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4.2 Metal Hardness Testing (Rockwell, Brinell, Vickers)

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FAA Aviation Mechanic — General AMG

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1 Chapter 1: Introduction & AMG Exam Overview

1.1 About the FAA Aviation Mechanic General Knowledge Test 1.2 How to Study for the AMG Exam

2 Chapter 2: Maintenance Forms, Records, Publications & Regulations

2.1 14 CFR Parts 43, 65, and 91 Regulatory Framework 2.2 Aviation Mechanic Privileges, Limitations & Recent Experience 2.3 Maintenance Recordkeeping & Return-to-Service Entries (14 CFR 43.9 & 43.11) 2.4 FAA Form 337 Major Repairs and Major Alterations 2.5 Airworthiness Directives (ADs) & Mandatory Compliance 2.6 Type Certificate Data Sheets (TCDS), STCs, ACs & Maintenance Manuals 2.7 FAA Forms, MMEL/MEL, Inoperative Equipment & Parts Effectivity

3 Chapter 3: Basic Electricity

3.1 DC Electrical Circuits, Ohm's Law & Power 3.2 AC Electrical Fundamentals, Reactance & Impedance 3.3 Aircraft Lead-Acid & Nickel-Cadmium Storage Batteries 3.4 DC Generators, Alternators & Electric Motors 3.5 Aircraft Wiring, Bonding & Circuit Protection Devices 3.6 Resistor Color Codes, Semiconductors, Digital Logic & Electrostatic Discharge

4 Chapter 4: Materials & Processes

4.1 Aircraft Ferrous and Nonferrous Metals & Heat Treatment 4.2 Metal Hardness Testing (Rockwell, Brinell, Vickers) 4.3 Nondestructive Testing (NDT/NDI) Inspection Methods 4.4 Aircraft Hardware, Fasteners & Safetying Methods 4.5 Composite Materials, Plastics & Control Cables 4.6 Soldering, Welding & Weld Quality Inspection

5 Chapter 5: Cleaning & Corrosion Control

5.1 Corrosion Types, Chemistry & Identification 5.2 Corrosion Removal, Chemical Conversion Coatings & Surface Treatments 5.3 Aircraft Cleaning Agents, Precautions & Preventative Maintenance 5.4 Aircraft Finishing Systems, Registration Markings & Placards

6 Chapter 6: Weight & Balance

6.1 Weight and Balance Terminology, Datum, Arm & Moment Principles 6.2 Aircraft Weighing Procedures, Scale Reactions & Tare Weight 6.3 Center of Gravity Computations, Extreme Loading & Ballast

7 Chapter 7: Ground Operation, Servicing & Safety

7.1 Engine Starting Procedures, Danger Areas & Ground Taxi Safety 7.2 Aircraft Servicing (Oxygen, Hydraulic, Pneumatic & De-Icing Fluids) 7.3 Aircraft Fueling Safety, Static Grounding & Hangar Fire Protection 7.4 Airport Movement Areas, Hazardous Materials, Tool Control & FOD Prevention

8 Chapter 8: Aircraft Drawings

8.1 Aircraft Drawing Types, Title Blocks & Orthographic Projections 8.2 Line Conventions, Dimensions, Tolerances & Material Symbols 8.3 Electrical Wiring Diagrams, Logic Flowcharts & Fluid Schematics

9 Chapter 9: Fluid Lines & Fittings

9.1 Rigid Metal Tubing Fabrication, Bending & 37° AN Flaring 9.2 Flexible Hose Construction, Pressure Ratings & Lay Lines 9.3 AN/MS/Quick-Disconnect Fittings, Identification & Installation

10 Chapter 10: Mathematics for Aviation Maintenance

10.1 Fractions, Decimals, Percentages & Ratio/Proportion 10.2 Geometric Computations (Area, Volume & Cylinder Displacement) 10.3 Algebraic Equations, Powers, Roots & Maintenance Conversions

