# Concepts and Terminologies in 4D Printing
## 3 Properties of 4D Printed Parts  
This section describes the properties of 4 DP, the factors that influence the 4 DP process, as well as the materials that are currently investigated and often used in the literature. In terms of the key properties, the shape fixity, shape recovery and repeatability are often used to describe the characteristics of 4 DP parts that are attributed to the quality of the components. In some instances, \"repeatability\" is an important aspect of 4 DP parts, having the ability to repeat the entire cycle without pronounced fracture or significant change to the permanent shape [19]. The \"shape fixity\" is the extent of a temporary shape being fixed for a \"shape memory polymer\" (SMP) [20]. The material properties of SMPs are described in Section 3.2. The \"shape fixity ratio\" (R\({}_{\beta}\)) is defined as the ability of a 4 DP part to instill a mechanical deformation applied during the programming process (1). The calculation is derived from the ratio of the strain \({}^{\mathrm{c}}\)\({}_{\mathrm{unload}}\) measured upon removal of the load after the cooling process, to the strain \({}^{\mathrm{c}}\)\({}_{\mathrm{load}}\) that was subject at a specific temperature above the T\({}_{\mathrm{g}}\)[21]. \[Rf\left(\%\right)=\frac{{}^{\mathrm{c}}\!\!\!\!\!\!\!\!\!\! of the material. If a heat source is involved, then the term \"thermomechanical degradation\" is used. Optical microscopy and Scanning Electron Microscope (SEM) can be used to evaluate the morphology of the material microstructure and to assess the severity of degradation. There are seven AM processes described in ISO-ASTM 52900:2017 [1] that establish the processes used in AM technology. Although, in principle, all seven AM processes could be adapted for 4 DP, most studies are related to polymers and composites that primarily use material extrusion, material jetting, and vat photopolymerization. Studies related to the use of metals often involve selective laser melting, which is classed as a laser powder bed fusion (L-PBF) process. The definitions of AM processes frequently used in 4 DP include \"material extrusion\", in which material is selectively dispensed through a nozzle or orifice; \"material jetting\", in which droplets of build material are selectively deposited; \"powder bed fusion\", in which thermal energy selectively fuses regions of a powder bed; and \"vat photopolymerization\" in which liquid photopolymer in a vat is selectively cured by light-activated polymerization [1]. Within the 4 DP framework, the \"interaction mechanism\" is also known as the \"programming\" process, in which the material is deformed into a temporary shape. When heat is involved in the programming process, the term \"thermomechanical programming\" or \"constrained-thermo-mechanics\" is used. In this approach, the stimulus is heat and the smart material retains the shape memory effect. The compressive strain during the loading process fixes the programmed state. The use of jigs, forming templates, and fixtures are required during the application of the mechanical load [23]. According to [PERSON] et al. [8], this constitutes a four-step cycle. First, the structure is deformed by an external load at a high temperature; second, the deformation (strain and constrain) is maintained; third, the structure is unloaded at the low temperature and the desired shape is achieved; fourth, the original shape can be recovered by reheating the structure. \"Deformation\" refers to the shape of an object being changed by applying force, and pressure known as \"loading\". The deformation of the object depends on the force, the material, and whether the material is in an elastic deformation or a plastic deformation. In certain studies, \"integrated programming\" is used to refer to programming steps that are instilled during the fabrication process, thereby eliminating the need for separate thermomechanical programming [24]. Studies described that the AM process may influence the 4 DP effect, leading to a phenomenon known as \"pre-strain\".