This article demonstrates basic knowledge understanding of tensile capacity of the drill pipe and some calculation. First of all we need to know basic of material strength and for our case is strength of metal

There are few simple terminologies which we would like to explain

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**(Stress (σ**

Stress (σ) equals to force divided by cross sectional area of the material (F/A). For our case, we will discuss about only stress in tensile because the drill pipe is almost always designed to work in a tensile condition

### Stress (σ) = F/A

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**(Strain**** (ε**

Strain (ε) is a change of material per an original length. From the definition, it equals to ∆L/L

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**Strain (ε) = ∆L/L**

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**(Young’s Modulus (Modulus of Elasticity**

Young’s modulus (the tensile modulus or elastic modulus) is a ratio of stress and strain along the axis and we can write into the following equation

### (Young’s modulus = Stress (σ) ÷ Strain (ε) = (F x L) ÷ (∆L x A

Where

F is pulling force.

L is an original length of pipe.

∆L is an amount by which the length of the pipe changes

A is a cross sectional area of object

The Young’s Modulus of material represents the factor of proportional in Hook’s Law therefore it will valid under the elastic zone. There are several units for Young’s Modulus as N/m2 (Newton), Maga Pascal (N/mm2) and Pound per Square Inch psi

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**Stress-Strain Curve**

A stress-strain curve is a graph derived from Stress (σ) versus Strain (ε) for a sample of a material. The nature of the curve varies from material to material. The following curve shows a behavior of metal

**Yield Point**or

**Yield strength**, is defined as the stress at which a material begins to plastically deform. Before the yield point the material will deform elastically and it will return to its original shape when the stress is released. If the tension applied is over the yield point, the deformation will be permanent and non-reversible

**Ultimate strength**is the maximum stress applied before the material is completely parted

**Young’s Modulus (modulus of elasticity)**is the slope of the Stress-Strain curve within the elastic limit (see Figure 4). It means that once tensile is less than Yield Point, the Young’s Modulus is valid for the calculation

Young’s Modulus of steel is 30 x 106 psi

In drilling operation, we must operate within Yield point because the metal will become the original shape. For example, if you get stuck, the maximum tension applied to free the stuck drillstring must be always under yield point with a designed safety factor for the operation

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**Drill Pipe Body Grade**

API RP7G classifies a grade of drill pipe body according to yield strength and tensile requirement (see Table 1 and Table 2). Four grades of drill pipe are “E”, “X”, “G” and “S”