Failure Load Data Research Articles

13.11: DSM design approach for hot‐rolled steel angle columns

ABSTRACTThis work reports the latest results of an ongoing investigation on the Direct Strength Method (DSM) design of hot‐rolled steel equal‐leg angle columns with fixed and pinned (cylindrical or spherical hinges) supports and short‐to‐intermediate lengths, i.e., buckling in flexural‐torsional modes. It extends the scope of similar studies carried out by the authors for cold‐formed (higher leg width‐to‐thickness ratios) steel angles with the same characteristics. After reviewing (i) the most relevant findings unveiled in the studies concerning hot‐rolled steel angles, including the role played by residual stresses in eroding their ultimate strength, and (ii) the available experimental and numerical failure load data, comprising fixed‐ended and pin‐ended columns with several geometries (cross‐section dimensions and length) and reported by various researchers, the paper briefly addresses the mechanical reasoning behind an efficient DSM‐based design approach proposed by the authors in the context of cold‐formed steel (slender legs) short‐to‐intermediate angle columns. Then, the available failure load data is used to assess the quality of their estimates provided by this DSM design approach, including the determination of the corresponding Load and Resistance Factor Design (LRFD) resistance factors. It is shown that the proposed design approach can be also successfully applied to predict hot‐rolled steel angle column failure loads, leading to LRFD resistance factors above the value currently prescribed, for compression member design, by the North American Specification (ϕc=0.85).

CFS lipped channel columns affected by L-D-G interaction. Part II: Numerical simulations and design considerations

This work deals with the structural behaviour, ultimate strength and design of fixed-ended cold-formed steel (CFS) lipped channel columns experiencing various levels of local-distortional-global buckling mode interaction (more or less close critical local, distortional and global loads). First, a comparison between experimental results, obtained from three tests carried out at The University of Hong Kong (UHK), and the corresponding Abaqus shell numerical simulations are presented and discussed – the UHK test campaign is reported in Part I of this two-part paper. The comparison (i) involves equilibrium paths, deformed configurations, failure loads and collapse mechanisms, and (ii) provides enough evidence to ensure that the numerical model developed may be deemed validated. Then, this numerical model is employed to perform a parametric study, aimed at obtaining numerical failure load data concerning lipped channel columns experiencing different levels of local-distortional-global interaction – 368 fixed-ended columns, exhibiting different geometries (cross-section dimensions and lengths) and various yield stresses, thus covering a wide slenderness range. Finally, the present (Part II) paper also assesses the quality of the failure load estimates provided by various design approaches, based on the Direct Strength Method (DSM), for CFS columns affected by the triple coupling phenomenon under consideration. This assessment is based on the comparison with both (i) experimental failure loads, namely those reported in Part I and others collected from the literature, and (ii) numerical failure loads, obtained from Abaqus shell finite element analyses (SFEA) and either reported in this work or gathered from previous publications by the authors.

Effect of an Outer Sleeve on an Inflatable Balloon Tamp in Terms of Height Restoration Under Simulated Physiological Load.

An in vitro biomechanical study. The aim of this study was to determine the effect of an optional sleeve on height restoration and compare it with the fracture reduction achieved by a commercially available inflatable bone tamp under simulated physiological load (110 N). Loss of reduction after bone tamp deflation before cement injection still remains a concern. The optional sleeve surrounds the bone tamp to help maintain height during the kyphoplasty procedure while filling the created cavity with bone cement on the contralateral side. Eighteen osteoporotic vertebral bodies (VBs) (T11-L4) were alternately assigned to 1 of the 2 treatment groups: group A: KYPHON (Kyphon Inc.) and group B: AFFIRM with sleeve (Globus Medical Inc.). The VBs were compressed axially at a rate of 5 mm/min until compressed to 40% of the initial anterior height. The fractured VBs then underwent kyphoplasty with cement augmentation while still maintaining load (110 N). The augmented VBs were then recompressed and anterior VB height (mm) and wedge angle (degrees) were measured initially after mechanically creating an anterior wedge fracture, and after repairing the compression fracture. The effect of kyphoplasty on vertebral height, kyphotic angle, cement volumes, and inflation pressures were compared between the treatment groups. Failure load (N) data were compared between intact and repaired VBs. Average percentage of lost VB height restored in group A was 30%, compared with 56% for group B. The mean changes in wedge angle were similar to those of vertebral height measurements. No significant difference in mean inflation pressures (group A: 175±37 psi; group B: 160±36 psi) were found between the 2 groups. Average percentage increase in failure load was 241% and 212% in groups A and B, respectively. Some height restoration was observed using the commercially available bone tamp in fractured VBs under simulated physiological load. The use of an outer sleeve significantly enhanced height restoration compared with the inflatable bone tamp alone.

