ASTM D 6671, Standard Test Method for Mixed Mode I - Mode II Interlaminar Fracture Toughness of Uni-Directional Fiber Reinforced Polymer Matrix Composites, was approved May 10 to enable more precise failure predictions. It was developed by members of ASTM Subcommittee D30.06 on Interlaminar Properties, part of Committee D30 on Composite Materials.

Subcommittee chairman James R. Reeder, Ph.D., NASA Langley Research Center, Hampton, Va., described its merit. “The test method measures a material property for which there was no standard test previously,” he said. “This test method was standardized rather than other possible tests because it offered several advantages. It can measure toughness over a large range of mixed-mode ratios; data from the test can be processed using simple closed-form equations (including the division of the Mode I and Mode II toughness components); and the preselected mixed-mode ratio stays essentially constant as the delamination grows.”

After Reeder and John H. Crews, Jr., Ph.D., developed the test method at NASA Langley, Reeder “conducted a round robin test activity that refined the test protocol which was eventually balloted for standardization” by ASTM.

Reeder explained why D 6671 was developed as well as the improvements it offers. “Delamination is a primary failure criterion of composite materials,” he began. “Delamination resistance is generally characterized by the fracture toughness parameter. In the past, the only standard for composite delamination growth was in Mode I where the delamination grows by pulling the layers apart. Delaminations in real structures would generally also be subjected to shear loading, and the fracture toughness was known to change dramatically with the addition of shear.

“This new test method allows for the fracture toughness of composite material to be measured with various amounts of opening and shear,” he noted. “The new material data is leading to much more accurate failure predictions.

“As in the standard Mode I test, the mixed-mode delamination test uses a unidirectional composite test specimen,” Reeder continued. “The results from the test are used to predict delamination growth in general laminates that are not unidirectional. Composite laminates are used in many different industries because of its high strength-to-weight and stiffness-to-weight ratios.”

D 6671 is available this month.

Technical comments may be directed to James R. Reeder, Ph.D., NASA Langley Research Center, Hampton, Va. (phone: 757/864-3456). Committee D30 meets in Santa Barbara, Calif., Oct. 16-18 in conjunction with MIL Handbook 17, and March 11-14 in Pittsburgh, Pa. For membership or meeting details, contact James P. Olshefsky, manager, ASTM Technical Committee Operations (phone: 610/832-9714). //

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