W.M. Banks

Tensile testing of cellulose based natural fibers for structural composite applications

A series of tensile tests were conducted on a Lloyd LRX tensile testing machine for numerous natural fibers deemed potential candidates for development in composite applications. The tensile tests were conducted on the fibers jute, kenaf, flax, abaca, sisal, hemp, and coir for samples exposed to moisture conditions of (1) room temperature and humidity, (2) 65% moisture content, (3) 90% moisture content, and (4) soaked fiber. These seven fibers were then tested for the four conditions and the mechanical properties of tensile strength, tensile strain to failure, and Youngs modulus were calculated for the results. These results were then compared and verified with those from the literature, with some of the fibers showing distinctly promising potential. Additionally, a study on the effect of alkalization using 3% NaOH solution was carried out on flax, kenaf, abaca, and sisal to observe impact that this common fiber pre-treatment process has on fiber mechanical properties. The result of the investigation indicated that over treatment of natural fibers using NaOH could have a negative effect on the base fiber properties. It is consequently apparent that a treatment time of less than 10 min is sufficient to remove hemicelluloses and to give the optimum effect.

Non-destructive examination of composite joint structures : a correlation of water absorption and high-frequency dielectric propagation

Applications of high-frequency dielectric measurements (300 kHz to 3 GHz) to the non-destructive examination (NDE) of composite structures are discussed. Preliminary data indicate that the method, previously used for aluminium bonded structures, can be applied to bonded carbon fibre composite structures. Water ingress into the bond structure was observed to influence both the time- and the frequency-domain data. Comparison of exposed and dry joints demonstrated that a good correlation exists between changes with time in the dielectric properties of the composites and the extend of water uptake in the matrix. Parallel gravimetric measurements were also performed. Diffusion of water into the composite structures is discussed in terms of various models. This paper demonstrates that this NDE method may be used for assessment of the water content in the matrix and has general applicability to the study of carbon-reinforced matrix materials.

Environmental Ageing of Adhesively-Bonded Joints. II. Mechanical Studies

The effects of exposure to moisture on the mechanical properties of a series of adhesively-bonded structures are reported. Changes observed in the maximum load, shear modulus, strain at maximum load, fracture energy, fracture toughness and stress are discussed and correlated with variation of the dielectric parameters. An initial increase in fracture toughness observed in the joints correlates well with the uptake of moisture having led to a lowering of the glass-rubber transition temperature. Differences in the ultimate strength and energy to failure for different surface pretreatments are observed. Loss in mechanical properties observed over the period of the study are paralleled by changes in the dielectric properties of the joints.

Non destructive evaluation of adhesively bonded composite structures using high frequency dielectric spectroscopy

Over the last ten years, the application of high frequency dielectric techniques for the assessment of adhesively bonded structures has been investigated. The technique has been used for the study of adhesively bonded aluminium structures and its application to carbon fibre reinforced plastic (CFRP) bonded structures forms the basis of this paper. The electrical conductivity of the carbon fibres in the CFRP composite materials is sufficiently high for adhesively bonded structures to exhibit the properties of a wave-guide. The non-conductive adhesive behaves as a dielectric. The time domain data allows the integrity of the structure to be explored and is sensitive to the orientation of the fibres at the adherent-adhesive interface. Furthermore, a good correlation is shown between time domain dielectric spectroscopy and gravimetric results. This study indicates that the success obtained in the application of high frequency dielectric measurements to adhesively bonded aluminium structures is also applicable to CFRP bonded structures. The dielectric studies not only indicate a new way to assess the state of such a structure but also are producing new insight into the application of dielectric time domain response (TDR) measurement to non-isotropic materials.

Dielectric studies of adhesively bonded CFRP/epoxy/CFRP structures : design for ageing

The ageing behaviour of carbon fibre-reinforced plastic/epoxy/carbon fibre-reinforced plastic (CFRP/epoxy/CFRP) adhesively bonded structures has been investigated by the use of high-frequency dielectric analysis and mechanical shear/cleavage experiments. The joints are aged at 70°C and 100% humidity and exhibit significant loss of strength over a prolonged period of exposure. The frequency-domain dielectric data indicate the rate at which water is absorbed in the joint structure. Time-domain data indicate whether the joint cross-section has undergone significant change with exposure to the ageing environment. A good correlation appears between the extent of change of the joint structure as characterised by the dielectric data and the loss of mechanical strength. There is evidence of toughening due to plasticisation of the adhesive from the fracture toughness data. This is the third in a series of papers dealing with dielectric studies on joints. The first looked at aluminium/epoxy/aluminium joints while the second looked at aluminium/epoxy/CFRP joints.

