Evaluating Stress, Strain, and Failure-Mode in Bending Test

With a simple sample geometry, a 3-point-bend test can be used to obtain stress-strain behavior, elastic modulus in bending (flexural modulus), and failure limits in bending. The standards ASTM E855, ASTM D790 or ISO 178, and ASTM C1684 or C1161 cover the properties of flat metallic spring materials, flexural properties of plastics, and 3-point bend testing for ceramics, respectively.

While carrying out a 3-point-bend test, the convex side of the plate or sheet is subjected to tension, with the outer fibers under maximum strain and stress (Figure 1). Failure occurs if the elongation or strain surpasses the limits of a material.

Figure 1. 3-point-bend test setup on UMT.

A 3-point flexural test can be used to determine the fracture toughness of a material. The test uses a pre-notched sample and can follow the ASTM E-1290 or ISO12135 procedures. The ASTM D7774 standard can be used to obtain fatigue properties. For medical applications, the ASTM F2606 standard can be utilized to conduct a 3-point-bend test on balloon-expandable stent systems and vascular stents.

To evaluate the different properties of sheet and laminate materials, an easy-to-use 3-point-bend module is integrated into Bruker’s UMT. These tests can be run under displacement-control or load-control modes.

Use of Acoustic Emission in 3-Point Bend Tests

When testing composite materials, failure can be caused due to the delamination between the matrix and the fibers, before complete specimen failure occurs. This phenomenon can be observed not only from the stress-strain curve data, but can also be detected with an acoustic emission (AE) sensor with a higher sensitivity. As illustrated in Figure 2, we can define the failure criteria as the first indication of the onset of significant AE signal, fiber delamination or breakage, or the increase to a particular predefined level of AE.

Figure 2. 3-point-bend data showing internal failure, identified by Acoustic Emission signal, prior to sample fracture.

Different composite research areas such as wetting chemistry of the matrix, surface activation of the fibers, and fiber surface roughness characteristics can be assessed by using the optional AE sensor.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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