Crash stable adhesives in application and simulation

Transcription

1 5. LS-DYNA Anwenerforum, Ulm 2006 Verbinungs- / Klebetechnik Crash stable ahesives in application an simulation Dr. Alexaner Droste DOW Automotive, Schwalbach, Germany Abstract: Structural ahesives have been introuce to increase the stiffness an crashworthiness, as well as to improve corrosion protection an sealing of the boy-in-white in upper class car moels an are now entering the segment of compact cars. Various numerical approaches are use to preict the mechanical responses for the ifferent loaing conitions applie to evelop an optimize the bone car-boy. Most often these approaches are limite to escribe the complex material behavior of ahesives, especially in crash an post-crash analysis. Thus, in the past, choosing the ahesive material an placing an optimizing the bonline was often base on testing an esign experience. As one of the leaing ahesive suppliers DOW Automotive is committe to support the OEMs an Tiers with suitable EA approaches to improve the material selection process an reuce testing costs. In aition a reliable numerical preiction allows to esign in the virtual evelopment phase an optimize joint to get the most benefits out of the ahesive material. This will improve the usage of structural boning also within the meium an lower car segments. This article will give an overview about current applications, evelopment an valiation of the moeling approach using MAT_Gurson (#120) within LS-DYNA for the crash stable ahesive BETAMATE 1496/1496V. Keywors: structural ahesives, strukturelles Kleben, Gurson C - I - 1

2 Verbinungs- / Klebetechnik 5. LS-DYNA Anwenerforum, Ulm Introuction New material evelopments for crash stable structural ahesives open up new applications in the moern car boy esign. Especially the improve amage characteristic of these ahesives allows their usage in high ynamic loaing conitions. To save evelopment cost as well to allow up-front optimization in the vehicle evelopment process, numerical tools like LS-DYNA nee a suitable material behavior constitutive formulation to escribe the material response. In aition to these material moels, the numerical parameters like maximum element size, element formulation, etc. especially for the full vehicle simulation, have to be consiere for balancing the accuracy an the calculation time. igure 1: Potential structural ahesive bonline in the car boy Structural ahesives have been applie for boy structures application as a thin layer between the substrates ( mm bonline thickness - BLT) with a varying length of up to 150 m (igure 1). Due to complex nature of ahesive material behavior an its influence on full vehicle response a nee for establishing CAE methoology exists. 2 Material behavior escription In general the quality of an ahesive can be characterize by its ability to ahere to ifferent often oily substrates. To save cost an to compete with other joining technologies like spot-weling, clinching, riveting minimal substrate surface preparation is require. The surface conition is one of the main aspects of the performance capability of a bone joint. If the interface between the substrate an the ahesive is weak, neither the substrate nor the ahesive can meet the expecte requirements. Within this article it is assume that interfacial failure oes not occur an a bone joint is always limite to its own material characteristic (cohesive failure) or to the substrate characteristic (substrate failure). 2.1 Different types of ahesive In orer to escribe the mechanical behavior of a bone part participating in vehicle components use for stiffness an energy management not only the (visco)-elastic-plastic behavior is significant. The mechanism of crack initiation, crack propagation an fracture leaing to softening an failure has to be consiere as well. The separation of bone joint is of importance, as the energy management coul be strongly influence if two or more substrates are kept together or are separating within the loa event. Ahesives can be classifie within ifferent types. The five most wiely use are epoxy, polyurethane, moifie acrylic, cyanoacrylate an anaerobic. Aitives or moifiers are use to improve certain properties like strength, loa, temperature, solvent an/or creep. Ahesives use for boy structrure applications are typicially one- or two component thermosets systems, which are curing at room- or elevate temperatures. Two component systems require careful proportioning an mixing. Some of C - I - 2

