Impact Testing for Metals

Impact Testing for Metals – Evaluation of Toughness and Resistance to Brittle Fracture

As an ISO/IEC 17025 accredited (CNAS) independent laboratory, we provide professional impact testing for metals to support the mining, oil and gas, construction, transportation, and manufacturing industries in Angola. Impact testing measures the ability of a metal to absorb energy during high‑rate loading, typically under a sudden blow or shock. This property is critical for components that may be subjected to accidental impact, low temperatures, or rapid loading events – such as railway tracks, crane hooks, mining equipment, pressure vessels, and structural steelwork. Unlike tensile or hardness tests, impact testing reveals a material’s transition from ductile (safe) to brittle (catastrophic) behavior, especially at low temperatures. Our laboratory performs Charpy V‑notch (CVN) and Izod impact tests on a wide range of ferrous and non‑ferrous metals, using calibrated pendulum impact testers with energy capacities from 5 J to 300 J. Tests can be conducted at temperatures ranging from ‑60°C to +100°C using controlled heating or cooling baths. The results – absorbed energy (J), lateral expansion (mm), and fracture appearance – help engineers select materials that resist brittle fracture, qualify welding procedures, and comply with international design codes for safety‑critical applications.

Types of Metal Samples We Test

• Structural steels (plates, beams, angles, channels) for buildings, bridges, and offshore structures
• Pressure vessel steels (boiler plates, reactor shells, storage tanks, LPG cylinders)
• Pipeline steels (seamless and welded pipe materials for oil, gas, and water transmission)
• Railway steels (rails, wheels, axles, couplers, and track fasteners)
• Cast steels and iron (valve bodies, pump casings, gear blanks, mining crusher parts)
• Forging steels (crank shafts, connecting rods, turbine discs, flanges, crane hooks)
• Tool steels and high‑speed steels (for impact‑resistant tools, punches, dies)
• Non‑ferrous alloys (aluminum, copper, brass, bronze, titanium, nickel alloys) where impact performance is required
• Weld specimens (butt welds, heat‑affected zone, weld metal – for welding procedure qualification)
• Heat‑treated specimens (quenched and tempered, normalized, annealed) to evaluate the effect of heat treatment on toughness

Fundamental Concepts – Impact Energy, Ductile‑Brittle Transition, and Fracture Appearance

Impact testing subjects a notched specimen to a single high‑velocity blow from a pendulum. The energy absorbed during fracture is a measure of the material’s toughness. Metals that behave in a ductile manner absorb significant energy (higher impact values) and show a fibrous, shear‑type fracture with plastic deformation. Brittle metals absorb little energy (low impact values) and exhibit a shiny, crystalline fracture with no visible deformation. Many body‑centered cubic (BCC) metals – including carbon and low‑alloy steels – undergo a ductile‑brittle transition as temperature decreases. Above the transition temperature, the material is tough; below it, the material becomes brittle and can fail suddenly with little warning. The Charpy impact test quantifies this transition by measuring absorbed energy over a range of temperatures. Key parameters derived from the test are:

• Absorbed energy (J or ft·lbf) – The total energy required to break the specimen.
• Lateral expansion (mm) – The increase in width of the specimen on the compression side after fracture; a measure of plastic deformation.
• Shear fracture appearance (%) – The percentage of the fracture surface that appears fibrous (shear) vs. crystalline (cleavage).
• Ductile‑brittle transition temperature – The temperature at which the absorbed energy drops by a defined amount (e.g., 50% of the upper shelf) or the fracture appearance becomes 50% shear.

Specimen Preparation and Conditioning

Proper specimen preparation is essential for repeatable impact test results. All specimens are machined from the parent material, from a weld coupon, or from a finished component.

• Specimen geometry – The standard Charpy specimen is a square bar 55 mm long, 10 mm × 10 mm cross‑section, with a 2 mm deep V‑notch (45° included angle, 0.25 mm root radius) machined at the mid‑span. For materials with insufficient thickness (e.g., plates thinner than 10 mm), sub‑size specimens (e.g., 7.5 mm × 10 mm or 5 mm × 10 mm) are used, and the results are noted with the specimen size.
• Notch machining – The notch is broached or milled with a sharp, standardized tool. The notch root radius is critical; too sharp or too blunt changes the energy. We verify the notch geometry with an optical comparator before testing.
• Orientation – For anisotropic materials (rolled plates, forgings), the notch orientation relative to the rolling or grain direction is recorded (e.g., longitudinal, transverse, or through‑thickness).
• Temperature conditioning – Specimens are soaked in a temperature‑controlled bath (liquid or air) for at least 10 minutes (or 30 minutes for low temperatures) to reach the target test temperature. The bath temperature is controlled within ±1°C. Common test temperatures range from room temperature down to ‑60°C (using methanol with dry ice or a refrigeration unit) and up to +100°C (using heated oil or water).
• Transfer time – After removal from the bath, the specimen is placed on the anvils and broken within 5 seconds to minimize temperature drift. For low‑temperature tests, special tongs are used to avoid warming the specimen by hand contact.

Test Procedure – Charpy V‑Notch Impact Test

The Charpy impact test is performed on a pendulum impact machine. The following steps describe the standard procedure.

