THICKNESS MEASUREMENT UNCERTAINTIES OF AEROSPACE PARTS BY ULTRASONIC: EXPERIMENTAL STUDY

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THICKNESS MEASUREMENT UNCERTAINTIES OF AEROSPACE PARTS BY ULTRASONIC: EXPERIMENTAL STUDY Brno César de Andrade Silva 1, Marício Saldanha Motta, José Edardo Ferreira de Oliveira 3 1 Centro Federal de

1 THICKNESS MEASUREMENT UNCERTAINTIES OF AEROSPACE PARTS BY ULTRASONIC: EXPERIMENTAL STUDY Brno César de Andrade Silva 1, Marício Saldanha Motta, José Edardo Ferreira de Oliveira 3 1 Centro Federal de Edcação Tecnológica Celso Sckow da Fonseca, Rio de Janeiro Brazil, brno.bcas@gmail.com Centro Federal de Edcação Tecnológica Celso Sckow da Fonseca, Rio de Janeiro Brazil, mmotta64@gmail.com 3 Institto Federal de Edcação, Ciência e Tecnologia de Pernambco, Pernambco Brazil, jefocefetpe@yahoo.com.br Abstract: The main prpose of this paper is to describe the thickness measrement of aerospace parts by nondestrctive ltrasonic method and discss the reslts obtained from an experiment in order to identify the sorce of measrement errors and metrological ncertainties from the process. Key words: measrement ncertainty, metrology, ltrasonic, aerospace strctres. 1. INTRODUCTION Ultrasonic testing has been practiced for many decades. Thickness gaging of indstrial parts is an example application of this test where instrments have been refined to make data collection easier and better [1]. Techniqes derived from the se of ltrasond are sed in several areas, especially applications in health and nondestrctive testing []. In the aerospace indstry, for example, several aircrafts stop its service to carry ot detailed inspections of their strctral members and systems, inclding the power plant. The stop to maintenance of the aircraft is responsible for ensring the correct operation of all components and to provide secrity, comfort and not to affect the pnctality of the services. Among several maintenance services to be performed in the aircraft dring a check, the corrosion prevent and control is the one of the most important since it incldes methods of inspection and tasks to prevent corrosion in all aircraft areas, sch as fselage, wings, stabilizers, flight control srfaces etc. Dring a rotine inspection, when a discrepancy is detected, some actions mst be immediately taken in order to correct the problem and avoid the loss of strctral strength. Corrosion is a major enemy of aircraft maintenance, since it redces the strctral member integrity and, if not corrected in time, may reslt in loss of member and flight safety. The general procedre of corrosion removal in strctral members consists in identifying the affected member thr aircraft maintenance manal, removing the damage sing a proper procedre, visally inspecting and performing nondestrctive inspections. After corrosion removal, the remaining thickness gaging is carried ot in order to estimate the portion of material that was lost. It will enable to verify the remaining static and fatige strength of the affected part, bt firstly, the vales have to be compared with allowable limits as stated in technical manals and aircraft maintenance handbooks. In the general procedre, the steps related with remaining thickness measrement by ltrasonic testing and reslts assessment are the critical steps. They reqire extreme care since can contain several errors coming from not estimated ncertainties of the measrement procedre, reslting in a wrong decision by technician or engineer. This paper discsses the se of ltrasonic method for remaining thickness gaging of an aircraft strctral member after corrosion removal, and also the inspection findings throgh an experiment objected to detect sorces of ncertainties in the ltrasonic measrement. In addition, this paper stdies those ncertainties, how they may reflect on the professional decision and establish a comfort zone of measrement reslts.. EXPERIMENT.1. General Thickness gaging sing ltrasonic test is based on the time taken by the ltrasonic plse inside the specimen. The condition to allow a correct measrement is a constant sonic velocity within the specimen. For all process of thickness gaging by ltrasonic, a reference standard mst be sed. The test reslts are, however, achieved when the instrment is calibrated in a reference standard block manfactred with the same material of the part being tested. Refer to Figre.1. Figre.1. Reference standard block [5]. To provide the thickness measrement of a part by ltrasonic, the instrment generates an electrical initial plse

