Failure Load Data Research Articles

DSM-Based failure load prediction of CFS pin-ended singly symmetric columns buckling in flexural-torsional modes

Recently, the authors investigated the post-buckling behaviour, strength and Direct Strength Method (DSM) design of cold-formed steel (CFS) fixed-ended singly symmetric columns buckling in major-axis flexural-torsional modes (FMT), some of which were shown to experience interaction with minor-axis flexural (Fm) buckling − FMT-Fm (global-global) interaction. The main fruit of this investigation was the development of an efficient DSM-based design approach capable of predicting the failure loads of such columns, regardless of their failure mode nature (pure FMT or FMT-Fm interactive). This work extends the scope of the above study to singly symmetric columns with three types of pin-ended support conditions, all fixed with respect to torsion and having warping fully prevented. Columns with seven cross-section shapes are considered, having their wall dimensions, lengths and yield stresses selected to ensure covering wide FMT slenderness ranges and various ratios between the Fm and FMT buckling loads. Following an investigation on the elastic and elastic-plastic post-buckling behaviours of the selected pin-ended columns, paying attention to the possible occurrence of FMT-Fm interaction, parametric studies are carried out to gather extensive pin-ended column failure load data, including some potentially associated with FMT-Fm interactive collapses. Then, the assembled numerical failure loads are used to show that (i) the available DSM-based strength curves are only able to predict adequately part of them and, thus, (ii) novel DSM-based design curves are needed to estimate the failure loads of the remaining pin-ended singly symmetric columns buckling in FMT modes. After developing such curves and assessing their merits (safety and reliability), it is concluded that different DSM-based design curves/approaches must be employed for columns with each type of pin-ended support conditions.

DSM design of cold-formed steel fixed-ended lipped channel columns undergoing distortional-global interaction

This work reports a numerical investigation dealing with the post-buckling behaviour, strength, and Direct Strength Method (DSM) design of cold-formed steel fixed-ended lipped channel columns experiencing coupling between distortional and global (flexural-torsional) buckling − distortional-global (D–G) interaction. Various cross-section dimensions and lengths are considered, to ensure that the columns selected undergo different levels and types (“true”, “secondary-bifurcation distortional” or “secondary-bifurcation global”) of D–G interaction. The results presented and discussed, determined by means of Abaqus shell finite element geometrically and materially non-linear analyses, consist of elastic and elastic-plastic post-buckling equilibrium paths, failure loads and collapse modes. The steel material behaviour is deemed elastic-perfectly plastic and several yield stresses are considered, thus making it possible to cover a wide column D–G slenderness range. Particular attention is devoted to identifying the most detrimental initial geometrical imperfection shapes, in the sense that they lead to the lowest column failure loads. The numerical failure load data gathered are subsequently used to assess the merits of the available DSM-based design approaches developed to handle cold-formed steel columns undergoing D–G interaction. Since these design approaches are shown to be either inefficient and/or improvable, the above failure load data are also used to propose modifications/improvements aimed at achieving an efficient failure load prediction in the specific context of fixed-ended lipped channel columns. The success of this endeavour provides encouragement to the authors in their search for a safe, accurate and reliable DSM-based design approach capable of handling cold-formed steel columns with arbitrary cross-section shapes and/or end support conditions that fail in D–G interactive modes.

Fatigue-life and stress distribution of a glass-ceramic under different loading conditions

This study aimed to evaluate the effect of different loading conditions on the mechanical behavior and stress distribution of a leucite-reinforced glass-ceramic. Plate-shaped ceramic specimens were obtained from leucite-reinforced glass-ceramic (1.5 × 8.4 × 8.3 mm) and adhesively cemented to a dentin analog substrate. Monotonic and cyclic contact fatigue tests were performed to simulate sphere-to-flat contact, using a 6 mm diameter spherical piston; and flat-to-flat contact, using a 3 mm diameter flat piston. For the monotonic test (n=20), a gradual compressive load (0.5 mm/min) was applied to the specimen using a universal testing machine. Failure load data were analyzed with Weibull statistics. The cyclic contact fatigue test was performed using protocols (load and a number of cycles) defined by the boundary technique (n=30). Fatigue data were analyzed using an inverse power law relationship and Weibull-lifetime distribution. The stress distribution was investigated using Finite Element Analysis (FEA). The monotonic and the fatigue Weibull modulus were similar among the two contact conditions. In fatigue, the slow crack growth exponent was greater for sphere-to-flat contact, which indicates that the load level had a greater effect on the specimen's probability of failure. In conclusion, FEA showed different stress distribution for the tested loading conditions. The stress distribution and probability of fatigue failure of specimens tested in sphere-to-flat contact showed greater dependency to load level.

