1.3 Mechanical & Non-Destructive Testing of Metals

1.3 Mechanical & Non-Destructive Testing of Metals

Airworthiness certification and continuing airworthiness management require quantitative verification of material integrity. Testing divides into two fundamental branches: Destructive Mechanical Testing, in which standardized test specimens are loaded to failure to establish baseline design properties, and Non-Destructive Testing (NDT), where operational aircraft components are inspected for defects without impairing their future serviceability.

Under EASA Part-66 Module 06, technicians must be proficient in interpreting tensile curves, hardness scales, impact energy data, fatigue mechanisms, and NDT inspection principles.

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Destructive Mechanical Testing: The Tensile Test

The tensile test (conducted per ASTM E8 or EN ISO 6892) measures the response of a metal to uniaxial tensile loading. A standardized machined "dog-bone" specimen with original gauge length ($L_0$) and cross-sectional area ($A_0$) is pulled at a controlled strain rate until fracture.

Stress (σ) [MPa]
     ^
 UTS |              * Peak (Ultimate Tensile Strength, Rm)
(Rm) |            /   \
     |           /     \  Necking begins
 Re/ |          * Yield Point (Rp0.2 / 0.2% Offset)
Rp0.2|         /         \
     |        /           * Fracture (Ru)
     |       / Elastic    |
     |      /  Region     | Plastic Deformation Region
     |     /              |
     |    / Hooke's Law   |
     |---* Proportional   |
     |   | Limit (σ = E·ε)|
     └───┴────────────────┴───────────────────────> Strain (ε) [mm/mm]
       0.002 (0.2% Offset)

Stress-Strain Terminology and Parameters

1. Engineering Stress ($\sigma$): Applied load ($P$) divided by the original cross-sectional area ($A_0$): $\sigma = \frac{P}{A_0} \quad [\text{MPa or psi}]$
2. Engineering Strain ($\epsilon$): Elongation ($\Delta L = L - L_0$) divided by the original gauge length ($L_0$): $\epsilon = \frac{L - L_0}{L_0} = \frac{\Delta L}{L_0} \quad [\text{dimensionless or } \%]$
3. Modulus of Elasticity (Young's Modulus, $E$): The slope of the linear elastic portion of the stress-strain curve where Hooke's Law applies ($\sigma = E \cdot \epsilon$). For virtually all aircraft steels, Young's modulus is approximately 205 to 210 GPa ($30 \times 10^6\text{ psi}$), irrespective of whether the steel is annealed, normalised, or heat-treated to maximum hardness.
4. Proportional Limit and Elastic Limit: The highest stress at which stress remains directly proportional to strain (proportional limit), and the maximum stress a material can withstand without permanent plastic deformation upon unloading (elastic limit).
5. Yield Strength / 0.2% Proof Stress ($R_{p0.2}$): Most high-strength alloy steels do not display a sharp, distinct yield point drop. Instead, yield strength is measured using the 0.2% offset method: a line is drawn parallel to the linear elastic slope starting at a strain of 0.002 (0.2%). The stress where this line intersects the stress-strain curve is designated the 0.2% Proof Stress ($R_{p0.2}$).
6. Ultimate Tensile Strength (UTS, $R_m$): The maximum engineering stress sustained by the specimen before localized cross-sectional thinning (necking) begins.
7. Ductility Measures:
   • Percentage Elongation (% EL): Measures overall plastic tensile stretch: $\%\text{EL} = \left( \frac{L_f - L_0}{L_0} \right) \times 100$
   • Percentage Reduction of Area (% RA): Measures localized necking capability: $\%\text{RA} = \left( \frac{A_0 - A_f}{A_0} \right) \times 100$

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Hardness Testing of Ferrous Metals

Hardness is defined as the resistance of a material to localized plastic deformation, penetration, or scratching. Because hardness correlates directly with ultimate tensile strength, it serves as the most widely used quality control inspection in aircraft maintenance.

Rockwell Hardness Testing Principles

The Rockwell test is the standard workshop test because it measures the differential depth of penetration, eliminating optical measurement errors and providing an immediate digital readout. The sequence is strictly standardized:

1. Apply minor load of 10 kgf: Seats the indenter through surface oxides and paint remnants into virgin metal, establishing the zero reference datum plane.
2. Apply major load (e.g., additional 140 kgf for HRC, bringing total load to 150 kgf): Causes plastic indentation.
3. Release major load back to minor load (10 kgf): The instrument measures the permanent increment in indentation depth ($e$), displaying the hardness number directly.

