ASTM D790

Introducing ASTM D790 – Industrial Physics

ASTM D790 is an ASTM industry standard that’s used to test the flexural strength properties of unreinforced and reinforced plastics, high modulus composites and electrical insulating materials. 

The American Society for Testing Materials (ASTM) is one of the most influential standard bodies in existence. Established in 1898, ASTM international exists to develop standardized test methods, specifications, practices, guides, classification, and terminology across a range of subject areas. 

The ASTM D790 flexural strength testing methods recommend a three-point loading system to apply a load to a simply supported beam sample. The method is generally applicable to both rigid and semi-rigid materials, but flexural strength can’t be determined for materials that don’t break or yield in the outer surface of the test specimen within a five percent strain limit. 

ASTM D790 flexural strength test specimens and process

The ASTM D790 test methods recommend that test specimens of a rectangular cross section are injection molded or cut from molded or extruded sheets or plates. They can also be cut from molded or extruded shapes.  

The specimens must be solid and uniformly rectangular. Once you have the right sample it rests on two supports which are loaded by means of a loading nose midway between the supports. 

You can measure deflection by using the crosshead position or through the use of a deflectometer. Deflection data obtained with a deflectometer will differ from data obtained using crosshead position and so the method of deflection measurement should be reported. 

ASTM D790 flexural testing properties

The flexural properties you measure with the ASTM D790 flexural strength testing method are especially useful for quality control and specification purposes and they include: 

• Flexural Stress (σf): when a homogeneous elastic material is tested in flexure as a simple beam supported at two points and loaded at the midpoint, the maximum stress in the outer surface of the test specimen occurs at the midpoint.  

• Flexural stress for beams tested at large support spans (σf): if support span-to-depth ratios greater than 16 to 1 are used – such that deflections in excess of 10 % of the support span occur – the stress in the outer surface of the specimen for a simple beam is reasonably approximated. 

• Flexural strength (σfM): maximum flexural stress sustained by the test specimen during a bending test.  

• Flexural offset yield strength: offset yield strength is the stress at which the stress-strain curve deviates by a given strain (offset) from the tangent to the initial straight-line portion of the stress-strain curve.  

• Flexural stress at break (σfB): flexural stress at break of the test specimen during a bending test.  

• Stress at a given strain: the stress in the outer surface of a test specimen at a given strain is calculated.  

• Flexural strain, ɛf: nominal fractional change in the length of an element of the outer surface of the test specimen at midspan, where the maximum strain occurs.  

• Tangent modulus of elasticity: the tangent modulus of elasticity, often called the “modulus of elasticity,” is the ratio, within the elastic limit, of stress to corresponding strain.  

• Secant modulus: the secant modulus is the ratio of stress to corresponding strain at any selected point on the stress-strain curve. 

• Chord modulus (Ef): the chord modulus is calculated from two discrete points on the load deflection curve.  

Our ASTM D790 flexural strength testing equipment

So, how can Industrial Physics improve your ASTM D790 flexural strength testing?  

United Testing 3-Point Bend Fixture 

Through our brand United Testing Systems we offer a wide selection of bend and flex fixtures for many different applications across a wide variety of industries. Common fixtures are generally three-point or four-point, but others are available that can accommodate a wide range of capacities. 

Industrial Physics is fantastic with plastics!

We’ve learnt a thing or two about plastic testing over the decades. Now Industrial Physics is recognized as a brand that likes to bring new, innovative flexural strength testing devices to market. 

We like to share our experience and lessons learnt across the knowledgebase section of our website. For example, there’s this blog that investigates thickness testing for plastic packaging films. Or you could check this one out, it’s on plastic density testing.  

If you have any questions about ASTM D790 testing or need help to analyze any property of any plastic, high modulus composite or electrical insulating material you’re examining, just get in touch!

Your global test and measurement partner for ASTM D790