The term yield strength is always bugging me deciphering the actual use in some cases. If I had a plate 3m x 2m 30mm material of Yield Strength 355N/mm², does this mean the maximum allowable force at one point on the plate is 10,650N (355 x 30mm), not taking into account safety factors, deflection, moments etc. Do you just multiply the Yield strength by the thickness?
In general, no. Just look at the units of stress: N/mm^2. If you multiply yield stress by thickness, you will be left with units of N/mm, which is not the units of force.
i don't think i am asking the question correctly. In other words what I mean is, say for instance you had a pipe of 355 MPa Yield Strength. If two pipes had the same outer diameter but one was 1 inch thick and the other 2 inches thick, does the 2 inch thick pipe have double the yield strength of the 1 inch pipe? Is there a relationship between yield strength and thickness?
Gary, you are not grasping the concept correctly. The yield strength of the material is actually an internal stress value in N/mm², usually gained from a tensile test. This doesn't mean if you apply a load i.e. 355 N then this will be fine for every mm of thickness you have. The yield strength is a measurment of the internal stress at which the material will start to yield, not the external load you can apply. No matter how thick your material is and the amount of Pressure/load you apply to it, you can't have an internal stress value over 355 N/mm2 for any thickness (excluding safety factors etc). The internal stresses will be less the thicker your material is, assuming the load is constant.
Many materials, such as metals, react to stress as if they are a very stiff spring. Compressve stress will shorten the material in the axis of the stress while bulging in the other axes. Tensile stress will stretch the material and thin it elswhere. When the stress is removed the material will spring back to the original size and dimensions. The spring rate is sometimes called Young's Modulus. Now, if the stress per cross section exceeds than the yield stength, the material does not spring back all the way if the stress is removed. It has yielded. Materials that have a yield strength much lower than their ultimate tensile strength are said to behave in a plastic manner. Those materials with a yield strength only a few percent lower than the ultimate tensile strength behave in a brittle manner. These behaviors may change for one particular material as temperature changes, also. It may sag of its own weight at high temperature and shatter in freezing temperatures.