Compression Test PDF

COMPRESSION TEST The uniaxial compression test is also carried-out on a Universal Testing Machine (UTM). The specimen is prepared in the form of a cube or in cylindrical form as shown in Figure. The uniaxial compression test has two main limitations: End restraint: When a cylindrical specimen is co...

COMPRESSION TEST The uniaxial compression test is also carried-out on a Universal Testing Machine (UTM). The specimen is prepared in the form of a cube or in cylindrical form as shown in Figure. The uniaxial compression test has two main limitations: End restraint: When a cylindrical specimen is compressed, it tends to bulge out on the sides. This is because the specimen is restrained at the ends by the platens of the testing machine. The bulging can cause the specimen to fail prematurely, and it can also make it difficult to measure the true compressive strength of the material. To remove the problem of bulging in compressive tests, some powder lubricants are used to reduce friction between samples and plates. Lateral instability: If the specimen is too long, it can buckle under the compressive load. This is known as lateral instability. Buckling can also cause the specimen to fail prematurely. To minimize the effects of end restraint and lateral instability, the specimen should be short and thick. A good rule of thumb is to keep the height-to-diameter ratio of the specimen less than 2. To conduct a uniaxial compression test, the following steps are typically taken: (1) Prepare the specimen. The specimen should be a cylinder with a flat top and bottom, and the ends should be perpendicular to the axis of the cylinder. The specimen should also be free of any defects or cracks. (2) Place the specimen in the testing machine. The specimen should be centered between the top and bottom platens of the testing machine, and the platens should be parallel to each other. (3) Apply the load. The load should be applied continuously at a constant rate until the specimen fails. The rate of loading should be chosen so that the specimen fails within a few minutes. (4) Record the data. The following data should be recorded during the test: The applied load,the displacement of the specimen, the stress and strain of the specimen (5) Calculate the results. The following results can be calculated from the data: Uniaxial compressive strength (UCS), Young's modulus, Poisson's ratio COMPRESSIVE STRESS-STRAIN DIAGRAMS The schematic stress-strain curve in compression shown in figure. These are also shown as (-σ vs-ε plot) or o-e in third quadrant of coordinate, and are the conventional curves. The plotting has been done using the data obtained by conducting compression test. Figure show the curves of cast iron and plexiglass, both in tension and compression. Cast iron and Plexiglass show much higher strengths in compression than in tension.  TENSILE TEST A tensile test is a destructive test that is used to determine the strength and ductility of a material. It is performed by pulling a sample of the material until it breaks. Tensile tests are performed on the following machines. a. Hounsfield Tensometer b. Universal Testing Machine c. Instron Testing Machine Hounsfield Tensometer The Hounsfield Tensometer Machine is a mechanically operated machine that is mainly used for testing thin sections and low tensile strength materials such as leather, rubber, thin wire, nylon, plastic composites, and plywood. Although this machine may also be used for higher tensile materials, it is less commonly used for this purpose. Universal Testing Machine The UTM's have large load capacity, generally ranging from 0-4 tonne and 0-40 tonne. Materials with large sections and high tensile strength such as steel and aluminium bars are tested on them. Tensometer is generally mechanically operated while UTM's are hydraulically operated. They can be controlled by computers or can be operated manually. Tensile Testing Standards and Specimen While conducting the experiment, a specimen is prepared in accordance to Indian standards BIS. Different tensile test specimens shown in Fig. 8.2a-d are in conformity with IS. Their grip ends may be cylindrical, pinholed, threaded or shouldered. Their cross-section can be either circular or rectangular. ASTM (American Society of Testing Materials), ASM (American Society of Metals), Japanese Standards JIS, British Standards BS, German Standards DIN, Chinese Standards GBJ, Russian Standards GOST or Indian Standards BIS. Procedure of Tensile Testing The tensile load P is applied gradually. For each incremental value of P, the elongation δl of the gauge length is observed by precision instruments such as a dial gauge. The observation of change in length beyond yield point may be done by a precision scale. Fluctuation of the live pointer on dial of the machine indicates occurrence of yield point. The specimen, finally, fractures with a thud sound. The two broken pieces of the specimen become hot after breaking. It is due to the conversion of internal friction into heat. The internal friction is caused due to the slipping of planes of atoms during loading. When the two pieces cool down, we measure the gauge length and also the diameter at the neck of fractured ends. The fractured ends are generally of cup and cone type.  Stress- strain Curve for ductile material (mild steel) A= Proportional Limit oa = linear deformation B = Elastic Limit ob = Elastic deformation C = Yield point (Upper Yield point) cd = perfect plastic yielding D = Lower Yield point de = strain hardening E = Ultimate strength ef = Necking F = Rupture strength LIMIT OF PROPORTIONALITY The limit of proportionality is the point up to which the stress is directly proportional to the strain and obeys Hooke's law. Beyond this point, the relationship becomes non-linear. Up to the limit of proportionality, the stress-strain curve is a straight line. ELASTIC LIMIT It is that point in the stress-strain curve upto which the material remains elastic, i.e. the material regains its original shape after the removal of the load. Thus, elastic limit represents the maximum stress that may be developed during a simple tension test such that there is no permanent or residual deformation after the removal of the