25878

پيش‌بيني عمر خستگي چندلايه‌هاي متعامد كربن اپوكسي با در نظر گرفتن اثرات خستگي‌استاتيكي تحت بار چرخه‌اي دامنه ثابت

دكتري

مهندسي مكانيك

1394

1400/5/11

دكتر فتح اله طاهري بهروز

مهندسي مكانيك

پديده خستگي‌استاتيكي دركامپوزيت‌ها به صورت يك بارگذاري خستگي با نسبت‌تنش1=R فرض شده و معادل با پديده خزش و وابسته به زمان مي‌باشد. در بارگذاري‌هاي چرخه‌اي با ميانگين تنش غيرصفر، اثرات خستگي‌استاتيكي در پاسخ چندلايه كامپوزيتي اثر مي‌گذارد. هدف اين رساله پيش‌بيني عمر خستگي چندلايه‌هاي متعامد كربن اپوكسي با درنظرگرفتن اثرات خستگي‌استاتيكي تحت بار چرخه‌اي كشش-كشش دامنه ثابت مي‌باشد. از اين رو بر مبناي نتايج تحقيقات موجود در منابع و همچنين مشاهدات تجربي پژوهشگر يك مدل آسيب پيش‌رونده خستگي با درنظرگرفتن اثرات خستگي‌استاتيكي پيشنهاد شده است. به‌اين‌منظور مدل آسيب پيش‌رونده خستگي بر پايه مدل افت تدريجي استحكام لايه‌هاي صفر درجه به عنوان لايه‌هاي بحراني، مدل تنش-كرنش هم‌زمان با افت تدريجي سفتي لايه‌هاي 90 درجه به عنوان لايه‌هاي زيربحراني و همچنين مدل پيش‌بيني عمر وابسته به چرخه و وابسته به زمان براي كامپوزيت تك جهته توسعه داده شده است. با انجام آزمايش‌هاي شتاب‌يافته خزشي تاثير بارگذاري استاتيكي بر افت تدريجي استحكام الياف لايه تك‌جهته مورد مطالعه قرار گرفت و يك مدل پديده‌شناختي براي پيش‌بيني استحكام باقيمانده خستگي‌استاتيكي (1=R) برحسب مدت زمان بارگذاري براي سطح تنش دلخواه توسعه داده شد. مي‌توان افت استحكام خستگي‌چرخه‌اي با نسبت‌تنش‌هاي دلخواه و افت تدريجي استحكام خستگي‌استاتيكي را به‌وسيله اين مدل پيش‌بيني كرد. براي پيش‌بيني عمر در وضعيت تنش دلخواه، يك مدل عمر تجربي به‌صورت تابعي خطي برحسب نسبت‌تنش (R) با استفاده از منحني‌هاي تجربي عمر خستگي‌چرخه‌اي (0/1=R) و عمر خستگي‌استاتيكي (1=R) در مقياس زمان پيشنهاد شد. يك مدل تنش-كرنش هم‌زمان براي پيش‌بيني تأثير رفتار وابسته به زمان و وابسته به چرخه بر محاسبه توزيع تنش در هر چرخه بارگذاري خستگي توسعه داده شد. در اين مدل خستگي‌چرخه‌اي به دليل رشد آسيب‌ها منجر به كاهش شيب ابتداي نمودار تنش-كرنش شده و تاثير خستگي‌استاتيكي به شكل غيرخطي‌شدن منحني تنش-كرنش ظهور مي‌كند. مدل آسيب پيش‌رونده خستگي با درنظرگرفتن اثرات خستگي‌استاتيكي به‌صورت يك زيرروال ماده در نرم‌افزار اجزامحدود آباكوس يكپارچه شده و امكان تحليل خستگي اجزامحدود براي سازه‌هاي پيچيده را نيز فراهم ميكند. پس از صحت‌سنجي عملكرد اجزا مدل، مدل اجزامحدود توسعه داده شده بر پايه مدل آسيب پيش‌رونده خستگي براي پيش‌بيني عمر چندلايه‌هاي متعامد [0/903]s كربن اپوكسي T300/L20 در دو نسبت‌تنش 0/1 و 0/25 استفاده شد. مقايسه منحني تجربي تنش-عمر خستگي چندلايه متعامد [0/903]s كربن اپوكسي T300/L20 در دو نسبت‌تنش مختلف با نتايج پيش‌بيني شده به‌وسيله مدل اجزامحدود دقت پيش‌بيني 88 و 82 درصدي به ترتيب براي دو نسبت تنش 0/1 و 0/25 به‌دست مي‌آيد كه نشان‌دهنده كارآمد بودن مدل توسعه‌داده‌شده براي پيش‌بيني عمر خستگي با در نظر گرفتن اثرات خستگي‌استاتيكي مي‌باشد.