Design approach against column local–distortional interactive failures based on the Direct Strength Method

This paper deals with the development and merit assessment of an efficient and rational design approach intended to handle local–distortional (L–D) interactive failures in cold-formed steel columns (CFS) and based on the Direct Strength Method (DSM). Initially, a brief overview of the mechanical aspects underlying local–distortional interaction is provided, for the benefit of readers not familiar with this coupling phenomenon, and its relevance in CFS column design is addressed. Then, the available experimental failure load data, obtained for columns exhibiting various cross-section shapes (plain, web-stiffened and web-flange-stiffened lipped channels, hats and racks) are collected and used as the basis for the development, calibration and validation of this DSM-based design approach, which relies also on the current DSM local and distortional strength curves and existing numerical failure loads. It covers local–distortional​ interactive failures occurring in columns with several ratios between the distortional and local buckling loads, which are grouped in three families, denoted as “true L–D interaction”, “secondary-distortional bifurcation L–D interaction” and “secondary-local bifurcation L–D interaction”. The failure load prediction quality (accuracy and safety) and reliability of the proposed DSM-based design approach are assessed and compared with those exhibited by the existing methodologies to design CFS columns against L–D interactive failures. The reliability assessments follow the procedure prescribed by the current North American Specification for the Design of Cold-Formed Steel Structural Members and indicate that the design approach proposed in this work should be ready for codification in the near future.

Global–global interaction in cold-formed steel plain and lipped channel columns

While investigating how accurately the current Direct Strength Method (DSM) global strength curve predicts the major-axis flexural–torsional failures of cold-formed steel columns, the authors (Dinis et al., 2019; 2020) found strong evidence that, for some column lengths and end support conditions, the ultimate strength is eroded by the coupling between major-axis flexural–torsional (FMT - critical) and minor-axis flexural (Fm) bucking (the first two global buckling modes) - i.e, a global–global (G–G) coupling/interaction. This finding is at the root and provided the motivation for the ongoing numerical investigation whose first results are reported in this work. The paper presents and discusses results concerning the behaviour and DSM design of cold-formed steel (CFS) fixed-ended plain and lipped channel columns that buckle in FMT modes and undergo G–G interaction. After a brief review of the DSM-based design approach that was proposed to handle column FMT failures (Dinis et al., 2020), attention is turned to investigating the column (i) buckling behaviour, essential for the selection of column geometries susceptible to G–G interaction, and (ii) FMT non-linear (post-buckling) behaviour — special care is devoted to the identification of the most detrimental initial geometrical imperfection shape. Then, the FMT failure loads of the selected columns are obtained for five yield stresses (to cover wide slenderness ranges) and the gathered failure load data are used (i) to clearly show that the existing DSM-based design approach cannot adequately predict them and (ii) as guidance for its modification, which brings about the Fm-to-FMT buckling load ratio and enables the adequate handling of also G–G interactive failures. The excellent failure load prediction quality exhibited by this modified design approach is very encouraging and makes the authors believe that its domain of application can be extended to CFS columns with other cross-sections and/or end support conditions — research is under way to assess whether this belief is well founded.

