r f a c e w i t h c o n t a c t c r a c k (left) a n d f r a c t u r e s u r f a c e w i t h H e r t z i a n c o n e to b e s e e n s i m u l t a n e o u s l y ; f r a c t u r e s t a r t e d at l o w e r c ge of c o n e . Fig. 4. S c a n n i n g e l e c t r o n m i c r o g r a p h of f r a c t u r e o r i g i n ; i m p a c t w a s w i t h 1.04 m m d i a m e t e r s t e e l b a l l b e a r i n g at 150 k m / h ; n o t e l o s s of m a t e r i a l a r o u n d c o n t a c t c i r c l e a n d s p l i t t i n g of c o n e c r a c k b y r a d i a l c r a c k . S t r e n g t h m e a s u r e m e n t s of a n n e a l e d s p e c i m e n s i m p a c t e d w i t h b a l l b e a r i n g s d i d n o t s h o w a n y o b v i o u s d e p e n d e n c e of s t r e n g t h on t h e r a d i u s of t h e c o n t a c t c r a c k . T h e s t r e n g t h as a f u n c t i o n of i m p a c t v e l o c i t y (energy) f o l l o w e d t h e r e l a t i o n s h i p t h a t is e x p e c t e d f r o m t h e o r y - - a s u d d e n d r o p at l o w v e l o c i t i e s , t h e n a n e a r l y c o n s t a n t v a l u e at h i g h e r v e l o c i t i e s . A f u r t h e r s h a r p d r o p in s t r e n g t h w a s s e e n at t h e h i g h e s t v e l o c i t y (181 km/h) . T h i s can be e x p l a i n e d b y t h e f a c t t h a t r a d i a l c r a c k s a r e f o r m e d d u r i n g i m p a c t at t h i s v e l o city, a n d t h e s e a r e m o r e c r i t i c a l l y o r i e n t e d w i t h r e s p e c t to the applied stresses. T h e d e p t h of t h e c o n e crack, i.e. t h e v e r t i c a l d i s t a n c e f r o m t h e s u r f a c e to t h e l o w e r e d g e of t h e c r a c k , as m e a s u r e d
J.R. Varner et al. / Tempered Flat Glass
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on f r a c t u r e s p e c i m e n s , i n c r e a s e s s h a r p l y f r o m 60 k m / h to 76 km/h, but u n d e r g o e s m u c h s m a l l e r c h a n g e s at h i g h e r v e l o c i t i e s . In fact, a m a x i m u m is o b s e r v e d at 150 km/h, w h i c h c o r r e s p o n d s to the m a x i m u m in the r a d i u s of the c o n t a c t c r a c k a l r e a d y m e n t i o n e d . T h i s is a f u r t h e r i n d i c a t i o n t h a t it is the r a d i a l c r a c k f o r m a t i o n w h i c h c a u s e s the s h a r p d r o p in s t r e n g t h at 181 km/h. S p e c i m e n s w h i c h had b e e n t e m p e r e d but not i m p a c t e d g a v e an a v e r a g e s t r e n g t h of 315 M N / m 2. T h o s e i m p a c t e d at 60, 92, and 181 k m / h y i e l d e d s t r e n g t h s of 115, i00, and 90 M N / m 2 r e s p e c t i v e l y . These v a l u e s s e e m to f o l l o w the t h e o r e t i c a l curve. A n n e a l e d s p e c i m e n s imp a c t e d at t h e s e s a m e v e l o c i t i e s h a d s t r e n g t h s of 66, 36, and 21 M N / m 2 respectively. Thus, it is s e e n t h a t t e m p e r i n g d o e s p r o v i d e s o m e p r o t e c t i o n a g a i n s t s t r e n g t h d e g r a d a t i o n due to i m p a c t . N o t i c e t h a t the r a t i o of t e m p e r e d s t r e n g t h to a n n e a l e d s t r e n g t h i n c r e a s e s as the imp a c t v e l o c i t y i n c r e a s e s , i.e. the c o m p r e s s i v e s t r e s s e s s e e m to be e v e n m o r e e f f e c t i v e at h i g h e r i m p a c t e n e r g i e s . Corresponding results w e r e o b t a i n e d for o t h e r m e a s u r e m e n t s . The r a d i u s of the c o n t a c t c i r cle was s m a l l e r for the t e m p e r e d s p e c i m e n s (e.g. 210 vs. 244 ~m at 92 k m / h ) . Of course, the d e p t h and s i z e of the c o n e c r a c k are v e r y i m p o r t a n t f a