00008

above this you start having that first region of nonlinear elastic response followed by perfectly plastic and then further yield response why don't we do this first of all because we know that the unloading line from any point follows is a straight line parallel to this loading line and because we know that we're looking for the yield stress by its property of having two thousand micro strain permanent deformation why don't we mark here 2000 micro strain and from this point we draw a line parallel to the loading line the same slope e and where the to

cross that is where your yield is that's where the ill start so this becomes the linearly elastic range of the response this becomes the minion nonlinear the elastic part of the response and this becomes the plastic or the healed part of the response so the response of the material has three regions the nearly elastic not be nearly elastic and plastic were the yield regime with this region being the perfectly plastic response and then this means the further yield response this method is called the mm microstrain offset method for yields determination or the 0.2% offset method

either one or the other so the method of determining the yield stress is based on this approach a is the 2000 micro strain or the 0.2% offset and you're going to be using it for your lab report now this is one way to determine yield there's another method to determine yield based on the following I think if this is the response in stress versus strength that you measure and the the slope here is e what you observe is that as you move from linear elastic response towards perfectly plastic response force field the slope the local

slope the tangent to the line decreases so here's another definition of yield where the slope becomes 1 versus e divided by 2 where the slope reduces to 1/2 that's the point where we call that yield setting this is called Johnson's apparent yield map so this method of determining the yield stress is called Johnson's apparent yield method it simply uses this the local slope of the line to determine the yield point obviously these are two separate definitions of yield stress done and as a consequence this value will yield stress returning as such and this value reveals

stress determined asaji may not be the same in fact they are not exactly the same but they're close enough for your experiment once you have the data you're going to determine the yield stress according to this method and according to dismiss and you're going to compare the two values of yield stress and report the difference report the two values and the difference between the two of them the important thing is for you to remember that whenever you read the properties of a given material including its yield stress you need to make sure that you understand

which method was used this method or this map this is the standard method that worked more frequently used method of determining the yield stress for a piece of material the 0.2% offset now we're going to be performing this experiment and you're gonna find out for yourselves I mentioned during the first lecture that this is the power of experiments you're going to know firsthand what the constitutive equation for the given material is relationship between stress and strength you're no longer going to need to rely upon what the textbook states or what somebody told you you're gonna

see for yourselves furthermore once you perform this experiment you are going to know how to do the same for any piece of material that you're going to be dealing with so if there is no information for a given material that you come across you can always perform a test now this is something that the determination of the responsible material is something that is very important it is the foundation those properties that you measure characterize the material and represent the foundation of all design work afterwards it is extremely important for everybody to be on the same

page and do a good job and report values that can be confirmed by others and can be relied upon for the purpose of design for that purpose given the importance of the best and given the importance of everybody coming up with the same answers for the same material the process of actually testing the piece of material and determining the problem is the tensile test actually is a process that is standardized the American Society of testing and materials a s/m American Society for testing and materials has a number of standards including a standard for performing the

tensile test to determine this property this set of properties for a material so I want you to be aware of the fact that actually testing to do in order to determine material properties for a given material needs to follow prescribed SPM standards and those ASTM standards prescribed all the elements that need to be considered even care to and followed all the processes and procedures that need to be followed in order for the outcome the results the measurements to be useful and to be reliable we that the purpose of this class is not to emphasize the

ASTM standards and we're not going to necessarily pay attention to all the aspects of that although the specimen the shape of the specimen that you're gonna be testing is according to the ASTM standard so it's not just some dog bone specimens type shape and size it is the specimen according cut according to ASTM standard requirements so that's the basis and that's the foundation of what you're going to be doing next time what is left these do actually the important work which is to measure stress and measure strength and then figure out how in the world

do you actually perform a test how do you actually apply a load especially okay so why don't we talk first about how do you perform such a test so I'll take a break here will stop and that will reposition the camcorder and continue limited