3 5. LS-DYNA Anwenerforum, Ulm 2006 Verbinungs- / Klebetechnik the structural ahesives are ifficult to remove an/or repair. Structural ahesive bons can be stresse to a high proportion of maximum failure loa uner service environments. In summary there are various chemical families with varying mechanical- as well as process parameters an also with a wie range of costs Selection factors: To select a suitable material for the require joint performance, ifferent necessities an conitions have to be consiere. Depening on its usage the following list shoul give an overview of selection factors for the ahesives: - Substrates to be bone: porosity, harness, surface coating - En-use requirements: stresses (construction, service), joint-types - Temperature requirements: cure, en-use - Exposure conitions: humiity, UV raiation, outoor weathering - lexibility requirements - Aging stability - Aesthetic requirements: color, gloss - Manufacturing conitions: materials hanling system, processing - Cost: waste, rejects, packaging, availability To evelop a new ahesive formulation, these factors will also be use for choosing the baseline polymeric system as well as the corresponing aitives Ahesives mechanical behavior from brittle to uctile A esigner can select ahesives varying from brittle to uctile in their general mechanical behavior. By aing a toughener, the failure behavior can be moifie towars a more stable post-critical material response (igure 2). brittle-unstable brittle-stable BETAMATE TM less tough uctile-unstable uctile-stable igure 2: Ahesive CTS: crack initiation an propagation of ifferent ahesive types With the CTS (compact tension specimen) the crack initiation process as well as the crack propagation of ifferent types of ahesives is examine quasi-statically. or the non-toughene materials it can be conclue that after reaching a certain loa level, the crack initiates but also propagates instantaneous through the specimen leaing to complete fracture an separation. or the stabilize (toughene) ahesives, the crack propagation behavior is moifie towars a ecelerating process an much more energy can be sustaine before total failure occurs. In aition it is not favorable for a suen loss of loa bearing capacity in crash relevant joints. Thus, the uctile-stable behaving ahesives seem suitable for applications requiring higher egree of practical toughness an loacarrying capability. BETAMATE 1496/1496V is a one component epoxy ahesive treate with a special multiphase-rubber-toughene technology (Patent: EP ) to support high loa bearing capacity an ecelerate failure mechanisms, especially esigne for joining structures participating in ifferent crash scenarios. C - I - 3

4 Verbinungs- / Klebetechnik 5. LS-DYNA Anwenerforum, Ulm 2006 As an example of the benefit using such a crash stable ahesive, igure 3 compares a brittle epoxy bone profile, a spot-wele profile an a profile bone with BETAMATE1496 of a rop-tower testing. a) brittle epoxy b) spot-wels c) toughene epoxy igure 3: Test results of ynamic rop-tower tests on a ouble hat section profile using a brittle ahesive, spot-wels an the crash stable BETAMATE 1496 In this example using a brittle epoxy or spot-weling has lea to separation of the parts uring the impact, not being able efficiently absorb the impact energy. One the other han, the toughene epoxy shows that the most efficient energy absorption take place when the steel profile unergoes axial plastic eformation without separation of bone flanges. 2.2 Material characteristics for a crash stable structural ahesive Depening on the requirements, the material characteristics have to be measure an benchmarke to support the material selection process as well as to ientify the behavior an parameters require for CAE simulations. It shoul be taken into account that, relate to complex stress fiels in the ahesive layer, most ahesives act very local. Especially for benchmarking it has to be ensure that the loa transfer into the joint is comparable. or the characterization an parameter ientification process it is suggeste to reuce the substrate influence as much as possible. In aition to bulk ahesive characteristics examine by tension, compression an shear tests, static as well as ynamic component tests representing the application are the supporting information neee to ientify an classify the performance behavior of the material moel Lap-shear To characterize an ahesive joint, most often the simple lap shear test is use (DIN 54451). or the material parameter ientification the bone area (overlap) of such a stanar test is slightly extene an thick steel substrates are use to minimize rotational influences cause by the substrate eformation. igure 4 shows the experimental an the simulation results of thick lap shear specimens with ifferent bonline thickness. t = 0.5 mm t = 1.0 mm t = 2.0 mm Tests Simulation igure 4: Experimental results for thick lap shear specimen tests of a crash stable ahesive an the simulation response using a etaile Gurson type material moel, see Wocke [2], [3]. C - I - 4

