Principles of Dairy Machine Design

Module 5. Properties of material, failures and factor of safety

Lesson 23
MATERIAL AND PROPERTIES: STATIC STRENGTH, DUCTILITY, HARDNESS, FATIGUE, DESIGNING FOR FATIGUE CONDITIONS

23.1 Introduction

The knowledge of materials and their properties is of great significance for a design engineer. The machine elements should be made of such a material which has properties suitable for the conditions of operation. We will discuss the commonly used engineering materials and their properties in Machine Design.

23.2 Classification of Engineering Materials

The engineering materials are mainly classified as:

1. Metals and their alloys, such as iron, steel, copper, aluminum, etc.
2. Non-metals, such as glass, rubber, plastic, etc.

The metals may be further classified as:

(a) Ferrous metals and (b) Non-ferrous metals.

The ferrous metals are those which have the iron as their main constituent, such as cast iron, wrought iron and steel.

The non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminum, brass, tin, zinc, etc.

23.3 Physical Properties of Metals

The physical properties of the metals include luster, colour, size and shape, density, electric and thermal conductivity, melting point, etc.

23.4 Mechanical Properties of Metals

The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness. We shall now discuss these properties as follows:

23.4.1 Static strength

It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a material to an externally applied force is called stress.

23.4.2 Stiffness

It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.

23.4.3 Elasticity

It is the property of a material to regain its original size and shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines.

23.4.4 Plasticity

It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.

23.4.5 Ductility

It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms-percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice are mild steel, copper, aluminum, nickel, zinc, tin and lead.

23.4.6 Brittleness

It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads snap off without giving any sensible elongation. Cast iron is a brittle material.

23.4.7 Malleability

It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron, copper and aluminum.

23.4.8 Toughness

It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture. This property is desirable in parts subjected to shock and impact loads.

23.4.9 Machinability. It is the property of a material which refers to a relative case with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. It may be noted that brass can be easily machined than steel.

23.4.10 Resilience

It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume with inelastic limit. This property is essential for spring materials.

23.4.11 Creep

When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow a permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines.

23.4.12 Fatigue

When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of a progressive crack formation which are usually fine and of microscopic size. This property is considered in designing shafts, connecting rods, springs, gears, etc.

23.4.13 Hardness

It is a very important property of the metals and has a wide variety of meanings .It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness is usually expressed in numbers which are dependent on the method of making the test. The hardness of a metal may be determined by the following tests:

1. Brinell hardness test
   
2. Rockwell hardness test
3. Vickers hardness
   
4. Shore scleroscope.