Notch Toughness — Charpy V-Notch Testing & Brittle Fracture Prevention
Notch toughness is the capacity of steel to absorb energy and resist brittle fracture when a crack or notch is present. Unlike ductility (measured in smooth-bar tensile tests), notch toughness specifically evaluates fracture behavior in the presence of a stress concentration — a far more realistic condition for real structures with bolted holes, weld toes, fatigue cracks, or fabrication notches.
The Charpy V-notch (CVN) test is the universal standard for measuring notch toughness. It directly determines whether a steel grade is suitable for use at low temperatures, in thick sections, or in dynamically loaded structures.
PRELIMINARY — NOT FOR CONSTRUCTION. All content is for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.
The Charpy V-Notch Test — Procedure
The CVN test (ASTM E23, ASTM A370) uses a specimen with standardized dimensions:
Dimensions: 10 mm × 10 mm × 55 mm
Notch: 45° V-notch, 2 mm deep, 0.25 mm root radius
Orientation: Notch perpendicular to rolling direction (typically)
Procedure:
- Machine the V-notch in the specimen
- Cool or heat the specimen to the test temperature (± 2°C)
- Place specimen on anvil supports (span = 40 mm), notch facing away from hammer strike
- Release pendulum hammer — strikes opposite the notch
- Measure energy absorbed from the difference in pendulum height before and after impact
- Record CVN value in ft-lb (US) or joules (SI): 1 ft-lb = 1.356 J
The test measures absorbed energy, NOT strength or elongation. A brittle fracture absorbs little energy (low CVN); a ductile fracture absorbs much more (high CVN).
Ductile-to-Brittle Transition
All structural steels exhibit a ductile-to-brittle transition as temperature decreases:
CVN Energy (ft-lb)
↑
80├── Upper Shelf (100% ductile fracture — dimpled, high energy)
│
60├ ╲
│ ╲
40├ ╲ ← Transition Zone (mixed ductile/brittle)
│ ╲
20├ ╲
│ ╲
0├─────────────────────── Lower Shelf (100% brittle — cleavage, low energy)
└──┴────┴────┴────┴────┴───→ Temperature (°F)
−80 −40 0 +40 +80
Key temperatures:
- Upper shelf: Fully ductile behavior — high energy absorption, shear fracture surface
- Lower shelf: Fully brittle — cleavage fracture, crystalline surface, low energy
- Transition temperature: Usually defined as the temperature at which CVN = 15 or 20 ft-lb, or where fracture surface is 50% shear
Different steel grades have different transition curves. Fine-grain steels (A572, A992) have lower transition temperatures than coarse-grain steels (older A36). Normalized and quenched-and-tempered steels typically have the best (lowest) transition behavior.
Factors Affecting Notch Toughness
| Factor | Effect on Toughness |
|---|---|
| Temperature | Lower temperature → lower toughness (most important factor) |
| Strain rate | Higher loading rate → lower apparent toughness |
| Thickness | Thicker sections → more constraint → lower toughness |
| Grain size | Finer grains → higher toughness at all temperatures |
| Notch acuity | Sharper notch → lower measured energy (standardized in CVN test) |
| Heat treatment | Normalizing, Q&T improve toughness vs as-rolled |
| Welding | Coarse grain HAZ may have reduced toughness |
| Chemical composition | Lower carbon, higher manganese: nickel improves toughness |
| Triaxial stress state | Constraint (thick sections, notches) promotes brittle behavior |
AISC 360 — CVN Requirements (Section A3.1c)
AISC 360 Table 2-1 prescribes CVN values based on:
- Service temperature zone (lowest anticipated service temperature, LAST)
- Member thickness (thicker = more constraint = tougher steel needed)
- Connection type (welded connections need higher toughness than bolted)
- Loading type (tension members need higher toughness than compression members)
Service Temperature Zones
| Zone | LAST (°F) | Typical Location |
|---|---|---|
| 1 | ≥ +50 | Indoor, heated, Southern US |
| 2 | +2 to +49 | Enclosed but not heated, moderate climates |
| 3 | 0 to −30 | Exterior, Northern US/Canada |
Simplified Requirements (Typical Building Columns)
| Thickness | Zone 1 | Zone 2 | Zone 3 |
|---|---|---|---|
| ≤ 2 in, bolted | None | None | 15 ft-lb @ LAST |
| ≤ 2 in, welded | None | 15 ft-lb @ LAST | 20 ft-lb @ LAST |
| 2-4 in, bolted | None | 15 ft-lb @ LAST | 25 ft-lb @ LAST |
| 2-4 in, welded | 15 ft-lb @ LAST | 25 ft-lb @ LAST | 35 ft-lb @ LAST |
For seismic applications (AISC 341), additional CVN requirements apply to members in the seismic force-resisting system, typically 20 ft-lb at the lowest anticipated service temperature.