2 at first which is led to the transmitter element of the probe where it is converted into a mechanic ltrasonic plse. Using a coplant, the ltrasonic plse is transmitted from the probe to the material to be tested throgh which it passes at a velocity typical of that material ntil it enconters a change in the material. Part of the plse energy is reflected from this point and retrned to the probe (echo). If the signal reaches the backwall of the test object before this, the reflected plse is called backwall echo [3]. Refer to Figre...4. Experiment overview When a strctral damage is reported in an aircraft dring rotine inspections, some standard procedres mst be performed in order to identify the damaged strctre, if it is a principal strctral element (P.S.E.) or a secondary element, and the correct assessment of the damage. If the damaged strctre is a P.S.E., it will directly affect the flight safety. The Figre.3 shows a typical damage by corrosion in an Airbs A300 airplane. This damage, when detected, reqires an immediate action to clean p and control the corrosion on order not to affect the strctral integrity of the member and conseqently the flight safety. The damages exposed in the Figre.3 are located on the pper skin of the right wing and were cased de to dissimilar material contact (material of fastener with material of skin) in presence of moistre. Flaws in the srface of protection provided sch contact casing corrosion. Figre.. Thickness measring sing a dalelement probe [3]... Relevant sorce of errors Several sorces of error are present in a thickness measrement by ltrasonic, sch as temperatre (by affecting the sond velocity of the material), probe selection, coplant sage, operator training, process knowledgement, adeqate method of calibration etc..3. Instrment calibration From the calibration, it can be identified the confidence of a measrement device or reference standard, comparing the reslts of calibration with standard specifications. To calibrate an instrment, it is necessary to express the ncertainty associated with the nmerical reslt declared, considered a key parameter in qantifying the qality of the final measrement reslt []. In the aerospace indstry, the thickness measrement by ltrasonic is widely sed with dal-element probe. Using a dal-element probe, the best instrment calibration is obtained by taking as reference two points of vale. These points mst be a vale above and one below the nominal thickness of the part to be measred. The vale of the nominal and its tolerances mst be known throgh engineering drawings. As mentioned before, the sonic velocity of the part being measred varies with the temperatre of material. This implies that the instrment mst be calibrated sing a reference standard block immediately before the inspection begins, in fieldwork, so that the removal of the reference temperatre is minimized. After the calibration procedre, the instrment is ready for se. Figra.3. Corrosion detected in several áreas of pper srfasse of na aircraft wing. Areas later reworked. The procedre of corrosion removal is described on the specific Strctral Repair Manal of the aircraft [4], which basically consists of removing the srface protection (decorative paint and epoxi base primer), removal of fasteners installed in the affected area, sanding the corroded area to remove all evidences of corrosion, smoothing the rework srface contor, measrement of remaining thickness by ltrasonic inspection and assessment of findings. The procedre for remaining thickness measrement is stated in the Nondestrctive Test Manal of the aircraft [5]. The procedre of corrosion removal, cleaning p and smoothing of rework srface is known as blend ot. The Figre.4 shows a typical shape of a plate after blend ot procedre. Figra.4. Blend ot, where T= Removed material, Y= Rework length and Y mst be greater or eqal to 40T.

3 .5. Experiment ncertainties and measrements VIM [6] describes the measrement ncertainty as a non-negative parameter that characterizes the dispersion of the vales attribted to a measrand, based on sed data. The measrement ncertainty incldes components coming from systematic effects sch as components associated with corrections and vales assigned to standards, as well as definitional ncertainty. Sometimes estimated systematic effects are not corrected, bt, instead of this, associated components of measrement are embedded. The stdy listed some sorces of ncertainty inherent in the thickness measrement by ltrasonic inspection in reworked areas located at pper srface of the aircraft wing. These ncertainties may directly affect the measrement reslts. Unlike the sal procedre, were randomly chosen three areas in the affected srface of the wing. It was drawn a grid of 10 x 10 mm and performed seven measrements with ltrasonic instrment in each sqare of mapping, totaling 189 measrements. The Figre.5 and.6 show the time of the measrements. After this procedre, it was observed the point with the highest dispersion vales. engineer who will assess the readings and define the actions to be taken to maintain the flight safety of the airplane. Table.1. Ultrasonic measrements of the point with the highest dispersion Reslts. Remaining srface x thickness of reworked area (in inches) 1 0,695 0, , , , , ,697 Mean 0,698 x Standard Deviation 0,008 S x For acceptance of the measred vales it was sed the Chavenet Criterion thr Eqation (1). xi X r* (1) S x Figre.5. Remaining thickness measrement by ltrasonic inspection. Several readings in a 10 x 10 mm grid. Where r * is the index to be fond, x i is the vale to be tested, X the mean of sample and deviation. Chavenet Criterion states that: R C S x the standard r* () Where RC is the rejection limit of Chavenet. From the Table.1 was extrated the vales with the highest dispersion. In the experiment, the vale 0,704 is considered sspect de to its dispercion in face of the mean of sample. Testing this vale by Chavenet by Eqation (1) we fond 1,9508 inches. Applying the Eqation (): 1,9508 (3) R C Figre.6. Remaining thickness measrement by ltrasonic inspection. This point was chosen to make for the calclations of ncertainty according to ISO GUM [7]. The Table.1 shows the vales fond after readings. ISO GUM [7] method allows to determine the expanded ncertainty of the process and establish a comfort zone for the technician or Checking the Table. for comparison of R C : Table.. Chavenet Rejection Limits. For n 7, R 1, 80. Then, C