c t o r s in the s t r e n g t h . In a f e w cases, the r a d i u s of the c o n e c r a c k at its b a s e w a s d e t e r mined from scanning electron micrographs. This allowed geometrical r e l a t i o n s h i p s to be u s e d to c a l c u l a t e the s u r f a c e a r e a of the c o n e crack. The a r e a of the c y l i n d r i c a l c i r c u l a r c r a c k w h i c h e x t e n d s a short d i s t a n c e into the g l a s s b e f o r e t u r n i n g and s p r e a d i n g to f o r m the c o n e was i g n o r e d , s i n c e it is o n l y a b o u t 1% of the c o n e c r a c k s u r f a c e area. The s u r f a c e a r e a was t h e n m u l t i p l i e d by two, b e c a u s e two s u r f a c e s are c r e a t e d d u r i n g f r a c t u r e . These calculations2Yielded an a v e r a g e of 1.30 m m 2 for t w o a n n e a l e d s p e c i m e n s and 0.12 mm for two t e m p e r e d s p e c i m e n s i m p a c t e d at 92 km/h. T h i s m e t h o d of c a l c u l a t i n g the f r a c t u r e s u r f a c e a r e a w i l l u n d e r e s t i m a t e the a c t u a l area, b e c a u s e it d o e s not a c c o u n t for s u c h t h i n g s as m u l t i p l e c r a c k i n g and d e v i a t i o n s f r o m ideal g e o m e t r y . H o w e v e r , it d o e s p r o v i d e an e s t i m a t e of the o r d e r of m a g n i t u d e of the f r a c t u r e s u r f a c e area. O n c e again, it is c l e a r t h a t t e m p e r i n g has p r o v i d e d m u c h m o r e d a m a g e r e s i s t a n c e . U s i n g the v a l u e s Qf f r a c t u r e s u r f a c e a r e a s h o w n a b o v e and a s s u m i n g a v a l u e of 3.82 J / m z for the f r a c t u r e s u r f a c e e n e r g y (6), the e n e r g y n e e d e d to p r o d u c e t h e s e a r e a s can be c a l c u l a t e d . For the a n n e a l e d s p e c i m e n s this v a l u e is 5×i0-6j, and for the t e m p e r e d s p e c i m e n s it is 4.6×i0-7j. At 92 km/h, the k i n e t i c e n e r g y ~f the i m p a c t i n g p a r t i c l e (1.04 nml d i a m e t e r s t e e l sphere) is 1 4 . 6 × 1 0 - J. In o t h e r words, o n l y a b o u t 0 . 3 4 Z of the a v a i l a b l e e n e r g y is u s e d to p r o d u c e t h e s e i m p a c t sites in the a n n e a l e d s p e c i m e n s , and o n l y a b o u t 0.03Z is n e e d e d to p r o d u c e the i m p a c t d a m g e in the t e m p e r e d s p e c i m e n s . This analysis has b e e n c a r r i e d out for t h e s e few s p e c i m e n s at this one v e l o c i t y only. Nonetheless, it is c l e a r that, e v e n a l l o w i n g for the u n d e r e s t i m a t e of the f r a c t u r e s u r f a c e area, the p e r c e n t a g e of a v a i l a b l e k i n e t i c e n e r g y t h a t is u s e d to p r o d u c e i m p a c t d e f e c t s is v e r y small. The r e s u l t s of this w o r k and of o t h e r e f f o r t s (7,8) s h o u l d e n c o u r a g e f u r t h e r i n v e s t i g a t i o n s of the e f f e c t of p r e - s t r e s s i n g on r e d u c i n g strength degradation. ACKNOWLEDGEMENTS T h i s w o r k was p e r f o r m e d at the I n s t i t u t fuer W e r k s t o f f w i s s e n s c h a f t e n III, U n i v e r s i t a e t E r l a n g e n - N u e r n b e r g , E r ] a n g e n , W e s t G e r m a n y , and the a u t h o r s w i s h to t h a n k the D i r e c t o r of the I n s t i t u t , P r o f e s s o r Dr. H. J. Oel, for his e n c o u r a g e m e n t and s u p p o r t . T h a n k s are a l s o due to Dr. H. E n g e l k e , w h o d e s i g n e d the c e n t r i f u g e . M o s t of the s c a n n i n g
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electron m i c r o s c o p y J. Mecha.
was p e r f o r m e d
by K.