11 Chapter 11: Physics for Aviation Maintenance

11.1 Matter, Mass, Weight, Density & Specific Gravity 11.2 Work, Energy, Power & Simple Machines (Mechanical Advantage) 11.3 Fluid Mechanics, Pascal's Law & Bernoulli's Principle 11.4 Thermodynamics, Heat Transfer, Gas Laws, Sound & Aerodynamics 11.5 Standard Atmosphere, Density & Pressure Altitude, Flight Controls & Aerodynamic Devices

12 Chapter 12: Inspection Concepts, Techniques & Precision Measurement

12.1 Aircraft Inspection Programs & Inspection Concepts 12.2 Precision Measuring Tools & Tool Calibration

13 Chapter 13: Human Factors in Aviation Maintenance

13.1 Human Error, the Dirty Dozen & Maintenance Safety Culture 13.2 Human Performance, Limitations, Fatigue & the Work Environment 13.3 Communication, Shift Turnover, Event Investigation & Professionalism

4.2 Metal Hardness Testing (Rockwell, Brinell, Vickers)

Key Takeaways Explain This Topic AI

Indentation hardness testing measures a material's resistance to localized permanent plastic deformation, providing a non-destructive verification of heat treatment and correlating directly to ultimate tensile strength ( U T S ≈ 500 × B H N UTS \approx 500 \times BHN U TS ≈ 500 × B H N for carbon and low-alloy steels).

Rockwell hardness testing measures the net increase in indentation depth between a 10 kg minor preload and a major load (60 kg, 100 kg, or 150 kg), utilizing a 1/16-inch steel ball (B Scale, red dial) for soft metals and a 120° diamond Brale penetrator (C Scale, black dial) for hardened steels.

Brinell hardness testing applies a 10 mm hardened steel or carbide sphere under 500 kg (soft metals), 1500 kg, or 3000 kg (steels) loads, requiring optical measurement of the indentation diameter with a calibrated microscope to compute the Brinell Hardness Number (BHN).

Vickers (136° square-base pyramid) and Knoop (elongated rhombic diamond) microhardness testers use ultra-low loads (1 g to 1000 g) to evaluate thin foils, electroplated surfaces, and shallow nitrided case depth profiles.

Accurate testing mandates that test specimens have a minimum thickness of at least 8 to 10 times the indentation depth (to avoid the anvil effect) and maintain edge/spacing margins of at least 2.5 to 3 indentation diameters from edges and previous indentations.

Last updated: August 2026

4.2 Metal Hardness Testing (Rockwell, Brinell, Vickers)

Hardness testing is the primary non-destructive quality control method utilized by aviation maintenance technicians and metallurgical inspectors to verify that structural metals, forgings, engine components, and landing gear assemblies have achieved their specified heat treatment, temper, and case depth. Indentation hardness directly reflects a metal's yield strength, wear resistance, and ultimate tensile strength.

1. Principles of Indentation Hardness Testing

In mechanical metallurgy, hardness is defined as the resistance of a material to localized permanent plastic deformation, indentation, scratching, or abrasion. In standard aviation testing, a standardized penetrator (indenter) is pressed into the surface of the test specimen under a precisely controlled static load for a specified dwell time. Hardness Indentation Mechanics & Stress Field:

  • [ Applied Static Test Load (P) ]
  • │
  • ▼
  • ▼ [ Penetrator / Indenter ] ▼
  • ═════════════════════════════════════════ <-- Specimen Surface
  • ░░░░░░░░░░░░░( Indentation )░░░░░░░░░░░░░
  • ▒▒▒▒▒▒▒[ Plastic Deformation Zone ]▒▒▒▒▒▒ <-- Work-Hardened Zone
  • ░░░░░░░[ Elastic Deformation Zone ]░░░░░░
  • ═════════════════════════════════════════ <-- Anvil Support
  • Tensile Strength Correlation

