Maximum Load Level Research Articles

Fatigue Behavior of Concrete-Filled Fiber-Reinforced Polymer Tubes

To date, research on concrete-filled fiber-reinforced polymer (FRP) tubes (CFFT) has focused on the effect of static loads, simulated seismic loads, and long-term sustained loads. Dynamic fatigue behavior of CFFTs, on the other hand, has received little or no attention. This paper reports on an experimental study to evaluate damage accumulation, stiffness degradation, fatigue life, and residual bending strength of CFFT beams. A total of eight CFFT beams with four different types of FRP tube were tested under four point bending. Test parameters included reinforcement index, fiber architecture, load range, and end restraints. Fatigue performance of CFFT beams is clearly governed by characteristics of the FRP tube and its three phases of damage growth: matrix cracking, matrix delamination, and fiber rupture. Lower reinforcement index increases stiffness degradation and damage growth, and shortens fatigue life. End restraints, e.g., embedment of FRP tube in adjacent members, promote composite action, arrest slippage of concrete core, and enhance fatigue life of CFFT beams. It is suggested that a maximum load level of 25% of the static capacity be imposed for fatigue design of CFFTs. With proper design, CFFTs may withstand repeated traffic loading necessary for bridge girders.

Fracture analysis of laser beam welded superalloys Inconel 718 and 625 using the FITNET procedure

This paper presents results of the analysis for residual strength prediction of two laser beam welded superalloys (Inconel 625 and 718) using the fitness-for-service (FFS) procedure FITNET. The analysis is based on Option 3 of the Fracture Module of the procedure arising from the European Community funded project FITNET ( www.eurofitnet.org). Analysis option 3 requires full stress–strain curves of the materials as input data. Since the strength mismatch ratio ( M) between the weld and the base metals for both alloys was less than 10%, a homogeneous solution route has been used for welded tension loaded specimens containing through thickness central cracks. The FITNET predictions of the attained maximum load levels of the specimens were verified with the generated experimental results. The analysis methodology, as well as comparison between predictions and experimental results are provided. Accurate yet conservative estimation of the maximum load carrying capacities have been achieved depending on the input data; as the input data becomes more detailed, the degree of conservatism decreases.

Characterizing Fatigue Crack Closure by Numerical Analysis

The crack closure phenomenon has attracted great attention in the prediction of fatigue crack growth. The finite element analysis of fatigue crack growth has been conducted by many researchers mainly emphasized on the technique implementation of the simulation. In this paper the behavior of plasticity induced fatigue crack closure was analyzed by the elastic-plastic finite element method for middle crack tension (MT) specimen. The material was assumed as linear-kinematic hardening. The crack growth was simulated by releasing the “bonded” node pairs ahead of crack tip in stepwise. The calculations focused on the effects of load cases and crack length on crack opening/closure levels. For constant amplitude cyclic loadings with different load ratios, the crack opening/closure levels increases for a while and then decreases continuously, with the increase of crack length. For the loadings with invariable maximum stress intensity factors (briefly the constant-K loading), however, the crack tip plastic zone sizes at different crack lengths remain unchanged and the crack opening and closing load levels normalized by the maximum load levels keep constants as well. The results indicate that the crack length does not affect the relative opening and closure levels and numerical analysis for the constant-K loading case should play a key role in characterizing the fatigue crack growth behavior.

Glass FRP-Bonded RC Beams under Cyclic Loading

Ten beams bonded with glass fiber reinforced polymer (GFRP) laminates were tested under cyclic loading with the load range and the FRP reinforcement ratio as test parameters. The maximum load level during cyclic loading was 55%, 65% and 75% of the static flexural strength while the minimum load level was kept constant at 35%. Deflections of the beams at the end of 525000 cycles were found to increase by 16% and 44% when the maximum load level was increased from 55% to 65% and 75% of the static flexural strength, respectively. Beams with FRP reinforcement ratios of 0.64% and 1.28% were found to exhibit lesser deflections of about 15% and 20%, respectively, compared to a similar beam without FRP reinforcement. An analytical approach based on cycle-dependent effective moduli of elasticity of concrete and FRP reinforcement is presented and found to predict the deflections of the test beams well.

