Yasushi Miyano

Prediction of Flexural Fatigue Strength of CRFP Composites under Arbitrary Frequency, Stress Ratio and Temperature

This paper deals with fatigue strength of a class of CFRP laminates satisfying certain hypotheses and proposes a prediction method of the fatigue strength at an arbitrary combination of frequency, stress ratio and temperature. Three-point bending tests were conducted for satin-woven CFRP laminates T400/3601 under static, creep and fatigue loadings. The experimental data support the prediction method and the hypotheses.

Loading Rate and Temperature Dependence on Flexural Fatigue Behavior of a Satin Woven CFRP Laminate

Generally, the mechanical behavior of polymer resins depends remarkably on time and temperature, which is called viscoelastic behavior. Thus, it can be presumed that the mechanical behavior of a CFRP using polymer resins is also time and temperature dependent. In this paper, the loading rate and temperature dependence on flexural fatigue behavior of satin woven CFRP laminates consisting of a matrix resin with a high glass transition temperature is investigated. The results show that not only flexural static strength but also flexural fatigue strength of the CFRP laminate depends on time and temperature even at temperatures that are much lower than the glass transition temperature. The reciprocation law of time and temperature was found to be applicable for both the flexural static and fatigue strengths. The fatigue and static fracture modes were found to be similar for the wide ranges of time and temperatures tested. The slope of the S-N curves also remains the same despite the large temperature range and two frequencies tested. From the above results a prediction method for flexural fatigue strength at an arbitrary temperature and frequency is proposed.

Role of Matrix Resin on Fracture Strengths of Unidirectional CFRP

As a common feature of composite materials, prominent anisotropy in mechanical prop erties is observed in unidirectional CFRP, which has higher fracture strength and stiffness along the carbon fiber strengthening component. Since it is well known that a resin matrix exhibits characteristic time and temperature dependence on mechanical behavior, that is, viscoelastic behavior, the CFRP is expected to exhibit similar behavior.

Accelerated Testing for Long-term Durability of FRP Laminates for Marine Use:

The prediction of long-term fatigue life of various FRP laminates combined with resins, fibers and fabrics for marine use under temperature and water environments were performed by our developed accelerated testing methodology based on the time-temperature superposition principle (TTSP). The base material of five kinds of FRP laminates employed in this study was plain fabric CFRP laminates T300 carbon fibers/vinylester (T300/VE). The first selection of FRP laminate to T300/VE was the combinations of different fabrics, that is flat yarn plain fabric T700 carbon fibers/vinylester (T700/VE-F) and multi-axial knitted T700 carbon fibers/vinylester (T700/VE-K) for marine use and the second selection of FRP laminates to T300/VE was the combinations with different fibers and matrix resin, that is plain fabric T300 carbon fibers/epoxy (T300/EP) and plain fabric E-glass fibers/vinylester (E-glass/VE). These five kinds of FRP laminates were prepared under three water absorption conditions of Dry, Wet and Wet C Dry after molding. The three-point bending constant strain rate (CSR) tests for these FRP laminates at three conditions of water absorption were carried out at various temperatures and strain rates. Furthermore, the three-point bending fatigue tests for these specimens were carried out at various temperatures and frequencies. The flexural CSR and fatigue strengths of these five kinds of FRP laminates strongly depend on water absorption as well as time and temperature. The mater curves of fatigue strength as well as CSR strength for these FRP laminates at three water absorption conditions are constructed by using the test data based on TTSP. It is possible to predict the long term fatigue life for these FRP laminates under an arbitrary temperature and water absorption conditions by using the master curves.

Mechanism of Strength Improvement of Foamed Plastics Having Fine Cell

The influence of cell size on tensile fracture strength of foamed plastics is examined in this study. Foamed specimens having various cell sizes with the same foaming magnification are molded. The tensile fracture strength of foamed plastics is measured and the relationship between the cell size and tensile fracture strength is discussed. It is found that the tensile fracture strength of foamed polyethyleneterephthalate and polypropylene which are crystalline resins increases with decrease in cell size more than the strength presumed from the foam magnification. The decrease in cell size results in an increase in cell density. An increase in cell surface area causes molecular orientation thereby effecting strength improvement.

Prediction of Tensile Fatigue Life for Unidirectional CFRP

Tensile fatigue strength in unidirectional CFRP depends on time and temperature as well as number of cycles. A prediction method of fatigue strength proposed [14] for polymer composites meeting certain conditions and confirmed for flexural fatigue strength of satin-woven CFRP laminates is applied to the tensile fatigue life of unidirectional CFRP. The method is based upon the four hypotheses: (A) same failure mechanisms for constant strain-rate (CSR), creep, and fatigue failure, (B) same time-temperature superposition principle for all failure strengths, (C) linear cumulative damage law for monotonic loading, and (D) linear dependence of fatigue strength upon stress ratio. Data are provided for tensile CSR, creep, and fatigue tests at various temperatures in the longitudinal direction of unidirectional CFRP. Experimental verification of the prediction method for the tensile fatigue strength of the unidirectional CFRP is presented.

Prediction of tensile fatigue life under temperature environment for unidirectional CFRP

A method for prediction of fatigue strength under temperature environment was proposed for polymer composites and its validity was confirmed for the flexural fatigue strength of satin-woven CFRP laminates and others. This method is based upon the four hypotheses: (A) same failure process under constant strain-rate (CSR), creep, and fatigue loadings, (B) same time-temperature superposition principle for all failure strengths, (C) linear cumulative damage law for a nondecreasing stress process, and (D) linear dependence of fatigue strength upon stress ratio. Data are provided for tensile CSR, creep, and fatigue tests at various loading rates, frequencies, and temperatures in the longitudinal direction of unidirectional CFRP. In this paper, experimental verification of the prediction method is discussed for the tensile fatigue strength of unidirectional CFRP.

Formulation of Long-term Creep and Fatigue Strengths of Polymer Composites Based on Accelerated Testing Methodology

The applicability of our developed accelerated testing methodology (ATM) based on the time—temperature superposition principle (TTSP) and others for the prediction of long-term fatigue life of polymer matrix composites is performed experimentally and theoretically. A formulation method of master curves of creep and fatigue strengths for polymer matrix composites is introduced based on ATM. The master curves of creep and fatigue strengths for the typical three directions of unidirectional CFRP, that is the longitudinal tensile and compressive loadings and the transverse tensile loading, are formulated using the data measured based on ATM.

Applicability of Fatigue Life Prediction Method to Polymer Composites

A prediction method of fatigue strength under an arbitraryfrequency, temperature, and stress ratio is proposed for polymercomposites and its validity is confirmed for the flexural fatiguestrength of satin-woven CFRP laminates. This method is based upon fourhypotheses: (a) same failure process under constant strain-rate (CSR),creep, and fatigue loadings, (b) same time-temperature superpositionprinciple for all failure strengths, (c) linear cumulative damage lawfor nondecreasing stress process, and (d) linear dependence of fatiguestrength upon stress ratio. This method was applied to the flexuralfatigue strength of various unidirectional CFRPs, and the verificationand limitations of this method were discussed.

Residual-stress analysis of an epoxy plate subjected to rapid cooling on both surfaces

This paper presents theoretical and experimental methods of finding the residual stress in an epoxy plate subjected to rapid cooling on both surfaces. The theoretical residual-stress distributions in a plate are calculated by using the fundamental equations based on the linear viscoelastic theory. The specimens in the experiment are subjected to rapid cooling. The residual stresses are measured by the layer-removal method. The theoretical and experimental results are compared and discussed.