Hollow bar anchor manufacturers carry out tensile test to ensure the self-drilling anchor rods are strong enough. The values are plotted on a coordinate graph – with strain elongation along the x-axis and stress, typically in MPa, along the y-axis. Figure 1 shows a typical tensile stress/strain curve.
Figure 1. Tensile Stress/Strain Curve
Figure 2. Tensile capacity from self-drilling anchor tensile curve
For small strains, the curve is actually linear. The change in the self-drilling anchor hollow bar in this region is referred to as elastic, because when the stress is removed the object returns to its original length.
After the steel material strains a certain amount, it reaches its yield point. At this point the slope of the curve decreased drastically and ceases being linear, and deformations to the hollow drill bar become permanent. This is an important distinction between different hollow bar anchors.
According to the test standards, the self-drilling anchor hollow bar specimen should be 500 to 2000 mm in length. Tensile test result would be fairly inaccurate and even invalid if the specimen length is too short.
Table 1 is a standard result of tensile test from SGS.
Table 1. tensile test result of ONTON self-drilling anchor hollow bar
The results fully achieve the client's requirements:
1. The test of bars is performed full size.
2. The minimum tensile strength is 237 MPa above the minimum yield strength.
It is better than the specified 25 000 psi (172 MPa) at least by F432.
3. Strength and ductility performance are up to the standards simultaneously.
In fact, ONTON has a range of standard self-drilling anchor rods. These rods are special treated to gain high resistance capacity. That means:
1. For a same cross section of steel, the load bearing capability of ONTON rods is up to 40% higher, allowing for a larger nailing grid or a smaller size (diameter) of bar rods. Project cost saving.
2. For a same tensile strength, our rods are lighter. Labor cost saving.
In the end of tensile test, if the strain increased even more, eventually the object will neck. During elastic and plastic deformation the entire object's cross-sectional area decreases slightly, but at the onset of necking the weakest part of the material suddenly becomes noticeably thinner. It takes less force to finish pulling the steel pipe apart.