Charpy and Izod Impact Testing: CVN Energy, DBTT, and Fracture Toughness Correlation
Charpy V-notch (CVN) impact testing is the most widely specified toughness qualification test in pressure vessel, pipeline, structural steel, and weld procedure qualification codes worldwide. It quantifies a material's resistance to rapid fracture under notch constraint — a property that cannot be inferred from tensile or hardness data alone. This article provides a rigorous treatment of test machine mechanics, specimen geometry, the physics of the ductile-to-brittle transition, fracture surface interpretation, empirical correlations to fracture toughness KIC, and the specific code requirements of ASME, EN, API, and AWS that define when and how Charpy testing must be performed.
Key Takeaways
- The Charpy test measures absorbed energy (joules or ft·lb) when a pendulum hammer fractures a 10×10×55 mm notched beam; the Izod test uses a cantilever-clamped specimen — the Charpy configuration dominates in international codes.
- Absorbed energy = mg(h1 − h2) = mgL(cosθ2 − cosθ1); a calibrated machine corrects for friction and windage losses automatically.
- The ductile-to-brittle transition temperature (DBTT) is a characteristic of BCC metals (steels, ferritic stainless); FCC metals (austenitic stainless, aluminium, copper) show no DBTT and remain tough to cryogenic temperatures.
- DBTT is most commonly defined at 27 J (EN/European codes) or at the 50% shear fracture appearance transition temperature (50% FATT); lateral expansion ≥ 0.38 mm is a supplementary criterion in ASME VIII.
- Grain refinement, controlled rolling, and microalloying (Nb, V, Ti) are the primary metallurgical tools for lowering DBTT; nickel addition lowers DBTT by approximately 10°C per wt% Ni.
- The Barsom–Rolfe correlation (KIC²/E ≈ 2×CVN on the upper shelf) allows preliminary KIC estimation but carries ±30–50% uncertainty; ASTM E1820 testing is required for design-critical applications.
Pendulum Machine Mechanics and Energy Measurement
The Charpy impact test uses a pendulum of known mass m and arm length L, released from a measured initial angle θ1. The pendulum falls, strikes and fractures the specimen, and rises to a follow-through angle θ2. By conservation of energy, the absorbed energy is:
CVN = mgL(cosθ₂ − cosθ₁)
Where: m = pendulum mass (kg)
g = 9.807 m/s²
L = arm length from pivot to striker centreline (m)
θ₁ = initial release angle (typically 140° — 160° from vertical)
θ₂ = follow-through angle (measured from vertical)
Striker impact velocity:
v = √(2gL(1 − cosθ₁))
= √(2 × 9.807 × L × (1 − cosθ₁))
Typical machines: L = 0.8 m, θ₁ = 150°
v = √(2 × 9.807 × 0.8 × (1−cos 150°)) = √(19.36) ≈ 5.2 m/s
ASTM E23 requires impact velocity 3–6 m/s; ISO 148-1 requires 5.0–5.5 m/s
Friction in the pivot bearings and aerodynamic drag on the pendulum arm introduce small systematic losses, typically < 0.5 J on a well-maintained machine. ASTM E23 and ISO 148-1 require verification of these losses and correction of the scale reading accordingly. Machines are verified annually against certified reference specimens traceable to national standards laboratories (NIST in the US, NPL in the UK).
Machine Capacity and Striker Geometry
Standard Charpy machines have a rated capacity of 300 J (220 ft·lb) or 450 J (330 ft·lb). Using a high-capacity machine to test a material with low CVN energy (say, 10–20 J) introduces large relative errors because the angle measurement precision is the same whether 20 J or 290 J is absorbed. For this reason, ASTM E23 specifies that when the measured energy exceeds 80% of the machine capacity, a smaller capacity machine should be used. The striker geometry — either 8 mm radius (Charpy C-type, ASTM E23) or 2 mm radius (Charpy U-notch variant) — is specified in the test report and applicable code.
