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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal
Single lap shear stress in hybrid CFRP/Steel composites Single lap shear stress in hybrid CFRP/Steel composites
XV Portuguese Conference on February 2016,c Paço de Arcos, Portugal a Fracture, PCF b 2016, 10-12 a
J. Lopesa, D. Stefaniakb, L. Reisa*, P.P. Camanhoc J. Lopes , D. Stefaniak , L. Reis *, P.P. Camanho
IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Rovisco Pais, 1, 1049-001 DLR, Institute of Composite Structures and Adaptive Systems,Av. Ottenbecker Damm 12, 21684Lisboa, Stade, Portugal Germany DLR, Institute of Composite Structures DammPortugal 12, 21684 Stade, Germany IDMEC, Pólo FEUP,and RuaAdaptive Dr. Roberto Frias,Ottenbecker 4200-465 Porto, airplane gasSystems, turbine engine IDMEC, Pólo FEUP, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal a
a b2 b2
c c
a
b
c
Abstract P. Brandão , V. Infante , A.M. Deus * Abstract a Engineering, Instituto Superior Técnico, Universidade Lisboa, Av.and Rovisco 1049-001 A criticalDepartment parameter of in Mechanical a hybrid CFRP/steel composites is the shear stress between de CFRP plies steelPais, foils.1, This paperLisboa, presents Portugal A critical parameter a hybrid CFRP/steel is the shear stress CFRP pliesspecimens and steel foils. This paperwith presents an experimental andinnumerical research on composites the single lap shear stress in between hybrid CFRP/Steel in accordance DIN b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, an anddifferent numerical research on the single lap shear stress basting in hybrid CFRP/Steel specimens in accordance with DIN ENexperimental ISO 1465. Four types of specimens were tested: Vacuum surface treatment and pickling treatment. For each Portugal c EN ISO 1465. Four different types of specimens were tested: Vacuum basting surface treatment and pickling treatment. For each type CeFEMA, of surfaceDepartment treatmentoftwo types ofEngineering, specimens were tested: DryTécnico, specimens and specimens immersed in water 1000 h.Lisboa, The Mechanical Instituto Superior Universidade de Lisboa, Av. Rovisco Pais, 1,for 1049-001 type of show surfacethat treatment types of specimens wereistested: Dry specimens and aspecimens immersed water for 1000 h. The Portugal results vacuumtwo blasting surface treatment the one that withstands higher shear stress in and is simpler and less results show thatpickling vacuumsurface blastingtreatment. surface treatment is the thatthat withstands shear stress and sensitivity is simpler to andwater less hazardous than The results alsoone show this typea higher of composite has low hazardous pickling surface treatment. also show that this of image composite has lowand sensitivity to water absorption. than A significant difference betweenThe theresults displacement measured by type digital correlation the displacement Abstract absorption. between the displacement measured by digital image correlation the displacement measured byAa significant LVDT wasdifference observed. Finite element models are able to predict the maximum shear stress and of hybrid composites measuredthat by athe LVDT was observed. Finite element elements models are to predict the maximum shear stress of hybrid composites provided constitutive parameters of cohesive areable validated by experimental results. Duringthat their aircraft engine components are subjected to increasingly demanding operating conditions, provided theoperation, constitutivemodern parameters of cohesive elements are validated by experimental results. especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent © 2016 The Authors. Published by Elsevier B.V. © 2016 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd.element All rightsmethod reserved.(FEM) was developed, in order to be able to predict degradation, one ofPublished which is creep. A model using the finite © The Authors. by Elsevier B.V. Peer-review under responsibility the Scientific Committee of PCF 2016. Peer-review under responsibility of theof Scientific Committee ofrecords PCF 2016. the creep behaviour of HPT blades. Flight data for a specific aircraft, provided by a commercial aviation Peer-review under responsibility of the Scientific Committee of(FDR) PCF 2016. company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model Keywords: Single Lap Shear, Composites, Experimental Techniques, Cohesive Elements needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were Keywords: Single Lap Shear, Composites, Experimental Techniques, Cohesive Elements obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The Nomenclature overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a Nomenclature model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
CFRP Carbon Fibre Reinforced Polymer CFRP FibrePublished Reinforced PolymerB.V. DIC Digital Image Correlation © 2016Carbon The Authors. by Elsevier DIC Digital Image Correlation Peer-review responsibility FEM Finiteunder Element Method of the Scientific Committee of PCF 2016. FEM Finite Method SLS SingleElement Lap Shear Keywords: High Lap Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. SLS Single Shear
* Corresponding author. Tel.: +351 966415585; fax: +351 218417951. * E-mail Corresponding Tel.: +351 966415585;[email protected] fax: +351 218417951. address:author. [email protected]; E-mail address: [email protected]; [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V. 2452-3216 © 2016 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby Scientific Committee of PCF 2016. Peer-review underauthor. responsibility the Scientific Committee of PCF 2016. * 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 © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. Peer review under responsibility of the Scientific Committee of PCF 2016. 10.1016/j.prostr.2016.02.009
