Method and apparatus for evaluating soft material properties using nonlinear vibrations
An apparatus for measuring elastic properties of materials excites the material into nonlinear vibration to reveal, in a free and forced vibration signals, information about Young's modulus, internal stress, and material thickness obtained from nonlinear and linear vibration information.
1 . An apparatus for measuring elastic properties of a material comprising:
a stimulating energy source coupled to the material to excite the material into vibration;
a vibration sensor adapted to provide an electrical signal measuring vibration of the material; and
a controller executing a stored program to control the stimulating energy source and vibration sensor to:
(a) excite the material with the stimulating energy source into linear and nonlinear vibration;
(b) cease excitation of the material to measure the linear and nonlinear vibration with the vibration sensor during a free vibration period;
(c) extract a value of elasticity from the measured linear vibration; and
(d) extract at least one of a thickness of the material and an internal static stress of the material from the measured nonlinear vibration.
2 . The apparatus of claim 1 wherein the stimulating energy source provides a dominant vibration of material along an axis and the thickness is along the axis.
3 . The apparatus of claim 2 wherein the controller excites the material with the stimulating energy source to an amplitude at least one tenth of a thickness of the material along the axis.
4 . The apparatus of claim 2 further including a rigid sample holder defining a cylindrical volume aligned with the axis and holding the material in disk form.
5 . The apparatus of claim 1 wherein the stimulating energy source is an electromechanical actuator coupled to the material.
6 . The apparatus of claim 1 wherein the vibration sensor is a noncontact vibration sensor.
7 . The apparatus of claim 1 wherein the vibration sensor is a laser vibrometer.
8 . The apparatus of claim 1 wherein the controller further receives an input indicating a density of the material.
9 . The apparatus of claim 1 wherein the controller further receives a dimension of the material perpendicular to an axis of vibration.
10 . An apparatus for measuring elastic properties of a material comprising:
a stimulating energy source coupled to the material to excite the material into vibration;
a vibration sensor adapted to provide an electrical signal measuring vibration of the material; and
a controller executing a stored program to control the stimulating energy source and vibration sensor to:
(a) excite the material with the stimulating energy source into linear and nonlinear vibration;
(b) cease excitation of the material to measure the linear and nonlinear vibration with the vibration sensor during a free vibration period;
(c) extract a value of elasticity from the measured linear vibration; and
(d) extract at least one of a thickness of the material and an internal static stress of the material from the measured nonlinear vibration; and
wherein (a) includes an initial operation of identifying a at least one resonant frequency of the material and the controller operates the stimulating energy source to provide a narrowband excitation of the material specific to the identified at least one resonant frequency.
11 . The apparatus of claim 10 wherein the initial operation identifies the natural resonant frequency of material by applying a broadband impulse force to the material with the stimulating energy source and analyzing a spectrum of the vibrations received by the vibration sensor.
12 . The apparatus of claim 1 wherein (d) extracts thickness of the material and not internal static stress.
13 . The apparatus of claim 1 wherein (d) extracts internal static stress and not thickness of the material.
14 . The apparatus of claim 1 further including (e) of repeating (a)-(d) to provide composite values of elasticity and at least one of thickness of the material and the internal static stress of the material combining individual measurements.
15 . The apparatus of claim 1 further repeating (e) to provide an output indicating an evolution of at least one of elasticity, thickness of the material, and internal static stress of the material over time.
16 . The apparatus of claim 1 further including at least one of a lamp, a heater, and a fan controllable by the controller to provide an environmental stress to the material from illumination, heating, or dehydration.
17 . The apparatus of claim 1 wherein the material has a Young's modulus between 1 Pa and 1 MPa.
18 . The apparatus of claim 1 wherein the measure of elasticity is Young's modulus.
19 . A method of measuring elastic properties of a material employing an apparatus providing:
a stimulating energy source coupled to the material to excite the material into vibration;
a vibration sensor adapted to provide an electrical signal measuring vibration of the material; and
a controller executing a stored program to control the stimulating energy source and vibration sensor; the method comprising operating the controller to:
(a) excite the material with the stimulating energy source into linear and nonlinear vibration;
(b) cease excitation of the material to measure the linear and nonlinear vibration with the vibration sensor during a free vibration period;
(c) extract a value of Young's modulus from the measured linear vibration; and
(d) extract at least one of a thickness of the material and an internal static stress of the material from the measured nonlinear vibration.