آنتروپي توليدي مبنايي براي پيش‌بيني كمانش ناشي از جوشكاري در پوسته‌هاي نازك

WELDING BUCKLING PREDICTION OF THIN SHELLS BASED ON ENTROPY GENERATION

در اين پژوهش روشي براي پيش‌بيني امكان كمانش ناشي از جوشكاري پوسته‌هاي نازك برمبناي آنتروپي توليدي به دست آمده است. در اين روش با بهره‌گيري از تحليل حرارتي سه‌بعدي و گذرا، كل آنتروپي توليدي در فرايند جوشكاري تعيين مي‌‌شود و با استفاده از تحليل سه‌بعدي مقادير ويژه‌ي كمانش، ميزان آنتروپي لازم براي شكل‌گيري اولين مد كمانش قطعه كار محاسبه مي‌شود. با مقايسه‌ي اين دو آنتروپي مي‌توان احتمال كمانش قطعه كار را بررسي كرد. درستي پيش‌بيني براساس تحليل سه‌بعدي اجزا محدود جوش (حرارتي ـ مكانيكي) ــ كه خود توسط نتايج آزمايشگاهي تاييد شده ــ بررسي و تاييد شده است. زمان لازم براي اجراي روش پيشنهادي در مقايسه با روش‌هاي تحليلي موجود كم‌تر است، زيرا در اين روش تحليل مكانيكي جوش حذف شده است.

Welding has been employed at an increasing rate for its advantage in high joint efficiency, water and air tightness, weight saving, reduction in fabrication cost and design flexibility. However, welded structures are not free from problems. Residual stress and distortion are major difficulties with welded structures. The buckling distortion of welded structures caused by welding stress is seen in relatively thin welded structures. Buckling distortion causes loss of dimensional control, structural integrity and increased fabrication costs. Thus, it is necessary to achieve a predictive analysis technique to determine the susceptibility of a particular design to buckling distortion. This paper presents a predictive buckling distortion method to determine if welded structures buckle during welding or not. In this method, a 3D thermal transient process is coupled to a 3D eigenvalue analysis to determine the total entropy generation due to welding and critical total entropy generation, respectively. In this work, because of the small size of the melted region (weld pool), the heat source model developed by the "moving isotherm pool" is used. Welding time in this work is calculated by dividing the welder speed obtained from the practical welding characteristics data by the length of the welded region. The temperature dependent thermal properties of material were also incorporated into the model. In this research, temperature distribution producing the minimum value of longitudinal stress to cause the buckling of welded shells is predicted by 3D eigenvalue analysis. This temperature distribution creates negative longitudinal strains in the welded region, causes tensile longitudinal stress in regions close to the weld line and compressive stress in regions away from the weld line. Total entropy generation, due to this temperature distribution, which is named critical entropy (Scri), is calculated. The comparison between total entropy generation due to completely welding shells (Sw) and critical entropy is a good criterion to predict the possibility of buckling welded shells. In this method, the computational time is much less than in the previous methods because of omitting the mechanical analysis of welding. The correctness of this predictive method is proved by analytical results.