Impact Testing

Written By Cassandra Painter

Refer to ASME SA-370 Section 19 through 28 for more details

Impact testing is a crucial aspect of ensuring the safety and integrity of pressure vessels, piping systems, and other pressure components in industrial settings, where designs need to have good toughness characteristics.

Impact Testing is a destructive test that can be performed on base metal, weld metal or both. And is often used to confirm that welding parameters used to qualify welding procedures yield acceptable toughness properties.

Significance of Impact Testing

Ferritic alloys exhibit a significant transition in behavior between ductile or brittle fracture depending on their temperature.

The specific temperature where toughness characteristics change is known as the transition temperature.

Above the transition temperature the impact specimens fracture by a ductile mechanism, absorbing relatively large amounts of energy.

Below the transition temperature the impact specimens fracture by a brittle manner absorbing less energy.

Within the transition range, the fracture will generally be a mixture of areas of ductile fracture and brittle fracture.

The temperature range of the transition from one type of behavior to the other varies according to the material being tested. This transition behavior may be defined in various ways for specification purposes.

Application

Certain design conditions and welding codes require impact testing due to the in-service application.

Materials that fall under specific temperature ranges and thicknesses are subject to impact testing, reference the applicable requirements for your specific application.

Common Testing Temperatures

The temperature range for impact testing depends on the material and its intended service conditions. Some common temperatures include the following:

  • Carbon Steels (CS) -20°0 F

  • Low Temperature Carbon Steels (LTCS) -50° F

  • Stainless Steels (SS) -320° F (196° C) occasionally may require -425° F

  • Nickel (Ni) -320° F occasionally may require -425° F

  • Aluminum (AL) -320° F occasionally may require -452° F

Testing Media

For testing at temperatures less than room temperature the following media is commonly used to condition the specimens.

  • Ice Water

  • Dry Ice + Solvents

  • Liquid Nitrogen

    • Most commonly used due to commercial availability

    • Capable of -320° F (196° C) and warmer depending on exposure time of sample

  • Liquid Helium

    • Very rare and costly

    • Capable of -452° F (269° C or 4.2 Kelvin) and warmer depending on exposure time of sample

Charpy Impact Test

A Charpy V-notch impact test is a dynamic test in which a notched specimen is struck by a weighted pendulum which is dropped from a specified height to strike the specimen.

Measurement of energy absorbed, the percentage shear fracture, the lateral expansion opposite the notch, or a combination thereof can be made from this test.

Sample Dimensions: The Charpy test specimen is typically a 10 x 10 mm bar with a notch machined into one face. The notch can be either V-shaped or U-shaped.

Energy Absorption Measurement: The energy transferred to the material is inferred by comparing the pendulum’s height before and after fracture.

Temperature Range: Testing can be performed at both ambient and reduced temperatures, including sub-ambient conditions.

Standards: Common standards include ASTM E23, ASTM A370, and ISO 148. There are also specific standards for plastics and polymers.

Test Results

  • Impact Energy: which is recorded as a value from the impact testing apparatus and reported in foot pounds (ft. lbs.) or Joules (J)

  • Fracture Appearance: the area of the brittle fracture face is determined and reported in percent shear area by referring to Table 7 or 8 of SA-370

  • Lateral Expansion: is the increase in specimen width, measured in thousandths of an inch (mils), on the compression side, opposite the notch of the fractured Charpy V-notch specimen as shown in Fig. 15 of SA-370.

Acceptance Criteria

  • The minimum acceptable impact energy or lateral expansion varies based on material type, thickness, temperature, service conditions, contract specifications, or welding codes.

  • Refer to the requirements of your application it may be specified in the engineering documents, material specifications or welding codes to determine the minimum acceptable values.

Summary

Impact testing measures the toughness of a material at a given temperature, and helps determine if the base metal or weld metal is suitable to safely operate per the design requirements.

Cassandra Painter