An experimental study on laminated panels in compression

Abstract The theoretical approach adopted in this paper is applied to the buckling and post-buckling behaviour of laminated plates subjected to unidirectional in-plane loading. Full details of the theoretical approach are given in a companion paper ( Comp. Struct. , 19 (1991) 41–65). The membrane boundary conditions considered were those which allowed the unloaded edges to wave in the plane of the plate. The flexural boundary conditions considered were simple supports on the loaded ends with the unloaded edges rotationally restrained, thus giving the theoretical approach more flexibility in allowing the unloaded edges to be simply supported and clamped as the limiting cases. An extensive complementary experimental investigation was carried out to confirm the theoretical findings. The test set-up gave simply supported boundary conditions on the unloaded edges and fully fixed conditions on the loaded ends. Large deflection results and post-buckling strain distributions were obtained for laminated panels composed of 14 layers of Grafil XAS/914C Fibredux, and comparisons are made with the theory.

Dielectric and mechanical assessment of water ingress into carbon fibre composite materials

Abstract High frequency dielectric measurements (300 kHz–3 GHz) have been used to assess the water uptake by bonded composite resin joint structures. Comparison of exposed and dry joints demonstrated that a correlation exists between changes with time in the dielectric properties of the joints and the extent of water uptake in the matrix as determined gravimetrically. Diffusion of water into the joints is discussed in terms of various models. The changes in mechanical properties as a function of water uptake are reported over an extended period of time. The use of dielectric measurements as a non-destructive examination method for the assessment of water ingress into joint structures is demonstrated.

Advances in the fatigue assessment of wire ropes

This paper presents the findings from an in-depth analysis of the (axial) stiffness data recorded during tension-tension fatigue tests on wire ropes, particularly in relation to how changes in stiffness during testing relate to changes in rope strength. A linear relationship between stiffness and strength is shown to exist and a methodology presented for quantifying residual strength with applied cycles. New lower bound fatigue lines for six-strand rope and spiral strand are presented which are based on a 10% loss of strength. These new lines have the advantage of having been established using a common discard criterion for wire ropes.

The instability behaviour of laminated panels with elastically rotationally restrined edges

Abstract In this contribution the analytical approach based on a semi-energy method was developed to study the instability behaviour of symmetrically laminated panels in pure compression. A four-term polynomial function is used to postulate the out-of-plane deflection form of a laminated panel. A one-term post-buckling function is used in conjunction with the semi-energy method of analysis. The von Karman type large deflection compatibility equation governing the behaviour of laminated panels is used together with a consideration of the total strain energy of the system. From the large deflection compatibility equation the deflection function is related to the stress function in terms of the unknown coefficients in the assumed deflection function. These are then substituted in the total strain energy expressions and the theorem of minimum total potential energy is applied to solve for the unknown coefficients.

A study of the effect of strain rate and temperature on the characteristics of quasi-unidirectional natural fibre-reinforced composites:

The responses of quasi-unidirectional vegetable fibres (sisal and flax) – styrene polyester matrix composites under 6-J nominal strike energy at different sample temperatures and at higher impact energies of 9 and 12 J for samples at room temperature have been studied and indentation properties realized from static loading at different cross-head displacement speeds. The findings are explained in terms of the impact characteristics. At temperatures above ambient, there were changes in the impact characteristics. The drop impact tests were carried out on samples (70 × 70 × 5 mm3) at an ambient temperature of 18 °C and elevated temperatures of 40 °C, 60 °C, 80 °C, and 100 °C using an instrumented drop tester with a 12.1-mm diameter hemispheric tup and the load history and energy profile were obtained. The actual damage response depends on many intrinsic and extrinsic factors, including the thickness of the laminate, the exact stacking sequence, the shape and kinetic energy of the impactor, and the degree to which the laminate is supported against bending. The examination of the impact and post-impact characteristics at elevated temperatures revealed a plastic mode of failure and the performance was assessed in terms of contact time, post-impact displacement, and total energy. Composites are generally brittle in nature and respond elastically with little or no plastic deformation, but this is not the case for natural fibre – styrene polyester matrix composites, especially at high temperatures, as there exists some degree of plasticity seen in the after-impact state of the samples. Low energy impact (6 J) results in cracking of the matrix leading to reduction in the strength of the composite. However, higher energy impact strikes produce cracking of the matrix and splintering of the fibres. From the load–indentation curves, average power coefficients of 3.6 and 15.4 were obtained for sisal and flax composites, respectively, as against 1.5 for the special Hertz contact of two elastic bodies.