# How do you identify the elastic limit and yield point on a stress/strain graph? I understand what these points are, but I'm struggling to identify E and Y on certain graph shapes.

With graph B I can identify Y, but there doesn't seem to be a clear elastic limit between P and Y.

With graph C there doesn't seem to be a clear yield point or elastic limit, so I'm not sure where I would place them.

Graph D doesn't follow Hooke's Law at all, so there is no limit of proportionality. In this case, where would the elastic limit and yield point go?

Note: This is not a specific homework question, in case anyone thinks the question is too specific. This is a general question about the placement of points on a graph that I need to be able to do for my A-Level course.

• Possible duplicate of Yield Point and Elastic Limit – sammy gerbil Jul 3 '17 at 23:06
• It's not. I read that question before posting this one. They're about the same topic (materials and stress strain graphs) but are about different elements of those topics. That question was about what Y and E are, and this question is about how to identify them on a graph. – Pancake_Senpai Jul 4 '17 at 8:06

## 1 Answer

The elastic limit is the point beyond which the material will not return to its original length when the load is removed. Yield point is essentially the same, except that it is usually defined when the permanent strain reaches a particular level such as 0.2%.

Neither the elastic limit nor the yield point can be identified from a graph in which the load is continuously increased. In order to identify these points the load must be removed. So there is no way of identifying these points from the graphs you have been given.

The elastic limit is not related to the limit of proportionality, which can be identified from such a graph. In some cases, of which graph D is an example, the approximately-linear region does not pass through the origin. The limit of proportionality will then be at the end of this region.

• At which point would you say the elastic limit is on D, roughly? Is it even possible to tell? Also, I understand from your answer that the limit of proportionality is marked as the end of any proportionality, not just the part that follows Hooke's Law. Is that what you meant? – Pancake_Senpai Jul 4 '17 at 8:03
• @Pancake_Senpai 1. It is not possible to tell where E is on the graphs provided, because there is no way of knowing whether the material will return to its initial length when the stress is removed. You would have to do other tests to find out where E is. 2. In my definition the limit of proportionality is at the end of the linear region, which is quite far to the right in graph D. However, it all depends on what definitions you use. The linear region for D does not pass through the origin, so the behaviour is not strictly proportional. – sammy gerbil Jul 4 '17 at 10:37