For plain carbon and low-alloy aircraft steels (such as 1025, 4130, and 4340), indentation hardness exhibits an almost perfect linear correlation to Ultimate Tensile Strength (UTS) : UTS (psi) ≈ 500 × BHN \text{UTS (psi)} \approx 500 \times \text{BHN} UTS (psi) ≈ 500 × BHN UTS (MPa) ≈ 3.45 × BHN \text{UTS (MPa)} \approx 3.45 \times \text{BHN} UTS (MPa) ≈ 3.45 × BHN

This empirical relationship allows technicians to verify the tensile strength class of critical airframe fittings non-destructively without machining destructive tensile test coupons.

2. Rockwell Hardness Testing (ASTM E18)

The Rockwell hardness test is the most widely applied hardness testing method in aviation manufacturing and maintenance due to its rapid operation, direct digital/dial readout, and elimination of optical measurement errors. Rockwell Hardness Test Cycle (Minor vs. Major Load):

  • Step 1: Apply Minor Load (10 kg)
  • Seats indenter, pierces surface oxide/oil --> Establishes Datum Line (0)
  • Step 2: Apply Major Load (60, 100, or 150 kg)
  • Indenter penetrates to total depth under full load
  • Step 3: Release Major Load (Retaining Minor Load 10 kg)
  • Elastic recovery occurs --> Net permanent depth increase (e) is measured
  • Rockwell Number = E - e (Directly displayed on dial/digital screen)
  • The Two-Step Load Principle

Minor Preload (10 kg): An initial static preload of 10 kg 10\text{ kg} 10 kg is applied. This seats the penetrator into the specimen, breaks through superficial oil, grease, or surface roughness, and establishes a zero-reference baseline datum ( 0 0 0 set on the dial gauge).

Major Test Load (60 kg, 100 kg, or 150 kg): The major load is applied smoothly via a dashpot mechanism, driving the penetrator into the metal. Once the dwell period ( 2 to 4 seconds 2\text{ to }4\text{ seconds} 2 to 4 seconds ) is complete, the major load is released while the 10 kg 10\text{ kg} 10 kg minor preload is maintained.

Measurement: The dial gauge or digital sensor measures the net permanent increase in penetration depth ( e e e ) between the minor load and major load states. Each single Rockwell point on the dial corresponds to a vertical penetration depth of 0.002 mm 0.002\text{ mm} 0.002 mm ( 0.00008 inches 0.00008\text{ inches} 0.00008 inches ) .

Rockwell Scales, Penetrators & Colors

Rockwell Scale Penetrator Type Major Load Dial Color Aviation Material Applications

Rockwell C (HRC) 120 ∘ 120^\circ 12 0 ∘ Diamond Spheroconical ( Brale Penetrator ) 150 kg 150\text{ kg} 150 kg BLACK Hardened alloy steels ( 4130 , 4340 4130, 4340 4130 , 4340 ), heat-treated landing gear components, crankshafts, turbine shafts ( 20 − 70 HRC 20 - 70\text{ HRC} 20 − 70 HRC ).

Rockwell B (HRB) 1 / 16 -inch 1/16\text{-inch} 1/16 -inch ( 1.588 mm 1.588\text{ mm} 1.588 mm ) Hardened Steel Ball 100 kg 100\text{ kg} 100 kg RED Annealed steels, structural aluminum alloys ( 2024 -T3 , 7075 -T6 2024\text{-T3}, 7075\text{-T6} 2024 -T3 , 7075 -T6 ), brass, bronze, soft metals ( 0 − 100 HRB 0 - 100\text{ HRB} 0 − 100 HRB ).

Rockwell A (HRA) 120 ∘ 120^\circ 12 0 ∘ Diamond Brale 60 kg 60\text{ kg} 60 kg BLACK Extremely thin sheet steel, shallow case-carburized surfaces, tungsten carbide inserts ( 60 − 88 HRA 60 - 88\text{ HRA} 60 − 88 HRA ).

