.' .'

Indian Journal of Pure & Applied Physics Vol. 40. November 2002. pp. 8 16-827

( , ( I

Pilot-type scientific experimental device using optical method and ~omputer vision technology

Chern-S hengi Li n

1 I. Depart ment of AutomaticlCont ro l Engineering, Feng Chi a Uni versity. Taic hung, Taiwan

Received 5 Febru ary 2002; revised 15 May 2002 ; accepted 19 August 2002

Digital image and multimedia technology projected in a modified laser s hado~ spot set-up to guide people to engage in the scient ific model of the crack growth has been described . This- novel device can be operated automati call y. and econOlmcall y in a participat ing-displ ay. A new method to process the experimental data and predi ct the fra ctu re time in the PC-hased digit al signa l processing system has been developed . With one video CCD camera and frame grabber anal yzi ng a seri es of images of laser shadow spot during crack growth . the computers obtain the image size of laser shadow spot to eva lu ate stress in tensity factor in real time. An auto-range Iinding algorithm is used to lind the location of a laser shadow spot. Six special experimental conditions have been considered to imp rove thi s scientil"ic model. Participant s can operate the model themselves. so that they can observe the change of sizes of the laser shadow Spot images and can respond in timc to know the statu s of specimen in tension. It also can remind the attenti on of the participant with the sign of a speech sound befo re the break of spec imen to achi eve the crfect during recreation to learn.'

I Introduction

The application of the multimed ia techno logy has been spreading widely and constantly due to the impro vement on the speed and effic iency of the compute rs in recent yea rs. The compute rs have been mak ing a breakthrough from simple ca lcul ati ons to compl ex di splays of interne t and multimedia. Here, a no ve l scientific mode l is c reated, experiment of laser shadow spot, by using opti ca l method, and

multimedi a tec hnology (Fig. I ).

transparent shell

Fig. I - The set-Llp o f a pil ot-type scientific experiment al devi ce

This novel device IS a lso pilot-type and networked multimed ia. It means, one who is engaged in the ex periment can obta in the ass istance from the pilot system of this dev ice and the multimedia information in this experiment can offer

communi cati on services at a d istance. The fo llow ing merits have been set-up fo r thi s sc ientifi c ex perimental mode l, in a participating display:

• The participants should be inte rac ti ve and creative by observing the change of s izes of the laser shadow spot images and comparing the status of spec imen in tensi on.

• The pilot-type sys tem should be fl ex ible and easy to use. Participants ca n operate the mode l themse lves with out any other attendant's he lp .

• The sys tem should be used to educate the participants or other vi sitors In an educati ona l situation and improve learning.

• The system should provide access to the inte rnet as a means for presenting rea l-time experimenta l informati on.

• The participants are a ll owed to squeeze the hand lebar to add tensile force and ~~ee the breakage of specimen. The c rac k growth speed in the specimen is larger than SOO m/sec. So, the sudden rupture will bring amazement to the parti c ipants. They can rea li ze the importance o r precaution and re inforcement o f a structure.

• T he sys tem can pred ic t the fracture-time, with the sign of a speech sound and kee p magical

LIN : EX PERIMENAL DEVICE USING COMPUTER VISION TECHNOLOGY 8 17

impress ion in the visitor's mind .

• The requirements of waste mate ri a l and operation cost are very low.

Fig. 2 - Material ne3r cr3ck ti p forms a circular ~ h ape of pl asti c zo ne and shrink in th ickness under ten sion

tensile snaaow

force t ,,~t ,,/" ..E)

'" '"

" " " "

6::: divergent

laser light

./ ./

" ./

Fig. J - The gco illctry relati onsh ip of laser li ght and shadow ~ pOl p81tern

Today, many researchers around the world use the method of computer VISIOn and opti ca l technology in problems of stress analys is '·

