Improvement of the fatigue crack growth resistance in long term operated steel strengthened with CFRP patches

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Procedia Structural (2017) 912–919 Structural IntegrityIntegrity Procedia500 (2016) 000–000

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2nd International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, Madeira, Portugal

Improvement of theonfatigue crack growth resistance in long term XV Portuguese Conference Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal operated steel strengthened with CFRP patches a a b blade of an Thermo-mechanical modeling high pressure turbine G. Lesiuka*, M. Katkowski , M. Dudaa,of A.aKrólicka , J.A.F.O. Correia , A.M.P. De b c Jesusgas , J. Rabiega airplane turbine engine a aFaculty

Materials Science and Engineering, Wrocław University of Science and Faculty of Mechanical Engineering, Department of Mechanics, a b c Technology, Smoluchowskiego 25, 50-370 Wrocław, Poland b bINEGI/Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal cc a Faculty of Civil Engineering, Faculty of Bridges and Railways, Wrocław University Science andPais, Technology, Wybrzeże Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa,ofAv. Rovisco 1, 1049-001 Lisboa, Wyspiańskiego 27, 50-370 Wrocław, Poland Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract

P. Brandão , V. Infante , A.M. Deus *

Abstract

The maintenance of the old steel structures is one of the main challenges in engineering practice – mainly due to the lack of the Abstract th and 20th th centuries. In this paper the experimental data performed for existing old steel structures erected at the turn of the 19 th th th fatigue crack growth behavior in structural components from the old 19th (and early 20th) centuries structures (e.g. bridges) has During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, been investigated. The delivered material for investigation was extracted from a beam made from puddled iron and mild rimmed especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent steel, commonly used 100 years ago. It been confirmed in element author’s method experimental that the fatigue crack growth in degradation, one of which is creep. A has model using the finite (FEM)works, was developed, in order to be able torate predict this ancient type of steel is higher than in its modern equivalent. One of the fundamental engineering task is the problem of the the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation extension of were the pre-critical fatigue crack and growth in such data a type steel.different One of flight the promising hybrid company, used to obtain thermal mechanical forofthree cycles. Inapproach order to is create theapproach 3D model (experimental-numerical) based ona the energy mechanisms in fatigue crack growth process. One of the needed for the FEM analysis, HPTadditional blade scrap wasdissipation scanned, and its chemical composition and material properties were successful is the CFRP (Composite Fibermodel Reinforced Polymer) patching along the fatigue crack paths. The3D obtained.strengthening The data thatmethod was gathered was fed into the FEM and different simulations were run, first with a simplified presented approach been discussed in the thismodel, paper on background of the and experimental data. As The it rectangular block has shape, in studied order toand better establish andthe then with the real 3D numerical mesh obtained from the blade scrap. was expected, the proposed strengthening method is efficient and promising in caseatofthe thetrailing “immediate” of critical members overall expected behaviour in terms of displacement was observed, in particular edge ofrepairs the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. with cracks © 2017 The Authors. Published by Elsevier B.V. ©© 2017 TheThe Authors. Published by Elsevier B.V. B.V. 2016 Authors. Published Peer-review under responsibility ofby theElsevier Scientific Committee of ICSI 2017. Peer-review under responsibility of the Scientific Committee of ICSI 2017 Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: fatigue crack; CFRP; old mild steel; fatiguelifetime improvement Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

* Corresponding author. Tel.: +48 713203919; fax: +48 713211235. E-mail address: [email protected] 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review underauthor. responsibility the Scientific Committee of ICSI 2017. * Corresponding Tel.: +351of218419991. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017 10.1016/j.prostr.2017.07.109