Flexural–torsional failure and DSM design of fixed-ended cold-formed steel columns at elevated temperatures

Recently, Dinis et al. (2019,2020) reported in-depth numerical investigations showing that the current Direct Strength Method (DSM) global design curve may severely underestimate the flexural–torsional (FT) failure loads of fixed-ended cold-formed steel (CFS) columns with slenderness above 1.5. This finding was based on the analysis of columns exhibiting various cross-section shapes and geometries, and covering wide slenderness ranges. Moreover, these authors used the failure load data gathered to develop, and propose a novel DSM column strength curve set that was shown to improve considerably the FT failure load prediction for those columns. The purpose of this work is to extend the scope of the previous investigations to fixed-ended CFS cold-formed columns failing in FT modes under elevated temperatures (up to 800 °C). The results presented and discussed consist of column FT post-buckling equilibrium paths and failure loads, obtained through geometrically and materially non-linear Ansys shell finite element analyses — to ensure covering wide FT slenderness ranges, several room-temperature yield stresses are considered. The model prescribed in Part 1–2 of Eurocode 3, for cold-formed steel, is adopted to describe the temperature-dependence of the steel material properties. Lastly, the numerical failure loads assembled in this work, together with numerical and experimental ones collected from the literature, are used to propose a modification of the DSM-based FT strength curve set developed by Dinis et al. (2020), making it capable of handling FT failures under elevated temperatures. Since the merit assessment of the modified strength curve set shows a visible improvement in FT failure load prediction quality, it is fair to argue that it constitutes a good starting point to search for an efficient general DSM-based design approach for CFS columns failing in FT modes at elevated temperatures.

Numerical and experimental study on CFS spherically-hinged equal-leg angle columns: stability, strength and DSM design

This paper reports a numerical and experimental investigation on the stability/buckling, elastic and elastic-plastic behaviour and strength, and Direct Strength Method (DSM) design of cold-formed steel (CFS) spherically-hinged short-to-intermediate equal-leg angle columns. It extends the scope of similar studies recently carried out for CFS fixed-ended and pin-ended (cylindrically-hinged) columns with the same characteristics. Initially, the paper addresses the stability/buckling behaviour of spherically-hinged columns − the results presented and discussed, which include the selection of the column geometries to be analysed (numerically and experimentally), are obtained by means of Generalised Beam Theory (GBT). Then, the post-buckling behaviour and strength of columns containing initial geometrical imperfections is investigated, by means of Ansys shell finite element analyses, and an experimental study, carried out at UFRJ (Federal University of Rio de Janeiro) and whose results are subsequently used to validate the previously developed numerical model, intended to perform a parametric study to obtain numerical failure load data covering a wide slenderness range for the columns considered. Such results comprise (i) initial imperfection measurements, (ii) load-displacement equilibrium paths, (iii) deformed configurations (including the collapse modes) and (iv) failure loads. The experimental and numerical failure loads obtained are then used to develop, validate and assess the merits of a DSM-based rational design approach that adopts the same concepts and procedures that were previously employed for cylindrically-hinged columns. The need for modifications arises from the new major-axis flexure support conditions, which affect the column flexural-torsional behaviour significantly. It is shown that the proposed/modified DSM strength curves lead to safe and reliable failure load estimates for spherically-hinged short-to-intermediate equal-leg angle columns exhibiting a wide slenderness range, thus making it possible to extend the scope of the existing DSM-based design approach to cover also columns with these end support conditions. The very high prediction quality achieved is attested by the fact that the LRFD resistance factors obtained are well above ϕc = 0.85 (value prescribed by the North American Specification for compression members).

A systematic framework for formulating convex failure envelopes in multiple loading dimensions

The failure envelope approach is widely used to assess the ultimate capacity of shallow foundations for combined loading, and to develop foundation macro-element models. Failure envelopes are typically determined by fitting appropriate functions to a set of discrete failure load data, determined either experimentally or numerically. However, current procedures to formulate failure envelopes tend to be ad hoc, and the resulting failure envelopes may not have the desirable features of being convex and well-behaved for the entire domain of interest. This paper describes a new systematic framework to determine failure envelopes – based on the use of sum of squares convex polynomials – that are guaranteed to be convex and well-behaved. The framework is demonstrated by applying it to three data sets for failure load combinations (vertical load, horizontal load and moment) for shallow foundations on clay. An example foundation macro-element model based on the proposed framework is also described.

