Previous experiments have shown a high degree of variation in viscoelastic creep strain measurements of polymer matrix composites (PMCs). Engineering design involving material properties of PMCs should be performed with this variability in consideration. Therefore, this paper presents an approach to characterize the longitudinal relaxation modulus of a vinyl ester polymer (DERAKANE 441-400) with variations in its material properties. A spectrum approach is used to describe viscoelastic material functions; a distribution function with two parameters is selected. Short-term tensile creep experiments were conducted at four temperatures below the glass transition temperature. In-plane longitudinal and transverse strain-time histories were measured using a digital image correlation technique. The variation in the relaxation modulus was described by formulating the probability density functions (PDFs) and the corresponding cumulative distribution functions (CDFs) of the moduli using a twoparameter Weibull distribution. Both Weibull shape (γ) and scale (β) parameters of the relaxation moduli distributions were shown to be time and temperature dependent. Two-dimensional quadratic Lagrange interpolation functions were used to characterize the Weibull parameters to obtain the PDFs and subsequently the CDFs of the creep compliances for the complete design temperature range (24°C ≤ T ≤ 60°C) during steady state creep (t ≥ 1000 s). At each test temperature, relaxation modulus curves were obtained for constant CDF values and compared with the experimental data. The predicted relaxation moduli of a vinyl ester polymer in the design space are in good agreement with the experimental data.
In this study, the fatigue behavior of a polyether ether ketone (PEEK) polymer was investigated under uniaxial strain-controlled conditions with and without mean strain. Experimental study included a series of fully-reversed fatigue tests at five strain amplitudes (2%, 2.5%, 3%, 3.5%, and 4%) and fatigue tests at three mean strain values (2%, 2.5%, and 3%) at various frequencies. Stress responses of PEEK obtained from the tests under fully-reversed cyclic loading condition indicated a significant cyclic softening, while a stress relaxation was observed for the PEEK specimens under mean strain cyclic loading. Two different types of fatigue models, including a strain-based (Coffin-Manson) and energy-based, were employed to correlate fatigue life obtained from all experimental data. Among the two fatigue models, the fatigue life prediction using the energy-based approach was found to provide a better correlation to PEEK experimental data for both fully-reversed and mean strain conditions when compared to the Coffin-Manson model.
An overview on recent research efforts is presented to obtain an understanding on the fatigue behaviour and failure mechanisms of metallic parts fabricated via powder-based additive manufacturing (AM) processes, including direct energy deposition (DED) and powder bed fusion (PBF) methods, utilizing either laser or electron beam as an energy source. Some challenges inherent to characterizing the mechanical behaviour of AM metals under cyclic loading are discussed, with emphasis on the effects of residual stresses on their fatigue resistance. In addition, an aspect pertaining to the structural integrity of AM parts relating to their fatigue behaviour at very high cycles is presented and compared with those of the conventionally-manufactured counterparts.
In this article, the data obtained from the uniaxial fully-reversed fatigue experiments conducted on polyether ether ketone (PEEK), a semi-crystalline thermoplastic, are presented. The tests were performed in either strain-controlled or load-controlled mode under various levels of loading. The data are categorized into four subsets according to the type of tests, including (1) strain-controlled fatigue tests with adjusted frequency to obtain the nominal temperature rise of the specimen surface, (2) strain-controlled fatigue tests with various frequencies, (3) load-controlled fatigue tests without step loadings, and (4) load-controlled fatigue tests with step loadings. Accompanied data for each test include the fatigue life, the maximum (peak) and minimum (valley) stress–strain responses for each cycle, and the hysteresis stress–strain responses for each collected cycle in a logarithmic increment. A brief description of the experimental method is also given.
This paper describes the details of an experimental investigation focusing on the vibration characteristics of a composite fuselage structure of an ultralight unmanned aerial vehicle (UAV). The UAV has a total empty weight of 70.3 kg and 6.3 m in length. The fuselage structure consists of the fuselage body with an integrated vertical stabiliser. All structural components are fabricated from oven-cured laminated carbon composite materials using uniaxial and biaxial prepreg fabric. The modal characteristics of the fuselage structure are determined for a free-free configuration which is simulated by suspending the structure from its wing attachment points through the use of springs. A centrally located shaker system is used to induce vertical oscillations in the structure, which is instrumented with nineteen dual axis accelerometers. Dynamic properties such as the frequency, damping and associated mode shapes are obtained for aeroelastic analysis. The design and implementation of the vibration tests along with the experimental results are presented.
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