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4.3 Tensile Testing:

Test specimens are accurately measured before testing, then fitted into the jaws of a tensile testing machine and subjected to a continually increasing tensile force until the specimen fractures. The tensile strength (Rm) is calculated by dividing the maximum load by the cross-sectional area of the test specimen, measured before testing. The test is intended to measure the tensile strength of the object or joint and thereby show that the basis for design, the base metal properties, remain the valid criterion.   In this stress strain curve, strain is plotted along the horizontal axis while the stress is plotted on the vertical axis. O to A: In this region, the stress and strain are directly proportional to each other and the behaviour of the material is said to be linear. A to B: Beyond point A, the linear relationship between stress and strain no longer exist. Hence, the stress at A is called the PROPORTIONAL LIMIT or LIMIT OR PROPORTIONALITY. Beyond this, the curve becomes slightly curved until point B is reached. With an increase in the load beyond the proportional limit, the strain begins to increase more rapidly for each increment in stress. The stress-strain curve then has a smaller slope until point B the curve becomes horizontal. However, up to the point B, the removal of the load would result in complete recovery by the specimen of its original dimensions (Elasticity). Elastic Modulus (Young’s Modulus): The measure of stiffness or rigidity of the material within the elastic range. B to C & C to D: Point B is the elastic limit of the material. Beginning at this point, considerable elongation occurs, with no noticeable increase in the tensile force. This phenomenon is known as YIELDING of the material. Stress at point C is called the YIELD STRESS or YIELD POINT (The stress at which the material begins to exhibit permanent deformation.). In this region, the material becomes perfectly plastic which means that it can deform without an increase in the applied load. The strains increasing at an accelerating rate is represented by C to D. The elongation of mild steel specimen in this region is 10 to 15 times the elongation that occurs between the onset of loading and the proportional limit. Extension in this region is known as DUCTILE EXTENSIONS. Ductility: The ability of the material to deform plastically before fracturing. It is typically measured by elongation or reduction in cross-sectional area. D to E: After undergoing the large strains that occur during the yielding in the region C to D, the steel begins to strain harden. During strain hardening, the material undergoes change in its atomic and crystalline structure resulting in increased resistance of the material to further deformation. Thus, additional elongation requires an increase in the tensile load and the stress-strain curve has a positive slope form D to E. The load eventually reaches its maximum values and the corresponding stress is called the ULTIMATE STRESS (The maximum stress the material can withstand before it fractures). E to F: Up to the maximum load, the bar extends uniformly over its parallel length but if straining is continued, a local reduction in cross-sectional area occurs. Further stretching of the bar is actually accomplished by a reduction in the load and fracture finally occurs at a point such as F. Fracture Toughness: The resistance of the material to fracture under stress. 3.3.1 Ductile Versus Brittle Material The main difference between the stress-strain curves of ductile and brittle materials lies in their deformation characteristics and ultimate failure. Ductile materials, such as most metals, exhibit a characteristic “yield point” on their stress-strain curve. Initially, these materials undergo elastic deformation, meaning they can return to their original shape when the applied stress is removed. As the stress increases, the material reaches its yield point, beyond which plastic deformation occurs. Ductile materials can undergo significant plastic deformation before ultimate failure, exhibiting a gradual necking effect in the stress-strain curve. This behaviour is associated with their ability to redistribute stress and undergo strain hardening, resulting in increased toughness and energy absorption before fracture. In contrast, brittle materials, lack a distinct yield point in their stress-strain curve. They exhibit limited plastic deformation and are prone to sudden, catastrophic failure once the applied stress exceeds their strength. The stress-strain curve for brittle materials typically shows a linear or slightly curved elastic region followed by an abrupt drop in stress upon fracture. This behaviour is characteristic of their limited ability to absorb energy and their tendency to fail without significant warning or plastic deformation. In summary, the key difference between the stress-strain curves of ductile and brittle materials is the presence of a yield point and significant plastic deformation in ductile materials, whereas brittle materials exhibit limited plasticity and tend to fail abruptly without warning. After a tensile test, ductile materials typically exhibit a necking phenomenon followed by a ductile fracture. The necking effect is characterised by localized reduction in cross-sectional area of the material (cup and cone shape), resulting in a narrow region before ultimate failure. Ductile fractures occur through the progressive formation and propagation of microvoids and void coalescence, leading to the material pulling apart. On the other hand, brittle materials typically undergo a sudden and catastrophic failure without significant necking or deformation. The fracture surface appears relatively flat and perpendicular to the applied load. Brittle fractures occur through the rapid propagation of cracks, often along planes of weakness or pre-existing flaws within the material. The fracture is characterized by a clean, smooth surface with little plastic deformation. Tensile ductility is measured in two ways:

The figure below illustrates these two ductility measurements.