Piper,
with
assistance
from
REFERENCES (i)
Evans, J. Am.
A. G., Ceram.
S t r e n g t h d e g r a d a t i o n by p r o j e c t i l e Soc., 56 (1973) 405-409.
impacts,
(2)
Wiederhorn, S. M. and Lawn, glass r e s u l t i n g from impact 60 (1977) 451-458.
(3)
Wiederhorn, S. M. and Lawn, B. R., Strength d e g r a d a t i o n of glass impacted with sharp particles: I, annealed surfaces, J. Am. Ceram. Soc., 62 (1979) 66-70.
(4)
Varner, J. R. and Oel, H. J., Einfluss yon O b e r f l a e c h e n b e s c h a e d i g u n g e n auf die F e s t i g k e i t von Glasstaeben, Glastechn. Ber., 48 (1975) 73-78.
(5)
Varner, J. R. and Oel, H. J., characterization, and effects Solids, 19 (1975) 321-333.
(6)
Wiederhorn, S. M., F r a c t u r e surface Ceram. Soc., 52 (1969) 99-105.
(7)
Kirchner, H. P. and Gruver, R. M., L o c a l i z e d impact damage stressed members, Mater. Sci. Eng., 28 (1977) 249-255.
(8)
Lawn, B. R., Marshall, D. B., and Wiederhorn, S. M., Strength d e g r a d a t i o n of glass impacted with sharp particles: II, t e m p e r e d surfaces, J. Am. Ceram. Soc., 62 (1979) 71-74.
B. R., S t r e n g t h d e g r a d a t i o n with spheres, J. Am. Ceram.
of Soc.,
Surface defects: their origin, on strength, Journ. Non-Cryst.
energy
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J. Am.
in
I M P A C T D A M A G E ON A N N E A L E D A N D ON T E M P E R E D F L A T G L A S S J.
R. V a r n e r ,
W.
Hallwig,
and
A. W a l t e r
Materials Department S c h o o l of E n g i n e e r i n g and A p p l i e d S c i e n c e U n i v e r s i t y of C a l i f o r n i a , Los A n g e l e s Los A n g e l e s , C a l i f o r n i a U.S.A.
I m p a c t d a m a g e was p r o d u c e d on a n n e a l e d a n d on tempered flat glass specimens using a variety of q u a r t z p a r t i c l e s and s t e e l b a l l b e a r i n g s . I m p a c t s w i t h q u a r t z p a r t i c l e s r e s u l t e d in d a m a g e w h i c h is t y p i c a l for s h a r p i n d e n t e r s , a l t h o u g h c o n s i d e r a b l e s c a t t e r in the s i z e of s i t e s was present. Ball b e a r i n g s c a u s e d H e r t z i a n f r a c t u r e . It w a s p o s s i b l e to c o r r e l a t e the r e s u l t i n g s t r e n g t h d e g r a d a t i o n w i t h i m p a c t e n e r g y and d a m a g e m o r p h o l o g y . T e m p e r i n g was e f f e c t i v e in r e d u c i n g d a m a g e and loss of s t r e n g t h . The r e q u i r e d f r a c t u r e s u r f a c e e n e r g y to p r o d u c e the H e r t z i a n d e f e c t s was e s t i m a t e d to be o n l y a small f r a c t i o n of the a v a i l a b l e k i n e t i c (impact) e n e r g y . INTRODUCTION It has b e e n k n o w n for s o m e t i m e t h a t f l a w s on g l a s s s u r f a c e s are r e s p o n s i b l e for the low p r a c t i c a l s t r e n g t h s of g l a s s p r o d u c t s ; h o w ever, the s y s t e m a t i c i n v e s t i g a t i o n of i m p a c t d a m a g e and the r e s u l t ing s t r e n g t h d e g r a d a t i o n is a r e l a t i v e l y r e c e n t d e v e l o p m e n t (1,5). T e c h n i q u e s h a d b e e n d e v e l o p e d to c h a r a c t e r i z e the c o m p l e x i m p a c t s i t e s and the r e s u l t i n g f r a c t u r e o r i g i n s (4,5). T h e s e e a r l i e r exp e r i m e n t s h a d u s e d SiC p a r t i c l e s to p r o d u c e the damage, and the imp a c t m e