load. ELASTIC RANGE This is the region of the stress-strain curve between the origin and the elastic limit. The elastic range in which only elastic deformations take place. These deformations disappear on the removal of the load. PLASTIC RANGE This is the region of the stress-strain curve between the elastic limit and point of rupture, the plastic deformations take place. These deformations are permanent deformations which do not disappear even after the removal of the load. The plastic range consists of three regions: (i) perfect plastic yielding region (ii) strain hardening region (iii) necking region YIELD POINT Yield point is the point just beyond the elastic limit, at which the specimen undergoes an appreciable increase in length without further increase in the load. The phenomenon of yielding is more peculiar to structural steel; other materials do not possess well defined yield point. YIELD STRENGTH The yield strength of a material is closely associated with the yield point. Yield strength is defined as the lowest stress at which extension of the test piece increases without further increase in load. ULTIMATE STRENGTH The ultimate stress or ultimate strength, as it is more commonly called, is the highest point on the stress-strain curve. RUPTURE STRENGTH The rupture strength is the stress corresponding to the failure point of the stress-strain curve.  STRESS-STRAIN DIAGRAM FOR BRITTLE MATERIAL 1. Fig. 1 shows the stress-strain diagram for brittle material, in which strain is are plotted along x-axis and corresponding stress is plotted along y-axis. 2. This diagram is obtained by performing a tensile test on the specimen. 3. The end of the test piece is fixed into grips connected to a straining device and load measuring device. The load on the test piece is increased slowly and corresponding extension is measured. 4. From these readings, the curve OAB is obtained. 5. In this curve, from O to A the stress is proportional to the strain and this is known as elastic stage. 6. Upto point A, Hook's law is applicable. The stress at A represents the proportional limit. 7. If the load is increased further the elongation becomes more rapid and diagram becomes curved. The stress will not be proportional to the strain. 8. At point B, suddenly the specimen breaks into pieces. 9. The total elongation OC before fracture is very small as compared to total elongation of ductile material.  TORSION TEST Purpose and method of test- The ability of a material to resist twisting moment (torque) is determined by a torsion test. In this test, the specimen of a solid circular section or a hollow tube (torsion tube) is taken. It is held in a Torsion Testing Machine at one end, and the other end is given the torque T. Fig. 8.60. This torque cause’s twist in the specimen. The twist is shown by deformed dotted lines which were rectangular grids before application of torque. The torque is increased gradually and the corresponding angle of twist θ is measured. The angle of twist is measured by a precision angular instrument or by a graduated dial for different values of torque. Torque vs twist diagram- From the observed data, the torque versus twist diagram is plotted whose likely shape is shown in Fig. 8.6c. Its variation is linear upto point A which implies that the elastic limit is from 0 to A. Calculation to determine torsional shear stress- The effect of torque is to create shear stress in the specimen. The torsional shear strength τ at the outer periphery of the solid circular specimen of diameter d is obtained from 16 𝑇 𝜏= 𝜋𝑑3 For hollow cylinder specimen 16 𝑇𝐷0 𝜏= 𝜋(𝐷𝑜4 − 𝐷𝑖4 ) Plot between shear stress and shear strain- A plot between τ and γ is shown in Fig. 8.6d. The elastic limit is upto A, and material obeys Hooke's law in OA range. Slope of this curve yields τ/γ = G, the shear modulus. https://www.youtube.com/watch?v=KcLZ95bhZ9E&ab_channel=SampurnaEngineering Relation between True stress and Engineering stress When a ductile material is subjected to tensile stress, beyond a certain stress, the cross sectional area of the material decreases at a particular position in the material. The true stress at any time of loading, is the force divided by the instantaneous cross sectional area A, at the instant of time Hardness: Hardness is a surface property. It is defined as the resistance of a material against permanent deformation of the surface in the form of scratch, cutting, indentation, or mechanical wear. Diamond is the hardest known material. In various hardness tests, different types of indentors are used, including: - A spherical ball - A cone - A pyramid There are different types of hardness tests, some of which are: - Mohs hardness test - Brinell hardness test - Rockwell hardness test - Vickers hardness test - Knoop hardness test. Mohs Test: The Mohs test is one of the oldest techniques to determine hardness. The Mohs test was conducted on rocks and minerals. Brinell Test: In this test, a diamond or hardened steel ball is used as an indentor. The ball is placed suitably in the upper housing of the Brinell hardness test machine. Before conducting the test, the surface of the specimen is made free from oil, grease, dust, and dirt. The indentor is applied to the load on the specimen gradually for 30 seconds. The load, with the diameter of the ball, is varied. The indentor is a ball made of steel or tungsten carbide having a diameter, D. Consider the ball indentor pressed at a load or force value of P newtons. The equation gives the Brinell hardness number: Rockwell Hardness Test: This test is more common due to its quick and simple method. There is no need for calculation, as the Rockwell hardness can be directly read on the dial. The test involves the application of an initial load of 10 kgf on the specimen, so that the effects of dust, dirt, oil, etc. are nullified. This makes the Rockwell test more accurate than Brinell's test. Vickers Hardness Test: This test is similar to Brinell's test but uses a different type of indentor, a square-based pyramid. The indentor has a diamond shape with an angle of 136° between the pyramid's opposite faces. Knoop Hardness Test: An orthorhombic pyramid is used as an indentor in this test. The included angle of the pyramid is 72.5º and 130º.