Evaluation of fracture toughness of ceramic matrix compositesusing small specimens

J.S. Park a,�, Y. Katoh b, A. Kohyama b, S.P. Lee c, H.K. Yoon c

a Graduate School of Energy Science, Kyoto University, Uji, Kyoto 611-0011, Japanb CREST-ACE and Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

c Mechanical Engineering Major, Division of Mechanical and Industrial System, Dong-Eui University, 24, Gaya Dong, Pusan Jin-Gu,

Pusan 614-714, South Korea

Abstract

Ceramic matrix composites (CMC) are being developed for high temperature utilization in aerospace and other

industrial application. They offer several attractive features including high strength and modulus, improved fracture

toughness, light weight, low thermal expansion coefficient and high thermal conductivity which contribute to good

thermal shock resistance and high temperature stability in chemical and oxidative environments. Though the

enhancement of fracture toughness is one of key issues for the development of these materials, the standardized

evaluation method for the characterization of fracture toughness has not been established yet. Miniaturization of the

test specimen for these tests has been also considered to be one of the most necessary issues because of the high cost of

materials and the restriction of experimental environments. In this study, the influences of specimen size on the fracture

resistance properties of plain-woven (P/W) Carbon/Carbon and Tyranno-LoxM/SiC composite were investigated. The

fracture toughness tests with the unloading�/reloading sequences were conducted with the compact tension specimens of

different thicknesses and widths. The initiation fracture toughness JQ of both materials increased with increasing of

specimen thickness. It seems that interlayer frictional sliding between layered fabrics is one of energy dissipating

mechanism in these materials. The initiation fracture toughness, JQ, of both materials also increased with increasing of

specimen width. The specimen width and the fabric condition affect the size of fracture process zone in 2D woven

CMC.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ceramic matrix composites; C/C composites; SiC/SiC composites; Tyranno-LoxM SiC fiber; Fracture toughness; Compact

tension specimen; Size effect

1. Introduction

Ceramic matrix composites (CMC), such as

silicon carbide fiber reinforced silicon carbide

matrix (SiC/SiC) composites and carbon fiber

reinforced carbon matrix (C/C) composites, are

the ideal candidate materials for high temperature

� Corresponding author. Tel.: �/81-774-38-3463; fax: �/81-

774-38-3467

E-mail address: jspark@iae.kyoto-u.ac.jp (J.S. Park).

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structural applications due to their excellentthermo-mechanical performances. Especially,

SiC/SiC composites have been developed as struc-

tural materials for fusion applications because of

its inherently low induced radioactivity under high

energy neutron irradiation [1,2]. Moreover, CMC

provides an enhanced degree of damage tolerance

and high toughness as a result of a crack arrest

capability driven by various stress redistributionmechanism such as extensive matrix crack multi-

plications, fiber�/matrix debonding and sliding

[3,4].

Although the enhancement of fracture tough-

ness is one of key issues for the development of

these materials, the standardized evaluation

method for these materials have not been estab-

lished yet. Recently the fracture toughness ofCMC has been evaluated by using various test

specimen geometries (such as single edge notched

beam, single edge notched tension, double canti-

lever beam and compact tension (CT)) and

approaches (i.e. linear elastic fracture mechanics,

J -integral and strain energy release rate G ). The

CT specimen offers some advantages of being less

likely to promote compressive yielding and moreforward-straight crack extension compared to the

bend specimen. Miniaturization of the test speci-

men for the fracture toughness test has been also

considered to be one of the most needed issues

because of the high cost of materials and the

restriction of experimental environments. There-

fore, it is strongly required to investigate the

influence of specimen size on fracture behaviorand the optimum specimen size for the evaluation

of fracture toughness in CMCs. Unloading�/re-

loading sequence is a potential method to probe

the damage of materials and has been widely used

to measure not only the damage of materials but

also the crack length of the fracture toughness

specimen. Even though full unloading lead to the

surface damages of reinforced fiber because ofinterfacial wear between fiber and matrix, it had

been used to measure compliance at the un-

loading�/reloading curve [5]. It is desired to apply

less unloading amount in order to reduce the

frictional damage of reinforced fiber.

In this study, the influences of specimen size on

the fracture resistance properties of C/C and SiC/

SiC composite materials were investigated. Frac-ture toughness tests with the unloading�/reloading

sequences (10% unloading amount) were con-

ducted by using CT specimens with different

thicknesses and widths. Based on the correlation

between the size of specimen and fracture beha-

vior, the applicability of small specimens to the

fracture toughness determination in CMCs was

investigated.

