Thermoelastic expansion mechanism

In addition to the temperature dependence of the properties such as strength and modulus, which we will discuss individually for each material class, there are two fundamental topics that are often described in the context of heat transfer properties or thermodynamics of materials—for example, thermal conductivity or specific heat—but are related more to mechanical properties because they involve dimensional changes. These two properties, thermoelasticity and thermal expansion, are closely related, but will be described separately. 

Thermoelastic Effect A mechanical phenomenon that involves the thermal expansion coefficient is the thermoelastic effect, in which a material is heated or cooled due to mechanical deformation. The thermoelastic effect is represented by the following relation ... 

From the dynamic mechanical investigations we have derived a discontinuous jump of G and G" at the phase transformation isotropic to l.c. Additional information about the mechanical properties of the elastomers can be obtained by measurements of the retractive force of a strained sample. In Fig. 40 the retractive force divided by the cross-sectional area of the unstrained sample at the corresponding temperature, a° is measured at constant length of the sample as function of temperature. In the upper temperature range, T > T0 (Tc is indicated by the dashed line), the typical behavior of rubbers is observed, where the (nominal) stress depends linearly on temperature. Because of the small elongation of the sample, however, a decrease of ct° with increasing temperature is observed for X < 1.1. This indicates that the thermal expansion of the material predominates the retractive force due to entropy elasticity. Fork = 1.1 the nominal stress o° is independent on T, which is the so-called thermoelastic inversion point. In contrast to this normal behavior of the l.c. elastomer... 

A third source of stress waves derives from the expansion of any gases (CO, CN, N2, CH3 etc.) produced by thermal or photochemical decomposition within the substrate [104]. This factor, for instance, has been invoked to account for the transient stresses of about 0.1 MPa detected in the UV irradiation of polyimide below the ablation threshold [106]. In the case of doped PMMA, irradiation with 150-ps pulses at 1064 nm, Hare et al. [104] estimate that at the ablation threshold, the thermoelastic mechanism and the expansion of the decomposition products contribute about equally to the generated pressure. For specifically designed polymers that upon irradiation form a high enough concentration of volatile products, the generated pressure has been suggested to be the primary cause of material ejection [68-69]. 

Returning now to uniaxial fibre orientation, we can use the same models of mechanical coupling to predict thermoelastic properties of the composite. If the coefficients of linear expansion of fibres and matrix respective) are a/ and a , the coefficients aj and 02 for the composite parallel to axes 1 and 2 can be shown to be... 

---