Saturday, January 12, 2013

Torsion test

 Torsion test
EXPERIMENT    :-Torsion test on a steel shaft of circular section using the Torsion test machine

AIM                     :- To determine the Modulus of rigidity of mild steel

APPARATUS      :-      01) Mild steel shaft ( circular cross section )
                                     02) Testing machine
                                     03) Telescope
                                     04) Scale
                                     05) Mirrors

THEORY             :-  
                                    T = Torque applied
                                    J  = Polar moment of inertia
                                                (  J = nd4 /32  , d = diameter of the bar  )
                                    G = Modulus of rigidity
                                    θ  = Angle of rotation over length L
                                     L = Length of the specimen  between two gauge points

                                               tan 2θ =  (H-h)/l
                                        when θ is small
                                                   θ =  (H-h)/2l

                        let                 l              = Horizontal distance from each mirror to the respective scale
                                   hR and h L       = Initial reading of the scales ( Right and left )
                                   HR and HL      = Readings of the scales after applying the torque

                                    θR   =    (HR-hR)/2l -  (HL-hL)/2l

                        There for the equation could be written as,

                        TL = GJ[ (HR-hR)/2l -  (HL-hL)/2l]

                        On arrangement

                                                HR-hR   =  (2lL/JG) T + HL-hL
             This is from    ;             Y        =     m        X  +   C

Therefore, G can be calculate from the gradient of the graph


                         Firstly the two telescopes were placed at a point 1000mm away from the relevent
           mirrors. Each telescope were adjusted so that the relevant scale was reflected in the mirror.
          Then the balancing arm was adjusted by moving the poise to the left end.
                              At that time torque was zero. though the telescope the readings ( hL & hR ) were
          taken. Next the toque was increased by 400 pounds each step and 6 sets of readings
          ( hL & hR )  were taken as above. Finally the diameter of specimen and the length were
          measured  .


          Torsion test is applicable for testing brittle materials such as mild steel a and the
       test has also been used to determine the forge ability of the materials by means of torsion testing
       at elevated temperature 
                             The torque is the product of tangential force multiplied by the radial distance from the twisting axis and the tangent, measured in a unit of N.m. 1b. in torsion testing, the relationship between torque and degree of rotation is graphically presented and parameters such as modulus of rigidity, modulus of rapture and shear strength at proportional limit are generally investigated more ever fracture surfaces of  specimens tested and torsion can be used to determine the characteristics of materials whether it would fail in a brittle or a ductile manner.
                             we did this experiment to determine the modulus of rigidity of mild steel. But when we are doing this experiment there are some mistakes can be happened. This apparatus has been used for a long time for this experiment so the rod has been subjected to torque many times. It may have been more twisted so the apparatus has to put more effort to twist the rod.
                             When we adjusted the telescopes the scales may be change. because the telescopes were placed so closer to each other so we must pay our special attention on adjusting the telescopes. And also we must focus the telescope properly. Otherwise we can't read the scale well.
                             When apply the load we must be careful otherwise on additional force will occur and it cause to the error readings. In this experiment we had to take the scale readings using telescope. When we doing that, human errors also happened.
                             If we be more careful during conduct of the experiment we can minimize the mistakes and errors in this experiment and we can get a more accurate result. In many areas of engineering applications, materials are some subjected to torsion in service. For example drive shaft , axial and twisted drill. More ever structural applications such as bridges, springs car bodies, air plane fuselages and boat hulls are randomly subjected to torsion. The material used in this case should not required only adequate strength but also be able to withstand torque in operation. Even through torsion test is not as universal as torsion test and do not have any standardized testing procedure, The significance lies on particular engineering applications and for the study of plastic flow materials.