CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
CHARACTERIZATION OF IMPACT IN COMPOSITE LAMINATES
Karel Minnaar and Min Zhou*
School of Mechanical Engineering Georgia, Institute of Technology, Atlanta, GA303 32-0405
Abstract. A new experimental technique is developed to determine the onset and evolution of
delamination in fiber-reinforced composites. The configuration uses a split-Hopkinson bar for lowvelocity impact loading and two Polytec laser vibrometer systems for real-time monitoring of the
initiation and progression of delamination. The experiment allows the histories of load, displacement,
and velocity of impacted specimens to be recorded and analyzed. Numerical simulations are conducted
using a cohesive finite element method. The method employs a cohesive zone model to simulate in-ply
cracking and interlaminar delamination and a transversely isotropic, elastic model to characterize the
bulk behavior of each ply. The simulations provide time-resolved characterization of damage during the
impact loading. The time at which delamination is detected decreases as the impact velocity is increased,
and delamination is detected at similar surface displacements. The progression of damage changes as the
bonding strength between plies is increased. The speed of delamination decreases as the bonding strength
is increased.
INTRODUCTION
apparatus is used for loading and for the
determination of histories of applied load, contact
point velocity displacement, and mechanical work.
The Hopkinson bar setup used an incident bar for
loading and no transmitter bar is used. Further, a
system of two laser interferometers is used to obtain
differential surface velocity and displacement
measurements at opposites sides on an impacted
specimen. This combination of diagnostics offers a
novel capability that allows real-time detection of
the onset and progression of interlaminar delamination along with time-resolved analysis of full
impact response.
The cohesive finite element method (CFEM)
provides a unique and powerful tool for analyzing
damage and fracture in a material through explicit
account of fracture. Since the discrete model
possesses the attributes of both deformation inside
elements and separation along embedded cohesive
Fiber-reinforced polymeric (FRP) composite
laminates are susceptible to damage due to
transverse impact. Such damage can lead to
significant reduction in the strength and stiffness of
the material. While mechanisms of impact damage
are relatively well understood, there is still a lack of
experimental tools that allow time-resolved analysis
of damage initiation and progression. Recently,
Hallett (2000) used a modified Split Hopkinson bar
apparatus and high-speed photography to correlate
the failure of impacted beams to abrupt changes in
measured deflection. Also, acoustic emission
techniques have been combined with microscopic
observations to continuously monitor damage
growth (Benmedakhene et al., 1999).
The first objective of the current investigation
is to develop an experimental approach for timeresolved analysis of impact response and damage
development. To this end, a split Hopkinson bar
f
To whom all correspondence should be addressed, 404-894-3294, min.zhou(5)me.gatech.edu.
Citation: Minnaar, K. and Zhou, M., 2001, " Characterization of Impact in Composite Laminates", Conference
Proceedings of the APS Topical Conference on Shock Compression of Condensed Matter, Atlanta, GA, June
24-29.
1208
surfaces, fracture is an inherent attribute of the
mode and this approach does not require any crack
initiation and propagation criteria.
The cohesive zone formulation allows fracture
to evolve as a natural outcome of the combined
effects of bulk constituent response, interfacial
behavior, and applied loading. The second objective
of this paper is to use the CFEM to develop a
framework for analyzing impact response and
impact-induced damage for composite laminates.
Impact Surface
FIGURE 1. Detection of Delamination
EXPERIMENTAL SETUP
The material tested is a [0°/90°/0°] laminate
made
from
NCT-301-1G150(50K)
epoxy
impregnated tape. The specimen is a rectangular
strip 75 mm x 18 mm in size and 3 mm in thickness.
Its two ends are placed on two roller pins. The
specimen is impacted at the center by a shaped
indentor. A starter crack with a width of 0.3 mm is
machined across the center of the longitudinal ply
and extends to the transverse ply. Two Polytec laser
vibrometers are used to measure the surface
velocities at one point on the front surface (impact
side) and one point on the back surface of the
impacted specimen. The lasers are aligned such that
the two beams are co-linear and perpendicular to the
specimen surface before impact.
The lasers are aligned at a distance L from the
center of the specimen. The difference between the
surface displacements, S=D2-Di, is used to detect
delamination, see Fig. 1. Where DI is the
displacement of the impacted surface and D2 is the
displacement of the rear surface. Positive
displacement is defined to be in the direction of
impact for both impacted and rear surfaces. Upon
impact, the starter crack initiates a matrix crack in
the transverse ply that propagates in a direction
perpendicular to the ply interfaces. The matrix crack
propagates towards the impacted surface until it
reaches the 0°/90° ply interface where delamination
initiates. The delamination propagates as a mode I
crack away from the impact site. The response from
an experiment with an impact velocity of
approximately 6.7 ms"1 is shown in Fig. 2. The
interferometer was placed 11.71 mm from the
impact site. The surface displacement and
differential displacement histories are shown. There
is a significant non-zero signal in the relative
displacement at 0.8 ms indicating that the
2
^
Time (ms)
3
2
Onset of delamination
2
Time(ms) 3
FIGURE 2. Surface displacement
delamination front has reached the point of
measurement. An increase in impact velocity
decreases the time at which delamination is detected
at a constant distance away from the impact site.