2. The

technique of laser shadow spot in fracture ana lys is has ex tensively been used for dete rmining stress­intens ity facto rs in c rack probl ems. Material near crack tip forms a c ircul ar shape of pl asti c zone and shrink in thickness, under tens ion (Fig. 2). When the c rack tip of a test spec imen is irradiated by a

paralle l opti cal beam, the pathway of its re fl ec ti on and refraction wi II change from paralle l to non-para llel as a result of the influence from vari ety of pressures generated by the irradiation. The surface of the c rack tip in a specimen will s lant and produce di scontinuous geometric shapes. So, the laser shadow dimension can be derived (di ameter) by geometrica l optics formulation , which leads to the resu lt that, the light-intensity in the lase r shadow spot is zero (Fig. 3). The rays of re fl ec ti on and refrac ti on, c han ge to an enve lope-type of patte rn in a three-dimensiona l space ' . When this enve lope patte rn is projected on a screen, one bri ght curve ca lled the caustic curve (a curve surrounded by a dark zone or spot) is produced, as shown in Fig. 4.

transmitted 1 ight

-..l----II----~tic

crack D

screen

Fig. 4 - Light deflection form s the spot

The laser shadow spot's appl icat ion to fracture studies was origin a ll y pe rformed by Manogg". He used on ly the transmitted laser shadow spot. whi ch is sometimes ca lled the laser shadow spot. Theocari s~ made a bi g step in the development of the laser shadow spot method and menti oned use of the reflected caustics . A re fl ected causti c can prov ide an e lementary understanding o \" how the fracture occurs··". The re fl ec ted caustic inc ludes two pieces of informatio n (front- face caust ic and rear-face causti c) concerning the va lues of the stress-intensity factor K,. Another type o f information prov ided by the re fl ec ted causti c outside the laser shadow spot curve lies in the interference fringes and concerns the thickness va ri ation of the speci men. The appl icat io ll of the lase r shadow spot set-up for stress intens ity factor

8 18 INDIAN.J PURE & APPL PHYS, VOL 40, NOVEMB ER 2002

reference plane

specimen

CCI)

Fig. 5 - Laser shadow spot experimental set-up

r----- - 1

~ I I _~==:J D

'-- __ . ________________ .. ..1 ( a ) ... -------Tb5- - ----·-J

In ~-=l L----____________ J

( c )

I---'--~

k'--J~ run, I -.~. " I .-- - 'r:::::J:

l ______ :==-~ -----. __ J II -- :.~B~)--l __ ~--- _n_ .'! I

( e )

_w'~ _____ ._ •• __ ._ ... _---- _._-, I

! _ _ .LC-...., i ___ .J.

,~

( d )

Fig. 6 -- Six special experimental conditi ons

KI and plastic zone measurements has been widely discussed . For exampl e, Gdoutos & A ifanti s7

studi ed the lase r shadow spots around c racks under mi xed-mode loading conditi ons. Kamath & Kim' used the laser shadow spo t set-up to study three­dimensional deformati on fi e lds very near the c rack t ip. Theocari s'} has ex tended the laser shadow spots method in reso lving the eva luati on of cod and the core region of plane strain. Chang & Lin lo.11 had done beautiful dynami c K measurements by utili zing only transmilled laser shadow spot set-up .

fn a laser shadow spot set-up. a set of suitable apertures and lenses a rc usuall y used to achie ve the required magnificati on . The laser beam afte r a spatial filter can be regarded as a pointed light source. The ex pe rimente r here, introduces two

lenses, one lens converges the laser light leav ing the spati a l filt er to a parallel beam and the o the r one conve rges the li ght again. to inc rease the fl ex ibility of the opti ca l me thod . Many types of laser s hadow spot ex pe riments can be arranged by pass ing the laser li ght through so me advanced optica l filte ring, but these are not easy to se t-up for an experime nte r and are unsuitable fo r scientific model l'.u So he re, the above set-up was simplified to use onl y a di ode­pumping laser w ith di vergent lens on it s head . In othe r words , the inte nti on for thi s device is to design a museum apparatu s, and the xh ibiti on clerk need not worry over the moving a li gnment by experimenter o r othe r factors l-l-I, .

