Nonlinear oscillators have been given growing attention due to their ability to enhance the performance of energy harvesting devices by increasing the frequency bandwidth. Duffing oscillators are a typical type of nonlinear oscillator characterized by a symmetric hardening or softening cubic restoring force. In order to realize the cubic nonlinearity in a cantilever at reasonable excitation levels, often an external magnetic field or mechanical load is imposed, since the inherent geometric nonlinearity would otherwise require impractically high excitation levels to be pronounced. As an alternative to magnetoelastic structures and other forms of symmetric Duffing oscillators, in this paper, an M-shaped bent beam with clamped end conditions is investigated for bandwidth enhancement under base excitation. The M-shaped beam geometry can exhibit significantly asymmetric spring behavior: hardening in one direction and softening in the other. A particular advantage of the M-shaped structure is its well-pronounced nonlinear characteristics without needing an external component to create hardening or softening. The force-displacement relationship of the M-shaped beam with a central lumped mass attachment is experimentally identified and asymmetric nonlinear behavior is verified. The purely elastic system parameters (such as the linear and nonlinear stiffness components) identified from the experiments are used in numerical simulations and compared with the experimental results. A quadratic damping term is included to account for nonlinear dissipative effects. Bandwidth enhancement with increasing base excitation is investigated experimentally and numerically. Very good agreement is observed between the simulated frequency response curves and experimental measurements.
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