• Machine calibration – The pendulum is checked for friction losses and zero position. A calibration test using a certified reference specimen (of known energy) is performed periodically (typically weekly).
• Specimen alignment – The specimen is placed on the anvils of the impact machine with the notch facing away from the pendulum striker (i.e., the striker hits the side opposite the notch, creating a three‑point bending configuration). Centering is ensured so that the striker contacts the specimen at the mid‑span of the notch.
• Test execution – The pendulum is raised to its initial height (which defines the potential energy). It is released and swings down, striking and breaking the specimen in one blow. After breaking, the pendulum continues to a final height that is recorded. The energy absorbed by the specimen is the difference between the initial height and the final height, corrected for friction.
• Recording results – The absorbed energy is read directly from the machine dial or digital display. For each test condition (material, temperature, orientation) we test a minimum of three specimens. The individual values and the average are reported.
• Fracture examination – After testing, the two halves of the broken specimen are examined. We measure the lateral expansion (the increase in width on the compression side) using a dial gauge or digital caliper. We also estimate the percentage of shear (fibrous) fracture area compared to cleavage (crystalline) area, using a standard comparator chart or by visual examination under good lighting.

Temperature Series – Determining the Ductile‑Brittle Transition Curve

For materials that will be used in low‑temperature environments (e.g., offshore platforms in Angola’s coastal waters, pipelines in highland areas, refrigeration equipment), we perform impact tests over a range of temperatures to generate a transition curve.

• Specimens are prepared from the same material batch and heat treatment condition.
• Tests are performed at typically 6 to 10 different temperatures (e.g., room temperature, 0°C, ‑10°C, ‑20°C, ‑30°C, ‑40°C, ‑50°C, ‑60°C, and sometimes +50°C or +100°C for upper shelf determination).
• For each temperature, three specimens are tested and the average absorbed energy is plotted against temperature.
• The transition temperature is defined in one of three common ways: (1) the temperature at which the absorbed energy is 27 J (20 ft·lbf) – common for structural steels; (2) the temperature at which the fracture appearance is 50% shear; (3) the temperature at which the lateral expansion reaches 0.38 mm (for pipeline steels).
• The full curve (upper shelf energy, lower shelf energy, transition region) is reported, along with the determined transition temperature.

Factors That Influence Impact Test Results

• Temperature – The most significant factor for BCC metals. As temperature decreases, absorbed energy drops sharply.
• Notch geometry – A sharp, precise notch is essential. A blunt notch (rounded root radius) produces higher, non‑conservative energy values.
• Striker geometry and machine alignment – Worn strikers or mis‑alignment can cause side loading and erratic results. Our machine is verified annually with reference specimens.
• Specimen orientation – Transverse specimens (notch perpendicular to rolling direction) generally give lower impact energy than longitudinal specimens.
• Grain size and microstructure – Fine‑grained steels have higher impact toughness than coarse‑grained steels. Undesirable microstructures (e.g., bainite, untempered martensite) can reduce toughness.
• Strain rate (impact speed) – The Charpy test uses a fixed pendulum speed (typically 5–6 m/s). Higher speeds would increase apparent energy; lower speeds would reduce it. We maintain the standard pendulum length and release angle.
• Specimen thickness – Full‑size specimens (10 mm thickness) give the most representative results. Sub‑size specimens absorb less energy; results cannot be directly compared to full‑size without correction.

Quality Control and Interpretation of Results

• For each test condition, we test three specimens. If any individual result deviates more than 20% from the average (and the deviation is not caused by a known specimen defect), we test an additional three specimens.
• Certified reference specimens of known impact energy (traceable to a national metrology institute) are tested monthly to verify machine calibration.
• Results are compared with client‑supplied acceptance criteria (e.g., “minimum absorbed energy of 27 J at ‑20°C for three specimens, average 40 J”). If the criteria are met, the material or weld is declared “pass”; otherwise “fail”.
• When no acceptance criteria are provided, we report the measured values together with typical minimum requirements for the material grade (e.g., for structural steel grade S355, typical requirement 27 J at 0°C).

Reporting and Deliverables

Each impact test report includes the following information:

• Sample identification (material grade, heat number, component description, orientation, specimen size)
• Test temperature (°C) and conditioning method (liquid bath, air bath, time soaked)
• Individual absorbed energy values (J) for each of the three (or more) specimens, average, standard deviation, and the minimum individual value
• Lateral expansion (mm) and shear fracture appearance (%) – for each specimen or as average, upon request
• For temperature series: a table and graph of absorbed energy vs. temperature, with the determined transition temperature clearly marked
• Photographs of the fracture surfaces (if requested), showing the characteristic shear lip and crystalline area
• Comparison with client‑supplied acceptance criteria (if provided) – pass/fail conclusion
• Statement that the test was performed in accordance with our accredited ISO/IEC 17025 procedures
• Raw data (temperature logs, machine calibration records) archived for 10 years

No statement of compliance with any external standard or regulation is made unless the client has provided specific acceptance criteria in writing. The report is intended for material qualification, welding procedure approval, and design verification.

Practical Applications for Angolan Industries

• Offshore oil and gas platforms: Charpy testing of structural steel and welds at low temperatures (down to ‑20°C) to ensure resistance to brittle fracture during cold weather or rapid gas decompression events.
• Mining equipment (shovel buckets, crusher mantles, grinding mill liners): Impact testing of cast steels and manganese steels to verify toughness under rock impact.
• Railway infrastructure: Testing of rail steel and wheel steel to prevent brittle fracture in Angola’s rail network, especially in regions with significant diurnal temperature variation.
• Pressure vessels and storage tanks: Impact qualification of steel used for LPG spheres, ammonia tanks, and high‑pressure gas cylinders, including post‑weld heat treatment verification.
• Construction (bridges, high‑rise buildings): Ensuring that structural steels (beams, columns, connections) have adequate toughness at the lowest expected service temperature in Luanda, Lubango, or other regions.