Tensile fatigue behaviour of PBO fibres

Poly(p-phenylene benzobisoxazole) (PBO) fibres are finding increasing applications on account of their exceptional stiffness and strength. This article presents results from tests on single PBO filaments, to characterize their intrinsic behaviour under quasi-static and cyclic tensile loading. Scanning electron microscopy is used to show the fibrillation mechanism leading to failure. Results are compared to data for polyester, aramid and high modulus polyethylene fibres. PBO fibres show shorter fatigue lifetimes than the other fibres when maximum stress is expressed as a percentage of quasi-static break load, but the absolute stress values applied are much higher for an equivalent lifetime.     

Tensile static and fatigue behaviour of sisal fibres

The work describes the quasistatic tensile and fatigue behaviour of as-received sisal fibres. Natural-based reinforced composites are gaining significant interest within the structural community, due to their interesting mechanical properties, recyclability and environmentally-sustainable production and use. Natural fibres such as sisal constitute an excellent reinforcement material, due to the low extraction costs from plants, and high level of recycling involved in their manufacturing process. In this work the diameter, Young’s modulus, strength and strain to failure over 15 different samples are measured and compared against data from open literature. Tensile cyclic fatigue loading at eight loading levels (from 0.6 to 0.95) has been carried out. The maximum forces involved (between 9 N and 22 N) are considerably higher than the ones used previously in open literature, and lead to significant dependence of the hysteresis loops, energy dissipation and S–N behaviour of the sisal fibres versus the cycle and loading ratio levels. The results obtained from this work can be used to predict from a fatigue and structural integrity point of view the behaviour of sisal-based reinforced composites with high load bearing capability, and extend the design envelope of this class of natural-reinforced materials.     

Tensile behaviour and strain hardening characteristics of constrained groove pressed nickel sheets

In this study commercially pure nickel sheets are severe plastically deformed at room temperature by constrained groove pressing (CGP) technique and the effect of pass number on the room temperature mechanical behaviour is investigated. Increase in strength observed after first pass is much higher than the increase observed during subsequent passes. Mechanical behaviour of constrained groove pressed sheets indicated negligible strain hardening ability during initial passes; gain in strain hardening ability is observed during latter passes resulting in enhanced ductility. The observation of shortened uniform elongation phase during tensile testing of CGP processed sheets could be linked to the lack of strain hardenability and change in deformation mechanism. Constitutive mechanical behaviour in uniform plastic deformation regime of nickel sheets in annealed condition obeys Hollomon relation whereas severely deformed sheets obey Voce relation closely. Strain hardening characteristics of groove pressed sheets analysed by Kocks–Mecking approach revealed stage-III hardening behaviour associated with high initial hardening rate when compared to annealed sheets. The influence of pass number on dislocation density evolution is assessed by Taylor’s expression. The synergistic effect of dislocation generation and recovery on the evolution of constitutive mechanical behaviour in the uniform elongation regime is described by applying single parameter based Kocks model. The kinetics of dislocation storage and dislocation annihilation in severely deformed nickel sheets during deformation corroborated with mechanical properties and dislocation density indicates the dominance of dislocation generation during earlier passes.     

Modelling of the bond behaviour of tuff elements externally bonded with FRP sheets

The performance of fibre reinforced plastic (FRP) materials used for external strengthening depends strongly on the bond behaviour at the FRP-substrate interface. In this paper, the results of an analytical model and of two Finite Element (FE) models (bi-and three-dimensional) for simulating bond behaviour in FRP-strengthened masonry elements using zero-thickness interface elements are presented. The primary parameters of bilinear and nonlinear bond-slip laws were determined from experimental results of single shear bond tests that the authors conducted on masonry blocks of yellow tuff bonded with FRP carbon and glass fabrics. Several parametric analyses were conducted to estimate the effect of the primary bond law parameters on the global behaviour of the specimens and to determine the effective bonded length for the investigated masonry support.     