Experimental investigation on cold-formed steel stiffened lipped channel columns undergoing local-distortional interaction

Abstract The objective of this work is to report a careful experimental investigation, planned at the University of Lisbon and carried out at The University of Hong Kong, concerning the behaviour and ultimate strength of cold-formed steel web-flange-stiffened (WFSLC) and web-stiffened (WSLC) lipped channel columns undergoing local-distortional (L-D) interaction. It involves 31 specimens (16 WFSLC+ 15 WSLC), brake-pressed from high-strength zinc-coated grades G450, G500 and G550 structural steel sheets, exhibiting critical distortional-to-local buckling load ratios ranging between 0.75 and 1.88. The column geometries were carefully selected to enable testing fixed-ended columns undergoing true L-D interaction and secondary distortional-bifurcation L-D interaction (ensuring evidence of the latter required selecting rather slender columns) − all tested specimens exhibited the expected L-D interactive failures. The specimen material properties were obtained from tensile coupon tests and their initial geometrical imperfections were measured prior to testing. The experimental results presented and discussed consist of column (i) load-displacement equilibrium paths, (ii) photos evidencing the evolution of the column deformed configurations along those paths (including the failure mode) and (iii) failure loads. Finally, the experimental failure load data obtained are compared with their estimates provided by the currently codified DSM design approaches for columns failing in L and D modes, showing their inadequacy to handle L-D interactive failures − the fresh light shed by this comparison will contribute to the timely codification, in the near future, of a DSM design procedure for columns affected by L-D interaction.

Distortional strength of end-bolted CFS lipped channel columns: Experimental investigation, numerical simulations and DSM design

This paper reports the results of an experimental investigation and numerical simulations concerning the influence of bolted end support conditions on the failure loads of cold-formed steel lipped channel columns buckling and failing in distortional modes, as well as on their prediction by means of the Direct Strength Method (DSM). The end supports consist of pairs of cylindrical high-strength bolts with hexagonal heads and nuts, which are inserted in standard-sized holes located at the intersections of the cross-section flanges with the minor principal centroidal axis (parallel to the web). After selecting the column geometries (cross-section dimensions and lengths) and yield stresses, the test set-up and procedure are described in detail, and the experimental results obtained are presented and discussed. Such results consist of initial imperfection measurements, equilibrium paths relating the applied load to key column displacements, deformed configurations (including the collapse mode) and failure loads. Next, those same experimental results are used to validate a previously developed shell finite model, which is subsequently employed to obtain additional numerical failure load data concerning the end-bolted columns under scrutiny. Then, attention is turned to assessing the merits of the available DSM column distortional design curves. Because it is concluded that these design curves clearly overestimate the failure load data obtained, new/modified strength curves are preliminarily proposed and shown to considerably improve the prediction quality (safety and reliability) of the end-bolted column failure loads, thus providing motivation and encouragement to continue the search for an efficient DSM-based design approach for end-bolted columns failing in distortional modes. • Experimental and numerical investigation on end-bolted lipped channel columns failing distortionally • Procedure and results of test campaign involving 13 column specimens described and discussed. • Validated SFE model employed to perform parametric study and gather distortional failure load data. • Available DSM design curves found to grossly overestimate the experimental and numerical failure loads. • Proposal of DSM-based design approaches that improve considerably the failure load prediction quality.

Improving the Direct Strength Method prediction of column flexural-torsional failure loads

