Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials

Theory:

The flexural properties of plastics are measured by most resin suppliers and some of the larger compounders. The three point bend test is used to determine the flexural strength and modulus of plastic material. The specimen is placed across two  bottom support spans and the third point contacts the sample in the middle from the top. 

Geometry:

There are two types of samples that may be used, either a bar which is longer than it is wide, or a square sheet. Bars are the most common and tend to be 1/8" thick. The distance between the two bottom supports is calculated using a ratio which can be found in the official ASTM specification. The ratio on length to width can also be found there. The key variable is the cross sectional area of the sample.

Solution:

The 3 point bend test is rather simple to set up. The plastic sample is placed across the two bottom spans which are adjustable according to the appropriate ratio. The two bottom span contact points can have rollers in order to better appropriate the normal force, however contact points with a 5mm radius, as pictured, are the norm. The 3rd loading point comes down on the UTM arm and bends the sample to the required amount of flexure.  

 

A deflectometer may be used if the position information from the crosshead of the Universal Testing Machine is not yielding results with enough precision. Deflectometers are contact measurement devices and are able to precisely measure exactly what is happening to the specimen during the test procedure.

 

Equipment List:

Benchtop Universal testing Machine

3 pt. Bend Fixture

Low force load Cell (50lbs.)

Graphworks 6 software

 

Analysis:

The test is run to 5% strain, of if the sample is brittle, until it breaks. Flexural Strength or Stress is calculated using the method laid out in the ASTM. The units are represented in MPa (1M newtons per square meter) or in PSI (lbs. per square inch). 

 

Flexural Modulus is a slightly different calculation but just involves dividing the stress delta by the strain delta from the beginning of the test. Units for flexural modulus are usually much higher and tend to reproted in KSI which is an often confused unit of measurment. KSI stands for 1,000 lbs. per square inch.

ASTM D790 Bend Test Fixture

ASTM D790 - Bend Fixture