The main objective of the current study is the investigation of the behavior of structural joints consisting of Shape Memory Polymers. Taking into account the large environmental impact caused by the massive production of structural materials, the change of the current structural design philosophy is becoming essential. Conventional structural engineering approach involves the design of structures fulfilling first strength requirements and second serviceability limit states. The latter applies strict deformation limits mostly related to the comfort of the users and their perception of safety. Adaptive structures get inspiration by organisms present in nature, which are able to adapt their shape and properties as a response to changing surroundings. The capability of a structure to counteract an external stimulus such as earthquake event or strong winds may lead to significant material savings. In particular, adaptive structures are able to react (deform) to applied loads through controlled shape changes and redistribution of internal forces. To this end members with variable length and variable stiffness are required.
The current study focuses on the behaviour of SMP joints which experience two states: locked and released. In the first state, the joint maintains its main function to transfer the loads between the connected elements whereas in the released state the joint becomes softer allowing thus a degree of movement between the elements. 3D printed Shape Memory polymers are chosen as main material due to their variable stiffness properties when exposed to external stimulus such as temperature and their ability to recover their original shape unconstrained. The thermomechanical cycle of thermally triggered SMPs constitutes of the glassy, glass transition and rubbery phase. Glass transition temperature is a threshold temperature, at which the material changes phase and a significant reduction of the young modulus occurs.
Shape Memory Polymers belong to the class of viscoelastic materials with time and temperature dependent properties. A number of experimental tests is performed for the thermomechanical characterization of the polymer. Namely, a rectangular sample is subjected to dynamic loading in a range of frequencies, while the temperature increases from 30oC to 90oC. The storage and loss modulus as well as the phase angle δ are measured as a function of temperature. The storage modulus describes the elastically stored and released energy and it is the actual modulus of the material while the loss modulus represents the dissipated energy due to friction between the molecular chains. From the storage and loss modulus, the Prony Series coefficients needed for the numerical simulation of the material, are calculated. Additionally, tensile tests are performed at various temperatures in order to define the strength and give an indication about the maximum elongation of the material. It is noted that at 25oC the strength of the material is approximately 36MPa. With the increase of the temperature the strength of the material drops while the maximum elongation increases. The shape memory effect of the polymer is also tested at different heating rates and ramp and isothermal temperature conditions. The aim of these tests is to define the factors that may influence the shape recovery phase of the polymer. It is observed that faster heating rates shift the transition phase of the polymer to higher temperatures causing delay on the onset of the recovery phase. On the other hand, slower heating rates enable the recovery process of the polymer at lower temperatures due to the intrinsic relaxation properties of the material. With respect to ramp and isothermal conditions, in both cases shape recovery occurs, with the difference that in first case the increase of the temperature triggers the recovery while in the second case the temperature is kept constant and the relaxation properties of the polymer contribute to the recovery of its original state.
The numerical simulation of the joint is based on the viscoelastic theory and makes use of the Prony Series coefficients calculated by the Dynamic Mechanical Analysis. For the validation of the numerical model, the DMA and the Shape Memory tests are numerically conducted. The numerical results are in good agreement with the experimental ones increasing thus the reliability of the results presented in the current study. Last, a truss structure is considered as a case study and the load bearing capacity of the SMP joint is assessed under permanent and variable loads. It is important to be mentioned that only the linear part of the material law is modelled numerically. Hence, the assessment of the load bearing capacity of the joint is realized until the end of the linear part of the stress – strain curve. It is concluded that at glassy state, the SMP joint is able to sustain 95% of the permanent loads and 45% of the Ultimate Limit State load combination. Upon heating, the load bearing capacity of the joint is decreased. At the temperature of 40ºC, the yield stress of the material is 26MPa which drops to 15MPa after necking. At the same temperature, the maximum strain of the material increases up to 150% meaning that it is able to accommodate large shape changes.
To conclude, the current research studies the mechanical behavior of SMPs and their potential use in the field of structural engineering and especially in the design of adaptive structure where large shape changes are required. Although the strength of the material at its glassy state is comparable to other structural materials such as wood its relaxation properties need to be studied carefully. Last, it is proved that SMP can accommodate large shape changes but reinforcement of the material is needed in order to increase its load
bearing capacity.
The present master thesis is part of the research program “Lighthouse Project 2017” entitled “Adaptive Joints with variable stiffness” with principal researcher Qinyu Wang from TU Eindhoven.