IMPACT ANALYSIS OF A CELLULAR PHONE

Transcription

1 4 th ASA & μeta Internatonal Conference IMPACT AALYSIS OF A CELLULAR PHOE We Lu, 2 Hongy L Bejng FEAonlne Engneerng Co.,Ltd. Bejng, Chna ABSTRACT Drop test smulaton plays an mportant role n nvestgatng mpact behavors and dentfyng weak ponts of a cellular phone at desgn stage snce actual testng s expensve and tme consumng. Ths paper presents the mpact study of a new cell phone desgn wth splt steel bands. The fnte element model of the assembly was developed by usng ASA, the state-of-the-art pre-processor, and analyzed wth LS-DYA. The unt was dropped on a grante floor from the heght of meter wth dfferent orentatons, such as face drop, edge drop and corner drop. Focus was pad on some key components. The ntegrty of the splt band was nvestgated carefully; the stresses for cover glass and LCD layers were evaluated numercally; and the shock absorbng performance of dfferent vsco-elastc pads attached on camera was compared n detals.. Introducton The most common falure of cellular phones s probably the mpact due to accdental drop, so the manufacturers am to develop products that are mpact resstant and can survve the drop test from specfc heght. Both physcal test and fnte element (FE) smulaton are employed n the ndustry at dfferent stages. At the desgn stage, FE method plays a more mportant role snce actual drop test can only be conducted at the end of desgn cycle and provdes lttle feedback to mprove the desgn. The FE smulaton wll help desgners understand how components nteract n the assembly and how the falure mode and mechansm s developed under the mpact loadng []. Physcal test should be conducted on prototypes snce feld tests are stll the most relable source of nformaton of the assembly durng drop test and the only method of fnal valdaton of the FE analyss. Hgh speed cameras are set up to montor the unt response, and the dropped unt s dsassembled carefully to check f cracks develop on some key components such as cover glass, back housng and lqud crystal dsplay (LCD) layers. Ths paper dscusses the drop test smulaton of a new cell phone desgn whose steel band s splt nto three segments. Attenton was pad on the ntegrty of the splt band, the stresses for the cover glass and LCD layers, and the mpact load on the camera module. LS-DYA, the non-lnear explct FEA code was selected for the analyss due to ts robust capablty of handlng mpact phenomenon. The FE model, ncludng around 550,000 elements wth more than 30 components, was developed by usng ASA, the state-of-the-art pre-processor n whch dfferent engneers mesh work was ncorporated and managed effcently; hgh qualty meshes were generated quckly, and the model was assembled and modfed convenently. Wth lttle modfcaton, the nput deck of ABAQUS for statc strength analyss was generated wth the same model usng ASA. The statc analyss wll not be dscussed n ths paper. 2. Development of the FE model The steel band was dvded nto three segments whch were connected by three plastc nserts as shown n Fg..Ths feature brought more flexblty to the assembly and consequently had effects on the unt s dynamc response durng drop tests.

2 4 th ASAA & μeta Internatonal Conference Fg. The splt band and plastc nsert. The unt was dropped on grante floor from the heght of m at dfferent orentatons, ncludng face drops, edge drops, corner drops, and tlt face drop as shown n Fg. 2. Fg. 2 Drop orentatons. The components were meshed by dfferent engneers and were assembled n ASA. Based on the geometrc characterstcs of each part, dfferent element types n LS-DYA were used. Hexahedral elements were used for the parts whch could be meshed wth brck elements wthn reasonable tme even though addtonal efforts were needed, snce hexahedral elements have better performance than tetrahedral elements n explct codes. Some parts wth hex meshes are shown n Fg.3.

3 4 th ASAA & μeta Internatonal Conference Fg. 3 Hex mesh for some components. It was very dffcult, f not mpossble, to hex-mesh some parts wth complcated geometry, so they were represented by second order tetrahedral elements. Shell elements were used by many engneers to model cover glass and LCD glass layers, but they were not employed n ths model. The reason lay n the fact that shell elements performed satsfactorly to epresent bendng modes of layers durng face drops, but caused numercal problem n the edge and corner drops n whch shell edges and corners contacted wth other components. Four layers of hex elements were desgned along the thckness of each glass layer to correctly epresent ts bendng stffness. Beam elements were used for smplfed jonts, whch was the only choce to nclude tny structures lke screws and clps nto a system level model [2]. The representaton was proven effectve and accurate enough. All meshes were verfed for consstences and refned to mprove ther qualty. Decsons regardng materal models, contact algorthms, multpont constrants (MPC), loadng and boundary condtons, and soluton parameters were determnedd to complete the whole model of the phone. Tme ncrement was set to E-8s wth mass scalng. It was found that the added mass was less than 2% of the total weght of the unt. The FE model was completely managed n ASA. Some components had several versons of desgn. It was convenent and effcent to change the model by replacng some parts wth ther modfed versons. It sgnfcantly reduced the perod of run-model, modfy-model and rerun-model process, and helped the desgners quckly fgure out the approprate desgn. Fg. 4 presents the FE model of the assembly.