A comparative study of the fatigue and post-fatigue behavior of carbon–glass/epoxy hybrid RTM and hand lay-up composites

The paper presents a study of the fatigue and post-fatigue behavior of a hybrid carbon–glass biaxial fabric reinforced epoxy composite manufactured by the resin transfer molding (RTM) and the hand lay-up (HL) processes, with the main objective of assessing whether a material characterization run at the prototype level of a handicraft technology could be significant for a mass production technology and whether a comparison on static properties (a viable task at an industrial level) could ensure the same level of agreement for the fatigue life and residual properties. Tensile and flexural static tests as well as displacement-controlled bending fatigue tests (R ratio of 0.10) were conducted on two sets of standard specimens, having fiber orientation parallel to the loading direction (on-axis specimens) and at 45° to the loading direction (off-axis specimens). Specimens were subjected to different fatigue loading, with the maximum load level up to 60% of the average ultimate flexural strength, and damage in the laminate was continuously monitored through the loss of bending moment during cycling. After 106 cycles, the fatigue test was stopped and residual properties were measured. Micrographs of sample sections revealed some voidage for HL specimens while resin rich areas were observed for RTM specimens. Results of the static tensile and flexural tests pointed out lower mechanical properties for the RTM specimens when tested on-axis and slightly higher properties when tested off-axis. Regardless of specimen fiber orientation, the fatigue and post-fatigue performance of RTM samples was inferior to that of HL specimens with the gap increasing for increasing fatigue load levels. The result was ascribed to the presence in RTM samples of resin-rich areas, which are reported to have limited influence on the laminate static properties but which may act as initiation sites for fatigue cracks.

Functionalization of magnetic nanoparticles with organic molecules: Loading level determination and evaluation of linker length effect on immobilization

A general method is introduced to immobilize organic molecules on magnetic nanoparticles through silanization reactions and determine the maximum loading level by UV-vis spectroscopy. Loading levels of 1.5 x 10(-3) mol per gram of nanoparticle were obtained with structurally diverse compounds such as rhodamine B and glucosamine. The length of the linker did not have a significant effect on loading as comparable maximum amounts of rhodamine B were immobilized on magnetic nanoparticles regardless of the linker length. Interestingly, rhodamine B derivatives lost conjugation during synthetic manipulations due to reversible spiroisobenzofuran formation. Full regeneration of conjugation was found to be slow with rhodamine B attached on magnetic nanoparticles. The results obtained from these studies will be useful for studying surface functionalization of MNPs in general.

Investigation of the most suitable location of insulation applying on building roof from maximum load levelling point of view

In this study, the maximum load levelling of periodic heat flux entering a room through a composite roof consisting of different insulation layer has been evaluated for different insulation positions in the roof. A numerical model based on implicit finite difference scheme was applied for 12 different roof configurations during typical winter and summer days. For this purpose, total insulation thickness was kept constant and insulation was placed as equally two pieces and as equally three pieces in different locations within the roof thickness. Then, insulation layers were moved 1 cm at a time across the roof thickness for 12 different configurations. Maximum and minimum values of periodic heat fluxes for each sweeping process in the roof were calculated for achieving the maximum levelling of the heat flux entering through the roof. It was found that the best load levelling was achieved in the case where three pieces insulation of equal thickness were placed one at the outdoor surface of the roof, the second piece of insulation was placed in the middle of the roof and third piece of insulation was placed at the indoor surface of the roof.

