"4

NASA

Technical Memorandura 103126

AVSCOM

Technical Report 90-C-009

Controlled Crack Growth Specimen

for Brittle Systems

Anthony M. Calomino

Lewis Research Center

Cleveland, Ohio

and

David N. Brewer

Propulsion Directorate

U.S. Army Aviation Research and Technology ActivityqAVSCOM

Lewis Research Center

Cleveland, Ohio

_'2/2 7

_,_._-__ _ : I, 3

May 1990

SYSTEMSCOMMAND

AVIATION R&T ACTIVITY

CONTROLLEI) CRACK GROWTH SPECIMEN FOR BRITTLE SYSTEMS

Anthony M. Calomino

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

and

David N. Brewer

Propulsion Directorate

U.S. Army Aviation Research and Technology Activity - AVSCOM

Lewis Research Center

Cleveland, Ohio 44135

1.0

I

LI3

SUMMARY

A pure Mode I fracture specimen and test procedure has been developed

which provides extendecl, stable, through-thickness crack growth in ceramics

and other brittle, nonmetallic materials. Fixed displacement loading, applied

at the crack mouth, promotes stable crack extension by reducing the stored

elastic strain energy. Extremely fine control of applied displacements is

achieved by utilizing the Poisson's expansion of a compressively loaded cylindrical pin. Stable cracks have been successfully grown in soda-lime glass and

monolithic A1203 for lengths in excess of 20 mm without uncontrollable catastrophic failure.

INTRODUCTION

The ability to measure fatigue crack growth and fracture properties for

brittle _aterials and material systems has been hindered by the inability to

induce and observe conlrolled crack growth. Sustaining stable crack growth in

most brittle materials is complicated by a low damage tolerance and relative

high material stiffness;. Elastic energy, stored in the loading fixture, is

transferred to the specimen during crack extension. This unavoidable transfer

of stored elastic energy is sufficient to cause unstable crack propagation.

A specimen design and loading technique has been developed which enables

controlled Mode I cract, growth studies to be performed in monolithic ceramics

and other brittle materials. The loading system approaches a "true" fixed displacement test by mainlaining high loading stiffness and proposes a solution to

gripping and grip alig_ment problems inherent in brittle testing. The unique

method used to load the specimen minimizes the influence that specimen stiffness has on the effective stiffness of the load train. The testing procedure

appears to be readily adaptable to a variety of test conditions such as elevated temperature test_;, fatigue crack growth tests, and dynamic loading tests.

Tests have thus far been conducted under monotonically increasing displacements, however fatigue crack growth studies are planned.

STABLE CRACK GROWTH SPECIMENS

The initiation and control of a sharp Mode I crack in ceramics has been an

experimental challenge due to low damage tolerance and relatively high bulk
modulus. Traditionally it has been thought that load frames must be very stiff

and possess a high degree of control on the loading rate to be used for ceramic

fracture testing. The problems associated with ceramic testing have led to

innovative specimendesigns and loading fixtures, However, currently available

techniques are not well suited for quantitative studies of stable crack growth

under true ModeI loading. This is particularly true if one wishes to study

stable crack growth in specimenswith a uniform cross section.

Chevron notched (CN) specimens (refs. 1 and 2) are someof the first test

geometries used to produce stable crack growth in brittle materials. Propagation stability is imposedthrough a geometrically increasing crack front width

which effectively increases the fracture resistance of the specimen. Chevron

notched specimens were originally employed to evaluate the work of fracture in

glasses and ceramics (refs. 3 and 4) and have proved to be very useful for a

variety of fracture studies. Unfortunately, quantitative studies are hampered

because crack lengths cannot be observed during propagation and the phenomenon

of crack arrest is influenced by specimen geometry.

Stable crack propagation also has been obtained by Suresh (ref. 5) and

co-workers by means of far field cyclic compression loading of double cantilever beam (DCB) type fracture specimens. Stable fatigue cracks have been

propagated from notch roots in monolithic and whisker reinforced ceramic specimens for distances of 1 to 2 mm. Crack propagation is theorized to occur during unloading as a result of residual tensile stresses generated from the

extremely high compressive fields near the crack tip. Stable crack extension

occurs within a zone of irreversible damage localized near the crack tip. Far

field cyclic compression provides a useful method of developing sharp throughthickness cracks, however, propagated crack lengths lie within a region of

material which has been damaged extensively. Fracture evaluations may suffer

from the unknown effects of the compressive damage zone.

Nose and Fujii (ref. 6) have developed a sharp crack initiation technique,

termed the Bridge Indentation (BI) method, for ceramic materials. The BI

method compressively loads standard MOR bar specimens in a direction transverse

to the longitudinal axis. A stress riser, for transverse Poisson's stresses,

is produced by either a fine saw cut or a microhardness indentation. A stable

crack is propagated from the stress riser and arrested in the beam. Precracked

beams are then loaded more conventionally through three- or four-point bending

for evaluation of the fracture toughness, a technique termed the single-edgeprecracked-beam method (SEPB). Results indicate that the SEPB method can be

used to accurately measure fracture toughness values, however, the method does

not allow quantitative study of stable crack growth.

SPECIMEN ,_ND LOADING SYSTEM DEVELOPMENT

The development of a stable crack growth test procedure in brittle materials resulted from an evolutionary series of several specimen designs and loading configurations. The design goal was an extremely stiff displacement

controlled loading system and a specimen geometry which produced a decreasing

energy release rate with crack extension. Displacement control was chosen

because a load controlled system affects the characteristics of both the energy

release rate function and the elastic energy distribution. Considerable effort
was also placed on reducing the dependencyof the test on grip aligtm_ent and

grip contact stresses knownsources of extraneous crack initiation.

Control of elastic displacements becomes crucial if the brittle fracture

specimen is included in the integral or primary load train. Measurement of the

total crack mouth opening displacement (CMOD) lead to the observation that many

prototype specimens failed within a few microns of crack opening displacement.

Due to the unpredictable and sensitive nature of crack growth in ceramic materials, the desired re_;olution of the displacement controlled system was two to

three orders of magnitude lower than the expected total displacement. Conventional closed loop te_t machines are incapable of this degree of displacement

control.

Another design objective was to minimize the transfer of elastic energy

into the fracture specimen during crack extension. Special consideration was

given to the distribu:ion of elastic energy stored in the load train and its

redistribution during crack extension. Unstable crack propagation rates in

ceramic materials are extremely high and the reaction time for conventional

closed-loop systems much too slow to be effective during crack extension.

Active closed-loop in:eraction of a servo-hydraulic load frame with a specimen

is not possible at th_ inception of unstable crack extension. The elastic

response of the load _ystem alone influences the test during fracture.

The final specimen design and loading scheme which satisfies the requirements for stable cracl_ growth in brittle materials is shown in figure l(a).

The loading scheme is unique in that the specimen is attached as a secondary

component to the load train. The primary component of the load train is a

solid cylinder which s axially compressed during loading.

The typical load ng configuration for fracture tests is, in principle,

similar to that shown in figure l(b) where the specimen is included as an integral, or primary part, of the load train. In this configuration, the link with

the greatest compliance, ideally the test specimen, contains the greatest portion of the strain energy. A redistribution of strain energy occurs as specimen compliance increa_;es with crack extension. The specimen absorbs all of the

redistributed energy _hich is then available for sustaining crack extension.

Instability occurs when the elastic energy" available to the specimen exceeds

that required to propagate a crack.

The "secondary" ioading scheme of figure l(a) shows the specimen coupled

in parallel with the primary loading component. The component with the greatest compliance contains the least strain energy in a parallel system. Although

an increase in specimen compliance causes an increase in the effective compliance of the parallel components, the resulting change in the distribution of

strain energy is minirlal compared to that for the primary loading scheme. In

addition, a decreasingly small amount of the energy that is redistributed is

apportioned to the fracture specimen. Most of the energy is absorbed by axial

compression of the loading pin. The benefits of a secondary loading scheme are

its isolation of the _¢pecimen compliance, its ability to minimize redistribution of energy during crack extension and a high degree of control on the displacements applied to the fracture specimen.
SPECIMENGEOMETRY

Overall dimensions of the design specimenswere limited by the size of the

bulk ceramic material which was 40 mmwide 50 mmlong and 10 mmthick. The

specimen was chosen to be loaded through displacements applied to the crack

mouth. In theory, fixed crack mouth displacement loading produces a selfarresting stress field in front of the advancing crack. The final geometry

chosen is detailed in figure 2,

High local stresses and strains produced near grip points from mismatched

surfaces were found to be problematic to the stability of crack extension.

Elastic deformations related to the point contact of loading rollers on the

flat surface of the specimen critically affected fracture stability. The

stored elastic energy was transferred to the specimen during crack extension

creating instability. It was thought that a conformed grip and specimen surface would reduce these damaging elastic deformations.