2. Experimental

Commercially available plain-woven (P/W) car-

bon/carbon composites (CX-31, Toyotanso,

Osaka, Japan) and SiC fiber reinforced SiC matrix

composites made by polymer impregnation and

pyrolysis method were prepared for this study.

SiC/SiC composites are reinforced by Tyranno-

LoxM Si�/Ti�/C�/O fiber (Tyranno-LoxM, Ube

Industries, Tokyo, Japan) that had undergone asurface modification in order to optimize the

fiber�/matrix interface [6,7]. Both composite sys-

tems have a layered structure of plain-woven

fabrics. The fabric condition of reinforced fiber

in Tyranno-LoxM/SiC is more loose, compared

with those of C/C composite. The number of fiber

bundles in unit length of C/C and Tyranno-LoxM/

SiC composites are 1.135 No./mm and 0.756 No./mm, respectively. The mechanical properties of

these materials are shown in Table 1.

CT specimens with various thicknesses and

widths were used for fracture mechanics evalua-

tion of crack growth (Fig. 1). A notch (a0/W�/0.6)

Table 1

Mechanical properties of C/C and Tyranno-LoxM/SiC compo-

sites

C/C Tyranno-LoxM/SiC

Tensile modulus (GPa) 47 70

Yield strength (MPa) 117 143

Tensile strength (Mpa) 127 199

Density (g/cm3) 1.60 2.10

Number of layer in unit

thickness (No./mm)

4.2 4.3

Number of fiber bundle in

unit length (No./mm)

1.135 0.756

J.S. Park et al. / Fusion Engineering and Design 61�/62 (2002) 733�/738734

was induced by a diamond wheel with 0.4 mm

thickness. And then the notch tip of each specimen

was carefully sharpened by means of a razor blade

and diamond paste to avoid the overestimation of

initiation fracture toughness [8].

Fracture toughness tests were performed with a

crosshead speed of 0.1 mm/min using a model 5581Instron in which the specimens were repeatedly

loaded and unloaded. During the test, the fracture

behavior of materials was simultaneously observed

by a video microscope. In this work, 10% of

applied force was unloaded at each loading�/

unloading cycle in order to reduce the frictional

wear damage on the surface of reinforced fiber.

Effective crack lengths at every unloading pointwere calculated by using the compliance which was

taken from the unloading curve. The R -curve was

determined from the J -integral based on the

ASTM standard. Detailed calculation procedures

have been reported elsewhere [9,10]. After test, the

fracture surfaces of specimens were observed with

a scanning electron microscope (JSM-6700F,

JEOL, Tokyo, Japan).

3. Results and discussion

Both material systems of two different kinds of

widths (W�/20 and 15 mm) fractured in a slow,

stable manner and exhibited apparent nonlinear,

inelastic responses. Prior to any extension of a

macroscopic crack from the initial machined notch

tip, distributed damage could be observed in thenotch frontal in the form of matrix cracks, intra-

and inter-bundle delamination, which were asso-

ciated with the large pores in the microstructures.

Subsequently, the extension of a macroscopic

crack occurred from the vicinity of the notch tip.

On the contrary, small specimens of narrow

width (W�/10 mm), showed a different fracture

behavior. Beyond the maximum force Pmax, thecompressive failure spreading from the back face

to the notch tip in small specimen was observed.

High modulus fibers, such as C and SiC fiber,

provide excellent strength and stiffness in tension,

but are of limited value for compressive loading.

Furthermore, it is hardly expected that matrices of

both materials contribute to its compressive

strength, because large amount of porosity andfaults exist in both materials [11]. Thus, only the

data pertinent to specimen with widths 20 and 15

mm for both materials are considered in the

following.

The determination of initiation fracture tough-

ness, JQ, is very important, because the severe

service environment of CFCC never allows the

formation of crack and flaws, which promote theinfiltration of harmful environments as well as the

degradation of mechanical properties. After the

maximum force Pmax, most structure fracture due

to energy instability, the resistance cannot be

practically expected. When the macroscopic crack

contact with a weaker structure (i.e. macro pore,

flaws and the intersection of reinforced fiber

bundles), it is reflected and then extended. Thesekinds of multiple failures in fracture process zone

lead to some peak in the force�/displacement

curve. In this study, the initiation fracture tough-

ness, JQ, was defined at the maximum force Pmax

or at the first peak force, where the macroscopic

crack was initiated.