t h o d h a d not a l l o w e d c a l c u l a t i o n of the i m p a c t e n e r g y . Acc o r d i n g l y , it w a s d e c i d e d to e x t e n d this w o r k by u s i n g p a r t i c l e s t h a t w o u l d be m o r e like t h o s e t h a t are e n c o u n t e r e d in s e r v i c e c o n d i t i o n s and to use an i m p a c t m e t h o d t h a t was w e l l d e f i n e d . Spherical p a r t i c l e s w e r e a l s o i n c l u d e d , b e c a u s e t h e y are g e o m e t r i c a l l y r e g u l a D and w o u l d aid in the s t u d y of the e f f e c t of t e m p e r i n g on d a m a g e r e s i s t a n c e and s t r e n g t h d e g r a d a t i o n . EXPERIMENTAL
PROCEDURE
S p e c i m e n s w e r e p r e p a r e d f r o m a c o m m e r c i a l s o d a - l i m e - s i l i c a flat glas~ d r a w n in the F o u r c a u l t p r o c e s s ( s u p p l i e d by F l a c h g l a s AG D e l o g - D e t a g Weiden, West Germany). A n n e a l e d s p e c i m e n s w e r e 5 0 x 5 0 x 2 0 mm, the t e m p e r e d s p e c i m e n s w e r e t h i c k e ~ 3.75 mm, to a s s i s t the d e v e l o p m e n t of an a p p r o p r i a t e t e m p e r a t u r e g r a d i e n t d u r i n g q u e n c h i n g . Temp e r i n g w a s c a r r i e d out by air q u e n c h i n g the s p e c i m e n s f r o m 6 0 0 ° C (Tg of this g l a s s is 540 o C), a p r o c e d u r e t h a t r e s u l t e d in c o m p r e s sive s t r e s s e s of a b o u t 21 M N / m 2 at the s u r f a c e . I m p a c t i n g was d o n e u s i n g a c e n t r i f u g e t h a t had b e e n d e s i g n e d and c o n s t r u c t e d for t h i s p r o j e c t . The s p e c i m e n s w e r e m o u n t e d on the arms of the c e n t r i f u g e , and the p a r t i c l e s w e r e d r o p p e d i n t o the p a t h of
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the r o t a t i n g g l a s s p l a t e s . The i m p a c t v e l o c i t i e s t h a t w e r e a v a i l a b l e w i t h t h i s m a c h i n e w e r e 60, 76, 92, 117, 150, and 181 k m / h (17, 21, 25, 32, 45, a n d 50 m/s). T h e n u m b e r of i m p a c t s i t e s on e a c h s p e c i m e n w a s k e p t s m a l l by u s i n g a l i m i t e d n u m b e r of p a r t i c l e s for e a c h test. Also, the s p e c i m e n s w e r e m a s k e d , l e a v i n g o n l y a small c i r c u lar a r e a e x p o s e d . Quartz particles were sands from three sources. O n e of t h e s e was m i l l e d p r i o r to c l a s s i f i c a t i o n in o r d e r to p r o d u c e s h a r p p a r t i c l e s . E a c h of the s a n d s was c l a s s i f i e d into v a r i o u s p a r t i c l e - s i z e f r a c t i o n s by u s i n g s i e v e s . The s a n d s w e r e s e l e c t e d b e c a u s e t h e y p r o v i d e d v a r ious p a r t i c l e m o r p h o l o g i e s (very s h a r p to r o u n d e d ) . The f i n e s t s i z e f r a c t i o n was 0 . 1 - 0 . 2 5 mm, and the c o a r s e s t was 0 . 6 3 - 1 . 