4 1,9508 1,80 (4) Then, the vale 0,704 mst be removed from the sample since it was disapproved by Chavenet. A new table (Table.3) is written wihot the vale excled by Chavenet. Table.3. Ultrasonic measrements of the point with the highest dispersion Reslts exclding the vale rejected by Chavenet. Remaining srface x thickness of reworked area (in inches) 1 0,695 0, , , , ,697 Mean 0, x Standard deviation 0, S x For less than the error can be present, it is always present in any measrement [8]. Dring the measrement process, several sorces of ncertainty can come p and mst be checked, since they can inflence on a component acceptance after evalation. In the stdy, this main sorces of ncertainty are coming from the ltrasonic instrment resoltion, de to the removal of reference temperatre (0 C), de to the statistic (Type A) and ncertainty inherited from reference standard block. The Eqation (5) describes the statistic ncertainty, known as type A, associated to the mean of readings. S x S xi i A n (5) The Eqation (6) describes the ncertainty de to the removal of reference temperatre. i t l 0 p.. T 3 The Eqation (7) describes the ncertainty inherited from reference standard block. i P ih (7) k P The Eqation (8) describes the ncertainty de to ltrasonic instrment resoltion. (6) r i r (8) 3 Applying all eqations described above, it is obtained the ncertainties contained in the process. After knowing all ncertainties, it is possible to calclate the combined ncertainty throgh Eqation (9), assming the coefficients of sensivity ( c ) are eqal to 1: c A t r c i i i i (9) Knowing the valor of h nmber of effective degrees ( described below: c, it is possible to calclate the eff ) throgh Eqation (10) as 4 c eff (10) A r h n 1 Table.4. Vales of coverage factor k for distribtion t for degrees of freedom in fnction of confidence [8]. Degrees P= 68,7 P= 95,45 P= 99,73 of freedom 35 1,01,07 3,3 40 1,01,06 3,0 45 1,01,06 3, ,01,05 3, ,005,05 3,077 1,000,000 3,000 From the Table.4 it is extracted the approximate vale of k : k,05 (11) Knowing the vale of k it is possible to calclate the expanded ncertainty U throgh Eqation (1): U k (1) Then, c R 0, ,00546 inch. for/ k, RESULTS In the stdy several aspects cold be observed and tested. Among these aspects, some were relevant for the