Flexural behaviour of concrete corbels containing steel fibers or wrapped with FRP sheets

In the present paper an analytical and experimental investigation referring to the flexural behaviour of reinforced concrete corbels subjected to vertical forces is presented. For fixed shape and dimensions of the corbels the experimental investigation analyses the effects of the following: longitudinal and transverase steel reinforcements; fiber reinforced concrete (FRC) with hooked steel fibers; external wrapping retrofitting technique with a thin layer of carbon fiber sheet (CFRP). The analytical model based on equivalent truss structures, allows one to determine the bearing capacity of corbels, distinguishing the different ultimate states reached. The analytical results are then compared with experimental values, showing good agreement.     

Tensile fatigue behaviour of tightly woven carbon/carbon composites

The tensile fatigue behaviour of a tightly woven carbon/carbon composite was investigated as a function of stress level. Load-controlled fatigue tests were performed in tension-tension mode with a stress ratio, R, of 0.1 under ambient laboratory conditions. Results of composite behaviour are discussed in terms of the relationship of the stress/strain behaviour to the fatigue life of these composites as well as the effects of applied stress levels. It is shown that these composites exhibit good resistance to cyclic loading. No fatigue failures were obtained after 10⁶ cycles when the maximum tensile load in the fatigue cycle is less than or equal to 80% of the static tensile strength. Evidence of textural changes related to fatigue was observed in the matrix region of these composites.     

Tensile and fatigue fracture behaviour of a spot welded Al- Mg alloy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile properties and fatigue characteristics of hybrid composites with non-woven carbon tissue

The mechanical characteristics of hybrid composites with non-woven carbon tissue (NWCT) are investigated under static tensile and tension–tension fatigue loadings. The hybrid composites are made by stacking NWCT and CFRP prepregs. Thirteen kinds of composites are studied; i.e. NWCT composites, CFRP longitudinal [0]₈, transverse [90]₁₂, off-axis [45]₁₂ and angle-ply [±45]_3S, hybrid longitudinal ([0/0]₄, [/0/0/]₄), transverse ([90/90]₆, [/90/90/]₆), off-axis ([45/45]₆, [/45/45/]₆), and angle-ply ([+45/−45]_3S, [/+45/−45/]_3S). The symbol ‘/’ means that the NWCT is located between the CFRP layers. To estimate the stiffness of hybrid composites, the rule of mixtures is used. The effects of NWCT on the tension–tension fatigue life, the residual strength and stiffness of hybrid specimens are evaluated. The fatigue damage and failure mechanisms of the hybrid composites are analyzed with an optical microscope.     

Tensile and fatigue behaviour of wrought magnesium alloys AZ31 and AZ61

The mechanical behaviour of two hot rolled magnesium alloys, namely the AZ31 and AZ61, has been evaluated experimentally under both monotonic and cyclic loading. Both longitudinal (L) and long transverse (LT) directions were evaluated. The tensile behaviour of the L and LT directions is similar and differs only in the offset 0.2% yield strength for both materials. This difference is attributed to the angular spread of basal poles toward the rolling direction and is more pronounced for the case of AZ31. A distinct hardening response is obvious in both directions. Twinning formation was observed; it is more pronounced in the longitudinal direction while the fracture mode is intergranular and equiaxed facets are present in the fracture surfaces of the specimens. The S–N curves exhibit a smooth transition from the low to high cycle fatigue regime. AZ61 exhibits an overall better fatigue behaviour compared to AZ31. A transgranular crack initiation mode is observed in all tested specimens while the propagation of the cracks is characterized as intergranular.     

