Impact Testing and Fracture Toughness

## Why Tensile Tests Alone Are Insufficient A specimen pulled slowly in a tensile machine absorbs energy through plastic deformation before fracture. The same material struck by a sudden impact has far less time for dislocations to move and for plastic deformation to occur at the crack tip. The result can be brittle fracture at a stress well below the tensile strength—exactly the failure mode that destroyed Liberty ships in World War II, where low-temperature brittle fracture propagated through welded hull sections. Impact tests and fracture toughness measurements quantify resistance to exactly this mode of failure. ## The Charpy V-Notch Test The Charpy test strikes a notched specimen (10 mm × 10 mm × 55 mm with a 2 mm deep V-notch) with a swinging pendulum at a fixed impact velocity. The energy absorbed by the specimen as it fractures is measured from the difference in pendulum height before and after the strike. Results are reported in joules (J) or foot-pounds (ft-lb). The test is governed by ASTM E23 (US) and ISO 148 (international). Test temperature can be varied from -196 °C (liquid nitrogen) to elevated temperatures by conditioning the specimens in controlled baths. ### Ductile-to-Brittle Transition BCC metals (carbon steels, ferritic stainless steels) exhibit a ductile-to-brittle transition temperature (DBTT) below which absorbed energy drops sharply. This transition is absent in FCC metals (austenitic stainless steels, aluminum alloys, copper alloys), which maintain good toughness to very low temperatures. Typical Charpy energies for common engineering steels at -20 °C: | Material | Condition | Charpy Energy (-20 °C) | |----------|-----------|------------------------| | AISI A36 structural steel | As-rolled | 20–40 J | | AISI 1045 | Normalized | 10–20 J | | AISI 4140 | Q&T to 900 MPa | 80–120 J | | AISI 4340 | Q&T to 1000 MPa | 60–100 J | | AISI 316 stainless | Annealed | >200 J | Pressure vessel codes (ASME VIII, EN 13445) specify minimum Charpy values at design temperature for carbon and alloy steels based on design wall thickness and operating temperature. A common requirement is 27 J minimum at the lowest operating temperature. ### Fracture Appearance Charpy specimens are also examined for fracture mode: shear (fibrous, ductile) vs. cleavage (flat, crystalline, brittle). The percent shear fracture (also called percent fibrous fracture) is reported alongside absorbed energy. The transition from predominantly ductile to predominantly brittle fracture appearance correlates with the energy transition but may occur at a somewhat different temperature. ## Fracture Toughness: K₃c The Charpy test gives an energy value but not a parameter directly usable in design calculations for components containing cracks or crack-like defects. Linear elastic fracture mechanics (LEFM) provides the plane-strain fracture toughness K₃c (in MPa√m), which quantifies the stress intensity at a crack tip required to propagate the crack catastrophically. The governing relationship is: **K = Y σ √(πa)** where K is the stress intensity factor, Y is a geometry factor, σ is the applied stress, and a is the crack half-length. Fracture occurs when K reaches K₃c for the material. This allows an engineer to calculate the maximum allowable stress for a component known to contain a defect of a given size, or conversely, the maximum tolerable crack size at a given stress level. Typical K₃c values: | Material | Condition | K₃c (MPa√m) | |----------|-----------|------------| | AISI 4340 steel | 1800 MPa tensile | 50 | | AISI 4340 steel | 1400 MPa tensile | 80 | | AISI 4340 steel | 1000 MPa tensile | 110 | | Ti-6Al-4V | Annealed | 55–70 | | 7075-T6 aluminum | | 24–30 | | 2024-T3 aluminum | | 34–44 | | AISI 316 stainless | Annealed | 200 | The trend with high-strength steel is clear: as tensile strength rises, K₃c falls sharply. Ultra-high-strength steels above 1600 MPa tensile strength have K₃c values in the range 50–80 MPa√m—they fracture with very little crack growth if a defect exists. This is why non-destructive inspection requirements are especially stringent for high-strength components. ## Izod Test The Izod test is similar in principle to Charpy but uses a vertically clamped specimen struck near the top. It is more common in older British and some European standards (IS0 180) and less commonly specified in current engineering practice than Charpy. The results are not directly comparable to Charpy values.