Abstract This paper reports the latest results of an ongoing investigation on the accuracy of the codified Direct Strength Method (DSM) global strength curve in predicting the ultimate strength of cold-formed steel columns failing in flexural-torsional modes. The first part of the paper is devoted exclusively to fixed-ended columns and continues recent work by the authors [1, 2] on the improvement of the flexural-torsional failure load estimation, in the moderate or high slenderness ranges − it is shown that the use of a novel set of strength curves, dependent on a cross-section normalised geometric parameter (involving the area, major and minor moments of inertia, and warping constant), leads to excellent failure load predictions, eliminating the large scatter stemming from the codified design curve and improving the DSM-based design approach proposed in [2]. The second part of the paper aims at extending the investigation to columns with other support conditions, namely three types of pinned supports, all fixed with respect to torsion and having warping prevented: end cross-sections attached to rigid plates resting on spherical or cylindrical hinges (i.e., pinned with respect to major and/or minor-axis flexure). Initially, a parametric study is performed, aimed at gathering failure loads concerning columns (i) with the cross-section shapes considered earlier (plain channels, unstiffened and stiffened lipped channels, return lipped channels, hat-sections and rack-sections), (ii) with various geometries (cross-section dimensions and lengths) and (iii) covering a wide slenderness range. These failure load data are then used to assess the quality of their estimates provided by the codified global DSM strength curve and by the strength curve set developed for fixed-ended columns. It is found that neither of them yields consistently good failure load estimates, which prompts (i) an in-depth comparative study on the elastic post-buckling strength of fixed-ended and pin-ended columns, and (ii) the proposal of modifications that lead to an efficient failure load prediction for the pin-ended columns − although the failure load set obtained in this work is necessarily limited, the fact that their predictions by the proposed DSM global design curves (i) exhibit very high quality and (ii) clearly outperform those yielded by the current design curve provides strong encouragement to search for further experimental and numerical validation.

Predicting experimentally-derived failure load at the distal radius using finite element modelling based on peripheral quantitative computed tomography cross-sections (pQCT-FE): A validation study.

Dual energy X-ray absorptiometry, the current clinical criterion method for osteoporosis diagnosis, has limitations in identifying individuals with increased fracture risk, especially at the distal radius. Peripheral quantitative computed tomography (pQCT) can provide volumetric bone density data, as well as information on bone geometry, which makes it possible to establish finite element (FE) models of the distal radius from which bone strength and stiffness can be calculated. In this study, we compared experimental mechanical failure load data of the forearm with pQCT- based FE (pQCT-FE) modelling properties. Sixteen cadaveric forearm specimens were experimentally loaded until failure. Estimated stiffness and strength variables of compression, shear, bending and torsion were calculated from pQCT-FE modelling of single cross-sections of 0.2 × 0.2 × 2.4 mm of the radius pQCT image. A moderate-to-strong coefficient of determination (r2) was observed between experimental failure load and pQCT-FE variables. The highest r2 was observed for bending stiffness (r2 = 0.83). This study validates the use of pQCT-FE in the assessment of distal radius bone strength for future studies.

A comprehensive modelling, analysis and optimisation of adhesive bonded single lap glass fibre reinforced composite joints

This paper presents a study on joint strength of adhesively bonded single lap glass fibre reinforced composite (GFRC). Taguchi L27 orthogonal array with four factors (adhesive thickness, overlap length, surface roughness, and adherend thickness) operating at three levels was used for conducting experiments and recorded the failure load and shear strength data. The results reveal that, adhesive thickness being the most dominant and surface roughness has less dominant on failure load and shear strength of composite joints. Multi objective particle swarm optimisation based on crowding distance (MOPSO-CD) was used to optimise the bi-objective functions simultaneously under the input variable constraints. The fitness value obtained for bi-objective functions were found to be close to unity (0.944). The confirmation tests were conducted for optimised conditions to know the appropriateness of MOPSO-CD tool. It has been found that at the optimised conditions the improvement is 52.94% and 61.12% respectively, for failure load and shear strength.

A comprehensive modelling, analysis and optimisation of adhesive bonded single lap glass fibre reinforced composite joints

This paper presents a study on joint strength of adhesively bonded single lap glass fibre reinforced composite (GFRC). Taguchi L27 orthogonal array with four factors (adhesive thickness, overlap length, surface roughness, and adherend thickness) operating at three levels was used for conducting experiments and recorded the failure load and shear strength data. The results reveal that, adhesive thickness being the most dominant and surface roughness has less dominant on failure load and shear strength of composite joints. Multi objective particle swarm optimisation based on crowding distance (MOPSO-CD) was used to optimise the bi-objective functions simultaneously under the input variable constraints. The fitness value obtained for bi-objective functions were found to be close to unity (0.944). The confirmation tests were conducted for optimised conditions to know the appropriateness of MOPSO-CD tool. It has been found that at the optimised conditions the improvement is 52.94% and 61.12% respectively, for failure load and shear strength.