4 4 th ASAA & μeta Internatonal Conference Fg. 4 FE model of the assembly. 3. Dscusson of the results 3. Stresses for plastc nsertss and glass layers From the smulaton results, t was found that the splt band ddd cause hgher stresses for the cover glass and other LCD layers compared wth the results from the model wth a contnuous band. However, wth a good desgn of the plastc nserts and the thckness of glass layers, the peak maxmum prncpal stresses for glass layers could be controlled under the materal s break stress for all drop orentatons. Fg. 5 shows the stress contour of a plastc nsert and Fg. 6 showss the stress contour of the cover glass. Fg. 5 Stress contour for a plastc nsert durng back face drop. Fg. 6 Stress contour for the cover glass durng back face drop.

5 4 th ASA & μeta Internatonal Conference In drop test smulaton, the band splts were typcally under a combned loadng condton of bendng and twstng. These loadng condtons are more representatve what the assembly wll experence durng ts testng and/or usage. For the splt at top as shown n Fg., the typcal loadng condton s bendng along longtudnal and/or vertcal axs, and twstng along transverse axs. For the two splts at sde, the typcal loadng condton s bendng along transverse axs and twstng along longtudnal axs. 3.2 Impact loadng on the camera module Hgh mpact force on camera module was observed n face tlt drop. In order to ensure the camera stll functoned properly after the mpactng, a pad made of vsco-elastc materal was attached on the camera module to attenuate the shock. The camera module s shown n Fg. 7. Fg. 7 Camera module. In LS-DYA, the key words *MAT_GEERAL_VISCOELASTIC s used to defne vscoelastc materal by nputtng the Prony seres [3]: g R t / τ () t = g ( e ) Where g R s dmensonless relaxaton modulus, and g and τ are materal parameters. The vsco-elastc behavor of a materal s often determned from dynamc vbraton experments. In these experments the materal s exposed to small stran vbratons and the resultng storage modulus G and loss modulus G are determned as a functon of the appled frequency ω. The tested storage modulus and loss modulus for the selected materal s shown n Fg. 8. () Fg. 8 Tested storage modulus and loss modulus for the materal.

6 4 th ASA & μeta Internatonal Conference The relatonshp between dynamc frequency data and Prony seres data s [4]: ' G ( ω) = G 0 '' G ( ω) = G 0 = g + G g τ ω = 0 = 2 g τ ω τ ω τ ω When the storage and loss modulus are known, the parameters G 0, g and τ are optmzed so that the resdual between the calculated modulus and expermental modulus reaches mnmum. A more convenent method s to use ABAQUS s materal evaluaton feature that can quckly perform the data converson. Fg. 9 shows the acceleraton hstory of the camera lens durng face tlt drop. 0,000Hz flter was appled to the data to get rd of nose. (2) Fg. 9 Acceleraton hstory of the camera lens durng face tlt drop. The peak acceleraton s around 30,000g whch s 30% lower than the value when non vscoelastc materal was used. 4. Summary and Conclusons In ths project, drop test smulatons of a new desgn of cell phone were conducted by usng ASA and LS-DYA. The FE model was developed and managed n ASA, an advanced CAE pre-processng tool that provded all the necessary functonalty for full-model buld up and management, from CAD data to ready-to-run solver nput fle. It sgnfcantly speeded up the desgn process. Dfferent drop orentatons were analyzed. The ntroducton of splt band dd ncrease the stresses for the cover glass and other glass layers. However, by approprately desgnng the splts and plastc nserts and carefully selectng the thckness of glass layers, the peak tensle stresses could be controlled below the materal s break stress. Shock absorbng pad was attached on the camera to reduce the mpact loadng on the lens. The dynamc testng data of the materal were converted to Prony seres as the nput for LS-DYA. The results showed that the vsto-elastc pad sgnfcantly reduced the mpact loadng on the camera lens.

7 4 th ASA & μeta Internatonal Conference References []. Anders Harrysson, drop test smulaton of cellular phone, Master s thess, [2]. Jason Wu, Advanced modelng and drop smulaton wth new features of LS-DYA. 9 th Internatonal LS-DYA users conference, p [3]. LS-DYA Theoretcal Manual, Lvermore Software Technology Corporaton, Lvermore, Calforna [4]. Abaqus Analyss User's Manual (verson 6.0), Dassault Systèmes Smula Corp., Provdence, RI, USA. 200.