I used 5 dumb- bell shaped specimen to do the tensile test of High speed steel with hardness 59-61HRC. Every time it fractured at the radius between gage and shoulder. Is there any special shape of the specimen when doing the tensile test of brittle material?
Possible, there is a stress concentrator in your specimen at the radius between gage and shoulder or some distortion which provokes the undesirable failure of brittle materials.
1. Try to minimise this distortion. For example, introduce the hinge for the axial transmitting of tensile load on the specimen.
2. Try to improve the shape of specimen for uniform stress distribution obtaining. You may fabricate the end parts of working part of the specimen in form of ideal liquid jet which is flowing from the the hole in bottom of vessel under gravity. The form of this jet coincides with uniform stress distribution in specimen under tension.
Best regards
Can you do drilling and tapping in the ends of the sample. If possible test some pieces with a drilled and tapped hole at each end face. Put a threaded rod into the drilled holes at the ends and grip the rods in the machine grips. Ensure that the threaded rod does not fail before the sample This will eliminate the possibility of off axis loading.
Best Regards
In addition to inhomogenity in the sample another cause can be the pulling speed. What speed are you pulling (strain rate). At high speeds, it is more probable to fail between shoulder and gauge length.
Thank you for the answers. Followings are the addition information Mr. Niazi and Mr. Bolton asked.
1. The material used was JIS- SKH61 (AISI M2).
2.This was heat treated to get the hardness of HRC 59-61.
3. After heat treatment final dimension was achieved by CNC lathe.
4. Strain rate in the experiment was 0.017 (1/60).
I am thinking about to add a curvature in the middle of the specimen where there will be 2% of diameter reduction (ex: dia 10mm at end and dia 9.8mm at the middle) to bring the fracture point to middle. What do you think on this?
Preparing the specimen with lathe is not one of the best methods. You need to specify very high surface finish, high tolerance on the diameter and concentricity to get a good specimen. The strain rate is low so the dynamic effects can be excluded. If you make a notch in the middle your results will change compared to the standard test. Adding a notch increases the triaxiality so the stress state will not remain uniaxial (locally). I do not know what is the purpose of the test so i cannot say if its a good idea to add a notch. By the way are you following a standard procedure for testing?
Dear Mr. Niazi,
Thank you for your quick reply. I am going to ask for grinded finish of the specimen surface this time. Purpose of this test is to get the ultimate tensile stress and youngs modulus for SKH51 with 59-61HRC. To my knowlage I am following the standered procedure.
If the aim of these tests is to get the mechanical properties, I would suggest to follow a testing norm such as ASTM E8. Everything is specified on this norm. I have attached a snapshot from Wikipedia which shows the table for dimensions and tolerances for standard specimen according to E8.
Yes, the procedure of shaping with high quality of surface treatment is difficult enough but this approach gives optimal result in sense of uniform stress distribution as prescribed by standards on mechanial tests. Good luck!
Thank you for all your support. I will do the test again base on ASTM E8 with better surface finish.
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