0 mm, w i t h s e v e r a l o t h e r f r a c t i o n s in b e t w e e n t h e s e two. The s p h e r i c a l parti c l e s w e r e h a r d e n e d s t e e l b a l l b e a r i n g s w i t h a d i a m e t e r of 1.04 mm. C h a r a c t e r i z a t i o n of the i m p a c t s i t e s w a s b e g u n u s i n g an o p t i c a l microscope. T h i s was f o l l o w e d by s c a n n i n g e l e c t r o n m i c r o s c o p y of PVC r e p l i c a s of the s u r f a c e s . A f t e r f r a c t u r e , the o r i g i n s w e r e d i r e c t l y o b s e r v e d in the s c a n n i n g e l e c t r o n m i c r o s c o p e , and m e a s u r e m e n t s of H e r t z i a n c o n e s and s u r f a c e c o n t a c t c r a c k s w e r e p e r f o r m e d on these specimens. F r a c t u r e s t r e n g t h s w e r e d e t e r m i n e d u s i n g the c o n c e n t r i c - r i n g test. The m a s k i n g w h i c h w a s d e s c r i b e d e a r l i e r e n s u r e d t h a t f r a c t u r e o c c u r red w i t h i n the s m a l l e r of the two rings, i.e. t h e r e was no p r o b l e m in a p p l y i n g the s t r e n g t h f o r m u l a . T a p e w a s a p p l i e d to the t e n s i l e s u r f a c e p r i o r to l o a d i n g . T h i s h a d no e f f e c t on the s t r e n g t h , but it did h o l d the b r o k e n p i e c e s t o g e t h e r so t h a t the f r a c t u r e o r i g i n c o u l d be r e c o v e r e d for e x a m i n a t i o n . RESULTS
AND DISCUSSION
All t h r e e s a n d s p r o d u c e d d e f e c t s w h i c h are t y p i c a l for s h a r p i n d e n ters, i.e. m e d i a n c r a c k s w h i c h are p e r p e n d i c u l a r to the s u r f a c e and e x t e n d i n t o the glass, and l a t e r a l v e n t s w h i c h o f t e n r e a c h the surface, c a u s i n g s p a l l i n g . F i g u r e 1 s h o w s an i m p a c t s i t e w h i c h i l l u s trates these features. In this case, the l a t e r a l v e n t s are not f u l l y d e v e l o p e d and do not r e a c h the s u r f a c e . In Fig. 2 s a n d p a r ticles of the s a m e t y p e t h a t c a u s e d the d e f e c t seen in Fig. 1 are shown. V e r y f e w s h a r p p o i n t s are to be seen, yet this s a n d p r o d u c e s s h a r p - p a r t i c l e i m p a c t sites. A c o m p a r i s o n of the s i t e s p r o d u c e d by the t h r e e s a n d s shows t h a t the s a n d s h a v i n g the m o r e i r r e g u l a r , s h a r p e r g r a i n s w e r e m o r e e f f e c t i v e in p r o d u c i n g f u l l y d e v e l o p e d l a t e r a l v e n t s . Strength measurements to be d i s c u s s e d l a t e r i n d i c a t e t h a t the r o u n d e d g r a i n s w e r e less e f f e c t i v e in p r o d u c i n g m e d i a n v e n t s as well. The steel ball b e a r i n g s p r o d u c e d H e r t z i a n d a m a g e w h i c h is t y p i c a l of blunt indenters. O n l y the c o n t a c t c r a c k on the s u r f a c e was f o r m e d at the l o w e s t v e l o c i t y used. At h i g h e r v e l o c i t i e s , the H e r t z i a n cone developed, then multiple surface cracking occurred. At v e r y h i g h v e l o c i t i e s , m a t e r i a l s p a l l e d off a r o u n d the c o n t a c t c r a c k and radial cracks developed. Figure 3 shows a specimen after fracture. The l o w - v e l o c i t y i m p a c t (60 km/h) p r o d u c e d a s i m p l e c o n t a c t c r a c k and f u l l y d e v e l o p e d subs u r f a c e cone. N o t i c e t h a t the f r a c t u r e m a r k i n g s c l e a r l y i n d i c a t e t h a t f r a c t u r e s t a r t e d at the l o w e r e d g e of the cone, e x t e n d e d into the g l a s s and t u r n e d b a c k to the s u r f a c e on b o t h sides of the e x p o s e d
J.R. Yarner et al. / Tempered Flat Glass
cone
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surface.
Fig.
1
Fig.
