Design - composites Flashcards
Ground aspects
- Integrate as many functions as possible
- Design for manufacturing
- The direction of the fibers
- Make sure loadings transverse to fiber direction and out-of-plane are low
- Avoid stress concentration
- Varify the design by experimental testing
Composite laminates - definition and properties
Assembly of layers of composite materials, which are join to provide required properties:
- In-plane stiffness: matrix distribute load throughout fibers and are strong in-plane but weak out-of-plane
- Bending stiffness
- Strength
Composite laminates - calculations
Stiffness in fiber direction = E1 (longitudinal modulus):
E1 = Vf x Ef + Vm x Em
Stiffness transverse to fiber direction = E2:
E2 = Ef x Em / (Vf x Em + Vm x Ef)
V(f/m) = volume (of fiber/matrix)
E(f/m) = Young’s modulus (of the fiber/matrix)
Composite laminates - guidlines
- Symmetric lay-ups = prevents residual stresses and warpage
- Avoid several layers with same fiber direction (3-4)
- Smooth thickness variations - not above 3% = prevents microcracks and delamination
- At least 5 mm internal radius = too small risk breaking fibers and are weak
- Joining with 20 mm of overlap
- Avoid sharp corners = sensitive to impact
- Still fibers = movement during manufacturing will cause loss of stiffness, strength and slight warpage
Composite laminate - Free-edge effects
- Stress concentration at the free edges of a laminate between two layers - avoid free edges by all-covering layer at outside
- Important for fatigue loading - ultimate failure starts at the edges
Composite laminates - drapeability
- Depend on type of fiber and resin, and stacking sequence
- Wanted during and after forming : no wrinkled fibers, minimum shear deformation, correct fiber angles
Composite laminates - residual stresses
- Can cause microcracking = transverse/matrix cracking
- Formed due to: chemical and thermal shrinkage during curing, absorption of mositure, or differemces in thermal expansion between fiber and matrix
Joining of composites and metal
- CFRP + metal = galvanic corrosion due to conductivity of the fiber
Impact damage
- Worst type of damage/loading for FRP
- Signs of impact damage:
1. matrix cracks
2. Delamination
3. Fiber fracture - Thin materials can flex and take up energy, hence they are good against impact
Impact damage - factors affecting size
- Impact energy (force, speed etc)
- Geometry of the damage-tool (sharp, round etc)
- Type of matrix material and fiber
- Amount of fiber in the composite
- Bonding between fiber and matrix (called sizing)
- Fiber lay-up (weave, chopped strand mat etc)
- Manufacturing method
- Material thickness
Matrix cracks (microcracks)
- A flexural failure
- More likely with thin composite or large support span
Delaminations
- Cracks between fibre layers
- Occurs after matrix cracks and are peanut-shaped
- They gros along fiber direction
Fiber fracrures
- Flexural failure
- An actual hole, not “just a crack”
- Fracture resistance depend on bonding between fiber and matrix
Experimental testing/characterization
- Tensile test
- Compression test
- Flexural test
- Shear test
- Fatigue testing
- Impact testing
- Thermal analysis
Tensile testing
- Asymmetric lay-up will lead to bending and elongation during testing
- Information about:
1. Tensile modulus = stiffness
2. Failure stress = tensile strength
3. Failure strain = max. elongation
Compression testing
Information about:
1. Compressive modulus = stiffness
2. Failure stress = compressive strength
3. Failure strain = max. compression
Flexural testing
- Test if material will bend and fail
- 3-pont bending (most common) and 4-point bending
- Information about:
1. Flexural modulus = bending stiffness
2. Failure stress = bending strength
3. Failure strain
Shear testing
- To determine shear properties
- Different types of test ex. ILSS which tests interlaminear shear strength
Fatigue testing
- Determine fatigue properties by cyclic loading
1. Max. stress and strain for a number of loading cycles
2. Stiffness reduction as a function of loading cycles
Impact testing
- Drop-weight for composite materials
- Charpy or Izod for plastics
- Information about:
1. Energy for damage initiation and for penetration
2. Initiation and propagation of delaminations for different energy levels
Thermal analysis
-
TGA (thermalgravimetric analysis): weight changes measured while temp. changes
1. Quality control
2. Measure amount of fibers and additives -
DSC (differential scanning calorimetry):
1. Measure Tg, crystallization and degree of cure -
DMTA (dynamic mechanical thermal analysis):
1. Meassure Tg (usually)
Tg temp becomes curing temp. ex. curing at 120 degrees means you can use AFO up to 120 degrees (usually 30 degrees under)
Non-destructive testing (NDT)
- Visual inspection: Just look
- Coin tapping: Delaminations - makes dull sound
- Aucustic emission: Locates cracks, cheap
- Ultra sound: Delaminations, expensive
- X-ray: Delamination and matrix cracks
- Thermography: Delamination - cools faster, cheap
- Shearography: Rare but good for large areas
- Penetrant liquid: Good for metals not composites