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J. Lopes et al. / Procedia Structural Integrity 1 (2016) 058–065 Author name / Structural Integrity Procedia 00 (2016) 000–000
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1. Introduction CFRP materials have outstanding mechanical properties. However, CFRP bolted joints still represent a challenge in design (Camanho & Matthews 1997). Its low bearing strength, high notch sensitivity, and dependence on lay-up configuration all contribute to difficult design solutions that are solved by increasing the thickness of the CFRP in the vicinity of a bolted area. The German Aerospace Agency has been developing a hybrid solution of CFRP and metallic foils in the vicinity of the bolted areas that significantly increases its bearing strength (Fink & Kolesnikov 2005), (Kolesnikov et al. 2008). Initially the metallic material was Ti alloy (Fink et al. 2010), (Camanho et al. 2009), (Fink & Camanho 2011). Economic consideration led to the replacement of Ti alloys for austenitic steel 1.4310. The adhesion between CFRP and the steel foils is critically important. The critical factor for this property is the surface treatment of the steel foils. Previously, a test program involving a 3 point bending of hybrid beams fibre demonstrated that vacuum blasting surface treatment is the best surface treatment available and also ensures shear stress values almost identical (≈99%) to the shear stress between CFRP layers (Lopes et al. 2014). This paper presents the research of Single Lap Shear (SLS), tests between CFRP and austenitic steel. Two different types of surface treatment with two different types of environmental conditions, a total of 4 different types of specimens were tested. 2. Specimen manufacturing The specimens were manufactured in accordance with DIN EN ISO 1465. The materials used were CFRP 8552/AS4 UD prepreg from Hexcel with 134 g/m2 (Hexcel Composites 2000) and austenitic steel 1.4310 (X10CrNi18-8). The Tensile Stiffness of austenitic steel 1.4310 is E = 178GPa (De Freitas et al. 2006). The mechanical properties of the CFRP are presented in Table 1. Table 1 - CFRP Material properties Material
Tension
8552/AS4 UD
E1 = 131.606GPa E1 = 115.543GPa E2 = 9.238GPa E2 = 9.858GPa = 0.302 12 12 = 0.335 G12 = 4.826GPa
Compression
The austenitic steel was subjected to two types of surface pre-treatment: vacuum blasting and pickling. Each of which was subjected to two different environmental conditions: dry specimens and specimens immersed in water for 1000 hours. Thus 4 different types of specimens were tested with 7 specimens for each type. The vacuum blasting of the steel foil was performed with 105 µm corundum particles. The pickling is performed in a H2SO4-HF-H2O2bath. Essential constituents of this nitrate-free solution are hydrofluoric acid and an oxidizing agent. All metallic sheet surfaces were treated with an AC-130 sol-gel post-treatment after pre-treatment and then added to the laminate stacking within one hour. Two plates of austenitic steel were bonded with a CFRP layer in between. The length of the overlap is 5 mm. The width of the specimens is 10 mm. The thickness of the CFRP layer is 0.13 mm. The dimensions of the specimen are presented in Figure 1.
Figure 1 – Dimensions of the SLS specimens (mm).
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3. Experiments 3.1. Tests at DLR Two test programs were performed. The first at DLR’s facilities in Braunschweig and the second test program at IST’s Department of Mechanical Engineering facilities in Lisbon. The tests at DLR were performed using a Zwick Z005TN testing machine with a load cell of 5 kN, and a LVDT was used to measured the displacement. The test was performed at constant displacement of 0.5mm/min until failure of the specimen. In Table 2 the average maximum load, shear stress, and shear stress standard deviation are presented. Table 2 - Experimental results of SLS tests Tests
Max SLS (MPa)
Average SLS (MPa)
SLS standard deviation (MPa)
Vacuum blasting (dry)
52.58
47.29
3.70
Vacuum blasting (1000h in H2O)
46.48
44.00
2.34
Pickling (dry)
46.80
42.03
3.79
Pickling (1000h in H2O)
46.61
43.46
3.03
According to Table 2 the specimens with vacuum blasting surface treatment present a higher maximum shear stress than the pickling surface treatment specimens. The 1000 h immersion in water has several important effects: The vacuum blasting specimens immersed in water present a maximum load 7% lower than the dry vacuum blasting specimens. Therefore water absorption does not have a significant effect in the hybrids CFRP/steel with vacuum blasting surface treatment. This is a very important property because it shows that in an actual structure with CFRP/steel hybridization with no protective coat such as a primer and/or paint is not significantly affected by humidity environment. Another notable effect is that the fact that the standard deviation of the specimens immersed in water is slightly lower than the standard deviation of the dry specimens. This observation is of little relevance because the purpose of water immersion is to test the tolerance of an actual structure to a humidity environment. Figure 2 to Figure 5 present the plots of each type of test.