2
Fig. i. T r a n s m i t t e d l i g h t p h o t o m i c r o g r a p h of i m p a c t s i t e p r o d u c e d u s i n g q u a r t z s a n d (size f r a c t i o n : 0 . 4 0 - 0 . 6 3 mm) at i m p a c t v e l o c i t y of 92 km/h; m e d i a n c r a c k s f o r m the s t a r p a t t e r n ; l a t e r a l v e n t s b e t w e e n p a i r s of m e d i a n c r a c k s are not f u l l y d e v e l o p e d ; 275X. Fig. 2. P h o t o m i c r o g r a p h of s a n d of s a m e t y p e u s e d to m a k e i m p a c t site s h o w n in Fig. i; s i z e f r a c t i o n : 0 2 5 - 0 . 3 1 5 mm; 47X. The s i z e of the d e f e c t s was m e a s u r e d p r i o r to s t r e n g t h t e s t i n g . In the c a s e of i m p a c t sites p r o d u c e d by q u a r t z g r a i n s , this w a s d o n e for all s i z e f r a c t i o n s of all t h r e e sands but at just one i m p a c t v e l o c i t y - - 9 2 km/h. T h e s u r f a c e size of t h e s e i m p a c t s i t e s was t a k e n as the d i s t a n c e f r o m the c e n t e r of i m p a c t to the e d g e of the s p a l l e d off r e g i o n . U s i n g this c r i t e r i o n of size, it was seen t h a t the f r e s h l y m i l l e d s a n d p r o d u c e d the l a r g e s t d e f e c t s and the least s c a t ter w i t h i n a s i z e f r a c t i o n . It was t y p i c a l to h a v e a v a r i a t i o n in b o t h size and s h a p e for s i t e s p r o d u c e d by any p a r t i c u l a r size f r a c tion. The g r a i n s v a r y in s i z e and shape, and any s i n g l e g r a i n m a y s t r i k e the s u r f a c e w i t h a corner, an edge, or a flat s u r f a c e . The t e n d e n c y was to h a v e i n c r e a s e d size of d e f e c t s for i n c r e a s e d s i z e of sand particles. The s u r f a c e m e a s u r e of the size of i m p a c t s i t e s p r o d u c e d by s p h e r e s is the d i a m e t e r of the c o n t a c t crack. For the a n n e a l e d s p e c i m e n s , the d i a m e t e r of the c o n t a c t c r a c k i n c r e a s e d w i t h i n c r e a s i n g i m p a c t v e l o c i t y , but t h e r e s e e m e d to be a m a x i m u m at 150 km/h. Surface s p a l l i n g m e n t i o n e d e a r l i e r m i g h t a c c o u n t for the s m a l l e r a v e r a g e v a l u e o b t a i n e d at 181 km/h. A l t h o u g h t h e r e is c o n s i d e r a b l e of the s p e c i m e n s i m p a c t e d w i t h
s c a t t e r in the d a t a for the s t r e n g t h s s a n d p a r t i c l e s , t h e t r e n d is that the
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Varner et al. / T e m p e r e d Flat Glass
s t r e n g t h d e c r e a s e s as t h e s i z e of t h e d e f e c t s i n c r e a s e s . T h e r e is an i n d i c a t i o n t h a t t h e s h a p e of t h e p a r t i c l e s h a s an i n f l u e n c e in that the sand with rounded grains had the highest strength values for particular defect sizes. This would mean that the median vents a r e n o t as deep, e v e n t h o u g h t h e s u r f a c e s i z e is t h e same. The opp o s i t e t r e n d is s e e n w i t h t h e f r e s h l y m i l l e d , s p l i n t e r y sand.
Fig.
3
Fig.