Digital image-process ing me thod was usual ly used in evaluating the obta ined opt ica l pattern ,

LIN: EXPERIM ENAL DEVICE USING COM PUTER VISION TECHNOLOGY

whic h is mainl y due to the ex ist ing singularity at the crack tip . The ray through the crack tip will form a c ircular e nve lope known as a causti c surface l

!>. One can move the sc reen towards the specimen to see the size of the circular image spot, which assumes a different diameter. As the caustics obta ined by a sphere are we ll defined , so the deviation of the

surface of the spec imen can be calcu lated , i.e., the charac ter ization of the deformed surface near the c rack tip is achi eved by the descripti on of the curve of the causti cs. Class ica l laser shadow spot patte rn s a re easy to be inte rpreted by the distance from crac k tip to the side of spot (Fig. 5) . A method to obta in these values is from the di stance between respecti ve peaks in the hi stograms of the image for evaluati on of KI of the c racks l7. When the direct ion of the max imum diamete r of the laser shadow spot curve is along the x or y-axi s, thi s measure ment is usua ll y accurate, since the peaks are very sharp . The va lue of the stress-intens ity factor is very sensitive to the measurement of the maximum diamete r of the laser shadow spot curve, and the fracture-ti me can be predi cted according to the va lue of the stress-intensity factor. The width of an ideal c rack in the spec imen is less than 0.2 mm. It is bett er to make an artificial c rack by a thin metallic di sk-cutte r. But he re, an artificial crack was made by a sawmill for sake of economy. The irregul arity of the sawn slit inc reases the difficulty in frac ture prediction . Furthe rmore, the exact locati on of laser shadow spot is ambiguous because of opti ca l diffraction effects . Consequentl y, the ex act position of the crack tip is also observed when the variation of the lase r shadow spot is too fast. So, new image­

process ing a lgorithm mu st be used to obtain the va lue of the maximum spot diamete r. The author des igned an auto-range findin g a lgorithm, to find the loca ti on o f lase r shadow spo t. As the participants will operate the mode l by the mselves, six spec ia l ex perimenta l conditi ons are considered to improve the re liabilit y of the mode l.

2 Laser Shadow Spot Theory

Here, a crack in a pl ate, subjec ted to in-plane mode I load ing is cons idered . The stress fi e ld in the vic inity o/" the crack tip is governed by the va lues o f the opening-mode KI stress-intens ity fac tor. B rown & Srawleyll deri ved a stress-intensity fac tor KI as a solut ion for a sing le-edge c racks in tens ion, as

follows :

K I = S ·~r,;-t l .12 .../rr" - 041 (if-- ) + IR7 ( i-r - 18 48 ( i- )' + 53 85 ( if--r ] where the K, for a doubl e-edge c rack in tension is:

K,=S ";;

[198 - 0 36 ( ~:' ) + 2 12 ( ~:' r -) 4 2 ( ~:l r] and the K, for the c rack , in pure bending of a beam IS :

K = 6M .,J;; I III' 2

[ 1 99 - 247 ( ~:' ) + 12 97 ( ~:' r -23 . 17 ( ~:' )' + 2~gC;: n where S is the appli ed stress; S = ~ ; P is the

rW tensile stress; r is the thickness of the spec imen: W is the width of the specimen; M is the momentum; and a is the c rack length.

An equ ati on is es tabli shed by transmi ss ion laser shadow spot theory and is expressed by KI(" as follows:

K Ie __ I _.6_7_I_

c_, ( -3 -~-6- r) ( z + " )')

where 0 is the transverse di amete r of the refl ected caust ics , c, is the stress-optical coeffic ient and I is the thickness of spec imen. ::. and '-, are constants dependin g on the charac teri sti c distances (such as in Fi g. 5) of the ex periment a rrangement. By simplifying the above equation , the relati on of the value of transverse diamete r D and the stress­intensity fac tor is g ive n by:

3 Image Processing for Auto-Range Finding

3.1 General description

As menti oned in the beginning of thi s paper, the compute r program has to be connected to a sophi st icated learning environme nt. Here, a new a lgorithm is deve loped for process in g the lase r shadow spot images . There a re SIX specia l experimental conditions (Fig. 6) .

820 INDIAN J PURE & APPL PHYS. VOL 40, NOVEMBER 2002

(a) spec imen is placed on the frame and crack tip is in ROJ (region of inte res t) .

(b) no specimen is pl aced on the frame .

(c) the force is acting a long a wrong direction (that means, one mu st apply the counte r-c lockw ise mo ment).

Cd) image of crack tip is out of ROI.

(e) the va ri at ion of the lase r shadow spot is larger than a thresho ld value .

( f) the c rack wi II be out o f image.

3.2 Judgement for each case

Here, many judg ing a lgorithms a re c reated to determine what conditi on the ex pe rimenta l operation is attributed to.

(a) No spec imen is placed on the frame

X,'mJ

xaddpix(j) = L darkpix(i , j ) = 0 i = X \lfll"l

where

.r 1'1(/11 is the x-axi s coordinate of start po int of ROI

x (, lit! is the x-axis coordinate of end po int of ROJ

xaddpix(j): the accumul ation of dark po int 111 a hori zontal line

i£'1/(1

xaddpix(j) = I darkpix(i, j ) i=istart

when getpix(i,j)2 T. darkpix(i.j)=O

gelpi.x( i,j)< T, dar/':'pix(i,j )= I

where gelpix(i,j) : get the gray leve l of pixe l(i.j)

T: thresho lding value

(b) Specimen is pl aced on the frame and crack-tip is

in ROI

... 0.2)

As shown in F ig .7 ,

Cd" ",n: the y-ax is coordinate of lower boundary in

obj ect

CliP the v-axi s coord in ate of uppe r boundary in obj ect

' w,de : the vert ical w idth o f obj ect, here the authors

set Cwide = 5

.' .

Ii ---l V=c

. ;-L--*-l-t-. up

~ ..... 'c I ~ reO"'"

crack

Fi g. 7 - Specimcn is pJ accd on the rr:lIn~ and crack tip is in ROJ

original range r -............. , _.... (X,y)

•

Fi g. 8 - Image of c rac k tip is out o r RO J

J

Otherwise, image of c rack tip may be out of ROI

(Fig . 8) .

(c) Shrinkage of ROJ

X end >Cri~hl + )'1

... (3.3)

LIN : EXPERIM ENAL DEVICE USING COM PUTER VISION TECHNOLOGY 82 1

V-\I • - J end

/ ----- old ROJ ( .

crack tip

Fig. 9 - Shrinkage of ROI

As shown in Fig. 9,

Y'I:''' is the v-ax is coordinate of start point of ROJ

Y Olld is the v-ax is coordin ate of end point of ROJ

Cr;~hl IS the x-ax is coordinate of ri ght boundary of ROJ

Fig. 10- Minimum di slance and maximum distance in ROJ

(d) extension in ROJ

... (3.4)

As shown in Fig. 10, the va lue of Yo, YI depend on the changi ng speed of the shadow spot, here the au thors set Yo =25 , YI=40.

(e) The crack tip will be out of image.

Y SI:trl - Ymin < Yhtl llmi

Ymax - Yclltl < Yh<lund

... (3.5)

old crack image

, ,,- ,, --", Y-Y;nax

/

old ROr

new RO l

X=X max

v=v . J .Jtnm

r~~

:<

Fig. II - The crack will be out or image

As shown in Fig. I I ,

J

X m;1I is the x-ax is coordinate of the left margll1 of Image

XII"" is the x-ax is coordinate of the ri ght margin of Image

)'m;1I is the y-ax is coordinate of the lower margin of Image

Ym:.x is the y-ax is coordinate of the upper margin of Image

X h",,,,d and )'1" ,"1<1 are the safe di stance from ROJ to the margin of image, here the authors set Xh" ulld = I 0 and YhiJUIllI =10.

