Method and apparatus for non-destructive measurement of elastic properties of structural materials

A method for determining the elastic properties of a single plate of a thin composite of either isotropic or orthotropic structural material non-destructively where it is unknown before testing whether the structural material is isotropic or orthotropic, comprising the steps of:

• suspending the single plate of either an isotropic or orthotropic material from a rigid support by a fine suspending medium so that a small angle is defined between the suspending medium and the material and so that edges of the material are free of appreciable restraint by the suspending medium;
• providing a vibration sensor in communication with the material to detect a vibration response;
• imparting an impulse to the material by an impulse means;
• connecting a force transducer to the impulse means;
• detecting by a vibration sensor a response signal in the material as a result of the impulse imparted;
• transforming the response signal into a frequency domain by an analyzer connected to the vibration sensor;
• obtaining resonance frequencies of the plate from the analyzer;
• deriving modal shape indices of the plate at each resonance frequency;
• communicating the resonance frequencies and the modal shape indices to a computing means;
• computing and displaying (E.sub.x, E.sub.y, G.sub.xy, and .nu..sub.xy) four elastic constants using the computing means;
• and
• determining after the computing and displaying step that the material is orthotropic or isotropic by inspection of the four elastic constants.

An apparatus for determining the elastic properties of a panel of a thin composite material non-destructively, comprising:

• a rigid support;
• a fine suspending medium extending from the support, the suspending medium having a low support stiffness, thereby engaging the material so that edges of the material are essentially free of restraint by the suspending medium;
• an impulse means for imparting an impulse to the material;
• a force transducer connected to the impulse means;
• a vibration sensor for detecting the response in the material as a result of the impulse imparted and for generating a signal in proportion thereto;
• a spectrum analyzer connected to the vibration sensor for transforming the signal into a frequency domain and obtaining resonance frequencies and modal shape indices of the panel;
• a computing means in communication with the analyzer for computing from the resonance frequencies and the modal shape indices, all four elastic constants and for displaying said constants so that a determination can be made that the material is orthotropic or isotropic;
• the rigid support including:
• a vibration isolation table;
• a support stand rigidly connected to the vibration isolation table;
• a cross-piece extending from the stand, the cross-piece having opposite ends;
• two cantilever bars, one bar being clamped to one end of the cross-piece, and the other bar being clamped to the other end of the cross-piece.