Thermal Prestress in Composite Compliant Shell Mechanisms

Abstract

Due to the anisotropic nature of fibre-reinforced laminates, thermally-induced internal stresses can remain in the material after manufacture. Mismatches between coefficients of thermal expansion are especially prominent in thin shells with fewer plies or large angle variations. Such stresses cause out-of-plane warping and are therefore often deliberately avoided. Utilising their effects on structural behaviour however, can enable stiffness-tailored composite compliant mechanisms. Work detailed in this paper aims to exploit thermal prestress to reduce the torsional stiffness of cross-ply tape laminate springs. An extension of an analytical tape spring model with composite thermal analysis is presented, which shows that thermal effects cause significant changes to the energy landscapes of thin composite shells. Tape springs that would otherwise be monostable structures become bistable and exhibit greater ranges of low-energy twisting when thermally-induced prestress is present. Predicted shell geometries are compared with finite element models and manufactured samples, showing good agreement between all approaches. The limited feasibility of zero torsional stiffness composite tape springs is discussed, as well as wider challenges involved in manufacturing prestressed composite compliant mechanisms such as fibre misalignment and moisture ingress.

On the source of the thermoelastic response from orthotropic fibre reinforced composite laminates

“…Therefore, as IM7/8552 is a very well characterised material, the thermal properties were taken from the literature [22][23][24] as 𝐶 𝑝 = 857 J kg −1 K −1 [22] and the coefficients of thermal expansion were taken as the mean of the values found in [23] and [24] shown in Table 6, i.e. 𝛼 1 = −0.3 ⋅ 10 −6 K −1 and 𝛼 2 = 28.4 ⋅ 10 −6 K −1 , as the 𝑉 𝑓 are close to the measured 𝑉 𝑓 = 0.63 ± 0.03.…”

Visualising Compliance of Composite Shell Mechanisms

“…For example, a scoliosis brace must be of an anatomically desirable form, fitting closely to the patient's body [9,10]. Advanced composite materials offer a potential solution in these instances by providing a light-weight, anisotropically tailorable material system that offers increased design freedom and deformation control, without changing the shell's geometry [11].…”