Mechanical Properties of Metals, Modulus of Elasticity, Yield Strength, etc on Pipe Systems

Mechanical Properties of Metals is one of the important part on pipe systems in this case is piping materials which is contain Modulus of Elasticity (Young's Modulus), Yield Strength, Ultimate Tensile Strength, Elongation and Reduction Area, and that's all of thing will explain in this chapter only at Piping & Fabrication.

Mechanical properties are critically important to the design process. They are defined as the characteristic response of amaterial to applied force. The standardized test methods for measuring these properties are described in ASTM specifications. Properties fall into two general categories, strength and ductility. Some properties, such as material toughness, are dependent on both strength and ductility. The most widely known and used material properties, as defined by ASTM, are described in the following paragraphs.

FIGURE A3.2 Tension-test specimens

Modulus of Elasticity (Young’s Modulus). 

The modulus of elasticity is the ratio of normal stress to corresponding strain for tensile or compressive stresses. This ratio is linear through a range of stress, known as Hooke’s law. The material behavior in this range is elastic (i.e., if the applied load is released the material will return to its original, unstressed shape). The value of the slope in the elastic range is defined as Young’s Modulus. The modulus of elasticity is measured using the tension test, the most widely used test applied to engineering materials. The test consists of applying a gradually increasing load in either tension or compression, in a testing machine, to a standardized test specimen (Fig. A3.2). The applied load is continuously monitored, as is test specimen elongation or contraction under load. These measured quantities are generally represented on a coordinate axis, called a stress-strain curve (Fig. A3.3). The modulus of elasticity and other strength properties are established from this curve. Values of the modulus of elasticity for a number of construction materials are given in Table A3.1

FIGURE A3.3 Stress-strain diagram

Yield Strength. 

When a specimen is loaded beyond the point where elastic behavior can be maintained the specimen will begin to deform in a plastic manner. Most materials do not abruptly transform from purely elastic to purely plastic behavior. Rather,

a gradual transition occurs as represented by a curve, or knee, in the stress-strain curve. Lacking an abrupt and easily definable point representing transition from elastic to plastic behavior, several standardized methods have been defined by ASTM to determine the yield strength used as the engineering property. The most common is termed the 0.2 percent offset method. In this approach a line is drawn parallel to the elastic portion of the curve anchored to a point displaced 0.2 percent along the strain axis. (Fig. A3.4). The yield strength corresponds to the calculated value of the load indicated at the intersection point of the drawn line, divided by the original cross-sectional area in the gauge length of the tensile specimen. By convention, this test is performed at a constant rate of strain, and is reported as newtons per square meter, or as pounds per square inch of cross section in English units.

Ultimate Tensile Strength. 

Upon further increase of applied load under constant strain rate, the specimen will continue to stretch until the loss of load-carrying cross section caused by specimen thinning during the test (due to Poisson’s ratio) cannot withstand further load increase. The ultimate tensile strength constitutes the maximum applied load divided by the original specimen cross-sectional
area.

Elongation and Reduction of Area. 

The ductility of the test specimen can be established by measuring its length and minimum diameter before and after testing. Stretch of the specimen is represented as a percent elongation in a given length (usually 2 or 8 in) and is calculated in the following manner:

percent of elongation

TABLE A3.1 Modulus of Elasticity U.S. Units for Metals

The diameter of the test specimen will decrease, or neck down, in ductile materials. Another standard measure of ductility is the reduction of area of the specimen, defined as follows: