Static and dynamic analysis of the fracture tests of fiber composites in hydraulically servo-controlled testing machines currently in use shows that their grips are much too soft and light for observing the postpeak softening. Based on static analysis based on the second law of thermodynamics, confirmed by dynamic analysis of the test setup as an open system, far stiffer and heavier grips are proposed. Tests of compact-tension fracture specimens of woven carbon-epoxy laminates prove this theoretical conclusion. Sufficiently, stiff grips allow observation of a stable postpeak softening, even under load-point displacement control. Dynamic analysis of the test setup as a closed system with proportional-integrative-differential (PID)-controlled input further indicates that the controllability of postpeak softening under crack-mouth opening displacement (CMOD) control is improved not only by increasing the grip stiffness but also by increasing the grip mass. The fracture energy deduced from the area under the measured complete load-deflection curve with stable postpeak is shown to agree with the fracture energy deduced from the size effect tests of the same composite, but the size effect tests also provide the material characteristic length of quasibrittle (or cohesive) fracture mechanics. Previous suspicions of dynamic snapback in the testing of stiff specimens of composites are dispelled. Finally, the results show the stress- or strain-based failure criteria for fiber composites to be incorrect, and fracture mechanics, of the quasibrittle type, to be perfectly applicable.

References

1.
Tsai
,
S. W.
, and
Wu
,
E. M. A.
,
1972
, “
General Theory of Strength for Anisotropic Materials
,”
J. Compos. Mater.
,
5
(
1
), pp.
58
80
.
2.
Hinton
,
M. J.
, and
Soden
,
P. D.
,
1998
, “
Predicting Failure in Composite Laminates: The Background to the Exercise
,”
Compos. Sci. Technol. J.
,
58
(
7
), pp.
1001
1010
.
3.
Soden
,
P. D.
,
Hinton
,
M. J.
, and
Kaddour
,
A. S.
,
1998
, “
A Comparison of the Predictive Capabilities of Current Failure Theories for Composite Laminates
,”
Compos. Sci. Technol. J.
,
58
(
7
), pp.
1225
1254
.
4.
Hinton
,
M. J.
,
Kaddour
,
A. S.
, and
Soden
,
P. D.
,
2002
, “
A Comparison of the Predictive Capabilities of Current Failure Theories for Composite Laminates, Judged Against Experimental Evidence
,”
Compos. Sci. Technol. J.
,
62
(
12–13
), pp.
1725
1797
.
5.
Bažant
,
Z. P.
,
1984
, “
Size Effect in Blunt Fracture: Concrete, Rock, Metal
,”
J. Eng. Mech. ASCE
,
110
(
4
), pp.
518
535
.
6.
Bažant
,
Z. P.
, and
Cedolin
,
L.
,
1991
,
Stability of Structures: Elastic, Inelastic, Fracture and Damage Theories
,
Oxford University Press
,
New York
.
7.
Bažant
,
Z. P.
, and
Cedolin
,
L.
,
2003
,
Stability of Structures: Elastic, Inelastic, Fracture and Damage Theories
, 2nd, ed.,
Dover Publication
,
New York
.
8.
Bažant
,
Z. P.
, and
Cedolin
,
L.
,
2010
,
Stability of Structures: Elastic, Inelastic, Fracture and Damage Theories
, 3rd, ed.,
World Scientific
,
Hackensack, NJ
.
9.
Bažant
,
Z. P.
, and
Planas
,
J.
,
1998
,
Fracture and Size Effect in Concrete and Other Quasibrittle Material
,
CRC Press
,
Boca Raton, FL
.
10.
Bažant
,
Z. P.
,
Daniel
, I
. M.
, and
Li
,
Z.
,
1996
, “
Size Effect and Fracture Characteristics of Composite Laminates
,”
ASME J. Eng. Mater. Technol.
,
118
(
3
), pp.
317
324
.
11.
Green
,
B. G.
,
Wisnom
,
M. R.
, and
Hallet
,
S. R.
,
2007
, “
An Experimental Investigation Into the Tensile Strength Scaling of Notched Composites
,”
Composites—Part A
,
38
(
3
), pp.
867
878
.
12.
Bažant
,
Z. P.
,
1976
, “
Instability, Ductility and Size Effect in Strain-Softening Concrete
,”
J. Eng. Mech. Div. ASCE
,
102
(
2
), pp.
331
344
.
