The ASTM D6641/D6641M test standard specifies the characterization of compressive strength and stiffness properties of polymer matrix composite materials. The test is performed by applying a combined end-load and shear-load to a standard specimen by means of a combined loading compression (CLC) test fixture. Suitable specimen materials include unidirectional composites (0° ply), multi-directional composite laminates, fabric composites and chopped fiber composites. Common properties that reference ASTM D6641/D6641M include the compressive modulus, compressive strength and strain at yield or failure (if observed), engineering compressive stress-strain curve, true compressive stress-strain curve (calculated), and offset yield stress and strain in compression.

- ASTM D695
- ASTM D3410

DatapointLabs offers a number of tests that address different properties covered under the ASTM D6641/D6641M standard. These include the following characterization measurements [bracketed links refer to relevant tests in the DatapointLabs Test Catalog]:

- Nadcap: compressive ultimate strength at failure and reported failure type, as per failure codes in standard; meets Nadcap accreditation and reporting requirements [M-209C_strength]

- Non-Nadcap: compressive modulus, engineering compressive stress-strain curve, true compressive stress-strain curve (calculated), compressive strength and strain at yield or failure (if observed), offset yield stress and strain in compression [M-209]

- Nadcap: compressive modulus, engineering compressive stress-strain curve, true compressive stress-strain curve (calculated), compressive strength and strain at yield or failure (if observed), offset yield stress and strain in compression; meets Nadcap accreditation and reporting requirements [M-209C]

The specific test procedure will vary by test and the individual test webpages (linked to above and below) should be consulted for relevant descriptions. In general, a universal testing machine (UTM) is used to determine the appropriate compressive property or properties under measure. The test specimen is placed in the two halves of a combined loading compression (CLC) fixture with its ends flush to the top and bottom of the fixture. The specimen is then clamped in the fixture to a specified torque sufficient to prevent either slippage or crushing. The fixture maintains perfect axial alignment and prevents compressive buckling of the specimen under test. The specimen and fixture are next placed between the platens of the UTM and jointly compressed at a constant cross-head speed to the point of yield or failure. In addition to direct compressive loading of the specimen at its ends, shear loading is also induced in the specimen from the compressive loading of the CLC fixture. The combined loading enables a greater total loading than that achievable from compressive loading only. For CLC tests that characterize strain, the strain is measured using non-contact extensometry.

Sample conditioning, if required, is carried out following specimen preparation in accordance with ASTM D618. Typical conditioning selections are ‘dry as molded’ (low humidity storage at 23°C) and 40 hours at 23°C and 50% relative humidity. Non-ambient testing may be carried out at any temperature between -53°C and 200°C.

Specimen specifications under ASTM D6641/D6641M are as follows:

- Compression bar specimens as specified by ASTM D6641/D6641M; contact DatapointLabs for further details regarding specimen geometry and preparation

Typical characterization measurements under ASTM D6641/D6641M include:

- Compressive Modulus: The ratio of compressive stress (load) to the corresponding strain (compression), measured below the elastic limit of a material

- Compressive Strength at Yield: The maximum compressive stress at the point where a material transitions from elastic to plastic behavior (Yield Point)

- Compressive Strain at Yield: The maximum strain at the point where a material transitions from elastic to plastic behavior (Yield Point)

- Compressive Ultimate Strength: The maximum compressive stress at the point of material failure

- Engineering Compressive Stress-Strain Curve: The graphical relationship between stress and strain in a material under compressive load, based on the original cross-section of the test specimen

- True Compressive Stress-Strain Curve: The graphical relationship between stress and strain in a material under compressive load, calculated from the engineering compressive stress-strain curve

- Offset Yield Stress in Compression: The offset yield method determines the yield point by the intercept of the stress-strain curve and a line with slope corresponding to the elastic modulus and a strain offset (nominally 0.2%); the offset yield stress is the stress at the determined intercept

- Offset Yield Strain in Compression: The offset yield method determines the yield point by the intercept of the stress-strain curve and a line with slope corresponding to the elastic modulus and a strain offset (nominally 0.2%); the offset yield strain is the strain at the determined intercept

The entire list of tests in the DatapointLabs test catalog that reference ASTM D6641/D6641M are as follows:

Standard | Description |
---|---|

M-209C_strength | Compressive Strength using Combined Loading Compression (CLC) for composite materials |

M-209 | Combined Loading Compression (CLC) |

M-209C | Combined Loading Compression (CLC) for composite materials |