MENG 302L Lab 10: Composite Beams

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Introduction: Laminated composites are useful in applications requiring high rigidity and light

weight. Common examples include aircraft fuselages and floors, skis and tennis rackets.

This lab will determine the structural properties of three composite beams. One beam

(P Beam) is made of aluminum skins with a tubular polycarbonate core. (The beam is

actually a drop from panels used to make feed housings on VLA Antennas(1)

.) Another (H

Beam) has aluminum skins with aluminum honeycomb core. The third beam (C Beam) is a

graphite epoxy rectangular tube.

Equipment:

Composite beams:

Composite Beam Dimensions

Beam Depth D Skin Thk. t Ctr. Dist. c Width W Weight (lbf) Length L

P Beam 3.08 .04 1.520 2.75 1.83 42.125

H Beam 1.875 .04 0.918 2.615 .957 40.125

C Beam 3.69 .119 - 3.11 3.41 45.375

Structural aluminum beam, 3” deep x 2.509 flange width 42” lg, Ixx = 2.93 in4.

100 lbs lab weights and hanger.

Strain gages centered on the lower flanges of all beams.

Vishay P3 strain indicator.

Dial indicator with magnetic base.

Units: US Customary

Objectives: Predict structural performance of composite beams, and compare the predictions

to actual performance in the lab. Compare structural efficiencies of the composite and

structural aluminum beams to that of a steel beam of similar size.

MENG 302L Lab 10: Composite Beams

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Pre-Lab:

Assuming the cores of the composite beams carry no load and are infinitely rigid in

shear, calculate the moment of inertia I of the beams about their neutral axes. (See Appendix:

Formulas.)

The beams will be simply supported, span~36”, with a 100 lb (50 lb for H Beam) load

applied at the center. Predict stress and deflection of all beams at the load point. (See

Appendix: Formulas.)

P Beam H Beam C Beam Alum. S 3x2.509

E 10.4 Mpsi 10.4 Mpsi 10.2 Mpsi(2) 10.4 Mpsi

I (in4)

2.93

σ (psi)

δ (in)

MENG 302L Lab 10: Composite Beams

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Procedure:

1. Work in teams of 3.

2. Do the following with the P Beam:

a) Measure and record the span (Distance between supports.)

b) Set the beam on the supports with the strain gage down and centered.

c) Attach the weight hanger & position it at the center of the beam.

d) Connect the strain gage to the P3, set gage factor, and zero the P3.

e) Set the dial indicator to bear on the top face of the beam, next to but not touching the

weight hanger. Zero the indicator.

f) Carefully add weights (50 lbs for H Beam, 100 lbs all others) to the hanger.

g) Read and record strain gage and dial indicator readings.

h) Carefully remove the weights.

i) Read and record strain gage and dial indicator readings.

j) If the final strain gage &/or dial indicator readings are more than a few µε or

thousandths of an inch from zero, repeat steps e through i.

3. Repeat steps b through j with the H, C and Aluminum beams.

Data Table

Date: Team Members:

P Beam H Beam C Beam Al S 3 x 2.509

Span (in.)

Initial strain reading

(µεi)

0 0 0 0

Initial indicator reading

(0.001 in) 0 0 0 0

Loaded strain reading

(µε)

Loaded indicator reading

(0.001 in)

Final strain reading

(µεf)

Final indicator reading

(0.001 in)

MENG 302L Lab 10: Composite Beams

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Worksheet 1) Calculate measured I and σ for all beams. Compare to the values from the Pre-lab.

P Beam H Beam C Beam Al S 3 x 2.509

E 10.4 Mpsi 10.4 Mpsi 10.2 Mpsi 10.4 Mpsi

(Pre-lab) IPre (in4)

2.93

(Measured) IMea (in4)

IMea/IPre

(Pre-lab)

σPre (psi)

(Measured)

σMea (psi)

σMea/σPre

2) Define allowable load W as that load which causes a deflection equal to 1/240th of the

span. Using formula d, it can be shown that:

Calculate the allowable load W for all beams. (Use IMea for all beams.)

P Beam H Beam C Beam Alum. S 3x2.509

Allowable load W

(lbs)

MENG 302L Lab 10: Composite Beams

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3) By the span/240 criteria above, with span = 36.5”, W for steel beam S3@7.5 is:

Define Structural Efficiency η as Allowable Load W divided by Beam Weight w.

For the steel beam,

Calculate Structural Efficiency η for the composite and structural al beams. (Use beam

weight per inch times span for beam weight w. The Al S beam weighs 0.212 lbf/in)

P Beam H Beam C Beam Al S 3 x 2.509

Beam Weight w (lbs)

Allowable Load W(lbs)

Structural Efficiency η=W/w

4) The 6061-T6 aluminum skins of the composite beams have a yield stress of 40 kpsi.

What is the factor of safety of the H beam when loaded with 100 lbs?

σMea

5) Plastic yielding of the skins under axial load is one possible failure mechanism of

composite beams. Name another possible failure mechanism.

Composite Beam Failure Mechanism

Write-up: (Worksheet.)

- The Executive Summary should include predicted and observed deflections and

stresses and structural efficiencies for all beams.

- The Results consist of the completed handout.

- In the Discussion, compare predicted to measured values of I and σ and structural

efficiencies of all beams. What might have caused the differences between measured

and predicted values? Mention anything else you deem worthwhile.

- For the Conclusion, recap everything in 50 words or less.

H Beam Factor of Safety (Plastic yielding of skins)

MENG 302L Lab 10: Composite Beams

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References:

1) http://www.aoc.nrao.edu/~jruff/9690103sh1%20EVLA%20FEED%20CONE.pdf

http://www.aoc.nrao.edu/~jruff/9690103sh4.pdf

http://www.aoc.nrao.edu/~jruff/A13Vertex.jpg

2) http://www.performance-composites.com/carbonfibre/mechanicalproperties_2.asp

Appendix: Formulas

a) Moment of inertia of a rectangle about its neutral axis:

b) Parallel Axis Theorem:

The moment of inertia of a section about axis x is given by:

Where axis x is parallel to axis xx and y is the distance between axes.

c) Moment of inertia of a rectangular tube about its neutral axis:

d) Deflection at the center of a simply supported beam:

e) Stress at the center of a simply supported, vertically symmetric beam:

Notes: