Statics and Strength of Materials | The effects of forces on materials

 

A good knowledge of some of the constants used in the study of the properties of materials is vital in most branches of engineering, especially in mechanical, manufacturing, aeronautical and civil and structural engineering. The engineer must not only take into consideration the ability of the chosen material of construction to do the job, but also its cost. Similar arguments lie in manufacturing engineering, where the engineer must be able to estimate the ability of his/her machines to bend, cut or shape the artefact s/he is trying to produce, and at a competitive price! This chapter provides explanations of the different terms that are used in determining the properties of various materials

 

Introduction

A force exerted on a body can cause a change in either the shape or the motion of the body. The unit of force is the newton, N. No solid body is perfectly rigid, and when forces are applied to it, changes in dimensions occur. Such changes are not always perceptible to the human eye since they are so small. For example, the span of a bridge will sag under the weight of a vehicle, and a spanner will bend slightly when tightening a nut. It is important for engineers and designers to appreciate the effects of forces on materials, together with their mechanical properties.

 

The three major types of mechanical force that can act on a frame are:

 

(i) tensile

(ii) compressive

(iii) shear

 

 

Tensile force

 

Tension is a force that tends to stretch a material, as shown in Figure 2.1. For example,

 

(i) The rope or cable of a crane carrying a load is in tension

(ii) Rubber bands, when stretched, are in tension.

(iii) When a nut is tightened, a bolt is under tension. A tensile force, i.e., one producing tension, increases the length of the material on which it acts.

 

 

Compressive force

 

Compression is a force that tends to squeeze or crush a material, 

 

(i) A pillar supporting a bridge is in compression.

(ii) The sole of a shoe is in compression.

(iii) The jib of a crane is in compression. A compressive force, i.e., one that produces compression, will decrease the length of the material on which it acts.

 

Shear force

 

Shear is a force that tends to slide one face of the material over an adjacent face. For example,

 

(i) A rivet holding two plates together is in shear if a tensile force is applied between the plates

(ii) A guillotine cutting sheet metal, or garden shears, each provide a shearing force. 

(iii) Transmission joints on cars are subject to shear forces.