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13 Nov 2007 Halpin & H. Kim 1
Does An Appropriate Test Protocol Exists
for Characterizing the Hail Ice Threat
Environment in Support of the Damage
Tolerance Requirements?
Comparison of current test
concepts and a recommendation.
13 Nov 2007 Halpin & H. Kim 2
Issue: Test Protocol Simulating
Hail Ice Damage• Real hail ice in its naturally occurring state is nearly
impossible to test– complex internal structure and shape – typically oblate
spheroid that is spherically layered or agglomerated
– dense frozen, porous, and liquid regions – density (reported inliterature)
– layers 170 – 920 kg/m3
– overall observed 870 – 920 kg/m3
• SIMULATED HAIL ICE (SHI) is manufactured in the lab:frozen water cast into spheres; density ~ 920 kg/m3
– unfilled ice – monolithic, flat, and spherically layered
– cotton filled per ASTM F320 spec (windshield testing)
Monolithic, or Single Cast Flat-Wise Layered Spherically Layered
Firing
Orientation
Objective of study: comparison of
impact damage initiation to composite
panels by cotton-filled and unfilled
simulated hail ice (SHI)
50.8 mm Ball with 8 g Cotton Fill
Cotton-Filled Ice
13 Nov 2007 Halpin & H. Kim 3
Comparison of Two Hail Ice Test Protocols:
The Cotton Reinforced Ice Does Not Reproduce Typical Hail Induced
Material-structural Damage (data by H Kim)
ASTMF320
FiberRupture &Puncture
P&W Hail Ice
Test, INITIAL
Delamination &
Resin Cracking
13 Nov 2007 Halpin & H. Kim 4
Impact DamageSize vs. KE
(data by H Kim)
0 40 80 120 160 200
Projectile Kinetic Energy (ft-lbf)
0
2
4
6
8
10
12
14
16
De
l am
ina
tio
n C
i rcu
mf. o
r C
r ack L
en
gth
(in
.)
Unfilled - No Damage
Unfilled
Clean Hole - DiameterSame as Ice
1.68 in. Ice
2.0 in. Ice
1.5 in. Ice
Woven Carbon/Epoxy Panels:0.065 to 0.075 in.
Ice:1.68 in. Unfilled1.5 and 2.0 in. Cotton-Filled
Cotton-Filled
- Penetration
FTE - cotton
FTE - Ice
- Delamination
Unfilled - Cracking
Cotton-Filled - No Damage
Cotton-Filled Cracking
13 Nov 2007 Halpin & H. Kim 5
A Head-to-Head Diagnostic Test: Private
Communication
(The Nature of the Impactor Makes a Difference!)
• Simulated hail ice, no damage observed at 500 in-lbs
– Damage threshold ~380 ft-lbs (4567 in-lbs)
• ASTM F320 composite reinforced ice
– Damage threshold ~44 ft-lbs (528 in-lbs)
– Damage size estimate >1 inch
• FFA-2 lead ball simulated FOD & runway debris.– Damage threshold ~10 to 18 ft-lbs (~180 to 220 in-lbs)
13 Nov 2007 Halpin & H. Kim 6
Comparison: Parametric Estimates and Diagnostic Test Conditions for the
ASTM F320 2.4 Inch Diameter Ball, & Simulated Natural Ice using the P&W.
ASTM 320
Test
Natural Ice Test
Prediction
SHI Test, No Damage
•FFA-2 Lead Ball Simulated FOD &Runway Debris.
13 Nov 2007 Halpin & H. Kim 7
Illustration of the Influence of Test Simulation Methodology on the Thickness
of a Typical Pressurized Structure: The ASTM F320 Cotton Composite
Reinforced Test Ball Is Significantly More Damaging That Simulated Hail Ice.
An Inappropriate Test Protocol, ASTM F320 Adds Weight but May Not
Provide Improved Damage Tolerance (Reduced CAI & Fatigue Sensitivity).
13 Nov 2007 Halpin & H. Kim 8
In Early 1980’s Pooled Data From Falling Weight Testing Was Used to
Correlate CAI Capability Resulting From a Combination of
Delamination, Resin Cracking & Fiber Breaking
• Delamination & Resin Cracking Produces Lower CAI CapabilityThan Fiber Breaking
13 Nov 2007 Halpin & H. Kim 9
Different Damage Morphologies May Have Different CAI Strengths:
Notional Illustration
ASTMF320
FiberRupture &Puncture
P&W Hail Ice
Test, INITIAL
Delamination
& Resin
Cracking
13 Nov 2007 Halpin & H. Kim 10
Cotton-Filled Ice PenetrationType V Damage Mode – Clean Hole (data by H Kim)
• Target: 8-ply woven carbon/epoxy plate, quasi-isotropic, 0.065 in. Thickness, held by picture-
frame fixture having 11x11 in. Opening and providing clamped b.C. (With in-plane freedom)
• Projectile: 2.0 in. Dia. Ice with cotton fill per ASTM F320, vo = 283 ft/s, KE = 186.6 ft-lbf
• Result: type V damage mode – clean hole penetration of size roughly same as projectile dia.