13.
ASTM
,
2007
, “
Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites
,” ASTM International, West Conshohocken, PA, Standard No. ASTM D5528.
14.
Salviato
,
M.
,
Kirane
,
K.
,
Ashari
,
S. E.
,
Bažant
,
Z. P.
, and
Cusatis
,
G.
,
2016
, “
Experimental and Numerical Investigation of Intra-Laminar Energy Dissipation and Size Effect in Two-Dimensional Textile Composites
,”
Compos. Sci. Technol.
, (in press).
15.
Bažant
,
Z. P.
,
1999
, “
Size Effect on Structural Strength: A Review
,”
Archive Appl. Mech.
,
69
(
9
), pp.
703
725
.
16.
Bažant
,
Z. P.
,
Kim
,
J.-J. H.
,
Daniel
,
I. M.
,
Becq-Giraudon
,
E.
, and
Zi
,
G.
,
1999
, “
Size Effect on Compression Strength of Fiber Composites Failing by Kink Band Propagation
,”
Int. J. Fract.
,
95
, pp.
103
141
.
17.
Bažant
,
Z. P.
,
Zhou
,
Y.
,
Novak
,
D.
, and
Daniel
, I
. M.
,
2004
, “
Size Effect on Flexural Strength of Fiber-Composite Laminates
,”
ASME J. Eng. Mater. Technol.
,
126
(
1
), pp.
29
37
.
18.
Salviato
,
M.
,
Kirane
,
K.
,
Ashari
,
S.
,
Bažant
,
Z. P.
, and
Cusatis
,
G.
,
2016
, “
Experimental and Numerical Investigation of Intra-Laminar Energy Dissipation and Size Effect in Two-Dimensional Textile Composites
,” Cornell University Library, Ithaca, NY,
Report No. SEGIM 16-05/707E.
19.
Bažant
,
Z. P.
,
Chau
,
V. T.
,
Cusatis
,
G.
, and
Salviato
,
M.
,
2016
, “
Direct Testing of Gradual Postpeak Softening of Notched Specimens of Fiber Composites Stabilized by Enhanced Stiffness and Mass
,” Cornell University Library,
Ithaca
,
NY
,
Report No. 16-07/d
.
20.
Rüsch
,
H.
, and
Hilsdorf
,
H.
,
1963
, “
Deformation Characteristics of Concrete Under Axial Tension
,” Voruntersuchungen Bericht (preliminary report) Report No. 44.
21.
Hughes
,
B. P.
, and
Chapman
,
G. P.
,
1966
, “
The Complete Stress-Strain Curve for Concrete in Direct Tension
,”
RILEM Bull. (Paris)
,
30
, pp.
95
97
.
22.
Evans
,
R. H.
, and
Marathe
,
M. S.
,
1968
, “
Microcracking and Stress-Strain Curves in Concrete for Tension
,”
Mater. Struct.
,
1
(
1
), pp.
61
64
.
23.
Heilmann
,
H. G.
,
Hilsdorf
,
H.
, and
Finsterwalder
,
K.
,
1969
, “
Festigkeit und Verformung von Beton unter Zugspannungen
,” Deutscher Ausschu s für Stahlabeton, Heft 203.
24.
Wawersik
,
W. R.
, and
Fairhurst
,
C.
,
1970
, “
A Study of Brittle Rock Fracture in Laboratory Compression Experiments
,”
Int. J. Rock Mech. Min. Sci.
,
7
(
5
), pp.
561
575
.
25.
Hudson
,
J. A.
,
Brown
,
E. T.
, and
Fairhurst
,
C.
,
1971
, “
Optimizing the Control of Rock Failure in Servo-Controlled Laboratory Tests
,”
Rock Mech.
,
3
(
4
), pp.
217
224
.
26.
Fairhurst
,
C.
, private communication on June 23, 2016 to Z. P. Bazant of the Photo of the First MTS Stiff Machine Built in 1968 in Collaboration With Fairhurt and Wawerskik.
27.
ASTM
,
1999
, “
Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials
,” ASTM International, West Conshohocken, PA, Standard No. ASTM D5045.
28.
Bažant
,
Z. P.
, and
Tabbara
,
M. R.
,
1992
, “
Bifurcation and Stability of Structures With Interacting Propagating Cracks
,”
Int. J. Fract.
,
53
(
3
), pp.
273
289
.
You do not currently have access to this content.