8: Design and Validation Flashcards
What are the sections of the composite testing pyramid (from bottom to top)?
-Coupons (different fibres/resins/orientations/ect.)
-Elements (Overall Geometry)
-Details (Regions of interest)
-Sub-components (joining of elements)
-Component
What is the purpose of ‘Coupons’ in the Building Block approach?
-Generate material design allowables and ‘basis’ values
-2D loading
What is the purpose of ‘Elements’ in the Building Block approach?
-Determine most strength-critical failure mode for each feature
-Select strength-critical failure mode test environment
-Matrix-sensitive failure modes (compression, out-of-plane shear)
-‘hot spots’ caused by out-of-plane loads
-3D loading
What is the purpose of ‘Details’ in the Building Block approach?
-Specimens testing a single loading condition and failure mode
-Compare to analytical predictions
-Adjust design allowables and analytical methods if necessary
What is the purpose of ‘Sub-components’ in the Building Block approach?
-Increasingly complicated tests evaluating complex loading situations
-Failure possibility from several failure modes
-Compare to analytical predictions and adjust models if necessary
What is the purpose of ‘Component’ in the Building Block approach?
-Full-scale component static and fatigue testing for validation
-Compare to analysis
Explain Coupon Testing in the Building Block approach?
-2D test laminates are machined/water-jet cut
-Composites more variable than metals, therefore more data uncertainty
-Hundreds of test specimens are required
-A-basis and B-basis values used to calculate safe designs (allowables)
Explain A-basis and B-basis allowables
A-basis:
-Safety critical components
-95% lower bound on the 1st percentile of test population (0.05%)
B-basis:
-Less critical structural components
-95% lower bound on the 10th percentile of test population (0.5%)
Explain the mechanical tests of ‘coupons’
-Conducted according to a standard
-Performed by universal testing frame (force measured by load cell)
Test types:
-Tension
-Compression
-In-plane Shear
-Inter-laminar fracture toughness (Double Cantilever beam (DCB) testing)
-Fatigue resistance (Cyclic testing)
-Damage tolerance (Open & filled hole tensile/compressive strength, Compression after impact (CAI))
How is strain measured during mechanical testing of ‘coupons’?
-Strain gauge on the surface of the composite (surface strain)
-Contacting Extensometer clamp around the sample (~1micrometer, can’t measure strain to failure)
-Non-contact (video) Extensometer
Full-field strain measurement:
-Digital Image Correlation (DIC)
-Speckle pattern required
-Tracks displacement of dots per frame
-Sensitive to ambient lighting
Explain Compression After Impact (CAI) testing
-Evaluates through-thickness damage tolerance
-Falling drop-weight induces controlled damage (impact energy set based on specimen thickness)
-Damaged specimen is C-scanned observing damage prior to further testing
-Specimens are compression tested in bespoke rig (avoids bending)
-Compared to non-impacted specimen (evaluates damage tolerance)
Explain ‘Element Testing’ in the building block approach
-Shapes are formed from flat laminates
-Testing evaluates ability of the material to withstand common laminate discontinuities
-Tests validated with FEA and analytical data
eg:
-Test a carbon stringer (T-joint)
Explain ‘Detail Testing’ in the building block approach
-Testing of assemblies
eg:
-Test stability (buckling) of stiffened composite panels
Explain ‘Sub-component Testing’ in the building block approach
-Evaluates behaviour and failure modes of more complex structures
-Tests after controlled damage (to understand performance if part is damaged)
-Application specific
Explain ‘Component Testing’ in the building block approach
-Properties dependent on manufacturing process
-Strain gauges applied before testing, measurements mustn’t exceed design allowables
-Tests conducted under requisite environmental conditions
What size should a testing coupon and its strain gauges be?
Coupon:
-Width >2x the unit cell size
Strain gauge:
-long enough to measure uniform strain
Why does strength vary with composite specimen size but modulus does not?
