Charpy V-Notch Impact Test: Measuring Toughness

What the Charpy Test Is and Where It's Used

The Charpy V-notch (CVN) impact test quantifies a material's notch toughness: how much energy a small notched bar absorbs when broken by a single, fast blow. It is deliberately severe — a sharp notch to concentrate stress, low temperature options to embrittle, and a high strain rate to suppress plastic flow — so it exposes brittle behavior that a slow tensile test would miss.

It is a mainstay of quality control and design qualification across heavy industry:

• Structural steel for bridges, ships, offshore platforms and pipelines, where cold-weather brittle fracture is a life-safety concern.
• Pressure vessels and boilers qualified under ASME Section VIII, which sets minimum CVN energy at the minimum design metal temperature.
• Welds and heat-affected zones, tested at −20 °C or −40 °C to prove the weld is as tough as the base plate.
• Pipeline steels (API 5L), where CVN shear area governs crack arrest.

Because a set of three specimens costs little and breaks in seconds, Charpy testing is the cheapest, fastest go/no-go screen for fracture-critical metal.

How It Works: The Pendulum and the Notch

A calibrated pendulum hammer is raised to a fixed starting height h₁, giving it a known potential energy (a common machine stores 300 J). Released, it swings through the bottom of its arc at roughly 5 m/s and strikes the specimen directly opposite the notch. The bar, simply supported at both ends, bends and fractures in about a millisecond.

The pendulum continues upward to a lower height h₂. The energy it lost to breaking the bar is:

• E = m·g·(h₁ − h₂), where m is the effective pendulum mass, g = 9.81 m/s², and h₁, h₂ are the drop and rise heights of the center of strike.

The machine reads this energy directly off a pointer or encoder in joules. The V-notch (2 mm deep, 45° included angle, 0.25 mm root radius) is the crux: it creates a triaxial stress state at its root, raising the local stress well above the yield strength and forcing the material to choose between ductile tearing and brittle cleavage. A blunter notch reads tougher; this is why root radius is controlled to ±0.025 mm.

Key Quantities and a Worked Example

Consider a 300 J machine with an effective pendulum mass m = 20.4 kg swinging on a 0.75 m arm. Suppose a mild-steel specimen tested at room temperature lets the pendulum rise so that (h₁ − h₂) = 0.60 m.

• E = 20.4 × 9.81 × 0.60 ≈ 120 J — a tough, fully ductile result.

The same steel at −40 °C might absorb only 8 J, with the pendulum barely slowing — a brittle cleavage failure. Plotting absorbed energy against temperature traces the classic S-curve with three zones:

• Lower shelf (~5–15 J): flat, brittle, cleavage fracture.
• Transition region: energy rises steeply; the DBTT is often set where energy = 27 J (a common code minimum) or at 50% of the upper-shelf value.
• Upper shelf (100–250 J): flat, fully ductile, fibrous fracture.

Two supporting measurements refine the result: lateral expansion (mm of specimen widening opposite the notch, e.g. a 0.38 mm / 15 mils minimum) and percent shear fracture area (fibrous vs. crystalline), which pins the fracture-appearance transition temperature.

Running the Test in Practice

Doing a valid Charpy test is about controlling the variables that make the number meaningful:

• Specimen prep: machine to 10 × 10 × 55 mm with the notch broached or milled to spec. Notch quality dominates scatter, so cutters are inspected regularly.
• Temperature: bars are soaked in a liquid bath (alcohol/dry ice, or a heated bath) and must be struck within 5 seconds of removal to limit warming per ASTM E23.
• Set count: three specimens per condition; the reported value is the average, with limits on how low any single result may fall.
• Machine verification: pendulums are checked against certified reference specimens; friction and windage losses are subtracted.

Codes then map the number to a decision. ASME and API set a minimum CVN energy at a specified test temperature — for example, 27 J at −20 °C for many structural grades, or a lateral-expansion floor for low-temperature vessels. If the plate or weld misses the minimum, it is rejected, re-heat-treated, or the material grade is upgraded (e.g., normalized rather than as-rolled).

Charpy vs. Izod and Fracture Toughness

Charpy has close cousins that engineers must not confuse:

• Izod test: same pendulum idea, but the specimen is clamped as a cantilever and the notch faces toward the striker. Izod dominates polymer testing (ASTM D256); Charpy dominates metals because loading and cooling a horizontal, simply-supported bar is faster and more repeatable.
• Fracture toughness (K_IC, J-integral, CTOD): these measure a design-usable material property — the critical stress intensity in MPa·√m — from a pre-cracked, quasi-static specimen. They tell you how large a crack a structure can tolerate at a given stress; Charpy does not, directly.

Charpy's virtue is that it is cheap and fast; its weakness is that its energy is empirical, not a transferable material constant. Engineers bridge the two with correlations — e.g., the Rolfe–Novak–Barsom relation estimates K_IC from upper-shelf CVN energy. Charpy screens and ranks; fracture mechanics quantifies for design. Instrumented Charpy machines add a load-time trace, splitting absorbed energy into crack-initiation and crack-propagation parts to partly close the gap.

Failure Modes, Trade-offs, and Historical Significance

The Charpy test earned its place by explaining catastrophes. The Liberty ships of World War II — nearly 1,500 of them cracked and about a dozen broke fully in two — failed because their welded, notch-sensitive steel dropped onto its lower shelf in cold North Atlantic water. Charpy curves showed the steel had a DBTT above its service temperature; the same lesson is often cited for the Titanic's brittle hull plates. These disasters made low-temperature CVN minimums mandatory in ship and bridge codes.

Key limitations and trade-offs to respect:

• Not a design number: you cannot compute allowable stresses or crack sizes from joules alone.
• Geometry-sensitive: notch radius, specimen size (sub-size 5 mm and 7.5 mm bars read lower), and strike alignment all shift results.
• Scatter in the transition: near the DBTT, results swing widely, so multiple specimens and statistics matter.
• Rate-specific: the ~5 m/s impact raises the apparent DBTT versus slow loading, giving a conservative (safe) but not exact estimate.

Despite these caveats, no other test packs so much fracture insight into a two-second, low-cost break — which is why every fracture-critical steel spec still calls for it.

Charpy vs. Izod vs. related toughness measures

Property	Charpy V-notch	Izod	Fracture toughness K_IC
Specimen support	Simply supported (both ends)	Cantilever (clamped one end)	Pre-cracked SENB/CT specimen
Notch orientation	Faces away from striker	Faces toward striker	Fatigue-sharpened crack tip
Result units	Energy, joules (J)	Energy, joules or ft·lbf	Stress intensity, MPa·√m
Specimen size	10 × 10 × 55 mm	10 × 10 × 75 mm (typ.)	Thickness-dependent, larger
Strain rate	High (~5 m/s impact)	High (~3.5 m/s impact)	Slow, quasi-static
Main use	Steel DBTT, weld QA	Polymers, some metals	Design-relevant crack tolerance