Correlation Between Hardness and Tensile Strength

For carbon and low-alloy steels (such as SAE 4130 and 4340), there is a direct empirical relationship between Brinell hardness and ultimate tensile strength:

$\text{UTS (psi)} \approx 500 \times \text{BHN}$ $\text{UTS (MPa)} \approx 3.45 \times \text{BHN}$

Example: An SAE 4340 component testing at 300 BHN has an approximate ultimate tensile strength of $300 \times 500 = 150,000\text{ psi}$ (approx. 1035 MPa).

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Impact Testing & Ductile-to-Brittle Transition

Impact testing evaluates a material's notch toughness—its capacity to absorb mechanical energy during sudden, high-velocity dynamic fracture in the presence of a sharp stress concentration.

Charpy V-Notch vs. Izod

• Charpy V-Notch (ASTM E23 / ISO 148): The standard test piece (10 mm × 10 mm × 55 mm with a 2 mm deep 45° V-notch) is supported horizontally at both ends as a simple beam. A heavy pendulum hammer is released from height $h_1$ and strikes the specimen directly behind the notch. The hammer fractures the specimen and swings up to height $h_2$. The energy absorbed by the fracture (in Joules or ft-lb) is calculated from the height difference.
• Izod Test: The specimen is clamped vertically as a cantilever beam and struck by the pendulum on the notched face above the clamp.

Ductile-to-Brittle Transition Temperature (DBTT)

When impact specimens of Body-Centered Cubic (BCC) metals (such as carbon steels, low-alloy 4130/4340, and ferritic/martensitic stainless steels) are tested across a range of temperatures, they exhibit a sudden, dramatic drop in energy absorption over a narrow temperature band known as the Ductile-to-Brittle Transition Temperature (DBTT).

• Above DBTT: The fracture surface is dull, fibrous, and displays shear lips (ductile microvoid coalescence, absorbing high energy, e.g., 60–100 J).
• Below DBTT: The fracture is bright, highly reflective, crystalline, and flat (brittle cleavage along crystallographic planes, absorbing very low energy, e.g., < 10 J).

Critical Aviation Rule: Commercial jet aircraft cruise at altitudes of 35,000 to 43,000 feet where ambient outside air temperatures reach -55°C to -60°C (-67°F to -76°F). Any ferrous alloy used in external structures must have its DBTT well below -65°C to prevent catastrophic brittle shatter during landing impacts. Crucially, Face-Centered Cubic (FCC) metals (austenitic stainless steels 304/316/321, aluminum alloys, titanium) DO NOT exhibit a DBTT; they maintain high toughness and ductility even at cryogenic temperatures (-196°C).

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Fatigue Testing and the S-N Curve

Fatigue is the progressive, localized structural damage that occurs when a component is subjected to fluctuating cyclic stresses well below its static tensile yield strength. Fatigue accounts for more than 80% of all in-service mechanical failures in aircraft structures.

The S-N Curve (Wöhler Diagram)

Fatigue performance is mapped by testing specimens at various cyclic stress amplitudes ($S$) and recording the number of cycles to failure ($N$, plotted on a logarithmic scale).

Cyclic Stress Amplitude (S)
     ^
     |
     |  * (High Stress - Low Cycle Fatigue)
     |    \
     |     \   Aluminium / Non-Ferrous (No Fatigue Limit)
     |      \            *-------------------
     |       \            \
     |        \            \
  Se |---------*============\======================== Ferrous Steels (True Fatigue Limit, Se)
     |          \            \
     |           \            *
     └────────────┴────────────┴─────────────────────> Cycles to Failure (N, log scale)
                 10^6         10^7

Ferrous Endurance Limit vs. Non-Ferrous Alloys

• Ferrous Metals (Steels): Display a distinct horizontal asymptote on the S-N curve known as the Fatigue Limit (or Endurance Limit, $S_e$), typically occurring between $10^6$ and $10^7$ cycles. For polished steel specimens, $S_e$ is approximately 40% to 50% of the Ultimate Tensile Strength (UTS). If cyclic stress amplitudes remain below this endurance limit, the component can theoretically withstand an infinite number of cycles without failing.
• Non-Ferrous Alloys (Aluminium, Titanium, Copper): Do not possess a true fatigue limit. Their S-N curves continue to slope downward indefinitely. For non-ferrous parts, engineers must define fatigue strength at a specific design life (e.g., $10^7$ or $10^8$ cycles).