AWS D1.1 — Welding Toughness Requirements
AWS D1.1 Structural Welding Code imposes CVN requirements on weld metal and heat-affected zone (HAZ):
- Weld metal CVN: Typically 20 ft-lb at the lowest service temperature for statically loaded structures, higher (25-40 ft-lb) for dynamically loaded structures
- HAZ CVN: Base metal specifications ensure HAZ toughness — the HAZ typically has the lowest toughness in a welded connection
Design Strategies for Brittle Fracture Prevention
1. Material Selection
Select steel grades with adequate CVN values for the service temperature. A992 typically satisfies most building requirements without special specification.
2. Detail Design
- Avoid sharp notches and re-entrant corners
- Use generous fillet radii at section changes
- Grind weld toes smooth (improves fatigue AND fracture resistance)
- Avoid welding transverse to the tension stress direction when avoidable
3. Redundancy
Provide alternate load paths so fracture of one element doesn't cause progressive collapse. This is a key strategy — even brittle fracture is acceptable if the structure survives with one failed member.
4. Stress Relief
Post-weld heat treatment reduces residual stresses, lowering the triaxial constraint that promotes brittle fracture.
Historical Context — The Liberty Ship Failures
The importance of notch toughness became tragically clear during WWII. Of approximately 2,700 Liberty ships built, over 400 suffered serious brittle fractures, and 12 broke completely in half — some while sitting at dock in cold water. Investigation revealed:
- Steel with adequate room-temperature ductility became brittle at North Atlantic temperatures (≈ 30°F)
- Sharp hatch corner details acted as crack initiators
- Welded construction (no crack-arresting features of riveted ships) allowed cracks to propagate across entire hull
The Liberty ship failures drove the development of the CVN test as a standard material acceptance criterion and led to modern fracture control plans in structural codes.
Frequently Asked Questions
What CVN value does my project need?
For a typical heated building in the US (Zone 1) using A992 W-shapes up to 2 inches thick with bolted connections — no CVN testing required. For an unheated warehouse in Minnesota (Zone 3) with welded moment connections — 20-25 ft-lb at the lowest service temperature, plus AWS D1.1 weld metal requirements. Always check AISC 360 Table 2-1 for your specific application.
Is all A992 steel notch-tough?
Most modern A992 steel from North American mills has CVN values exceeding 40 ft-lb at +70°F and 20+ ft-lb at +40°F, well above minimum requirements for typical applications. However, CVN testing is not mandatory unless specified. For critical applications (bridges, seismic members, thick welded sections), CVN testing should be specified on the purchase order and verified by mill test reports.
How does the CVN test differ from the tensile test?
The tensile test measures strength and ductility (Fy, Fu, %EL) under slow, uniaxial loading on a smooth specimen — it evaluates general mechanical properties. The CVN test measures energy absorption under dynamic (impact) loading on a notched specimen — it specifically evaluates resistance to brittle fracture. A steel can have excellent tensile elongation but poor CVN values (brittle at low temperature), or vice versa. Both are required for a complete material characterization.
Related Terms and Pages
- Ductility — Elongation, R-Value & Seismic Performance
- Yield Strength (Fy) — Definition & Values
- Tensile Strength (Fu) — Definition & Values
- Residual Stress — Hot-Rolled vs Welded
- Steel Fatigue Design — Reference Guide
- Steel Charpy Values — Reference Table
- AISC Steel Manual — Reference
Educational reference only. Notch toughness requirements per AISC 360 Table 2-1, AISC 341, and AWS D1.1 must be verified and specified by a licensed Professional Engineer for all structural steel projects.
Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.