of Siege Screws Composites CFRP Friction Relaxation pid her ents during tightening and preload relaxation during operation in product life time. Friction coefficients are ateria ries or ondly, threaded fasteners clamping CFRP laminates show high loss of preload, as the laminates are loaded perpendicular to the fiber direction. Especially when exposed to elevated temperature, com- posite materials show significant preload relaxation. This effect is driven by a high thermal expansion coefficient combined with itch P, mean head ning Torq 0 is reduce and in the contact zone lth. The diagram in Fig. 1 displays the relations tween friction coefficient and tightening preload for di screw diameters (M6 to M16) and a constant tightening of Ttot = 20 Nm. It becomes clear that slightly higher friction coefficients (for comparison see also Fig. 5) lead to quite low preloads, which might be insufficient to ensure a required clamp force. Strongly rising preloads for friction coefficients lower than about 0.06 risk breaking of the screw because of too high axial load. The small circles at the end of the thick lines in the diagram below indicate the threshold for a 90% utilization of a screw with ⇑ Corresponding author. Tel.: +49 2717404558. E-mail addresses:
[email protected] (C. Friedrich), hendrik.hubbertz@ uni-siegen.de (H. Hubbertz). Composite Structures 110 (2014) 335–341 Contents lists availab Composite S sev 1 MVP–Chair of Machine Elements, Fastening Systems, Product Innovation. als. First of all the friction behavior is mostly unknown. Friction strongly influences the preload when threaded fasteners are tight- ened. The friction coefficients are uncertain and possibly change differently with the conditions of the tribological system [6]. Sec- geometric conditions (flank diameter d2, thread p bearing diameter Db) and a constant tighte Ttot = 20 Nm the generated tightening preload FP rising friction coefficients in head bearing area lb 0263-8223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compstruct.2013.11.024 ue of d with thread hip be- fferent torque products, but always require fastening. The benefit of mechanical fastening of CFRP materials compared to bonding is that the joint remains detachable. Threaded fasteners permit flexibility and dis- assembly for maintenance, repair and end of life. However the use of CFRP laminates leads to some major diffi- culties, as the material’s behavior is completely different frommet- Friction is very important for the assembly of bolted joints, be- cause it directly influences the transformation of the tightening torque into the generated preload [14]. The basic calculative rela- tionship [1,21] between tightening torque and preload describes the equation in Fig. 1. It can be recognized that for the same 1. Introduction The use of semi finished CFRP m priced solution especially for small se responsible for preload level and preload deviation. For established material combinations, standards with defined ranges are available. What about friction coefficients with composite materials in compo- nent contact? The contribution shows results from numerous measurements, such as different combina- tions of materials at various lubrication states, development of friction with preload magnitude, friction development with number of retightening, influence of tightening speed, deviations in the system. Furthermore every bolted joint, which has to transmit forces between components, can only work with a reliable and sufficient preload. This is critical due to the temperature dependency of the type of lami- nate matrix (yield point and thermal expansion; here carbon fiber reinforced Epoxy (EP) matrix and Poly- ether ether ketone (PEEK) matrix are investigated). Conclusions refer to improvement of related products. � 2013 Elsevier Ltd. All rights reserved. ls allows a reasonable individual engineering deteriorating mechanical properties of the plastic matrix at higher temperatures. 