Rockwell D (HRD) 120 ∘ 120^\circ 12 0 ∘ Diamond Brale 100 kg 100\text{ kg} 100 kg BLACK Thin case-hardened steel surfaces.

Rockwell E (HRE) 1 / 8 -inch 1/8\text{-inch} 1/8 -inch Steel Ball 100 kg 100\text{ kg} 100 kg RED Cast iron, soft aluminum/magnesium alloys.

CRITICAL EXAM RULE: Never test soft metals (like annealed aluminum or copper) on the Rockwell C scale because the severe 150 kg 150\text{ kg} 150 kg load will drive the diamond Brale into the backing anvil, destroying the delicate 0.2 mm 0.2\text{ mm} 0.2 mm diamond tip radius. Always select the B scale for aluminum and unhardened alloys, and the C scale for hardened aircraft steels.

Loading diagram... Rockwell Scale Dial and Penetrator Configuration

3. Brinell Hardness Testing (ASTM E10)

The Brinell hardness test is the primary industrial method for testing heavy structural forgings, sand castings, engine master rods, and coarse-grained metals where a large indentation is necessary to average out microscopic metallurgical variations. Brinell Optical Indentation Measurement:

  • [ 10 mm Hardened Steel / Carbide Ball (D) ]
  • │ (Load P: 500, 1500, or 3000 kg)
  • ▼
  • ┌─────────────────────────────────────┐
  • │ ╲ ╱ │
  • ──────┘ ╲ ╱ └────── <-- Surface
  • ░░░░░░░░░░░░░░░░( Indentation d )░░░░░░░░░░░░░░░░░░
  • |<------- d -------->|
  • Brinell Test Mechanics

Penetrator: A polished 10 mm 10\text{ mm} 10 mm ( 0.3937 inch 0.3937\text{ inch} 0.3937 inch ) diameter hardened steel ball (for materials under 450 HBW 450\text{ HBW} 450 HBW ) or tungsten carbide ball (for materials up to 650 HBW 650\text{ HBW} 650 HBW ).

Standard Static Test Loads & Dwell Times:

3000 kg 3000\text{ kg} 3000 kg Load: Standard for ferrous metals, structural steels, and high-strength titanium alloys. Dwell time: 10 to 15 seconds 10\text{ to }15\text{ seconds} 10 to 15 seconds .

1500 kg 1500\text{ kg} 1500 kg Load: Used for brass, bronze, and intermediate copper alloys.

500 kg 500\text{ kg} 500 kg Load: Standard for structural aluminum alloys, magnesium alloys, and soft nonferrous metals. Dwell time: 30 seconds 30\text{ seconds} 30 seconds (longer dwell allows plastic creep to stabilize).

Optical Diameter Measurement & BHN Formula

Unlike Rockwell testing, Brinell hardness is calculated by measuring the spherical surface area of the indentation . The diameter of the round impression ( d d d ) is measured in two mutually perpendicular directions using a calibrated optical microscope with an internal reticle ( 0.05 mm 0.05\text{ mm} 0.05 mm divisions), and the average diameter is inserted into the Brinell equation: BHN = 2 P π D ( D − D 2 − d 2 ) \text{BHN} = \frac{2P}{\pi D \left(D - \sqrt{D^2 - d^2}\right)} BHN = π D ( D − D 2 − d 2 ​ ) 2 P ​

Where:

P = Applied static test load (in kilograms, kg) P = \text{Applied static test load (in kilograms, kg)} P = Applied static test load (in kilograms, kg)

D = Diameter of the ball penetrator (standard 10 mm) D = \text{Diameter of the ball penetrator (standard } 10\text{ mm)} D = Diameter of the ball penetrator (standard 10 mm)

d = Average measured diameter of the indentation (in millimeters, mm) d = \text{Average measured diameter of the indentation (in millimeters, mm)} d = Average measured diameter of the indentation (in millimeters, mm)

