Material Testing & Characterization
Every number in a material datasheet comes from a standardized test. Understanding how these tests work — and what their results actually mean — is essential for selecting materials, specifying quality requirements, and interpreting failure analyses.
Tensile Testing (ASTM E8 / ISO 6892-1)
The most fundamental mechanical test. A dog-bone specimen is gripped at both ends and pulled at a controlled rate until fracture.
What You Get
- Young's modulus (E) — Slope of the linear elastic region
- 0.2% offset yield strength (σy) — Where permanent deformation begins
- Ultimate tensile strength (σu) — Maximum engineering stress
- Elongation at break (%) — Total strain at fracture (ductility)
- Reduction of area (%) — Cross-section decrease at fracture (another ductility measure)
Specimen Types
| Geometry | Standard | Application |
|---|---|---|
| Flat dog-bone | ASTM E8 (sub-size or standard) | Sheet metals, composites |
| Round bar | ASTM E8 (standard) | Bar stock, forgings, castings |
| Miniature | Custom | Weld zones, HAZ testing, research |
Important Details
- Strain rate matters: Most standards specify quasi-static rates (~10⁻³/s). High strain rates (10²–10³/s) are tested with split Hopkinson bar for crash simulation data.
- Temperature effects: Hot tensile tests (ASTM E21) characterize elevated-temperature properties for jet engine alloys.
- Engineering vs. true stress: Datasheets report engineering stress (force/original area). True stress (force/instantaneous area) is higher and used in FEA material models for large-deformation simulations.
Hardness Testing
Quick, non-destructive (or minimally destructive) test that measures resistance to indentation. The most common quality control test in manufacturing.
Rockwell (ASTM E18)
An indenter is pressed into the surface under a minor load, then a major load is applied. Hardness is read directly from the dial (or digital display) based on depth of penetration.
| Scale | Indenter | Major Load | Range | Use |
|---|---|---|---|---|
| HRC | Diamond cone (120°) | 150 kg | 20–65 | Hardened steels, tool steels |
| HRB | 1/16" steel ball | 100 kg | 20–100 | Soft steels, aluminum, brass |
Brinell (ASTM E10)
A 10mm hardened steel or tungsten carbide ball is pressed into the surface under a known load (typically 3,000 kg for steels). The diameter of the resulting impression is measured.
Best for: Castings and forgings with coarse microstructures where Rockwell would be inconsistent. Rule of thumb for steels: σu (MPa) ≈ 3.45 × HBVickers (ASTM E92)
A diamond pyramid (136° angle) is pressed into the surface. The diagonal of the square impression is measured under a microscope. Works at any load from microhardness (10g) to macro (50 kg).
Best for: Thin sections, surface layers (case depth profiling), weld microhardness traverses.Microhardness (Knoop/Vickers)
Very low loads (1–1,000 gf) for testing individual microstructural phases, thin coatings, or small heat-affected zones.
Application: Measuring the hardness profile across a carburized case, verifying coating hardness, characterizing weld zones.Impact Testing (ASTM E23)
Charpy V-Notch Test
A notched specimen is struck by a pendulum. The energy absorbed during fracture is measured in joules. This tests toughness — the ability to absorb energy before fracturing.
Specimen: 10 × 10 × 55 mm bar with a 2mm deep, 45° V-notch.Why It Matters
The Charpy test reveals the ductile-to-brittle transition temperature (DBTT) for BCC metals (steels). Above the DBTT, the fracture surface is fibrous (ductile, high energy). Below it, the fracture is cleavage (brittle, low energy).
| Steel Type | Typical DBTT | Charpy Energy at RT |
|---|---|---|
| Structural mild steel | ~-20°C | 40–100 J |
| HSLA pipeline steel (X70) | ~-60°C | 200+ J |
| Austenitic stainless (304) | No DBTT (FCC) | 200+ J |
| High-carbon tool steel | ~+50°C | 10–20 J |
Fatigue Testing (ASTM E466 / E606)
What Is Fatigue?
Fatigue is failure under cyclic loading at stresses well below the static yield strength. It's responsible for an estimated 80–90% of all structural failures in service.
S-N Curves (Stress-Life)
Specimens are subjected to cyclic loading at different stress amplitudes. The number of cycles to failure (N) is recorded for each stress level. Plotted on a log scale: S (stress amplitude) vs. N (cycles to failure).
Key concepts:- Endurance limit (Se): For ferritic steels and titanium, there exists a stress level below which fatigue life is essentially infinite (>10⁷ cycles). Approximately Se ≈ 0.5 × σu for steels (up to ~1,400 MPa UTS).
- No endurance limit: Aluminum, copper, and austenitic stainless steels do NOT have a true endurance limit — the S-N curve continues to decrease. Designers must specify a fatigue life at a given number of cycles.
Strain-Life (ε-N) Testing
For low-cycle fatigue (high strains, <10⁴ cycles), strain-controlled testing is used. Produces the Coffin-Manson relationship used in powertrain and structural design where components experience plastic strains (engine mounts, suspension components, pressure vessels).
Fatigue Crack Growth (ASTM E647)
Once a crack exists, how fast does it grow per cycle? Measured as da/dN vs. ΔK (stress intensity factor range). The Paris Law describes the linear region: da/dN = C(ΔK)^m.
