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
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
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
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)
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 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
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
