compression after impact

7/31/2019 COMPRESSION AFTER IMPACT

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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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THANK YOU

compression after impact

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