So, now let's look in more detail at what's happening beyond the tensile strength, and the fact that the material is not getting weaker even though the engineering stress is dropping off. And this sort of narrowing of the diameter occurs. Again, if we recall that up in this area, even with Poisson's ratio we're taking into account, the cross-sectional area here is approximately still equal to that initial cross-sectional area. That is the ongoing, even though we're distorting this material now, and again, deforming it severely at this point, we're going to show you this stress concentration in the center, in the neck down. We're still defining the engineering stress as the load over A0. We're continuing to define the strain as the Delta L over l knot. This A knot continues to be our normalizing factor. It's not taking into account the true cross-section areas. What's happening at this point is that the actual cross-sectional area at this point, A, is significantly less than A naught. So, as a practical matter the true stress, Is now significantly greater than the engineering stress. And so if we were to plot, maybe we'll color code this. If we were to plot the true stress. I'll just add my little red line here as the true stress. That would really be about the same up to this point, but will continue to go up. So the material is really not becoming any weaker or softer as the engineering stress strength plot implies. In fact would continue to show higher strength if you will. More and more resistance to deformation right up to the point of failure. But again we were convenienced historically, have defined engineering stress and engineering strain in this way. And so we live with this fact, but nonetheless, it is a practical consideration, because again, we're designing these systems. You mechanical engineers, you civil engineers, are defining systems in terms of the initial dimensions of the system. And these gross degrees of deformation are, if you will, worse case scenarios in which it is not the anticipated dimension of the system in the original design. So it's not inappropriate that we do monitor stresses and strains in that sense, so when we do get into the overload condition where we are stretching the material to extensive degrees of plastic defamation, then this tensile strength Or T.S., is a very useful parameter to indicate, within the dimensions of the original design, this is the maximum load, the maximum stress that can be withstood by this particular material. So it becomes a very important kind of ultimate strength indicator in the context of the dimensions of the overall part.