4
Fig. 3. S c a n n i n g e l e c t r o n m i c r o g r a p h of f r a c t u r e o r i g i n ; i m p a c t w a s w i t h 1.04 m m d i a m e t e r s t e e l s p h e r e at 60 k m / h ; s p e c i m e n w a s t i l t e d in m i c r o s c o p e to e n a b l e o r i g i n a l s u r f a c e w i t h c o n t a c t c r a c k (left) a n d f r a c t u r e s u r f a c e w i t h H e r t z i a n c o n e to b e s e e n s i m u l t a n e o u s l y ; f r a c t u r e s t a r t e d at l o w e r c ge of c o n e . Fig. 4. S c a n n i n g e l e c t r o n m i c r o g r a p h of f r a c t u r e o r i g i n ; i m p a c t w a s w i t h 1.04 m m d i a m e t e r s t e e l b a l l b e a r i n g at 150 k m / h ; n o t e l o s s of m a t e r i a l a r o u n d c o n t a c t c i r c l e a n d s p l i t t i n g of c o n e c r a c k b y r a d i a l c r a c k . S t r e n g t h m e a s u r e m e n t s of a n n e a l e d s p e c i m e n s i m p a c t e d w i t h b a l l b e a r i n g s d i d n o t s h o w a n y o b v i o u s d e p e n d e n c e of s t r e n g t h on t h e r a d i u s of t h e c o n t a c t c r a c k . T h e s t r e n g t h as a f u n c t i o n of i m p a c t v e l o c i t y (energy) f o l l o w e d t h e r e l a t i o n s h i p t h a t is e x p e c t e d f r o m t h e o r y - - a s u d d e n d r o p at l o w v e l o c i t i e s , t h e n a n e a r l y c o n s t a n t v a l u e at h i g h e r v e l o c i t i e s . A f u r t h e r s h a r p d r o p in s t r e n g t h w a s s e e n at t h e h i g h e s t v e l o c i t y (181 km/h) . T h i s can be e x p l a i n e d b y t h e f a c t t h a t r a d i a l c r a c k s a r e f o r m e d d u r i n g i m p a c t at t h i s v e l o city, a n d t h e s e a r e m o r e c r i t i c a l l y o r i e n t e d w i t h r e s p e c t to the applied stresses. T h e d e p t h of t h e c o n e crack, i.e. t h e v e r t i c a l d i s t a n c e f r o m t h e s u r f a c e to t h e l o w e r e d g e of t h e c r a c k , as m e a s u r e d
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on f r a c t u r e s p e c i m e n s , i n c r e a s e s s h a r p l y f r o m 60 k m / h to 76 km/h, but u n d e r g o e s m u c h s m a l l e r c h a n g e s at h i g h e r v e l o c i t i e s . In fact, a m a x i m u m is o b s e r v e d at 150 km/h, w h i c h c o r r e s p o n d s to the m a x i m u m in the r a d i u s of the c o n t a c t c r a c k a l r e a d y m e n t i o n e d . T h i s is a f u r t h e r i n d i c a t i o n t h a t it is the r a d i a l c r a c k f o r m a t i o n w h i c h c a u s e s the s h a r p d r o p in s t r e n g t h at 181 km/h. S p e c i m e n s w h i c h had b e e n t e m p e r e d but not i m p a c t e d g a v e an a v e r a g e s t r e n g t h of 315 M N / m 2. T h o s e i m p a c t e d at 60, 92, and 181 k m / h y i e l d e d s t r e n g t h s of 115, i00, and 90 M N / m 2 r e s p e c t i v e l y . These v a l u e s s e e m to f o l l o w the t h e o r e t i c a l curve. A n n e a l e d s p e c i m e n s imp a c t e d at t h e s e s a m e v e l o c i t i e s h a d s t r e n g t h s of 66, 36, and 21 M N / m 2 respectively. Thus, it is s e e n t h a t t e m p e r i n g d o e s p r o v i d e s o m e p r o t e c t i o n a g a i n s t s t r e n g t h d e g r a d a t i o n due to i m p a c t . N o t i c e t h a t the r a t i o of t e m p e r e d s t r e n g t h to a n n e a l e d s t r e n g t h i n c r e a s e s as the imp a c t v e l o c i t y i n c r e a s e s , i.e. the c o m p r e s s i v e s t r e s s e s s e e m to be e v e n m o r e e f f e c t i v e at h i g h e r i m p a c t e n e r g i e s . Corresponding results w e r e o b t a i n e d for o t h e r m e a s u r e m e n t s . The r a d i u s of the c o n t a c t c i r cle was s m a l l e r for the t e m p e r e d s