Bending fatigue stiffness and strength degradation in carbon–glass/epoxy hybrid laminates: Cross-ply vs. angle-ply specimens

A hybrid glass–carbon non-woven fabric reinforced epoxy matrix composite, constituted by layers of biaxial carbon fabrics, biaxial glass fabrics and hybrid carbon–glass fabrics, was considered for its bending fatigue behaviour. Tensile and flexural static tests as well as displacement-controlled bending fatigue tests (ratio of 0.10) were conducted on two sets of standard specimens, having fibre orientation parallel to the loading direction (cross-ply specimens) and at 45° to the loading direction (angle-ply specimens). Specimens were subjected to different fatigue loading, with the initial maximum load level up to 85% of the laminate ultimate flexural strength, and damage in the laminate was continuously monitored through the loss of bending moment during cycling. After 10 6 cycles the fatigue test was stopped and residual properties were measured on tested specimens. Stiffness-based stress-number of cycle (SN) curves were drawn for the two sets of specimens. The amount of stiffness loss for cross-ply and angle-ply specimens was observed to depend on the fatigue load level. In particular, cross-ply specimens were observed to damage more significantly than angle-ply specimens only at high fatigue loading. This was attributed to different damage mechanisms for the two sets of specimens. Reduction in material strength and elastic modulus as measured after 10 6 cycles was also found to depend on the level of fatigue loading and to follow different trends for the two sets of specimens.

Complementarity model for load tap changing transformers in stability based OPF problem

The paper presents a stability optimal power flow (OPF) based problem that incorporates complementarity models for load tap changing transformers (LTCs). The complementarity formulation models the change of behavior of LTC controlled buses when the tap becomes blocked at one of its limits and the voltage level can no longer be regulated. The incorporation of proposed model in stability OPF based problem leads to a more accurate estimation of the maximum or critical loading level.

Incorporation of multi-walled carbon nanotubes into high temperature resin using dry mixing techniques

As part of an ongoing effort to develop multifunctional advanced composites, blends of PETI330 and multiwalled carbon nanotubes (MWNTs) were prepared and characterized. Dry mixing techniques were employed and the maximum loading level of the MWNT chosen was based primarily on its effect on melt viscosity. The PETI330/ MWNT mixtures were prepared at concentrations ranging from 3 to 25 wt %. The resulting powders were characterized for homogeneity, thermal and rheological properties and extrudability as continuous fibers. Based on the characterization results, samples containing 10, 15 and 20 wt % MWNTs were chosen for more comprehensive evaluation. Samples were also prepared using in situ polymerization and solution mixing techniques and their properties were compared with the ball-mill prepared samples. The preparation and characterization of PETI330/ MWNT nanocomposites are discussed herein.

Bending fatigue behavior of glass–carbon/epoxy hybrid composites

A hybrid glass–carbon fiber reinforced epoxy matrix composite, characterized by the presence of intraply biaxial glass–carbon laminae as well as biaxial glass laminae and biaxial carbon laminae, was considered for its bending fatigue behavior. Displacement-controlled bending fatigue tests with R ratio of 0.10 were conducted on standard specimens and damage in the composite was continuously monitored through the loss of bending moment during cycling. The specimens were subjected to different fatigue loading, with the maximum load level up to 85% of the material ultimate flexural strength. Early damage was observed after a few hundred loading cycles causing degradation of material stiffness with cycling. The amount of stiffness reduction was observed to be a function of the magnitude of applied fatigue loading on the specimen. After 1 million cycles the fatigue test was stopped and residual properties were measured on tested specimens. Reduction in material strength and elastic modulus was found to depend on the level of fatigue loading. However, the reduction in stiffness did not exactly correlate with the reduction in strength. At last, possible use of the obtained data for the design of structural components is briefly addressed.