Figure 2 - Vacuum Blasting dry surface treatment.
Figure 3 - Vacuum blasting wet surface treatment.
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Figure 4 - Pickling dry surface treatment.
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Figure 5 - Pickling wet surface treatment.
From the observation of the previous figures it can be seen that all specimens have similar behaviour: An almost linear behaviour with some irregularities caused by heterogeneities of the bonding between CFRP and steel followed by a sudden failure. 3.2. Tests at IST The SLS tests were replicated at the Department of Mechanical Engineering of IST. The tests were performed in an Instron 5566 testing machine with a capacity of 10 kN. The load was measured with a load cell and the displacement was measured with a LVDT. The rate of the tests were set at 0.2 mm/min. A Digital Image Correlation equipment (DIC), (Pan et al. 2009) was used to measure the displacement of these specimens in order to compare it with the displacement measured by the LVDT. Figure 6 presents the plots of the tested specimens with the displacement measured by a LVDT (dashed line) and DIC (solid line).
Figure 6 - SLS tests with LVDT and DIC displacement
Table 3 presents both the LVDT displacement and the DIC displacement at peak load of each specimen. The DIC displacement is lower than the LVDT displacement by a factor of approximately 0.64.
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Table 3 - Comparison between LVDT displacement and DIC displacement Specimens
Max SLS (MPa)
DisplacementLVDT (mm)
DisplacementDIC
Ratio
(mm)
DIC/LVDT
st
43.63
0.39
0.24
0.617
2nd
37.20
0.28
0.18
0.637
rd
3
41.17
0.35
0.22
0.632
4th
33.58
0.23
0.15
0.671
1
4. Numerical model ABAQUS finite element code was used in the numerical analysis of the SLS research. The goal of the finite element model was to assess what are the parameters of the cohesive elements, mainly the damage threshold values t0n, t0s, t0t (respectively threshold normal stress and threshold shear stress in two orthogonal directions), that can provide a good correlation between experimental results and numerical results. In this simulation the fracture energy release rate, GC, remained unchanged and set at G IIC =1.0 N.mm-1 given that was the value that enabled the best correlation between numerical and experimental results in the ILSS research (Lopes et al. 2014). The model itself is composed of three parts: A core with the length of the overlap and the two remaining parts of the steel beams. The core is composed of the bottom steel layer, the CFRP layer, and the top steel layer. Between the two CFRP/Steel interfaces there is a layer with zero thickness cohesive elements (Camanho & Davila 2002), (Camanho et al. 2003). Figure 7 shows a general view if the FEM model and Figure 8 shows a schematic representation of the constituent parts of the SLS FEM model. The core of the model is composed of a steel part with 6 hexahedral C3D8R elements across the 1.5mm thickness; one layer of zero thickness cohesive elements, 4 C3D8R elements across the 0.13mm thickness in the CFRP layer; another layer of zero thickness cohesive elements, and another steel part of 6 hexahedral C3D8R elements across the 1.5mm thickness. The model has a total of 26225 nodes and 21888 elements, 20736 C3D8R solid hexahedral elements and 1152 COH3D8 cohesive hexahedral elements.
Figure 7 – General view of the FEM model
Figure 8 – Constituent parts of the FEM model
4.1. Results and analysis The maximum load that the FEM model can withstand is limited by the damage threshold values of the cohesive elements: t0n, t0s, t0t. A good match between peak experimental load and peak numerical load can be obtained by adjusting these parameters to their proper values. Figure 9 presents the plot of several FEM simulations of the SLS specimens. Table 4 presents the parameters of each simulation and Table 5 presents the SLS FEM simulations.
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Figure 9 - Detailed view of numerical simulations for several settings of
63
t n0 ,t s0
Table 4 - FEM simulation parameters Simulation
Threshold of normal stress (MPa)
Threshold of shear stress (MPa)
1
70
30
2
70
42
3
90
30
4
120
30
Table 5 - Results of SLS FEM simulations Settings
1- t0n = 70 MPa; t0s = 30 MPa 2 - t0n = 70 MPa; t0s = 42 MPa 3- t0n = 90 MPa; t0s = 30 MPa 4- t0n = 120 MPa; t0s = 30 MPa
Max SLS (MPa)
Displacement at Max SLS (mm)
45.24
0.1925
44.91
0.1894
48.43
0.2065
48.76
0.2079
Table 5 and Fig. 9 show that with increasing threshold values there is an increasing peak stress. The numerical simulations can predict the experimental peak stress provided that stress threshold values are properly set. Table 6 presents a comparison between the average of the experimental peak stresses and the numerical peak stresses. Despite the good correlation in Table 6, a significant difference was detected between the slopes of the numerical plots and the slopes of the plots of experimental data with the displacement measured by a LVDT. Figure 10 presents a comparison between the numerical results and the experimental results measured by DIC. Observing Figure 10 it can be concluded that there is a good correlation between numerical and experimental results in spite of the numerical results having a slight higher slope than the experimental results.
