An advanced methodology for windscreen modelling in ls dyna

The present study is carried out on Ls-Dyna solver for pedestrian head impact on windscreen. Methodology for correlation of acceleration pulse is developed by using the material cards in Ls-Dyna. This new material card has the capability to handle tension and compression strengths differently.

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AN ADVANCED METHODOLOGY FOR WINDSCREEN MODELLING IN LS DYNA

Amol Jaware

Applus IDIADA Automotive Technologies India Pvt Ltd., India

Sudip Chandratre

Applus IDIADA Automotive Technologies India Pvt Ltd., India

Mario Perez

Applus IDIADA Automotive Technologies Spain

Jayadip Narule

Applus IDIADA Automotive Technologies India Pvt Ltd., India

ABSTRACT

Laminated Safety Glass (Glass/PVB/Glass) is widely used for the automobile windscreens Poly vinyl butaryl (PVB) placed in between two glass layers constructs the laminated glass The very purpose of using the interlayer is to avoid separation of glass pieces after breakage and thereby reduce the possibility of injury to occupant and pedestrians

The stiffness of the windscreen plays vital role in roof crush, frontal impact and pedestrian load cases Pedestrian load cases are inherent part of the EURO NCAP, KNCAP and JNCAP Head Injury Criteria (HIC) value decides the rating of pedestrian load case To perform head impact tests on windscreen is tedious task since

it involves the replacement of the whole windscreen for each impact point To avoid this and for the purpose of cost saving it is very important to develop the reliable windscreen failure methodology in CAE, to model the behavior of windscreens after impact

The present study is carried out on Ls-Dyna solver for pedestrian head impact on windscreen Methodology for correlation of acceleration pulse is developed by using the material cards in Ls-Dyna This new material card has the capability to handle tension and compression strengths differently The propagation of crack in glass layers can be visualized and hence the crack pattern of the glass can be predicted Glass laminates fails when the stress exceeds the maximum principle normal stress The crack pattern in the region of impact becomes visible once the energy criteria is reached PVB interlayer continues to provide stiffness after the impact Head-form impacts are simulated at four locations of the windscreen and results are validated with test The correlation activity is performed by monitoring the HIC values, pulse trend and crack pattern Similar trend of acceleration pulse is obtained with the good agreement of first and second peaks with test

Abbreviations

EURO NCAP The European New Car Assessment Program

Keywords: Laminated safety Glass, LS-Dyna, HIC

Cite this Article: Laura A Beltrán, Oscar F Avilés S, Mauricio Mauledoux, Oswaldo

Rivera, Robinson Jiménez, Prototype of a Human Upper Limb Driven by Pneumatic

Muscles International Journal of Mechanical Engineering and Technology 10(12),

2019, pp 380-389

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=12

1 INTRODUCTION

Modelling the failure behavior of windscreen is still under development Many attempts have been made till the date to establish the methodology, which would help in predicting the failure behavior of windscreen The study of single layer shell model to three layered laminated glass model is available in literature Out of two, three layered laminated glass model is found to give promising results in comparison of single layer model Three layered laminated glass model has the two layers of glass and PVB layer is sandwiched in between The layers of glass are generally modelled with shell elements where as PVB can be modelled with shell or solid elements Laminated glass fracture shows very complex nature during the impact Laminate experiences the bending load condition at point of impact The glass layer

on opposite side undergoes the tension and glass layer on same side of impact experiences compression [1,2] The initial behavior after impact is almost linear elastic and no fracture can

be observed The glass layer on opposite side of impact breaks first, since glass being brittle material is weak in tension The crack formation takes place and it propagates to edge from center of impact Afterwards, the glass layer on same side of impact fails [1]

MAT_MODIFIED_PIECEWISE_LINEAR_PLASTICITY are some most widely used material cards for glass modelling in LS-Dyna The detail capabilities of each of card is given

in table 1

Table 1 Capabilities of material models

Sr

No

Mat STRAIN

RATE

FAILURE Anisotropy thermal DAMAGE Tension/co

mpression

Element

1 Laminated

Glass

2 Johnson

Holmquist

Ceramics

3 Piecewise

linear

plasticity

SOLID

The complex asymmetric behavior of laminated glass during fracture, demands the

provision of giving the tensile strength and compressive strength separately The material

cards mentioned above does not have this capability [3] In addition, anisotropy of glass can’t

be modeled with this existing materials The newly released material card has these

capabilities

In the present paper, methodology has been developed to model the glass by using LS

