Piping Interface – Page 3

Tensile strength is the maximum stress up to which a material can be loaded without failure. When the tensile strength is exceeded, the material breaks or fails. This is also known as ultimate tensile strength or UTS in abbreviated form. The Tensile Strength of a material is a very important parameter in mechanical design as all components are designed in such a way that the generated stress in that part does not exceed tensile strength during the design life of the component. In this article, we will discuss more regarding Tensile Strength, its definition, its significance, measuring methods, and typical values. Definition of Tensile Strength Mathematically, the tensile strength of a material is defined as the ratio of the maximum load that the material can support to its original cross-sectional area. So, UTS=Maximum force to create failure of the body/Cross-Sectional Area=F/A. From the above equation, it is evident that the unit of Tensile strength is N/m2 or Pa (pascal) in SI systems. In the FPS system, the unit of tensile strength is psi or lb/in2. Usually, the tensile strength of materials is expressed in MPa or psi. In the stress strain curve of a material it is the maximum stress value before failure as shown in Fig. 1 below. Fig. 1: Tensile Strength in Stress-Strain Curve Significance of Tensile Strength Tensile strength is the maximum stress generated before fracture. So, it can be stated as for a given cross-section, the maximum load that the material can be subjected to can be easily calculated. This maximum load or stress value is very important. Tensile Strength data is required for various purposes as mentioned below: Structural and Mechanical DesignMaterial EvaluationPreparing Material SpecificationQuality ControlFailure AnalysisModeling to predict material behavior under complex loading conditions. The tensile strength is the material’s resistance to tension. The more the tensile strength of a part, the more difficult it is to stretch, and more force is required to stretch it. Factors Affecting Tensile Strength Tensile Strength is an intensive property of a material and independent of the size of the specimen. The value of tensile strength varies from one material to the other. The factors that affect the tensile strength in metals are: Temperature: With an increase in temperature of a material the tensile strength decreases.Surface Defects: Specimen with defects have less tensile strength as compared to materials without defects.Specimen Preparation: Tensile strength of a material depends on the surface preparation of the materials. This is the reason that a standard philosophy as outlined in Codes and Standards is followed for specimen preparation.Heat Treatment: Heat treatment can modify the tensile strength of metals.Amount of Cold Work on the material Measuring Tensile Strength Tensile strength measures a material's resistance to breaking. The tensile test for metals is measured by performing tensile testing in a universal testing machine. Various ASTM codes provide guidelines for the tensile testing procedure. For example, ASTM E8 provides standard test methods for metallic materials and ASTM D638 provides tensile testing procedures for plastic materials. A universal testing machine has two crossheads where the specimen is fitted and an increasing tensile load is applied to extend the specimen till it breaks. the elongation against the applied force is plotted automatically by the instrument from which the stress-strain curve is generated. The tensile strength is decided from that curve (Refer to Fig. 1) as the maximum stress point in the curve during failure. Common codes and standards that are followed for tensile testing are: ASTM E8/E8M, ISO 6891, JIS Z2241, ASTM D3039, ASTM D638, ASTM D828, ASTM D882, ISO 37, MPIF Test Standard 10, etc. The tensile strength of a material can be measured by the above-mentioned process. But Scientists in research organizations have already standardized tensile strength values for all common materials in various codes and standards like ASME BPVC codes, ASME B codes, etc. So, Engineers need to simply follow the relevant code and standard and find out the tensile strength of the material from standardized tables provided in those codes. Tensile Strength vs Yield Strength