Reliability

It is defined as the probability that a given system will perform it’s function adequately for it’s specified period of lifetime under specified operating conditions.

Common measures are :

1. Failure rate.

2. Mean time between failures(MTBF)

3. Survival percentage.

Failure Rate:

Rate which components of population fail.

R(t)=N_s(t)/N_f(t)

Where,

N_s(t)- No. of components that survived during time ’t’

N_f(t) – No. of failures that occurred during the same time.

Mean time between Failures(MTBF):

The reciprocal of the failure rate(1/λ).

Where

λ —-Failure rate.

Failure rate = (No.of failure )/(Time period during all components were exposed to failure)

 

Reliability Analysis:

Failure Mode and Effect Analysis(FMEA)

Purpose :

1. To recommend design changes.

2. To identify design weakness.

3. To help in choosing alternatives.

Four Stages:

I^st Stage – System boundaries and the scope of the analysis is decided.

II^nd Stage – Data Collection

Ex: Specification,Operating Procedure,Working Conditions.

III^rd Stage– Preparing the component or parts list.

IV^th Stage – Failure frequency and the functions of the part identified,causes of       failures,Failure detection.

 

 

 

CRITICALITY

CRITICALITY is a measure of the frequency of occurrence of an effect.

                       – May be based on qualitative judgement or

– May be based on failure rate data (most common)

Qualitative analysis:

–Used when specific part or item failure rates are not available.

Quantitative analysis:

–Used when sufficient failure rate data is available to calculate criticality numbers.

Qualitative Approach:

• Because failure rate data is not available, failure mode ratios and failure mode probability are not used.
• The probability of occurrence of each failure is grouped into discrete levels that establish the qualitative failure probability level for each entry based on the judgment of the analyst.
• The failure mode probability levels of occurrence are:

–Level A – Frequent

–Level B – Reasonably Probable

–Level C – Occasional

–Level D – Remote

–Level E – Extremely Unlikely

Quantitative Approach

Failure Mode Criticality (CM) is the portion of the criticality number for an item, due to one of its failure modes, which results in a particular severity classification (e.g. results in an end effect with severity I, II, etc…).

• Category I – Catastrophic: A failure which may cause death or weapon system loss (i.e., aircraft, tank, missile, ship, etc…)
• Category II – Critical: A failure which may cause severe injury, major property damage, or major system damage which will result in mission loss.
• Category III – Marginal: A failure which may cause minor injury, minor property damage, or minor system damage which will result in delay or loss of availability or mission degradation.
• Category IV – Minor: A failure not serious enough to cause injury, property damage or system damage, but which will result in unscheduled maintenance or repair.

The quantitative approach uses the following formula for Failure Mode Criticality:

Cm = βαλpt

Where

Cm = Failure Mode Criticality

β = Conditional probability of occurrence of next higher failure effect

α = Failure mode ratio

λp = Part failure rate

T = Duration of applicable mission phase

MECHANICAL TESTING

Various tests:

• Tensile Test

A tensile test, also known as a tension test, tests a material’s strength. It’s a mechanical test where a pulling force is applied to a material from both sides until the sample changes its shape or breaks. It’s is a common and important test that provides a variety of information about the material being tested, including the elongation, yield point, tensile strength, and ultimate strength of the material. Tensile tests are commonly performed on substances such as metals, plastics, wood, and ceramics.

Tensile testing systems use a number of different units of measurement. The International System of Units, or SI, recommends the use of either Pascals (Pa) or Newtons per square meter (N/m²) for describing tensile strength. In the United States, many engineers measure tensile strength in kilo-pound per square inch (KSI).

• Tensile test with electronic extensometer

This instrument is to be used on Tensile or Universal testing machines to find out Proof stress & Young’s modulus values. In case of many brittle materials such as high carbon steels, alloy steels, light aluminium & magnesium alloys, it is difficult to get yield values. For such materials stress corresponding to a certain allowable amount of plastic deformation is termed as proof stress say 0.1% or 0.2% proof stress. The measuring range is up to 5mm & resolution is 0.001mm.

• Tensile testing at elevated temperature.

High temperature tensile testing is a procedure to test the properties of a material at above room temperature. It will determine the following parameters:

• Tensile strength (breaking strength)
• Yield strength
• Elongation
• Reduction of area

Specialist testing, measurement and control equipment is required to perform this test.
The results of such a test will provide a good indication of the static load bearing capacity of the material and therefore establishes the suitability of a material for its intended purpose.

• Tensile test on Tor steel Bars

TOR steel is one of the best grade of steel used in concrete reinforced. It’s a kind of high adherence steel. Other types of steel are used for less resistance concrete. Thermo mechanically Treated (TMT) bars are a type of corrosion resistant steel reinforcing bar used in concrete construction.

• Bend test on plates

A bend test is used to determine whether a specific piece of metal in question will break or fracture under pressure. This is important in the construction of any project using metal, otherwise the building or the item being made could collapse from the immense pressure exerted on it. Every piece of metal made cannot be tested, therefore certain pieces are tested and if they pass, the other pieces are made using the same process. The results of a bend test are reported differently depending on the type of material tested. There is no standard method for reporting the durability that applies to all materials, rather each group has its own set by which it is judged and compared to other metals in that group.

The bend test is essentially measuring a metal’s ductility. Ductility defines how easily a metal can bend without breaking. The higher the ductility of a metal, the more it can bend without breaking or becoming deformed from its original shape. This is important because certain metals must handle pressure without snapping yet still be ductile enough to bend slightly and not lose their support or shape. Copper and steel are two metals that have a high ductility and do well under pressure.

• Bend test on pipes

Bending tests are carried out to ensure that a metal has sufficient ductility to stand bending without fracturing. A standard specimen is bent through a specified arc and in the case of strip, the direction of grain flow is noted and whether the bend is with or across the grain.

• Bend Test on Tor steel

The purpose is to make certain the weld and the base metal are properly fused, and that the weld metal and the heat affected zone (HAZ) have appropriate mechanical properties

• Re-Bend test on Tor steel

The purpose of re-bend test is to measure the effect of strain ageing on steel. Strain ageing has embrittlement effect which takes place after cold deformation by diffusion of nitrogen in steel. Hence, there is limitation stated in some design codes to restrict the nitrogen content of steel to 0.012%.

• Nick Break Test

The NICK-BREAK TEST is useful for determining the internal quality of the weld metal. This test reveals various internal defects (if present), such as slag inclusions,  gas  pockets,  lack of  fusion,  and  oxidized  or burned metal. To accomplish the nick-break test for checking a butt weld, you must first flame-cut the test specimens from a sample weld.

MECHANICAL TESTING

Various tests:

• Tensile Test

A tensile test, also known as a tension test, tests a material’s strength. It’s a mechanical test where a pulling force is applied to a material from both sides until the sample changes its shape or breaks. It’s is a common and important test that provides a variety of information about the material being tested, including the elongation, yield point, tensile strength, and ultimate strength of the material. Tensile tests are commonly performed on substances such as metals, plastics, wood, and ceramics.

Tensile testing systems use a number of different units of measurement. The International System of Units, or SI, recommends the use of either Pascals (Pa) or Newtons per square meter (N/m²) for describing tensile strength. In the United States, many engineers measure tensile strength in kilo-pound per square inch (KSI).

• Tensile test with electronic extensometer

This instrument is to be used on Tensile or Universal testing machines to find out Proof stress & Young’s modulus values. In case of many brittle materials such as high carbon steels, alloy steels, light aluminium & magnesium alloys, it is difficult to get yield values. For such materials stress corresponding to a certain allowable amount of plastic deformation is termed as proof stress say 0.1% or 0.2% proof stress. The measuring range is up to 5mm & resolution is 0.001mm.

• Tensile testing at elevated temperature.

High temperature tensile testing is a procedure to test the properties of a material at above room temperature. It will determine the following parameters:

• Tensile strength (breaking strength)
• Yield strength
• Elongation
• Reduction of area

Specialist testing, measurement and control equipment is required to perform this test.
The results of such a test will provide a good indication of the static load bearing capacity of the material and therefore establishes the suitability of a material for its intended purpose.

• Tensile test on Tor steel Bars

TOR steel is one of the best grade of steel used in concrete reinforced. It’s a kind of high adherence steel. Other types of steel are used for less resistance concrete. Thermo mechanically Treated (TMT) bars are a type of corrosion resistant steel reinforcing bar used in concrete construction.

• Bend test on plates

A bend test is used to determine whether a specific piece of metal in question will break or fracture under pressure. This is important in the construction of any project using metal, otherwise the building or the item being made could collapse from the immense pressure exerted on it. Every piece of metal made cannot be tested, therefore certain pieces are tested and if they pass, the other pieces are made using the same process. The results of a bend test are reported differently depending on the type of material tested. There is no standard method for reporting the durability that applies to all materials, rather each group has its own set by which it is judged and compared to other metals in that group.

The bend test is essentially measuring a metal’s ductility. Ductility defines how easily a metal can bend without breaking. The higher the ductility of a metal, the more it can bend without breaking or becoming deformed from its original shape. This is important because certain metals must handle pressure without snapping yet still be ductile enough to bend slightly and not lose their support or shape. Copper and steel are two metals that have a high ductility and do well under pressure.

• Bend test on pipes

Bending tests are carried out to ensure that a metal has sufficient ductility to stand bending without fracturing. A standard specimen is bent through a specified arc and in the case of strip, the direction of grain flow is noted and whether the bend is with or across the grain.

• Bend Test on Tor steel

The purpose is to make certain the weld and the base metal are properly fused, and that the weld metal and the heat affected zone (HAZ) have appropriate mechanical properties

• Re-Bend test on Tor steel

The purpose of re-bend test is to measure the effect of strain ageing on steel. Strain ageing has embrittlement effect which takes place after cold deformation by diffusion of nitrogen in steel. Hence, there is limitation stated in some design codes to restrict the nitrogen content of steel to 0.012%.

• Nick Break Test

The NICK-BREAK TEST is useful for determining the internal quality of the weld metal. This test reveals various internal defects (if present), such as slag inclusions,  gas  pockets,  lack of  fusion,  and  oxidized  or burned metal. To accomplish the nick-break test for checking a butt weld, you must first flame-cut the test specimens from a sample weld.