Residual Stresses - Definition, Properties, Effect on Behavior of Tension Members

Residual Stresses

Tension members response to load is much similar to that of tensile-test coupon . Member behavior may differ from coupon behavior because of: Slip in bolted & riveted connections Non-linear behavior of connections Residual stresses in member Residual stresses result principally from non-uniform cooling of hot rolled or welded shapes and from cold straightening of bent members.

• They have more surface exposure per unit volume, the flange tips tend to cool faster than flange-to-web junctures.
• Similarly, Central portion of web tends to cool faster than junctures
• As a result, metal at junctures continues to contract as it cools after flange tips and web interior have cooled to temperature of surroundings.
• This contraction is partially restrained by cooler metal.
• Developing tensile stresses in regions of junctures & compressive stresses in remainder.
• These are called residual Stresses
• Magnitude and distribution of thermal residual stresses are influenced to considerable degree by geometry of x-section
• 20 W’s were investigated
• An investigation on W’s Revealed that flange tip stress varied from 4.1 to 18.7 Ksi
• Residual stresses in web varied from 41Ksi comp; to 18.2Ksi tension.
• Showing some W’s develop residual tension over entire web, instead the pattern shown.
• Only one out of 20 sections was thicker than 1 in.
• Residual stresses tend to increase in magnitude with increase in thickness
• Because of high concentration of heat, tensile residual stresses at the weld in welded members usually equal the yield strength of the weld metal itself.
• Residual stresses in welded shapes are determined by section geometry and method of preparation of components.
• Example H may be fabricated from Universal-mill plates or from plates flame-cut to width.
• Because they are quenched and tempered, A514 rolled steel shapes are partially stress-relieved, so residual stresses are small
• Quenching is the act of rapidly cooling the hot steel to harden the steel.
• Quenched steel is hard and brittle. Often it is just too brittle and must be made more malleable, This is achieved by a process known as tempering.
• The quenched steel is heated again but this time to a temperature between 200 °C and 300 °C.
• When the metal reaches the tempering temperature, it is quenched again in cold water or oil. The result is a steel that is still hard but is more malleable and ductile
• Fabricating operations such as cambering and straightening by cold bending also induce residual stresses.
• These stresses are superimposed on the thermal residual stresses.
• If member is straightened by rotorizing, which is a continuous procedure, the residual stress distribution will be changed along entire length of member.

UTM - Universal Testing Machine

HASEEB JAMAL

ENGINEERING MECHANICS

A machine used to test specimens for tensile strength, compressive strength, shear strength and to perform bend test along other important laboratory tests. The primary use of the testing machine is to create the stress strain diagram. Once the diagram is generated, a pencil and straight edge or computer algorithm can be used to calculate yield strength, Young’s Modulus, tensile strength or total elongation.

Components of UTM:

It consists of two main parts, called:

• Loading Unit
• Control Unit

Loading unit

In this unit actual loading of the specimen takes place - consists of three cross heads namely upper head, middle head and lower head. Using appropriate cross heads tensile, compressive, shear, bending load with the help of different attachment can be applied. Loading unit of a UTM consists of:

1. Upper cross head – To clamp testing specimen from top
2. Lower cross head – To clamp testing specimen from below
3. Table – to place the specimen, used for compression test

Control Unit:

The load is applied and recorded by this unit. The load is applied with control valve and released by release valve. The load is applied with the help of hydraulic pressure.

Extensometer

An instrument used to measure elongation in the material

Tests UTM can perform

1. Tensile Tests
2. Adhesion Tests
3. Cycle tests with momentary stops
4. Pull-Out Tests
5. Creep Tests
6. Hysteresis Tests

Displays Test Traces and Values

1. Test Traces: An ongoing test can be displayed as either:
2. Load/Displacement
3. Load/Position
4. Load/Time
5. Position/Time
6. Displacement/Time

Digital Indicating Windows: The following are displayed:

1. Maximum Load (peak hold)
2. Current Load (during a test)
3. Cross head Position
4. Displacement (from the start of a test)
5. Applications of Universal Testing Machine

Use of Universal Testing Machine:

Universal Testing Machine can be used and applied to perform tests on the following samples.

• Rope
• Steel Rope
• Winches
• Steel Wire
• Electrical Wire
• Webbing
• Spring
• Slings
• Cable
• Nylon Rope
• Links
• Chain
• Steel Chain