Testing, Characterization, Failure Analysis, and Degradation
Mechanical testing
Know specimen geometry and standards for tension, hardness, Charpy impact (absorbed energy and the ductile-to-brittle transition), fracture toughness K_IC and J-integral, and fatigue testing (S–N curves, fatigue limit, crack-growth da/dN versus delta-K and the Paris law). Understand scatter and the need for multiple specimens.
Characterization and NDT
Optical metallography, SEM with EDS, XRD for phases and residual stress, DSC and TGA for polymers. For NDT: liquid penetrant finds only surface-breaking defects, magnetic particle works only on ferromagnetic materials, ultrasonic and radiography find internal defects, and eddy current suits conductive surface and near-surface flaws.
Failure analysis
Follow the sequence: collect background, preserve the fracture surface, macroscopic and microscopic examination, and only then destructive testing. Recognize ductile overload (necking, dimples), brittle fracture (cleavage facets, no deformation), fatigue (beach marks from load changes, striations one per cycle), and intergranular paths (embrittlement, SCC, creep cavitation).
Degradation
Corrosion forms: galvanic, pitting, crevice, intergranular, erosion-corrosion, stress-corrosion cracking, and hydrogen embrittlement. Oxidation and sulfidation dominate at high temperature, with protective scales following parabolic kinetics.
Traps
Calling beach marks striations (beach marks are macroscopic and mark load changes; striations are microscopic and mark cycles); using penetrant on a porous or painted surface; assuming magnetic particle works on austenitic stainless; confusing hydrogen embrittlement with hydrogen attack.
Worked example
At delta-K = 20 MPa*m^1/2 with C = 1e-11 and m = 3 (da/dN in m/cycle), crack growth is 8e-8 m/cycle, about 0.08 mm per 1000 cycles. Use such rates to set inspection intervals before a crack reaches critical size.