Retell the textMechanical Properties

Theme 1. Development of Mechanics

1. Topical vocabulary:

2. Read and translate the text into your native language:

Mechanical Properties

By mechanical properties, we mean those properties that determine the behavior of food materials when subjected to external forces. As such, mechanical properties are relevant both to processing (e.g. conveying, size reduction) and to consumption (texture, mouth feel).

The forces acting on the material are usually expressed as stress, i.e. intensity of the force per unit area . The dimensions and units of stress are like those of pressure. Very often, but not always, the response of materials to stress is deformation, expressed as strain. Strain is usually expressed as a dimensionless ratio, such as the elongation as a percentage of the original length. The relationship between stress and strain is the subject matter of the science known as rheology (Steffe, 1996 ).

We define three ideal types of deformation (Szczesniak, 1983 ):

Elastic deformation: deformation appears instantly with the application of stress and disappears instantly with the removal of stress. For many materials, the strain is proportional to the stress, at least for moderate values of the deformation. The condition of linearity, called Hooke’s law (Robert Hooke, 1635–1703, English scientist) is formulated in Eq. (1.1):

where

Plastic deformation: deformation does not occur as long as the stress is below a limit value known as yield stress. Deformation is permanent, i.e. the body does not return to its original size and shape when the stress is removed.

Viscous deformation: deformation (flow) occurs instantly with the application of stress and it is permanent. The rate of strain is proportional to the stress (see Chapter 2). The types of stress are classified according to the direction of the force in relation to the material. Normal stresses are those that act in a direction perpendicular to the material’s surface. Normal stresses are compressive if they act towards the material and tensile if they act away from it. Shear stresses act in a direction parallel (tangential) to the material’s surface (Figure 1.1).

The increase in the deformation of a body under constant stress is called creep. The decay of stress with time, under constant strain, is called relaxation.

1. Translate the following sentences from English into Russian:

1. Типы стресса классифицируются в соответствии с направлением силы по отношению к оборудованию.
2. Штамм обычно экспресс в качестве безразмерного отношения, таких как удлинение в процентах от первоначальной длины.
3. Механические свойства актуальны для обработки (например, транспортировки, измельчения) и потребления (текстуры, ощущение во рту).
4. Деформация является постоянной, то есть организм не возвращается к исходному размеру и форме, когда снимается напряжение.

2. Answer the following questions:

1. What does the mechanical properties of food materials mean?
2. How do you understand the “stress”?
3. How many types of deformations of food materials do you know?
4. What is the difference between compressive and tensile stress?

Retell the textMechanical Properties

Theme 2.Sewing machines

Task 1. Topical Vocabulary