Structural behaviour of RC beams with external flexural and flexural–shear strengthening by FRP sheets

This paper presents experimental research on reinforced concrete (RC) beams with external flexural and flexural–shear strengthening by fibre reinforced polymer (FRP) sheets consisting of carbon FRP (CFRP) and glass FRP (GFRP). The work carried out has examined both the flexural and flexural–shear strengthening capacities of retrofitted RC beams and has indicated how different strengthening arrangements of CFRP and GFRP sheets affect behaviour of the RC beams strengthened. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the RC beam. In addition theoretical calculations are developed to estimate the bending and shear capacities of the beams tested, which are compared with the corresponding experimental results.     

Tensile static, fatigue and relaxation behaviour of closed cell electret PVDF foams

We present the experimental results related to the mechanical behaviour under tensile static, fatigue and compressive relaxation loading of closed cell PVDF electret foams under different loading ratio conditions. The specimens are statically loaded until 60% of their ultimate displacement, and subsequently subjected to cyclic loading under displacement control. The static tests show a stress-strain behaviour and failure mode similar to the one of other polymeric closed cell materials, such as polymetacrylimide-based foams. The fatigue tests show an evolution of the stiffness degradation over the levels of cycles characterised by three distinct phases. The behaviour of the energy dissipated versus the loading ratios and numbers of cycles applied is discussed in this paper, as well as the comparison between the compressive fatigue behavior recorded on similar classes of foams. The compressive relaxation behaviour shows a two-phase dependence over the loading time, with increasing modulus for higher loading ratios, and absence of an asymptotic modulus for long time exposure.     

Tensile fatigue behaviour of glass plain-weave fabric composites in on- and off-axis directions

The tension–tension fatigue behaviour of woven fabric composites was investigated in the on-axis and off-axis directions. Similar to the static tensile properties of woven fabric composites, the tension–tension fatigue behaviour of woven fabric composites was also observed to be anisotropic. The fatigue damage development in both the directions was studied using scanning electron microscopy and acoustic emission techniques. The influence of the applied fatigue frequency and the fatigue stress ratio in both the directions was investigated. At a fatigue frequency of 5 Hz, the hysteresis heating will occur more severely in the off-axis direction than in the on-axis direction. Increasing fatigue stress ratio will lead to a lower hysteresis heating.     

Tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC)

The tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC) under constant amplitude fatigue cycles is presented. Three series of uniaxial tensile fatigue tests up to a maximum of 10 million cycles were conducted with the objective to determine the endurance limit of UHPFRC that was supposed to exist for this material. The fatigue tests reveal that an endurance limit exists in all three domains of UHPFRC tensile behaviour at S-ratios ranging from 0.70 to 0.45 with S being the ratio of the maximum fatigue stress to the elastic limit strength of UHPFRC. Rather large variation in local specimen deformations indicates significant stress and deformation redistribution capacity of the UHPFRC bulk material enhancing the fatigue behaviour. The fatigue fracture surface of UHPFRC shows features of the fatigue fracture surfaces of steel, i.e. fatigue crack propagation is identified by a smooth surface while final fracture leads to rather rough surface. Various fatigue damaging mechanisms due to fretting and grinding as well as tribocorrosion are identified.     

Tensile behaviour of grass

The tensile behaviour of ryegrass (Lolium perenne L.), which was grown, harvested and tested under controlled conditions, is described. Whereas some of the grass leaf specimens behaved in a predominantly brittle manner, others evinced a semi ductile mode such that a proportional limit could be identified. Results indicated that the tensile properties depended upon specimen location and the tensile test strain rate. The data showed that as strain rate was increased, the stiffness, toughness and strength increased, while ductility decreased. Comparison of test results as a function of water content did not reveal statistically significant differences in any of the mechanical parameters. Analysis of the leaf structure suggests that the epidermal cells play a major role as a load-bearing component.     

