Destructive vs. Non-destructive Testing: Materials Evaluation Techniques, Lecture notes of Materials science

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MM 212

Materials Evaluation Techniques

Fall Semester 2018, FMCE, GIKI

Instructor: Muzammil Irshad Research Associate

Lecture 43 : Destructive Vs Non-destructive Testing

Introduction :

 Non-destructive testing is not the only approach to evaluate materials.

 Traditionally, most materials evaluation methods involve the use of test samples cut from the materials or components of interest.

 In some cases, destructive tests are more economical than non-destructive tests because the cost of the destructive test is usually lower.

 For low-cost parts, the sacrifice of a few components can be tolerated.

 For high-added-value parts, the cost of sacrificing some parts may outweigh the additional cost of non-destructive testing

 Materials characterization as a special case of testing can be distinctly different from non-destructive evaluation.

 Value added is the difference between the price of product or service and the cost of producing it.  Examples of value added costs are the direct materials, direct labour, and installation costs associated with a sale.

Worked Example:

The cost of inspecting a plant is $50,000/d, the time needed for inspection is 10 d, and the cost of downtime at the plant is $400,000/d; the cost of a failure at the plant has been calculated to be $500,000,000, and the cost of replacement parts is $20,000,000. If the probability of failure is 1% without inspection, but is effectively zero after inspection, is it economic to inspect the plant? Is it economic to inspect if the probability of failure is 0.8%?

Solution:

Inspection cost = ($50,000 + 400,000). 10 = $4,500,000. The cost of failure = $520,000,000. If the probability of failure is 1%, the probable cost of failure = cost of failure times probability of failure = $5,200,000. On this basis, economics dictates that it is cost-effective to inspect the plant. If the probability of failure is 0.8%, the probable cost of failure = $4,160,000. On this basis, economics suggests that it is not cost-effective to inspect the plant, unless a cheaper method of inspection can be found.

So, decision can be affected by the probability of failure. As the probability of failure decreases, there is less incentive to inspect. Also, as the cost of failure increases, there is more incentive to inspect.

 (Probability is the measure of the likelihood that an event will occur).

Factors to consider in selecting tests:

Economics:

 Destruction of high-added-value parts may be unacceptable

 More testing can be justified for high-value components if it reduces the probability of failure and thereby the probable cost of failure.

 Speed of inspection and cost of downtime is also important for in-service inspection

Safety:

The cost of failure can become very high if there are safety issues or potential injuries or fatalities that would be involved if there were a failure.

Safety issues can turn into economic issues as a result of fines, litigation, etc.

The following is a list of some standard destructive tests that are routinely used on materials:

• Tensile testing
• Fatigue testing
• Charpy impact testing
• Creep testing
• Metallography (sectioning, polishing, and viewing)
• Hardness testing
• Corrosion testing
• Compact tension testing (for crack growth)

 Non-destructive testing is therefore mostly directed towards expensive, high added-value parts

 Non-destructive testing is much preferred on the production side of industry for online evaluation of parts coming from a manufacturing line.  Also for in-service evaluation of parts in which they continue to perform their normal function during the inspection.

Economics of testing:  The use of non-destructive testing to validate a part adds to its value by providing increased confidence in its ability to perform its intended function.

 It is very difficult to know in practice what the probability of failure will be, either before or after inspection, and therefore exact calculations are difficult to make.

Added Value vs. Cost of Non-destructive Testing:

 Manufacturer may just need to know that the product meets certain specifications (No failure or safety issues).

 However, there has to be some added value associated with performing the test.

 If the initial value of the part is A$, and the final value of the validated part is B$, then the added value is B$ − A$.  If the cost of conducting the non-destructive inspection for validating the part is C$.  Then this inspection only makes economic sense if the added value exceeds the cost of the test that is to be used for the purpose of quality control/assurance

In-Service Testing If non-destructive testing can reduce the probability of failure from pi to pf, then for the test to be economically justified, the cost of the test must be less than the decrease in the probable cost of failure.

Added Value vs. Costs of Nondestructive Testing and Failure

In the more general case, if nondestructive testing is performed at a cost of C$ to reduce the probability of failure from pi to pf, then the inspection still only makes economic sense if the following inequality is satisfied:

Clearly, the more effective the nondestructive test is in reducing the probabilityof failure pf,

Destructive Tests with Cost of Failure

 The argument that destructive tests can reduce failures is dependent on the assumption that the parts tested and destroyed are identical to the parts that are not tested.

 In many instances this is questionable? particularly in the case of failures due to defects that may be present in only a fraction of the parts.

 In other cases this argument may be valid, for example, when the destructive tests are designed to establish intrinsic properties of the material, such as ultimate tensile stress.

 If the associated cost of failure is D$, and the probability of failure after the test is pf , whereas the probability of failure before the test was pi, then the following inequality needs to be satisfied for destructive tests in order that the testing is economically justified:

In the limiting case, as the added value B$ − A$ decreases towards zero, there is no incentive for testing.

Economic considerations in destructive Vs. Nondestructive testing

 For either form of test we can define a net expected economic benefit (NEEB),which needs to be greater than zero for the test to be economically viable.

 For a non-destructive test, we can define the NEEB as:

 Whereas for a destructive test, we can define the NEEB of testing as:

 Generally, the value C$ is higher for nondestructive testing than it is for destructive testing.

Worked example:

The cost of a power plant failure due to malfunction of a critical component is $1,500,000. If by the use of nondestructive testing, which costs $100,000, it can be established that a critical power plant component will not fail, whereas without testing the probability of failure over a given period is 10%, determine the NEEB of doing the nondestructive testing.

Solution:

Intrinsic properties vs. Performance:

 Techniques that generally come under the heading of nondestructive evaluation and testing relate performance to the three areas of synthesis/processing, structure/ composition, and properties, whereas material characterization links properties to the three other areas of synthesis and processing, structure/composition, and performance.

Examples of characterization methods:

• Mechanical tests
• Ultrasonic tests
• Optical tests
• Thermal tests
• Conductivity tests
• Magnetic tests
• Radiation tests

In some cases, correlations between materials properties and failure-related mechanisms, such as the build up of dislocations and residual stress, can be exploited to allow materials characterization of intrinsic properties to be used as an indirect measure of the condition of the material and its likely performance.