Charpy Impact

The Charpy impact test is a standardized pendulum fracture test that measures the energy absorbed by a notched specimen when it breaks under a sudden impact load. That absorbed energy is used as an indirect measure of notch toughness, or resistance to brittle fracture. A standard Charpy V-notch specimen is 10 x 10 x 55 mm with a closely controlled V-notch.

In a CNC machining workflow, Charpy impact testing rarely runs on the production part. Instead, a machinist prepares test coupons from the same heat, weld, or forging, often machining them to 10 x 10 x 55 mm and cutting a precise V-notch. The coupon's orientation and notch geometry control where fracture starts, so tool wear, chatter marks, burrs, or an overcut notch can turn a valid test into a misleading one. Once machined, the coupon is deburred, verified against drawing tolerances, marked for traceability, and sometimes conditioned to sub-zero temperature before the lab swings the pendulum. The recorded absorbed energy then tells the engineer whether material or weld procedure meets fracture-critical job requirements. For the machinist, the practical stakes are simple: stable fixturing, sharp tooling, controlled depth passes, and careful handling ensure the test result reflects the material rather than the machining process.

• ⚠️Wrong notch geometry: An undersized root radius or overcut notch changes the stress concentration, causing premature or delayed fracture and invalidating the energy comparison against ASTM-style requirements.
• ⚠️Poor orientation and thermal handling: If the notch faces wrong in the fixture or low-temperature coupons warm up before the swing, the striker hits a different stress zone and toughness appears artificially high or low, invalidating comparison.
• ⚠️Machine verification neglect: A worn anvil, dull striker, or unverified pendulum scale shifts the measured joules, so a good specimen can look bad or a bad specimen can pass, and scatter hides true material performance.