What is Hammer Testing ?

Typical Impulse Hammer Analysis System

Hammer Testing
Something we are asked from time to time is “What exactly is hammer testing and how can it help me?” In this article we will try to explain the basics.

Hammer testing is also sometimes referred to as Modal Testing, and it is a way of testing to determine the dynamic behaviour of a structure in response to an applied stimulus. A transfer function of a mechanical system describes this dynamic behavior, and there are six major transfer functions used in mechanical systems analysis, with each describing the dynamic behavior in a different fashion, and each involving the ratio of the output response to the input stimulus.

Response can be measured most conveniently with accelerometers placed at important points throughout the structure under test, and Spectrum analyzers perform the necessary mathematical ratio transform on the input and response signals to produce, almost instantaneously, the desired transfer function.

The input stimulus (forcing function) may be applied to the structure by various methods. One way to excite structures is by use of an electrodynamic or hydraulic shake with a force sensor attached to the armature driving the test object, to measure the input force amplitude. However when the test object is too large to be excited in this fashion, or when it is physically impossible to do so, the structure can be hit with a calibrated dynamic impulse hammer.

A force sensor mounted in the head of the hammer transforms the input force pulse into a waveform that contains the necessary amplitude and phase information to completely describe the forcing function. Impact tip material stiffness helps determine the frequency content of the input forcing function by controlling the impact pulse duration. By defining the frequency and amplitude of the forcing function, impulse hammers present a fast, simple way to excite structures in a well-defined fashion.

The user can combine various head extenders to alter the hammer head mass, and select different impulse tips, to create different impulse waveforms. Fourier analysis shows that faster rise time pulses with short pulse duration contain the highest frequencies, so use the metal (aluminum or steel) impact tips and no head extender for quickest rebound to produce impulses with the highest frequency content, and the opposite for low frequency excitation. Heavier structures with lower frequency responses (resonances) require lower frequency excitation at higher input force levels.

The result of the impact from the hammer would ideally be measured with an array of accelerometers mounted on the structure, but one accelerometer could be fitted and then the structure impacted at several points.

Selection of Impulse Hammers for different applications - from miniature to large sledge

March 2014


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