-Material strength is strongly linked to defects, failures occur at the defects (stress concentrations)
-High variability of the microstructure
How does strain to failure change (tensile & flexural testing) with specimen volume?
-Strain to failure decreases
Explain flexural testing and compare it to Tensional testing
-Test rig span is a function of sample thickness
-Span to stiffness ratio is adjusted depending on the in-plane stiffness of the material
-Short spans increase through-thickness shear, reducing flexural stiffness
What issues are there with manufacturing large parts (scale effects)?
-Scale is related to manufacturing (eg. kayak to shipping hull)
-Difficult to have a consistent impregnation of resin (increased defects)
-Harder to consolidate with heat and pressure (Lower Vf, longer cycle times)
-Hard to uniformly heat, especially with changing cross-section (Uneven cure produces stress concentrations at the weld lines)
-Large fabric plies are hard to handle (fabric shear and crimp)
State the Weibull Theory probability of failure equation
-Larger material volumes have higher probability of containing a critical flaw
-Assumes scatter of test data due to material variability (not the test rig)
-Only valid if the failure mode is constant
What are typical ‘Weibull modulus’ (m) values for: Ceramics, Composites & Metals
Ceramics: 5-20
Composites: 5-30
Metals: >30
State the Weibull Theory filament tensile strength equation
What is the effect Filament length has on the filament tensile strength?
-Filament strength decreases with filament length
What are ideal rules for Aerospace laminate manufacture?
-Fibres aligned in direction of principal loads or stresses
-Constant fibre angles are difficult to maintain on double curved surfaces
-Quasi-isotropic plies are isotropic in in-plane stiffness only (not for strength and bending)
What are the Aerospace Laminate Design Guidelines? (Contiguity, 10% rule, damage tolerance)
Contiguity:
-No more than 2 plies of same orientation stacked together
-Angle difference between adjacent plies <45 degrees (reduce inter-laminar shear stresses)
10% Rule:
-minimum of 10% of plies oriented at 0, +-45 & 90 degrees
-0 degree unidirectional plies have poor transverse properties (off-axis necessary)
-Angle plies required to carry shear load (+-45 is the best)
Damage Tolerance:
-No 0 degree plies on the outer surfaces
-Surface plies should ideally be +-45 degrees
How is delamination at ply-drop locations (thickness reduction through part) avoided with ply layup sequence?
Resin pocket at ply-drop site leads to defects (stress concentrations)
Covering plies (surfaces):
-Never dropped, keeping continuity at the free surface
Angle (at the drop):
-Maximum taper angle of 7 degrees
Max dop number:
-2 plies at the same thickness increment
-Try to minimise number of ply drops (minimise resin rich voids)
Stagger:
-Staggered through the through-thickness and length (at least 8 times the thickness)
Order:
-Drop plies in order of stiffness to ensure a smooth transfer of load and reduce stress orientations
Why are composites vulnerable to delamination?
-Low transverse strength, due to plies bonded with matrix in through-thickness
-Failure due to inter-laminar stresses
-Out of plane forces introduced by: Attachments, joints & impacts
-Interlaminar tension from curved sections
-Stress concentrations from dropped plies, matrix cracks and free edges
State the Interlaminar Shear Strength equation
Explain the Short Beam Shear Test (coupon test)
-Assumes shear stress distribution in a cantilevered beam is parabolic through the thickness
-Shear stress max at the mid-plane, 0 at -surfaces
-3 Point connection to the beam
What are the common failure modes in the Short Beam Shear Test (coupon test)?
-Interlaminar shear (delamination)
-Compression (Buckle Fracture)
-Tension (Tear Fracture)
-Inelastic Deformation
Explain the L-Bend Test Test (Element test)
-Uniform thickness L-Bend specimens required (difficult to manufacture)
-4 point connection to beam
-Similar parabolic shear stress distribution to short beam test
What through-thickness reinforcements can a laminate have to improved delamination resistance?