Specimen Temperature Control
Temperature conditioning is critical for transition temperature testing. Specimens are soaked in a temperature-controlled bath (liquid nitrogen, dry ice/acetone, or heated silicone oil) for a minimum time that ensures thermal equilibration through the full 10×10 mm cross-section:
Minimum soak time (ASTM E23 Table A1.1):
At temperatures below −60°C: ≥ 5 min in liquid coolant
−60°C to 0°C: ≥ 5 min in liquid coolant
0°C to +200°C: ≥ 5 min in heated bath or furnace
Transfer time: maximum 5 seconds from bath to anvils (per ASTM E23 A1.5.2)
— specimen temperature change during transfer ≤ 2°C for steel
Temperature measurement: ±1°C of target using calibrated thermocouple
in the bath, not in the specimen (risk of notch damage)
Transfer time is the most common source of test error. Steel specimens at −60°C can warm by 5–10°C in 10 seconds of air exposure. An operator who takes 8–10 seconds from bath to machine will test at a temperature 3–6°C higher than intended — a shift that can raise the apparent CVN energy by 5–20 J in the transition region, potentially causing a marginal specimen to incorrectly pass. ASTM E23 permits no exception to the 5-second limit.
Specimen Types: Charpy, Izod, and Sub-size Variants
Standard Charpy V-Notch (CVN)
The standard specimen per ASTM E23 and ISO 148-1 is 10×10×55 mm with a 45° V-notch, 2 mm deep, root radius 0.25 ± 0.025 mm, centred at mid-length. The anvil support span is 40 mm. The striker strikes the face opposite the notch at mid-span. Dimensional tolerances are tightly controlled because the absorbed energy is highly sensitive to notch root radius: a radius of 0.30 mm (versus the nominal 0.25 mm) can increase absorbed energy by 5–10% in the transition region.
| Specimen | Standard | Size (mm) | Notch Type | Notch Depth (mm) | Root Radius (mm) | Notes |
|---|---|---|---|---|---|---|
| Charpy V-notch (CVN) | ASTM E23 / ISO 148-1 | 10×10×55 | V, 45° | 2.0 | 0.25 | Universal standard; dominant in codes |
| Charpy U-notch (CUN) | ISO 148-1 / DIN EN 10045 | 10×10×55 | U, 5 mm radius | 5.0 | 1.0 | Higher absorbed energy; used in some European standards |
| Sub-size CVN 7.5×10 | ASTM E23 Annex A4 | 7.5×10×55 | V, 45° | 2.0 | 0.25 | Thin plate or HAZ sampling |
| Sub-size CVN 5×10 | ASTM E23 Annex A4 | 5×10×55 | V, 45° | 2.0 | 0.25 | Very thin plate; correction factor required |
| Sub-size CVN 2.5×10 | ASTM E23 Annex A4 | 2.5×10×55 | V, 45° | 2.0 | 0.25 | Rarely used; low constraint; large scatter |
| Izod V-notch | ASTM E23 / ISO 180 | 10×10×75 | V, 45° | 2.0 | 0.25 | Cantilever; dominated by polymers and cast iron testing |
Charpy vs. Izod: Key Distinctions
The Izod specimen is clamped vertically as a cantilever at one end, with the notch at the clamp face; the striker impacts the free end 22 mm above the notch. The Charpy specimen is simply supported at both ends and struck from the opposite face. The Charpy configuration is preferred in international metal testing codes for two reasons: (1) the simply-supported geometry is more reproducible because it is insensitive to clamping force; (2) the Charpy specimen can be tested at sub-ambient temperature without the clamping mechanism being affected by thermal expansion changes. The Izod test finds primary use in the plastics and polymer industry (ISO 180) and for cast iron quality control.
Notch Preparation and Its Importance
The notch is machined by broaching (preferred) or milling to the specified geometry. Root radius must be within ± 0.025 mm of the nominal 0.25 mm. A sharper root radius (say 0.15 mm) increases stress concentration and locally reduces fracture energy; a blunter root (0.35 mm) reduces stress concentration, increasing apparent toughness. Notch geometry is verified using a toolmaker's microscope or optical comparator on production tooling. ASTM E23 requires notch verification at the start of each machining session and after every 200 specimens.
