Bush65 Posted July 8, 2009 Share Posted July 8, 2009 Could it be something to do with the lumps on the ends of the sample? The lumps have a differing area to the actual bit you're testing, so with Pressure/Area = Force, then with the bigger area of the top bits, it will give the impression of requiring more Force? Maybe? I'm not sure, I do electronics! Yes the lumps on the end will affect the test, but possibly not in the way you are thinking. This is why I asked the following: ...If the tensile testing machine was in reasonable calibration, then I question your 25mm value for the gauge length of the test specimen.Edit: was your test specimen, prepared properly, to the standard dimensions? And is 25mm the gauge length for calculating strain? It is many years since I have done any tensile testing, and the samples used then were prepared far differently. But no doubt there are differences in testing machines and how the samples must be prepared for each. Quote Link to comment Share on other sites More sharing options...
muddy Posted July 8, 2009 Share Posted July 8, 2009 I presume you used a hounsfield tensometer? This is a simple observation and highly unlikely due to the numbers looking good but you did have the correct beam in didnt you? I cant find my graphs from when we did similar testing but the whole group got very accurate results from a number of samples just like yours (0.4, 0.6 and 0.8 carbon steel IIRC) over a range of 3 or 4 testing machines. Will. Quote Link to comment Share on other sites More sharing options...
clbarclay Posted July 8, 2009 Share Posted July 8, 2009 Well your results look vastly more usable than what I got during the first. The test samples included a roughly straigtened paper clip and an aluminium rod which slipped in the clamps more than it stretched. The results were not useable in any practical sense, but for the purpose of the coursework report it provides lots to talk about in the discussion which is worth more marks. Quote Link to comment Share on other sites More sharing options...
Paddy_SP Posted July 10, 2009 Share Posted July 10, 2009 I'm afraid my brain shut down hours ago, so I haven't got the focus to read all the detail, however, I have what I hope is a pertinent question - are you sure the tensile tester had the right load cell fitted? Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted July 11, 2009 Author Share Posted July 11, 2009 I'm afraid my brain shut down hours ago, so I haven't got the focus to read all the detail, however, I have what I hope is a pertinent question - are you sure the tensile tester had the right load cell fitted? AFAIK there was no load cell. It worked on a screwthread to extend the sample. We have concluded that there was something very wrong the either the machine or the sample. I'll let you know what the feedback is. Quote Link to comment Share on other sites More sharing options...
Paddy_SP Posted July 19, 2009 Share Posted July 19, 2009 AFAIK there was no load cell. It worked on a screwthread to extend the sample. We have concluded that there was something very wrong the either the machine or the sample. I'll let you know what the feedback is. I think we've got our wires crossed here - most tensile testers use a screwthread to extend the sample - I was referring to the means of measuring the forces... From bitter experience, I can tell you that it's very common for the wrong load cell to be fitted by unknown people who then make a point of not telling anyone... Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted July 19, 2009 Author Share Posted July 19, 2009 I think we've got our wires crossed here - most tensile testers use a screwthread to extend the sample - I was referring to the means of measuring the forces... From bitter experience, I can tell you that it's very common for the wrong load cell to be fitted by unknown people who then make a point of not telling anyone... That's very interesting to read. I've since re-tested a sample just out of curiosity and the same result repeated. There is definitely something very wrong with the machine - this may explain it. I'll suggest it to the administration. Quote Link to comment Share on other sites More sharing options...
Cheesy Posted July 19, 2009 Share Posted July 19, 2009 That's very interesting to read. I've since re-tested a sample just out of curiosity and the same result repeated. There is definitely something very wrong with the machine - this may explain it. I'll suggest it to the administration. I dont think this is the problem since you UTS and yeild look about right, its the elongation that is the problem, most likely is that the sample is too big for the machine so you cant get an accurate elongation, the elongation needs to be measured in at least hundredths of a mm Quote Link to comment Share on other sites More sharing options...
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