Universal Testing Machine

Universal testing machine is to be distinguished from a special purpose machine in the sense that such a machine is used for performing several tests. Tests that are performed in a universal testing machine are tension, compression, bending, transverse shear and hardness. On the other hand, a special machine like torsion testing machine or compression machine can be used only for particular test as indicated by name.

Two types of universal testing machines are in common use. One is mechanical type operated by screw and gear as shown in Figure  and the other is hydraulic universaltesting machine as shown in Figure 21. Both these machines are basic in nature. The screw type varies in size and capacity over wide range of a few newtons to a few tones. Hydraulic machines are generally of high capacity. These machines have limited speeds of movement of cross-heads so that loads applied are within the definition of static. The strains rates greater than 10– 6 mm/mm.s are usually not available on universal testing machines. Modern computer operated machines now-a-days make wide range of strain rates available and they can be even used for cycling stresses. Instron and MTS machines are examples.

Whatever by the type of machine general features remain same, a universal machine applies axial load which can act as tension, compression, shear or bending load upon the specimen depending upon positioning of the specimen. The application of load is effected by relative movement between two cross-heads. Generally machines have two cross-heads rigidly connected with each other and a third cross-head is positioned between them. Each machine has proper specimen holding and supporting devices to apply desired type of load upon the specimen. The load applied upon the specimen has to be balanced and measured and as such each machine should have this device.

Figure shows essential features of a mechanical type of universal testing machine. The upper and lower cross-heads are rigidly connected with middle cross-head can be moved down by a screw and gear arrangement driven by an electric motor. While moving down the middle or movable cross-head can apply a tension load on a specimen gripped between the moving cross-head and upper cross-head. Alternatively, the middle cross-head can apply a compressive load on a specimen placed on lower cross-head. The fixed cross-heads are supported over a compound lever system at whose end a poise or balance weight can be shifted to achieve balance of the lever and thus measure the applied force. The balance weight is often replaced by a swinging pendulum whose rotation is transmitted to an indicator through gear train for direct indication of load.

In a hydraulic universal testing machine the rigid frame containing upper and lower cross-head is attached to an accurately finished piston which exactly matches with the hydraulic cylinder. The middle cross-head, whose position is adjustable remains stationary when the load is applied upon the specimen. An electric motor rotates a hydraulic pump that forces the oil into the cylinder whereby the rigid assembly of lower and upper cross-head is lifted up. During this movement a tensile load will act upon the specimen held between upper and middle cross-head and a compressive force will act upon a specimen placed on the lower cross-head. The hydraulic pressure on the bottom of the hydraulic cylinder is transmitted to a hydraulic capsule which is connected to a Bourdon tube gauge which can be calibrated to read the force directly. The middle cross-head can be moved up or down by separate positioning screws and during its positioning movement the hydraulic pump is not operated.

Both types of machine often incorporate the mechanical graphical devices to plot the load displacement curves. Modern machines incorporate load cell & displacement gauges whose electrical signals respectively proportional to load and displacement may be plotted automatically on X-Y plotter when the test is proceeding.

                                      Figure: Screw-Gear Type Universal Testing Machine

                             Figure: Schematic of Hydraulic Universal Testing Machine