Fracture Mechanisms and the Ductile-to-Brittle Transition
The most important application of Charpy testing in materials selection is characterisation of the ductile-to-brittle transition temperature (DBTT) — the temperature range over which BCC metals shift from high-energy ductile fracture to low-energy brittle fracture. The transition is not a sharp temperature but a band of 50–100°C width governed by competing fracture mechanisms at the microstructural scale.
Upper Shelf: Ductile Fracture (Microvoid Coalescence)
At temperatures well above the DBTT, fracture occurs by microvoid coalescence (MVC). Voids nucleate at inclusions and second-phase particles (MnS, Al₂O₃, carbides) when the local stress exceeds the particle–matrix interface strength. Voids grow by plastic deformation of the surrounding matrix, elongate, and coalesce to form a fibrous, rough fracture surface. The absorbed energy on the upper shelf (USE) is typically 150–350 J for modern pressure vessel steels and is a measure of the material's resistance to slow crack growth.
Lower Shelf: Brittle Fracture (Transgranular Cleavage)
At temperatures well below the DBTT, fracture occurs by transgranular cleavage — propagation of a crack along specific crystallographic planes of low fracture energy, the {100} planes in BCC iron. Cleavage fracture is characterised by:
- River marks: Chevron-shaped ridges on the fracture surface, converging toward the crack origin (visible under SEM or stereo microscope). River marks form where cleavage cracks pass through sub-grain boundaries and change propagation plane slightly.
- Flat, shiny, faceted surface: Reflecting crystallographic regularity; appears granular and bright to the naked eye.
- Very low absorbed energy: Typically 2–15 J on the lower shelf for ferritic steels.
The cleavage fracture stress σF is grain-size-dependent, following a Hall–Petch type relationship because a cleavage crack must traverse each grain boundary and re-nucleate on the other side:
Cleavage fracture stress:
σ_F = σ_F0 + k_F × d^(−1/2)
Where: σ_F0 = lattice friction stress for cleavage (MPa)
k_F = microstructural cleavage resistance (MPa·m^(1/2))
d = grain diameter (m)
For ferritic steel:
σ_F0 ≈ 150 MPa, k_F ≈ 0.3 – 0.5 MPa·m^(1/2)
At d = 50 μm: σ_F ≈ 150 + 0.4/(50×10⁻⁶)^(1/2) = 150 + 57 = 207 MPa
At d = 10 μm: σ_F ≈ 150 + 0.4/(10×10⁻⁶)^(1/2) = 150 + 127 = 277 MPa
Finer grain (lower d) → higher σ_F → cleavage requires more energy → lower DBTT
Transition Region: Mixed Mode Fracture
In the transition region, the fracture surface shows a mixture of fibrous (ductile) and crystalline (cleavage) zones. The competing fracture mechanisms — MVC requiring dislocation motion (thermally activated, rate-sensitive) and cleavage requiring only elastic stress (athermal above a critical stress) — are both active, and their relative contributions depend on local temperature, strain rate, constraint, and microstructural heterogeneity.
Why BCC but not FCC Shows a DBTT
BCC metals (α-iron, ferritic steels, chromium, molybdenum, tungsten, niobium) exhibit a DBTT because: (1) dislocation motion in BCC is governed by the Peierls–Nabarro stress — a lattice friction stress that increases sharply as temperature falls, so BCC metals become very difficult to deform plastically at low temperatures; and (2) the cleavage fracture stress is relatively low on {100} planes because bonding is less isotropic than in FCC. FCC metals (austenite, copper, aluminium, nickel) have low Peierls stress at all temperatures — their yield stress rises only modestly with cooling — so dislocations remain mobile and can blunt crack tips even at cryogenic temperatures. FCC materials therefore maintain high toughness to −196°C and below, making them the material of choice for cryogenic storage vessels, LNG equipment, and liquid hydrogen applications.
