ASTM C365 Standard Test Method for Flatwise Compressive Properties of Sandwich Cores

Get Certified ASTM C365 Testing for Structural Sandwich Core Performance 

Sandwich structures serve various fields, including aerospace, transit, marine, and wind energy, and require a light core to take up compressive loads and keep the facings apart. If either the compressive strength or the modulus is too low, the material may collapse under load, crush the core, and wrinkle the faces. ASTM C365 testing provides standardized compressive performance data to certify that core materials meet design allowances. Independent laboratory testing ensures material compliance, reduces product liability, and optimizes structural weight. 

The scope of ASTM C365 includes :

• Material type: Honeycomb cores (aluminum, paper, aramid/Nomex), rigid cellular plastics, balsa wood, foam cores, and open-cell or closed-cell core structures.
• Purpose: Determination of flatwise compressive strength, compressive stress at specified strain, and flatwise compressive modulus. 
• Properties evaluated: Ultimate flatwise compressive strength, yield compressive strength, compressive modulus of elasticity, and failure modes (core crushing, buckling, or cell wall shear).
• Testing method: Uniform compressive loading of block specimens at a controlled crosshead speed in a direction perpendicular to the plane of the core thickness at self-aligning loading platens.
• Results: Load-displacement curves, calculated compressive stress in megapascals (MPa) or pounds per square inch (psi), and compressive modulus values. 
• Test limitations: Assesses material behaviour in bare form without facing skins; may not necessarily reflect the behaviour of a full sandwich panel, depending on the bond integrity of the skin to the core.
• Use conditions: Under standard laboratory temperature and humidity, or in high- and low-temperature environments (high or low humidity) chambers.
• Applications: Structural components for aerospace, cores for wind turbine blades, sandwich panels for marine hulls, and structures for transportation.

 What are the Uses of ASTM C365 Testing?

ASTM C365 testing allows a structural design engineer or a composite materials lab, to assess the mechanical properties of core materials when they experience direct compression. This standard:

• Measures final compression strength and elastic modulus of flatwise core materials.
• Offers sandwich panel finite element analysis (FEA) and structural modeling design allowables.
• Checks and monitors batch quality and consistency for critical manufacturing processes.
• Compares performance characteristics of various core densities, cell sizes and material types.
• Supports material qualification for aerospace and defence material specifications.
• Helps assess the effects of the environment (i.e. temperature and humidity) on compressive properties.

What Materials Can Be Tested Under ASTM C365? 

The standard conducts tests on honeycomb cores, foam cores, and thermoplastic polymers for compressive strength and stiffness under flatwise loading, thus ensuring adequate suitability in terms of the essential strength-to-weight ratio of such sandwich composite structures.

Why is ASTM C365 Important? 

It is an important method for determining sandwich core material compressive properties in structural applications. It therefore enhances design, performance, and quality control in order to minimize failure risks of lightweight yet durable materials, thereby fulfilling industry-specific requirements.

ASTM C365 Testing Process and Data Collection

The technicians measure the initial length, width, and thickness of the core sample to determine the cross-sectional area and density of the sample. The technicians load the sample on the lower platen of the Universal Testing Machine. The technicians move the upper self-aligning platen down until they reach uniform contact. The loading is applied in a monotonous manner at a prescribed loading rate. An LVDT or compressometer measures deflection of the thickness to eliminate platen compliance errors. The test continues until the last load or until the specimen undergoes plastic deformation/densification. The force and deflection data are continuously recorded to draw the stress-strain curve. 

Common Challenges and Troubleshooting

Non-parallel specimen loading results in non-uniform stress distribution and early local crushing, and thus artificially low compressive strength values. Precision-machined or sanded parallel specimen faces are required. Due to the slight variations in the surface, it is necessary to use the self-aligning spherical platen seat. In testing honeycomb cores, damage to edge cells during cutting may lower apparent strength, and the size of the specimen should be adequate to minimize the percentage of edge cells. 

• The flatwise compressive strength is the maximum compressive stress a core can withstand before failure, and the lab reports it in the following units: MPa or psi. The higher the value, the more load-bearing the material.
• The laboratory reports the compressive modulus in terms of MPa or psi, and this is an indication of the stiffness of the core in compression. The higher, the more resistant to deformation.
• Proportional limit stress is also taken into account as it gives the stress at which the core starts to experience plastic deformation.
• The laboratory reports deformation data, which can help compute the applied force value as a function of deformation from the entire curve.
• Data that the test method provides is applicable for sandwich core structural design properties, material specifications, research and development applications and quality assurance.
• The compressive strength and modulus results are compared to relevant product specifications and/or design parameters to determine the ability of the core material to perform the required functions.

Link to ASTM C365