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CDM > D8: Laminate - Service > Flashcards

D8: Laminate - Service Flashcards

1
Q

Why are composites bad at absorbing impact damage?

A
  • Because they are brittle & exhibit very little plastic deformation
  • Means they have low energy absorption compared to materials with a plastic zone (area under stress strain curve)
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2
Q

What does BVID stand for? What does it mean as a design technique? Why is it necessary?

A
  • Barely visible impact damage
  • Assume that there is internal damage that we cannot see (1inch diameter hole corresponding to delaminated zone)
  • Because impact damage shows little sign on surface but propagates through the material, delaminating and shattering fibres in a cone. Can’t be detected with visual inspection, so have to ensure components are strong enough even if BVID is present.
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3
Q

What 8 things can we do to design for impact?

A
  • Use toughened epoxies or TPs for matrix
  • Minimise grouping of plies
  • Use kevlar & glass hybrids with CF
  • Use fabric plies as outer surface (more compliant)
  • Use 45 deg outer plies to protect 0 deg plies
  • Avoid minimum gauge
  • Design for replaceability & reparability
  • Model impact dmg as a hole of equiv. thickness for initial analysis
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4
Q

What are reduced allowables? Why are they necessary?

A
  • A limit on general strain, typically 0.4% at ultimate load
  • To ensure that BVID doesn’t grow throughout the structure’s lifetime
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5
Q

What are the limits for visible and non-visible damage, respectively?

A
  • Component must be repaired before strength degrades to less than limit load
  • Damage must not degrade strength to less than limit load
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6
Q

How is impact testing carried out? What is the most limiting test for composites?

A
  • Indenter dropped onto a clamped specimen, followed by residual strength tests to assess the effect on the component’s structural integrity.
  • Residual hot wet compressive strength after impact test
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7
Q

How does composite fatigue performance compare with metals?

A

Better, provided that the laminate is fibre dominated in direction of loading & loading is in-plane.

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8
Q

What are the 3 phases of composite fatigue? How long do they last relative to each other?

A
  • Wear in, stable growth, wear out
  • Short, long, short
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9
Q

What does the wear-in fatigue phase consist of? When does it stop?

A
  • Failure of weak fibres
  • Failure of weak matrix regions (e.g. areas of porosity)
  • Failure of weak interfaces
  • Material essentially stress-relieving
  • When the material reaches a stable “characteristic damage state”
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10
Q

What does the stable-growth fatigue phase consist of?

A
  • Damage accumulation slows down
  • Matrix cracks and interface disbonds slowly grow and couple into larger cracks
  • Occurs particularly at high stress concentrations
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11
Q

What does the wear-out fatigue phase consist of?

A
  • Areas of cracks amalgamate to form delaminated zones
  • Zones grow as 2D cracks between adjacent plies
  • Propagation accelerates until area is weak enough to fail under the cyclic load
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12
Q

What’s different about the location of fatigue damage accumulation in composites vs metals?

A

In composites, damage accumulates throughout the material, in metals damage tends to be a singular predominant crack at a high stress concentration

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13
Q

What fatigue loading conditions are required to degrade composite stiffness?

A

Very high stresses at very high cycle numbers

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14
Q

What shape are composite S-N curves in tension?

A

Characteristically very fact, even as cyclic stress approaches material static ultimate stress.

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15
Q

How does the fatigue performance under compressive loading compare to tensile? Why?

A
  • Less impressive
  • Fatigue by delamination growth can be significant
  • Temperature and moisture can be significant because of weakened matrix support of fibres
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16
Q

What type of cyclic loading is the most damaging in composites? Why?

A

Cycling between tension and compression, because of interactions between different failure modes

17
Q

How do we account for statistical scatter when designing for fatigue?

A
  • Extreme value statistics (Weibull)
  • Use a stress factor (instead of a life factor, because of significant scatter at high stress levels & flat SN curve)
18
Q

What are 5 ways to design against fatigue damage? Why are they necessary?

A
  • Use conservative designs to lower stress & strain levels
  • Use damage-tolerant design
  • Demonstrate no damage growth of impact specimens
  • Verify fatigue life by testing to several lifetimes, and in different conditions
  • Avoid out-of-plane loads
19
Q

What are the 4 levels of fatigue test performed and how many are performed?

A
  • Major test, complete airframe (usually one)
  • Sub component assembly test (3 per component)
  • Structural features, joints & notches etc (many, to verify scatter)
  • Coupons (many, to verify scatter)
20
Q

Why are fatigue tests for composites longer and more expensive than those for metals?