Metallurgical Factors Controlling CVN Energy and DBTT
Grain Size
Grain refinement is the most effective and well-understood method for simultaneously increasing yield strength and reducing DBTT — the only strengthening mechanism that achieves both objectives. The DBTT shift per unit change in grain size for structural steels follows empirically:
DBTT shift from grain refinement (Pickering, 1978):
ΔDBTT ≈ −40 × Δ(ASTM grain size number) [°C]
Or equivalently:
DBTT (°C) ≈ −18 − 14 × log(σ_y) − 22 × (ASTM No.) [empirical, low C steels]
Effect of grain diameter:
Halving grain diameter (d → d/2) lowers DBTT by ∼ 10–15°C
Controlled rolling: ASTM 10–12 vs. normalised ASTM 7–8 → ΔDBTT ≈ −40 to −60°C
Composition Effects
| Element | Typical Range (wt%) | Effect on DBTT | Mechanism |
|---|---|---|---|
| Carbon (C) | 0.10–0.30 | +13°C per 0.1 wt% C increase | Increased pearlite fraction raises cleavage propagation stress; carbides act as crack nuclei |
| Manganese (Mn) | 0.5–1.8 | −5°C per 0.1 wt% Mn increase | Solid-solution strengthening + grain refinement; desulphurises (converts MnS from stringers) |
| Nickel (Ni) | 0–9.0 | −10°C per wt% Ni | Lowers stacking fault energy in BCC; raises cleavage fracture stress; transforms Mn-S morphology |
| Silicon (Si) | 0.1–0.5 | +6°C per 0.1 wt% Si increase | Solid-solution hardening raises Peierls stress; promotes cleavage |
| Phosphorus (P) | < 0.025 | +70°C per 0.1 wt% P increase | Strong embrittler; grain boundary segregation reduces cohesive strength; strict limit in pressure vessel steels |
| Sulphur (S) + MnS inclusions | < 0.010 | Highly directional; worst in S–L orientation | MnS stringers act as crack initiation sites; vacuum degassing and Ca treatment reduce elongated MnS |
| Niobium (Nb) | 0.02–0.06 | −20 to −40°C total | Grain refinement via NbC/NbN pinning; retardation of recrystallisation during controlled rolling |
| Vanadium (V) | 0.04–0.12 | −10 to −20°C | VC precipitation strengthening + grain refinement; less effective than Nb at controlled rolling temperatures |
Inclusion Cleanliness and Anisotropy
MnS inclusions elongate during rolling, creating directionality in impact toughness. Testing specimens machined in the short transverse direction (S–L orientation) can yield CVN energies 30–60% lower than specimens in the longitudinal direction (L–T) from the same plate. Codes specify the required test orientation explicitly — weld procedure specifications for pressure vessels per ASME Section IX typically require specimens from the heat-affected zone in the transverse–longitudinal (T–L) orientation for base metal and cross-weld for weld metal. Calcium treatment and vacuum arc degassing reduce MnS stringer length and improve through-thickness toughness.
Heat Treatment Effects
The relationship between heat treatment condition and CVN behaviour is central to the selection of quench-and-temper parameters for pressure vessel steels. Higher tempering temperatures produce coarser carbide dispersions with greater interparticle spacing, reducing void nucleation density and improving USE, but also increasing grain size slightly. Quenched-and-tempered steels consistently outperform normalised steels of the same composition in terms of CVN, because the fine martensite lath structure created by quenching — described in detail in the martensite formation article — presents more grain boundary area per unit volume to retard cleavage propagation. The quench and temper processing sequence is therefore the standard route for low-temperature service pressure vessels and structural components.
Strain Ageing
Strain ageing occurs when carbon and nitrogen atoms, freed into solution by plastic deformation, migrate to dislocations and pin them. In low-alloy steels with free nitrogen (above approximately 60 ppm dissolved N), strain ageing at 100–300°C raises the yield stress and raises the DBTT by 15–30°C. This is relevant in pressure vessel steels subjected to cold-forming operations (rolling, pressing, flanging) before service. ASME Section VIII Div. 1 UCS-79 requires post-cold-forming heat treatment (stress relief or normalise) when the cold strain exceeds specified limits (typically >5% or >3% for thicker sections) specifically to eliminate the strain-ageing embrittlement risk.
Fracture Appearance Assessment: SFA and Lateral Expansion
Shear Fracture Appearance (SFA or %SA)
The fracture surface of a broken Charpy specimen is assessed visually to determine the percentage of the fractured area showing shear (fibrous, ductile) appearance versus cleavage (crystalline, brittle) appearance. This assessment is made with the naked eye or a low-power hand lens (×10). ASTM E23 provides reference photographs for correlating visual appearance to percentage shear values.
