 So the next few videos are going to move on to talk about stresses in soils But first let's just recap quickly what a stress is now It's important to think about Stresses in soils because that's what soils respond to It's not the forces, but it's the the area over which the forces are spread So that's what stress is. That's the definition of stress. It's the force over the area So stress is equal to force over area So the units of stress would really be in force over area Canonitin per meter squared When we're talking about normal forces in soils We use the symbol sigma When we're talking about shear stresses the symbol we use is tau But we'll come back to shear stresses later So how do materials respond to changes in stress? Well what they do is they deform so they change shape or they change volume And you can measure that change in shape through something called strain So if we're just looking at strain one dimension So strain and that's equal to the change in length over the original length So if I had a spring and I added stress to that spring so I pulled it apart The strain that the spring would be subjected to would be the change in length So how much I displaced it but divided by its original length And that's given the symbol epsilon or gamma Gamma if we're talking about shear strain And epsilon if we're talking about fully metric strain So a stress strain diagram, a stress strain graph is plotted on these axes So where we have stress on the y-axis and strain on the x-axis And before we subject a material to any stress it would be down here So there would be zero stress on it and it will have zero strain When we subjected to some stress it would follow some sort of relationship between stress and strain And that relationship is given by something called Jung's modulus of elasticity So this straight line part of the graph is elastic deformation So for instance if I loaded a spring up, if I increased the length of the spring by putting stress into it If I then let that spring go it should deform back down to its original point So this is elastic deformation and the gradient of that line So the change in stress over the change in strain is equal to Jung's modulus of elasticity Which we give the symbol E So if we keep adding stress to it most materials will then go through a point where they eventually start straining with very small increments of stress We will reach something called plastic deformation where if we then if we got to this point And we took the load off and it would not return back down to its original position But to form back to some point here where there was irreversible strain So that's the plastic deformation and eventually you would get to a point where the material would fail or fracture So one way I think is quite helpful to think about it is if you take a stress value And you multiply it by the change in volume that the material is exerting Or is experienced, the change in volume that the material is experiencing And if you look at the units of those, so stress has units of the kilonewtons per meter squared And the change in volume has units of meters cubed The units that we are left with are kilonewton meters Which is the units of energy, the kilonewton meter is the same as the kilodule So the relationship between stress and the change in volume and the strain in the material Is really a way of talking about how much energy that the material has absorbed I think that's a useful way of thinking about it So we're going to come back to thinking about displacements when we talk about settlement and consolidation within soils But let's think a little bit more about stress