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Failure Analysis and Prevention Dr. Dheerendra Kumar Dwivedi Department of Mechanical and Industrial Engineering Indian Institute of Technology, Roorkee Lecture - 21 General Procedure of Failure Analysis: NDT for Failure Analysis Hello I welcome you all in this presentation related with the subject failure analysis and prevention. And we are talking about the general practice and procedures for the failure analysis. And in this connection we have already talked about the 2 points, one was like the collection of the background information of the failed component and the second one was the preliminary examination of the failed component, and after this we have got the failed component we need to study the failed component with regard to the presence of discontinuities or the stresses which have been experienced by the component during the service. (Refer Slide Time: 01:00) So, for that purpose we need to perform the non-destructive testing. So, non-destructive testing, so non-destructive testing for failure analysis. So, what is the kind of a role of the NDT in failure analysis? So, you know the primary purpose of conducting the NDT on the failed component is a to basically identify the presence of discontinuities in the component if they are there. So, establishing establishing presence of discontinuities, defects, cracks, pores etcetera if they are present if any present in the present where. So, this presence is to be identified especially with regard to the certain locations like; I it may be near fracture surfaces or in the bulk base metal itself. So, there will be the, and this will be done for the 2 different purposes. the NDT of the bulk material will help us to see whether the material is sound or not or if it is having the inherited discontinuity due to the use of imperfect base metal itself. Or if the discontinuities are present or defects are present like cracks, and other things are present near the fracture surface then it may indicate that either these have been caused by the service conditions or these were presented due to the various regions related with the improper manufacturing. Ah. So, once these are identified identification and establishment of these discontinuities is done, basically with regard to the shape and the size of a discontinuities ah. So, the main aim is a to relate the possible contribution of these discontinuities with the failure or fracture. So, if the discontinuities are there of the large size at the critical location then they of course, can be considered as a major possible contributor towards the failure, but that will be established only through the fracture mechanics analysis of the entire situation in light of the size and shape and the location of these discontinuities in the main component which has failed. So, it provides the basis for the failure analysis due to the presence of the discontinuities if they are established through the NDT. So, this is one of the aspects and another one is to do the analysis with regard to the approximate loading of the machine component, or the stress induced in a particular part it also indicates the kind of the strain especially in especially in the high strain areas. So, these are some of the things apart from that it can also help us to see if there is any presence of the residual stresses in the component and if they have contributed towards the failure. So, these are the 2 types of the things which can be done through the use of NDT in connection with the failure analysis. So, establishing the presence of discontinuities and then investigating their role towards the failure. And the second one is like the kind of the load which has acted on to the machine or a particular component kind of stresses which have been is induced or what is the effect of the service conditions in terms of the load on to the strain which has been induced or the kind of residual stresses where which were there which were present. So, apart from the design load if we are able to establish this through the NDT; that the component has been subjected to the certain kind of the load and certain kind of the stresses which you are beyond the acceptable level for that particular component beyond the allowable level of the stresses for that component. So, this can be used as a possible way to relate with the failure. So, and it can also indicate the possibility of the accidental load or the abnormal load under which the failure has taken place with respect to the design load or the expected load. So, these are the 2 main objectives of conducting the NDT of the failed and one we are able to identify and establish the size, shape and the type of discontinuity is it is location, where they are present and then efforts will be made to relate them with the failure. So, and the second one is to identify that it will help to estimate the kind of the load stresses strain which have been induced as a result of the loading during the service. So, that can be related with the possibility of the normal load or abnormal load accidental load so likewise. So, now how it is conducted? So for establishing the defects and the discontinuities. (Refer Slide Time: 07:45) In the failed component, we have one category of the methods and for establishing the load stresses strain we have another category of the method. So, this falls under the experimental stress analysis. And a for determining or establishing the defects and discontinuities in the failed component we have the another category of the methods. So, we will be talking about these methods in detail. So, there are 4 5 common methods of the establishing the discontinuities and defects in the failed component ah, like liquid penetrant test this is also known as DPT dye penetrant test this is one. Second is the magnetic particle test, third is et current test, 4th is ultrasonic test, and the fifth is like radio graphy test wherein we have like X ray or the gamma ray testing of the metals. So, each type of the test of for certain advantages and disadvantages over the others so, we will take up each type of the test one by one sequentially. So, like say the first is liquid penetrant test. (Refer Slide Time: 09:40) This is also known as dye penetrant test. This test is primarily used for the surface defects, which may be in form of the like cracks, pores which are open or the blow holes are like in weld joints, crater cracks, crater cracks or undercuts all these things can be easily established with the help of the dye penetrant test. So, this is primarily for the surface cracks. So, very fine pores and the cracks etcetera can be easily identified and established with regard to their size and location where they are present. (Refer Slide Time: 10:46) So, in this set test what we do basically like say if this is the component and it is having some kind of crack like this. (Refer Slide Time: 10:55) So, first of all the surface to be inspected of the failed component is cleaned. And after the cleaning we apply the dye over the surface. So, dye is basically thin liquid metal with the color so kerosene kind of thing can be used for this purpose with the color. So, whenever it is a spread over the surface so, thin liquid by the capillary action seeped into or gets filled into those fine pores or cracks and once this is given sometime after spraying the this one is cleaned. So, after cleaning all these things are basically removed from the surface. And thereafter we apply developer, so developer is sprayed is spread over the surface. So, when the developer is applied developer or the chalk powder is spread over like the chalk powder n is one of the form of applying the developer. So, since the dye has already been cleaned from the surface so all those regions where from it has been completely removed. The developer or chalk powder will not have any change of it is color, but wherever there is a pore or there is a crack at that location the developer will absorb the liquid dye which has seeped into these locations of the cracks and the pores. So, and these areas your there will be change of colour. So, the location where the change of color in the developer is taking place or the location where or the length up to which this change in color is taking place these 2 things will be indicating the size and the location. So, the distance up to which there is a change in color that will be indicating. The size of the discontinuities similarly where the change in color is taking place that will be indicating the location where such kind of the discontinuity is present. So, this kind of method can be used on any kind of the material whether it is a magnetic, non-magnetic, metallic, non-metallic, electrical conducting, non-electrical conducting etcetera. (Refer Slide Time: 13:55) The next method is the magnetic magnetic particle test. this test is basically used for the surface and near surface defects mostly these may be in the form of the, like say mostly these may be in the form of like cracks or pores etcetera ah, but, but very the deep surface defects cannot be established and identified through the magnetic particle test this is one thing, second mostly the ferromagnetic materials. So, the discontinuities and the defect in ferromagnetic materials can be evaluated or assessed by this method. (Refer Slide Time: 14:55) For non-magnetic materials and the poorly magnetic materials it cannot be applied with and therefore, mainly this one is applied for in the ferromagnetic materials. (Refer Slide Time: 15:19) So, mostly for like say the steels the cast iron kind of the components their discontinuities can be easily checked through the this magnetic particle test. To understand this test we need to see the certain basics of the magnetism. (Refer Slide Time: 15:37) Where we know that in mag each magnet there is a north pole and there is a south pole. So, the magnetic lines of the forces take exit from the north pole and...