Composite Karten

- Glass fiber polyamid composite: GF-PA

- Carbon fiber epoxy composite: CF-EP

• Fibers are parallel to each other

• Even fiber distribution

• Ideally straight and unbroken fibers

• Ideal adhesion between fibers and matrix

• Number of layers

• Fiber volume fraction

• Fiber orientations of the individual layers

• Layer thicknessess

• Stacking sequence

• Fiber and matrix materials

• Aircraft/ Helicopter —> aviation industry

• Posthetics

• Sport Equipment (Bike, sailing)

• Bridges (low maintenance, high fatigue)

e.g. GLARE

• Bones

• Muscles

• Wood (Bamboo)

Wood engineering

Mirror symmetry w.r.t. the x1-x2-plane, the x1-x3-plane and the x2-x3-plane

—> Basic assumption for lecture

Isotropy w.r.t. the x2-x3-plane

The material shows 3 planes of symmetry, lying in the 3 principal axis.

—> In the layer itself, shear cuppling and bending twisting coupling are avoided

A transversal istrotpy in the x2 - x3 plane

From G12, G13 & G23, G23 will be the lowest because only the Matrix is load carrying

Because a strain in 1 will yield a higher strain in 2 than vice versa

Typical value: 60%

If utilized further, fibre-fibre contact starts existing, which cannot bare any load. Further, a vf of 0,6 isn´t far of from the max. possible:

• Kirchhoff Plate Kinematics - Crosssections remain plane and normal to middle plane (analog to Euler-Bernoulli beam)

• Plane state of stress

• Plane state of strain -> Thickness doesn´t change

• Thin layered structures (b/h >10)

• Not applicable if the transverse shear stiffness is low

• Errors are non conservative

With symmetric laminates

A16 & A26

B11

D16 & D26

By utilizing orthotropic layers —> Cross-ply laminate (only 0 and 90°)

By using balanced laminates —> for every layer angle not equal to 0 or 90, there is a layer with an opposite angle (always +/- pairs)

The basic modell is springs in parallel —> Same stress:

• Crackling noises

• Getting milky

• Stiffness reduction

It describes how to develop effective material properties for a given mixture from the basic material properties and the volume fractions.

The basic model is springs in series

The basic model is springs in series:

Usually the matrix is more influenced by mositure. (However Aramid and natural fibres will take up moisture)

• Mass and dimensional increase due to moisture absorbtion

• Lower electrical and thermal resistance

• Lower Modulus of elasticity

• Resulting Eigenstresses

• Lower barable load

—> Especially the combination of high humidity and temperature is detremental

The start at Micro damages, e.g. a single fibre fracture or Matrix micro crack (1mü)

They develop to Micro cracks (10mü), e.g. fracture of fibre groups, micro delaminations, buckling of fibres, growth of micro cracks

Macro cracks are complete layer damage, e.g. Fiber layer fracture. buckling of layers or delaminations (100mü - 1mm)

Strucutral failure describes the unstable growth of cracks, the structure cannot bear the load annymore.

• Individual layers are considered

• Inplane state of stress

• Xt - Axial tensile strength

• Xc - Axial compressive strength

• Yt - Transverse tensile strength

• Yc - Transverse compressive strength

• S - shear strength

—> They are usually derrived via experiment

Fibre Fracture

Fibre buckling

Matrix failure

Matrix failure

Matrix failure

Xt

• Maximum stress criterion —> Don´t use, doesn´t consider stress interactions (remember mehrdimensionaler Spannungszustand, Vergleichsspannung)

• Maximum strain criterion —> Don´t use, same issue as max. stress

• Tsai-Hill —> Considers interaction, but doesn´t distinguish between failure modes and compressive, tensile failure

• Tsai-Wu —> Considers interaction and compressive, tensile failure, doesn´t consider different failure modes

• Puck´s criterion —> Gives different criteria for different failure modes —> Full consideration

• Hashin´s criterion —> Gives different criteria for different failure modes —> Full consideration

Failure that differentiate failure mode will evaluate for fibre and matrix failure. They have an evaluation for compressive and tensile load for both sigma-11 and -22

• Always used balanced laminates —> No shear coupling and no Eigenstresses

• Min 10% of layers in a certain orientation, max. 70%

• Always use symmetric laminates —> No bending-extension coupling

• balance distribution of layers (don´t accumulate a single orientation) and ensure that adjacent layer angle differences aren´t too large

• Assemble +/-45° layers (violates last rule)

• Do not assemble layers of equal orientation (bad fatigue and crack resistence)

• Outer layers: For Bolts and shear loads - 45°; for buckling - 0°

Lateral support

General issue: Loads of manual work

Series connection

Multilayered

Hybrid loop (GFRP more compliant, stiff CFRP “attracts stresses”)

• Different materials can be joined

• No heat influence

• No notch effect as with bolt

• Sealing effect

• Rough tolerances possible

• High damping - energy absorption

  
  

• Non - uniform stresses and stress peaks in adhesive

• Sensitive to peeling

• Adhesives are limited in regard to temperature and humidity

• Production and QA nor fully reliable

• Material overlaps needed

• Shafting

• Dobule lap

• Single lap (bending)

• Doubler (used for repair)

>85°

There is a minimum overlap length which will minimize stress peaks, beyond that, no further improvement to be expected

Graded adhesive layer

Optimice edges

Secure against peeling #Angschtniet

• Different materials can be joined

• Screws (screw rivets) are detachable

• Low scatter of properties for screws and rivets

• Bolts and joints allow for good QA

• High energy absorption

• Holes reduce strength in laminate

• Screw and rivet heads may be disadvantageous

Single & Double lap joint

Bending is introduced

Bearing failure (safe, desired, energy absorption)

Tensile failure (unsafe)

Shear failure (unsafe)

Splitting failure (unsafe)

Idealy they are pierced and not drilled

Rigid bolts, compliant part —> Forces only transferred by end bolts

Compliant bolts, rigid part —> Forces equally transferred by all bolts