A circular loading pin and circular hole in the test specimen best satisfied testing needs. Both the pin and the receiver hole can be machined to high

degree of accuracy and the circular geometry of the pin is maintained under

load. An added benefit of a radially expanding, circular loading pin is the

self-alignment of the test speci_:_en and grips with increasing load. Compressive contact loads are distributed over the inside area of the receiver hole,

thereby reducing contact stresses and the probability of failure at the load

point. The only elastic strain energy stored in the loading system which can

be transferred during crack extension results from the radial compressive

strains superimposed on the pin by crack mouth opening forces. These radial

forces, and subsequent displacements in the pin, are extremely small. Reduced

contact displacements and forces results in a minimal amount of stored elastic

energy.

SPECIMEN PREPARATION AND LOADING TECHNIQUES

Specimens are first machined as flat plates with a width of 35 ram a length

of 45 mm and a thickness of 1.25 mm. The receiver hole is rough cut with a

diamond coring bit and then surfaced with a fine diamond coated grinding mandrel. The final diwneter of the receiver hole varies by 25 gm about a nominal

5.00 mm. A 0.25 mm wide prenotch is machined to a depth of 15 mm with a diamond slicing saw. The tip of the prenotch has a tapered cross-section through

the specimen thickness extending a distance of 3 mm to facilitate initiation of

a sharp crack.

Proper selection of the initial diameter of the unstressed load pin is

important for successful testing. A maxim_n 22 kN axial load on the pin produces a radial expansion of 8 pm. Approximately 5 >m of CMOD is required to

complete a test. The available displacement is more than sufficient to fail

the fracture specimen, however, axial compressive stresses of 1.0 GPa may be

produced if the entire 8 pm displacement range is needed. A set of 25 loading

pins, having a size range from 30 >m above to 30 >m below the nominal 5.00 mm

diameter, is used to obtain the proper initial pin diameter. The difference

between pin diameters is 2.5 >m. The load pin possessing the closest fit with

the receiver hole is selected to initiate a test. Since high loads may be necessary to complete a test, careful alignment of the pin center is required to
minimize bending and the probability of buckling. The unsupported length of

the pin was kept at a mtnimumto reduce the risk of buckling by utilizing a

socket assembly shown in Figure 3.

Crack mouth opening displacements are measuredwith conventional extensometry. Crack lengths were obtained from the electrical potential drop induced

in a thin film of gold sputtered onto the ceramic surface. Crack lengths were

also monitored optically.

The test procedure provides for a very high degree of applied displacement

control, although the displacement range of the pin is limited to 8 >m. Assuming axial pin loads can be controlled to less than 20 N over a 22 kN range,

applied displacements can be theoretically controlled within 7 nm over the full

displacement range. Finer control is capable at smaller ranges. Extremely

slow applied displacement rates can be achieved by ramping the axial load.

Load rates of 30 nm/hr are achievable with standard closed loop servo hydraulic

test frames.

fifSC[!SS ION OF RES[_LTS

Controlled crack growth was achieved in A1203 specimens to an a/W ratio in

excess of 0.9 (approx. 25 am.) An A1203 specimen with a stable crack grown to

an a/w ratio greater than 0.80 is shown in figure 4. The results of a typical

test are shown in figure 5. Tests were also performed satisfactorily in sodalime glass with similar results. Crack propagation occurs at a pseudostatic

rate where crack extension occurs by incremental jumps and arrests. Uncontrollable catastrophic failure was not observed during crack growth in either the

ceramic or glass material. Fracture properties will be calculated using CMOO

and crack length measurements together with the elastic solution for specimen

compliance.

The testing procedure is theoretically insensitive to specimen thickness

and thicker specimen should be easily accommodated without increased pin loads.

Larger applied displacements can be achieved by increasing pin diameters when

testing more compliant materials.

CONCLUSIONS

1. A pure Mode I fracture specimen has been developed which provides

stable, constant cross-section, through thickness cracks in ceramic material

systems.

2. Cracks can be propagated stably to an a/W ratio in excess of 0.9 (a

distance of 25 mm with the present specimen geometry.) Longer cracks can be

achieved by simply scaling up the dimensions of the specimen.

3. Specimen geometry is simple and easy to machine. The testing procedure

provides grip self-alignment and reduces grip contact stresses.

