5.1 Susceptibility to delamination is one of the major design concerns for many advanced laminated composite structures. Complex structural geometries can result in out-of-plane stresses, which may be difficult to analyze. When curved structural details are loaded such that the deformation results in an increase in the radius of curvature, interlaminar tensile stress and delaminations can result. Knowledge of a laminated composite material’s resistance to interlaminar fracture is useful for product development and material selection. Failure criteria and design allowables involving out-of-plane stresses may not be readily available or may be poorly validated, requiring additional experimental data.

5.2 This test method can serve the following purposes:

5.2.1 To measure a curved-beam strength;

5.2.2 To measure an interlaminar strength when using a unidirectional specimen where all fibers are oriented 0° relative to the long straight edges of the specimen;

5.2.3 To establish quantitatively the effect of fiber surface treatment, local variations in fiber volume fraction, and processing and environmental variables on the curved beam strength or the interlaminar (through-the-thickness) tensile strength of a particular composite material;

5.2.4 To compare quantitatively the relative curved-beam strength or interlaminar tensile strengths of composite materials with different constituents;

5.2.5 To compare quantitatively the values of the curved-beam strength or interlaminar tensile strengths obtained from different batches of a specific composite material, for example, to use as a material screening criterion, to use for quality assurance, or to develop a design allowable;

5.2.6 To produce out-of-plane structural failure data for structural design and analysis; and

5.2.7 To develop failure criteria for predicting failures caused by out-of-plane stresses.

1.1 This test method determines the curved beam strength of a continuous fiber-reinforced composite material using a 90° curved beam specimen (Figs. 1 and 2). The curved beam consists of two straight legs connected by a 90° bend with a 6.4 mm [0.25 in.] inner radius. An out-of-plane (through-the-thickness) tensile stress is produced in the curved region of the specimen when force is applied. This test method is limited to use with composites consisting of layers of fabric or layers of unidirectional fibers.

FIG. 1 Test Specimen Geometry (SI units)

FIG. 2 Test Specimen Geometry (inch-pound)

1.2 This test method may also be used to measure the interlaminar tensile strength if a unidirectional specimen is used where the fibers run continuously along the legs and around the bend.

1.3 This test method is limited to use with composites consisting of layers of fabric or layers of unidirectional fibers.

1.4 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.

1.4.1 Within the text, the inch-pound units are shown in brackets.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.