Characteristic of strength of materials

Two (2) type specimen (RA & RB) was subjected to tension in accordance with the MS 146:2006 provisions by using universal testing machine. The results of the tensile tests indicating the yield stress and ultimate stress is shown in Table 1 below.

Table 1: Table 2: Average Value for  Yield Stress and Ultimate Stress

Sample Designation	Yield Stress (N/mm2)	Ultimate Stress (N/mm2)
RA	518.18 ± 7.12	585.64 ± 4.32
RB	432.16 ± 5.32	570.08 ± 3.12

i)         Given f_k = f_m -1.86s. Calculated the characteristic of strength for those concrete rebar steel.

ii)   Given the minimum provisions of MS 146:2006 - Specification for Carbon steel bars for the reinforcement of concrete is as Table 2 below. Determine the safety of those samples as reinforcement steel bar. 

Table 2: The minimum provision of MS 146: 2006

Standard	Characteristic Strength (N/mm2)	Ultimate/Yield Strength Ratio
MS 146:2006	460	1.05

Answer

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i)   Given f_k = f_m -1.86s

      Characteristic of strength for RA sample è f_{k =} 518.18 – 1.86(7.12) = 504.94

      Characteristic of strength for RB sample è f_{k =} 432.16 – 1.86(5.32) = 422.26

ii)   Characteristic of strength for RA sample è = 504.94 (above the code) = Safe #

       Characteristic of strength for RB sample è = 422.26 (below the code) = Unsafe #

Preventing stress concentrations.

High local stresses can cause objects to fail more quickly, so engineers must design the geometry to minimize the stress concentrations. Propose the alternative preventive actions to eliminate or minimise the stress concentration.

Answer

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• Drill a large hole at the end of the crack. This is however, a temporary solution that must be corrected at the first opportune time.

• Systematically check for possible stress concentrations caused by cracks

• Apply a metal stitch to lock the crack or metal paste technology

• Creating the fillet at the sharp edges.

• Providing a fillet radius so that the cross-section may change gradually

• Using an elliptical fillet

• Using a number of small notches rather than a long one, if a notch is unavoidable

• Using narrow notches rather than wide notches, if a projection is unavoidable

• Using stress-relieving grooves