material-testing

name	material-testing
description	Material testing — tensile testing (ASTM E8), hardness (Brinell/Rockwell/Vickers), Charpy impact, creep testing, fracture toughness KIC, fatigue S-N curve generation, DIC.
metadata	{"priority":7,"promptSignals":{"phrases":["material testing","tensile test","hardness test","Charpy test","material characterization","stress strain curve","coupon test"],"minScore":4}}

Material Testing — Complete Skill

Tensile Testing (ASTM E8 / ISO 6892-1)

Specimen

Round or flat bar specimens (dog-bone shape) Gauge length L₀ (standard: 50mm or 4D round) Reduced section diameter D (round) or width W × thickness t (flat)

Measurements

Engineering stress: S = F/A₀ (original area) Engineering strain: e = (L-L₀)/L₀ = ΔL/L₀

True stress: σ = F/A (instantaneous) = S(1+e) True strain: ε = ln(L/L₀) = ln(1+e)

Properties Extracted

Elastic modulus: E = slope of linear region [GPa] Proportional limit: highest stress in linear region 0.2% offset yield strength (Sy): stress at 0.2% permanent strain (most used) Ultimate tensile strength (Sut): maximum engineering stress Fracture strength: stress at fracture (lower than Sut due to necking) Elongation at fracture (A%): (L_f - L₀)/L₀ × 100% Reduction of area (Z%): (A₀ - A_f)/A₀ × 100%

Strain Rate Effects

Standard rate: strain rate 0.01-0.1 mm/(mm·min) [quasi-static] High rate: split-Hopkinson bar (SHPB) for 10²-10⁴ s⁻¹ Rate sensitivity: σ = K ε̇^m (power law); m = strain rate sensitivity

Ramberg-Osgood Model (Fitting)

ε = σ/E + (σ/K)^(1/n) K = strength coefficient, n = strain hardening exponent Fit from plastic portion of σ-ε curve (log-log: log(σ_pl) vs. log(ε_pl)) n = slope, K = intercept (true fracture stress)

Hardness Testing

Brinell (HB) — ASTM E10

10mm tungsten ball, 3000 kgf (steel), 500 kgf (Al) Dwell: 10-15 seconds HB = 2F/(πD(D-√(D²-d²))) where d = indent diameter Measures: large area average (forgiving of inclusions) Cannot use on: case-hardened surface (ball penetrates through)

Rockwell (HRC, HRB) — ASTM E18

HRC: 150 kgf, diamond cone (Brale) — hard steels (>HRC 20) HRB: 100 kgf, 1/16" ball — soft materials (70HRB ≈ HRC 0) HRA: 60 kgf, diamond — cemented carbide, thin case Fast, direct readout, minor vs. major load system Cannot use thin sections: HRC requires >1.5mm

Vickers (HV) — ASTM E92

136° diamond pyramid, loads 1-120 kgf HV = 1.854 F/d² (d = average diagonal, mm, F in kgf) Scales from HV 1 (microindentation) to HV 30 (macro) Most versatile — works on any material, any thickness, case-hardened layers HV and HB agree numerically below HB 350: HV ≈ HB × 1.05

Knoop (HK) — Microhardness

Elongated diamond pyramid; small load (1-200 gf) Used for: anisotropic materials, thin coatings, individual phases/grains HK ≈ HV × 1.1 (typically)

Hardness Conversion (Approximate)

ASTM E140 tables are the reference — formula below is approximate only: HRC ≈ 60 - 12.5 × log(HB) [rough, for HB 200-400] Sut (MPa) ≈ 3.45 × HB [carbon/alloy steels, HB < 400]

Charpy V-Notch Impact Test (ASTM E23)

Procedure

10×10×55mm specimen, 2mm V-notch (or U-notch) Pendulum energy 300 J absorbed = energy absorbed [J] or [ft-lb] Test at multiple temperatures → ductile-brittle transition curve

