Glossary term

Impact Hammer Test

Instrumented modal test method that uses a calibrated hammer force pulse and measured structural response to estimate frequency response functions.

Branch: Mechanical Engineering
Glossary type: method
Content: Glossary term
Updated: Jun 25, 2026
Revision: v1.0.1 · reviewed

Definition

method

An impact hammer test is an experimental modal test that applies a measured hammer force pulse to a structure and records response so frequency response functions can be estimated.

Impact hammer testing uses an instrumented hammer, usually with a force transducer in the hammer head, to excite a structure with a short broadband force pulse. The measured force and response signals are processed into frequency response functions, coherence and modal information. The useful bandwidth depends on hammer tip stiffness, contact duration, force level, sensor bandwidth, sampling rate and hit quality.

An impact hammer test excites a structure with a measured hammer force pulse and records the structural response. The force signal is usually measured by a force transducer inside the hammer head. The response may be acceleration, velocity, displacement or strain.

For a measured force $F(\omega)$ and response $Y(\omega)$ , the test is commonly used to estimate a frequency response function:

\displaystyle H(\omega)=\frac{Y(\omega)}{F(\omega)}

The hammer impact approximates an impulse, but it is not an ideal mathematical impulse. The force pulse has finite duration, finite bandwidth, finite amplitude and practical limitations from the hammer tip, operator technique, structure, sensor range and data acquisition system.

Engineering Role

Impact hammer testing is useful when a structure is small or medium sized, portable excitation is needed, the test must be quick, or shaker attachment would alter the boundary condition. It is common for brackets, machine bases, panels, piping spans, floors, laboratory structures, aircraft components, fixtures, frames and troubleshooting work.

The method can provide:

point and transfer FRFs;
natural frequency estimates;
damping estimates from curve fitting or decay review;
mode-shape information from roving hammer or roving sensor tests;
coherence and repeatability checks;
evidence for finite-element correlation;
operating-response interpretation when compared with operating deflection shapes or order-tracked data.

The hammer tip matters. A soft tip gives a longer contact time and lower useful high-frequency content. A hard tip gives a shorter pulse and more high-frequency energy, but can overload sensors, excite local nonlinearities or damage delicate structures.

Hit Quality and Acceptance Gates

Impact hammer data should be accepted or rejected at acquisition, not only after curve fitting. A useful test plan defines the target frequency band, hammer tip, force range, trigger settings, response range, number of averages and rejection rules before the first hit is kept.

Common acceptance gates include:

\gamma^2(f)\geq\gamma^2_{min}

over the decision band, and an estimator agreement check such as:

\displaystyle \frac{|H_2|-|H_1|}{|H_1|}\leq\delta_H

where $\gamma^2_{min}$ and $\delta_H$ are project-specific thresholds. For a release test near an important mode, a team might require coherence above 0.90 and H1/H2 magnitude agreement within 10 percent. Those values are not universal, but stating them prevents a clean-looking FRF from being accepted after double hits, weak force energy or response clipping.

The force time record and force spectrum should be reviewed with the response. A valid hit has a single clean contact, enough force energy through the frequency band of interest, no overload, no sliding impact and a repeatable direction. If several hits violate those rules, averaging them usually makes the report look smoother while making the engineering evidence worse.

Worked Example: Check Pulse Bandwidth and Impulse

An engineer tests a machine bracket with an instrumented hammer. The target frequency band is:

0\leq f\leq 300\ \text{Hz}

A medium hammer tip gives an approximately triangular force pulse with peak force:

F_{pk}=800\ \text{N}

and contact duration:

t_c=2.0\ \text{ms}=0.0020\ \text{s}

A rough bandwidth screen for a short contact pulse is the first spectral zero:

\displaystyle f_0\approx\frac{1}{t_c}=\frac{1}{0.0020}=500\ \text{Hz}

Engineering comment: the first zero is above the required $300\ \text{Hz}$ band, so the hammer tip is likely adequate for this frequency range. The engineer would still inspect the measured force spectrum. If the force spectrum has a deep notch or poor energy in the band of interest, the FRF will be weak even if this simple screen passes.

For a triangular force pulse, the impulse is:

\displaystyle J\approx\frac{1}{2}F_{pk}t_c

Substitute the measured values:

\displaystyle J=\frac{1}{2}(800)(0.0020)=0.80\ \text{N s}

If the local effective mass responding initially is roughly:

m=12\ \text{kg}

the idealized initial velocity increment is:

\displaystyle \Delta v\approx\frac{J}{m}=\frac{0.80}{12}=0.0667\ \text{m/s}

Engineering comment: this is only a scale check, not a modal result. The actual measured response depends on flexibility, boundary conditions, damping, local modes, sensor direction and the force location. If the response channel clips or the structure behaves nonlinearly, the hit should not be used for linear FRF estimation.

If a softer tip produces:

t_c=5.0\ \text{ms}

the rough first zero becomes:

\displaystyle f_0=\frac{1}{0.0050}=200\ \text{Hz}

That tip would not be suitable for a $300\ \text{Hz}$ FRF review unless the measured force spectrum still has adequate usable energy. The solution is usually to use a harder tip, a different hammer, a shaker, or a narrower frequency band.

Impact hammer test is not a frequency response function. The hammer test is the excitation and measurement method. The FRF is the processed response/input result.

Impact hammer test is not modal analysis by itself. Modal analysis uses FRFs, curve fitting, mode-shape assembly and validation. A hammer test may provide the data, but it does not guarantee correct modal parameters.

Impact hammer test is not an ideal impulse response. The force pulse has finite duration and measured shape. The engineer must use the measured force signal, not assume a unit impulse.

Impact hammer test is not a ground vibration test. A GVT is a controlled aircraft or structural modal campaign that may use shakers, hammers or other excitation. Hammer testing is one possible excitation method.

Impact hammer test is not operating deflection shape testing. ODS reviews forced response under operating conditions. Hammer testing applies an artificial force input for modal or FRF measurement.

Validation and Common Mistakes

A defensible impact hammer test states hammer model, force range, tip type, force calibration, impact location and direction, response sensors, response units, sampling rate, anti-alias filtering, windows, averaging, trigger settings, acceptance rules, frequency range, boundary condition, hit repeatability and coherence.

Common mistakes include:

accepting double hits or sliding hits;
using a tip that does not excite the frequency band of interest;
clipping the force or response channel;
ignoring poor coherence at anti-resonances or low-response bands;
moving the hammer direction or impact point between averages;
comparing FRFs from different boundary conditions;
using too much force and exciting nonlinear joints, backlash or contact changes;
treating a clean-looking FRF as valid without checking force spectrum, phase, reciprocity and repeatability.

REF

Disciplines

Impact Hammer Test

Definition

Engineering Role

Hit Quality and Acceptance Gates

Worked Example: Check Pulse Bandwidth and Impulse

Validation and Common Mistakes

See also

Engineering Role

Hit Quality and Acceptance Gates

Worked Example: Check Pulse Bandwidth and Impulse

Distinction from Related Terms

Validation and Common Mistakes