3.2 Auto-range finding

Step I - define ori ginal region, at first a 100"-' 300 rectangul ar region is selected as a ROJ

Step 2 - if getp ix(i,j)<Threshold

then setp ix(i.j)=O,e lse setpix(i,j)=255

Step 3 - the accumul ati on of clark poi nt In a horizontal and vertica l line

822 INDlAN J PURE & APPL PHYS , VOL 40, NOVEMBER 2002

ca lcu late yaccpix(i) if X c lid < C ",gh, + Yo

calcu late xaccpix(j) then X CI <I = X ,'"<1 + I ° Step 4 - find the edge o f object

where xaccpix(cy »xaccmlll

where xaccpix(c ) » X(lCC"';1I

Cr;g h, =111ax c,

where yaccpix( cJ> xaccm ;1I

where )WCC"';II is an adjustable constant and I ° in the system,

Step 5 -judgement for the boundary

If Y~I :t rt - Ymin<Y onund

end

nell' ROJ

,... - - - - - - - -: 1 aser shadow

~ot

i 1

\. crac~---

1

1

1

-... ....... 1 Y=CdO\\1J 1'-...... _ ... _--

original RO [

Fig. 12 - Extension in boundary

l

Step 6 - extension III boundary , as shown III

Fig,12 ,

then x"""=x ,(;,,.,-I °

Step 7 - shrinkage of boundary

original ROI new RO[

/

Ii 1- -- --

1

___ __ 1

............ crack

Fig. 13 - The moving of RO!

Step 8 - set the left margin , as shown in Fig, 13,

X start= X clUl - (Y\,! IllI - Y start)

goto Step 2

where

setpix(i,j):set the g ray leve l of pixel(i,j)

Threshold : thresholding value

xaccpix(j): the accumu lation of da lrk point III a horizontal line

;£'1/(/

xaccpix(j) = I zeropix(i, j) ;=;XI(lr,

when getpix( i,j) =1= 0, zeropix( i,j )==0

when getpix(i,j)=O, zeropix(i,j)= I

yaccpix(i): the accumulation of dark point III a vertical line

LIN: EXPERIM ENAL DEVICE US ING COM PUTER VlSION TECHNOLOGY 823

jl'lId

yaccpix(i) = I z.eropix( i , .I) ;= ;Slarl

when gelpi.r(i.j) * O,zeropix(i.j)=O

when ge fpix(i.j) =O,zeropix(i.j)= I

Cy : the verti ca l coordinate in object

C, : the hori zonta l coordinate in object

3.4 Calculating the diameter of shadow spot

The meth od of ca lculati on for the diameter of shadow spot Cd,,,m"", is I isted as fo ll ows:

Step I - finding the lower edge of shadow spot

where xaddpix(j» d );mj,

Step 2 - finding the upper edge of shadow spot

C" I' =max (c)

where .raddpix(j» d,jn,jl

Step 3

where F is the actua l length of a pixel.

The author set the value of d,jmjl = 10.

___ -in

" II iI 0

~Mk 'I ! / : L

JLi l (A) (B)