Energy Absorbed (CVN) Results

Upper shelf: ductile fracture, high CVN (100-250 J for structural steel) Lower shelf: brittle fracture, low CVN (<20 J) Transition range: 15-85% shear fracture appearance DBTT: temperature at 50% ductile appearance (or 27 J criterion)

Code Requirements

AWS D1.1 Supplementary Requirements: CVN ≥ 20 ft-lb at -20°F (Zone II bridges) ASTM A709 Gr 50W: 15 ft-lb at +10°F ASME BPVC: impact testing required for t > 1/2" at T < RTNDT+60°F

Fracture Toughness Testing — K_IC (ASTM E399)

Compact Tension (CT) or Three-Point Bend Specimen

Pre-crack by fatigue cycling to sharp crack Load to fracture (or 5% load drop) P_Q from load-displacement record → K_Q → validity checks → K_IC

Validity Requirements

a ≥ 2.5(K_Q/Sy)² — plane strain (thick specimen) B (thickness) ≥ 2.5(K_Q/Sy)² W - a ≥ 2.5(K_Q/Sy)² If all met: K_Q = K_IC (valid plane strain)

K_IC typical values: see fracture-mechanics skill

J-Integral Testing (ASTM E1820)

For materials too tough for ASTM E399 (very high Sy or low K_IC) Elastic-plastic fracture mechanics J_IC = K_IC²/(E') where E' = E (plane stress), E/(1-ν²) (plane strain)

Fatigue S-N Curve Generation (ASTM E466, E468)

Test Setup

Rotating bending (R.R. Moore) or axial (servo-hydraulic) or bending fatigue Stress ratio R = σ_min/σ_max (R=-1 fully reversed, R=0.1 tension-tension) Test until fracture or 10⁷ cycles (runout)

Number of Specimens

Minimum 6-8 per stress level; 3-5 stress levels Staircase method for endurance limit estimation (Dixon-Mood) Log-normal distribution assumed for cycles to failure at each stress level

Data Reduction

Log-log fit: log(N) = A - B×log(σ) → N = C/σ^m Or: σ = σ_f' × (2N)^b (Basquin) — fatigue strength coefficient and exponent Statistical: mean curve + 2σ design curve (95% confidence, 50% reliability)

Endurance Limit (Se)

For steels: Se ≈ 0.504 Sut (Shigley, unnotched, R=-1) Apply Marin factors for actual conditions — see fatigue-life skill

Digital Image Correlation (DIC)

Principle

Random speckle pattern on specimen surface → track displacements optically 2D DIC: in-plane displacements (u, v) from single camera 3D DIC (stereo): full 3D surface displacement (u, v, w) Full-field strain: ε_x, ε_y, γ_xy computed from displacement gradients

Applications

Strain concentration visualization (compares to FEA predictions) Crack initiation and propagation tracking Displacement at inaccessible locations Large-deformation testing (rubber, soft tissue, metals at high strain)

Accuracy

Typical: strain accuracy ≈ 50-200 με (0.005-0.02%) Spatial resolution: tied to facet size (typically 5-20× speckle size) Calibration: stereo setup requires calibration grid

Creep Testing (ASTM E139)

Constant load applied at elevated temperature Record strain vs. time at constant T and σ Extract: secondary creep rate ε̇_min, rupture time t_r Use Larson-Miller analysis for extrapolation — see creep-relaxation skill

Test Report Requirements

• Specimen ID, heat/lot number, chemical composition
• Test date, temperature, humidity (if relevant)
• Equipment calibration status, calibration date
• Number of specimens, statistical reduction method
• Individual results + mean ± standard deviation
• Comparison to specification minimum requirements
• Qualified engineer signature

Output

Provide: test standard (ASTM/ISO), specimen geometry, property extracted (E/Sy/Sut/KIC/CVN/N_f), statistical analysis (n, mean, std), code acceptance criteria, plot type (S-N, σ-ε, load-displacement).