Fracture Assessment of Welded Aerospace Aluminium Alloys using SINTAP Route

Analytical assessment levels of the Structural Integrity Assessment Procedure (SINTAP) were applied to middle cracked thin Al 6013 T6 base material and friction stir welded (FSW) wide plates to predict their residual strength. The welded joints exhibit strength undermatching and, hence, plastic strain localization occurs within the lower strength FSW weld region which requires special considerations when applying the SINTAP based failure assessment route. Two R-curve types were used in the analysis: in terms of the J-integral obtained from the load-CMOD (Crack Mouth Opening Displacement) test record and in terms of directly measured crack tip opening displacement (CTOD) δ5 at the crack tip. The SINTAP estimations of maximum load level for the base material panels were remarkably accurate predicting almost exactly the experimental residual strength value in the highest analysis level. The weld strength mismatch analysis level applied to the FSW wide plates has successfully provided a conservative estimate of the maximum load (being 16% lower then the experimental value).

Commercial vehicle EGR coolers for maximum load levels

It is to be expected that the new emission standards will lead to increased utilization of cooled exhaust gas recirculation, and necessitate substantially higher recirculation rates. In the case of engines for heavy commercial vehicles, this could mean that up to 120 kW will need to be removed from the exhaust gas, which will necessitate not only new EGR coolers, but also improved performance from the engine cooling system. The EGR coolers currently being developed by Behr are designed to achieve double the recirculation rate and a much higher pressure level in comparison with the systems currently in use. To achieve this, the power density and absolute performance, pressure strength, and resistance to vibration need to be increased, and fouling characteristics must be maintained at least at current levels, if not improved further.

A comparison in performance of greenhouses with various shapes: a parametric study

SYNOPSIS This paper presents a comparison in the performance of greenhouses with various shapes for a composite climate (New Delhi). The shapes under study are: even span, uneven span, modified arch, vinery and modified IARI. The performance of the greenhouses has been evaluated with the help of an experimentally validated model. A parametric study has also been presented. The effect of a north wall has been numerically evaluated in an even span, uneven span and modified arch shaped greenhouse. Based on numerical computations, which were carried out for a typical day in summer and one in winter, it could be concluded that: i. greenhouse shape affects the room temperature and thermal load levelling significantly, which is higher in winter as expected; ii. The maximum room temperature (15°C and 20°C) and thermal load levelling (0.10 and 0.12) were increased due to the north wall in winter and summer conditions, respectively; iii. the fluctuations in room temperatures were reduced due to the increase in isothermal mass (water); iv. an increase in the number of air changes along with the use of movable insulation considerably reduced the room temperature in an uneven span greenhouse with a north wall and v. in terms of room temperature and thermal load levelling, the modified arch and vinery shaped greenhouses presented the best performance in summer and winter conditions, respectively.

The fracture behavior of a Ti-6242 alloy deformed in bending fatigue

In this paper, the fracture behavior of the Ti-6242 alloy subjected to bending fatigue deformation is presented and discussed. Samples of the alloy were deformed to failure in bending over a range of maximum load levels. The macroscopic fracture mode was found to be marginally different over the range of load levels examined. Microscopically the fracture surfaces revealed features reminiscent of locally ductile and brittle mechanisms. The kinetics governing crack advance and fracture is rationalized in light of maximum load, local stress concentration effects, and contributions from intrinsic microstructural features.

Estudo clínico duplo-cego de extrato padronizado (BNT-08) das raízes de Pfaffia glomerata (Spreng.) Pedersen: avaliação do efeito tônico em atividade física

Realizou-se estudo clínico duplo-cego controlado com placebo, buscando-se comprovar os efeitos tônicos em atividade física atribuídos às raízes de Pfaffia glomerata. Foram triados 38 voluntários normais e realizou-se testes ergoespirométricos, com medição de VO2 máximo, carga máxima atingida, freqüência cardíaca submáxima e níveis sanguíneos de lactato no início e no fim do teste. Os voluntários receberam o extrato das raízes ou placebo por seis meses. Ocorreu apenas uma desistência no grupo placebo, não se verificou alterações clínico-laboratoriais e anotou-se manifestações mínimas de efeitos adversos. O teste ergoespirométrico realizado no início e no fim do estudo clínico não evidenciou alterações positivas ou negativas nos vários parâmetros analisados. Concluiu-se que, nos termos do protocolo utilizado e com o número de voluntários envolvidos, que o extrato padronizado das raízes de Pfaffia glomerata não promoveu benefícios aos voluntários em termos de aptidão e atividade física.

