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Tensile Tests - notes
Engineering Studies
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Engineering Studies - 17/10/2024Preview text
Tensile Testing
What is Tensile Testing?
A tensile test, also known as tension test, is probably the most fundamental type of mechanical test you can perform on material. Tensile tests are simple, relatively inexpensive, and fully standardised. By pulling on something, you will very quickly determine how the material will react to forces being applied in tension. As the material is being pulled, you will find its tensile strength along with how much it will elongate.
Why Perform a Tensile Test or Tension Test?
Tensile tests are performed for several reasons. The results of tensile tests are used in selecting materials for engineering applications. Tensile properties frequently are included in material specifications to ensure quality. Tensile properties often are measured during development of new materials and processes, so that different materials and processes can be compared. Finally, tensile properties often are used to predict the behaviour of a material under forms of loading other than uniaxial tension.
The tensile strength of a material often is the primary concern. The tensile strength of interest may be measured in terms of either the stress necessary to cause appreciable plastic deformation or the maximum stress that the material can withstand. These measures of strength are used, with appropriate caution (in the form of safety factors ), in engineering design.
Also of interest is the material’s ductility , which is a measure of how much it can be deformed before it fractures. Rarely is ductility incorporated directly in design; rather, it is included in material specifications to ensure quality and toughness. Low ductility, in a tensile test, is often accompanied by, low resistance to fracture under other forms of loading. Elastic properties also may be of interest, but special techniques must be used to measure these properties during tensile testing, and more accurate measurements can be made by ultrasonic techniques.
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Gauge Length
Shoulder Shoulder
Tensile Test Specimens
Consider the typical tensile specimen shown below. It has enlarged ends or shoulders for gripping. The important part of the specimen is the gage section.
The cross-sectional area of the gauge section is reduced relative to that of the remainder of the specimen so that deformation and failure will be localised in this region. The gauge length is the region over which measurements are made and is centred within the reduced section. The distances between the ends of the gauge section and the shoulders should be great enough so that the larger ends do not constrain deformation within the gauge section, and the gauge length should be great relative to its diameter. Otherwise, the stress state will be more complex than simple tension.
Sample test specimens
Graphing a Tensile Test
The modulus of elasticity is a measure of the stiffness of the material, but it only applies in the linear region of the curve. If a specimen is loaded within this linear region, the material will return to its exact same condition if the load is removed. At the point that the curve is no longer linear and deviates from the straight-line relationship, Hooke's Law no longer applies and some permanent deformation occurs in the specimen. This point is called the elastic or proportional limit. From this point on in the tensile test, the material reacts plastically to any further increase in load or stress. It will not return to its original, unstressed condition if the load were removed.
Yield Strength
A value called yield strength of a material is defined as the stress applied to the material at which plastic deformation starts to occur while the material is loaded.
Ultimate Tensile Strength
One of the properties you can determine about a material is its Ultimate Tensile Strength (UTS ). This is the maximum load the specimen sustains during the test. The UTS may or may not equate to the strength at breakage. This all depends on what type of material you are testing; brittle, ductile or a substance that even exhibits both properties. Sometimes a material may be ductile when tested in a lab, but, when placed in service and exposed to extreme cold temperatures, it may transition to brittle behaviour.
Offset Method
The Slope of the Offset Line is equivalent to Young’s Modulus or the Modulus of Elasticity
R
r
0 m
Specified Offset = 0 - m
For some materials, the departure from the linear elastic region cannot be easily identified. Therefore, an offset method to determine the yield strength of the material tested is allowed. An offset is specified as a % of strain (for metals, usually 0%, for plastics a value of 2% is used). The stress (R) that is determined from the intersection point (r) when the line of the linear elastic region (with slope equal to Modulus of Elasticity ) is drawn from the offset (m) becomes the Yield Strength by the offset method.
Stress - Strain diagram
Stress
Stress (or Applied Force)
Breaking or Rupture Point
Strain (or Change in Length)
A typical stress-strain curve for plain carbon steel is shown below.
Stress-strain curve for plain carbon steel
NOTE: - There is a clearly defined point where the material yields, therefore the yield strength is easily identifiable unlike the previous stress-strain curve where we had to use the offset method to determine this.
A true stress-strain curve and an engineering stress-strain curve for plain carbon steel is shown below.
Fracture
Strain hardening Necking
Yield Strength
Young’s Modulus = = Slope
Rise Run
Stress
Strain
Ultimate Tensile Strength
P Proportional Limit - This is when an increase in stress no longer has a linear relationship with an increase in strain. Sometimes this is known as the elastic limit.
E Elastic Limit - The elastic limit is the limit beyond which the material will no longer go back to its original shape when the load is removed, or it is the maximum stress that may e developed such that there is no permanent or residual deformation when the load is entirely removed.
Y Yield Point - The yield point is the point at which the material will have an appreciable elongation or yielding without any increase in load.
U Ultimate Strength - The maximum ordinate in the stress-strain diagram is the ultimate strength or tensile strength.
R Rupture Strength - The nominal stress developed in a material at rupture. It is not necessarily equal to ultimate strength. And, since necking is not taken into account in determining rupture strength, it seldom indicates true stress at rupture.
A Actual Rupture Strength - Taking necking into account determines the actual rupture strength.
Elastic and Plastic Ranges - The region in the stress-strain diagram from O to E is called the elastic range. The region from E to R/A is called the plastic range.
P
E
Y
Stress (σ)
U
Strain (ɛ) R
A
O
Tensile Tests - notes
Subject: Engineering Studies
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