ASTM C393

Overview

ASTM C393 is the standard test method used to determine the flexural stiffness, shear stress, and core shear modulus of sandwich-structured composite panels. These structures typically consist of two strong face sheets bonded to a lightweight core, such as honeycomb, foam, or balsa wood.

This test helps engineers evaluate how well a sandwich panel resists bending loads, especially in aerospace, automotive, and marine applications where weight and strength are critical.

Test Equipment and Fixtures

ASTM C393 is performed using a universal testing machine and a three- or four-point bending fixture, depending on the desired failure mode (face sheet or core failure).

Key Equipment Components:

• Upper loading noses: Two rounded loading anvils (for four-point bending) or a single center nose (for three-point bending)

• Lower support spans: Two fixed supports to hold the sandwich beam specimen

• Load cell: To record the applied force until failure

• Deflection measurement: May be taken via crosshead displacement or a separate extensometer/deflectometer

Recommended Dimensions:

• Specimen width (b): Typically 75 mm or wider

• Length (L): Must provide adequate span-to-depth ratio, often 150 mm or more

• Core thickness (c) and overall thickness (t): Varies depending on the material system being tested

The loading and support noses should have a radius of 6.35 mm (0.25 inch) to minimize local crushing.

Specimen Preparation

• Cut specimens with clean, square edges

• Ensure uniform bonding between the face sheets and the core

• Remove surface defects or delaminations before testing

It is critical to align the sample so that loading is perpendicular to the face sheets and symmetric across the span.

Test Procedure

1. Place the specimen on the support rollers.

2. Align the loading nose(s) with the center (for 3-point) or 1/3 spans (for 4-point).

3. Begin loading at a constant crosshead speed—typically 5 mm/min.

4. Record maximum force (P) and deflection (δ) at the center.

5. Observe and note the failure mode: core shear, face sheet rupture, or adhesive failure.

Calculations

1. Core Shear Stress (τ)
τ = (3 × P × S) / (4 × b × c)

Where:

• τ = core shear stress (Pa or psi)

• P = total applied load (N or lbf)

• S = support span (mm or in)

• b = specimen width (mm or in)

• c = core thickness (mm or in)

2. Facing Bending Stress (σ)
σ = (3 × P × S) / (4 × b × t_f²)

Where:

• σ = facing stress (Pa or psi)

• t_f = thickness of one face sheet (mm or in)

• t = total thickness of sandwich panel (used in modified formulas if needed)

3. Core Shear Modulus (G_c)
G_c = (P × S) / (b × c × δ)

Where:

• G_c = core shear modulus (Pa or psi)

• δ = deflection at the center of the beam (mm or in)

These calculations allow engineers to characterize both the flexural performance of the composite and the specific contribution of the core material.

Applications and Use Cases

• Aerospace composite panels used in aircraft flooring, interior walls, and wing structures

• Automotive panels in hoods, doors, and trunk liners

• Marine hulls and decking structures

• Wind turbine blades and infrastructure panels requiring high stiffness-to-weight ratios

Related ASTM Standards

Several other ASTM standards are closely related to ASTM C393 and are often used in tandem when analyzing sandwich structures:

• ASTM D7249 – Test method for facing properties of sandwich constructions under flexural load

• ASTM C365 – Compressive strength of sandwich cores

• ASTM C273 – Shear properties of sandwich core materials by tension loading of core specimens

• ASTM D790 – Flexural properties of unreinforced and reinforced plastics (not for sandwich structures, but often used for face sheet material testing)

• ASTM C394 – Edgewise compressive strength of sandwich constructions

These methods help build a complete mechanical profile of a sandwich panel system, from in-plane face strength to out-of-plane shear and core behavior.

Summary

ASTM C393 provides a critical framework for evaluating the flexural stiffness and core shear properties of sandwich panel constructions. By simulating real-world bending loads, this test helps engineers design stronger, lighter composite systems for demanding structural applications. Whether you’re manufacturing aerospace interior panels or automotive composites, this standard is essential for quality control and material validation.