5 5. LS-DYNA Anwenerforum, Ulm 2006 Verbinungs- / Klebetechnik Comparing the test response with the simulation base on a Gurson material moel an a etaile mesh representing the ahesive, the force-eflection behavior can be moele accurately incluing the local propagating cohesive failure of the joint. Changing the specimen geometry an/or the substrate material in a lap-shear test will lea to ifferent shear strength levels ue to changes of the local stress fiel in the bone region. One has to take care in comparing ocumente material parameters of ifferent sources, for example comparing the shear strength of an aluminum-steel with a steel-steel lap shear or comparing a bonline thickness of 0.5 mm with a 2.0 mm one Impact Peel Another test, especially to juge the boning capability uner a ynamic loaing, is the impact peel test (DIN ISO 11343). As the wege is impelle ynamically through the two bone steel substrates, very high local tension stresses occur in the bon - a critical loa case for ahesives. igure 5 sketches the testing setup an the results to evaluate a non-stabilize ahesive with a crash-stabilize one. Ahesive (20 mm x 30 mm) Substrate Specimen grip Wege Wege retaining shackle igure 5: impact peel test an results for a crash stabilize an a non-stabilize ahesive Evaluating a crash-stabilize ahesive against a non-stabilize one with the impact peel test, it can be easily seen that the crack growth is ifferent. Non-stabilize ahesives fail after crack initiation instantaneously an the substrates separate. or the stabilize ahesive the crack front is more or less close to the wege front. As the two substrates are still partially bone, the steel has to eform an more impact energy can be absorbe. urthermore, by changing the impelling spee, rate epenencies can be observe an a potential rate epenency coul be taken into account for the material moeling. igure 6 is giving a summary of ifferent ahesive types for their impact peel strength epening on the stabilization treatment incluing the low temperature effects C -20 C RT Strength (N/mm) General Purpose Ahesive 0 0 Structural Ahesive Semi racture Toughene ully racture Toughene Increase crash resistance Increase urability Increase flexibility to own gauge metal & reuce wels igure 6: Impact peel strength of ifferent ahesives incluing low temperature behavior C - I - 5

6 Verbinungs- / Klebetechnik 5. LS-DYNA Anwenerforum, Ulm 2006 Regaring ynamic loaing on safety relevant structures, general-purpose ahesives will not keep the substrates together. Non-stabilize structural ahesives can withstan a low energy impact, but at lower temperatures their resistance capacity is minimal. Structural-semi crash-stable ahesives coul resist meium impact energy levels aequately. or higher impact levels, like they occur in the bons of components participating in the crash loa path, the crash stable ahesives fulfill the neee performance requirements an supporting also lower temperatures. 3 Material moel Screening of the available material moels in LS-DYNA by extensive material testing an presimulation, the Gurson material moel (MAT_GURSON #120) has been chosen to represent DOW Automotive s BETAMATE crash stable ahesives. The Gurson approach inclues an acceptable representation of the general elasto-plastic response as well as the feasibility to inclue the complex amage mechanisms for the crash-stabilize ahesives. In aition, the Gurson approach is implemente in all main explicit EA-programs, which offer a broa spectrum of usage. Within the next LS-DYNA 971 releases also strain rate epenency is inclue. The Gurson flow function in LS-DYNA is efine as follows: σ Φ = σ 2 M 2 Y + 2 q 1 f * 3 q2σ cosh 2σ M H 1 * 2 ( q f ) = 0 1. (1) Herein σ M is the equivalent von Mises stress, σ Y is the yiel stress an σ H is the mean hyrostatic stress, q 1 an q 2 are material constants. The amage behavior is relate to the effective voi volume fraction f*. Within the voi volume fraction upate, f 0 -the initial voi volume fraction, f C -the critical voi volume fraction an f -the voi volume fraction at total failure control the porosity evelopment. The voi nucleation is escribe by f N which is the voi volume fraction of nucleating particles, ε N is the mean nucleation strain an σ N is the stanar eviation of the normal istribution of ε N. The intereste reaer is referre to the LS-DYNA keywor manual [1]. In aition, Wocke iscuss the Gurson approach for DOW structural BETAMATE ahesives in [2], [3]. 3.1 Moel valiation As shown in igure 4, the etaile approach (etaile by a fine iscretization of the bonline) is representing the material behavior very well also for a varying geometry like the influence of the bonline thickness. or full car crash simulations a etaile mesh representing the bonline isn t acceptable, as the critical time step size woul ramatically rop by the small ahesive element as well as by the number of elements use to represent the ahesive Macro-element ocusing a macro-element size of 5 mm x 5 mm x BLT mm representing the ahesive, a full car simulation seems feasible. Nevertheless, the local crack initiation an propagation couln t be examine like it woul be with the etaile approach using macro-elements. The big element is representing a homogenize failure on a large bon area, which also leas to less accuracy. The material constants neee for the Gurson moel have been slightly ajuste to represent the average failure mechanisms for such a larger bon area. igure 7 is inicating the general approach of such a macro-element setup within LS-DYNA. length Substrate: shell element (miplane) Tie_Noes_to_Surface_Offset BLT Ahesive: soli element (MAT_Gurson) Tie_Noes_to_Surface_Offset Substrate: shell element (miplane) igure 7: Macro-element approach Connecting the ahesive representing soli element to the substrate representing mi-plane shells C - I - 6

