Composite failure analysis

Hand layup procedure and effect of holes on composite strength Paul Peavler 4/22/2010

Outline Background -motivation Goals and Objectives -develop hand layup procedure -test effect of holes on composite tensile strength Methodology -hand layup equipment and procedure -sample preparation and testing Results, analysis, and discussion -laminate layup -characteristic length of open hole composites Conclusions -material differences

Motivations Hand layup provides a simple, cost-effective composite construction method Use in schools Use in small to medium businesses One-time applications Composites often have holes for fasteners, clearance, or other parts Stress concentration Characteristic length Varies by laminate properties

Goals and Objectives Develop hand layup procedure Use on-campus laboratory Develop set of instructions Make instructions available to students Test effect of holes on composite tensile strength Determine tensile strength of modified composites Determine characteristic length

Methodology Hand layup procedure Cut prepreg tape into 12”x12” plies, using a [0/45/0/-45/0/45/90/-45] S configuration Place laminates on tool with release agent and release film on tool side Place perforated release film and bleeder ply above laminate Fully assembled laminate

Methodology Hand layup procedure Enclose tool in vacuum bag Use vacuum pump to remove all air around laminate Cure laminate in press Complex cure cycle takes approximately 4 hours Carver heated press

Methodology Test samples Cut laminates into test samples 1, 1.25, and 1.5 inches wide by 10 inches long Drill holes of 3/16, 1/4, 5/16, and 3/8 inches Apply increasing tensile loads in MTS to test samples until fracture Obtain laminate properties from tests of uniaxial layups Test sample in MTS machine

Results & Analysis Material properties Laminates with only 0 degree and 90 degree plies tested by another student Resulted in E 1 =13.1Msi, E 2 =1Msi Values are approximately 1.5x lower than those provided by Daniel [1] Use Daniel text to approximate ν 12 =0.27 and G 12 =0.667Msi Use program CLT to determine E x , E y, ν xy , v yx , G xy laminate properties E x =6.57Msi, E y =3.81Msi, v xy =0.394, v yx =0.227, G xy =2.03Msi [1] Daniel, Isaac M. Engineering mechanics of composite materials . 2nd ed. New York: Oxford UP, 2005. Print. CLT program

Results & Analysis Theoretical failure Use material properties from CLT in program LAMFAIL Predicts first ply failure of composites; must be modified for ultimate failure LAMFAIL predicts ultimate last-ply failure of layup at 9260 lbs/in With a laminate thickness of 0.1 in, this results in an unnotched stress of 92.6 ksi LAMFAIL program

Results & Analysis Measured failure and results Measured failure loads and resulting stresses are shown at right Must correct for plate not of infinite width Presents K/K ∞ factor Stress concentrations Stress concentration factor for composites cannot be calculated by strength ratio Measured results Stress concentration Width correction factor Strength correction Width (in) Hole Size (in) Max Load 1 (lbs) Max Load 2 (lbs) Max Stress 1 (psi) Max Stress 2 (psi) 1 0.188 5571.2686 5849.9775 54526.19598 57181.17706 0.250 5008.7554 5287.3491 48715.71934 50890.78598 0.313 4908.5039 49134.17317 0.375 4392.7212 4297.4004 42221.46482 43276.9426 1.25 0.188 6798.6235 7524.1182 53273.65085 59874.89018 0.250 6527.7026 6418.4214 51735.71893 50951.98381 0.313 6291.9258 5896.4907 49223.73751 47502.92599 0.375 6134.5381 5618.6167 47915.22311 43603.84227 1.5 0.188 8202.583 8455.9316 55097.48512 57250.72173 0.250 7938.1162 8154.3813 53781.27507 55072.61154 0.313 7118.3252 7619.48 48372.318 52073.37243 0.375 7478.0078 7527.5117 50418.06769 50409.24475

