ASTM C393_Standard Test Method for Shear Strength

The following was adapted from ASTM - C393/C393M

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Equipment required/apparatus

Sampling & Geometry:

• Sampling:

  • Make sure to test at least five specimens per test condition unless you are confident that the results obtained are valid through the use of fewer specimens.

• Standard Specimen Configuration:

  • The specimen should be rectangular in cross-section, with a width of 75mm [3.0 in.], a length of 200mm [8.0in.] and the depth of the specimen should be equal to the thickness of the sandwich construction.

• Non-Standard Configuration:

  • When we have a non-standard configuration, the following shall be satisfied:

    • Width shall not be less than twice the total thickness nor more than six times the total thickness, not less than three times the dimension of a core cell, nor greater than one half the span length.

    • The specimen will be equal to the support span length plus 50mm [2.0 in.], or plus one half of the sandwich thickness, choose whichever one would be greater

• Geometry:

  • The standard specimen configuration should be used whenever the equations shown below will produce the desired modes being core/core-to-core facing bond failure.

  • If the equations do not indicate the desired mode, a non-standard configuration will be used.

Procedure

• The following parameters must be specified before the test.

  • The sampling method, the specimen geometry and conditioning travellers ( if required).

    • A traveller is a test specimen that measures moisture, it must also be identical to the material being tested in every way.

  • The desired properties and data reporting format.

  • The parameters for environmental conditioning.

  • The nominal thickness of the facing materials.

• General Instructions

  • If the test environment is different from a conditioned environment, condition the specimens in a chamber.

  • Before testing, you must measure the length, width and thickness at three places in the test section.

    • The length and width will be measured with an accuracy of +/- 0.254 mm [+/- 0.010 in.].

    • The thickness will be measured with an accuracy of +/- 0.25 mm [+/- 0.001 in.].

    • Measure and record the length of the support and loading spans.

    • All these measurements will be done using the tools outlined in the equipment section.

  • The speed of testing should produce failure within 3 to 6 minutes.

    • If the ultimate strength cannot be reasonably estimated, you should conduct initial trials using standard speeds until the ultimate strength of the material can be determined.

    • The standard speed is 6mm/min [ 0.25 in./min].

  • If it is possible, it is recommended to test the specimen under the same fluid exposure level used for conditioning.

    • In cases of elevated temperature testing of a moist specimen, the test environment may need to be modified.

      • An example of this is when testing at elevated temperatures with no fluid exposure controls that have a specified limit on time to failure from withdrawal from the conditioning chambers.

  • The fixture shall be installed in accordance with the loading fixture arrangement shown in the equipment section

  • Place the specimen into the test fixture and align the fixture so the longitudinal axis of the specimen is perpendicular (within 1 degree) to the longitudinal axes of the loading bars, the bars must also be parallel (within 1 degree) to the plane of the specimen facings.

  • Attach the deflection transducer to the fixture and the specimen, connect to the recording instrumentation. Zero the strain gages and balance the deflection transducer.

  • Apply a compressive force to the specimen until failure or until a deflection equal to the thickness is reached.

  • The following data shall be recorded:

    • Force versus crosshead displacement, and force versus deflection data continuously, or at frequent regular intervals(around 2-3 recordings per second, with a target minimum of 100 recorded data points).

    • If any failures are noted initially, record the various values such as force, displacement, and mode of damage at the points the failure occurred at.

    • Some initial (non-catastrophic) failures that should be reported include:

      • Facesheet delamination, core-to-core facesheet disbond, partial core fracture, and local core crushing.

         

    • Record the mode, area and location of each initial failure.

    • use the failure identification codes (from the following table) and record the method used to determine the initial failure whether it is visual, acoustic emission, etc.

    • Record the maximum force, failure force, the head displacement and the deflection at the moment of ultimate failure.

    • Finally for the ultimate failure modes, record the mode, area and location of ultimate failure for each material. Use the failure identification codes shown below. The only acceptable failure modes are shear failures of the sandwich core/failures of the core-to-facing bond. Failure of one or both of the facings preceding the failure of the core or core-to-facing bond is unacceptable.

Calculation

• The following are calculations that shall be done during the procedure:

  • Force-Displacement Behavior:

    • Plot the force-displacement data to determine if there’s any significant compliance change prior to ultimate failure which means if there is a change in the slope of the curve, this is referred to as a transition region. The following is an example the document gave of a transition region:

      • “Determine the slope of the force-displacement
        curve above and below the transition point using
        chord values over linear regions of the curve. Intersect the
        linear slopes to find the transition point. Report the force and
        displacement at such points along with the displacement values
        used to determine the chord slopes. Report the mode of any
        damage observed during the test prior to specimen failure.”

3-Point Mid-Span Loading:

4-Point, Quarter-Point loading:

4-Point (Third Point) Loading: