ENGR 45, SRJC, FALL 2010, SAM WILMARTH

THE MATERIALS SCIENCE OF A SKIMBOARD

Homemade wooden boards: $20 - $50

Professional foam-core, composite-laminated

boards: $155 - $450

Why glass is used for fiber reinforcement in composites

•Easily drawn into high-strength fibers from the molten state•Readily available and economically fabricated into glass-reinforced plastic composites•Fiber is relatively strong, and when embedded in plastic matrix, creates composite with high specific strength (ratio of tensile strength to density)•Chemically inert, good for corrosive environments

Supplies: Extruded Polystyrene (XPS) foam blocks, polyester resin and hardener, medium density (6 oz.) fiberglass cloth, mixing cups, brushes, stirrers, and scissors

Left: Polystyrene foam cores next to fiberglass cloth

Right: Foam cores laminated with fiberglass and polyester resin

Foam

Cor

e

1 La

yer Fi

berg

lass

(after

ben

d te

st)

2 La

yers

Fib

ergl

ass

3 La

yers

Fib

ergl

ass

THREE POINT BEND TESTD = deflection of the center of the beam

m = slope of the elastic region of the load-deflection curve

ϵ f = flexural strain

E f = flexural modulus of elasticity

Foam core before plastic deformation

Foam core after break point

Foam core sample with 1 layer of fiberglass, before and after plastic deformation

SAMPLES AFTER BEND TESTING

Because of their strength, the samples laminated with 2 and 3 layers of fiberglass had to be tested on a different apparatus, and did not yield any usable data.

Composite failure is not catastrophic because: fibers do not all fracture at once, and fractured fibers are still embedded in the matrix (resin).

Ecl = EmVm + EfVf

Ecl : Modulus of elasticity of a continuous and aligned fibrous composite in the longitudinal direction

Em ,Ef : Modulus of elasticity for matrix, fiber phases

Vm ,Vf : Volume fraction of matrix, fiber phases

One layer of fiberglass and polyester resin increased the flexural modulus of elasticity by 40%. There is no data for two and three layers. However it is likely that this number is artificially low, because there was evidence that the resin reacted with the foam and softened it. In fact, post-experimental research confirmed that polyester resin dissolves polystyrene.

Flexural Modulus, Ef (MPa) Polystyrene Foam Core 5.91

One Layer Fiberglass 8.25

THE INCOMPATIBILITY CAN BE AVOIDED EITHER BY REPLACING THE POLYSTYRENE WITH POLYURETHANE OR ANOTHER TYPE OF FOAM, OR BY REPLACING THE POLYESTER RESIN WITH AN EPOXY RESIN. EPOXY RESIN ALSO HAS SUPERIOR STRENGTH CHARACTERISTICS.

After a cure period of seven days it can be seen that the tensile strength of the epoxy resin is 20 to 30% higher than those of polyester and vinyl ester.

TYPICAL RESIN PRICING BY THE QUART

+ → Epoxy Resin System:$4.03/ounce

→ Polyester Resin:$0.50 - $1.09/ounce

• HTTP://S4.HUBIMG.COM/U/1455635_F520.JPG• HTTP://WWW.MAUISKIMMERS.COM/IMAGES/TEAM/FOWLER.JPG• HTTP://WWW.VICTORIASKIMBOARDS-STORE.COM/• HTTP://EN.WIKIPEDIA.ORG/WIKI/THREE_POINT_FLEXURAL_TEST• HTTP://WWW.C-CYACHTS.COM/IMAGES/COMPANY_ASSETS/

512F1C7F-0D64-4A5E-9D91-785DC064755F/16CF4_ADV_OF_EPOXY_V_POLY.PDF

• HTTP://GLASSATTIC.COM/POLYMER/OTHER_MATERIALS.HTM• HTTP://WWW.WESTMARINE.COM/WEBAPP/WCS/STORES/

SERVLET/WESTADVISORVIEW?LANGID=-1&STOREID=11151&CATALOGID=10001&PAGE=EPOXY-SYSTEMS

• HTTP://EN.WIKIPEDIA.ORG/WIKI/POLYSTYRENE#EXPANDED_POLYSTYRENE

• HTTP://SKIMONLINE.COM/BASICS/SKIMBOARDING-EQUIPMENT/BUILDING-YOUR-OWN-SKIMBOARD/

• CALLISTER, WILLIAM D. MATERIALS SCIENCE AND ENGINEERING : AN INTRODUCTION, SEVENTH EDITION.

WORKS CITED