NDE Digital Methods for Predicting Stiffness and Strength of Wind Turbine Blades - Hybrid Anisotropic Materials for Wind Power Turbine Blades - page 182

Layup. laminate. stress-strain. curves. are. highly. nonlinear,. especially. at.
elevated.temperatures.
We. predicted. FRP. fatigue. stress. (σ s ). from. noncontact. measurement. of.
delaminating.cracks.using.Equation.(6.30)

n

2 n n

. σ = E ε e n ∂ + 1 E α Te n ∂ . (6.30)

s 11 11

1

where:
σ s .is.the.fatigue.stress;
E 11. is.the.modulus.of.elasticity.in.the.iber-reinforced.direction;
Δε 2 .is.the.strain.applied on .the.delaminating.layer;
n.is.the.number.of.stress.cycles.per.minute,.(change.from.1.to.1,000.cycles);
T.is.the.temperature.gradient;
α.is.the.coeficient.of.thermal.expansion;
.e n .is.a.exponential.function.of.natural.logarithm.

Previous. works. described. how. to. measure. strain. (Δε 2 ). in. a. resin. matrix.
using.embedded.iber.optic.sensors, 24 .and.how.to.measure.temperature.on.
the. boundary. surface. of. an. FRP. object 14 . with. infrared. thermography. cam-
eras..Our.previous.work 4 .described.how.to.measure.the.modulus.of.elasticity.
using.an.NDE.ultrasonic.method..In.this.chapter,.we.replace.the.integrals.of.
Equation.(6.30).with.summations,.Equation.(6.31):

n n

∑ 11

+ ∑

2 n n

σ = E ε e E α Te

. . (6.31)

s 11

1 1

Thermography.is.the.use.of.an.infrared.imaging.and.measurement.cam-
era. to. “see”. and. “measure”. thermal. energy. emitted. from. an. FRP. object..
Thermal,.or.infrared.energy,.is.light.that.is.not.visible.because.its.wavelength.
is.too.long.to.be.detected.by.the.human.eye;.it's.the.part.of.the.electromag-
netic.spectrum.that.we.perceive.as.heat..Unlike.visible.light,.in.the.infrared.
world,. everything. with. a. temperature. above. absolute. zero. emits. heat.. The.
higher.Infrared.thermography.cameras.produce.images.of.invisible.infrared.
or. “heat”. radiation. and. provide. precise. noncontact. temperature. measure-
ment.capabilities.
The.internal.heat.of.a.tested.FRP.object.increases.during.fatigue.testing. 25 .
If.the.test.frequency.is.above.1,000.cpm.(cycles.per.minute),.the.internal.heat.
of. the. sample. typically. rises. to. 50. to. 70°C.. For. test. frequencies. below. 300.
cpm,.the.internal.heat.decreases.to.25.to.30°C..The.effect.of.thermal.cycling-
induced. microcracking. in. iber-reinforced. polymer. matrix. composites. is.
studied.on.the.console.loading.model.cantilever.beam.(Figure 6.6).