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CAI-2
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Damage Tolerance: Investigation of Effect of damage on the mechanical
properties of laminated composite structures.
It refers to the experimental determination or the
numerical prediction of the residual mechanicalproperties of the damaged structure.
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U.S. Air Force draft requirements
for damage tolerance for low-velocity impacts:
Laminates should maintain a minimum design strength
after impacts with 100 ft-lb kinetic energy by a 1-in
diameter hemispherical indenter or after impacts
resulting in a 0.10 indent, whichever is less severe
(Schoeppner 1993).
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Why only CAI?
Compression is critical for impact-damaged specimens
because under this type of loading, strength reductions
are the largest.
Procedures must specify both how the impact test is tobe performed and how the compression test is to be
conducted
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NASA Reference Publication 1142 (1985) or
Boeing Standard Specification BSS 7260 (1982)
United Kingdom, the CRAG method.
Differences between the NASA and Boeing methods
have to do with specimen size and the exact manner in
which the impact tests are performed prior to
compression testing.
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Standards: NASA
Boeing BSS 7260(ASTM D 7137 and Boeing BSS 7260)
SACMA
CRAG
AITM 1-0010 Duarte et al
ASTM
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During CAI tests, the specimens were clamped along
the top and bottom edges and supported along the two
sides in a fixture .
The lateral support is designed to prevent overall
buckling of the specimen.
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CAI strength depends on the energy absorbed by the
specimen, which can be strongly affected by the
particular design of the test fixture.
For the same impact energy level, damage is lessextensive if the holding fixture is more flexible (Prandy
et al. 1991) or if some of the other parameters affecting
the impact dynamics are different.
Therefore, the method for performing the impact testmust be specified in detail.
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Literature on Test Parameters:1. If the undamaged strength of the material is 400 MPa, testing of a
NASA-type specimen requires the application of a 452 kN force, and fora Boeing-size specimen, 163 to 204 kN are needed.
2. With the method proposed by Sjoblom and Hwang 1989, the 76.2 mm x177.8 mm (3 in. x 7 in.) specimen is equipped with 1.59 mm (1/16 in.)thick glass-epoxy end tabs that leave a 3 in. x 4 in. test section.
To prevent overall/Global buckling (or macro-buckling) of these thin specimens(2 mm), 12.7 mm (1/2 in.) thick anti-buckling plates placed on either side of the
specimen are held in place by four bolts. These bolts are hand-tightened so asto allow the specimen to compress freely.
A 31.75 mm (1.25 in.) diameter hole is made in the center of the anti-bucklingplates because impact damage creates surface deformation and damage.
The specimen equipped with the anti-buckling plates is then gripped in an MTSmachine and tested.
3. Similar fixtures were designed by Sarma Avva and Padmanabha (1986),Nettles and Hodge (1991), Breivik et al. (1992).
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Fixture for CAI:
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4. Test results are used to screen materials, but caution isadvocated when attempting to extrapolate to actual
components (Sjoblom and Hwang 1989).
5. CAI strength depends on the size of impact-induceddelaminations.
Smaller damage areas lead to smaller reductions in residual
strength (Ghasemi Nejhad and Parvizi-Majidi 1990).
6. In CAI tests, Srinivasan et al. (1992) observed localizedbuckling of the sublaminates formed by impact damage,delamination growth, and final failure by buckling.
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Buckling: Buckling and delamination growth are thought to be
the first steps in the compressive failure process.
Because of the presence of delaminations, the load-
carrying capacity of the damaged structure will be
lowered and
In addition, once buckling occurs, delaminations might
extend and further decrease the load-carrying capacity
of the structure.
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Local Buckling:
The delamination is located near the surface of the
specimen and will buckle while the rest of the laminate
remains straight called a local buckling mode, as
opposed to global or mixed modes.
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Global Buckling:
When a short delamination is located near the mid-
plane of the laminate, its effect on the stability of the
laminate is small, the laminate buckles as if
undamaged, and in this case we have a global
buckling mode.
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Mixed Buckling:
For longer delaminations that are not located near the
surface, buckling of the delamination reduces the
overall rigidity of the laminate, and the remaining
portion is no longer symmetric and will buckle at a
much lower load in a different mode , called a mixedmode.
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A f R id l
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Assessment of Residual
Properties:1. Pure Compression Methods:
a) CAI
b) EWC
2. 4-Point Bending Method:-Bending Strength
-Bending Stiffness
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CAI, EWC, 4.B
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