Yield Strength is the property of ductile materials. For brittle materials, distinct yield strength is not obtained. Hence, proof stress is determined by drawing a parallel line to the straight elastic region of the stress-strain curve starting from 0.2% of strain. The main differences between Tensile Strength and Yield Strength are tabulated below: Tensile StrengthYield StrengthTensile strength is measured at the point of fracture.Yield Strength is measured at the point of plastic deformation.Tensile strength is design parameter for brittle materials.Yield strength is design parameter for ductile materials.Value of tensile strength is always higher than yield strength.Value of Yield Strength is lower than tensile strength.After tensile strength, material fails.After Yield Strength, material loses its elastic behaviour.Table 1: Tensile Strength vs Yield Strength Tensile Strength of Steel Tensile strength of steel provides the value of tensile stress that a steel component can withstand until it leads to a failure by any of the two ways: ductile or brittle failure. Click here to know more about ductile and brittle failure.The tensile strength of common steels of industrial use is provided in the following table (Table 2): Steel MaterialTensile Strength (MPa)Yield Strength (MPa)Tensile Strength (ksi)Yield Strength (ksi)Structural Steel, ASTM A36400-55025058-8036Mild Steel, 109084124712236Maraging Steel, 280026932617391380Chromium Vanadium Steel94062013690API 5L X655314487765ASTM A514760690110100Chromium Nickel Austenitic Stainless Steel520-72021075-10430Molybdenum Chromium Nickel Austenitic Stainless Steel520-67022075-9732Duplex Steels640-85046093-12367Stainless Steel AISI 30286050212573Ferritic Stainless Steel5002807341Martensitic Stainless Steel6503509451Precipitation Hardening Stainless Steel11001000160145Ordinary Carbon Steel315-610195-27546-8828-40Tool Steel-Annealed280-700180-36041-10226-52Tool Steel-Normalised530-760380-43077-11055-62Tool Steel-Cold Hardened750-1200-109-174-Alloy Structural Steel500-750-73-109-API 5L X424142906042API 5L X524553596652API 5L X605174147560A106 B4142416035API 5L B4142416035A333-64142416035A516-704832627038Table 2: Tensile and Yield Strength of Steel Tensile Strength of Other Materials The following table (Table 3) provides the tensile strength of some common materials. MaterialTensile Strength (MPa)Yield Strength (MPa)Diamond28001600Bamboo265142Brass250 Cast Iron, ASTM A48200130Copper22070HDPE3726-33Marble15 Rubber15 Tungsten1510 Aluminum40-5015-20Gold100 Iron35080-100Lead12 Nickel140-19514-35Silver170 ABS plastics40 A53 Seamless and Welded Standard Steel Pipe - Grade A331207A53 Seamless and Welded Standard Steel Pipe - Grade B414241A106 Seamless Carbon Steel Pipe - Grade A400248A106 Seamless Carbon Steel Pipe - Grade C483276A252 Piling Steel Pipe - Grade 1345207A252 Piling Steel Pipe - Grade 2414241A252 Piling Steel Pipe - Grade 3455310A501 Hot Formed Carbon Steel Structural Tubing - Grade A400248A501 Hot Formed Carbon Steel Structural Tubing - Grade B483345A523 Cable Circuit Steel Piping - Grade A331207A523 Cable Circuit Steel Piping - Grade B414241A618 Hot-Formed High-Strength Low-Alloy Structural Tubing - Grade Ia & Ib483345A618 Hot-Formed High-Strength Low-Alloy Structural Tubing - Grade II414345A618 Hot-Formed High-Strength Low-Alloy Structural Tubing - Grade III448345API 5L Line Pipe310 - 1145175 - 1048Acetals65 Acrylic70 Aluminum11095Boron 3100Brass250 Cast Iron 4.5% C, ASTM A-48170 Cellulose, cotton, wood pulp and regenerated80 - 240 Cellulose acetate, sheet30 - 52 Cellulose nitrate, celluloid50 Chlorinated polyether39 Concrete, High Strength (compression)40 Copper22070Douglas fir Wood50 Epoxy resins26 - 85 Marble15 Nylon-645 - 9045Nylon-6660 - 80 Phenolic cast resins33 - 59 Phenol-formaldehyde molding compounds45 - 52 Pine Wood (along grain)40 Polyacrylonitrile, fibers200 Polycarbonates52 - 62 Polyethylene HDPE (high density)15 Polyethylene Terephthalate, PET55 Polyamide85 Polyisoprene, hard rubber39 Polyimide aromatics68 Polypropylene, PP28 - 36 Polystyrene, PS30 - 100 Polyurethane cast liquidOct-20 Polyurethane elastomer29 - 55 Silicon Carbide 3440Steel, High Strength Alloy ASTM A-514760690Steel, stainless AISI 302860502Steel, Structural ASTM-A36400250Titanium Alloy900730Table 3: Tensile Strength of materials