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

The requirement of design

State the point of view regarding the requirement of design that should be considered in the first stage of structural design process.

Answer

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Combined economy safety and pleasure:

Maintain its function or characteristic without high maintenance costs;

Give adequate warning of danger in event of an overload;

Keep and acceptable appearance at working load with no public unease about its safety. (e.g.: due to distortion, deflection and vibration).

Creep: Microstructure & preventive action

Creep is one of the deterioration mechanisms that could be detected by in-situ metallography.

  i)  Sketch the schematic diagram the microstructure of steel that experienced creep.

 ii)  Give your idea of the metallurgical design solutions that should be carried out to prevent a creep failure.

Answer

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i)  

Schematic diagram of creep microstructure 

ii)   Creep can be prevented by

Reduce the effect of grain boundaries: 

• Use single crystal material with large grains.

• Addition of solid solutions to eliminate vacancies. 

Employ materials of high melting temperatures.

Apply a new technology of ceramic thermal barrier

Consult Creep Test Data during materials Selection

• Type of service application

• Set adequate inspection intervals according to life expectancy.  

Critical parts & components in industry

In heavy industry such as petrochemical and power generation plant,

i)  Classify the type of components that susceptible to failure in relation of function of materials, pressure and thermal efficiency.

ii) Identify the critical point in boiler component that should be periodically inspected from creep  failure.

Answer

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i)   Boiler tubes, Furnace, Superheaters, Steam piping, Turbine, Heat exchanger & Pressure Vessel

ii)  Bends, T-section, Reducing sections, Valves, Rigidity-held pipes & Hangers

The sources of failure

One of the practical aspects of materials selection in the design, development and production of new components (or parts/ structures) is the possibility of failure of the component (or parts/ structures) under normal operation. Classify and analyse all the sources of failure that must be considered before designing a component (or parts / structures).

Answer

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1)        Deficiencies in design,

§  Safety factors are usually included in the design to account for uncertainties in the operational conditions.

§  The designer consider the possible failure mechanisms that could be induced during service

2)        Deficiencies in materials

§  Selection of an inadequate material during the design process (design deficiency).

§  The deliberate or accidental substitution of an alternate material

§  The presence of defective material.

3)        Service conditions that have been modified from those considered in design,

§  Modifying service condition includes changes in

ü  Temperature and/ or pressure,

ü  ambient environment,

ü  speed,

ü  power or loading,

ü  lubricant, and process stream.

§  If it cannot be determined that the modified conditions were included in the design process,

ü  a systematic evaluation of the potential effects of the modification should be undertaken.

4)        Errors in assembly or maintenance.

§  Service failures often result from errors in original assembly, errors in maintenance procedures or errors in reassembly following maintenance.

§  Failures due to assembly or maintenance errors are most often found in the moving parts of mechanical assemblies.

Welding failure by stress corrosion cracking

A schematic diagram of a welded elbow pipe illustrated in Figure 3(a) is failed pipe that located at first stage superheater tubes in the boiler. It had been moderately cold-bent during installation. Non-destructive testing result shows the longitudinal cracking was located on the welding heat affected zone, along the weld beads (Figure 3b). The failure occurred after only 3 months of operation. The steam drum lacked adequate devices for separation of steam and water, and load swings were frequent, possibly causing carryover of boiler water. Microstructural analysis revealed plastically deformed grains from the cold bending. The cracks were highly branched and ran between the grains (intergranular) as they passed through the tube wall. The corrodent contains a slight sodium chloride from boiler-water carryover. The tube was then removed and sends to your consultancy firm for further failure investigation.

Figure 3: Schematic diagram of  (a) welded elbow pipe & (b) the cracking heat affected zone along the weld bead

(i)                 Give hypothesis the root cause of this pipe failure.

(ii)               Justify your hypothesis.

(iii)      If metallographic analysis performed on the failed area, determine the characteristics of the microstructure that will be observed under optical microscope.

(iv)             Recommend a few possible solutions to prevent the occurrence of this defect in the future?

Answer

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(i)                 Give hypothesis the root cause of this pipe failure.

            The root cause of failure is hypothesized as Stress Corrosion Cracking

(ii)               Justify your hypothesis.

            Generally welding contains internal stress.

            “Moderately cold-bent during installation” results residual/ internal stress”

            “The steam drum lacked adequate devices for separation of steam and water, and load        swings were frequent, possibly causing carryover of boiler water” results hot water          enter the tube & forces the acceleration of oxidation reaction

           “Micro structural analysis revealed plastically deformed grains from the cold bending”    shows an elongated grain proves the presents of residual/internal stress 

           “The cracks were highly branched” shows the presents of stress ran between the grains  (intergranular)” shows the presents of corrosion

           “The corrodent was consist of small content of sodium chloride from boiler-water  carryover” shows chloride ion from NaOH acts as active corrosion agent

          

(iii)      If metallographic analysis performed on the failed area, determine the characteristics of the microstructure that will be observed under optical microscope.

Schematic diagram of optical metallograph for SCC shows hairline, branching, Intergranular & Transgranular cracking

(iv)             Recommend a few possible solutions to prevent the occurrence of this defect in the future?

Stress corrosion cracking (SCC) requires three conditions to occur simultaneously,

      (a) a susceptible alloy,

      (b) tensile stresses, and

      (c) a specific corrosive environment.

For instance, the necessary tensile stress can be either externally applied or existing in the material due to forming or welding. If one condition is eliminated, SCC will not occur.

Prevention can be achieved by:

- Reducing the overall stress level and designing out stress concentrations

- Selection of a suitable material not susceptible to the environment

- Design to minimise thermal and residual stresses

- Developing compressive stresses in the surface the material

- Use of a suitable protective coating

Welding Symbol: Fillet weld

Draw the welding symbol for the following desired weld

Figure 1:  Schematic diagram of desired weld.

Answer

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