2. Friction Keywords: Threaded fasteners untightening possibilities. Special criteria for design in the flow of preload have to be met. Very important are friction behavior Friction behavior and preload relaxation with composite structures C. Friedrich, H. Hubbertz ⇑ Dep. of Mech. Eng., Institute of Engineering Design MVP, University of Siegen, D 57068 a r t i c l e i n f o Article history: Available online 12 December 2013 a b s t r a c t Composite materials are ra maintained and repaired. T ers for clamping compon journal homepage: www.el fastening systems n, Germany1 ly replacing conventional materials and components need to be installed, efore they also have to be fastened mechanically e.g. with threaded fasten- together. Threaded fasteners offer a high loading capacity along with le at ScienceDirect tructures ier .com/locate /compstruct 336 C. Friedrich, H. Hubbertz / Composite property class 10.9 [5]. The on-going thin lines illustrate the calcu- lated values without regarding the material strength. It is to be kept in mind that friction coefficients are always statistically influenced values of a system with different local conditions in a contact zone [13]. Fig. 2 shows the main influencing parameters for a head bearing area of a screw. Major factors are the local contact pressure pcclocal and the local sliding velocity v which are strongly influenced by the geometry of the head support com- ponent contact (radius of contact) and change within the same contact zone. Their mean values pccmean and vmean can be regarded as rough estimations to make them calculable analytically. The tightening angle # defines the sliding distance and causes wear which can lead to changing friction coefficients even in one tight- ening procedure. For repeated tightening normally a strong change of friction coefficients occurs (see also Fig. 8). The friction coefficients demonstrated in this paper were measured on a multichannel assembly test stand (see Fig. 3) for measurements according to DIN EN ISO 16047 [3]. The screws are fastened by a tightening spindle on exchangeable head bearing plates. During the assembly the variables time t, tightening angle #, preload FP, total tightening torque Ttot and thread torque Tth are simultaneously recorded. Knowing the fasteners geometry both friction coefficients (lth and lb) can be calculated from the preload and torque measurements. The screws and bearing plates on which the experimental results of this chapter are based on, are also displayed in Fig. 3. Hexagon screws with flange head M8x50 10.9 were fastened on bearing plates of CFRP with Epoxy matrix and PEEK matrix (Poly- ether ether ketone). The results were compared to bearing plates made of unreinforced Epoxy as well as steel (42CrMo4) and alumi- num (EN AW-7075). The CFRP and Epoxy plates were also tested for relaxation (Section 3). Typical lubrication conditions are as supplied, lubricated, dry and Fig. 1. Calculated relation between preload FP0 and friction coefficient lth = lb for constant tightening torque. retightened which lead to strongly scattering tightening behavior and thus to uncertainties in the assembly process. Fig. 4 displays exemplarily the measured tightening diagrams with preload over tightening torque for CFRP with Epoxy matrix and the conditions Fig. 2. Key variables for assessment of bolted joint tribology at head support [13]. as supplied (in this case blank steel, slightly oiled), dry (degreased) and lubricated with MoS2 (Molybdenum disulfide) grease. The retightening behavior is covered separately in Fig. 8. The curves for the lubricated (with MoS2) and as supplied (slightly oiled) condition show similar behavior. The lubricated screws generate slightly higher preloads at the same torque, indi- cating a little lower friction coefficient (difference Dlb � 0.04). The dry (degreased) bolts need considerably higher torque values to reach the same preload as well as they strongly differ from each other (large deviation for high friction coefficients). This implies a high and strongly varying friction coefficient in head bearing area and at nut thread component. 2.1. Friction coefficients Basically the friction coefficients in the contact zones of threaded fasteners lb and lth can have any value (up to approxi- mately l � 1). This is why the guideline VDI 2230 [21] defines five classes of friction (A–E) whose values start at l = 0.04 for class A and the end of class E is not defined (P0.30). For this reason the friction coefficients of fasteners must be guaranteed to be in a specified range. Only by such definition it is made sure that a reli- able assembly process can be set up which is especially important for the manufactures of high series. Thus particularly automotive manufacturers release standards which define the permitted range of friction that the supplied fasteners must possess. Fig. 5 provides an overview of the ranges which are defined by the VDI 2230 (fric- tion classes) and selected automotive manufacturers. It will be noted that the friction ranges defined by the manufac- turers are all in the same range from l = 0.08 to l = 0.18. Some standards define the coefficients in head bearing area lb and in the thread contact zone lth separately but