Concept of Stress – Stress and Strain – Strength of Materials


hello friends here we are starting with a new subject called as strength of

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materials the name of the subject is strength of materials now when we talk about this subject as it is clear from the name this subject will deal with materials materials those are used in engineering applications and it is strength of materials that is how much would be the load carrying capacity of a certain material with the help of this subject we are able to calculate how much would be the load carrying capacity of various materials the knowledge of this subject is very important in the designing of some components example bridges roof trusses brackets plates bolts nuts everything for that the designing of that components will deal with strength of materials first so this is a basic subject which is studied before studying the designing of machine elements so we need to have deep knowledge of this subject when we are going for design of any new component next when I talk about strength of material it is also called as song which is the short-form of the subject some is very much important for civil engineering and mechanical engineering in both these branches the concept of Psalm should be very much clear and a person or an engineer should have a deep knowledge of this subject before he goes for designing some of the components so this is the stepping stone now when we talk about Psalm in that discussing the strength of materials the first thing which comes to my mind that is called as stress in Psalm the most common or the first term which we should study that is called as stress now stress if I define it it is it is the internal resistance to external loading it means stress will be the internal resistance or I can say the internal property of a material which will resist the external loading next I can even say that stress is defined as the ratio of external load to the cross-sectional area this is the definition of stress that stress is defined as the ratio of external load to the cross-sectional area this I can explain it with the help of a diagram so here in this diagram here I am drawing a rod which is subjected to external load P now when we are applying an external load over this rod and the load is trying to pull the rod in that case this rod will try to resist the external load that is if I draw this diagram by breaking the section so this external loading will try to pull the rod in its direction but there will be internal resistance set up that is for this load which is acting towards left there will be a resistance towards right for this load which is acting towards right there will be resistance towards left in other words I can say that when the rod is trying when the load acting on the rod is trying to pull the rod in that case there are resistance set up in the rod which will move towards the inner direction and it will try to avoid moving out because of the load so this internal resistance which is set up in the material is called a stress and here I have defined stress is the ratio of external load to cross-sectional area so I can write down the relation mathematically stress is denoted by letter Sigma and this is equal to P upon a that is external load upon cross-sectional area and here stress it’s unit will be in terms of Newton per mm square because load is in terms of Newton area is in terms of mm square and this area is nothing but the area of the rod that is if we have I am assuming that the rod is circular so your area would be corresponding to this diameter of the rod so here we have seen what is meant by stress what is the definition of stress next I’ll write the various units of stress and those are if suppose the stress in term is in terms of kilopascal 1 kilo Pascal because normally we denote stress by Newton per mm square but if it is in terms of 1 kilo Pascal then how to convert it so here 1 kilo means tennis – 3 Pascal and 10 raised to 3 Pascal means Newton per meter square next I keep 10 raised to 3 Newton as it is instead of meter I’ll write down 10 raised to 3 mm square so this will be 1 kilo Pascal is equal to 10 raised to 3 Newton divided by 10 raised to 6 mm square so hence 10 raise to 3 10 is 2 3 will get cancelled out here in the denominator I’ll have 10 raised to 3 so therefore the relation becomes 1 kilo Pascal is equal to 10 raised to minus 3 Newton per mm square so I’ll say that for example if the stress is given in terms of kilo Pascal I’ll take that if it is 50 kilo Pascal then this will be equal to 50 into 10 raise to minus 3 Newton per mm square similarly if it is 100 kilo Pascal then it will be 100 into 10 raised to minus 3 Newton per mm square so like this we have to convert so this was regarding kilo Pascal now if suppose the stress is in terms of mega Pascal that is 1 mega Pascal and we have to convert this into Newton per mm square so how we are going to convert this 1 mega means 10 raise to 6 Pascal then 10 raise to 6 Pascal is Newton per meter square next therefore 1 mega Pascal is equal to 10 raise to 6 Newton divided by 1 meter is 10 raise to 3 mm full square so here I have 10 raise to 6 Newton divided by 10 raise to 6 mm square so therefore 1 mega Pascal becomes 10 raise to 6 10 raise to 6 will get cancelled out so it is equal to 1 Newton per mm square now suppose for example if stress is given as 70 mega Pascal then we can directly write it as 70 Newton per mm square because 1 mega Pascal is 1 Newton per mm square next if it is 90 mega Pascal then it can be written as 90 Newton per mm square so we have seen the conversion of mega Pascal now if the stress is in terms of Giga Pascal so 1 Giga Pascal is equal to Giga main string raised to 9 Pascal so it is 10 raised to 9 and 1 Pascal is Newton 1 Newton per meter square so therefore 1 Giga Pascal is equal to 10 raised to 9 Newton upon 4 meter I will write down 10 raised to 3 mm square so here we have 10 raised to 9 Newton upon 10 raise to 3 to the power square so this would be 3 2 3 into 2 that is 6 10 raise to 6 mm square next 10 raise to 6 gets canceled out here we have 10 raised to 3 left so therefore 1 Giga Pascal is equal to 10 raised to 3 Newton per mm square so this is the relation which we have so suppose for example if stress is given as 60 Giga Pascal then it will be equal to 60 multiplied by 10 raised to 3 Newton per mm square if it is 84 Giga Pascal then this will be 84 into 10 raised to 3 Newton per mm square so here in this video we have seen the description the concept and the definition of stress along with its unit and how to convert that unit

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