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Table 6 - Comparison between numerical and experimental results Settings
Average Maximum Experimental Results (MPa)
1- t0n = 70 MPa; t0s = 30 MPa 2 - t0n = 70 MPa; t0s = 42 MPa 3- t0n = 90 MPa; t0s = 30 MPa 4- t0n = 120 MPa; t0s = 30 MPa
47.29
Difference between numerical and experimental
Difference between numerical and experimental (%)
-2,05
-4,3%
-2,38
-5,0%
1,14
2,4%
1,47
3,1%
Comparison between numerical simulation and DIC experimental data 55 50 45
Shear Stress (MPa)
40 35 30 25 20 15
70/30 70/42 90/30 120/30 #1 DIC #2 DIC #3 DIC #4 DIC
10 5 0
0
0.05
0.1
0.15 Displacement (mm)
0.2
0.25
0.3
Figure 10 - Comparison between numerical data and DIC experimental data
5. Concluding remarks The results on the single lap shear stress confirm that vacuum blasting is the best surface treatment available. It is also simpler and less hazardous than pickling. Hybrid CFRP/Steel are resistant to water absorption . This is a very important property. The fact that an unprotected CFRP/Steel hybrid can have such low sensitivity in water absorption demonstrates the robustness of CFRP/steel hybrids composites. The SLS test is more suited to measure shear stress than the three point bending test. Because the former imposes an actual shear stress whereas in the latter there is an induced stress imposed by transverse shear. The FEM model of the SLS test is also less complex than the FEM model of the three point bending stress. Unlike this test, the FEM model does not require contact formulation between specimen and fixture. The comparison between numerical and experimental results has shown that with proper settings of the cohesive element properties a good agreement between maximum experimental and numerical SLS can be obtained. The comparison between numerical and experimental is satisfactory when the numerical results are compared with DIC measured displacement. This observation demonstrates that DIC is a very important technology when specimens with small absolute displacements are being tested.
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Acknowledgements The first author’s research is supported by the research grant BD/51597/2010 provided by the Portuguese Foundation for Science and Technology. The support of the German Aerospace Agency in the manufacture of all the specimens is acknowledged. References Camanho, P.P. et al., 2009. Hybrid titanium–CFRP laminates for high-performance bolted joints. Composites Part A: Applied Science and Manufacturing, 40(12), pp.1826–1837. Available at: http://linkinghub.elsevier.com/retrieve/pii/S1359835X09000396 [Accessed October 24, 2012]. Camanho, P.P. & Davila, C.G., 2002. Mixed-Mode Decohesion Finite Elements for the Simulation of Delamination in Composite Materials, Camanho, P.P., Davila, C.G. & De Moura, M.F., 2003. Numerical simulation of mixed-mode progressive delamination in composite materials. Journal of composite materials, 37(16), pp.1415–1438. Camanho, P.P. & Matthews, F.L., 1997. Stress analysis and strength prediction of mechanically fastened joints in FRP: a review. Composites Part A: Applied Science and Manufacturing, 28(6), pp.529–547. Available at: http://www.sciencedirect.com/science/article/pii/S1359835X97000043 [Accessed October 24, 2012]. Fink, A. et al., 2010. Hybrid CFRP/titanium bolted joints: Performance assessment and application to a spacecraft payload adaptor. Composites Science and Technology, 70(2), pp.305–317. Available at: http://dx.doi.org/10.1016/j.compscitech.2009.11.002. Fink, A. & Camanho, P., 2011. Reinforcement of composite bolted Joints by means of local metal hybridization. In P. Camanho & L. Tong, eds. Composite joints and connections. Woodhead Publishing. Fink, A. & Kolesnikov, B., 2005. Hybrid titanium composite material improving composite structure coupling. In Spacecraft Structure, Materials and Mechanical Testing 2005. {ESA} {Special} {Publication}. Noordwijk, The Netherlands, p. 135. Available at: http://adsabs.harvard.edu/full/2005ESASP.581E.135F [Accessed October 24, 2012]. De Freitas, M., Reis, L. & Li, B., 2006. Comparative study on biaxial low-cycle fatigue behaviour of three structural steels. Fatigue & Fracture of Engineering Materials and Structures, 29(12), pp.992–999. Available at: http://doi.wiley.com/10.1111/j.1460-2695.2006.01061.x [Accessed November 12, 2014]. Hexcel Composites, 2000. HexPly® 8552 Product Data. , pp.1–6. Kolesnikov, B., Herbeck, L. & Fink, a., 2008. CFRP/titanium hybrid material for improving composite bolted joints. Composite Structures, 83(4), pp.368–380. Lopes, J. et al., 2014. Inter-laminar shear stress in hybrid CFRP/austenitic steel. Frattura ed Integrità Strutturale, (31). Pan, B. et al., 2009. Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review. Measurement Science and Technology, 20(6), p.062001.