in the same range. Fur- thermore a total friction coefficient ltot is specified assuming that lb is equal to lth which can only be a rough estimation without the possibility to determine the real stress state in the screw shank. Anyway several standards only refer to ltot. Since nut threads are normally not cut into CFRP, it is of special interest to look at the friction coefficient in head bearing area lb when the tightening behavior of CFRP is analyzed. The head bear- ing area normally contains the highest contact pressure. Fig. 6 shows the results for the tightening conditions introduced above. The coefficients are analyzed at a preload level of FP0 = 15 kN. The bearing plates of CFRP laminates with EP and PEEK matrix are compared to Epoxy (without fibers), steel and aluminum. The friction coefficients in head bearing strongly differ with changing head bearing materials and lubrication states. When tightening the supplied, slightly oiled screws on the CFRP lami- nates, the head friction coefficient is very low. Compared to the standardized ranges from Fig. 5 the values are on the lowest level. Lubricated with MoS2, the coefficients become even lower (minus 60%), so there is the danger of breaking of screws at normal torque level as well as danger of self loosening. Quite high coefficients were measured at CFRP laminates tightened with dry/degreased bolts showing high deviation at the CFRP with EP matrix. The rea- sons are surface changes and wear grooves because of fibers break- ing out of the low-strength fiber bonding (non-woven fabrics). The fiber bonding is higher at the woven CFRP with PEEK matrix lead- ing to much smaller deviations even in dry lubrication state. It can be summarized that the friction coefficients of plastics are rather low when slightly lubricated. Regarding fiber reinforcement, the friction and wear is dependent on the bonding of the fibers. 2.2. Deviations and uncertainties Structures 110 (2014) 335–341 Instead of analyzing the head friction coefficient at a certain level (of FP = 15 kN in Fig. 6), the following Fig. 7 shows its Screws Plates (for assembly and relaxation) Assembly test stand Steel 42CrMo4 CFRP with Epoxy Matrix bidirectional 0/90° fibers T300, fiber value content ≈ 60% CFRP with PEEK Matrix satin (atlas) weave fibers T300, fiber value content ≈ 50% Epoxy without fibers Poxy Systems Epoxy Resin L with hardener EPH 161 tightening spindle multichannel measuring head CFRP bearing plate C. Friedrich, H. Hubbertz / Composite Structures 110 (2014) 335–341 337 Hexagon screws with flange M8x50 –10.9 acc. DIN EN 1665 development over increasing preload. The diagram displays selected typical curves of different materials of head bearing plates. The metals provide a stable friction coefficient even at high preloads (the coefficient of aluminum increases moderately). In Fig. 3. Assembly test stand for friction measurem 0 5 10 15 20 25 0 20 40 60 80 100 Torque Ttot [Nm] Pr el oa d F P [k N ] bearing plate: CFRP with Epoxy Matrix lubricated with MoS2 as supplied (blank, slightly oiled) dry (degreased) Hexagon screws with flange M8x50 - 10.9 DIN EN 1665 (plane head support) tightening speed 50 min-1 1st tightening Fig. 4. Measured tightening diagrams (preload over tightening torque) for different lubrication states using a CFRP bearing plate with Epoxy Matrix. Fig. 5. Standardized range of friction coefficients defined by VD Aluminum EN AW-7075 (AlZnMgCu1,5) contrast the configurations with plastic bearing plates (CFRP EP, CFRP PEEK, EP) show strongly decreasing head friction coefficients with increasing preload. The extremely low values have to be rated critical concerning the danger of self loosening. ents as well as screws and plates for tests. I 2230 and automotive manufacturers [2,10,15–17,19–22]. 0,000 0,050 0,100 0,150 0,200 0,250 0,300 0,350 0,400 CFRP with EP Matrix CFRP with PEEK Matrix Epoxy without fibers Steel 42CrMo4 Aluminum AW-7075 be ar in g fr ic tio n co ef fic ie nt µ b [ -] as supplied (blank, slightly oiled) dry (degreased) lubricated (MoS2) error bars min/mean/max from 3 measurements each Hexagon screws with flange M8x50 - 10.9, DIN EN 1665, tightening speed 50 min-1, analysis at FP = 15 kN condition: Fig. 6. Measured head friction coefficients for materials in contact and lubrication states. 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 1 6 11 16 21 be ar in g fr ic tio n co ef fic ie nt µ b [ -] Steel 42CrMo4 Epoxy without fibers Aluminum AW-7075 CFRP with EP Matrix CFRP with PEEK Matrix Hexagon screws with flange M8x50 -10.9, DIN EN 1665, as supplied (blank, slightly oiled), analysis at FP = 15 kN, tightening speed 50 min -1, as supplied (blank, slightly oiled) 0,000 0,020 0,040 0,060 0,080 0,100 0,120 0,140 CFRP with EP Matrix CFRP with PEEK Matrix Steel 42CrMo4 be ar in g fr ic tio n co ef fic ie nt µ b [ -] 15 min-1 50 min-1 100 min-1 Hexagon screws with flange M8x50 - 10.9, DIN EN 1665, as supplied (blank, slightly oiled), analysis at FP = 15 kN tightening speed: error bars min/mean/max from 3 measurements each 338 C. Friedrich, H. Hubbertz / Composite Structures 110 (2014) 335–341 As stated above, another important friction state is retightening. Normally the friction conditions change as a result of abrasive wear, generation of grooves or leveling of micro-contacts. Fig. 8 op- poses the head friction coefficients at first tightening to the coeffi- cients at the eighth tightening procedure. The screws were used as supplied (blank, slightly oiled) at the different head bearing mate- rials. Photos of the head bearing areas show the different occur- rence of visible wear. In general the mean values rise as well as the scattering. This consequently leads to an uncertain level of preload when tighten- ing with the same tightening torque. The biggest changes were ob- served in the CFRP materials. After penetration of the deck surface, exposed fibers were found that can be associated with this obser- vation. The Epoxy without fibers shows quite low variance but the visible wear is very high (see photo in Fig. 8). The increase of the friction coefficient with steel bearing plates is mainly caused by roughness of hard surface with high stability. In contrast to its usual behavior, the aluminum plates show only a slight increase of the friction coefficient caused by slightly oiling and high strength aluminum alloy (AW-7075). The overview in Fig. 9 shows the influence of the tightening speed (and so the tightening velocity, see Fig. 2) on the head fric- tion coefficient on the CFRP and steel bearing plates. This consider- ation is important for plastic materials. It is obvious that the stable steel surface and CFRP with PEEK matrix does not showmajor changes at different tightening speeds. Using a CFRP bearing plate, the head friction coefficient signifi- cantly falls up to 45% with higher tightening speed. The PEEK ma- Preload FP [kN] Fig. 7. Development of friction with preload magnitude. trix is more temperature-stable than the Epoxy and thus shows no change; similar to steel and dissimilar to normal plastic materials. Finally, in Fig. 10 the magnitude of the tightening torque is compared to the torque at same preload for untightening 0.000 0.100 0.200 0.300 0.400 0.500 CFRP with EP Matrix CFRP with PEEK Matrix Epoxy without fibers Steel 42CrMo4 Aluminum AW-7075 be ar in g fr ic tio n co ef fic ie nt µ b [ -] 1. tightening 8. tighteningerror bars min/mean/max from 3 measurements each Hexagon screws with flange M8x50 - 10.9, DIN EN 1665, as supplied (blank, slightly oiled), analysis at FP = 15 kN, tightening speed 50 min-1 Fig. 8. Change of head friction coefficient from first tightening to eighth tightening. procedure. Because of the thread pitch, the untightening torque at a definite preload level is normally in the range of 70% of the tightening value at the same preload for same friction. The diagram shows that this percentage between tightening/untightening is al- most in this range for all combinations. The conclusion from this is that the friction coefficient does not change significantly between tightening/untightening. The effects of changing friction will be further investigated; more information on instability of friction at bolted joints can be found in [12]. 3. Relaxation Preload relaxation is the effect of preload loss over time without rotation of screw (difference to self-loosening). This preload loss occurs in every preloaded fastening system, but differs widely in its level as it is influenced by time, temperature loads, mechanical loads, surface conditions, surface pressure and involved materials (screw, clamped parts, nut). Preload relaxation can be divided into three main effects, explained in Fig. 11: (a) Seating FZ means short time relaxation, dependent on strain hardening of all surfaces in the flow of preload in the fasten- ing system. (b) Load Plastification DFPplast takes place when the fastening system is strongly (over-) loaded with local yielding (plastif- ication). As a result from this the stable preload in the sys- tem is reduced after unloading [4,18]. (c) Creeping DFPcreep is a time-dependent deformation of mate- rials under the influence of stress. It leads to preload loss over a long time period. The mechanisms of creeping regard- Fig. 9. Influence of tightening speed on head friction coefficients. ing bolted joints are investigated in [11]. 0 5 10 15 20 25 CFRP with EP Matrix CFRP with PEEK Matrix Epoxy without fibers To rq ue T to t [ N m ] tightening untightening error bars min/mean/max from 3 measurements each Hexagon screws with flange M8x50 - 10.9, DIN EN 1665, as supplied (blank, slightly oiled), analysis at FP = 15 kN, tightening speed 50 min-1 -62% -61% -62% Fig. 10. Absolute value of tightening toque and untightening toque at a preload level of FP = 15kN. materials. It seems that it does not make any difference if the tem- pr el oa d- re le xa tio nC FR P. ..d sf single flow of preload (without sleeve) -25000 -20000 -15000 -10000 -5000 0 5000 10000 15000 20000 25000 temperature state ax ía l f or ce F z [ N ] 100 °C RTRT 100 °C 100 °C RT RT Fsmax = 22 kN clamp force in CFRP preload relexation at RT 35% preload in screw FSA = 12 kN Fp0 Fastening System CFRP-plates 8.0 mm, screw TiAl6V4 M8, Grade 5, support diameter 16 mm, tightening 10 kN FE-Analysis non-linear, orthotropic material properties, contact modeling, thermal-mechanical- coupled analysis site Structures 110 (2014) 335–341 339 a) mainly refers to the assembly process, b) to irregular loads and c) to regular loads in life cycle. One common parameter influ- encing all three effects from above is temperature level (as normal operating temperature or irregular thermal load). Elevated temperature leads to preload loss because of two effects enforcing each other. On one hand, the preload increases especially when clamping CFRP, because the thermal expansion coefficients per- pendicular to the fibers are about five to ten times higher than for metals (steel or titanium; own measurements). Consequently the contact pressure on the laminate strongly increases. On the other hand, at the same time the material strength decreases, espe- cially of the plastic matrix, also including the creep strength. Both effects lead to instable preload at elevated temperatures, as the re- sults below indicate. The following results of a non-linear finite element calculation (Fig. 12) display the influence of load plastification without consid- ering seating and creeping. Just by heating up to 100 �C the preload in the screw rises up to 220% of its initial level. Within three tem- perature cycles the preload falls by 35% only because of plastic Fig. 11. General behavior of preload relaxation in bolted joints [8]. C. Friedrich, H. Hubbertz / Compo deformations. These are the reasons why the relaxation tests following in this chapter are performed at elevated temperatures of 80 �C and 100 �C. This is a temperature level that standard applications in engineering are normally exposed to. 80 �C can be obtained for standard applications without thermal load (exposed to sunlight); 100 �C is the limiting temperature of mechanically loaded Epoxy materials. 3.1. Method of preload measurement For relaxation studies it is necessary to measure the preload remaining in the preloaded shank of each screw. A common way is to measure the screws elastic elongation originating from the preload. Using a calibration factor, which was determined at a hydraulic test stand, the length difference can be converted into a preload value. Optimized mechanical preload measurement by a micrometer spindle has been proved as a robust solution, when the specimens have to be measured and thus coupled multiple times. Nevertheless the coupling of the surfaces stays very crucial, because the change in length of short preloaded screws is quite low. Thus a micrometer with measuring tips was used and the screws were equipped with strain hardened centering holes, as it can be seen in Fig. 13. The clamped parts tested are the same plas- tic plates that are used in the tightening tests in Section 2. However for the relaxation analysis titanium screws with countersunk angle of 100� according to aviation standard LN 29956-08018A were em- ployed. This represents a realistic configuration for an application of CFRP materials in aviation. 3.2. Influence of temperature The results for relaxation of CFRP materials at different temper- atures are shown in Fig. 14. The clamped CFRP laminates were ex- posed to a temperature of 80 �C and 100 �C, the clamped Epoxy plates without fiber reinforcement to 100 �C, for a duration of 500 h. The preload was measured in increasing intervals (1 h, 24 h, 48 h, 130 h, 500 h) because of the decreasing relaxation ef- fects over time (see Fig. 11). Map of relative residual preload shows the percentage preload FP remaining at the time of measurement from initial preload FP0. It is apparent that CFRP laminates with the quite temperature- stable PEEK matrix have the best preload stability of the examined markings: increments of FE simulation Fig. 12. Calculated preload deviation with thermal load (preload change and short time preload relaxation by plastification for temperature cycles with CFRP [9]. perature is 80 �C or 100 �C. Whereas this temperature variance of 20 �C is critical for CFRP with the thermosetting EP matrix. Thermal exposure to 100 �C leads to a preload loss of app. 90% after 24 h, whereas 80 �C still has over 50% of its initial preload. Even after screw centering hole Micrometer spindle Screws Clamped Parts* M8x30, Ti-6Al-4V, driveTORQ-SET, countersunkangle 100°, standardLN 29956-08018A PoxySystems Epoxy Resin L with Hardener EPH 161 Epoxy without fibers: atlas weave, fibers T300, Vf ≈ 50% CFRP with PEEK Matrix: bidirectional 0/90°, fibers T300, Vf ≈ 60% CFRP with Epoxy Matrix: *same materials like in chapter 2 precise determination of screw-elongation resolution 1µm calibration necessary [µm/kN] coupling of surfaces important (centering hole and micrometer with measuring tip) Fig. 13. Principle of mechanical preload measurement and parts for relaxation analysis. 0% 20% 40% 60% 80% 100% Tightening 1h 24h 48h 130h 500h R el at iv e re si du al p re lo ad F P/ F P 0 CFRP with PEEK Matrix, 80°C and 100°C CFRP with Epoxy 80°C Screws: MJ8 Ti6Al4V, Countersunk angle: 100°, support diameter 15.5 mm error bars min/mean/max from 3 measurements each 100°C Epoxy without fibers tightening preload FP0 = 10kN 340 C. Friedrich, H. Hubbertz / Composite 500 h at 80 �C there is a remaining preload from about 35%. At 100 �C CFRP with Epoxy matrix shows a similar behavior to the non-reinforced Epoxy having almost no residual preload left after 500 h of thermal exposure. Further measurements show that pre- load relaxation is slightly higher for screws with 100� countersunk angle compared to screws with higher countersunk angles, this is valid for all materials shown. These results emphasize that the influence of the matrix mate- rial of CFRP is remarkable for relaxation at elevated temperatures. Depending on the material used, the preload relaxation can be so severe that there is practically no preload left and the fastening system looses its function. A loss of preload can also lead to other failure events in operation such as material rupture, corrosion and self loosening [9]. Altogether preload relaxation with clamped parts of (reinforced) plastic materials is often significantly higher in contrast to metals, which is not in the focus of this paper. Tests with low temperatures show that an additional cooling down cycle after heating-up can also have influence on preload relaxation [7]. 3.3. Influence of preload Assessing metals, higher stressing of materials normally leads to higher creep effects. To analyze this connection, the CFRP mate- rials were tested for preload relaxation at different levels of initial preload. Besides a preload of 10 kN in Fig. 14, the CFRP plates were tightened to 25 kN and 35 kN, the results are shown in Fig. 15. Interestingly the opposite of the effect described above can be observed: The relative residual preload after 500 h of exposure to Fig. 14. Preload relaxation over time for two CFRP laminates and Epoxy without fibers. 100 �C is significantly higher, when the fastening system started with higher preloads. The reason for this behavior is probably that 0% 20% 40% 60% 80% 100% 10kN 25kN 35kN Initial tightening preload FP0 re la tiv e re si du al p re lo ad F P/ F P 0 Screws: MJ8 Ti6Al4V Countersunk angle 100° Temperature Cycle: 100°C, 500h error bars min/mean/max from 3 measurements each CFRP EP CFRP PEEK Fig. 15. Evaluation of relaxation measurement focusing preload loss vs. initial tightening preload. with higher pressure more of the laminate’s matrix flows out of the compressed region increasing the local fiber content in the flow of forces. In general carbon fibers have a higher creep resistance com- pared to the plastic matrixes. The investigations on these effects are continued [7]. 4. Summary and conclusions The friction coefficients in head bearing area of steel screws and bearing plates of CFRP, non-reinforced plastic and metals were compared. The friction coefficients show different behavior from metals and are in the range between uncritical and critical low at first tightening as well as very dependent on the state of lubrica- tion. The stability of the friction coefficient is quite reliant to the fiber bonding as well as it is sensitive for retightening. Preload relaxation is a critical effect for CFRP laminates, especially when exposing to elevated temperatures. Nevertheless the relaxation is dependent on the fastened laminates, especially on the matrix material. This is important to the preload behavior in CFRP fasten- ing systems as the preload is induced perpendicular to the fibers. CFRP laminates should not be retightened. Possibly the wear resistance of the head bearing areas can be improved by sleeves or washers. Like in every fastening system, defined friction coeffi- cients are crucial and degreased tightening can be dangerous. In operation of fastened CFRP materials, the engineer should consider high safety margins in calculation. Furthermore the corrosion between the carbon fibers and the metallic fasteners must be prevented (not focused in this paper). Overall the extension of guidelines is recommended to cover CFRP fastening systems regarding friction, preload relaxation and wear. It can be dangerous to use the same threaded fastener for metals and composites without special design recalculation. 5. Acknowledgment The authors would like to thank DIN Deutsches Institut für Normung e. V. for partly funding the project on screw relaxation with CFRP materials and Boysen GmbH & Co. KG for the supply of titanium screws used for relaxation tests. References [1] Bickford JH. An Introduction to the Design and Behavior of Bolted Joints. New York: Dekker; 1981. [2] BMW GS9003-2. Anziehdrehmomente/Vorspannkräfte für Schrauben und Muttern mit metrischem Gewinde. 2009. [3] DIN ISO EN 16047. Fasteners - Torque/clamp force testing. Beuth: Berlin; 2005. [4] Friedrich C. Designing fastening systems. In: Totten GE, Xie L, Funatani K, editors. Modeling and Simulation For Material Selection And Mechanical Design. New York: Marcel Dekker; 2004. [5] Friedrich C. Screw-Designer v2.0 - innovative software for non-linear analytical simulation of threaded fastening systems. www.screw-designer.de. [6] Friedrich C, Hubbertz H. Engineering calculation of threaded fastening systems considering deviations in advanced design. ASME: Houston; 2012. IMECE2012- 86898. [7] Friedrich C, Hubbertz H. Product innovation needs improved design process of screw joints. ASME: San Diego; 2013. IMECE2013-62927. [8] Friedrich C, Hubbertz H. Relaxation of clamp force in fastened carbon composite structures with thermal load.In: International Conference on Composites Engineering ICCE-21, Santa Cruz de Tenerife; 2013. [9] Friedrich C, Hubbertz H, Dinger G. Standardized calculation of bolted Joints for future requirements, ASME: Denver; 2011. IMECE2011- 63774. [10] GM GME-00150. General Specification All Vehicle. Bolted Joints. 2001. [11] Granacher J, Kaiser B, Hillenbrand P, Dünkel V. Relaxation von hochfesten schraubverbindungen BEI Mäßig erhöhten temperaturen. Konstruktion 1995;47:318–24. [12] Hörnig T, Kopfer H, Friedrich C. Instability of friction at bolted joints – measurement and evaluation. Karlsruhe, Germany: DGM Friction, Wear and Wear Protection; 2011. [13] Kopfer HW, DeAgostinis M, Friedrich C, Croccolo D. Friction characteristics in Structures 110 (2014) 335–341 light weight design focusing bolted joints. ASME: Houston; 2012. IMECE 2012–85940. [14] NASA. Fastener Design Manual. reference publication: 1228; 1990. [15] PSA B15 4102. Electrolytic zinc coatings and associated finishes – sherardizing process. 2009. [16] Renault 01–50-005/–D. Fasteners coefficient of friction test. 2003. [17] SAE USCAR-11. Torque-tension testing and evaluation of fastener finishes. 2007. [18] Sakai T. Bolted Joint Engineering – Fundamentals And Applications. Berlin: Deutsches Institut für Normung; 2008. [19] VDA 235–101. Reibungszahleinstellung von mechanischen Verbindungselementen mit metrischem Gewinde. 2009. [20] VDA 235–104. Cr(VI)-freie Oberflächenschutzarten für Verbindungselemente mit metrischem Gewinde. 2004. [21] VDI 2230. Systematic Calculation of Bolted Joints. Design Guideline Verein Deutscher Ingenieure. Berlin: Beuth; 2003. [22] VW 01129. Grenzwerte der Reibungszahlen. Mechanische Verbindung selemente mit Metrischem ISO-Gewinde. 2007. C. Friedrich, H. Hubbertz / Composite Structures 110 (2014) 335–341 341 Friction behavior and preload relaxation of fastening systems with composite structures 1 Introduction 2 Friction 2.1 Friction coefficients 2.2 Deviations and uncertainties 3 Relaxation 3.1 Method of preload measurement 3.2 Influence of temperature 3.3 Influence of preload 4 Summary and conclusions 5 Acknowledgment References