An investigation of framework–sorbate interactions in p-dibromobenzene/ZSM-5 and bromobenzene/ZSM-5 systems by FT-Raman spectroscopy

The host–guest interactions of two related sorbates, p-dibromobenzene (pdbb) and bromobenzene, adsorbed on completely siliceous ZSM-5 have been examined by FT-Raman spectroscopy. For the pdbb/ZSM-5 system, a phase transition was identified at a loading of 4 molecules per unit cell (u.c.). The results suggest that the adsorption and phase transition behavior of pdbb/ZSM-5 are similar to those of p-dichlorobenzene/ZSM-5 and p-xylene/ZSM-5 except that the maximum loading level is 7 molecules per u.c. rather than 8 molecules per u.c. For bromobenzene, a sorbate-induced phase transition also occurred at a loading of 4 molecules per u.c. This is different from chlorobenzene which induces two successive transitions in the ZSM-5 framework upon adsorption.

Fatigue life prediction of fiber reinforced concrete under flexural load

This paper presents a semi-analytical method to predict fatigue behavior in flexure of fiber reinforced concrete (FRC) based on the equilibrium of force in the critical cracked section. The model relies on the cyclic bridging law, the so-called stress–crack width relationship under cyclic tensile load as the fundamental constitutive relationship in tension. The numerical results in terms of fatigue crack length and crack mouth opening displacement as a function of load cycles are obtained for given maximum and minimum flexure load levels. Good correlation between experiments and the model predictions is found. Furthermore, the minimum load effect on the fatigue life of beams under bending load, which has been studied experimentally in the past, is simulated and a mechanism-based explanation is provided in theory. This basic analysis leads to the conclusion that the fatigue performance in flexure of FRC materials is strongly influenced by the cyclic stress–crack width relationship within the fracture zone. The optimum fatigue behavior of FRC structures in bending can be achieved by optimising the bond properties of aggregate–matrix and fiber–matrix interfaces.

Powder densification. 1. Particle–particle basis for incorporation of viscoelastic material properties

The present investigation was undertaken to examine the basic unit of densification: the particle–particle indentation. The true interparticle contact area that is established during densification ultimately determines the quality of the tablet compact. By examining the interfacial contact between mutually indenting viscoelastic particles, the process of contact evolution may be represented in mathematical form through extension of the classical Hertzian elastic contact description to encompass material viscoelastic terms. In this way, the time‐dependent response of materials to applied loads may be addressed explicitly. The effects of rates of applied loading and maximum load levels were also considered. This analysis was based on viscoelastic stress data collected using an instrumented Instron analyzer during the densification of PMMA/coMMA, a pharmaceutical polymeric coating material. A crossed cylinder matrix compaction geometry was used to simulate the geometry of two mutually indenting spherical particles. Numerical and graphical solutions delineating the relationship between contact area evolution and the prescribed loading force are presented. This particle–particle description of the contacting interface serves as a unit basis for describing the entire powder bed. The powder bed may ultimately be modeled as a collection of these particles in contact.

Optimization of imperfection-sensitive symmetric systems for specified maximum load factor

A method is presented for optimum design of a symmetric structure which reaches an unstable bifurcation point as the load factor is increased. Reduction of the maximum load level due to the antisymmetric imperfection is considered, and a straightforward algorithm is proposed for calculating the magnitude of reduction of the load factor corresponding to the most critical mode of antisymmetric imperfection. The sensitivity coefficients of the bifurcation load factor are calculated by using the interpolation method developed by the authors, and an approximate formulation is presented for sensitivity analysis of the maximum load factor. It is shown in the examples that the errors in the sensitivity coefficients do not lead to any significant difference in the optimum design.