بررسي اثر نقص هندسي ريزساختار در رفتار تك محوري فوم هاي سلول‌ باز

Effect of Geometrical Imperfection of the Microstructure on the Uniaxial Behavior of Open-Cell Foams

امروزه فوم ها به‌منزله‌ي نسل جديدي از مصالح سبك در صنايع خودرو سازي و هوافضا مورد توجه قرار گرفته اند. در اين پژوهش، اثر نقص در هندسه ي ريزساختار در رفتار فوم سلول باز مورد مطالعه قرار گرفت. به اين منظور، از روش همگن‌سازي، به عنوان راهكاري مناسب براي ارتباط ويژگي هاي مكانيكي مواد در مقياس هاي بزرگ (جسم) و كوچك (ريزساختار) بهره گرفته شد. به اين ترتيب كه ابتدا با بازتوليد يك مدل همگن سازي موجود، رفتار تك محوري نمونه ي فوم با ساختار منظم محاسبه شد. در ادامه، با اعمال آشفتگي در هندسه ي ريزساختار، تغييرات خصوصيات تك محوري فوم نسبت به حالت منظم مورد بررسي قرار گرفت. نتايج به‌دست‌آمده، محدوده ي تغيير كميت هاي تك محوري فوم را با افزايش آشفتگي در ريزساختار مشخص كرد. همچنين، حالت هايي از تغيير هندسه ي ريزساختار، كه سبب بيشترين تغييرات در كميت هاي تك محوري فوم مي شوند، شناسايي شدند.

Solid foams are novel materials with outstanding characteristics that have found numerous applications in the construction of light, stiff structures during the past two decades. At the micro-level, solid foams benefit from a cellular structure, which is composed of either closed or open cells. Open-cell foams, which were investigated in this study, are described as a network of inter-connected struts (or ligaments) with beam-like action. The mechanical design of foams requires their mechanical properties to meet certain criteria. These criteria can be satisfied by careful selection of the base material and the microstructural geometry. However, the selected material has to be manufactured, and manufacturing is by no means a perfect process. As a result, there is always a threat that the properties of the final material differ from the ones expected. The current study addresses this issue by exploring the effect of geometrical imperfections on the uniaxial properties of regular open-cell foams. Such an investigation requires an appropriate approach, capable of relating the microstructural characteristics to the effective behavior of the material. An existing homogenization model was used for this purpose. The model assumes the open-cell foam to be constructed from a repetition of the same unit-cell. The unit-cell in its original state has a regular structure, i.e. it comprises four identical members forming equal angles with respect to each other; the point at which these members coincide is called the unit-cell vertex. Imperfection was introduced in the form of small perturbations in the position of the unit-cell vertex. Accordingly, homogenization was employed to obtain the uniaxial stress-strain response of various perturbed microstructures. The results were then quantified in terms of four uniaxial parameters. The next step was to compare these parameters with their counterparts that had been obtained for the regular microstructure. The consequence of this was to identify how the uniaxial response of originally regular, open-cell foam could vary when its microstructure was subjected to geometrical imperfections. In particular, this investigation revealed that different uniaxial parameters altered linearly (or pseudo-linearly) as the perturbation of the microstructure increased.