- 100% shear (fibrous): Fully ductile fracture; no cleavage facets visible. Corresponds to upper shelf behaviour.
- 50% shear: Defines the 50% FATT (fracture appearance transition temperature) — a widely used DBTT criterion, particularly for pipeline steels (API 5L, DNV-ST-F101).
- 0% shear (crystalline): Fully cleavage; lower shelf behaviour. Bright, flat, faceted surface.
The 50% FATT is preferred over the 27 J criterion for high-strength pipeline steels because the energy criterion can be met by a specimen with mixed fracture mode (50% cleavage) if the yield strength is sufficiently high, giving a misleadingly optimistic impression of toughness. The FATT criterion directly measures the fracture mode transition and is therefore more physically meaningful.
Lateral Expansion
Lateral expansion (LE) measures the plastic bulge on the compression face of the broken specimen (the face struck by the hammer) perpendicular to the notch plane. It is measured using a vernier gauge or optical comparator per ASTM E23 Annex A3:
Lateral Expansion (mm) = (W_after − W_before)
= total width increase of compression face after fracture
ASME Section VIII UG-84(c)(2) minimum:
Lateral Expansion ≥ 0.38 mm (15 mil) at test temperature
for pressure vessel plate steels > 1/2 in. (12.7 mm) thick
At the upper shelf: LE typically 1.5 – 3.0 mm
At 50% FATT: LE typically 0.5 – 1.0 mm
At lower shelf: LE typically < 0.1 mm
Correlation Between CVN and Fracture Toughness KIC
Fracture toughness KIC (units MPa·m1/2) is the fundamental material property governing crack propagation in structural integrity assessment. Direct KIC measurement requires large, fatigue-pre-cracked specimens (ASTM E399), which are expensive and cannot always be machined from production material. Several empirical correlations allow preliminary KIC estimation from CVN data.
Barsom–Rolfe Correlations
Upper shelf (Barsom-Rolfe, 1977) — US customary units:
(K_IC / σ_y)² = 5 [CVN/σ_y − 0.05]
K_IC²/E ≈ 2 × CVN (approximately, for σ_y in ksi, CVN in ft·lb, K_IC in ksi√in)
SI unit form:
K_IC (MPa√m) ≈ 0.54 × (CVN_upper × σ_y)^(1/2)
where CVN in J, σ_y in MPa
Transition region (lower shelf) — Roberts-Newton modification:
K_IC (MPa√m) ≈ (5 × CVN)^(1/2) [CVN in J, approximate lower bound]
Example calculation (API 5L X65 pipeline steel):
CVN (upper shelf) = 150 J, σ_y = 448 MPa
K_IC ≈ 0.54 × (150 × 448)^(1/2) = 0.54 × 259 = 140 MPa√m
This is a rough estimate; actual measured K_IC may range 110–180 MPa√m
Uncertainty: ± 30–50% (use for preliminary screening only)
The uncertainty in these correlations arises because: (1) the Charpy specimen is much smaller than a KIC specimen and experiences a different stress state; (2) the Charpy test measures absorbed energy including both crack initiation and propagation components, while KIC measures only propagation toughness; and (3) the Charpy test is a dynamic (high strain rate) test, whereas KIC is quasi-static. For any structural integrity assessment involving leak-before-break analysis, fitness-for-service (API 579 / BS 7910), or critical flaw size calculation, KIC or CTOD testing per ASTM E1820 is required. See also the related article on fracture toughness testing methods for a full treatment of KIC, CTOD, and J-integral methods.
Code Requirements: When and How Charpy Testing Is Mandated
The Charpy test is the primary toughness qualification test in virtually every major structural and pressure equipment code. The trigger conditions, minimum energy values, test temperatures, and specimen orientations vary by code and material group.
