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Fatigue Testing of Semi-Structural Composite Materials

Tim Langschwager

Product Manager

A. Schulman Engineered Composites

20-February, 2018

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Agenda

• What is Fatigue and why is it important?• Test methods and process• Key application areas• Test data

• Vinyl ester vs. Epoxy• Carbon Fiber vs. E-glass Fiber• 3K Carbon Fiber vs. 12K Carbon Fiber

• Conclusion

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Fatigue

Fatigue testing in composites is used to determine a life cycle capability at load of a material in a tension-tension method. This is done in a positive load method to evaluate any potential crack propagation, fiber fractures, de-laminations. ASTM D3479 copyright ASTM Int’l, February 5, 2018

By stressing the material repeatedly in a constant stress environment you are able to evaluate the life cycle capabilities of your material whether it be through material specifications, research and development, quality assurance, or structural design.

Fatigue as defined by English Oxford Living Dictionary “Weakness in metal or other materials caused by repeated variations in stress.”

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Fatigue Test Methods

• ASTM D3039 Test Method for Tensile Properties of Polymer Matrix Composite MaterialsASTM D3039 Copyright ASTM Int’l

• ASTM D3479 Standard Test method for Tension-Tension Fatigue of Polymer Matrix Composite Materials ASTM D3479 copyright ASTM Int’l, February 5, 2018

Testing consisted of 3 lots of material, 12 specimen per lot for a total for 36 test bars to determine the Tensile properties. ASTM D3039 methodology consist of using specimen machined from flat panels.

-Using the ASTM D3039 Data, 3 specimen per lot- 3 lots per load parameter were tested per load from 40% up to 90% of maximum load.-This results in 54 test bars per material to develop fatigue results to achieve a S-N curve (stress life).-Testing proceeded in Tension-Tension at 3 Hz.-This testing for 1 material can take over 45 days of test time when evaluation out through 1 million + cycles.

Data is from a controlled environment.

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Test Procedure-

250 KN Hydraulic Instron (56,000 lbf)

Instron WaveMatrixsoftware

Test point Time Required Test Mode Total Time

100% Minutes per sample ASTM D3039 36 specimen 3 hours

90% Minutes per sample ASTM D3479 9 specimen 1 hour

80% 0-20 minutes per specimen ASTM D3479 9 specimen 2 hour

70% 20-3 hours per specimen ASTM D3479 9 specimen 4-8 hours

60% 1 day per specimen ASTM D3479 9 specimen 9 days

50% 4 days/specimen ASTM D3479 9 specimen 36 days

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Graphs

• Monitoring the testing through 3 different graphs• Graph 1. (top right) shows the cyclic frequency of the

oscillation over a period of time. This is to ensure consistency of the oscillatory testing.

• Graph 2. (bottom right) shows the force in relation to distance. This is used to monitor the force maximum and minimum as well as the force line remain consistent. Cracking, slipping of the specimen can alter the values.

• Graph 3. (bottom middle) is used to monitor the maximum and minimum distance during the testing over the total cycles. This shows any changes is elongation which may occur.

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Test Data- Epoxy Carbon vs. E-glass

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Life (Cycles)

S-N curve

E Glass Epoxy

CF Epoxy

As a % Maximum Load there is little difference found between Carbon Fiber or E-glass fiber products using the same resin system at 40-45% fiber volume.

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Test Data- 3K vs. 12K PAN Carbon fiber Vinyl Ester resin

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S-N curve

3K CF-SMC

12K CF-SMC

As a % Maximum Load there is little difference found between 3K and 12K carbon fiber products using the same resin system at 40-45% fiber volume even though there is a difference in Ultimate Tensile Strength

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Test Data- Epoxy vs. Vinyl Ester Carbon Fiber

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S-N curve

CF Vinyl Ester

CF Epoxy SMC

As a % Maximum Load there is a notice-able difference found between Carbon fiber SMC using Vinyl Ester or Epoxy resin at 40-45% fiber volume.

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3K Carbon Fiber vs. 12K Carbon Fiber Vinyl Ester resin

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S-N curve

3K CF-SMC

12K CF-SMC

AL6061-T6

Tension – Tension 3Hz

When comparing based on Ultimate Tensile Strength there is a difference in Fatigue performance however when comparing to AL6061-T6 Aluminum the results show promising evidence of similar fatigue performance at 40-50% of maximum load.

Langschwager, Tim, “Carbon Fiber SMC Technology for Lightweight Structures” SPE-ACCE, Novi, Michigan September, 2013.

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Summary-

• It’s important to consider fatigue life in structural and semi-structural components that are subjective to repetitive stresses.

• Aerospace, Automotive are primary concern areas which have benefited from this data.• Time consuming- Aerospace, Automotive-Structural/Semi-Structural Applications.

• For less critical applications engineers will have a tendency to us more robust safety factors as opposed to performing fatigue testing.

• Through our testing-• The resin system is showing to be more of the dominate factor vs. fiber.• Minor difference between % Maximum load testing when comparing Carbon fiber and E-glass fiber.• The Hybrid Vinyl Ester system performed slightly better than the Epoxy SMC system.• Regardless of Ultimate Tensile Strength- % Maximum load showed typical results for fatigue life in

chopped fiber SMC to be 40-50%.

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