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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal
Single lap shear stress in hybrid CFRP/Steel composites Single lap shear stress in hybrid CFRP/Steel composites
XV Portuguese Conference on February 2016,c Paço de Arcos, Portugal a Fracture, PCF b 2016, 10-12 a
J. Lopesa, D. Stefaniakb, L. Reisa*, P.P. Camanhoc J. Lopes , D. Stefaniak , L. Reis *, P.P. Camanho
IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Rovisco Pais, 1, 1049-001 DLR, Institute of Composite Structures and Adaptive Systems,Av. Ottenbecker Damm 12, 21684Lisboa, Stade, Portugal Germany DLR, Institute of Composite Structures DammPortugal 12, 21684 Stade, Germany IDMEC, Pólo FEUP,and RuaAdaptive Dr. Roberto Frias,Ottenbecker 4200-465 Porto, airplane gasSystems, turbine engine IDMEC, Pólo FEUP, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal a
a b2 b2
c c
a
b
c
Abstract P. Brandão , V. Infante , A.M. Deus * Abstract a Engineering, Instituto Superior Técnico, Universidade Lisboa, Av.and Rovisco 1049-001 A criticalDepartment parameter of in Mechanical a hybrid CFRP/steel composites is the shear stress between de CFRP plies steelPais, foils.1, This paperLisboa, presents Portugal A critical parameter a hybrid CFRP/steel is the shear stress CFRP pliesspecimens and steel foils. This paperwith presents an experimental andinnumerical research on composites the single lap shear stress in between hybrid CFRP/Steel in accordance DIN b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, an anddifferent numerical research on the single lap shear stress basting in hybrid CFRP/Steel specimens in accordance with DIN ENexperimental ISO 1465. Four types of specimens were tested: Vacuum surface treatment and pickling treatment. For each Portugal c EN ISO 1465. Four different types of specimens were tested: Vacuum basting surface treatment and pickling treatment. For each type CeFEMA, of surfaceDepartment treatmentoftwo types ofEngineering, specimens were tested: DryTécnico, specimens and specimens immersed in water 1000 h.Lisboa, The Mechanical Instituto Superior Universidade de Lisboa, Av. Rovisco Pais, 1,for 1049-001 type of show surfacethat treatment types of specimens wereistested: Dry specimens and aspecimens immersed water for 1000 h. The Portugal results vacuumtwo blasting surface treatment the one that withstands higher shear stress in and is simpler and less results show thatpickling vacuumsurface blastingtreatment. surface treatment is the thatthat withstands shear stress and sensitivity is simpler to andwater less hazardous than The results alsoone show this typea higher of composite has low hazardous pickling surface treatment. also show that this of image composite has lowand sensitivity to water absorption. than A significant difference betweenThe theresults displacement measured by type digital correlation the displacement Abstract absorption. between the displacement measured by digital image correlation the displacement measured byAa significant LVDT wasdifference observed. Finite element models are able to predict the maximum shear stress and of hybrid composites measuredthat by athe LVDT was observed. Finite element elements models are to predict the maximum shear stress of hybrid composites provided constitutive parameters of cohesive areable validated by experimental results. Duringthat their aircraft engine components are subjected to increasingly demanding operating conditions, provided theoperation, constitutivemodern parameters of cohesive elements are validated by experimental results. especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent © 2016 The Authors. Published by Elsevier B.V. © 2016 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd.element All rightsmethod reserved.(FEM) was developed, in order to be able to predict degradation, one ofPublished which is creep. A model using the finite © The Authors. by Elsevier B.V. Peer-review under responsibility the Scientific Committee of PCF 2016. Peer-review under responsibility of theof Scientific Committee ofrecords PCF 2016. the creep behaviour of HPT blades. Flight data for a specific aircraft, provided by a commercial aviation Peer-review under responsibility of the Scientific Committee of(FDR) PCF 2016. company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model Keywords: Single Lap Shear, Composites, Experimental Techniques, Cohesive Elements needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were Keywords: Single Lap Shear, Composites, Experimental Techniques, Cohesive Elements obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The Nomenclature overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a Nomenclature model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
CFRP Carbon Fibre Reinforced Polymer CFRP FibrePublished Reinforced PolymerB.V. DIC Digital Image Correlation © 2016Carbon The Authors. by Elsevier DIC Digital Image Correlation Peer-review responsibility FEM Finiteunder Element Method of the Scientific Committee of PCF 2016. FEM Finite Method SLS SingleElement Lap Shear Keywords: High Lap Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. SLS Single Shear
* Corresponding author. Tel.: +351 966415585; fax: +351 218417951. * E-mail Corresponding Tel.: +351 966415585;[email protected] fax: +351 218417951. address:author. [email protected]; E-mail address: [email protected]; [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V. 2452-3216 © 2016 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby Scientific Committee of PCF 2016. Peer-review underauthor. responsibility the Scientific Committee of PCF 2016. * 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 © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. Peer review under responsibility of the Scientific Committee of PCF 2016. 10.1016/j.prostr.2016.02.009
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1. Introduction CFRP materials have outstanding mechanical properties. However, CFRP bolted joints still represent a challenge in design (Camanho & Matthews 1997). Its low bearing strength, high notch sensitivity, and dependence on lay-up configuration all contribute to difficult design solutions that are solved by increasing the thickness of the CFRP in the vicinity of a bolted area. The German Aerospace Agency has been developing a hybrid solution of CFRP and metallic foils in the vicinity of the bolted areas that significantly increases its bearing strength (Fink & Kolesnikov 2005), (Kolesnikov et al. 2008). Initially the metallic material was Ti alloy (Fink et al. 2010), (Camanho et al. 2009), (Fink & Camanho 2011). Economic consideration led to the replacement of Ti alloys for austenitic steel 1.4310. The adhesion between CFRP and the steel foils is critically important. The critical factor for this property is the surface treatment of the steel foils. Previously, a test program involving a 3 point bending of hybrid beams fibre demonstrated that vacuum blasting surface treatment is the best surface treatment available and also ensures shear stress values almost identical (≈99%) to the shear stress between CFRP layers (Lopes et al. 2014). This paper presents the research of Single Lap Shear (SLS), tests between CFRP and austenitic steel. Two different types of surface treatment with two different types of environmental conditions, a total of 4 different types of specimens were tested. 2. Specimen manufacturing The specimens were manufactured in accordance with DIN EN ISO 1465. The materials used were CFRP 8552/AS4 UD prepreg from Hexcel with 134 g/m2 (Hexcel Composites 2000) and austenitic steel 1.4310 (X10CrNi18-8). The Tensile Stiffness of austenitic steel 1.4310 is E = 178GPa (De Freitas et al. 2006). The mechanical properties of the CFRP are presented in Table 1. Table 1 - CFRP Material properties Material
Tension
8552/AS4 UD
E1 = 131.606GPa E1 = 115.543GPa E2 = 9.238GPa E2 = 9.858GPa = 0.302 12 12 = 0.335 G12 = 4.826GPa
Compression
The austenitic steel was subjected to two types of surface pre-treatment: vacuum blasting and pickling. Each of which was subjected to two different environmental conditions: dry specimens and specimens immersed in water for 1000 hours. Thus 4 different types of specimens were tested with 7 specimens for each type. The vacuum blasting of the steel foil was performed with 105 µm corundum particles. The pickling is performed in a H2SO4-HF-H2O2bath. Essential constituents of this nitrate-free solution are hydrofluoric acid and an oxidizing agent. All metallic sheet surfaces were treated with an AC-130 sol-gel post-treatment after pre-treatment and then added to the laminate stacking within one hour. Two plates of austenitic steel were bonded with a CFRP layer in between. The length of the overlap is 5 mm. The width of the specimens is 10 mm. The thickness of the CFRP layer is 0.13 mm. The dimensions of the specimen are presented in Figure 1.
Figure 1 – Dimensions of the SLS specimens (mm).
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3. Experiments 3.1. Tests at DLR Two test programs were performed. The first at DLR’s facilities in Braunschweig and the second test program at IST’s Department of Mechanical Engineering facilities in Lisbon. The tests at DLR were performed using a Zwick Z005TN testing machine with a load cell of 5 kN, and a LVDT was used to measured the displacement. The test was performed at constant displacement of 0.5mm/min until failure of the specimen. In Table 2 the average maximum load, shear stress, and shear stress standard deviation are presented. Table 2 - Experimental results of SLS tests Tests
Max SLS (MPa)
Average SLS (MPa)
SLS standard deviation (MPa)
Vacuum blasting (dry)
52.58
47.29
3.70
Vacuum blasting (1000h in H2O)
46.48
44.00
2.34
Pickling (dry)
46.80
42.03
3.79
Pickling (1000h in H2O)
46.61
43.46
3.03
According to Table 2 the specimens with vacuum blasting surface treatment present a higher maximum shear stress than the pickling surface treatment specimens. The 1000 h immersion in water has several important effects: The vacuum blasting specimens immersed in water present a maximum load 7% lower than the dry vacuum blasting specimens. Therefore water absorption does not have a significant effect in the hybrids CFRP/steel with vacuum blasting surface treatment. This is a very important property because it shows that in an actual structure with CFRP/steel hybridization with no protective coat such as a primer and/or paint is not significantly affected by humidity environment. Another notable effect is that the fact that the standard deviation of the specimens immersed in water is slightly lower than the standard deviation of the dry specimens. This observation is of little relevance because the purpose of water immersion is to test the tolerance of an actual structure to a humidity environment. Figure 2 to Figure 5 present the plots of each type of test.
Figure 2 - Vacuum Blasting dry surface treatment.
Figure 3 - Vacuum blasting wet surface treatment.
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Figure 4 - Pickling dry surface treatment.
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Figure 5 - Pickling wet surface treatment.
From the observation of the previous figures it can be seen that all specimens have similar behaviour: An almost linear behaviour with some irregularities caused by heterogeneities of the bonding between CFRP and steel followed by a sudden failure. 3.2. Tests at IST The SLS tests were replicated at the Department of Mechanical Engineering of IST. The tests were performed in an Instron 5566 testing machine with a capacity of 10 kN. The load was measured with a load cell and the displacement was measured with a LVDT. The rate of the tests were set at 0.2 mm/min. A Digital Image Correlation equipment (DIC), (Pan et al. 2009) was used to measure the displacement of these specimens in order to compare it with the displacement measured by the LVDT. Figure 6 presents the plots of the tested specimens with the displacement measured by a LVDT (dashed line) and DIC (solid line).
Figure 6 - SLS tests with LVDT and DIC displacement
Table 3 presents both the LVDT displacement and the DIC displacement at peak load of each specimen. The DIC displacement is lower than the LVDT displacement by a factor of approximately 0.64.
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Table 3 - Comparison between LVDT displacement and DIC displacement Specimens
Max SLS (MPa)
DisplacementLVDT (mm)
DisplacementDIC
Ratio
(mm)
DIC/LVDT
st
43.63
0.39
0.24
0.617
2nd
37.20
0.28
0.18
0.637
rd
3
41.17
0.35
0.22
0.632
4th
33.58
0.23
0.15
0.671
1
4. Numerical model ABAQUS finite element code was used in the numerical analysis of the SLS research. The goal of the finite element model was to assess what are the parameters of the cohesive elements, mainly the damage threshold values t0n, t0s, t0t (respectively threshold normal stress and threshold shear stress in two orthogonal directions), that can provide a good correlation between experimental results and numerical results. In this simulation the fracture energy release rate, GC, remained unchanged and set at G IIC =1.0 N.mm-1 given that was the value that enabled the best correlation between numerical and experimental results in the ILSS research (Lopes et al. 2014). The model itself is composed of three parts: A core with the length of the overlap and the two remaining parts of the steel beams. The core is composed of the bottom steel layer, the CFRP layer, and the top steel layer. Between the two CFRP/Steel interfaces there is a layer with zero thickness cohesive elements (Camanho & Davila 2002), (Camanho et al. 2003). Figure 7 shows a general view if the FEM model and Figure 8 shows a schematic representation of the constituent parts of the SLS FEM model. The core of the model is composed of a steel part with 6 hexahedral C3D8R elements across the 1.5mm thickness; one layer of zero thickness cohesive elements, 4 C3D8R elements across the 0.13mm thickness in the CFRP layer; another layer of zero thickness cohesive elements, and another steel part of 6 hexahedral C3D8R elements across the 1.5mm thickness. The model has a total of 26225 nodes and 21888 elements, 20736 C3D8R solid hexahedral elements and 1152 COH3D8 cohesive hexahedral elements.
Figure 7 – General view of the FEM model
Figure 8 – Constituent parts of the FEM model
4.1. Results and analysis The maximum load that the FEM model can withstand is limited by the damage threshold values of the cohesive elements: t0n, t0s, t0t. A good match between peak experimental load and peak numerical load can be obtained by adjusting these parameters to their proper values. Figure 9 presents the plot of several FEM simulations of the SLS specimens. Table 4 presents the parameters of each simulation and Table 5 presents the SLS FEM simulations.
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Figure 9 - Detailed view of numerical simulations for several settings of
63
t n0 ,t s0
Table 4 - FEM simulation parameters Simulation
Threshold of normal stress (MPa)
Threshold of shear stress (MPa)
1
70
30
2
70
42
3
90
30
4
120
30
Table 5 - Results of SLS FEM simulations Settings
1- t0n = 70 MPa; t0s = 30 MPa 2 - t0n = 70 MPa; t0s = 42 MPa 3- t0n = 90 MPa; t0s = 30 MPa 4- t0n = 120 MPa; t0s = 30 MPa
Max SLS (MPa)
Displacement at Max SLS (mm)
45.24
0.1925
44.91
0.1894
48.43
0.2065
48.76
0.2079
Table 5 and Fig. 9 show that with increasing threshold values there is an increasing peak stress. The numerical simulations can predict the experimental peak stress provided that stress threshold values are properly set. Table 6 presents a comparison between the average of the experimental peak stresses and the numerical peak stresses. Despite the good correlation in Table 6, a significant difference was detected between the slopes of the numerical plots and the slopes of the plots of experimental data with the displacement measured by a LVDT. Figure 10 presents a comparison between the numerical results and the experimental results measured by DIC. Observing Figure 10 it can be concluded that there is a good correlation between numerical and experimental results in spite of the numerical results having a slight higher slope than the experimental results.
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Table 6 - Comparison between numerical and experimental results Settings
Average Maximum Experimental Results (MPa)
1- t0n = 70 MPa; t0s = 30 MPa 2 - t0n = 70 MPa; t0s = 42 MPa 3- t0n = 90 MPa; t0s = 30 MPa 4- t0n = 120 MPa; t0s = 30 MPa
47.29
Difference between numerical and experimental
Difference between numerical and experimental (%)
-2,05
-4,3%
-2,38
-5,0%
1,14
2,4%
1,47
3,1%
Comparison between numerical simulation and DIC experimental data 55 50 45
Shear Stress (MPa)
40 35 30 25 20 15
70/30 70/42 90/30 120/30 #1 DIC #2 DIC #3 DIC #4 DIC
10 5 0
0
0.05
0.1
0.15 Displacement (mm)
0.2
0.25
0.3
Figure 10 - Comparison between numerical data and DIC experimental data
5. Concluding remarks The results on the single lap shear stress confirm that vacuum blasting is the best surface treatment available. It is also simpler and less hazardous than pickling. Hybrid CFRP/Steel are resistant to water absorption . This is a very important property. The fact that an unprotected CFRP/Steel hybrid can have such low sensitivity in water absorption demonstrates the robustness of CFRP/steel hybrids composites. The SLS test is more suited to measure shear stress than the three point bending test. Because the former imposes an actual shear stress whereas in the latter there is an induced stress imposed by transverse shear. The FEM model of the SLS test is also less complex than the FEM model of the three point bending stress. Unlike this test, the FEM model does not require contact formulation between specimen and fixture. The comparison between numerical and experimental results has shown that with proper settings of the cohesive element properties a good agreement between maximum experimental and numerical SLS can be obtained. The comparison between numerical and experimental is satisfactory when the numerical results are compared with DIC measured displacement. This observation demonstrates that DIC is a very important technology when specimens with small absolute displacements are being tested.
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Acknowledgements The first author’s research is supported by the research grant BD/51597/2010 provided by the Portuguese Foundation for Science and Technology. The support of the German Aerospace Agency in the manufacture of all the specimens is acknowledged. References Camanho, P.P. et al., 2009. Hybrid titanium–CFRP laminates for high-performance bolted joints. Composites Part A: Applied Science and Manufacturing, 40(12), pp.1826–1837. Available at: http://linkinghub.elsevier.com/retrieve/pii/S1359835X09000396 [Accessed October 24, 2012]. Camanho, P.P. & Davila, C.G., 2002. Mixed-Mode Decohesion Finite Elements for the Simulation of Delamination in Composite Materials, Camanho, P.P., Davila, C.G. & De Moura, M.F., 2003. Numerical simulation of mixed-mode progressive delamination in composite materials. Journal of composite materials, 37(16), pp.1415–1438. Camanho, P.P. & Matthews, F.L., 1997. Stress analysis and strength prediction of mechanically fastened joints in FRP: a review. Composites Part A: Applied Science and Manufacturing, 28(6), pp.529–547. Available at: http://www.sciencedirect.com/science/article/pii/S1359835X97000043 [Accessed October 24, 2012]. Fink, A. et al., 2010. Hybrid CFRP/titanium bolted joints: Performance assessment and application to a spacecraft payload adaptor. Composites Science and Technology, 70(2), pp.305–317. Available at: http://dx.doi.org/10.1016/j.compscitech.2009.11.002. Fink, A. & Camanho, P., 2011. Reinforcement of composite bolted Joints by means of local metal hybridization. In P. Camanho & L. Tong, eds. Composite joints and connections. Woodhead Publishing. Fink, A. & Kolesnikov, B., 2005. Hybrid titanium composite material improving composite structure coupling. In Spacecraft Structure, Materials and Mechanical Testing 2005. {ESA} {Special} {Publication}. Noordwijk, The Netherlands, p. 135. Available at: http://adsabs.harvard.edu/full/2005ESASP.581E.135F [Accessed October 24, 2012]. De Freitas, M., Reis, L. & Li, B., 2006. Comparative study on biaxial low-cycle fatigue behaviour of three structural steels. Fatigue & Fracture of Engineering Materials and Structures, 29(12), pp.992–999. Available at: http://doi.wiley.com/10.1111/j.1460-2695.2006.01061.x [Accessed November 12, 2014]. Hexcel Composites, 2000. HexPly® 8552 Product Data. , pp.1–6. Kolesnikov, B., Herbeck, L. & Fink, a., 2008. CFRP/titanium hybrid material for improving composite bolted joints. Composite Structures, 83(4), pp.368–380. Lopes, J. et al., 2014. Inter-laminar shear stress in hybrid CFRP/austenitic steel. Frattura ed Integrità Strutturale, (31). Pan, B. et al., 2009. Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review. Measurement Science and Technology, 20(6), p.062001.