Determination Of The Characteristic Strength Properties Of Mild Steel Reinforcement
[A CASE STUDY OF ILORIN METROPOLIS]

2.7.2 CREEP
At elevated temperatures a metal in tension will continue to elongate under a constant stress which may be much less than the ultimate tensile stress. This phenomenon is called ‘creep’ and is measured by the rate of strain per hour under a certain stress at a given temperature.
If a ‘short time’ tensile test is carried out on a metal specimen at a specific temperature a fairly definite ultimate stress is obtained. The material may be made to fail by creep under a lower stress, however, provided sufficient time is allowed the rate of creep depending on the stress. At any temperature there is a limiting stress below which creep will not take place, i.e. the metal will not fractures if the stress is applied for an indefinite period.
This limiting creep stress may frequently be less than half the ultimate stress obtained in a normal test at that temperature. in designing any part which is stressed at high temperatures it is clearly necessary to base the working stress on the limiting creep stress.
2.7.3 COMPRESSION TEST
Specimens for compression tests on metals are usually circular, and for concrete square, in section. To prevent failure by buckling, the length should be of about the same order as the minimum width.
For a ductile material such as mild steel or copper later distortion takes place, and due to the restraining influences of friction at the load faces, the cross section becomes greatest at the centre, the test piece taking up a barrel shape failure finally occurs by cracks Brittle materials such as cast iron and cement usually frail by wearing along planes inclined  between 50^0 and 700 to the longititudinal axis appearing on the surface and Spreading inwards.
 2.7.4 FATIGUE
Many machine parts are subjected to fluctuating stresses, taking at relatively high frequencies and under these conditions failure is or occur at stress values much lower than would apply for static loading. This phenomenon is known as ‘fatigue’ failure.
Fluctuating stresses occur in practice under three main types of loading.
    Direct stresses (tension and compression).
    Bending stresses.
     Torsional stresses
Experiments show that, for a given means stress, there is a limiting rage of stress below which fracture will not take place for an indefinite number of cycles. This range is known as the endurance or fatigue limit and may be quoted as a maximum and minimum stress or as a range about a certain mean. The fatigue limit in reversed bending is found to be about 25% higher than in reversed tension and compression, probably due to the stress gradient. In reversed torsion the fatigue limit for sear stress is about 0.55 times the tensile fatigue limit.
In order to determine the fatigue limit at a given mean stress, it is necessary to carry out a series of tests on specimens subjected to a gradually decreasing range of stress. It will then be found that the number of cycles of stress required to fracture each specimen increases and as the fatigue limit is approached some hundreds of millions of reversals may be understood.
Factors of design which affect the fatigue strength are
a.     Surface treatment.
b.     Surface finish.
c.     The frequency of stress reversals.
d.     Stress concentrations caused by sudden changes in cross-sectional features such as screw threads and key ways.
2.8     EFFECT OF CARBON CONTENT
The variation of mechanical properties in plain carbon steel in the annealed condition is shown below.
It will be seen that the ultimate strength and hardness values increases together with the increased carbon content, the elastic limit (and similarly the yield point) increasing at a reduced rate. At ,the same time there is a marked falling off in ductility in indicated by the decrease in value for elongation and reduction in area, steel containing more than about 0.6% carbon exhibiting a “brittle” type of fracture.