Preventive maintenance of wood-framed buildings for hurricane preparedness

This paper presents a time-dependent reliability modeling and optimal maintenance planning approach for residential buildings subject to hurricane winds. A gamma process is employed to model the stochastic degradation of building components. The time-dependent fragility function and the failure probability of the roof system obtained from the degradation model are used to determine the retrofit timing of components that optimize an average cost per cycle time. The method is illustrated on determining the retrofit times of roof-to-wall connections based on actual component failure and hurricane wind load data. Effectiveness of the optimal retrofit age is compared to a replacement based on time-invariant failure probabilities. A simulation approach is presented to quantify the effect of wind speed uncertainty on maintenance cost.

Fatigue failure load of an adhesively-cemented lithium disilicate glass-ceramic: Conventional ceramic etching vs etch & prime one-step primer

ObjectivesTo evaluate the effect of different glass-ceramic surface treatments and aging on the fatigue failure load of a lithium disilicate glass-ceramic adhesively cemented to a dentin analogue material. MethodsOne hundred and twenty (120) disc-shaped lithium disilicate specimens (Ø=10mm, thickness=1.5mm) were produced and randomly allocated (n=20) into 6 groups, considering 2 study factors: “surface treatment” in 3 levels (SIL—silane application only; HF5+SIL—5% hydrofluoric acid etching and silane application; ME&P—etching with an one-step ceramic primer), and “storage” in 2 levels (baseline—storage for 7 days; aging—storage for 90 days+12,000 thermal cycles). Ceramic discs were adhesively cemented to discs of a dentin analogue material (Ø=10mm, thickness=2.0mm) following the manufacturers’ instructions. The fatigue failure load was determined by the staircase approach (250,000 cycles; 20Hz; initial load=1050N [∼70% of mean load-to-failure]; step size=52.5N [5% of initial load]). Micro-morphologic, fractographic, and atomic force microscope analysis were also performed. Fatigue failure load data were evaluated by one-way ANOVA, Bonferroni and t-tests for independent samples. ResultsHF5+SIL presented higher fatigue failure load in both conditions (baseline and aging); ME&P presented intermediary mean values, while the SIL group presented the worst performance. All groups had a statistically significant decrease in the fatigue performance after aging. SignificanceHydrofluoric acid followed by silane application showed the best fatigue performance for an adhesively-cemented lithium disilicate ceramic. Aging negatively influenced the fatigue performance for all tested groups.

On the distortional-global interaction in cold-formed steel columns: Relevance, post-buckling behaviour, strength and DSM design

This work reports the available results of an ongoing numerical (shell finite element) investigation on the post-buckling behaviour, strength and design of fixed-ended cold-formed steel columns undergoing distortional-global (D-G) interaction. Columns with different cross-section shapes are analysed, namely plain lipped channel (LC), web-stiffened lipped channel (WSLC) and zed-section (Z) columns, in order to investigate distinct D-G interaction natures: involving either distortional and (global) flexural-torsional buckling (LC and WSLC columns) or distortional and (global) minor-axis flexural buckling (Z columns). In particular, the relevance of these D-G interaction types is discussed, by assessing when they affect visibly the column ultimate strength and/or failure mode. The results presented and discussed concern columns with various geometries and yield stresses, thus ensuring a wide variety of range combinations involving (i) global-to-distortional critical buckling load ratios (RGD) and (ii) squash-to-non critical buckling (distortional or global) load ratios (Ry) and leading to non-negligible failure load erosion. The possible occurrence and failure load impact of “secondary (distortional or global)-bifurcation D-G interaction” (RGD<1.0 or RGD>1.0 and high Ry) are investigated - it is well known that such impact may be significant in columns experiencing “true D-G interaction” (RGD≈1.0). The above results consist of (i) relevant non-linear (elastic and elastic-plastic) equilibrium paths, (ii) deformed configuration evolutions along those paths, evidencing D-G interactive effects, and (iii) figures providing the failure mode characterisation. Then, the numerical failure load data obtained are compared with their predictions by (i) the currently codified DSM (Direct Strength Method) column global and distortional strength curves, and (when necessary) (ii) proposed DSM-based design approaches, specifically developed to handle D-G interactive failures - a few design considerations are drawn from these comparisons.

