Sandwich Construction Materials Mechanical Testing

Sandwich construction materials are versatile materials with high strength, high stiffness, and low density. They are commonly used in the aerospace, marine, and construction industries. This post discusses the construction of sandwich materials, definitions, and mechanical testing and equipment.

What are sandwich construction materials? 

Sandwich construction materials are composite structures composed of a sandwich material of at least two different materials such as foam, honeycomb, polymers, wood, and ceramic. The structure of a sandwich construction includes two face plates, optional adhesive layers, and a core. The facings are usually solid materials that are thin, dense, and high-strength whereas the core is a lower-strength, lower-density material. When these materials are joined together in the sandwich structure, the overall material is a highly efficient element. Sandwich construction materials are often used in industries such as aerospace, construction, and more.  

What are some examples of materials used in the outer facing layers and in the core of a sandwich construction?

Face materials can be selected from Aluminum, Steel, Titanium alloys, fiber-reinforced polymers, and wood. 

The core material may be a foamed polymer, synthetic rubber, honeycomb, or foam.

What are sandwich materials used for?

Sandwich construction materials can be designed from a combination of a variety of facing and core materials. They are commonly used in aerospace, marine, and construction industries. Depending on the final application, the materials selected will have different properties. For example, sandwich composite materials used in aerospace vehicles have high strength, high stiffness, are light in weight and need to have the ability to resist high velocity impacts. For packaging applications, high strength-to-weight ratio, as well as resistance to corrosion, moisture and chemical reactions, become a priority. 

What mechanical properties make sandwich construction materials preferred in certain applications? 

Overall, sandwich construction materials have a high strength-to-weight ratio, high stiffness-to-weight ratio, low density, and are resistant to corrosion. The face plates combined with the core material provide greater flexural rigidity while the core provides adequate shear strength and greater stiffness. In addition, sandwich construction materials have a uniform energy absorption capacity and provide good thermal insulation [1]. 

What are some testing standards for sandwich construction material mechanical testing?

Sandwich construction materials are versatile materials. Thus, the determination and analysis of mechanical properties play an important role in the selection of a material for its final use. Testing standards organizations have developed testing methods tailored to sandwich construction materials and the determination of their mechanical properties. Some standard methods include:

• ASTM C273 – Shear Properties of Sandwich Core Materials
• ASTM C297 – Flatwise Tensile Strength of Sandwich Constructions
• ASTM C365 – Flatwise Compressive Properties of Sandwich Cores
• ASTM C393 – Core Shear Properties of Sandwich Constructions by Beam Flexure
• ASTM C394 – Standard Test Method for Shear Fatigue of Sandwich Core Materials
• ASTM C480 – Standard Test Method for Flexure Creep of Sandwich Constructions
• ASTM D1781- Climbing Drum Peel for Adhesives
• ASTM D7249 – Facesheet Properties of Sandwich Constructions by Long Beam Flexure
• ASTM D7250 – Sandwich Beam Flexural and Shear Stiffness

What are some of the factors that need to be taken into account when preparing sandwich panel test specimens? 

Important aspects of sandwich panel specimen preparation that contribute to data scatter are incomplete or nonuniform core bonding to facings, misalignment of core and facing elements, the existence of joints, voids or other core and facing discontinuities, out of plane curvature, facing thickness variation, and surface roughness.

What are some of the test types to measure the mechanical properties of sandwich construction materials? 

Below is a list of different test types to measure the mechanical properties of sandwich construction materials.

Tension Testing

Tension testing on sandwich construction materials provides information on the strength and quality of core-to-facing bonds and produces flatwise tensile strength data for the core material.

Compression Testing

Compression testing in the flatwise direction or in the edgewise direction is performed to determine the core strength and modulus. 

Flexural (Bend) Testing

Sandwich construction materials can be tested in flexion to calculate the flexural stiffness, core shear strength, shear modulus, compressive, and tensile strength. The standard bending test setup for sandwich constructions is in a 4-point bend, thus a 4-point bend fixture is required to load the test specimen.

Shear Strength Testing

Shear properties are fundamental to the design of sandwich panels. By running shear strength tests, researchers and developers can gather data on the force-deflection behavior of the core material used in sandwich constructions. Shear strength testing can be conducted in tension or in compression. The short beam flexure test method is another method to measure the core shear strength.

Calculations include the core shear modulus and shear strength. In addition, if shear strain is calculated, a displacement transducer can be added to the test setup to acquire strain data. 

Peel Testing

Peel testing on sandwich construction materials is a special type of test called the climbing drum peel test and is run using a climbing drum peel fixture. The peel resistance of the adhesive bond between the facing plate and the core material is measured.

Featured Configuration, Versatility in Testing 

ADMET Sales and Engineering teams have worked on configuring a testing system that is able to run multiple ASTM test methods without breaking the budget. The featured configuration shown below is the 50kN capacity eXpert 2613 electromechanical frame equipped with MTESTQuattro controller and the PC-based MTESTQuattro software.

Grips & Fixtures To Run Multiple ASTM Test Methods

The versatile eXpert 2613 testing system was supplied with multiple grips and fixtures to run the following ASTM standards: ASTM C297, ASTM D1781, ASTM D7249, ASTM D695, ASTM D3846. Test specimens included sandwich construction composites, reinforced plastics, plastic tubes, and wood.

High, Mid, Low Load Ranges With Multiple Load Cells

The force capacity of the eXpert 2613 frame is up to 50kN (11,240lbf). Not all testing required such high forces, thus, the system was offered with multiple load cells to cover the whole force range for all the testing. One load cell matched the capacity of the frame at 50kN (11,240lbf) for high-strength material testing on sandwich composites. A mid-range load cell at 10kN (2,248lbf) capacity as well as a low-force range load cell that can measure loads up to 1kN (225lbf) was supplied with the necessary mounting adapters to run tests on the same machine. All load cells were calibrated per ASTM E4 in both tension and compression directions. Calibration files were saved in MTESTQuattro software. ADMET load cells are supplied with smartducer identification chips, thus, once set up in the software, MTESTQuattro will automatically identify the load cell that is connected to the Load channel and there is no need to reconfigure a load cell.

Additional Transducers

In order to measure strain and deflection, displacement transducers at the specified displacement range and accuracy were supplied. Both a clip-on extensometer and a deflection measurement device were needed for the different ASTM test methods. These devices were supplied with the necessary mounting hardware and calibrated in the required range and in the proper direction of testing.

Conclusion

ADMET is a leading manufacturer of material testing machines. We have experience working with complex structures such as the sandwich composite and with a variety of industries including aerospace, construction, and government.

Our sales approach is a consultative process where we mutually work with customers to diagnose their needs by having open, honest, and straightforward conversations. We will work with you to find the right solution for your testing whether it is to test one type of material per one test method or multiple materials following multiple methods, we are here to help.