7 5. LS-DYNA Anwenerforum, Ulm 2006 Verbinungs- / Klebetechnik As the bonline thickness has an influence on the 3D-stress fiel in the element, the material parameter set for the failure behavior of the Gurson approach is ientifie for a representative 3D volume. It is suggeste to moel the macro-element with the real geometrical value associate with the bon line thickness. It is certain that the irect connection of the macro element with the substrate representing mi-plane shell elements woul have a positive effect on the calculation time, but such an approach woul nee a material parameter fit each time a change in substrate thickness takes place. Memhar et al. [4] iscusse the mi-plane connection by calibrating the strength an failure parameter from the etaile approach to a replacement approach, however, further stuies of this approach will have to be one on sensitivity an accuracy. Also the macro-element approach nee ajustments cause by the ahesive representing volume, but for a bonline thickness below 1 mm the influence is negligible (compare igure 4), an sensitivity stuies show that using the real bonline thickness one material parameter set can be use. Once more, in general the bonline thickness for a structural ahesive is between mm. To connect the ahesive representing element with the (stanar) substrate shells, a tie noes to surface contact with offset is suggeste. The shell size of the substrates shoul be close to the soli contact surface size Application valiation As the approach is suggeste to be use in real applications, a valiation example was chosen to show the suitability of the Gurson material moel an the macro-element approach on there potential to represent the structural ahesive. As iscusse above, the crash-stabilize ahesive has been evelope to establish a joint allowing two metal substrates to eform without separation of bone flanges. To valiate the failure mechanism, a hat section simply close-off with a flat steel sheet was chosen. Uner high ynamic axial loa the profile part starts to fol, whereas the backplate starts to peel-off. In igure 8 the test setup for the axial crush is sketche. Support an Reaction ixture Ajustable Weight Impactor Sle Rubber Bumpers Axial Crush Tubular Specimen Rubber Bumpers Impact Direction Sle Track Armature Bening Specimen Configuration igure 8: Test setup for the ahesive moel application tests for axial crush an for a bening impact (small picture) A sle with ajustable weight an spee is impacting a specimen mounte on a rigi support. Moifying the specimen support, the specimen coul be impacte in a bening moe or, wele perpenicular to a support plate for axial loaing. The eformation of the specimen is strongly ominate by the eformation of the steel substrates, only the axial configuration is leaing to a cohesive failure of the ahesive, whereas the bening configuration was use to ajust the material moel for the steel. igure 9 show the force-eflection result of the testing an the simulation of the axial crush test. C - I - 7