Fig. 14 - (A ) the frame; (8) the specimen

4 Experimental Details

In order to ge t the exact va lue of a very short time duration, a CeD with electrical tri gger is used. The opti ca l set-up consists of a diode-pumping laser, operat ing at 530 nm wavelength with a lens, producing a divergent light beam. The frame is designed to support tensi on force and is stati onary, full y constrained on the sc ientifi c mode l structures [Fig. 14(a)]. Two hooks are attached on the upper and lower parts of the frame. It is easy for ch i Idren to load the spec imen and perforlll simple tensi on tes t. The spec imen was prepared from plexiglas plates (300 x 50 mm and of thi ckness 2.0 mm), whi ch contained an initi al edge crack of a 5 111m length [Fig. 14(b)]. The pointed li ght source transmitted into the crack tip , forming the laser shadow spot image. In order to obtain an estimati on of the laser shadow spot di ameter at fracture , the ex hibiti on clerk must take a ca librat ion berore normal operat ion. In the calibrati on operati on, errors introduced by the quality of the lens set and the accuracy in defining the distance between the diode-pumping laser, lens set, spec imen and the screen have been taken into account. It was assumed that, the va lue in measuring the diameter of the laser shadow spot in calibration stage is equa l to that of following experimental operati on stages because, the dimension of the spec imens are all the same. Thus , no precision al ignment of the components and no high quality lens set are required. The strain of the spec imen can also be calcu lated by the rou nds and the pitch of the frame.

After starting the system, the main graph is disp layed (Fig. 15). Participants can select a start­button to process the shadow spot image in real­time, when they place the specimen on the frame. A color-variable progressive bar indicates the status of the tensile test. The green, ye ll ow and red co lour bars represent , safety , danger and breakage. One can observe arbitrarily the magnificati on of the x-y plot shadow spot diameter and stress-intensity factor versus time if they select the K, button (Fig. 16). In the left part of the computer screen , the variation of the dimension of shadow spot is plotted and preserved by a group of curves. One can compare these curves to the numerical ray-tracing pattern (Fig. 17), which is simulated by computer.

824 INDIAN J PUR E & APPL PHYS. VOL 40. NOVEMBER 2002

i stop 1miJ clear curve ' fracture

Fig. 15 - The main graph in thi s co mputer program

CUfllfI I

r window i.(

.' zoom in l'

7 total 0 KI mm

1f

initial

~ fJ , , I 1 . , ]~ l~~ 1m ,.

r

time 0.1 sec

Fit!. 10 - Magnifi cati on in shadow spot diameter and stress ~ intensity fac tor

When someone selects the start-button , but no spec imen is placed on the frame (Fi g. 18): the computer will remind the parti cipants by a dI gital speech ' Please pl ace the spec imen on the frame' . In the mean time, a digital movie fil e in .AVr format will display on the screen to show how to pl ace the specimen. When someone twists alo ng a. wrong direc ti on (Fig. 19), the computer WIll remInd the parti cipants by a digital speech ' Pl ease twi.st counter-c lockwise'. Again , a digital movIe WIll display on the screen to show, how to apply the tensil e force . In case of wrong direction , the crack

ti p moves free ly and qui ckl y, but the bject can sti ll be tracked success fully by using the auto-range finding algorithm. Fi g. 20 shows the saturati on of the laser shadow spot and thi s is the time for co mputer to predict the fracture-time ' Please watch out , th e spec imen will brea k' . After a spec imen frac ture, a digital animati on will display the procedures of breakage.

Table I - The spot diameter on fra cture-ti me has a close rel ati ons hip to specimen thi ckness

Speci men Thickness Crack length Spot diameter (111In )

Lot # 1 1. 10 111 111 1.50111111 I:US Lot #2 1. ll n1l11 1.551111ll IS .2 i Lot #3 1.04 ml11 1.55 111111 13. 55 Lnt #4 1.90ml11 1.56111 111 15.1 5 Lot #S 2.05 111111 1.50 ml11 14.3,,( Lot #0 2.02111m I. S51ll111 1,,( 0.1 Lot #7 2.1 4111 111 1.57111111 15.SS Lot #8 2. 15111 111 1.55111 111 15 0 1 Lot #9 1.92ml11 1. 50111111 13.96