Dyna material card This card has the advanced capabilities like provision to give separate

tensile and compressive strengths and ability to model the damage behavior It is a smeared

fix crack model with different stress dependent failure criteria like Rankine, Mohr-Coulomb

or Drucker-Prager According to Rankine’s criteria, tensile strength and compressive

strength of material bounds the major principle stress and minor principle stress as

follows

Mohr-Coulomb criteria has the four different cases of failure according to the state of

major principle stresses, either tensile or compressive

(

(

Drucker-Prager criteria is given as follows

*(

) ( ) (

) √ + (6) Once the critical stress is reached in the element, two element degradation procedures are

available One procedure allows the degradation of element in required number of load step

cycles(NCYCR), another uses the damage model This damage model reduces the stress and

stiffness of the element Flag is provided to choose between the available damage models

MAT_ADD_EROSION card can be used with some of the above mentioned cards to

predict the crack pattern of windscreen This material card is intended to predict failure

mainly for laminated glass impact The parameters viz critical stress, critical energy and

critical radius are need to specify for laminated glass discretized with shell mesh Pyttel has

suggested a promising failure approach by using MAT_ADD_EROSION card parameters viz

critical stress, critical energy and critical radius According to this approach element deletion

takes place when the main failure criteria is reached in certain boundary region, along with

necessary condition of critical energy The main failure criterion is defined by principle stress

value at failure Once the main failure criterion is reached, that element is taken as center of

impact and circular boundary region is set around that element with radius equal to critical

radius The internal energy of shells is then tested in circular boundary region against the

product of critical energy and area factor

Area at the denominator is considered two times, because we expect the two shell layers

of laminate glass as the case in real scenario If this energy balance is achieved in the given

circular boundary, then and only then the elements are allowed to erode by main failure

criteria

MAT_MOONEY_RIVLIN_RUBBER, MAT_OGDEN_RUBBER and MAT_SIMPLIFIED_RUBBER are the material cards available for modelling the behavior of PVB Hyper elastic rubber material card is used to model the PVB in current methodology, because of its simplicity In this card, PVB can be modeled by giving material constants, force-displacement curve or Nominal Stress-Nominal strain curve [3]

2 EXPERIMENTS

Dynamic head form impact tests are carried out on laminated windscreen at four different locations Adult head form impactor with the mass of 4.5 kg, angle of impact 65 deg and velocity of impact 11.129 m/s is used The glass layers of used windscreen are 1.65 mm thick and PVB layer is 0.7 mm thick The positions of all the impact points are shown in fig 1 All the impacts are carried out in convex direction of windscreen The tests are conducted with actual test environment by fixing the windscreen on BIW The results of the test pulse for all positions are shown in fig 2 As shown in the Acceleration plots of different positions two peaks can be Clearly observed If you observe the plot b in the fig2, the multiple curves at the before failure stage are due to the presence of wiper in impact zone The magnitude of the first peak is dependent on the initial stiffness of windscreen, more the stiffness before failure more is magnitude of first peak The stiffness of PVB layer contributes majorly to magnitude

of second peak The crack patterns of all impact positions are shown in fig 3 It can be observed that crack patterns of position I and position II are spread on large area where as the crack pattern of position III and position IV are local

a) Position I b) Position II c) Position III d) Position IV

Figure 1 Impact Positions

3 DISCRETIZATION AND CONSTITUTIVE RELATION

a) Position I b) Position II c) Position III d) Position IV

Figure 2 Test acceleration curves for different impact positions

The CAE model is generated with reference to the actual test conditions The chosen discretization has the three layers of shell mesh, with element size of 10mm The contact between PVB and two glass layers is set to Tied contact The PVB interlayer is tied to frame with NRB connections Glass layers of laminate are modelled with mat glass The behavior

of glass before failure is isotropic defined by small strain linear elasticity with modulus of elasticity E and Poisons ratio µ A catastrophic asymmetric behavior occurs when the plain Stress condition set by Rankine’s criteria is reached, where major principle stress and minor principle stress are bounded by the tensile and compressive strength

( )

As soon as stress reaches the critical stress value given by the Rankine criteria, the crack occurs in the direction perpendicular to the normal stress direction The appearance of crack sets up the crack co-ordinate system with respect to element coordinate system The erosion

of elements within predefined radius will start when the main erosion criteria set by “sigma1” and energy threshold set by “critical energy” will reach This erosion of elements makes the crack pattern visible [5]

PVB interlayer is modelled with Hyper elastic rubber material card [6] The strain energy function used by this material card is defined as:

( ) ∑ ( ) ( ) ( ) (8)

Where

The material properties of glass and PVB interlayer are given in below table 2

Table 2 Material properties of glass and PVB

4 RESULTS AND VALIDATION

Non-local failure criteria with element erosion technique is used in current methodology A non-local failure is induced in the windscreen with the help of three parameters critical internal energy, critical radius and critical stress Glass is modelled as linear elastic material with MAT_ADD_EROSION and PVB is modelled as hyper elastic material Stress based

failure criteria is used in current model by applying Rankine theory of failure The strength values of the glass in tension and compression are assigned and different levels of non-local parameters are considered Initially the parameters in hyper elastic cards are assigned constant values and sensitivity of non-local parameters on peaks of acceleration pulse are analyzed Afterwards, the sensitivity of PVB card parameters are analyzed for decided sets of non-local parameters

