We study conversion of thermal energy to mechanical energy and vice versa in α-FPU chain with spatially sinusoidal profile of initial temperature. We show analytically that thermal expansion and temperature oscillations, caused by quasiballistic heat transport, excite mechanical vibrations with growing amplitude. This new phenomenon is referred to as "ballistic resonance". At large times, the mechanical vibrations decay monotonically. No recurrence, typical for the original FPU problem, is observed. We assume that the absence of recurrence is due to finite initial temperature of the chain.Energy of an isolated solid body can be converted from a mechanical form to a thermal form and vice versa. Conversion of mechanical energy to thermal energy leads, for example, to damping of mechanical vibrations (or waves). Thermal expansion and heat transport cause an opposite process, i.e. conversion of thermal energy to mechanical energy.In macroscopic continuum theories, the conversion is modeled by coupling between the equation of momentum balance and the equation of energy balance. An example of a continuum theory, describing the conversion, is the linear thermoviscoelasticity. Macroscopic theory of linear thermoviscoelasticity is well-developed [1]. However at micro-and nanoscale this theory faces significant difficulties, because macroscopic constitutive relations can be inapplicable at lower length scales. For example, recent theoretical [2-4] and experimental studies [5][6][7][8][9] show that at micro-and nanoscale the Fourier law of heat conduction can be violated. In particular, ballistic regime of heat transport is observed [10,11].A convenient model for investigation of thermomechanical processes at micro-and nanoscale is the Fermi-Pasta-Ulam (FPU) chain [12]. Despite the apparent simplicity of the model, analytical description of thermoelasticity, heat transport and energy conversion in the FPU chain remains a serious challenge for theoreticians.Several anomalies of thermomechanical behavior are observed for the FPU chain. The heat transport in the FPU model is anomalous, i.e. the Fourier law is not satisfied [13][14][15]. The Maxwell-Cattaneo-Vernotte law also fails to describe the heat transport in FPU chains [16,17]. Harmonic approximation allows to derive equations [18,19] and closed-form solutions [20, 21] describing heat transport in chains. However the question arises wether temperature field, obtained in harmonic approximation, can be used for estimation of thermoelastic effects (e.g. excitation of mechanical vibrations due to thermal expansion). We address this question below.Conversion of mechanical energy to thermal energy is even more challenging. Studies of this process have a long history, starting from the pioneering work of Fermi, Pasta, and Ulam [12]. In paper [12], initial conditions, corresponding to the first normal mode of a chain, were considered. It was shown numerically that energy of this normal mode demonstrates almost periodic behavior, i.e. the system does not reach thermal equilibrium...