Tensile Testing Procedure of Deformed Bars(Steel) as per ASTM

The tensile test of steel and its by-products is one of the most important tests used to access the ductility and ultimate tensile strength of steel deformed bars. The tensile testing procedure is yet a simple one but it requires due concentration of the performer in order to get satisfactory results.

The test must be carried out using some well-known standard procedure. In this article, a summarised version of the tensile test as per ASTM is discussed. The most important parameter and keynotes are presented in this article exclusively for the ease of the reader. You will also find out the most commonly asked questions related to tensile testing of steel rebars.

What is Tensile Testing?

A standard procedure to find out the tensile strength of the material, in which tensile load is applied on a material (e.g. Steel Bar) until it fails. The peak load is recorded at yield and ultimate points which is then divided by the cross-sectional area to obtain the yield strength or the ultimate strength.

Stress-Strain Curve for mild steel, Tensile Testing Procedure
Typical Stress-Strain Curve for mild steel

Where

  • A= Lower Yield Point
  • B = Upper Yield Point
  • D = Ultimate Stress
  • E = Fracture
  • OA = Linear region (Hooke’s law govern)
  • BC = Perfect plasticity or yielding
  • CD = Strain Hardening
  • DE = Necking

What is the Loading Rate for tensile testing of steel?

The specified loading rate as per standard ranges from 167 lb/s to 10,000 lb/s. The minimum load is 167 pounds per second and the maximum load should be less than 10,000 pounds per second.

Apparatus Required For Tensile Testing

  • Machine capable of applying consistent tensile load at a specified loading rate (e.g Universal Testing Machine UTM). The load cell of the machince should be of sufficient capacity.
  • PPE (Personal protective equipment) for the operators
  • Test Samples

Tensile Testing Procedure:

Following is the summary of the generalized tensile test of steel rebars and other similar alloys.

Sample Preparation:

The specimen should be of sufficient length and should match the dimensions of the testing machine. The minimum gauge length recommended by ASTM A 615 is 8 inches. Proper marking should be done to delineate the gauge length on the test specimen.

Note Guage length, Total Length, Diameter, and Weight of the test sample.

What is gauge length in tensile test

The gauge length is the length used in elongation computations. The gauge length, according to the test standard, is either 2 inches or 80 mm. The initial cross-sectional area is determined by multiplying the width and thickness within the gauge length. ASTM A615 recommends a minimum of 8 inches gauge length for the tensile testing of steel rebar.

Testing Machine Setup:

The testing machine must be calibrated and of sufficient capacity. The test specimen should be clamped in the tensile testing region of the machine such that all connections are firm. The load rate application should be smooth without any impact loading.

Loading Rate:

The minimum loading rate for tensile testing of steel rebar should be greater than 167 lb/sec as per ASTM. The maximum loading should be less than 10,000 lb/sec as per ASTM. It is important to avoid sudden kinks and jerks during the application of load. The machine should have a capacity greater than the failure load of the sample.

How to calculate tensile strength

After careful installation of the test sample, apply the tensile load. Proper safety procedures should be adopted during the test. The material will absorb the load for some time before yielding and it will completely collapse when its ultimate capacity is reached. Note down or calculate the following parameters:

  • Load at the yield point
  • Load at the ultimate point
  • Final Guage length of the sample by reconnecting the broken sample.

Calculations:

The following calculation is required in the final report:

  • Actual and Nominal Diameter of the test sample
  • The density of the Sample which is mass per unit volume
  • Initial and Final Guage length
  • Percentage Elongation by the formula:
Percentage Elongation formula, Percentage Elongation
  • Yield Strength is given directly by the machine or calculated by the following formula:
Yield Strength formula, Yield Strength
  • How to calculate ultimate tensile strength: Ultimate tensile strength is provided by the machine or calculate using:
ultimate Strength formula, ultimate Strength

Faqs related to tensile test

What is the Purpose of Tensile testing?

The goal of this test is to calculate the yield strength, ultimate strength and to check out the ductility (Material’s capacity to deform permanently) of material by calculating its percentage Elongation. The material can be then recommended for use according to its capacity.

In Structural and Civil Engineering it is always recommended to use ductile materials having sufficient yield strength. The steel deformed rebar is a classical example of ductile material having a yield strength of above 100 KSI depending on its grade. The grade is used to classify the steel reinforcing bars on the basis of yield strength. For instance Grade 40 means that the rebar has a yield strength of 40 Ksi.

What is Tensile strength?

The maximum strain a material can sustain without breaking when stretched over a given period of time is divided by the initial cross-sectional area of the material. Tension is the amount of force needed to pull on one end of a wire. Tensile strength, also known as tension, is a measurement that indicates how much force it takes to break or pull apart something. In English units, it’s commonly measured in pounds per square inch (psi).

Classification of steel by Grade
Classification of steel by Grade

Tear resistance is also measured by tensile strength. The amount of force that may be applied to a substance before it breaks is called tensile strength.

The ratio of maximum load a material can support without breaking when stretched to the original cross-sectional area of the material. When forces less than the tensile strength are relieved, a substance either totally or partially regains its original form and size.

What is tensile strain?

The ratio of extension or change in length of a material external force, divided by its original length. If a 1 m long thing is stretched to 1.1 m, the strain is 0.1/1.0 = 0.1/1

When strain is denominated in length units that are not relevant, it makes no difference what unit of measure you use to express your dimensions. Some people like those ancient imperial measures. Alternatively, micro-cubits and milli-cubits are perfectly acceptable.

What is tensile stress?

The tensile stress formed as a result of a tensile force put on somebody is known as tensile stress. The amount of direct strain caused by the tensile forces that cause an object to stretch is referred to as tensile stress. It is a measurable quantity that pertains to tensile forces and stretching operations.

It causes any material along the load’s application line to elongate. It is a condition in which a force applied to an object tries to pull or stretch the material along the force’s line of impact.

What is the ASTM Standard used for tensile test of steel?

ASTM A 615 is the most popular standard that is widely accepted as a standard for the tensile testing of steel products. ASTM A 615 has provisions both for tensile as well as bend test of steel rebar. In this article, the overall procedure, requirements, and important points from the standard are presented.

Conclusion

A very important test in the field of civil and structural engineering is discussed in a summarized manner. The test specimen fails under a controlled load and the maximum capacity of the material is thus recorded at yield and ultimate points. The steel rebars can be then recommended for further usage. The steel rebars are usually used in the tensile region of concrete. Concrete is weaker in tension, so this problem is overcome by installing reinforcing bars in the tensile region.

The steel rebars have some good capacity beyond the yield point, but to keep the factor of safety the design of structures is usually based on the yield tensile strength of steel rebars. The rebars are thus classified on the basis of yield strength which is referred to as Grade. The tensile testing verifies the grade of steel. Tensile testing can also conclude if a material is ductile or brittle. Brittle materials have very little strength beyond the yield point. It is always recommended to use ductile material in structures subjected to various types of loads.

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