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1. Introduction Fatigue and fatigue crack growth are one of the most frequent causes of machine or construction failures. On that canvas an application of CFRP patches to reinforce long-term operated structures made of steel, evaluated in fatigue crack growth context, seems to be a rational way to elongate their life time span and, hence, its further safe exploitation. The CFRP members also show an excellent impact resistance in various application: in mechanical and civil engineering (Khalili et al., 2009) as well in ballistic (Pach et al., 2017) area. Especially, this issue is vivid for stemming from 19th and the beginning of 20th century bridges which were usually built of components made of puddle or mild steels. According to Bień (2012), almost of 68% of currently exploited rail-road bridges are more than 50 years old and around 28% are over 100 years old since they were erected. Although fracture mechanics allows for effective and relatively sufficient estimation of subcritical fatigue growth period of pure metal alloy components without any reinforcement, it does not give answer for problems when reinforcement is applied. Therefore, many tests of many variants of reinforcement are needed to obtain reliable response how much they can elongate lifespan whether it is economically viable. 2. Experimental procedure 2.1. Material and specimen preparation Material for investigation was gained from the railway old riveted, metallic bridge (1899-1902) located in Kluczbork (Poland, rail line 143 – 67.749km). The general view of the girder structure is shown in Fig. 1.

Fig. 1. General view of the structural element from Kluczbork bridge

According to the experimental program, the chemical analysis of the five components (C, Mn, Si, P, S) was carried out. The obtained results are presented in Table 1. The basic mechanical properties (according to static tensile test results) are collected in Table 2. In addition, the referenced values for typical old steel have been presented in the mentioned tables.

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Table 1. Chemical composition (in % by weight) of tested steel, puddle iron and old mild steel. Materials

C

Mn

Si

P

S

investigated steel

0.1

0.52

0.004

0.028

0.03

typical values for puddle iron

<0.8

0.4

n/a

<0.6

<0.04

typical values for old mild steel

<0.15

0.2÷0.5

Variable

<0.06

<0.15

Table 2. Monotonic quasi-static tension properties of the tested material, puddle iron, old mild steel. Materials

y/0.2 (MPa)

u (MPa)

E (GPa)

investigated steel

304

416

212

typical values for puddle iron

220÷280

330÷400

170÷200

typical values for old mild steel

250-300

340÷450

200÷220

The microstructure of investigated material is shown in Fig. 2.

Fig.2. The microstructure of investigated material: (a) structure of non-metallic inclusions, non-etched state, light microscopy; (b) ferrite grain structure in low magnification, etched 3%HNO3, light microscopy; (c) ferrite grain (A) structure with perlite areas (D), thick envelope of Fe3C III (C) on ferrite grain boundaries with typical degradation processes separations (B) inside ferrite grains; etched 3%HNO3, light microscopy; (d) enlarged ferrite grains (A) with degradation symptoms (B) manifested by the numerous of brittle separations, etched 3%HNO 3, light microscopy.

According to obtained results of material and mechanical investigations it should be state, that the investigated material is typical old mild steel commonly used at the beginning of the 20 th century in civil engineering. The microstructure, non-metallic inclusions distribution confirm this statement. In case of the puddle iron, the microstructure and mechanical properties slightly differs from obtained results – see the differences in papers of

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Lesiuk et al. (2015), De Jesus et al. (2015), Correia et al. (2015) etc. The low carbon content (and low Si level) indicate that the tested steel from 1902 year can show the tendency to microstructural degradation processes described in papers of Pękalski (1999), Lesiuk and Szata (2011). According to LM images, the presence of microstructural degradation processes is confirmed (see. Fig. 2c and 2d) in numerous of brittle separations inside ferrite grains. It can generally support the global brittleness of metallic members of the girder. Therefore, this type of material is representative for the efficiency of the strengthening strategy in terms of fatigue crack propagation in similar longterm operated structures. As bonding, the Sikadur®-31 CF Normal glue was used. It is 2-component thixotropic epoxy adhesive. As product data sheet announces it is moisture tolerant, based on a combination of epoxy resins and extra filler. The bonding can be utilized in temperatures between +10˚C and +30˚C. It can be applied to join: concrete elements, natural stone, ceramics, bricks, mortar, masonry, steel, iron, aluminium, wood, epoxy and glass. The adhesive has the following advantages: easy to apply, good adhesion, High strength, hardens without shrinkage, no primer needed and good mechanical resistance. The adhesive can be applied easily with spatula and trowel. To reinforce steel specimens, the Sika® CarboDur® S1014/180 bands were applied which are pultruded carbon fibre plates on epoxy matrix for structural strengthening of structures. It presents very high strength, excellent durability, lightweight, outstanding fatigue resistance, service up to +150˚C and corrosion resistant (Sika, 2017). The basic mechanical properties of the used CFRP patches are collected in Table 3. Table 3. Mechanical properties of Sika® CarboDur® S1014/180 (Sika, 2017) Property

A (%)

u (MPa)

E (GPa)

[g/cm3]

t[mm]

Value

>1.7

>2800

>165

1.6

1.4

The specimen for FCGR (Fatigue Crack Growth Rate) investigation were extracted in longitudinal direction (LT). According to ASTM E647 (2015), the CT specimens (W=50mm, t=8mm) were prepared. Before the strengthening, specimens underwent process of pre-cracking as it is described in ASTM E647 (2015), which basic aim is to obtain maximally sharp tip of crack enabling proper fatigue crack growth propagation, because after machining of the mechanical notch using EDM (Electro Discharging Machine) process. When specimen is fully prepared – standard specimen without patches (Fig. 3a) - CFRP patches can be glued to the specimen surfaces using two-component epoxy adhesive (Fig. 3b) which is applied with spatula. The edge of a CFRP patch is coincident with mechanical notch tip and perpendicular to crack line – fibres of a patch are laid parallel to force axis. (a)

(b)

Fig. 3. Compact tension specimens ready for test: (a) without reinforcement; (b) with full CFRP patches

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2.2. Fatigue crack growth rate experiments The used tensile machine, apparatus (e.g. clevises, grips etc.) and specimens were prepared for experiments in accordance with the ASTM E647 (2015). General view on the measurement stand is shown in Fig. 4. For nonreinforcement specimens, the tests were performed using constant amplitude loading (R=0.1, Fmax=4.5kN). The stress intensity factor (SIF) for the CT specimen is specified using formula presented in ASTM E647 (2015):

a 2 3 4  W  0.886  4.64 a  13.32 a   14.72 a   5.6 a   , K= 3/ 2 W B W W  W   W   a    1    W F

2

(1)

where: 𝑎𝑎 − crack length, 𝐵𝐵 − specimen thickness, 𝑊𝑊 − specimen width, 𝐹𝐹 − applied force.

Fig. 4. The CT specimen during FCGR test: 1 – 50kN load cell, 2 – fracture mechanics clevis, 3 – CT specimen, 4 – clip gage (extensometer), 5 – displacement actuator

For non-reinforcement specimen, the crack length was monitored using compliance method, periodically controlled by visual observations. During survey following signals were registered: force, displacements, crack opening displacement (COD). Amid applying of monotonically arising loading, the crack length size was determined by compliance procedures ASTM E647 (2015). The function of plane stress elastic compliance for CT specimens is described by formula: 𝒂𝒂

𝑾𝑾

= 𝑪𝑪𝟎𝟎 + 𝑪𝑪𝟏𝟏 𝒖𝒖𝒙𝒙 + 𝑪𝑪𝟐𝟐 𝒖𝒖𝟐𝟐𝒙𝒙 + 𝑪𝑪𝟑𝟑 𝒖𝒖𝟑𝟑𝒙𝒙 + 𝑪𝑪𝟒𝟒 𝒖𝒖𝟒𝟒𝒙𝒙 + 𝑪𝑪𝟓𝟓 𝒖𝒖𝟓𝟓𝒙𝒙 .

(2)

Coefficients C0, C1, C2, C3, C4, C5 are fully described by ASTM E647 (2015) depending on measurement localization of COD. The ux quantity is defined as: 𝒖𝒖𝒙𝒙 =

√

𝟏𝟏

𝑩𝑩𝑩𝑩𝑩𝑩 +𝟏𝟏 𝑭𝑭

,

(3)

where:  represents crack opening displacement (COD) measured from clip gage and E is Young modulus. The proper tests were realized by constant amplitude of force range F. Before the main investigation, the fatigue pre-crack was made preserving all condition of loading described in ASTM E647 (2015). Since, the described elastic-compliance method application of equation (2) is impossible for strengthened CT specimens, due to significantly changed stiffness of specimen. Therefore, it was decided to apply beach marking method, based on the following active and passive blocks of loading. The beach marks are macroscopic – unlike striation which are microscopic – fatigue feature with explicit difference of colour from area which was created under other loading character, so that man’s eye is able to distinguish both blocks of loading and assume number of cycles N (Fig. 5). The beach mark method is based on

