ABE313 Topic 6 Mechanical Properties PDF

Topic 6 MECHANICAL PROPERTIES In this topic, only the mechanical properties of solids will be described as next topic deals with the rheological properties of AB materials. Definition: Rheological and mechanical properties According to a broad definition as described by Mohsehin (1970...

Topic 6 MECHANICAL PROPERTIES In this topic, only the mechanical properties of solids will be described as next topic deals with the rheological properties of AB materials. Definition: Rheological and mechanical properties According to a broad definition as described by Mohsehin (1970), mechanical properties are related to an applied force and the behavior of a material when a force is applied on it. Rheology on the other hand is defined as “a science of the study of deformation and flow”. It can be noticed that when the force, acting on a material, results in deformation and flow the rheological and mechanical properties are identical. Rheology takes also into consideration the time effect under loading. Then three involved parameters are force (𝐹), deformation (𝐷), and time (𝑡); according to a rheological point of view, mechanical behavior is also a function of force deformation and time. In engineering terminology about mechanical properties, the term stress (𝜎) is used to express the force per unit area [N/m 2 or Pa (Pascal)], considering the force is expressed in N. Stress can be further described as normal stress, which is perpendicular to force acting plane. According to its direction, normal stress can be tensile or compressive. Shear stress (𝜏) is another type of stress, which is tangential to the acting plane of the force. Torsional is a less common stress type in food applications and is a combination of shear stress and uniaxial compression or tensile stresses perpendicular to the applied forces. Finally, bulk stress is referred to as volume change. There are also other stress definitions such as true stress, fracture stress, and compressive strength described in the following paragraphs. Strain, on the other hand, is the dimensionless change of size or shape due to a force. The size change in one axis can be described as axial strain (parallel to the axis of the sample), transverse strain (perpendicular to the axis of the sample) both described with the symbol 𝜀 or shear strain (angular strain), described with the symbol 𝛾. Strain can be also found as true strain, later described. Destructive measurements involve usually different loads types. For example, in bending tests, both normal (compressive and tensile) and shear forces are applied. Solid foods can be classified into elastic and inelastic materials according to their response under loading. According to Fig. 6.1, the first category corresponds to time independent, the second one to time-dependent solid foods, which are further classified as viscoelastic and viscoplastic. Concerning the flow, it can be classified as plastic and viscous. In plastic materials, specific stress is required that the flow begins; in viscous the flow begins immediately under force applying. Figure 6. 1. Classification of material rheological behavior. The elastic solids represent as ideal behavior and are also described as ideal solids. An ideal solid is also known as a Hookean solid. Consequently, an ideal liquid behavior is described as a Newtonian liquid. In time-independent materials, their response under load does not depend on time or loading rate. A reverse deformation takes place, meaning that a full recovery of their original size and shape is observed when the applied force is removed, as long as the elastic limit is not exceeded. The pathway of shape and size recovering under force unloading can be differ between Hookean and non-Hookean solids, both elastic. In a Hookean elastic body, there is a linear stress/strain relationship and the pathway of loading and unloading is the same. In a non-Hookean elastic body, the stress/strain relationship is not linear and when the body returns to its original shape and size under unloading, a different pathway is followed. In reality, ideal (elastic) behaviors are extreme ones, and most foods are viscoelastic. More precisely, most foods are inelastic, meaning that they present a time-dependent behavior under loading. The conditions, the stress applied, and the time determine the degree of their solid or liquid character. Viscoelastic or viscoplastic behavior combines both such a solid-like (elastic) and liquid-like (viscous) behavior. The difference is that a viscoelastic material will recover its initial shape partially, whereas viscoplastic does not recover its deformation, as shown in Fig. 6.1. It is important to note the impact of the strain applied. Most food materials are viscoelastic at small deformations whereas viscoplasticity is a behavior found at large deformation. When deformation is small the basic mechanical principles are theoretically described with reasonable accuracy. A small deformation in engineering material corresponds to strain