Detailed Testing Information

Materials

Area of Nomex honeycomb (3 samples each): 30 ft^2

• flatwise tensile strength (3x3)

  • maximum 9 in^2

• face sheet compressive strength (3x3)

  • L < 8 * t

  • 2 < W < L, W > 2 * t

• compressive strength/modulus (3x3)

  • min area: 4 in^2

  • max area: 9 in^2

• flexure creep (3x8 and 3x24)

  • core/core-to-facing failure mode desired: width = 3 in, length = 8 in

  • facesheet failure mode desired: width: 3 in, length 24 in

• flexural strength/modulus

  • 2D flexural properties of sandwich subjected to distributed load (ASTM D6416)

    • L, W = 1.017 * (support span)

  • facesheet properties of sandwich by long beam exposure (ASTM D7249) (3x24)

    • width = 3 in

    • length = 24 in

  • determining sandwich flexural/shear stiffness (ASTM D7250)

    • width < L/2

    • length = (span length) + 2 in

Flatwise Tensile Strength

Summary

• Tests flatwise tensile strength of core, facing of sandwich panel, and core to face sheet bond

• Subjects sandwich to tensile force normal to plane of the sandwich, transmitted through loading blocks bonded to facings

Procedure

1. Bond loading blocks to face sheet

   1. Ensure adhesive does not produce thermal effects

   2. Bonding pressure should not exceed face-to-core bonding pressure

2. Produce failure between 3-6 minutes

   1. Standard head displacement rate is 0.5 mm/min

   2. Acceptable modes of failure are internal to sandwich: core tensile failure, core facing adhesive failure, face sheet tensile failure

3. Record force v. head displacement

   1. 2-3 data readings per second

   2. 100 data points minimum per test

4. Record max force, failure force, and head displacement at moment of fracture

5. Calculations

   1. Ultimate flatwise tensile strength = maximum force before failure / area

Face Sheet Compressive Strength

Summary

• Provides basis for load-carrying capacity

• Subjects sandwich to increasing compressive force parallel to plane transmitted through clamped or bonded end supports

Procedure

1. Measure specimen dimensions before test

2. Produce failure between 3-6 minutes

   1. Head displacement rate of 0.5 mm/min

3. Apply compressive force until 10% of anticipated ultimate force is achieved, then reduce compressive force to 150 N at equivalent unloading rate and check strain gage for proper alignment

   1. Look for evidence of bending: differences in stress-strain or force-strain on opposite faces of specimen, rapid divergence of strain readings on opposite faces

   2. Calculate percent bending to know the direction of bending (based on sign) and to know reason for bending

      1. If bending percent > 10% - examine fixture for misalignment, looseness, and gaps

      2. Rapid increase in bending percentage indicates panel instability

4. Record force v. head displacement and force v. axial strain

   1. 5-10 data recordings per second, 300 data points per test

5. Record force, displacement, strain, damage mode, compliance changes, and initial damage events

6. Calculations

   1. Ultimate compressive strength = ultimate force / (width * (2 * thickness of one face sheet))

Compressive Strength / Modulus

Summary

• Subjects sandwich core to uniaxial compressive force normal to plane

• Used to calculate compressive strength and modulus normal to plane

• Specimen centered in apparatus between platens that do not extend beyond specimen periphery

Procedure

1. Ensure specimen does not have cross sectional area (square/circle) exceeding 10000 mm^2 or less than 60 cells in the core

2. Apply initial load of 45 N, then zero and balance LVDT/compressometer

3. Produce failure between 3-6 minutes with a head displacement of 0.5 mm/min

4. Load until failure or until deflection measures 2% of initial core thickness

   1. Acceptable failure methods are uniform compressive failure

   2. Invalid failures: compressive failure at corner/edge of specimen

5. Record data at 2-3 recordings per minute for a minimum of 100 data points per test

6. Calculations

   1. Ultimate strength = ultimate force / cross sectional area

   2. Deflection stress = applied force at 2% thickness / area

   3. Compressive modulus = ((P_.003 - P_.001) * t)/((d_.003 - d.001) * A)

      1. P_.003 / P_.001 = applied force at percent thickness value

      2. d_.003 /  d_.001 = deflection value when thickness is .003 or .001 of initial value

      3. t = thickness

      4. A = cross sectional area

Flexure Creep

Summary

• Subjects sandwich panel to sustained normal force using 3-4 point loading fixture

Procedure

1. Measure specimen and length of support and loading spans

2. Test at least five specimens

3. If core/core-to-facing failure mode is desired use specimen configuration/proportions from ASTM test method C393

4. If facesheet failure is desired use specimen configuration/proportions from ASTM test method D7249

5. Calculations

   1. weight required to apply force using 3-point loading system

      1. ((P - p)A - wB) / M

      2. P = force applied to specimen

      3. p = mass of loading plate and rod

      4. w = mass of lever arm

      5. A = distance between pivot point and point of applied force

      6. B = distance from pivot point to center of gravity of loading arm

      7. M = distance between pivot point and weight point

   2. Creep deflection rate

      1. Difference in deflection / time

   3. Creep deflection percentage

      1. ((D - d) / d) * 100

      2. D = total deflection under constant load at time t

      3. d = initial static deflection under same load

   4. Average core shear stress

      1. F = P / ((d + c) * b)

      2. P = force?

      3. F = core shear stress

      4. b = sandwich width

      5. c = core thickness

      6. d = sandwich thickness

      7. t = nominal facing thickness

   5. Facing stress

      1. F = (PS) / (4 * (d - t) * bt)

      2. S = support span length

Flexural Strength and Modulus