8 Verbinungs- / Klebetechnik 5. LS-DYNA Anwenerforum, Ulm 2006 Profile thickness: 1.0 mm Bonline thickness: 0.35 mm Impactor mass: kg (72 lbs) Impactor spee: 8.81m/s (19.71 mph) igure 9: orce eflection results for the axial crush of a hat section close by boning with a steel plate By the peel-off, cohesive failure within the ahesive layer occurs. Regaring the force-eflection response, the force level as well as the whole event is preictable within an error range of less than 20%. The foling process of the profile is ominating the local force transfer to the ahesive elements an thus, controlling their failure. As the steel ominates the force-eflection response, the length of the faile bonline can be use to juge the quality of the Gurson approach with macro-elements. The experimental results show that the crack front stops close to the fols. This was epicte in the CAE simulations as well. It shoul be emphasize that the valiation example has been selecte to verify the computational moel at the extremes on its usefulness. A real boy structure woul be esigne an imensione to avoi the failure of the ahesive. 3.2 Outlook The accuracy an suitability of the ahesive moeling with the Gurson approach will be analyze an iscusse with the OEMs an Tiers on full-scale structures. These partners will have to efine an provie feeback if the metho can be use to get the benefits of moeling structural boning like weight optimization, spotwel reuction, etc. as useable an accurate in the virtual evelopment phase of a car. Development activities are unerway to elaborate a set of parameters neee for other graes of Dow Automotive ahesives incluing crash stable BETAMATE 1480 ahesive with the Gurson approach. 4 Summary Discussing the capabilities of ahesives in the virtual evelopment phase of structures participating in the crash energy management, it can be conclue that the accuracy level of a bone joint shoul increase. Due to the complex behavior of the ahesive material, especially in crash, not only the preiction of the maximum force but also the failure mechanism incluing the softening an fracture behavior shoul be integrate into escribing the behavior of such materials. As numerous ahesives are available, the material moel selection for crash simulation is iscusse comparing ifferent types of ahesives uner static an ynamic loaing. Within the material selection process, the benefits for crash-stabilize ahesives are relate to the performance expectations an crash requirements. As the structure material is ominating the energy issipation, the joint shoul keep the substrates together to get the most out of the energy issipation from the substrates as well as to keep the structure integrity as long as possible. C - I - 8

9 5. LS-DYNA Anwenerforum, Ulm 2006 Verbinungs- / Klebetechnik To preict this behavior in an appropriate material moel, material characterization an testing methos for the parameter ientification have been provie. or crash-stabilize BETAMATE ahesives, DOW Automotive suggests to use MAT_Gurson representing the ahesive behavior. Taking the critical timestep of a etaile iscretization into account, a macro-element approach is suggeste, to keep calculation times for e.g. full car crash simulations feasible. The representation of such a macro-element is iscusse. Results of the methoology are valiate with an axial crush test of a hat section profile close by boning with a crash-stable ahesive with a flat plate. This setup allows comparing the overall force-eflection (energy) as well as to appraise the quality of the failure moel. or this type of component the accuracy of the moel is acceptable, whereas for a full car usage the results are uner evelopment. DOW Automotive will continue to broaen the material moels to the main BETAMATE graes use in crash relevant applications. 5 ACKNOWLEDGMENTS Special thanks go to Colmar Wocke DOW Engineering Simulation performing the evelopment of the Gurson approach by using ABAQUS/Explicit as the evelopment coe. In aition the author want to thank Matt Turpin DOW Lea Designer, Mansour Miramai DOW Global Technology Leaer Boy Engineere Systems, Scott Burr DOW Automotive Material Science for their input an the testing work an finally Mustafa Ahme DOW Boy engineere systems for the sensitivity stuies. 6 Literatur [1] Hallquist, J. et al.: LS-DYNA Keywor User s Manual, Version 970, LSTC, April 2003, Livermore, USA [2] Wocke, C. P.: Moeling ailure in a Moern, Toughene Epoxy Ahesive using Abaqus/EXPLICIT,2004 Abaqus User s Conference, May 2004, Boston, USA. [3] Wocke, C. P.: Berechnung un Simulation crashstabiler Strukturklebstoffe, Ahäsion 3.06, Vieweg, 2006, p [4] Memhar, D. et al.: Entwicklung un Anwenung von Ersatzmoellen für ie Moellierung von Klebverbinungen unter Crashbelastung, 4. LS-DYNA Anwenerforum, Bamberg, Germany 2005 Crash I-Spotwel/Boning, p. B-I-29 DOW AUTOMOTIVE BODY ENGINEERED SYSTEMS Structural Ahesives Structural an Acoustical Reinforcing Soun Absorbing Sealing Systems Injection-moleDashmats Acoustical Damping Uner-the-hoo Engine an Noise Shiels Glass Glass Cleaner Glass Primer Urethane Ahesive C - I - 9

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