p e c i m e n s (e.g. 210 vs. 244 ~m at 92 k m / h ) . Of course, the d e p t h and s i z e of the c o n e c r a c k are v e r y i m p o r t a n t f a c t o r s in the s t r e n g t h . In a f e w cases, the r a d i u s of the c o n e c r a c k at its b a s e w a s d e t e r mined from scanning electron micrographs. This allowed geometrical r e l a t i o n s h i p s to be u s e d to c a l c u l a t e the s u r f a c e a r e a of the c o n e crack. The a r e a of the c y l i n d r i c a l c i r c u l a r c r a c k w h i c h e x t e n d s a short d i s t a n c e into the g l a s s b e f o r e t u r n i n g and s p r e a d i n g to f o r m the c o n e was i g n o r e d , s i n c e it is o n l y a b o u t 1% of the c o n e c r a c k s u r f a c e area. The s u r f a c e a r e a was t h e n m u l t i p l i e d by two, b e c a u s e two s u r f a c e s are c r e a t e d d u r i n g f r a c t u r e . These calculations2Yielded an a v e r a g e of 1.30 m m 2 for t w o a n n e a l e d s p e c i m e n s and 0.12 mm for two t e m p e r e d s p e c i m e n s i m p a c t e d at 92 km/h. T h i s m e t h o d of c a l c u l a t i n g the f r a c t u r e s u r f a c e a r e a w i l l u n d e r e s t i m a t e the a c t u a l area, b e c a u s e it d o e s not a c c o u n t for s u c h t h i n g s as m u l t i p l e c r a c k i n g and d e v i a t i o n s f r o m ideal g e o m e t r y . H o w e v e r , it d o e s p r o v i d e an e s t i m a t e of the o r d e r of m a g n i t u d e of the f r a c t u r e s u r f a c e area. O n c e again, it is c l e a r t h a t t e m p e r i n g has p r o v i d e d m u c h m o r e d a m a g e r e s i s t a n c e . U s i n g the v a l u e s Qf f r a c t u r e s u r f a c e a r e a s h o w n a b o v e and a s s u m i n g a v a l u e of 3.82 J / m z for the f r a c t u r e s u r f a c e e n e r g y (6), the e n e r g y n e e d e d to p r o d u c e t h e s e a r e a s can be c a l c u l a t e d . For the a n n e a l e d s p e c i m e n s this v a l u e is 5×i0-6j, and for the t e m p e r e d s p e c i m e n s it is 4.6×i0-7j. At 92 km/h, the k i n e t i c e n e r g y ~f the i m p a c t i n g p a r t i c l e (1.04 nml d i a m e t e r s t e e l sphere) is 1 4 . 6 × 1 0 - J. In o t h e r words, o n l y a b o u t 0 . 3 4 Z of the a v a i l a b l e e n e r g y is u s e d to p r o d u c e t h e s e i m p a c t sites in the a n n e a l e d s p e c i m e n s , and o n l y a b o u t 0.03Z is n e e d e d to p r o d u c e the i m p a c t d a m g e in the t e m p e r e d s p e c i m e n s . This analysis has b e e n c a r r i e d out for t h e s e few s p e c i m e n s at this one v e l o c i t y only. Nonetheless, it is c l e a r that, e v e n a l l o w i n g for the u n d e r e s t i m a t e of the f r a c t u r e s u r f a c e area, the p e r c e n t a g e of a v a i l a b l e k i n e t i c e n e r g y t h a t is u s e d to p r o d u c e i m p a c t d e f e c t s is v e r y small. The r e s u l t s of this w o r k and of o t h e r e f f o r t s (7,8) s h o u l d e n c o u r a g e f u r t h e r i n v e s t i g a t i o n s of the e f f e c t of p r e - s t r e s s i n g on r e d u c i n g strength degradation. ACKNOWLEDGEMENTS T h i s w o r k was p e r f o r m e d at the I n s t i t u t fuer W e r k s t o f f w i s s e n s c h a f t e n III, U n i v e r s i t a e t E r l a n g e n - N u e r n b e r g , E r ] a n g e n , W e s t G e r m a n y , and the a u t h o r s w i s h to t h a n k the D i r e c t o r of the I n s t i t u t , P r o f e s s o r Dr. H. J. Oel, for his e n c o u r a g e m e n t and s u p p o r t . T h a n k s are a l s o due to Dr. H. E n g e l k e , w h o d e s i g n e d the c e n t r i f u g e . M o s t of the s c a n n i n g
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