Placement of protective devices in distribution system considering uncertainties in loads, temporary and permanent failure rates and repair rates

Distribution system reliability is directly affected by the placement of various types of protective devices at different locations of the distribution system. To reduce the momentary interruptions, fuse-blow scheme is used which in turn increases sustained interruption. Utilities having more concern to the sustained interruptions prefer to use fuse-save scheme over fuse-blow scheme. Different models have been developed to solve the problems incorporating placement of various combinations of protective devices in a distribution network for increasing the profit of the utility. The uncertainties in temporary failure rates, permanent failure rates, repair rates and load data have been considered in the problem formulation using three-point estimate method. A generalised objective function capable of simulating different combinations of the protective devices has also been developed. The formulated problems have been solved for 58 bus and IEEE 123 bus distribution networks using mixed integer non-linear programming optimisation technique. After analysing the test results of the various scenarios for the two test systems, it is concluded that maximum profit for the utility is obtained by using the scenario corresponding to a combination of the four protective devices, namely reclosers, switches, fuse-blow fuses and fuse-save fuses.

The effect of hydrofluoric acid concentration on the fatigue failure load of adhesively cemented feldspathic ceramic discs

ObjectiveThis study investigated the influence of hydrofluoric acid (HF) etching at different concentrations on the fatigue failure load of adhesively cemented feldspathic ceramic discs (Vita Mark II). Besides, their effect on the micromorphology of ceramic surface was investigated. MethodsEighty ceramic discs (ϕ=10 mm; thickness=1.5 mm) were cemented to epoxy supporting discs (ϕ=10 mm; thickness=2.0mm) using different surface conditioning methods (n=20): nonetched control (CTRL), or etched for 60s with different HF concentrations: 1% (HF1), 5% (HF5), or 10% (HF10). All the ceramic discs received a silane application (Monobond Plus). The epoxy discs were etched with 10% HF for 60s and received a primer coating (Multilink Primer A+B). Adhesively cementation was performed (Multilink Automix), and the assemblies (ceramic discs/epoxy discs) were subjected to cyclic loads in water by a staircase approach (500,000 cycles; 20Hz; initial load=290N; step size=30N). Fatigue failure load data were analyzed using 1-way ANOVA and post-hoc Tukey’s tests (α=.05). ResultsMean failure load of the HF5 group (255.0±23.0N) was significantly lower; HF1 group (301.7±71.0N) presented intermediate values, and the highest values were achieved in CTRL (351.7±13.4N) and HF10 (341.7±20.6N) groups. All the failures were radial cracks starting from the bonding surface. SignificanceIn terms of fatigue failure load, etching with 1% and 5% HF had a deleterious effect on the fatigue behavior of an adhesively cemented feldspathic ceramic, while 10% HF had no negative influence.

On the direct strength design of cold-formed steel columns failing in local-distortional interactive modes

This paper present and discusses proposals for the codification of efficient design approaches for cold-formed steel columns affected by local-distortional (L-D) interaction. These proposals, based on the Direct Strength Method (DSM), were developed, calibrated and validated on the basis of experimental and numerical (shell finite element) failure load data concerning columns with several cross-section shapes (plain, web-stiffened and web-flange-stiffened lipped channels, hat-sections, zed-sections and rack-sections) and obtained from investigations carried out by various researchers. Three types of L-D interaction are taken into account, namely “true L-D interaction”, “secondary-local bifurcation L-D interaction” and “secondary-distortional bifurcation L-D interaction”. Moreover, previously available DSM-based design approaches developed to handle column L-D interactive failures are reviewed and their merits are assessed and compared with those exhibited by the present proposals. The paper also presents reliability assessments of the failure load predictions provided by the available and proposed DSM-based design approaches, following the procedure prescribed by the current version of the North American Specification for the Design of Cold-Formed Steel Structural Members.