Hybrid composites based on polyethylene and carbon fibres. Part 6: Tensile and fatigue behaviour

The tensile and fatigue behaviour of unidirectional carbon-high-performance polyethylene/epoxy hybrid composites has been studied, including the effect of hybrid design and surface treatment of the high-performance polyethylene (hp-pe) fibres. Results indicated that the tensile behaviour of carbon-hp-pe hybrids in both monotonic and fatigue testing can be interpreted, adopting the conventional ‘constant strain’ model for hybrid composites. Deviations from this constant strain model, so-called hybrid effects, were observed in monotonic tensile testing for those hybrid systems with the highest degree of fibre dispersion, incorporating either untreated or treated hp-pe fibres, whereas only the latter displayed synergistic fatigue performance. Hybrid effects under tensile loading conditions were in reasonable agreement with calculations accounting for statistical effects and stress concentrations as determined by finite element analyses.     

Evaluation of FRP and hybrid FRP cables for super long-span cable-stayed bridges

This paper evaluates the safety factors, the applicable lengths, and relative cost of FRP (fiber reinforced polymer) and hybrid FRP cables that are potentially suitable for cable-stayed bridges with a super long-span of between 1000 m and 10,000 m. Following previous studies on 1000-m scale cable-stayed bridges with FRP cables, two kinds of hybrid FRP cables – the previously discussed hybrid basalt and carbon FRP (B/CFRP) cable and the newly-developed basalt and steel-wire FRP (B/SFRP) cable – as well as conventional steel cable, CFRP cable, and BFRP cable are further investigated focusing on their promise in meeting potential requirements for super long-span bridges. Some major results are as follows: (1) a three-stage model for determining safety factors of cables with different kinds and lengths is proposed; (2) a threshold of λ² is suggested to achieve both high material and stiffness utilization efficiency, based on which the applicable lengths for different kinds of cables were evaluated; and (3) hybrid B/SFRP cables and BFRP cables are comparable in cost to steel cables within a 3000 m span, while hybrid B/CFRP cables and CFRP cables demonstrate a superior performance/cost ratio over a longer span.     

Tensile and fatigue properties of Fe-Mn-Al-C alloys

The tensile properties and fatigue behaviour of three solution-treated Fe-29 Mn-9 Al-C (wt%) alloys having various carbon contents leading to different volume fractions of austenite and ferrite phases were investigated. The carbon contents were 1.06%, 0.60% and 0.26%, respectively and the corresponding volume fractions of austenite were 100%, 90% and 45%, respectively. The alloy having 1.06% carbon possessed the best tensile properties but its fatigue behaviour was only comparable to the other two alloys with lower carbon contents. The alloy having 0.60% carbon possessed the lowest yield strength, but its fatigue life was slightly better than other two alloys. The alloy having 0.26% C possessed lowest elongation and medium strength, and its fatigue life was comparable to the other two alloys. Their tensile properties and fatigue behaviour were explained in terms of crack initiation, crack propagation, grain size, constituent arrangement and constituent fraction.     

环氧玻璃钢压力容器的疲劳性能分析

依据美国ASME标准的条件，对国内领先水平的玻璃钢压力容器的疲劳寿命进行了测试．从而对其疲劳性能进行了分析，补充了国内领先水平的玻璃钢压力容器性能评价内容．     

Acoustic fatigue and damping technology in FRP composites

There is considerable interest in the use of composite materials in aerospace structures. One important area is to develop a stiff, lightweight composite material with a highly damped, high-temperature polymer matrix material. This paper concerns the application of such material, in the form used in thin skin panels of aircraft, and investigates of its fatigue properties at both room and high temperature. Flexural fatigue tests have been carried out at two different temperatures and harmonic three-dimensional finite-element (FE) analyses were performed in order to understand the dynamic behaviour of plates. Random acoustic excitation tests using a progressive wave tube, up to an overall sound pressure level of 162 dB, at room temperature and high temperatures were also performed in order to investigate the dynamic behaviour of panels made of the materials. Parameter studies were carried out in order to examine various methods for including damping in the structure, and conclusions have been drawn concerning optimal incorporation of a highly damped matrix material into a high-performance structure.