Chapter 2: Testing, Inspecting and Repairing Steel Parts

Why materials are tested

Tests confirm that a material meets its specification and that heat treatment has produced the required properties. Some tests are destructive (a specimen is broken — tensile, impact, fatigue) and are done by manufacturers and laboratories; others are non-destructive and are used in maintenance to find damage without harming the part.

The tensile test

• Stress = load ÷ cross-sectional area; strain = change in length ÷ original length.
• In the elastic region stress is proportional to strain (Hooke's law); the slope is Young's modulus (stiffness). Steel's modulus is about three times aluminium's.
• At the elastic limit / yield point permanent (plastic) deformation begins. Design limit loads keep structure below yield.
• The highest point is the ultimate tensile strength (UTS). The specimen then necks and fractures.
• Percentage elongation and reduction of area at fracture measure ductility.

Hardness testing

• Brinell — a 10 mm hardened steel ball is pressed in with 3 000 kg for steel (500 kg for softer metals); the diameter of the impression is measured with a microscope and converted to a Brinell hardness number (BHN). Large impression — not for thin or finished parts.
• Rockwell — a minor load (10 kg) seats the indenter, the major load is applied, and the depth of penetration is read directly from a dial. The C scale (diamond "Brale" cone, 150 kg) is for hardened steel; the B scale (1/16 in ball, 100 kg) for softer steels and non-ferrous metals. Quick and widely used in workshops.
• Vickers — a 136° square diamond pyramid; the diagonals of the impression are measured. Works on any material, thin sections and case-hardened layers.
• Shore scleroscope — measures the rebound height of a diamond-tipped hammer; leaves almost no mark. Barcol is a portable tester used on aluminium and composites (Chapter 4 and 6).
• Because hardness and tensile strength are related, a hardness reading is a practical check that steel has been correctly heat-treated.

Impact and fatigue testing

• Charpy (specimen supported at both ends) and Izod (specimen held as a cantilever) break a notched bar with a swinging pendulum; the energy absorbed shows toughness and notch sensitivity. Steels become brittle at low temperatures — impact tests at several temperatures reveal this.
• Fatigue tests apply millions of load cycles. Plotting stress against cycles to failure gives the S-N curve. Ferrous metals show an endurance (fatigue) limit; aluminium alloys do not, so they must be given a safe life or inspected for cracks.
• Creep tests measure slow stretching under load at high temperature (turbine parts).

Non-destructive inspection of steel parts

• Visual — the first method: good lighting, magnifier, mirror and borescope; look for cracks, corrosion, dents, gouges, wear and heat discoloration.
• Magnetic particle — for ferromagnetic materials only. The part is magnetised and fine iron particles (often fluorescent, viewed under UV light) collect where a crack disturbs the magnetic field. It finds surface and near-surface cracks; the field must cross the crack, so parts are magnetised in two directions. Parts must be demagnetised afterwards.
• Dye (liquid) penetrant — finds defects open to the surface in any non-porous material.
• Eddy current — finds surface and near-surface cracks in conductive materials, and around fastener holes.
• Ultrasonic — finds internal flaws and measures thickness. Radiography (X-ray) shows internal flaws and assembly faults.

Repair and inspection procedures

• Highly stressed steel parts (bolts, gear components, fittings) are normally replaced, not repaired. Minor nicks and corrosion may be blended out within the limits of the SRM or CMM, followed by NDT to prove no crack remains, and the protective finish restored.
• Welding destroys heat treatment. Parts heat-treated to high strength must not be welded or straightened unless approved data allows it.
• Welded 4130 tube structures can be repaired by approved methods (sleeve splices, inner sleeves, fishmouth or scarf joints — AC 43.13-1B), usually followed by normalising.
• Hydrogen embrittlement: acid cleaning and electroplating can let hydrogen into high-strength steel, making it brittle. Plated parts are baked after plating, and acid strippers are kept off high-strength steel.
• Protection: cadmium plating (sacrificial — it corrodes first, leaving white deposits), zinc-nickel, chromium plating on wear surfaces such as landing-gear piston rods (ground parts are crack-checked), phosphate coatings, primers and paint. Rust is removed mechanically — abrasive paper or a wire brush of steel, never one used on aluminium.