ASTM D6272 PDF

The major difference between four point and three point bending modes is the location of the maximum bending moment and maximum axial fiber stress. In four point bending the maximum axial fiber stress is uniformly distributed between the loading noses. In three point bending the maximum axial fiber stress is located immediately under the loading nose. Any test specimen preparation, conditioning, dimensions, or testing parameters covered in the material specification, or both, shall take precedence over those mentioned in this test method. If there are no material specifications, then these default conditions apply.

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There are two test procedures, A and B, and the usage of them depends on the flexibility of the specimen material. The bottom span is set by a ratio, while the top span must be aligned so that the two contact points are equidistant from the center line.

Test method A is used for materials that are brittle and will snap at relatively weak deflection forces. This type of test is designed to be very slow. Test method B is more suitable for materials that are flexible. Carbon fiber and reinforced plastics may exhibit characteristics that allow them to be highly flexible and some will never fail through a crack propagation event - they will just fold.

The specimens are rectangular strips that are cut from larger pieces of sheet material. The strips should overhang the bottom spans slightly, and also be thinner in width than the width of the contact points. Specimen materials for test method A will snap without much deflection.

Therefore, a span to deflection depth ratio of can be used. For test method B, where the samples are much more flexible, a larger span ratio must be used. In these cases, span ratios can increase to or even higher. Solution: A 4 point bend fixture with adjustable bottom and top spans is the easiest to use solution. This allows for easy experimentation which speeds up testing, especially amongst different types of plastic. It may be wise to also use a deflectometer. Deflectometers are contact sensors which accurately measure the deflection strain during testing.

The deflectometers have a high Hz sample rate which deliver the data for acquisition. Some of the more advanced material analysis requires the use of sophisticated instrumentation such as the contact deflectometer. Analysis: Flexural strength is a factor of the maximum force the sample can withstand before snapping. Since the material must break, flexural strength can only be properly calculated in test method A.

Analysis for procedure B is more involved. Flexural Stress is calculated from a rectangular cross sectional area. The modulus of elasticity is also referred to as the flexural modulus. This calculation is a function of the flexural strength and is tabulated according to the specification.

Other derived equations may be calculated including offsets, tangents and secants. Modulus of Rupture is similar term that is used in bend test analysis and it commonly refers to the flexural strength. ASTM D

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ASTM D6272

Significance and Use 5. The major difference between four point and three point bending modes is the location of the maximum bending moment and maximum axial fiber stress. In four point bending the maximum axial fiber stress is uniformly distributed between the loading noses. In three point bending the maximum axial fiber stress is located immediately under the loading nose. Any test specimen preparation, conditioning, dimensions, or testing parameters covered in the material specification, or both, shall take precedence over those mentioned in this test method. If there are no material specifications, then these default conditions apply. Scope 1.

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