# Initial shallow gradient in measured stress strain curve

What's causing the initial shallow gradient of the stress strain curve of ceramic in this graph? The rest of the curve is a straight line alright until the yield point.

Edit: the material being tested is alumina ceramic.

Given the test configuration and the test specimen dimensions I would suspect that the initial gradient slope is due to the gradual engagement of the test specimen.

I.e. the material top and bottom surface will have a roughness associated with it. Something like in the following image.

As a result when this is being compressed initially there is less area in contact, but gradually it increases. As the area of contact increases the slope will increase.

Although its not clear in the curve that you provided, if its a stress-strain (SS) or force displacement (F-d). If it is the latter (F-d) and the units are in inches the 0.005 would correspond to about 100$$\mu m$$ which is a common roughness for many surface finishes.

Also, looking at the image, I wonder if the material you are testing has a foam structure. Like the following picture.

In that case, you would expect to get gradual collapse of the foam structure and to get a ever increasing response.

• The machine I used was Instron, which only has two compression platens. They seem very different from the self-locking mechanisms that you mentioned. Also the alumina ceramic, the material being tested was shaped like a cylinder. Does this still apply? Sep 16 '20 at 7:16
• the test was compressive. I have edited the question to include a picture of the rig. Sep 16 '20 at 7:43
• Now that does seem like a very plausible explanation. Thanks for the help! Sep 18 '20 at 3:34