Results & Analysis Characteristic length Based on laminate properties; dimension over which axial stress averages for failure Use Mathcad to solve for δ Can now easily solve for characteristic length Data to right shows all calculated factors Average characteristic length is 0.13387 inches Approximately 3.4mm Calculated characteristic values Strength ratio a 0 = characteristic length σ n1 / σ 0 σ n2 / σ 0 δ 1 δ 2 a 0 _1 a 0 _2 0.6127219 0.6425564 0.469 0.426 0.1061434 0.1263204 0.5662749 0.5915581 0.538 0.5 0.107342 0.125 0 0.5981258 0.49 0.1626276 0.5457216 0.5593638 0.57 0.549 0.1414474 0.1540301 0.5897767 0.6628571 0.503 0.398 0.0926317 0.1418028 0.5847738 0.5759152 0.51 0.524 0.120098 0.1135496 0.5721802 0.5521773 0.529 0.56 0.1391186 0.1227679 0.5773186 0.5253718 0.521 0.603 0.1723848 0.1234453 0.6051876 0.6288386 0.48 0.445 0.1015625 0.1169242 0.5990755 0.6134598 0.489 0.468 0.1306237 0.142094 0.5490296 0.5910369 0.565 0.501 0.1202987 0.1556262 0.5860632 0.5859606 0.508 0.508 0.1815945 0.1815945

Conclusions Material properties Material is approximately 1.5x weaker than typical unidirectional carbon fiber Stress concentration factor and strength ratio Stress concentration factor is 3.25; strength ratio is approximately 0.54 Characteristic length is 0.13387 inches This is approximately 1.5x lower than bookvalue of 5 mm with different layup Failed test sample

Recommended future work Complete hand layup video Video was captured of the hand layup process This video can be edited and used as a guide for future student projects and classes Test different layups Find characteristic length for various layups of same material Test different materials Test material with strengths closer to that of the general textbook values Determine how much of an effect strength has on concentration factor and characteristic length

Appendix Cure cycle Segment Force Dwell Time Temp SP 1 Temp SP 2 1 4300 lb 5.2 min 96 °F 96 °F 2 4300 lb 5.3 min 122 °F 122 °F 3 4300 lb 5.2 min 149 °F 149 °F 4 4300 lb 5.3 min 175 °F 175 °F 5 4300 lb 5 min 175 °F 175 °F 6 4300 lb 5 min 175 °F 175 °F 7 4300 lb 5 min 175 °F 175 °F 8 4300 lb 5 min 175 °F 175 °F 9 4300 lb 3.7 min 194 °F 194 °F 10 4300 lb 3.8 min 212 °F 212 °F 11 4300 lb 3.7 min 231 °F 231 °F 12 4300 lb 3.8 min 250 °F 250 °F 13 4300 lb 22.5 min 250 °F 250 °F 14 4300 lb 22.5 min 250 °F 250 °F 15 4300 lb 22.5 min 250 °F 250 °F 16 4300 lb 22.5 min 250 °F 250 °F 17 4300 lb 15 min 60 °F 60 °F 18 4300 lb 15 min 60 °F 60 °F 19 4300 lb 15 min 60 °F 60 °F 20 4300 lb 15 min 60 °F 60 °F

Appendix Measured sample dimensions Sample 1 width (in) Sample 1 thickness (in) Sample 2 width (in) Sample 2 thickness (in) 0.992 0.103 1.003 0.102 1.008 0.102 0.999 0.104 1.002 0.102 0.999 0.100 1.020 0.102 0.993 0.100 1.239 0.103 1.232 0.102 1.237 0.102 1.235 0.102 1.241 0.103 1.229 0.101 1.243 0.103 1.239 0.104 1.474 0.101 1.477 0.1 1.476 0.1 1.466 0.101 1.457 0.101 1.478 0.099 1.44 0.103 1.464 0.102

Appendix Finite width corrections and stresses K/K ∞ σ n1(∞) σ n2(∞) 1.0405649 56738.046 59500.726 1.0763889 52437.059 54778.277 1.1272491 0 55386.45 1.196875 50533.816 51797.091 1.0251471 54613.326 61380.568 1.0466667 54150.052 53329.743 1.0763889 52983.884 51131.622 1.1157143 53459.699 48649.43 1.0171131 56040.374 58230.459 1.0314815 55474.389 56806.379 1.0510173 50840.143 54730.015 1.0763889 54269.448 54259.951

Composite failure analysis

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