Category Archives: Failure Analysis

Factor of Safety, FOS

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It is common practice to size the machine elements, so that the maximum design stress is below the UTS (Ultimate Tensile Stress) or yield stress by an appropriate factor – the Factor of Safety, based on UTS(Ultimate Tensile Stress) or Yield Strength. The factor of safety
also known as Safety Factor, is used to provide a design margin over the theoretical design capacity to allow for uncertainty in the design process. Factor of safety is recommended by the conditions over which the designer has no control, that is to account for the uncertainties involved in the design process.

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The uncertainties include (but not limited to),

  • Uncertainty regarding exact properties of material. For example, the yield strength can only be specified in between a range.
  • Uncertainty regarding the size. The designer has to use the test data to design parts which are much smaller or larger. It is well known that a small part has more strength than a large one of same material.
  • Uncertainty due to machining processes.
  • Uncertainty due to the effect of assembly operations like riveting, welding etc.
  • Uncertainty due to effect of time on strength. Operating environments may cause a gradual deterioration of strength, leading to premature and unpredictable failure of the part.
  • Uncertainty in the nature and type of load applied.
  • Assumptions and approximations made in the nature of surface conditions of the machine element.

Selection of factor of safety

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The selection of the appropriate factor of safety to be used in design of components is essentially a compromise between the associated additional cost and weight and the benefit of increased safety or/and reliability. Generally an increased factor of safety results from a heavier component or a component made from a more exotic material or/and improved component design. An appropriate factor of safety is chosen based on several considerations. Prime considerations are the accuracy of load and wear estimates, the consequences of failure, and the cost of over engineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury or death usually use a safety factor of four or higher (often ten). Non-critical components generally have a safety factor of two. Extreme care must be used in dealing with vibration loads, more so if the vibrations approach resonant frequencies. The vibrations resulting from seismic disturbances are often important and need to be considered in detail. Where higher factors might appear desirable, a more thorough analysis of the problem should be undertaken before deciding on their use.

  • 1.25 – 1.5
    Material properties known in detail. Operating conditions known in detail. Loads and resultant stresses and strains known with with high degree of certainty. Material test certificates, proof loading, regular inspection and maintenance. Low weight is important to design.
  • 1.5 – 2
    Known materials with certification under reasonably constant
    environmental conditions, subjected to loads and stresses that can be determined using qualified design procedures. Proof tests, regular inspection and maintenance required.
  • 2 – 2.5
    Materials obtained for reputable suppliers to relevant standards
    operated in normal environments and subjected to loads and stresses that can be determined using checked calculations.
  • 2.5 – 3
    For less tried materials or for brittle materials under average
    conditions of environment, load and stress.
  • 3 – 4
    For untried materials used under average conditions of environment, load and stress. Should also be used with better-known materials that are to be used in uncertain environments or subject to uncertain stresses.

Usually the factor of safety is kept larger, except in aerospace and automobile industries. Here safety factors are kept low (about 1.15 – 1.25) because the costs associated with structural weight are so high. This low safety factor is why aerospace parts and materials are subject to more stringent testing and quality control. Now computers are being used to provide more accurate simulation of stresses that occur in components, particularly in the case of high value products where safety and saving weight is essential.

FAILURE ANALYSIS | CORROSION ANALYSIS | ROOT CAUSE FAILURE ANALYSIS

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• Why ?

As the standards of our industry rise due to increasing globalization and competition, there is an ever growing need for consistency and reliability. Breakdown of any unit, system or equipment is an avoidable and costly occurrence and must be prevented or minimized. Analysis of such failures becomes a resourceful and affordable tool in addressing such unwanted occurrences.

To establish whether the cause of component failure lay on:

a) Service conditions
b) Design considerations
c) Material and its specification
d) Improper processing and assembly procedures or
e)  Combinations of these.

01-RootCause-root cause analysis cycle-problem solving steps-avoidance of recurring problems

Only the real “Root cause” can ensure the effectiveness of corrective and preventive actions and avoid recurrence of failure.

01-CauseEffect-analysis-bottom up predictive-ishikawa - fishbone diagram-prediction analysis

• Stages Of Failure Analysis

1. Understanding and assimilation of background data and selection of samples.
2. Examination and documentation of the failed part by the following

1. Visual examination of parts, location (if necessary) and relevant photographs as well.

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2.  Non destructive testing by means of Radiography, Dye      penetrant, Magnetic particle testing etc.

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3. Mechanical Testing for various physical properties.

3. Vital specimens are selected, classified, and subjected to:

  1. Macroscopic examination and analysis. This involves examining the fracture surfaces, secondary cracks, deposits and other such elements
  2. Microscopic examination and analysis of fracture surface (by Scanning Electron Microscopy, if required).

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4. Chemical analysis of material for conformation to specifications.

5. Chemical analysis of corrosion products, deposits, contaminants etc.

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6. The actual state of the failed part and the failure mode are established.

7.  Fracture mechanics study if found necessary.

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8. A simulation of the identical working environment to determine if any external      factors have contributed to the failure

9. Conclusions are determined after compiling all evidences and analysis and       then the report is generated.
10. Follow-up recommendations are also provided.

 

 

 

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