Impulse Excitation vs Process Compensated Resonance Testing

Two Resonance Methods, One Question

Process Compensated Resonance Testing (PCRT) and GrindoSonic Impulse Excitation Testing (IET) belong to the same family. Both excite a whole part, record how it resonates, and sort it against a reference population. Neither localizes a defect; both judge the part as a whole. The difference is the signal each one reads.

PCRT applies a swept-sine input and runs statistical pattern recognition across a part’s resonance frequencies (ASTM E2534). GrindoSonic IET applies a single impulse and measures, at high resolution, both the resonance frequency and the damping (ASTM E1876). For additive manufacturing, where many defects are internal and microstructural, that second signal is often what separates a sound part from a borderline one.

Key takeaway: Frequency tells you a part’s stiffness and geometry are right. Damping tells you its microstructure is. PCRT reads the first reliably; GrindoSonic IET adds the second from the same tap.

Where Frequency Alone Is Enough

Resonance frequency is a strong, sensitive signal in its own right. It shifts with stiffness, mass, geometry, gross density, and many process parameters. For sorting parts by build settings, or for catching a stiffness or density outlier, a frequency-pattern method does the job. Your own process-monitoring case classified powder-bed-fusion parts by laser power and scan strategy using resonance frequencies and Z-score statistics, with no damping required. If the defect you fear changes a part’s stiffness, frequency will catch it.

What Damping Adds

Damping, also called internal friction or Q⁻¹, measures the energy a vibrating part dissipates, and it moves independently of stiffness. A microcrack, a lack-of-fusion network, a residual-stress state, or an incomplete heat treatment can leave resonance frequency almost untouched while raising damping sharply. ASTM E2534 itself notes that damage broadens resonance peaks, and peak broadening is rising damping, climbing from the onset of microcracking. The physics is established: dissipation tracks the microstructural features that govern fatigue and damage tolerance, not just bulk stiffness.

Two production cases show damping earning its keep. In LPBF aluminium heat treatment, damping signatures tracked precipitation and stress relief continuously, where frequency alone would have missed the staged microstructural change. In FDM polyamide inspection, damping discriminated delamination that frequency did not flag. Frequency confirms a part is stiff; damping confirms it is sound.

Verifying Post-Processing

Critical AM steps such as stress relief and Hot Isostatic Pressing must take effect uniformly across a build. Measure each part before and after: a frequency change confirms densification or a stiffness gain, while a damping change reveals residual-stress relaxation, crack closure, or microstructural healing that frequency cannot see. Parts that respond abnormally get pulled for review before they ship.

Model Validation and Reverse Engineering

Resonance testing also checks a part against a digital model. Matching frequencies confirm that geometry and elastic properties agree with the model’s assumptions; damping then exposes material-state or microstructural differences that a purely elastic simulation never captured. That second axis matters most for low-volume AM, spare-part runs, and qualifying replacements for legacy components, where no large reference population exists to train on.

Choosing Your Approach

For gross sorting (stiffness, geometry, density, build-parameter drift), resonance frequency alone does the job, whether it comes from PCRT or IET. For fatigue-critical parts, microstructure-sensitive alloys, or verifying post-processing, the damping axis is what catches the defects frequency misses. The two methods are less rivals than one signal versus two, and GrindoSonic IET reads both from a single tap.