Critical for damage-tolerant design — used to set inspection intervals for aircraft structures. If a crack is found at length a₀, how many flight cycles until it reaches the critical crack length (a_crit)?Fracture Toughness (ASTM E399)
KIC — Plane Strain Fracture Toughness
Measures a material's resistance to crack propagation. A pre-cracked specimen is loaded until the crack extends unstably.
Units: MPa√m
| Material | KIC (MPa√m) |
|---|---|
| Al 7075-T6 | ~29 |
| Al 2024-T3 | ~37 |
| Ti-6Al-4V (annealed) | ~75 |
| 4340 steel (Q&T, 1,500 MPa) | ~50 |
| 300M (Q&T) | ~60 |
| CFRP (delamination, GIC) | Mode I: ~0.25 kJ/m² |
Creep Testing (ASTM E139)
What Is Creep?
Time-dependent plastic deformation under constant load at elevated temperature. At room temperature, creep is negligible for metals. Above ~0.4 Tm (melting temperature in Kelvin), creep becomes significant.
The Creep Curve
Three stages:
- Primary creep — Decreasing strain rate (work hardening)
- Secondary (steady-state) creep — Constant strain rate (balance between hardening and recovery). The minimum creep rate from this stage is the key design parameter.
- Tertiary creep — Accelerating strain rate leading to rupture (void formation and necking)
Application
Jet engine turbine blades: Designed to a maximum creep strain (typically 1%) over the component's service life (e.g., 10,000 hours). This is why single-crystal superalloys exist — eliminating grain boundaries removes the dominant creep mechanism (grain boundary sliding). Power plant steam pipes: Cr-Mo steels (P91, P92) designed for 100,000+ hours of creep life at 550–600°C.Non-Destructive Testing (NDT)
NDT methods detect defects without damaging the part. Critical for quality control in manufacturing and in-service inspection.
Visual Inspection (VT)
The simplest and most common method. Trained inspectors examine surfaces for cracks, corrosion, porosity, incomplete welds, and other visible defects. Often aided by borescopes (internal inspection of engines), magnifying lenses, and cameras.
Detects: Surface cracks >~0.5mm, corrosion, deformation, assembly errors.Liquid Penetrant Inspection (PT)
A colored or fluorescent dye is applied to the surface, drawn into surface-breaking cracks by capillary action, then made visible with a developer.
Detects: Surface-breaking cracks in non-porous materials. Sensitivity down to ~1 μm wide cracks. Limitation: Only surface defects. Cannot inspect porous materials (uncoated castings with open porosity).Magnetic Particle Inspection (MT)
The part is magnetized. Surface and near-surface defects create magnetic flux leakage that attracts magnetic particles (dry or wet, visible or fluorescent).
Detects: Surface and near-surface cracks in ferromagnetic materials (steels, iron — NOT aluminum, titanium, or austenitic stainless). Application: Landing gear, crankshafts, gears — any critical ferromagnetic component.Ultrasonic Testing (UT)
High-frequency sound waves (1–25 MHz) are transmitted into the material. Reflections from internal defects (voids, inclusions, delaminations, cracks) are detected.
Detects: Internal defects, thickness measurement, bond quality. Application: Aerospace composite inspection (delamination detection), weld inspection, forging quality, thickness gauging of corroded pipes. Phased array UT uses multiple transducer elements to steer and focus the beam electronically, producing detailed cross-sectional images. Rapidly replacing conventional UT in critical applications.Radiographic Testing (RT)
X-rays or gamma rays pass through the part onto film or a digital detector. Dense areas (sound material) attenuate more radiation than defects (voids, porosity).
Detects: Internal volumetric defects — porosity, shrinkage, inclusions, slag in welds. Limitation: Poor at detecting planar defects (cracks) oriented parallel to the beam. Radiation safety requirements. Computed tomography (CT) is industrial 3D X-ray scanning — produces full 3D models of internal geometry and defects. Used for casting quality, additive manufacturing inspection, and reverse engineering.Eddy Current Testing (ET)
An alternating magnetic field induces eddy currents in a conductive material. Defects and property changes alter the eddy current flow, which is detected by the probe.
Detects: Surface and near-surface cracks, conductivity variations (heat treatment verification), coating thickness. Application: Aircraft skin inspection (detects fatigue cracks around fastener holes), tube inspection in heat exchangers.Thermographic Inspection
Infrared cameras detect heat patterns that reveal subsurface defects. Active thermography uses a heat pulse; passive thermography monitors parts during operation.
Application: Composite delamination and disbond detection, electrical hot spots, brake rotor integrity.NDT Method Selection
| What You're Looking For | Best Method(s) |
|---|---|
| Surface cracks in steel | MT (fastest), PT |
| Surface cracks in aluminum/titanium | PT, ET |
| Internal voids in castings | RT, UT |
| Delamination in composites | UT (phased array), thermography |
| Fatigue cracks at fastener holes | ET |
| Weld quality (volumetric) | RT |
| Weld quality (planar cracks) | UT |
| Corrosion/thickness loss | UT |
| Full 3D internal inspection | CT |
Key Takeaways
- Tensile testing (ASTM E8) provides the fundamental mechanical properties: E, σy, σu, elongation
- Charpy impact testing reveals the DBTT — critical for steels used in cold environments
- Fatigue causes 80–90% of structural failures; steels have an endurance limit, aluminum does not
- Fracture toughness (KIC) determines whether a material fails in a damage-tolerant or brittle manner
- Creep testing is essential for any component operating above ~0.4 Tm
- NDT method selection depends on the material (ferromagnetic or not), defect type (surface or internal), and geometry
