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Introducing ASTM D256 - Industrial Physics

ASTM D256 is an ASTM international standard test method to determine the Izod pendulum impact resistance of plastics.  

This ASTM test method reveals the energy needed to break standard test specimens of a specified size, under stipulated parameters of specimen mounting, notching, and pendulum velocity-at-impact. 

The ASTM D256 test method is essentially an impact test involving a pendulum-type hammer,  mounted in an impact testing machine. Your samples must first be prepared with a milled notch before they’re tested with one controlled pendulum swing.  

Reasons for Izod pendulum testing  

The Izod pendulum test is useful for quality control and for checking a material’s specification. If two groups of samples from the same source show significantly different energy absorptions, types of breaks, critical widths, or critical temperatures after this impact test, you can assume that they were made of different materials. Or they may have been exposed to different processing or conditions.  

What does the Izod pendulum test reveal? 

The energy lost by the pendulum during the pendulum test breakage of your specimen is the sum of the following: 

  • Energy to initiate fracture of the specimen 
  • Energy to propagate the fracture across the specimen 
  • Energy to throw the free end (or ends) of the broken specimen (“toss correction”) 
  • Energy to bend the specimen 
  • Energy to produce vibration in the pendulum arm 
  • Energy to produce vibration or horizontal movement of the machine frame or base 
  • Energy to overcome friction in the pendulum bearing and in the indicating mechanism, and to overcome windage (pendulum air drag) 
  • Energy to indent or deform plastically the specimen at the line of impact 
  • Energy to overcome the friction caused by the rubbing of the striker (or other part of the pendulum) over the face of the bent specimen. 

Types of failure shown through pendulum testing

The ASTM D256 approach lists a number of different types of sample failure. The type of failure for each specimen should be recorded as one of the following four categories: 

  • C = complete break: a break where the specimen separates into two or more pieces after Izod pendulum testing. 
  • H = hinge break: an incomplete pendulum test break, where one part of the specimen can’t support itself above the horizontal, when the other part is held vertically (less than 90° included angle). 
  • P = partial break: an incomplete pendulum test break that doesn’t meet the definition for a hinge break, but has fractured at least 90% of the distance between the vertex of the notch and the opposite side. 
  • NB = non-break: an incomplete pendulum test break where the fracture extends less than 90% of the distance between the vertex of the notch and the opposite side. 

If you’re working with tough materials, the pendulum impact test may not have the energy necessary to break any extreme fibers and toss the broken piece or pieces. Results obtained from “non-break” specimens should be considered a departure from the pendulum impact test standard and not be reported as a standard result.  

Impact test machines that make an impact!  

At Industrial Physics we’ve been making impact test devices for many decades. And we like to share the lessons we’ve learnt over the years with our customers! For example, why not check out this blog we’ve written on impact testing which includes a link to our brochure on the subject? 

ASTM D256 testing equipment

So, how can Industrial Physics improve your ASTM D256 testing? 

Across our portfolio of brands we manufacture several pendulum impact testing machines and accessories that are designed with ASTM test standards in mind.

The Ray-Ran Advanced Pendulum Impact System 

This pendulum impact tester uses microprocessor technology to determine the energy required to break or rupture plastic or ceramic specimens. You can use it for Izod pendulum testing, Charpy testing or tension impact testing.  


Ray-Ran Auto Cycle Test Sample Notching Cutter 

When you need to produce stress-concentrated notch profiles in samples to be used for impact testing to ASTM D256 standards, look no further than our industry-leading notching cutter. 

Your global test and inspection partner for ASTM D256