Morphology of Fatigue Failure

Examining an aircraft fatigue fracture under magnification reveals three distinct zones:

1. Crack Initiation Site: Typically at a surface stress concentration (sharp radius, corrosion pit, tool score mark, inclusion).
2. Fatigue Crack Propagation Area: Characterized by smooth, burnished surfaces displaying concentric "beach marks" (clamshell arrest marks) that represent the progressive advance of the crack front during successive flight cycles.
3. Fast Fracture Overload Zone: The final remaining cross-section suddenly fails in static overload (coarse, dull ductile shear or bright brittle cleavage) when it can no longer support the operating load.

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Non-Destructive Testing (NDT) of Aircraft Ferrous Hardware

Non-Destructive Testing allows maintenance engineers to inspect parts for crack initiation, corrosion, manufacturing defects, and structural degradation without compromising future airworthiness.

1. Magnetic Particle Inspection (MPI)

Magnetic Particle Inspection is the premier NDT method for detecting surface and near-subsurface discontinuities in ferromagnetic materials (carbon steels, alloy steels 4130/4340, and 400-series martensitic stainless steels).

Exam Warning / Common Trap: MPI CANNOT be performed on austenitic stainless steels (AISI 300 series) such as 304, 316, 321, or 347. Austenitic stainless steels have an FCC crystal structure and are non-magnetic; they cannot sustain magnetic flux fields.

Operational Principle

When a ferromagnetic part is magnetized, magnetic lines of force (flux lines) pass through the material. If a surface or near-surface crack interrupts these flux lines, the flux cannot jump the air gap easily. Instead, it detours around the defect, with some lines leaking out into the air above the crack, creating localized North and South magnetic poles (magnetic flux leakage field). When fine ferromagnetic oxide particles (colored or fluorescent) are applied, they are magnetically drawn to the leakage field, forming a sharp, visible indication.

Magnetization Techniques

• Circular Magnetization: Created by passing high-amperage current directly through the component (via headstock contact clamps) or through a central copper conductor bar placed through a hollow bore. The magnetic field circles around the part circumference. Because flux lines must cut across defects at approximately 90° (minimum 45°) to create leakage fields, circular magnetization detects longitudinal cracks parallel to the axis.
• Longitudinal Magnetization: Created by placing the component inside an encircling solenoid coil or wrapping a flexible current-carrying cable around it. The magnetic flux flows parallel to the longitudinal axis. Longitudinal magnetization detects transverse and circumferential cracks perpendicular to the axis.

Current Types and Application Methods

• Alternating Current (AC): Subject to the "skin effect," which concentrates magnetic flux in the outer 1 mm to 2 mm of the surface. Provides maximum sensitivity for fine surface fatigue cracks.
• Direct Current (DC) / Half-Wave Rectified (HWDC): Penetrates deeper into the cross-section, providing superior detection of near-subsurface flaws (such as subsurface forging inclusions).
• Continuous Method: Magnetic particle suspension is sprayed over the part while the magnetizing current pulse is actively energized. Mandatory for aircraft steels with low magnetic retentivity.
• Residual Method: Particles are applied after the magnetizing current has been shut off, relying on residual magnetism. Permissible only for high-retentivity, fully hardened steels.

Demagnetization

Following MPI, components retain residual magnetism, which would attract abrasive ferrous wear particles, freeze up instruments, cause electric arcing in bearings, and severely deflect the aircraft's primary magnetic compasses. Demagnetization is mandatory. It is performed by passing the component through an AC demagnetizing coil and slowly withdrawing it along the coil axis for at least 1.5 meters, or using decreasing step-down reversing DC current. The component must be checked with a calibrated field indicator (Gauss meter) to verify residual field is below 2 to 3 Gauss.

2. Overview of Complementary Aviation NDT Methods