4. Fracture tests can be performed on most conventional test frames since

relatively small compressive loads are required and a high degree of applied

displacement control can be produced.
REFERENCES

1. D. Munz, R.T. Bubsey, and J.E. Srawley, "Compliance and Stress Intensity

Coefficients for Short Bar Specimens With Chevron Notches," Int. ].

Fract., 16 [8] 359 (1982).

2. R.T. Bubsey, O. Munz, W.S. Pierce, and J.L. Shannon Jr., "Compliance

Calibration of the Short Rod Chevron Notch Specimen for Fracture Toughness

Testing of Brittle Materials," Int. J. Fract., 18 [2] 125 (1982).

3. J. Nakayama, "Direct Measurement of Fracture Energies of Brittle

Heterogeneous Materials," J. Am. Ceram. Soc., 48 [11] 153 (1965).

4. H.G. Tattersol, and G. Tappan, "The Work of Fracture and Its Measurement

in Metats, Ceramics and Other Materials," ]. Mater. Sci., ! [3] 296 (1966).

5. S. Suresh, and J.R. Brockenbrough, "Theory and Experiments of Fracture in

Cyclic Compression: Single Phase Ceramics and Ceramic Composites," Acta

Metall., _ [6] 1455 (1988).

6. T. Nose, and T. Fujii, "Evaluation of Fracture Toughness for Ceramic

Materials by a Single-Edge-Precracked-Beam Method," ]. &n. Ceram. Soc., 71

[5] 328 (1988).

C F Cs

I i

I_ CF

C S ).> C F

C s : SPECIMEN C_:_

COMPLIANCE

C F : FIXTURE

COMPLIANCE C F

(b) Linear system. (a) Parallel system.

Figure 1.

T35 mm L

,/'-- 2.5 mm

i

I

i

I

22.5 mm ---_

45 mm

Figure 2. - Specimen geometry.

---_ _- 1.25 mm

6
...--.........._>

I -- I

I n I

I -- I

LOAD CELL

THREADED

SOCKET

ASSEMBLY -.-.,

_ HSS LOAD PIN

"L

SPECIMEN

,,,'-- LOCKING NUT

I __ I

I __ I

L--. m m ._1

SE RVO-HYDRAULIC

RAM

i

Figure 3. - Socket assembly.
Figure 4 - Stable crack in AI203.

5 --

4

E

E

-,r

I-- 3 --

v 2 --

0

1

J

/-

f

J

I I I I I I

•1 .2 .3 .4 .5 .6

CRACK OPENING DISPLACEMENT, pm

Figure 5. Typical experimental results for monolithic AI203

ceramic specimen.
National Aeronautics and

Space Adminlslrallon

1. Report No. NASA TM-103126

AVSCOM TR 90-C-009

4. Title and Subtitle

Report Documentation Page

2. Government Accession No.

Controlled Crack Growth Specim,_n for Brittle Systems

7. Author(s)

Anthony M. Ca]omino and David N. Brewer

9. Performing Organization Name and Address

NASA Lewis Research Center

Cleveland, Ohio 44135-3191

and

Propulsion Directorate

U.S. Army Aviation Research anJ Technology Activity--AVSCOM

Cleveland, Ohio 44135-3127

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

and

U.S. Army Aviation Systems Command

St. Louis, Mo. 63120-1798

3. Recipient's Catalog No.

5. Report Date

May 1990

6. Performing Organization Code

8. Performing Organization Report No.

E-5454

10. Work Unit No.

505-63-1B

1L161102AH45

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15, Supplementary Notes

Anthony M. Calomino, NASA L,:wis Research Center. David N. Brewer, Propulsion Directorate, U.S. Army

Aviation Research and Technology Activity--AVSCOM.

16, Abstract

A pure Mode I fracture specimen and test procedure has been developed which provides extended, stable, throughthickness crack growth in cerami,:s and other brittle, nonmetallic materials. Fixed displacement loading, applied

at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine

control of applied displacements is achieved by utilizing the Poisson's expansion of a compressively loaded

cylindrical pin. Stable cracks have been successfully grown in soda-lime glass and monolithic A1203 for lengths

in excess of 20 mm without uncontrollable catastrophic failure.

17. Key Words (Suggested by Author(s))

Ceramic

Stable crack growth

Brittle fracture

19. Security Classif, (of this report)

Unclassified

18. Distribution Statement

Unclassified- Unlimited

Subject Category 27

20. Security Classif. (of this page)

Unclassified

21. No. of pages

10

22. Price*

A02

NASA FOaM lS2e OCT e6 *For sale by tt-e National Technical Information Service, Springfield, Virginia 22161