The time of detection decreases from 3.0 ms to 0.8
ms when the impact velocity is increased from 3.5
ms"1 to 6.7 ms"1, see Fig. 3. This indicates that the
average delamination speed increases as the impact
velocity is increased. From the response of a speci^ u.o
Point of measurement: 1 1 .7 mm away from impact site
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FIGURE 3. Time of onset of delamination
1209
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imen, it is possible to record DI at the time at which
delamination is detected. In Fig. 4, DI at the time of
detection is plotted as a function of impact velocity
for various values of L. It can be seen that
delamination is detected at similar surface
displacements for the impact velocities shown.
particular laminate layer. It is assumed that Tmax
varies between E22/5 to EH/10 depending on the
orientation of the ply, where E22 and EH are the
Young's Modulus of the composite in the transverse
and longitudinal directions respectively. A nominal
strength value of E22/20 is assumed for the interface
between two plies. Reported values of mode I and
mode II energy release rates (GIc and Gnc) are used
to estimate the characteristic length A n c . It is
assumed that Gic/Gnc is constant and A rtc = A f c .
PROBLEM ANALYZED
5
6
Impact Velocity ( m s )
,
Figure 5 shows the configuration used. This
configuration contains a small starter crack in the
lower longitudinal ply at the center of the beam that
extends to the transverse ply. This configuration is
chosen to study Mode I (opening mode)
delamination growth. A constant impact velocity of
6.7 ms"1 is applied at the center of the upper surface
of the beam. A 2D plain strain formulation is used.
7
FIGURE 4. Surface Displacement at onset of delamination
NUMERICAL SIMULATION
Numerical simulations of the experiment are
carried out using a CFEM. Each laminate is
considered to be homogeneous and transversely
isotropic. The fracture model is based on a cohesive
surface formulation of Xu and Needleman (1994)
and represents a phenomenological characterization
for atomic forces on potential crack surfaces.
Damage evolution is characterized using the time
history of crack length. In the current study, a
bilinear cohesive law implemented by Zhai and
Zhou (2000) is used to describe the constitutive
traction-separation relationship. Four material
parameters are required to describe the cohesive
relation: the original tensile strength of the interface
(^max)> me original shear strength of the interface
max IMPa
3004
2781
2559
2336
2113
1890
1667
1445
1222
999
776
553
331
108
-115
Time=50 jasec
Time=116|isec
FIGURE 5. Weak interface damage progression
(^max)» critical normal separation (A n c ), and the
critical tangential separation (A r c ). Discretization of
the specimen is based on triangular elements
arranged
in
uniform
"cross-triangular"
quadrilaterals. Cohesive elements are placed
between all element boundaries to model fracture in
the forms of interply cracking and delamination.
The cohesive parameters assigned to a cohesive
surface pair depend on its location and orientation.
There is also a distinction between cohesive
surfaces located on the interfaces between adjacent
laminates and cohesive elements located within a
Time=50 jasec
Time=128 jasec
3004
2781
2559
2336
2113
1890
1667
1445
1222
999
776
553
331
108
1-115
FIGURE 6. Strong interface damage progression
1210
impact are being conducted. An experimental
technique for real-time monitoring of delamination
progression in composite laminates has been
developed. A framework for the simulation of the
impact deformation and damage based on a
cohesive finite element method is presented. The
model allows fracture in the forms of interply
cracking and delamination to be tracked
individually. Calculated damaged modes and
progression of failure agree qualitatively with
experimental observations. The damage patterns are
controlled by the interface bonding strength and
delamination speed decreases as the bonding
strength increases. The onset and progression of
delamination is not sensitive to change in impact
velocity in the range tested.
RESULTS AND DISCUSSION
Currently, there exists no direct measurement
technique to determine the cohesive model
parameters. The interface strengths are varied to
understand how the strength values influence the
simulation. The evolution of deformation and
failure is compared to the experiment. Figures 5 and
6 show the progress of failure in an impacted
specimen with the distribution of ^max
superimposed on the deformed configurations. The
progression of damage changes as the interface
strength is increased by 50% from the nominal
values of T^ = 400MPa and 7^ = 500MPa .
The results show that, delamination along the lower
interface starts at the tip of the starter crack and
growths away from the point of impact. In the case
of the stronger interface (Fig. 6), the inply crack
begins at the tip of the starter crack and propagates
to the upper interface. Delamination along the upper
interface starts at this point and there is no
delamination in the lower interface. This
progression of damage at the higher interface
strength value compares well with the experiments.
Figure 7 shows the peak maximum delamination
speed reduces from 450 ms'1 to 230 ms"1 as the
interface strength is increased.
ACKNOWLEDGEMENT
Support of this work by the Office of Naval
Research through grant NOOO14-99-1-0799 to
Georgia Tech (Scientific Officer: Yapa D. S.
Rajapakse) is gratefully acknowledged.
REFERENCES
Benmedakhene, S., et al., 1999, Comps. Sci. and Tech.,
Vol. 59, pp. 201-208.
Hallett, S.R., 2000, Comps. Sci. and Tech., Vol. 60, pp.
115-124.
Xu, X. -P. and Needleman, A., 1994, J. Mech. Phys.
Solids, Vol. 42, pp. 1397-1434.
Zhai, J. and Zhou, M., 2000, Intl. Journal of Fracture, pp.
161-180.
500
450
Nominal Interface Strength
400
§_
300
150% Nominal Interface Strength
I 15°
50
Time (^sec)
100
FIGURE 7. Delamination Speed
CONCLUSION
Experimental and numerical studies on the
deformation and failure of fiber-reinforced
structural composites subjected to low-velocity
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