ASTM D7249

ASTM D7249 outlines a standardized method for determining the flexural stiffness and strength of sandwich panels using a long beam bending test. This test simulates distributed loading across a span, mimicking real-world conditions for structural sandwich composites used in aerospace, marine, automotive, and construction industries.

This method is particularly useful for testing large-scale sandwich panels where core shear strength, facing tensile/compressive strength, and overall flexural behavior are critical to performance.

Purpose and Scope

ASTM D7249 is designed to measure the structural performance of sandwich panels in long-span flexural applications. The primary results obtained from this method include:

• Apparent flexural stiffness (EI)

• Maximum load at failure

• Failure mode identification (facing yield, core shear, etc.)

The standard is intended for panels with metallic, polymeric, or composite facings and rigid core materials (e.g., foam, balsa, honeycomb). It can be applied to both flat and slightly curved panels, though curvature must be minimal.

Testing Equipment

To perform ASTM D7249 testing, the following equipment is required:

• Universal Testing Machine (UTM):
  Capable of applying loads with controlled crosshead speed and recording force-displacement data.

• Custom Long-Span Flexure Fixture:
  This includes a four-point or three-point bending setup, depending on the test variation (Method A or B):

  • Method A (Preferred): Four-point bending to produce a constant moment between inner supports.

  • Method B: Three-point bending, simpler but introduces a shear component at the loading point.

• Support and Loading Cylinders:
  Typically steel rollers with radiused edges to distribute force and avoid damage.

• Data Acquisition System:
  For measuring load, crosshead displacement, and optionally strain gauges for local facing or core deformation.

Test Specimen

• Panel Size:
  Specimen length should be 30 times the total panel thickness for four-point bending, or 20 times for three-point bending. This ensures that failure occurs in bending rather than local crushing or support interactions.

• Specimen Preparation:

  • Clean and deburr edges.

  • Maintain uniform thickness and width.

  • Edges may be reinforced to prevent localized crushing.

• Number of Specimens:
  Typically 5 or more are tested to ensure statistical validity.

Test Procedure

1. Specimen Mounting:
   Place the panel in the bending fixture with the support and loading spans accurately measured and aligned.

2. Loading:
   Apply load at a constant crosshead speed (usually 2.5 mm/min or 0.1 in/min) until failure.

3. Data Recording:
   Continuously record force and mid-span displacement. If strain gauges are used, record strain on the facings and core.

4. Observe Failure:
   Note the failure mode: core shear, facing wrinkling, debonding, etc.

Calculations

• Flexural Stiffness (EI):
  Calculated using beam theory based on the span and slope of the load-displacement curve in the elastic region.

• Maximum Bending Stress in Facing:
  Based on maximum load and geometry of the panel.

• Shear Stress in Core (Optional):
  Calculated if core shear failure occurs.

Reporting Requirements

A complete test report according to ASTM D7249 should include:

• Material and construction details of the sandwich panel.

• Dimensions of specimens.

• Test setup (Method A or B, span lengths, roller diameters).

• Load-displacement curves.

• Flexural stiffness, failure load, and failure mode.

• Any anomalies or deviations from the standard.

Common Applications

ASTM D7249 is commonly used for testing panels in:

• Aircraft flooring and interior panels

• Wind turbine blades

• Marine decking and bulkheads

• Automotive body panels

• Construction and architectural sandwich panels

Related and Similar Standards

If ASTM D7249 is not appropriate due to panel size, shape, or loading type, consider these related standards:

• ASTM C393 – Standard Test Method for Core Shear Properties of Sandwich Constructions by Beam Flexure
  (Shorter spans, focused on core shear)

• ASTM D790 – Flexural Properties of Unreinforced and Reinforced Plastics
  (For homogenous plastic or composite materials)

• ASTM D7250 – Standard Practice for Determining Sandwich Panel Properties from Flexure Testing
  (A more general practice that complements D7249)

• ASTM D7766 – Flexural Properties of Sandwich Constructions by 4-Point Bending
  (Alternative long-span sandwich testing method)

• ASTM C365 – Flatwise Compressive Properties of Sandwich Cores
  (To isolate the compressive performance of the core itself)

Summary

ASTM D7249 is a specialized and essential test method for determining the flexural stiffness and strength of large sandwich composite panels under long-span loading. It provides a realistic simulation of real-world structural applications and is widely used in industries where weight savings and structural performance are critical.

Careful control of fixture geometry, span lengths, and specimen preparation is key to obtaining reliable and consistent results. It complements other flexural test methods like ASTM C393 and D790 and is especially suited for engineers working in composite structure design and quality control.