A
  • Increased sample size required to account for increased scatter
  • Load frequencies must be kept low (5-10Hz) to prevent the build up of internal heat (bad dissipation)
  • Tests performed at higher stress levels for fewer cycles cannot be extrapolated down
21
Q

How does the notch sensitivity of metals and composites? Why?

A
  • Under static loading, metals are notch-insensitive, because the relatively high loads exceed the elastic limit, alleviating concentrations through ductile deformation.
  • Composites can’t deform plastically to alleviate the stress concentrations, so they’re notch-sensitive under static loading.
  • Under the relatively lower cyclic stresses, metals are notch-sensitive as the stresses aren’t high enough for ductility to alleviate concentrations.
  • Accumulation of damage at notches in composites can reduce stress concentrations under lower cyclic loading, making them notch insensitive.
22
Q

What does the crack propagation phase look like in metals vs composites?

A
  • Singular crack growth vs multiple regions of damage
  • Predominant under tension vs compression
  • Damage growth perpendicular to direction of loading vs relatively undefined direction
23
Q

What is the inspection window like in metals vs composites? Why?

A
  • Cracks in metals are very small for most of their lives, but growth accelerates. Therefore, window between a crack becoming big enough to see & becoming catastrophic is small.
  • Relatively large window for composites, because of BVID allowance we tolerate damage up to an inch wide which is easy to detect with NDT. Also, strict strain limit of 0.4-0.5% keeps damage growth slow and stable.
24
Q

How are the properties of metals and composites affected by fatigue damage respectively?

A
  • Metals affected in a small region at the crack tip (plastic zone)
  • Composites suffer from general degradation of the entire loaded material.
25
Q

How does the environment affect metals vs composites?

A
  • Metals relatively unaffected by moisture and temperature (affected by corrosion)
  • Composites suffer a significant reduction in compression ability (matrix support), and a long-term degradation of properties
26
Q

How do the testing spectra of metals and composites compare? Why?

A
  • Metals include high freq low cycles because they are sensitive to them. Neglect higher loads as they can have a beneficial crack blunting effect.
  • Composites neglect high freq low loads because they are insensitive to them. Have to include lower frequency high loads because sensitive.
27
Q

What is the first stage of classical phenomenological fatigue analysis? Why can’t it be applied to composites?

A
  • Load idealisation: load spectrum reduced to sets of constant amplitude cycles. Done based on stable hysteresis loops of plastic deformation in the damage initiation phase.
  • Because it doesn’t have a damage initiation phase and doesn’t exhibit plastic deformation.
28
Q

What is the second stage of classical phenomenological fatigue analysis? Why can’t it be applied to composites?

A
  • Data manipulation: limited S-N data extrapolated, under the assumption that diff. cycles of load at diff. combos of mean and alternating stress have equivalent damage effects.
  • The assumption doesn’t hold up for composites, different modes of failure can occur under the different types of loading.
28
Q

What is the second stage of classical phenomenological fatigue analysis? Why can’t it be applied to composites?

A
  • Data manipulation: limited S-N data extrapolated, under the assumption that diff. cycles of load at diff. combos of mean and alternating stress have equivalent damage effects.
  • The assumption doesn’t hold up for composites, different modes of failure can occur under the different types of loading.
29
Q

What is the third stage of classical phenomenological fatigue analysis? Why can’t it be applied to composites?

A
  • Damage accumulation: assume that the damage accumulation is non-interactive/linear, and is independent of: load mixing, current damage state, and waveform/frequency.
  • The exact opposite is true for composites.
30
Q

How does classical fatigue analysis hold up when applied to composites?

A
  • Pretty good for GFRP despite lack of validity of initial assumptions
  • Very limited agreement for CFRP
31
Q

What are the 3 design allowables, from largest to smallest?

A
  • Material means
  • Material allowable (knocked down using statistical methods (A or B) to account for scatter)
  • Design allowable (includes hot+wet property reduction, long term degradation, hole & defect tolerance etc)
32
Q

What are the 4 design limits, from smallest to largest?

A
  • Operating levels
  • Design limit (largest load likely to be encountered during lifetime)
  • Proof load (load that must be sustained with limited permanent effects, structure can still perform function after)
  • Ultimate load/design ultimate (catastrophic failure)
33
Q

When are A and B basis scatters used?

A
  • Primary and secondary structures respectively (A is more conservative)
34
Q

What are the design strain limits at ultimate load for CFRP (direct and shear) and GFRP (direct)? What’s different about how they’re defined?

A
  • CFRP: 0.4-0.45% for direct (comp-tens), 0.8% for shear. Driven by static strength.
  • GFRP: 0.4-0.5%. Driven by fatigue.

CDM (10 decks)

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