Image processing methods in ductile fracture of solids

MECHANICS RESEARCH COMMUNICATIONS 0093-6413/87 $3.00 + .00

Vol.14(1), 57-60, 1987. Printed in the USA Copyright (e) 1987 Pergamon Journals Ltd.

IMAGE PROCESSING METHODS IN DUCTILE FRACTURE OF SOLIDS

Y.J. Chao Department University Columbia,

of Mechanical Engineering of South Carolina SC 2 9 2 0 8

(Received

September

24

accepted for print

1986;

September

24

1986)

Introduction

Ductile fracture problem has been one of the major research a r e a s i n t h e m e c h a n i c s c o m m u n i t y i n r e c e n t y e a r s . Much e f f o r t has been devoted toward the development of a non-linear or ductile fracture mechanics methodology to characterize fracture behavior under elastic/plastic conditions. T h i s p a p e r r e p o r t s on c u r r e n t research work a t t h e U n i v e r s i t y of South Carolina i n w h i c h an innovative experimental a p p r o a c h i s d e v e l o p e d and a p p l i e d t o t h e elastic-plastic or fully-plastic fracture of a solid. The project's goal is to study the strain distribution in the nearcrack-tip field for various ranges of plastic zone s i z e and t h e conditions of J-dominance in plane-stress state. The p u r p o s e o f the research work i s t o p r o v i d e the needed experimental data toward a better understanding of the near-field behavior around the crack tip in ductile fracture. Some p r e l i m i n a r y results for the plastic zone s i z e and shape a r o u n d a n o t c h a r e g i v e n .

Basic

When

Concept

the

plastic metal

flow

increase

in a deformed

occurs.

changes.

polished planes

stress

before

a

result,

Furthermore,

if the

being

in surface

with

the

As

structural

loaded,

roughness

surface.

The

the

surface

plastic

of

the

is smooth

or

induces

an

flow

phenomenon

yielding,

texture

to the interaction

physical

57

exceeds

surface

object

the

due

metal

of

of the

slip

roughening

58

Y.J. CHAO

of an originally our

study

to quantify

are related

To extract

[lJ. image

digitized

to

construct

total such

intensity,

are

relations

can

as

shown

image

of

in Figure

root represents plastic the

distribution

Figure

2

shows

stress

hardening

the

symmetric

inside 600

four

is

the

Gyp,

0.235.

determined

size

monotonic is

33.5

Table

1

of

the

be

shape

in

Progress

A

loading

is

due to the By using and

a

the

comparison

plastic

a

304

by

the

The

and

zone

the

predicted. for

loadings.

is

tested.

determined ksi,

these field.

shape

can

and

specimen

average

region.

size,

zone

size

texture

near the notch

line of the specimen with a finite element

They show very good agreements.

Research

zone

zone

v-notched different

some

concentration

used

that

loads were under

plastic

are

between

that have occurred

plastic

plastic

specimen,

exponent

experimentally

the

tensile

the

specimens

stress

black dot pattern

changes

on

normalized assumes

by

computer.

loads

non-uniform

specimen

at the stress

the

steel,

of

under

a

surface

and

was

taken

relationships

then

to

The observed

curves,

strain

for

applied

the surface

calibration

experiments

statistical

contrast

is

performed

uniaxial

the

a pre-polished

1.

system

Smooth

The

One

specimens

deformation

stainless

be

in

then

curves.

normalized

also

vision

features. under

and

that

from a

parameters

form

are

relevant

established.

Single-edge-notched typical

digitized

techniques

strains

parameters

image of a specimen

in

calibration

texture

a computer

discontinuities

the

parameters,

the

using

motivates

Results

surface

surface,

extract

geometric

equivalent

useful

stored

flow occurs

changes.

and Preliminary

processing

image

without

texture

metal

and

as plastic deformation

In principle,

camera

Digital

to

Procedure

deformed

video

plastic

and quantify

plastically developed

surface

to the surface

Experimental

a

smooth

yield strain of

the

along

the

calculation.

IMAGE PROCESSING METHODS IN DUCTILE FRACTURE

59

A study of the strain distribution within the plastic

zone in a

notched element

specimen analysis

are used generated

with

is currently under

Experiments

elastic-plastic

investigation.

using

notched

These works

and data

specimens

finite

reduction

with

fatigue

cracks are currently underway to study the conditions

of J-dominance small

Various

comparison

to verify the proposed experimental

procedures.

both

and

for the HRR field in the plane

scale yielding

and

fracture parameters,

the measured near-field obtained

from

large

scale

stress

yielding

such as J-integral,

state

conditions.

calculated from

information will be compared with

far-field

measurements

and

for

the

those

approximate

theoretical solutions.

Acknowledgement

The w o r k

reported

Foundation acknowledge College

Grant their

here

was c o m p l e t e d

No.

MSM-8504976.

financial

of Engineering,

support

University

under

U.S.

The and

the

National

author support

Science

wishes

to

from

the

of South Carolina.

References

.

C. Lee, Y.J. Chao, W.H. Peters, and M.A. Sutton, "Surface Texture and Platic Deformation", Proceedings of 13th Southeastern Conference on Theoretical and Applied Mechanics, p. 275, (1986).

60

Y.J. CHAO

2

0

0 75Gvs

3

q

0 El50" s

! t

o

FIG.

i. Image of A L o a d e d Notched Plate

FIG.

FEM SOLUTION (IN)

LOAD

";4

L)2

2. E x p e r i m e n t a l l y Plastic Zones

0.0618

0_0700

0.75 O'ys

0.0687

0.0857

0-85ays

0.0847

0.0958

0.94 Gys

0.1108

0.1163

0.95 Qvs

0.1375

0.1400

F4 -- F T A B L E i.

Determined

EXPER. RESULT(IN)

17ys

0.69

X 3t/:4

C o m p a r i s o n of P l a s t i c Zone Size w i t h F i n i t e E l e m e n t Method.