ASME Section VIII Division 1 — Pressure Vessels
UG-84 governs impact testing for pressure vessel steels. Testing is required when: (1) the design temperature falls below the minimum design metal temperature (MDMT) exemption curve temperature for the plate thickness; or (2) the material is used for lethal-service vessels (UW-2). Key requirements:
- Test temperature: MDMT or 17°C below MDMT for certain conditions
- Minimum CVN energy: 20 J average (full-size, 3-specimen set); no single specimen < 14 J
- Lateral expansion: ≥ 0.38 mm (15 mil) for plates >12.7 mm thick
- Weld metal and HAZ: same energy requirements as base metal at the same temperature
- Post-weld heat treatment (PWHT) must not raise DBTT — separate impact tests required on PWHT-treated weld samples
EN 13445 — Unfired Pressure Vessels (European)
Impact testing is required for all P-numbers and material groups except certain thin sections and low-hazard categories. The acceptance criterion is typically 27 J at the test temperature, with the test temperature defined as the minimum allowable temperature (MAT) from the EN 13445 Annex B temperature chart. Weld procedure qualifications per EN ISO 15614-1 require CVN testing of weld metal and HAZ specimens at the test temperature specified in the design specification.
API 5L — Pipeline Steels
API 5L specifies CVN testing for all PSL2 (product specification level 2) line pipe in terms of both absorbed energy and shear fracture appearance (50% FATT). Typical PSL2 requirements for X65 grade at 0°C: CVN ≥ 40 J (body), ≥ 27 J (weld seam), and 50% FATT ≤ −10°C. These requirements are driven by the need to arrest running ductile fractures in gas pipelines — a phenomenon governed by the upper shelf energy level rather than the transition temperature alone.
AWS D1.1 — Structural Welding, Steel
Charpy testing of weld metal is required for fracture-critical members (FCM) per Clause 4.14 when specified in the contract documents. Standard acceptance criterion: 27 J at −18°C (0°F) for common structural steels, with more stringent requirements for offshore and seismic applications. The test specimens are machined with the notch oriented through the weld metal or HAZ as specified in the WPS qualification record. See the HAZ microstructure article for details of HAZ sub-zone behaviour that determines where in the HAZ impact specimens are most critically placed.
Test Temperature vs. Design Temperature
| Code | Test Temperature Basis | Min. CVN (Full-size, J) | Supplementary Criterion |
|---|---|---|---|
| ASME VIII Div. 1 | MDMT or MDMT −17°C | 20 J avg. / 14 J min. | Lateral expansion ≥ 0.38 mm |
| ASME VIII Div. 2 | MDMT | 27 J avg. / 20 J min. | Lateral expansion ≥ 0.38 mm |
| EN 13445 (Europe) | MAT from Annex B chart | 27 J avg. / 19 J min. | % Shear fracture optional |
| API 5L PSL2 X65 | 0°C (body); −10°C (weld) | 40 J (body); 27 J (weld) | 50% FATT specified |
| DNV-ST-F101 (offshore pipe) | Minimum design temperature | 35–45 J depending on OD | 85% USE for arrest per Appendix C |
| AWS D1.1 (FCM) | −18°C (standard) | 27 J | Notch orientation specified |
| NACE MR0175 / ISO 15156 | Ambient (sour service qual.) | Per base code; H₂S limits apply | Hardness HRC ≤ 22 governs |
Irradiation embrittlement in reactor pressure vessels: Neutron irradiation shifts the DBTT of reactor pressure vessel (RPV) steel upward by 50–200°C over the design life, depending on fluence and copper content. The reference temperature shift ΔRTNDT is tracked using surveillance capsule specimens — archive Charpy specimens irradiated in the reactor and periodically removed for testing per 10 CFR 50 Appendix H — to ensure the pressurised thermal shock (PTS) margin remains adequate throughout the vessel's operating licence. This is the most safety-critical Charpy application in the nuclear industry, described in detail in hardness testing context and in ASTM E185 / E900 standards.
Frequently Asked Questions
What is the difference between a Charpy and an Izod impact test?
What energy is absorbed in a Charpy test and how is it measured?
What is the ductile-to-brittle transition temperature (DBTT)?
How does grain size affect Charpy impact energy?
What is lateral expansion and why is it specified alongside CVN energy?
How is Charpy CVN energy correlated to fracture toughness KIC?
What are sub-size Charpy specimens and when are they used?
What causes the upper shelf energy of steel to decrease in service?
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Further Reading