Brinell vs. Rockwell Comparative Characteristics

Parameter Brinell Hardness Test Rockwell Hardness Test

Indentation Size Large ( 2.0 − 6.0 mm 2.0 - 6.0\text{ mm} 2.0 − 6.0 mm diameter impression) Small / microscopic ( < 0.5 mm <0.5\text{ mm} < 0.5 mm depth)

Measurement Basis Optical diameter of spherical impression ( d d d ) Mechanical depth of penetration ( e e e )

Speed of Test Slower (requires microscopic measurement & calculation) Rapid ( 5 − 10 seconds 5 - 10\text{ seconds} 5 − 10 seconds , direct dial/digital readout)

Best Application Heavy forgings, castings, non-homogeneous metals Precision machined parts, finished aircraft fittings, thin sheets

4. Vickers & Knoop Microhardness Testing (ASTM E92 / E384)

Microhardness testing applies ultra-light loads ( 1 gf 1\text{ gf} 1 gf to 1000 gf 1000\text{ gf} 1000 gf ) to evaluate microscopic metallurgical structures, extremely thin foils, electroplated coatings (such as cadmium, hard chrome, or nickel), and to map the hardness gradient across nitrided or carburized case layers. Vickers vs. Knoop Penetrator Geometries:

  • Vickers Indenter (136° Square Pyramid) Knoop Indenter (Elongated Rhombic)
  • ┌─────────┐ ┌───────────────┐
  • │ ▲ │ │ ▲ │
  • │ ◄ █ ► │ │ ◄─────█─────► │
  • │ ▼ │ │ ▼ │
  • └─────────┘ └───────────────┘
  • Square Impression (d1 = d2) Long Diagonal L = 7.11 × Width W
  • Equal Stress in All Directions Shallow Depth (Ideal for Thin Coatings)
  • 1. Vickers Hardness Test (HV)

Penetrator: A geometrically perfect 136 ∘ 136^\circ 13 6 ∘ square-base diamond pyramid.

Impression: Produces a sharp square indentation. Both diagonals ( d 1 , d 2 d_1, d_2 d 1 ​ , d 2 ​ ) are measured optically and averaged: H V = 1.8544 × P d 2 HV = \frac{1.8544 \times P}{d^2} H V = d 2 1.8544 × P ​

(where P P P is load in kgf and d d d is average diagonal length in mm).

Advantage: The square diamond indenter produces geometrically similar indentations regardless of test load magnitude, providing a continuous, seamless hardness scale from soft pure aluminum ( 20 HV 20\text{ HV} 20 HV ) to ultra-hard tungsten carbide ( 1500 HV 1500\text{ HV} 1500 HV ).

2. Knoop Hardness Test (HK)

Penetrator: An elongated, rhombic-base diamond pyramid with a longitudinal-to-transverse diagonal ratio of 7.11 : 1 7.11 : 1 7.11 : 1 .

Impression: Produces a long, narrow, shallow diamond impression. Only the long diagonal ( L L L ) is measured: H K = 14.229 × P L 2 HK = \frac{14.229 \times P}{L^2} HK = L 2 14.229 × P ​

Aviation Application: Because the penetration depth is only about one-third of a Vickers indentation under identical loads, Knoop testing is the industry standard for measuring the hardness profile of nitrided engine cylinder cases at 0.001 -inch 0.001\text{-inch} 0.001 -inch depth intervals without cracking the brittle outer white layer ( F e 2 N Fe_2N F e 2 ​ N ).

5. Portable Hardness Testers for Aviation Maintenance

When structural assemblies cannot be disassembled or transported to a laboratory bench tester, FAA-approved portable hardness testers are utilized on the flight line and in sheet metal repair shops.

1. Webster Hardness Tester (Model B)

Design: Handheld, plier-action portable tester.

