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 .Tangeton said: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?
PhanthomJay said: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 .
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.Tangeton said: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?
The UTS, or ultimate tensile strength, is the maximum stress that a material can withstand before it breaks. Breaking stress, on the other hand, is the stress at which a material breaks or fractures. In simpler terms, UTS is the highest point a material can reach before it starts to deform, while breaking stress is the point at which it actually breaks.
Understanding the difference between UTS and breaking stress is crucial in engineering and material science. It helps determine the strength and durability of a material and allows engineers to design structures that can withstand the expected stresses without failure.
The UTS is typically calculated by dividing the maximum load applied to a material during a tensile test by the original cross-sectional area of the material. This gives the value of stress in units of force per unit area, such as pounds per square inch (psi) or megapascals (MPa).
No, the UTS and breaking stress are not necessarily the same value. The UTS is the maximum stress a material can withstand without breaking, while breaking stress is the point at which a material actually breaks. In some cases, the UTS may be slightly higher than the breaking stress due to factors such as material imperfections or testing conditions.
The UTS is just one measure of material strength, and it specifically refers to the strength of a material under tension. Other measures of strength include yield strength, which is the stress at which a material begins to deform permanently, and compressive strength, which measures a material's ability to withstand a compressive force without breaking. Each measure of strength provides important information about a material's mechanical properties and is used for different purposes in engineering and material science.