Measuring in the shear zone is very, very difficult. One possibility is with a termographic camera, but the measure have poor precision and you dont use lubricant. Turning better than milling, because the contact of the tool is constant.

Other possibility is with a termocuple 0,2mm under the cutting surface of insert. But this measure is "far" from shear cut.

I have tried thermocoupled embeded into the workpiece during my Phd Studies. Not a fully accurate process.

How about combining the two embedded thermocouple position plus a contactless laser temperature measuring device and then try to corelating the three measurements to get a more accurate estimation of the shear zone temperature?

I think for research, this could be trialled!

How do measure the amount of phases in the casting of ductile iron after getting the intensity of that phase by XRD.

During machining operation, temperature may be measured by using infrared thermography. You may try for this;=.

There are three methods to do that:

1.Thermocouple

2.Thermo-camera(IR camera)

3.Resistance Temperature Detectors (RTDs)

Probably you already know the first 2 options, and the third one works like coating on the tool edge.

To sum up, personally, I prefer thermo-camera. Because of its simplicity in terms of experimental setup and in terms of interpretation of results. But the problem here is uncertainty of experiment result and zooming the camera on shear zones. But nothing is impossible with little bit modification you can get whatever you want.

Good luck

I thing changing in temperature can cause change in natural frequencies, so by using accelerometers, Ultrasonic sensors or microphones maybe estimating temperature can be possible. 

Good to know about varioua possibilities on temperature mesurement in shear zone. Thank you for sharing your ideas.