Impact-Side ViewBack-Side
View
Liberated Panel
Fragments,
Recovered Ice Ball
13 Nov 2007 Halpin & H. Kim 11
Cotton-Filled Ice PenetrationType IV Damage Mode – Large Cracking (data by H Kim)
• Target: 8-ply woven carbon/epoxy plate, quasi-isotropic, 0.065 in. Thickness, held bypicture-frame fixture having 11x11 in. Opening and providing clamped b.C. (With in-planefreedom)
• Projectile: 2.0 in. Dia. Ice with cotton fill per ASTM F320, vo = 183 ft/s, KE = 77.1 ft-lbf
• Result: type IV damage mode – thru penetration with large cracking patterns, ~ 13.1 totalcrack length
Impact-Side
ViewBack-Side
View
13 Nov 2007 Halpin & H. Kim 12
Comparison: Cotton-Fill vs. Unfilled Damage at ~180 ft-lbf(data by H Kim)
0.065 in. thick panel impacted byASTM F320 Cotton-Filled IceBall, 2.0 in. Dia.
vo = 283 ft/s, KE = 186.6 ft-lbf
Damage: clean hole penetration ~ 2in. diameter
0.072 in. thick panel impacted by
Unfilled Ice Ball, 1.68 in. Dia.
vo = 370 ft/s, KE = 179.4 ft-lbf
Damage: penetration with large
cracking ~ 13 in. tot. crack length
Both Images Panel Back-Side View
13 Nov 2007 Halpin & H. Kim 13
Fuel Tank - Tire Rupture Performance Requirements in AC25.963-1(A Positive Example)
13 Nov 2007 Halpin & H. Kim 14
Compliance Testing Performed at a Structural Configuration Level.
Properties of the Impacting Rubber Disk Comparable to Tire Tread
Used for the Aircraft Per the Performance Requirement.
13 Nov 2007 Halpin & H. Kim 15
Structural Capacity for Impact Threats Characterized With Test
Articles Representative of Aircraft Design and
Impacting Objects Relevant to the Threat
13 Nov 2007 Halpin & H. Kim 16
Suggested Test Standard:
Common Format - Performance Based &
Relevant to Damage Tolerance Needs
• Bird Strike– ASTM 330-89 modified
– Far Part 25 (no airframerequirement?)
• Weight-Size - 90%
• Velocity, VC
– Performance Focus• Bird carcass
• Gel Pack
– Document Threat
– Other
• Hail Strike– ASTM 330-89 modified
– Far Part 25 (no airframerequirement?)
• Weight-Size - 90%
• A/C Velocity, VC &
• Hail Terminal velocity
– Performance Focus• Simulated Hail Ice
– Document Threat
– Other
13 Nov 2007 Halpin & H. Kim 17
Comparative Test Program Recommended
• 1960 through the 1980 for bird strike
– Bird carcass & Gel Pack calibration
– Competitive test & simulation model development
– EU investments 1990’s into 2005
• AC5.963-1 good example; tire rupture performancerequirement
• Lightning requirements - performance focus?
• Activity similar to birdstrike recommended for the hail icethreat
– Natural ice versus Simulated Hail Ice
– Competitive test & simulation model development
• Document in common location
– Requirements for airframe (fragmented)
– Characterization of threat environments
– Acceptable test protocol(s)
13 Nov 2007 Halpin & H. Kim 18
!Sandwich" Construction:
Honeycomb Sandwich Construction Increases Vulnerability
13 Nov 2007 Halpin & H. Kim 20
SUMMARY
1. Damage Thresholds should be the bases for the dimensioning of laminatesexposed to hail and other impact threats (the damage thresholds will dependon the nature of the threat, FOD is different that hail ice, than the ASTM ball,than the rubber puck for tire damage, ---.
2. The threat environments need to be quantified, agreed to, and kept up todate. They are independent of platforms.
(Good data base for the US But need data for international locations.)
JOINT INTERNATIONAL EFFORT?
A data base need; hail & operational velocities, ---.
3. Defense of a design is the responsibility of the OEMs including test &analysis methods.
(Realistic test protocols are available.)
4. Inspection for Cause; Hail example, triggered by extreme Rogue weatherconditions, and a defined operating period allowing short term operationsbefore repair, dependent upon severity of damage as defined by the Pxxinternal load conditions or Allowable Damage Limits.
This perspective would utilize a “Self Evident Rogue Event” instead of “SelfEvident Damage” as a criteria condition. The dent has been a “Self EvidentDamage” criteria condition BUT is not a sufficient basis for a preventivedesign & maintenance concept.