-3D weaving (through-thickness filaments wound between plies)
-Tufting (Needle forms loops through to the exiting side of the laminate)
-Z-pinning (through-thickness metallic pins)
-Toughen the matrix material, increasing the strain to failure (add thermoplastic particles to the resin, non-woven veil (discontinuous) at inter-ply boundary)
What is a sandwich panel? and explain its properties
2 thin skins with high mechanical properties separated by a thick lightweight core
-Skins carry tensile/compressive loads in bending
-Core carries shear loads
-Separation gap of skins increases bending stiffness and torsional rigidity (second moment of area)
-Efficient panels with high bending stiffness at low density cores
Using the parallel axis theory calculate a sandwich panels Modulus
State the second moment of area and parallel axis theorem equations
What is the flexural rigidity equation for a sandwich panels where Es (skin modulus)» Ec (core modulus)?
What is the flexural rigidity equation for a sandwich panels where tc (core thickness)» ts (skin thickness)?
What are the potential failure modes in sandwich panels?
Face yield of skin in tension: High core shear modulus for thin skins
Shear failure of core: Low core shear modulus for thick skins
Wrinkling of skin in compression: Low core shear modulus for thin skins
Delamination: weak interference layer between plies/core
What are the properties for Nomex honeycomb core structure?
Density: 0.048 g/cm^3
Compressive strength: 2.4 MPa
Compressive modulus: 0.14 GPa
Shear strength (L&W directions): 1.2/0.7 MPa
Shear modulus (L&W directions): 40/25 MPa
What are the properties for Aluminium honeycomb core structure?
Density: 0.083 g/cm^3
Compressive strength: 4.52 MPa
Compressive modulus: 1.31 GPa
Shear strength (L&W directions): 2.48/1.45 MPa
Shear modulus (L&W directions): 448/241 MPa
What are the properties for PVC foam core structure?
Density: 0.075 g/cm^3
Compressive strength: 1.33 MPa
Compressive modulus: 0.73 GPa
Shear strength: 1.09 MPa
Shear modulus: 28 MPa
What are the properties for Balsa wood core structure?
Density: 0.15 g/cm^3
Compressive strength: 12.8 MPa
Compressive modulus: 4.07 GPa
Shear strength: 2.98 MPa
Shear modulus: 159 MPa
What do honeycomb sandwich core properties depends on?
Anisotropic
-Cell geometry (from l; wall length, and T; thickness)
-Wall thickness (t)
-Material properties of cell walls
How are honeycomb core structures manufactured?
-Lines of adhesive applied to flat sheets
-Stack sheets
-Cure adhesive to form block
-Slice block to required thickness
-Apply force to expand block (bond line pattern determines honeycomb structure)
Alternative:
-Corrugated rolls convert the sheet to a corrugated sheet
-Bond corrugated sheets together
-Slice bonded sheets to required thickness
Explain the ‘one step’ autoclave Sandwich panel manufacturing process
-Prepreg is laid up on core
-Moulded and cured
-Core and skins are bonded with: excess resin from the prepreg or an additional adhesive film
Explain the ‘two step’ autoclave Sandwich panel manufacturing process
-Both sandwich panel skins are moulded and cured separately
-Peel ply during curing leaves a textured surface for bonding
-Additional adhesive bonds the skins to the core
Explain the ‘two step’ wet lay-up/vacuum bagging Sandwich panel manufacturing process
-Skins laid up, vacuum bagged and cured separately
-Peel ply prepares rough surface for core bonding
-Additional adhesive bonds the skins to the core
How does Modulus and UTS vary with tow size for random and aligned discontinuous fibres?
How does UTS and deposition rate vary for: random, aligned and dry tow placement preforms
Discontinuous Carbon Fibre Preforms (DCFP) are described as notch insensitive, explain this
-First failure occurs away from the hole/notch
-Highly damage tolerant
