مطالعه ي تجربي رفتار نرم‌شوندگي پلي‌استال تحت بارگذاري محوري تناوبي جابه‌جايي ـ كنترل

Experimental Investigation On The Softening Behavior Of Polyacetal Under Cyclic Axial Strain-Controlled Loading

در اين مقاله، رفتار نرم‌شوندگي ماده‌ي پلي‌استال (POM) تحت بارگذاري متناوب تك‌محوره جابه‌جايي ـ كنترل مورد بررسي آزمايشگاهي قرار گرفته است. آزمايش‌هاي تجربي توسط دستگاه سروهيدروليك اينسترون 8802 و در چهار گروه با نسبت كرنش كمينه به بيشينه‌‌‌ي متفاوت انجام شده است. در اين آزمايش‌ها، كميت هاي تنش ميانگين، حدود تنش، شيب حلقه‌هاي هيسترزيس، چگالي انرژي كرنشي اتلافي، و دامنه‌ي چگالي انرژي كرنشي ـ كشساني بررسي شده است. نتايج حاصله نشان مي‌دهد كه رهايش تنش ميانگين در صورت صفربودنِ كرنش ميانگين مقداري ناچيز است، و كرنش ميانگين مثبت باعث افزايش سرعت رهايش تنش ميانگين مي‌شود. چگالي انرژي كرنشي اتلاف شده و دامنه‌ي چگالي انرژي كرنشي كشساني براي انواع بارگذاري با افزايش تعداد سيكل به مقدار پايداري مي رسند. همچنين چگالي انرژي كرنشي اتلافي در بارگذاري متناوب كششي ـ فشاري بيشتر از بارگذاري متناوب كششي است.

In this paper, the softening behavior of polyacetal (POM) under strain-controlled cyclic axial loading is studied. All experimental tests were performed in four groups with different ratios of minimum to maximum strains by a servo-hydraulic INSTRON 8802 machine, to a specified number of cycles. The system was attached to a computer for test control, as well as for data acquisition. For each selected loading cycle, the computer acquired 100 data points per cycle, and those data can be used for further analysis. In these tests, quantitative parameters of mean stress, stress range, slope of stress-strain hysteresis loops, dissipated strain energy density, and elastic strain energy density amplitude have been investigated. The mechanical properties of specimens were determined according to simple tension and compression tests. It is observed that there is different behavior in compression and tension for polyacetal. Under displacement control loading, softening behavior is observed. The results showed that relaxation of mean stress was negligible under cyclic loading with zero mean strain, and due to positive mean strain, the rate of relaxation of mean stress increased. Minimum compressive strain caused the stress range to be steady, and the rate of stress range became zero. Dissipated strain energy density and elastic strain energy density amplitudes have been stabilized with an increasing number of cycles for all types of loading. Also, dissipated strain energy density under tension-compression cyclic loading is higher than that under tension cyclic loading. The hysteresis loop slope is the gradient of the line that connects the extreme points of the hysteresis loop together. At the same ratio of minimum to maximum strain, the hysteresis loop slope is lower at any cycle for higher strain amplitude and higher mean strain. Also, it is observed that the maximum rate of hysteresis loop slope occurs under symmetric displacement control loading, with the ratio of minimum to maximum strain equal to -1.