Laminate Bearing Strength Test Fixtures (ASTM D5961, procedures A, C, D)

Model No. WTF-LB-D5961 (A, C, or D) (Stainless Steel)

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Fig. 1: ASTM D5961 Fixture Configurations A (upper center), B (right), C (left), D (lower center).

ASTM Standard D5961 (Reference 1) includes four distinct laminate bearing test methods, identified as Procedures A, B, C, and D and shown in Fig. 1.  Procedure B is actually a specimen support fixture and is described on its own page of this website.  The other three procedures are pin bearing fixtures, as will be described here.

PROCEDURE A:  DOUBLE-SHEAR TENSILE LOADING

Procedure A in ASTM D5961 is intended for testing composite laminates in double shear. That is, the pin in the fixture attempts to shear out a segment of the composite laminate test specimen by creating two shear planes in the pin.  The standard test specimen is 5.5 in. long and 1.5 in. wide, with a specified thickness in the range of 0.125 in. to 0.208 in. A 1/4 in. diameter pin or fastener is used.

This simple test fixture is shown in Fig. 2.  The two halves of the fixture are separated by a spacer (not provided) of the same thickness as the specimen being tested. Obviously, a piece of the same test material can be used as a spacer, if desired. The test specimen is pinned in the other end of the fixture using the 1/4 in. diameter hardened steel pin provided, or the users lightly torqued (20-30 in-lb) fastener, if desired.  Since a raised boss is present at the pin hole, the opposite end of the fixture is thicker by an equal amount so that the specimen is uniformly supported when gripped.

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Fig. 2: Standard ASTM D5961 Procedure A Fixture, shown assembled on left and disassembled on right.

The end of the fixture with the spacer in place is gripped in a standard tensile wedge grip. Each half of the fixture is 0.2 in. thick; thus this grip must have an opening range to accommodate 0.4 in. plus the spacer (specimen) thickness. The free end of the specimen is held in a second tensile wedge grip.

A tensile load is applied until a maximum loading has been reached, monitoring both applied force and hole deformation, as described in the ASTM standard.

Sometimes the customer has a requirement to test non-standard specimen sizes, requiring Wyoming Test Fixtures, Inc. to custom-design and fabricate a special test fixture.  An example is shown in Fig. 3.

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Fig. 3: Standard ASTM D5961 Procedure A Fixture (left) and special large-scale fixture with 1/2 in. diameter pin (right).

An alternate version of the test fixture shown in Figs. 1 and 2 that also satisfies all of the requirements of ASTM D5961 is the so-called SACMA test fixture (see Section H-2 of this catalog, and Reference 2).  As described in Section H-2, the SACMA fixture is much easier to use since the specimen holder is one piece, with adjustable bushings to accommodate variations in specimen thickness.

PROCEDURE C:  SINGLE-SHEAR, TENSILE LOADING

Procedure C of ASTM D5961 loads the laminate bearing specimen in single shear. That is, a single shear plane is developed in the loading pin as the tensile load is applied.  The test fixture is shown in Fig. 4.

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Fig. 4: Standard ASTM D5961 Procedure C Fixture

The specimen is placed on the right surface of the test fixture, aligning the hole in the end of the specimen with the hole shown in the fixture and a fastener inserted and lightly torqued.  Or a pin can be used.  The flat tang at the top of the test fixture is gripped in a wedge grip and a tensile force is applied to the lower end of the specimen.  The maximum force attained and the deformation of the hole in the specimen are recorded.

PROCEDURE D: DOUBLE SHEAR, COMPRESSION LOADING

 
Procedure D of ASTM D5961 loads the laminate bearing specimen in double shear through a close-tolerance, lightly torqued fastener (or pin).  The fixture is shown in Fig. 5.

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Fig. 5: Laminate Bearing Test Fixture Procedure D (ASTM D5961)

The two lower components of the fixture are spread apart to permit inserting the 1.50 in. wide laminated composite specimen, aligning the 1/4 in. diameter hole near the end of the specimen with.the holes in the fixture.  A pin or fastener is inserted in the hole, the fastener being torqued as desired.  The upper end of the fixture is held in a grip, as is the lower end of the test specimen.  The specimen is kept short enough to avoid buckling.  A 5.5 in. long specimen is specified in ASTM D5961.  The applied compressive loading induces a double shear in both the specimen and the loading fastener (or pin).  Load at failure and hole deformation are measured.

Source of Additional Information:

1) ASTM Standard D5961/D5961M-13 (2013), "Bearing Response of Polymer Matrix Composite Laminates," American Society for Testing and Materials, West Conshohocken, Pennsylvania (first issued in 1996).

2) SACMA Recommended Method SRM 9-89, "Bearing Strength Properties of Oriented Fiber-Resin Composites," Suppliers of Advanced Composite Materials Association, Arlington, Virginia, 1989 (now discontinued).