It is found that the first peak in acceleration pulse is highly affected by the glass material parameters compared to PVB parameters This is because they add the stiffness to the model before failure of glass It is observed that first peak increases with increase in values of critical internal energy of element deletion, tensile strength of glass material and decrease in critical radius of element erosion zone Slight increase in first peak is also observed for increase in PVB parameters Second peak in Acceleration pulse is governed by the PVB material constants It increases with increase in constant values The figure given below gives the summary of sensitivity analysis

Figure 4 Summary of sensitivity analysis

The results of acceleration pulse and crack pattern are given in following figures fig 5 and fig 6 respectively As seen from the figure good level of correlation has achieved with the proposed methodology The behavior of laminated glass before failure has obtained correctly

at all the impact locations Card parameters viz critical energy, critical radius and tensile strength are tuned to achieve the good correlation in before failure zone The after failure behavior is also in good agreement, trend wise Control on pulse is less in after failure zone The fracture pattern obtained is close to reality behavior at all points of impact Material property values are kept constant at each location of impact while the non-local parameters need retuning to catch the peaks of the pulse The curvature of glass, presence of hard points and impact location controls the non-local parameter values In addition, presence of micro flaws in actually tested laminated glass at surface and edges, which are not modeled in CAE, changes the stiffness at that location In actual case, Laminated glass may fail below the defined failure strength because of this micro flaws

a) Position I b) Position II c) Position III d) Position IV

Figure 5 Correlation Results at different impact positions (Use Colored Print)

a) Position I b) Position II c) Position III d) Position IV

Figure 6 Crack patterns at different impact positions

The Head Injury Criteria values are calculated at the all impact positions and shown in Fig.7 below The HIC value of position II is in good agreement with the test curve HIC value HIC values at the other positions shows the considerable variation This variation in HIC values is obtained because of post breakage behavior of laminated glass

Figure 7 Test and Simulation HIC values Comparison (Use Color Print)

5 CONCLUSIONS

In the current study, laminated glass (Glass + PVB + Glass) is modelled with mat glass and hyper elastic material cards for glass layer and PVB interlayer respectively MAT_ADD_EROSON card is used with glass material card to predict the crack propagation Stress based failure criteria is implemented for glass with Rankine theory Initially, the priority is given for matching HIC values and later the trend of acceleration pulse with test curve and crack pattern are given the attention

Critical energy, Critical radius, Critical stress and Tensile strength of glass are observed to play key role to achieve the initial stiffness of glass and hence the first peak of acceleration pulse These parameters are tuned carefully to achieve the first peak of pulse PVB material properties are responsible for second peak in acceleration pulse The attempt is made to tune the second peak, first by tuning the constants A & B and later by using the material curves for range of strain rates It is observed that fine tuning of the second peak is possible only after the use of proper strain rate curve Micro flaws present on the surface of laminated glass makes its behavior different than CAE model of same The brittle nature of glass and presence

of micro flaws on the surface and edges makes behavior of glass unpredictable Non local failure used for predicting crack pattern shows its dependence on curvature of glass and location of impact Therefore, a single material card cannot be released, which can predict behavior at all impact points The properties of glass and PVB are constant at each of impact point, but non local parameter values needs tuning for each point

REFERENCES

[1] Christian Alter, Stefan Kolling, Jens Schneider: A New Failure Criteria For Laminated

Safety Glass.11th European LS-DYNA Conference 2017, Salzburg, Austria

[2] M Timmel, S Kolling, P Osterrieder, P.A Du Bois: A finite element model for impact

simulation with laminated glass International Journal of Impact Engineering 34 (2007) 1465–1478

[3] LS-Dyna Keyword User’s Manual, Volume-II, Livermore Software Technology

Corporation

[4] Qi Lui, Junyong Lui, Qiang Miao, Dazhi Wang, Xiaodong Tang: Simulation and Test

Validation of Windscreen Subject to Pedestrian Head Impact

[5] Frederic Nub, Andreas Herkenhoff, Lutz Eckstein: Effect of the Non local Failure Criteria

on the Acceleration Signal of Head Impactors at Impact in Windscreen Centre LS-Dyna Forum 2014

[6] Camilla Fors: Mechanical Properties of Interlayers in Laminated Glass Experimental and

Numerical Evaluation ISRN LUTVDG/TVSM 14/5198 SE (1-52) | ISSN 0281-6679