Influence Of Specimen Length Research Articles

Experimental characterization and modeling of GF/PP commingled yarns tensile behavior

Three-dimensional (3D) fabrics of commingled yarns offer the possibility of a low-cost and fast manufacturing of complex-shaped composite parts. Textile-reinforcement behavior during the forming process is very important since the appearance of defects (for example wrinkles, yarn misalignment or breakage) can significantly affect the mechanical properties of the final part. Experimental characterization of the mechanical behavior of textile-reinforcements is expensive, time consuming, and a large scattering of results is often observed. To overcome this, meso-scale modeling is an interesting method to study and understand the textile behavior at the unit cell level. To perform realistic simulations, an accurate modeling and, therefore, knowledge of the yarn mechanical behavior are needed. In this paper, a simple protocol is proposed and validated in order to investigate the tensile behavior of commingled polypropylene/glass yarns. Influence of specimen length and strain rate are highlighted. A comparison of tensile behavior of yarns before and after weaving is carried out in order to evaluate the weaving damage effect. Finally, a model describing the commingled yarn behavior is proposed. The parameters of the model are defined. Their dependency to strain rate, specimen length, and weaving damage are highlighted.

Experimental Evaluation of the J or C* Parameter for a Range of Cracked Geometries

Abstract In the current ASTM Standard Test Method for Measurement of Creep Crack Growth Rates in Metals (E 1457) the experimental C* parameter is related to the load and creep load line displacement rate through the geometry related η factor. In this work η factors for a range of geometries are presented. The geometries examined are the compact tension specimen, C(T), single edge notch specimen in tension, SEN(T), and bending, SEN(B), double edge notch specimen in tension, DEN(T), middle crack specimen in tension, M(T) and the C-shaped specimen in tension CS(T). Calculations have been performed for a linear elastic-power law hardening material but the resulting η factors are applicable to either power law plastic or power law creeping materials. Values for ηLLD and ηCMOD, based on the load line displacement and crack mouth opening displacement, respectively, have been determined. A wide range of crack depths, 0.1⩽a/W⩽0.7, where a is crack length and W is specimen width, and hardening exponents, 3 ⩽N ⩽10, under plane stress and plane strain conditions have been examined using the finite element method. The influence of specimen length, crack length, material properties and out of plane stress state on the η factor has also been considered. It has been found that for shallow cracks the value of η depends quite strongly on the exponent, N in the material power law, regardless of whether η is defined based on the load line displacement or crack mouth opening displacement. The ηLLD factor has also been found to be strongly sensitive to plane stress/strain conditions imposed, a/W and specimen length, whereas ηCMOD depends more weakly on a/W and is almost independent of specimen length for the cases examined. There is, however, no clear trend in these variations over the range of specimen geometries and a/W examined. These results are found to be consistent with those in the literature. Recommendations are made regarding the most appropriate values for η, depending on the specimen type and geometry while taking into account the variability due to the material properties, out of plane stress state and variations between the numerical analyses.

Residual Stresses in Wires: Influence of Wire Length

Residual stresses are one of the causes of failures in structural components. These stresses may arise in the fabrication process from many causes. They cannot be easily accounted for because they are both difficult to predict and to measure. X-ray diffraction (XRD) is nowadays a widespread technique for measuring surface residual stresses in crystalline materials. Very small specimens are often used for this purpose due to geometrical restrictions of either the diffractometer sample holder or the component to be inspected. However, the cutting process itself may affect the residual stress state in these specimens, so measured stresses could be misleading. In this work, the influence of specimen length on residual stresses was investigated in cold-drawn ferritic and pearlitic steel wires by XRD measurements and finite element simulations. In the ferritic wires, numerical simulations coincide with experimentally measured stresses. However, in the pearlitic wires the effect of the stresses in cementite (which could not be measured by XRD) has to be taken into account to explain the observed behavior. The results obtained have shown that in both materials the cutting process affects residual stresses, so it is recommended that specimens larger than five times the wire diameter be used.

Evaluation of damping and elastic properties of composites and composite structures by the resonance technique

This paper describes the resonance technique for determining the stiffness and damping properties of a composite or composite structure. Pultruded GRP composites and optical fibre cables (multi-component structures) were investigated. The resonance frequencies (natural frequencies) of a material, or a system, are a function of its elastic properties, dimensions and mass. This concept is used to determine the stiffness of a vibrated material by the resonance technique, which applies only very low stresses through the application of acoustic energy. This makes it applicable to measure the stiffness of multi-element cables. The damping properties, in terms of Q−1 (internal friction) were determined by both a free exponential decay curve and half-peak bandwidth methods. The influence of specimen length and measurement set-up was investigated. The applicability and accuracy of the resonance technique for a composite structure were discussed. The measured elasticity of optical cables was found to be in good agreement with the derived theoretical value.

Behaviour of thin-walled steel box sections with or without internal restraint

In this paper a comprehensive series of tests is described and presented. The tests were performed to examine the behaviour under axial load of short, thin-walled, square steel tubes with or without internal restraint. The influence of specimen length on the local buckling strength was also investigated. The specimens had width-to-thickness ratios of between 37.3 and 130.7 and length-to-width ratios ranging from 0.77 to 2.91. Two test series were performed. In Series 1 the effect of changing the buckling mode by forcing the formation of outwards buckles was examined for a wide range of width-to-thickness ratios. In Series 2 the influence of specimen length on the formation of local buckles was examined by using thin-walled steel tubes with and without internal restraint. The material properties, residual stresses and geometric imperfections have been measured. The test strengths have been compared with strength models in design standards and specifications. Improved recommendations for design have been made.

The influence of specimen length on the tensile failure properties of tendon collagen

Connective tissues such as ligament, tendon and skin are composites of strength-bearing collagen fibers embedded in a hydrated matrix. The tensile response and failure properties of rat-tail tendon are thought to represent those of the collagen fiber itself. In this study, the tensile failure properties of rat-tail tendon (tendon collagen) were determined for specimens of various test length. The experimental results indicated that failure strain, based on the test grip-to-grip dimension, and failure strain energy density decreased as specimen length increased. The failure stress, on the other hand, did not change appreciably with specimen length. Thus, tensile failure data cannot simply be normalized by the grip-to-grip length of the test specimen. Experimental data from various laboratories must clearly document the length of the test specimen.

Easy glide of cubic metal crystals

The structural features associated with easy glide in high-purity aluminum crystals have been studied by microscopic and X-ray methods. The temperature dependence of the phenonemon has been examined on identically oriented crystals over the range − 196°C to 200°C, while the influence of specimen-length has been studied at room temperature. An examination of the orientation dependence of the stress-strain curves of copper single crystals confirms the results of Rosi in so far as easy glide ends at a value of the resolved shear stress σ 0 such that σ 0 − τ c (critical resolved shear stress) is constant. However, it is shown that the length of the easy-glide range varies because of the existence of a size factor, demonstrated by experiments on crystals of identical orientation, but of different sizes. The effect of thin electro-deposits and, in particular, of alloying additions on the extent of easy glide, has been investigated. The view is advanced that easy glide ends as a result of secondary slip arising prematurely in the vicinity of dislocation pile-ups. Examination of this model gives the result that τ c σ 0 is constant, in accord with the experimental results. The value of τ c determines the extent of easy glide, and from this follows an explanation of the temperature dependence. Variation in the length of the slip path and consequently of the number of pile-ups, is put forward as an explanation of the orientation dependence of the phenomenon.