1400/10/25

Fatigue Life Prediction of Carbon/epoxy Cross-ply Laminates Considering the Effects of Creep (Static-Fatigue)

8/2/2022 12:00:00 AM

Various modes of damage and degradation mechanisms are involved in the cyclic loading of composite laminates. The static-fatigue is assumed to be a fatigue load with a stress ratio of R=1 and is equivalent to the creep phenomenon, which is time dependent. In cyclic loads with non-zero mean stress, the static-fatigue affects the composite laminate response, including the mean strain growth and the rate of gradual degradation of mechanical properties. The aim of this thesis is to predict the fatigue life of crossply carbon/epoxy laminates taking into account the static-fatigue effects under tensile-tensile constant amplitude cyclic loading. Therefore, based on data in the literature as well as the experimental observations of the author, a progressive fatigue damage model has been proposed, taking into account the effects of static-fatigue. The model is developed based on the main damage modes of cyclic-fatigue and static-fatigue in crossply laminates to consider the effects of cycle- and time-dependent phenomena on the fatigue life of crossply, simultaneously. Therefore, the progressive fatigue damage model is developed based on a) the gradual strength degradation in fiber direction of on-axes plies as critical layers, b) a nonlinear isochronous stress-strain model with gradual stiffness degradation in matrix direction of off-axes plies as subcritical layers, and c) a cycle- and time-dependent fatigue life prediction model for unidirectional plies of laminates under arbitrary stress ratios. By designing fatigue experiments, loading parameters such as stress amplitude, mean stress and loading frequency for unidirectional and crossply [0/903]s of T300/L20 carbon/epoxy laminates have been studied experimentally and the effects of these factors on the short-term cyclic mean strain development, cyclic strain amplitude growth and sample surface temperature were investigated. From the results of cyclic mean strain growth which is related to the static-fatigue and strain amplitude as an indicator of cyclic damage growth, in different designed experiments, it was observed that static-fatigue is related to the average cyclic stress and loading time, while it is independent of cyclic stress amplitude and loading frequency. Sample surface temperature was also measured during cyclic tests using a thermography camera to separate thermal strains from mean strain and strain amplitude developments. Accelerated creep experiments were conducted to study the effect of sustained loading on the gradual strength degradation in fiber direction of unidirectional composites. A phenomenological model was developed to predict the static-fatigue (R=1) residual strength in terms of loading time for the arbitrary stress levels. The performance of this model was evaluated with experimental data of T300/L20 carbon/epoxy UD laminates and data available in the literature for Kevlar/epoxy and S-glass/epoxy strands. The results of cyclic-fatigue (R=0.1) residual strength of T300/L20 carbon/epoxy UD laminates were compared with the results of static-fatigue (R=1) residual strength data on the loading time scale. It was observed that by accurately predicting fatigue life in terms of time (in any arbitrary state of stress), the cyclic-fatigue and static-fatigue residual strengths could be predicted with a same equation. To predict life in the arbitrary state of stress, a fatigue life model was proposed as a linear function of stress ratio (R) using the experimental cyclic-fatigue life curve (R=0.1) and static-fatigue life curve (R=1). The capability of this model was evaluated separately with the results of glass/polyester laminates in several stress ratios. Also, the performance of the residual strength model in combination with the life prediction model was validated with the results of the residual strength experiments of T300/L20 carbon/epoxy unidirectional laminates under several stress ratios. An isochronous stress-strain model was developed to predict the effect of time- and cycle-dependent behavior on the calculation of the stress distribution in every simulated load cycle. In this model, the growth of damages due to cyclic-fatigue leads to a reduction in the initial slope of the stress-strain curve and the effect of static-fatigue appears in the form of nonlinearity of the stress-strain curve by time. The validity of this model has been examined with the available results in the literature for short fiber glass/polyamide6 and carbon/epoxy angle-ply laminates. The progressive fatigue damage model taking into account the effects of static-fatigue is integrated into the ABAQUS finite element software through a user-defined material (UMAT) subroutine. Finite element method allows fatigue analysis of complex structures. The nonlinear stress analysis, gradual strength degradation in fiber direction, gradual stiffness degradation in matrix direction and cyclic mean strain growth prediction of finite element model was verified. Then the developed finite element model based on the progressive fatigue damage model was employed to predict the fatigue life of T300/L20 carbon/epoxy [0/903]s laminates in two stress ratios 0.1 and 0.25. Comparison of the experimental S-N curves of T300/L20 carbon/epoxy [0/903]s laminates in two different stress ratios with the results predicted by the finite element model, shows the acceptable efficiency of the developed model for predicting fatigue life of carbon/epoxy crossply laminates considering the effects of static-fatigue. The r-squared of model predictions are 0.88 and 0.82 for stress ratios 0.1 and 0.25, respectively.

آسيب پيش‌رونده خستگي , خستگي‌استاتيكي , استحكام باقيمانده , سفتي باقيمانده , كامپوزيت كربن اپوكسي , اجزامحدود , خزش , خستگي

, Progressive fatigue damage , static-fatigue , residual strength , residual stiffness , carbon/epoxy composites , finite element , creep , fatigue