Experimental Study On Nonlinear Mechanical Properties Of Aluminum Foam

Aluminum foam is a good functional and structural material,widely used in transportation,building decoration,aerospace and other fields.Aluminum Foam structures in these applications are often in a dynamic load environment,the random load and periodic load excitation is inevitable,easily lead to forced vibration.In this paper,the method of optical measurement is employed to explore the forced vibration mechanical properties of the foamed aluminum cantilever beams with different porosity,The main contents are as follows:1.Traditional vibration test methods are acceleration sensor method and laser Doppler method,they are limited to point-by-point measurement,in contrast,optical measurement has many excellent features such as whole filed,high precision and non-contact characteristic.In this paper,a real-time subtraction of electronic speckle pattern interferometry method based on phase disturbance is proposed,and the detailed theoretical derivation is given.The dark stripes obtained by real-time subtraction are clearly correspond to the zeros of the zeroth-order Bessel functions.Compared with the fringe pattern modulated by the first-order Bessel function in the traditional subtraction method,the fringe pattern obtained by real-time subtraction has high contrast and is more convenient for quantitative calculation.The two phenomena in the experiment are explained reasonably:the contrast of the fringe pattern in the high-order mode is better than counterpart of the low-order mode;the fringe pattern generated by the amplitude fluctuation is co-modulated by the first-order Bessel function and the sine function.2.Using the real-time subtraction method proposed in Chapter 2,the electronic speckle interferometry system is established to study the amplitude-frequency characteristics and damping characteristics of the foamed aluminum cantilever samples with different porosity.The results show that the first order bending frequency and the porosity of the foamed aluminum cantilever exhibit a linear relationship.It can be considered that aluminum foam is a macroscopically homogeneous material in the study of the natural frequency of aluminum foam.Due to the complexity of the aluminum foam space,the damping properties and the porosity have no regular relationship,but the internal friction values of each group of samples are much larger than pure aluminum.3.During the forced vibration experiment,it was found that the foam aluminum cantilever has a strong nonlinear response,mainly reflected in the multiple-frequency response and amplitude-force characteristics:The natural frequency of the structure is an integer multiple of the excitation frequency,and the odd multiple-frequency response is the most obvious and the even multiple-frequency response is weak.With the increase of porosity,even frequency response quickly decay disappears,while the higher odd frequency response still exists.It can be considered that the odd multiple-frequency response is the intrinsic property of the material same as the natural frequency of the structure;The amplitude-force curve of linear forced vibration system shows that the amplitude of response increases with the amplitude of vibration,However,it was found that during the nonlinear vibration,the amplitude-force curve of the foam aluminum cantilever is no longer satisfied with the growth type of the linear system,Except for the triple frequency response,the amplitude-force curves of the other multiple-frequency responses all have the characteristics of fall back and spring back.