Fig. 17 - The ray-tracing pattern or laser shado w spot

Although, the ax is of symmetry or the laser shadow spot should be symmetri c to the crack-ax is with respect to the ax is of load ing of the pl ate, but in the rea l experiment , there will be small variati on in the profile and dimension of the lase r shadow spot image in fracture. The main reason is, th.e, thickness of all the spec imens are not the same, It the lot number of the plexiglas is different (Table I ). The size and geometry of the ex tremj ty of the slit

LIN : EXPERIMENAL DEVICE US I G COMPUTER VISION TECHNOLOGY 825

a lso influences the s ize of shadow spot on fracture­time. Indeed , for hi gh values of the appli ed stress when plasti c enc laves are es tabli shed the length of the lase r shadow spot starts to increase more rap idl y than its width so that, the shape of the laser shadow spot becomes more and more obl ong . So, the judgement of each spec ial conditi on is 99 % accurate but error-ratio of the predi cti on of fracture­time is 5 C;e (Tabl e 2) .

Tahle 2 - Tesl result s or th is sySICIll

Ilem Succcss Fail Error-rat io No specimcn is placeu on Ihe framc

300 0 00/"

The force is aCling along wrong direclion

2lJX 2 ON7~

The v;lI'ialion o fthc laser sh;ILlow

SpOI is 100 fasl 2lJX 2 0.6%

The crack wi l l he out o f image 2lJ9 lU%

Pred icli on in fraclll l"C- time 2X6 1-1 5%

Fig. I X - No spcc imen is placed on the frame

In accordance with the present sc ientifi c mode l, an inte lli gent system IS provided which can inte rface with a c li ent to guide and analyze the laser shadow spot experiments. In thi s new scie nti fic mode l, an audi o-video signal can be distributed from one ex hibiti on room to anywhere. Peopl e browse the WWW ho mepage to watch the event. A

multimedia presentati on process IS activated periodicall y by a 0.5 sec timer-s ignal to renew the presentati on of the next presentation obj ects. The author adopts automatic document , fetching upon world wide web link activation . However, the

documents presented to the users have been synthes ized by the world wide web ne twork at the time the reques t was served (Fig. 2 1).

I

fracture I slop ]§D clear curvol

Fig. I lJ - The force is aCl ing along wrong dircc li on

Fig. 20 - T he sa turat ion o f the laser shadow spOI

826 INDI AN J PURE & APPL PHYS , VOL 40, NOVEMBER 2002

Fig. 2 1 - Laser shadow spot ex periments of the WWW homepage

5 Conclusion

.::11.1 . iii

A combination of the laser shadow spot, on-line image process ing and multimedi a technologies as applied to scientific model is made. Although , the met hod can be onl y used for a crack which is visibl e, but, it is still useful in materi al fracture ex periment. From thi s model, fracture predicti on are determined, strain data are computed, and all the operations are guided. Once the data has been coll ected and entered, the system analyzes and puts the res ults in the internet serve r. One of the aims is to promote an interes t In the sC ience of electro-optics by encouraglllg children to participate in the experimental ac ti viti es . This challenging and interes ting experiment will II1crease sc ientific

knowledge and develop the educational fun ction .

There are four designing key po ints in the device as follows:

Computer controls the total system and accepts a modest pl ace as a particul ar aid within a specifi c educational fi eld; the technologies, inc lud ing opti cs, fracture mechanics, di gital image-process ing, multimedia and internet, are integrated; information is handled digitally ; the interface to users, permits interactivity.

Finall y, it must be menti oned that, in this ex periment , since the diode-pumping laser is adopted as the I ight source and the si mpl i fi ed lens sets are used to generate laser shadow spot patterns, the value of strain and stress-intens ity factor becomes less prec ise . But, thi s set-u p will be more suitable for scientific ex hibiti on as the li ght source has long life and the dev ice is easil y comprehensive.

Acknowledgement

Thi s work is sponsored by the National Sc ience Council , Taiwan, Republic of China, under grant number NSC-86-2S IS-S-035-00 I.

References

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17 Lin C S. Illdiall .I P/lre & Afipl Pin's, 37 ( 1999) 853.