3.1. Effects of specimen thickness

Fig. 2 presents the effect of specimen thickness

on the initiation fracture toughness, JQ, of 2D

woven Tyranno-LoxM/SiC and C/C composites.

The initiation fracture toughness JQ of Tyranno-

LoxM/SiC composite increases with increasing

specimen thickness, and that of C/C composites

Fig. 1. The specimen geometry and dimension of Tyranno-

LoxM/SiC and C/C composites.

J.S. Park et al. / Fusion Engineering and Design 61�/62 (2002) 733�/738 735

also increases slightly. This tendency indicates that

a certain fracture mechanism concerned with

specimen thickness contributes to increasing frac-

ture resistance of both materials.

As shown in Fig. 3, it could be observed that

delamination and frictional sliding between

layered fabrics occurred in the fracture process

zone of both materials. The strain mismatch

between layered fabrics may lead to the delamina-

tion and interlayer friction. Thus, it can be

considered that the interlayer frictional sliding

between layered fabrics is one of main reason for

the higher fracture resistance of thicker specimen.

3.2. Effects of specimen width

Fig. 4 presents the effect of specimen width on

the initiation fracture toughness, JQ, of both

materials. The experimental result of small speci-

mens (W�/10 mm) is regarded as invalid data due

to a compressive failure behavior at the back-face

of specimen. Initiation fracture toughness, JQ, of

both materials has a tendency to increase accord-

ing to the increase in specimen width. Especially,

the dependence of Tyranno-LoxM/SiC compo-

sites, which have looser fabric structure upon the

width, is higher than those of C/C composite. It

means that the size of fracture process zone, where

matrix crack multiplication and macroscopic crack

extension occur, is affected by the specimen width

and the condition of fabric structure. It is expected

that the size of fracture process zone is saturated at

a certain width of specimen. However, the satura-

tion level was not obtained clearly in this work.

Fig. 2. Effect of specimen thickness on the initiation fracture

toughness, JQ..

Fig. 3. Micrograph and schematic of delamination and frictional sliding between layered fabrics at notch tip.

J.S. Park et al. / Fusion Engineering and Design 61�/62 (2002) 733�/738736

3.3. Effects of specimen size

The dependence of specimen size on the fracture

resistance of the both materials are summarized

and shown in Fig. 5. The projected area means the

product of thickness and the distance from notch

tip to back-face of specimen. (Open circular

symbols indicate the valid experimental points

(specimen width 0/10 mm) and cross symbols

indicate invalid experimental points (specimen

width �/10 mm). Fracture resistance of both

materials is significantly affected by the specimen

thickness and the size of fracture process zone as

well as other fracture mechanism (i.e. matrix crack

multiplication, reinforced fiber bridging and pull-

out). The initiation fracture toughness of test

specimen linearly increases with increasing pro-

jected area. Because of the relatively low notch

sensitivity of 2D CMCs, the micro and macro

cracks are branched and spread out. From these

experimental results, it can be concluded that the

large size of fracture process zone in small speci-

mens (W0/20 mm) makes the fracture toughness

of CMCs overestimated and increases with in-

creasing specimen size.

4. Summary

The influences of specimen size on the fracture

resistance properties of Tyranno-LoxM/SiC and

C/C composite materials were investigated. The

fracture toughness tests with the unloading�/re-

loading sequence were conducted with the CT

specimens of different thickness and width. Theinitiation fracture toughness JQ of Tyranno-

LoxM/SiC composites increases with increasing

specimen thickness. That of C/C composites also

increases slightly. It seems that interlayer frictional

sliding between layered fabrics is one of energy

dissipating mechanism in 2D woven CFCC. The

Fig. 4. Effect of specimen width on the initiation fracture

toughness, JQ.

Fig. 5. Effect of specimen size on the initiation fracture

toughness, JQ.

J.S. Park et al. / Fusion Engineering and Design 61�/62 (2002) 733�/738 737

Initiation fracture toughness, JQ, of both materialsis increased according to the increase in specimen

width. It can be concluded that the size of fracture

process zone in these materials changed with the

width of specimen. However, this experiment is

conducted under limited width and thickness. It is

expected that JQ values is saturated at a certain

level of specimen size. The saturation level was not

obtained clearly in this work. The investigation onthe effects of specimen size of CFCC is summar-

ized above and it is evident that more work using

different extent in size and various materials is

necessary.

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