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changing stress ratio R (up to 0.6) by preserved Fmax value and there are always two values of R used for the test. For strengthened specimens, the active block loading (Fmax=6kN, Fmin=0.6kN) duration was Na=75 000 cycles, and duration of passive block loading (Fmax=6kN, Fmin=3.5kN) Np was equal 35 000 cycles. 2.3. Experimental results of FCGR For non-strengthened specimen the fatigue lifetime was obtained direct from experiment. However, in case of the CTs specimens with CFRP patches, the obtained fatigue lifetime (based on active and passive blocks) was incorrect. In order to calculate equivalent fatigue lifetime (for comparison) it need to eliminate the passive blocks from lifetime calculation. As a proof for the “passive” nature of the blocks with R=0.6 we can consider the fracture surface of the CT specimen with CFRP after tests, where the crack growth increment during passive block (dark area) is very small in comparison with the active block (bright area).

Fig. 5. Fracture surface of the specimen 1 with CFRP after FCGR tests

The crack growth increment for active blocks was calculated based on Fig. 5 and measuring the crack length (average) using digital microscope. The fatigue lifetime for (Fmax=4.5kN, no-reinforcement) and (Fmax=6kN, CFRP patches) is presented in Fig. 6. For better comparison, with the same magnitude of loading, the numerical calculation (based on Paris law) was performed. According to raw data for non-reinforcement specimen, the Paris (1960) law was involved: 𝑑𝑑𝑑𝑑

𝑑𝑑𝑑𝑑

= 𝐶𝐶(∆𝐾𝐾)𝑚𝑚 ,

(4)

where C, m are experimentally determined constants. In the presented approach, C=4x10-10, m=3.66 for steel from Kluczbork (calculated in mm/cycle - MPam axis configuration). The fatigue lifetime Nt (from initial a0=19mm to a1=31mm) under constant load amplitude (Fmax=6kN, R=0.1) was calculated by integrating Paris law: 𝑎𝑎

𝑁𝑁𝑡𝑡 = ∫𝑎𝑎 1

𝑑𝑑𝑑𝑑

𝑚𝑚 0 𝐶𝐶(∆𝐾𝐾)

.

(5)

As it was expected – only in initial stage of fatigue crack growth (a=19-21 mm) the kinetics of fatigue crack growth was similar in each cases. The strengthened Kluczbork steel presents higher resistance level against fatigue crack propagation. The required number of cycles (considering the same magnitude of loading Fmin=0.6kN, Fmax=6kN) for crack growth from a0=19mm to a1=31mm was equal N=450000 for strengthened specimen, and 168000 for nonreinforcement specimen. It means that fatigue lifetime was improved with factor 2.67. The fracture surface of specimen (Fig. 5) indicate that the main mechanism of CFRP strengthening consist in significant reduction of K with

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the increasing of the fatigue crack length. This effect is manifested by the larger bright areas associated with growing crack.

Fig. 6. Fatigue crack growth curves for CT specimen obtained by experiment (Fmax=4.5kN, no reinforcement), Paris law integration (Fmax=6kN, no reinforcement), experiment (Fmax=6kN, full CFRP patches).

3. Conclusions 1. 2. 3. 4.

The old mild steel from 1902 year was investigated under static and cyclic conditions. The basic material investigations were performed. The microstructural degradation processes (typical for this ancient type of steel) have been found; The CFRP patches strengthening strategy is suitable for old structural members (from puddle iron or old mild steel) with fatigue cracks. However, the wrong preparation of the CFRP and metal surface can cause premature failure of strengthened joint; The main mechanism of fatigue crack growth retardation is associated with local K reduction due to CFRP patches; Further experiments and numerical simulations are required in order to determine the optimal conditions of the CFRP patches (load state, fiber orientation etc.).

Acknowledgements This work was financial supported by the Wroclaw University of Science and Technology – Department of Mechanics, Materials Science and Engineering internal, fundamental research program. 2. The publication has been prepared as a part of the Support Programme of the Partnership between Higher Education and Science and Business Activity Sector financed by City of Wroclaw. 3. The authors also acknowledge the Portuguese Science Foundation (FCT) for the financial support through the post-doctoral grant SFRH/BPD/107825/2015. References 1.

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