Operation: The operator places an aluminum sheet between the anvil and the spring-loaded penetrator and squeezes the handles until bottomed. The penetrator consists of four flat-faced cylindrical points.

Readout: Dial scale reads directly from 0 to 20 Webster units 0\text{ to }20\text{ Webster units} 0 to 20 Webster units .

Aviation Role: Rapid verification of wrought aluminum tempers in sheet metal shops (e.g., distinguishing 2024-O, 2024-T3, and 6061-T6 sheet before riveting).

2. Barcol Impressor (Model 934-1)

Design: Handheld, spring-loaded direct-reading indentor conforming to ASTM D2583.

Penetrator: A hardened steel truncated cone with an included angle of 26 ∘ 26^\circ 2 6 ∘ and a flat tip diameter of 0.0062 inches 0.0062\text{ inches} 0.0062 inches ( 0.157 mm 0.157\text{ mm} 0.157 mm ).

Readout: Direct dial readout from 0 to 100 Barcol units 0\text{ to }100\text{ Barcol units} 0 to 100 Barcol units .

Aviation Role: In accordance with AC 43.13-1B , the Barcol Impressor is utilized for testing the hardness of aluminum alloys, soft nonferrous metals, and specifically for verifying the cure state of structural composite matrix resins (epoxies), fiberglass laminates, and transparent plastics after hot-bond repairs.

6. Testing Limitations, Precautions & Anvil Effects

Improper testing procedures yield invalid readings that can result in the rejection of airworthy components or the installation of unhardened parts. Anvil Effect & Indentation Spacing Constraints:

  • ═══════════════════════════════════════════════════
  • ◄─── 2.5d ───► ◄───── 3.0d ─────►
  • [Edge Margin] [Center Spacing]
  • ▼ ▼
  • ( d ) ( d )
  • ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░ ▲
  • ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ │ Thickness ≥ 8d to 10d
  • ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░ ▼
  • ═══════════════════════════════════════════════════ <-- Hard Anvil Support
  • 1. Minimum Specimen Thickness & The 'Anvil Effect'

Governing Rule: The specimen thickness must be at least 8 to 10 times 8\text{ to }10\text{ times} 8 to 10 times the indentation depth for Rockwell testing, and at least 10 times 10\text{ times} 10 times the indentation depth for Brinell testing.

The Anvil Effect: If a thin specimen is tested, the plastic deformation zone under the indenter penetrates completely through the sheet and contacts the hardened steel anvil. The indenter senses the rigid anvil rather than the test metal, yielding falsely high hardness readings . If an indentation mark or shiny spot is visible on the bottom surface of the test piece, the test is invalid due to anvil effect.

2. Edge Distance & Indentation Spacing

Edge Margin: The minimum distance from the center of any indentation to the edge of the test specimen must be at least 2.5 to 3.0 times 2.5\text{ to }3.0\text{ times} 2.5 to 3.0 times the indentation diameter ( 2.5 d 2.5d 2.5 d ) . Testing closer to an edge allows the metal to bulge outward laterally, yielding falsely low hardness values.

Spacing Between Impressions: The center-to-center distance between adjacent indentations must be at least 3.0 to 5.0 times 3.0\text{ to }5.0\text{ times} 3.0 to 5.0 times the indentation diameter ( 3 d 3d 3 d ) . Testing too close to a previous test site drives the indenter into the strain-hardened plastic zone of the prior impression, yielding falsely elevated hardness readings.

3. Surface Preparation & Decarburization

Test surfaces must be clean, smooth, parallel to the anvil, and free of paint, scale, rust, and heavy oil. Hot-rolled or heat-treated steels exhibit a surface decarburized layer that must be ground off before measuring core hardness.

Curved Surfaces: Testing on cylindrical shafts or curved tubing requires mathematical correction factors added to the dial reading to compensate for reduced lateral support.