4 Pt. Bend Fixture
ASTM D6272 - Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending
A four point bend fixture is used to test ASTM D6272. 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. Both Flexural Strength (because we have a sample break) and the Flexural Modulus can be calculated from Procedure A.
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.
Sample geometry for ASTM D6272 is described in full detail in the associated official publication. 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 16:1 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 1:32 or even higher.
A 4 point bend fixture with adjustable bottom and top spans is the easiest to use solution. UGC's universal bending jigs use a centered gear crank shaft to keep the spans at equidistant points. 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.
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. Generally, the test is stopped at 5% strain unless there is a known breaking stress. 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.