5 measrement reslts and others were nimportant to the process. A random point was chosen to be sed as base for variations of standard experiment. For each variable, it was taken three readings. It was observed the mode of action of each operator who performed the measrements, especially with regard to the pressre applied on the probe dring the measrement. When little pressre is applied to the probe, no significant reading variation was fond. Refer to Table 3.1 for findings. Table 3.1. Ultrasonic readings in a random point varying the pressre on the probe. Reading Little pressre applied Medim pressre applied A lot of pressre applied 1 st 0,75 0,750 0,750 nd 0,75 0,751 0,750 3 rd 0,75 0,750 0,750 It was also observed the inflence of the amont of coplant in the measrement process. It was noticed that the amont of coplant has little inflence in the procedre. Refer to Table 3. for findings. Table 3.. Ultrasonic readings in a random point varying the amont of coplant.. Reading Small qantity Medim qantity A lot of qantity 1 st 0,751 0,750 0,750 nd 0,750 0,75 0,751 3 rd 0,751 0,751 0,751 The operator was also observed. The readings were taken with three different operators. It was noted that the most experienced operator did not vary the measrements, while the other two operators, less experienced, varied their measrements. Refer to Table 3.3 for findings. Table 3.3. Ultrasonic readings in a random point varying the operator. Reading Operator 1 Operator Operator 3 1 st 0,75 0,751 0,751 nd 0,75 0,751 0,750 3 rd 0,75 0,75 0,751 Still, it was confirmed that the temperatre change indces an nreliable measrement reslt. Firstly, the eqipment was calibrated inside a room at a temperatre of 1ºC. Shortly after the experiment was performed on the airplane with the reslts as per Table 3.4 Table 3.4. Ultrasonic readings in a random point varying calibrating the eqipment in a room at 1ºC and after field measrement at 8ºC. Reading Operator 1 1 st 0,75 nd 0,753 3 rd 0,754 When the eqipment is calibrated and measred in field (at the same temperatre as the part, 8ºC) the variation becomes smaller. Refer to Table 3.5 for findings. Table 3.5. Ultrasonic readings in a random point calibrating the eqipment in the same temperatre as the part to be inspected (T= 8ºC). Medição Operador 1 1ª 0,750 ª 0,750 3ª 0,751 It was also noted that the srface finish of reworked area inflences the reslts, since the better finish becomes in a more accrated reslt. The experiment also shows that the largest inflence on the reslts (expanded ncertainty) came from the ncertainty inherited from the reference standard block. This shows that when we have a more accrate reference standard, more accrate the process will be. The difference between the crystal transdcer dilatation and the reference standard was not taken into accont in this experiment, and the inherited ncertainty coming from the certification of calibration of the ltrasonic eqipment. Tolerance is a vale that expresses the difference between the maximm and the minimm dimensions that a certain size mst present withot compromising the fnctional reqirements of the part with that dimension [8]. Taking the limit (+0,005 inch) and minimm (-0,003 inch) of the skin thickness (as per prodction drawing): Nominal thickness after rework is 0,698 inch. Upper dimension is 0,703 inch. Lower dimension is 0,695 inch. The tolerance (t) is the difference between the pper dimension and the lower dimension [8], as per Eqation (13): t D s D i (13) t 0,703 0,695 0,008 pol. (14) To check the sitability or noto f a measrement system, we compare their measrement ncertainty with the tolerance of the process, provided that the erros present in the system are minimized [8]. The ideal condition is pointed ot that:

6 1 1 U t 0,008 0,0008pol. (15) and 1 1 U t 0,008 0,0066pol. (16) 3 3 According to [9], the zone line mst take into accont the vale of measrement ncertainty as shown in Figre 3.1. [5] NTM Non-destrctive Test Manal, Airbs A300, Chapter Part 1. Measrement of the Thickness of Blended Ot and Paint Coates Alminm Sheet and plate, Oct/1997. [6] INMETRO Vocablário Internacional de Metrologia: conceitos fndamentais e gerais e termos associados (VIM 008). 1ª Edição Brasileira. Rio de Janeiro, p. [7] ABNT Associação Brasileira de Normas Técnicas. Gia para expressão da incerteza de medição ISO GUM. Rio de Janeiro: Inmetro e ABNT, p. [8] OLIVEIRA, José Edardo Ferreira de. A metrologia aplicada aos setores indstrial e de serviço. Principais aspectos a serem compreendidos e praticados no ambiente organizacional, Brasília: SEBRAE, 008. Figre 3.1. Comparison between measrement ncertainty and process tolerance. [9] ISO , 1998, Geometrical Prodct Specifications (GPS) Inspection by measrement of workpieces and measring eqipment Part 1: Decision rles for proving conformance or nonconformance with specifications. With this experimental stdy we can noted that a eqational model mst be sed to establish a zone line taking into accont the prodction tolerance and the ncertainties coming from the process. Doing like this, the technical evalator will be flly able to approve or reject a rework in accordance with allowable limits in the manfactrer technical manals. This method prevents the sitation when the rework is ot of bondary vales as per zone line and this has not been taken into accont, which cold reslt in an airplane retrning to service ot of the reqirements reqired by national and international reglatory agencies of aviation. REFERÊNCIAS [1] NDT Resorce Center, web site Introdction to Ultrasonic Testing. [] OLIVEIRA, E. G. de; COSTA-FÉLIX, R. P. B. da; ALVES, C. H. F., Validação de m sistema de calibração de transdtores de ltrassom por atoreciprocidade, Revista Brasileira de Engenharia Biomédica, v. 4, n.1, p , abril 008. [3] Kratkramer DMS E/ DMS / DMS TC, Manal. Important remarks abot thickness testing, Wall thickness measrement with the DMS and Calibrating, Isse 3, May/00. [4] SRM Strctral Repair Manal, Airbs A300, Chapter , Page Block 701. Repair of Corroded Areas, Mar/005.

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