Steel Fracture Toughness — Charpy V-Notch Testing, CVN Requirements & Brittle Fracture Prevention

Fracture toughness measures a material's resistance to crack propagation. In structural steel, low fracture toughness at service temperatures can cause sudden brittle fracture — catastrophic failure without warning or yielding. The Charpy V-Notch (CVN) impact test is the primary quality metric. AISC 341-22 mandates CVN testing for seismic connections, heavy shapes, and demand-critical welds because these locations experience high strain rates, constraint, and triaxial stress states that promote brittle behavior.

The Charpy V-Notch test

A standard CVN specimen (10 × 10 × 55 mm bar with a 2 mm deep 45-degree V-notch) is struck by a pendulum at a specified temperature. The energy absorbed before fracture, measured in foot-pounds (ft-lb) or Joules, indicates the material's toughness at that temperature.

Ductile-to-brittle transition temperature (DBTT): Steel transitions from ductile (high energy absorption, shear-type fracture surface) to brittle (low energy, cleavage fracture) over a temperature range. The goal is to ensure the DBTT falls well below the lowest anticipated service temperature.

AISC 341-22 CVN requirements

AISC 341-22 Section A3.3 specifies CVN testing requirements for seismic applications:

Heavy shapes (AISC 341-22 Section A3.3): Shapes with flange thickness tf ≥ 1.5 in used in seismic force-resisting systems must demonstrate CVN toughness of 20 ft-lb at 70 degrees F. Testing is performed at the core of the flange (the slowest-cooling region with the worst toughness) per ASTM A6 Supplementary Requirement S30.

Demand-critical welds (AISC 341-22 Section A3.4): Weld filler metal must achieve 20 ft-lb at -20 degrees F per AWS D1.8 Annex A. This is far more stringent than the heavy shape requirement because weld metal cools rapidly and develops more constrained microstructures.

Beam-to-column connections in SMF/IMF: Both base metal and weld metal must meet the CVN requirements. The column flange material in the k-area (where rolling-induced residual stresses are highest) is particularly vulnerable.

Factors affecting fracture toughness

Worked example — specifying CVN for a seismic column

Given: W14x398 column (tf = 2.845 in) in an SMF, SDC D, minimum service temperature = 0 degrees F.

Step 1 — Check heavy shape threshold: tf = 2.845 in > 1.5 in — heavy shape provisions apply per AISC 341-22 Section A3.3.

Step 2 — Required CVN for base metal: 20 ft-lb at 70 degrees F, tested at the core of the flange per ASTM A6 S30. Specify this in the structural steel material specification on the drawings.

Step 3 — Weld filler metal: Demand-critical CJP groove welds at beam-to-column connections require filler metal with CVN = 20 ft-lb at -20 degrees F per AWS D1.8.

Step 4 — Material selection: A913 Gr 65 is preferred over A992 for heavy W14 columns in seismic applications. A913 is produced by quenching and self-tempering (QST process), which provides superior CVN toughness in thick sections. A992 produced by traditional hot-rolling may have marginal toughness at the flange core of shapes with tf > 2 in.

Specification note on drawings: "W14x398: ASTM A913 Gr 65, Supplementary Requirement S30 (CVN 20 ft-lb at 70F at flange core). CJP welds at moment connections: AWS D1.8, demand-critical filler metal E71T-8Ni1 (CVN 20 ft-lb at -20F)."

KIc correlation from CVN

For fitness-for-service assessments (e.g., evaluating existing structures with discovered cracks), the plane-strain fracture toughness KIc can be estimated from CVN data using the Barsom-Rolfe correlation:

KIc² / E = 5 × CVN    (upper-shelf, in ksi²·in / ksi = ksi·in units)

For the transition region (lower shelf):

KId = 15.5 × (CVN)^0.5    (dynamic fracture toughness, ksi·sqrt(in))

These correlations are approximate and should be used for screening only. Actual KIc testing per ASTM E399 is required for critical fracture mechanics assessments.

Code comparison

AISC 341-22 (USA): Requires CVN testing per Section A3.3 for heavy shapes (tf ≥ 1.5 in) and demand-critical welds. Test temperatures: 70 degrees F for base metal, -20 degrees F for weld metal. AWS D1.8 governs welding procedure qualification for seismic connections.

AS 4100-2020 Section 2.5 (Australia): Specifies minimum impact properties by reference to AS/NZS 3678 and AS/NZS 3679.1. Steel grade designations include toughness subgrades (e.g., Grade 300L15 requires 40 J at -15 degrees C). The designer selects the appropriate subgrade based on the minimum design service temperature from AS 4100 Table 2.5.1. For thicknesses above 40 mm, L0 or L15 subgrades are typically required.

EN 1993-1-10 (Eurocode 3): Uses a maximum permissible thickness approach based on steel toughness subgrade (JR, J0, J2, K2), reference temperature TEd, and stress level. Table 2.1 of EN 1993-1-10 cross-references charpy temperature, member thickness, and reference temperature to determine whether a given subgrade is acceptable. The reference temperature TEd = Tmd + DTsigma + DTR + DTcf, accounting for minimum air temperature, stress level, strain rate, and cold forming effects.

Common mistakes engineers make

1. Specifying CVN for the wrong test temperature. The AISC 341 base metal requirement is 20 ft-lb at 70 degrees F (room temperature) — this is not a cold-weather test. The -20 degrees F requirement applies only to weld filler metal. Confusing these leads to either unnecessarily expensive base metal procurement or non-compliant weld metal.

2. Assuming all A992 steel has adequate toughness. ASTM A992 does not include a mandatory CVN requirement. Unless Supplementary Requirement S30 is specified on the purchase order, no CVN testing is performed. For heavy seismic columns, S30 must be explicitly called out on the drawings and purchase order.

3. Ignoring the k-area problem in heavy W-shapes. The k-area (the web-to-flange fillet region) of heavy W-shapes produced by rotary straightening can have reduced toughness due to cold work. AISC 341-22 Commentary Section A3.3 warns against welding in this region. Column stiffeners and continuity plates should be clipped to avoid welding in the k-area.

4. Using lamellar tearing-susceptible material for through-thickness loading. When tensile forces act through the thickness of a plate or flange (e.g., at column continuity plates), lamellar tearing can initiate along non-metallic inclusions. Specifying ASTM A770 (through-thickness tensile testing) or using Z-quality steel per EN 10164 prevents this failure mode.

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Related references

• How to Verify Calculations
• steel grade reference
• steel stress-strain curves
• steel fire resistance
• High Strength Steel
• Residual Stress
• Steel Grade Selection
• Steel Material Properties

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