What is the Charpy Impact Test and Why is it Important?

Steel is one of the most versatile building materials and it is essential for engineers and designers to understand material toughness tests like the Charpy impact test and its specification requirements.

You determined that steel is the best building material for your next design and construction project. How do you ascertain the toughness of the steel required to meet the structure’s longevity and performance?

Depending on the project and the environment the structure will be exposed to over time, toughness requirements will vary. Toughness is a material's ability to absorb energy without fracturing. For example, an engineer designing an office building may not need to consider a stringent requirement for the toughness of the steel used for the building’s interior. Conversely, toughness is an essential factor to consider when designing a stadium with steel members exposed to low exterior temperatures. Although the Charpy impact test is a globally used method for measuring the toughness of steel material, there are some misconceptions in the industry about when, where and how the Charpy impact test should be applied.

Tests like the Charpy impact test are essential for these types of structures. Although it is a globally used method for measuring the toughness of steel material, there are some misconceptions in the industry about when, where and how the Charpy impact test should be applied.

CHARPY IMPACT TEST PROCEDURE

Charpy impact testing procedures are detailed in the ASTM A673/A673M-2017 specification, “Standard Specification for Sampling Procedure for Impact Testing of Structural Steel.” Below is a general synopsis of the testing procedure. For additional test information, please reference ASTM A673/A673M.

Understanding CVN Impact Test Specifications

There are several key factors structural engineers should consider when specifying CVN requirements for their project. Different steel products — wide flange, bar, plate and sheet, for example — have different Charpy impact test requirements. Additionally, the ability to meet a required CVN test will vary depending on the thickness and production methods for each steel product.

ASTM A673/A673M requires the impact testing frequency, temperature, minimum average absorbed energy value and testing location (for wide-flange shapes, flange vs. core) to be included on the material purchase order and therefore must be specified on the contract documents. This article discusses impact testing requirements that are standard in the industry today. If a project warrants more stringent requirements, it is in the project’s best interest to consult the steel product producer, such as Nucor, to evaluate the feasibility of obtaining the desired toughness for the members

Charpy Impact Test Temperature and Minimum Average Absorbed Energy
When determining the temperature and energy requirements for the Charpy impact test, it is important to understand the environment each member will be subjected to. ASTM A709-2021/A709M-2021 Table 10, “Relationship Between Impact Testing Temperature Zones and Minimum Service Temperature” defines the temperature zones typically used when specifying the CVN toughness testing temperature. Below are the three zones based on the minimum temperature the structural members will see while in service:

Care should be taken to choose a temperature zone not for the environment of the geographic location the structure will be built in, but for the environment of the actual steel member after construction is complete. For example, if you are designing a steel-framed stadium in northern Alaska, you would likely need multiple Charpy impact tests requirements based on the temperatures the different steel members would be exposed to over the service life of the structure. The interior steel support beams within a conditioned space may fall within the Zone 1 temperature range, but the exterior beams making up the outer shell of the stadium that are exposed directly to the elements may fall within the Zone 3 temperature range requirements.

Once these determinations are made, ASTM A709/A709M-2021 may be used to determine the energy absorption requirements. Although it is primarily intended for bridge applications, ASTM A709/A709M Table 11 “Non-Fracture Critical Tension Component Impact Test Requirements” or Table 12 “Fracture Critical Tension Component Impact Test Requirements” may be used to set the Charpy impact testing requirements for that member. (For commonly specified wide-flange sections, ASTM A992 would be equivalent to Grade 50ST and 50SF and ASTM A913 would be equivalent to Grade QST 50T and QST 50F or QST 65T QST 65F in Tables 11 and 12.)

Test Location For wide-flange shapes, specimens for impact testing may be taken from a flange or core location.

Flange Test: The specimen to be tested will be taken from a location at the end of the member at a spot one-third the distance from the outer edge of the flange to the web of the shape (see Figure 3).

Core Test: Heavy rolled shapes may have a different grain structure at the intersection between the flange and the web than the balance of the section. This may result in a lower notch toughness in these areas. Therefore, per AISC 360-2022 Specification, Section A3.1d, for hot-rolled shapes with a flange thickness exceeding 2” and subject to tensile forces, an alternate core location test is required and must be specified on the contract documents and requested in the material purchase order. This Core Location test specimen will be taken from a location midway between the inner flange surface and the center of the flange thickness at the intersection with the web mid-thickness (see Figure 4).

For the Core Location test, it is recommended the minimum average absorbed energy shall be 20 ft-lb and the test temperature shall be 70° F, as noted in Supplement 30 from ASTM A6-2023. Should the requirements for absorbed energy greater than 20 ft-lb or a temperature lower than 70° F be needed for the Core Location test or for any CVN test questions, it is best to speak with a material manufacturer like Nucor during design or early in the procurement process prior to specifying. This will allow steel purchasers to carefully assess if the energy, temperature and toughness requirements being specified are achievable before a steel order is placed. Typically, the core test carries additional testing fees and may necessitate alternative designs of welded plate girders in lieu of structural steel rolled shapes if the required toughness is not achievable.

Your Partner in Construction Solutions

Nucor’s team of experts are here to answer your questions and help you understand how Charpy impact tests impact the type of materials you could specify for a project. Let’s work together early in the design process to evaluate your project’s material specifications, strengths, grades and temperature range requirements to provide the ideal steel solutions for your complex structure.

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