How is the UTS different from breaking stress and why?

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1. May 27, 2015

Tangeton

By definition the UTS is the maximum stress a material can take. But how exactly can a material not break after reaching the UTS if it is so? Why is there a breaking stress and how come on the graph of stress against strain the stress seems to decreases before the braking stress?

2. May 27, 2015

PhanthomJay

There is an engineering stress strain curve and a true stress strain curve. The engineering stress strain curve assumes that the member cross section area remains constant at all levels of tensile load, but in actuality, cross section starts to significantly reduce (called necking) at high stress values beyond the yield point, in which case if you plot true stress, which accounts for the reduced cross section area , versus strain, the true stress value always increases up to rupture, whereas if you use engineering stress, you get a peak on the curve prior to significant necking, and beyond that, the stress gets lower because it is assumed that cross section remains constant . The value of the engineering stress at this peak is called the ultimate tensile strength, whereas the breaking strength is the rupture stress at point of failure .

3. May 28, 2015

Tangeton

I understand the point you're trying to make about cross-sectional section remaining constant, but since Stress = F/A, if A is constant, should stress be still increasing with the force applied? I know that it is a stress vs strain graph, but strain is extension over length and the bigger the extension, the larger the force from Hooke's law, so the force must be increasing with stress it would seem to me?

4. May 28, 2015

PhanthomJay

But Hooke's Law does not apply after yielding, since the stress strain curve is no longer linear beyond the yield stress ( we're talking steel or aluminum as an example), and thus, increasing strain no longer implies increasing force. Ideally, when you apply an increasing tensile load to a test specimen, the elongation and strain increase as the applied load increases until yield, but then the elongation increases without much increase in the applied load, sort of like silly putty where stretching becomes extensive with no increase in applied load required.