Tensile strength and failure load of sutures for robotic surgery.

Robotic surgical platforms have seen increased use among minimally invasive gastrointestinal surgeons (von Fraunhofer et al. in J Biomed Mater Res 19(5):595-600, 1985. doi: 10.1002/jbm.820190511 ). However, these systems still suffer from lack of haptic feedback, which results in exertion of excessive force, often leading to suture failures (Barbash et al. in Ann Surg 259(1):1-6, 2014. doi: 10.1097/SLA.0b013e3182a5c8b8 ). This work catalogs tensile strength and failure load among commonly used sutures in an effort to prevent robotic surgical consoles from exceeding identified thresholds. Trials were thus conducted on common sutures varying in material type, gauge size, rate of pulling force, and method of applied force. Polydioxanone, Silk, Vicryl, and Prolene, gauges 5-0 to 1-0, were pulled till failure using a commercial mechanical testing system. 2-0 and 3-0 sutures were further tested for the effect of pull rate on failure load at rates of 50, 200, and 400mm/min. 3-0 sutures were also pulled till failure using a da Vinci robotic surgical system in unlooped, looped, and at the needle body arrangements. Generally, Vicryl and PDS sutures had the highest mechanical strength (47-179kN/cm2), while Silk had the lowest (40-106kN/cm2). Larger diameter sutures withstand higher total force, but finer gauges consistently show higher force per unit area. The difference between material types becomes increasingly significant as the diameters decrease. Comparisons of identical suture materials and gauges show 27-50% improvement in the tensile strength over data obtained in 1985 (Ballantyne in Surg Endosc Other Interv Tech 16(10):1389-1402, 2002. doi: 10.1007/s00464-001-8283-7 ). No significant differences were observed when sutures were pulled at different rates. Reduction in suture strength appeared to be strongly affected by the technique used to manipulate the suture. Availability of suture tensile strength and failure load data will help define software safety protocols for alerting a surgeon prior to suture failure during robotic surgery. Awareness of suture strength weakening with direct instrument manipulation may lead to the development of better techniques to further reduce intraoperative suture breakage.

Short-to-intermediate slender pin-ended cold-formed steel equal-leg angle columns: Experimental investigation, numerical simulations and DSM design

Angles exhibit a complex structural behaviour, responsible for the fact that, in the current North American Specification for Cold-Formed Steel Structures, short-to-intermediate equal-leg angle columns are (i) not yet pre-qualified for the Direct Strength Method (DSM) design and (ii) excluded from the application of the LFRD resistance factor ϕ=0.85, valid for all other cold-formed steel compression members. Recently, the specific behavioural features exhibited by the above angle columns were incorporated into the proposal of a novel DSM-based design approach, for both fixed-ended and pin-ended columns, and it was shown that this added rationality goes along with quite accurate and reliable failure load predictions. However, the investigation leading to this design proposal also unveiled that there are no available experimental failure loads of slender pin-ended columns with intermediate-to-high slenderness values, which implied that the design procedure was validated for such columns exclusively on the basis of numerical failure loads. The research work reported in this paper provides a contribution towards filling this gap, since it mainly consists of an experimental study, carried out at the Federal University of Rio de Janeiro, on the behaviour and collapse of short-to-intermediate slender pin-ended cold-formed steel equal-leg angle columns. After addressing the selection of the columns to be tested, the experimental set-up and test procedure are described in detail and the results obtained are presented and discussed. Such results involve (i) initial imperfection measurements, (ii) equilibrium paths relating the applied load to key column displacements, (iii) deformed configurations (including the collapse mode) and (iv) failure loads. Next, those same experimental results are used to validate a shell finite model previously developed by the authors, which is subsequently employed to obtain additional numerical failure load data concerning the pin-ended angle columns under scrutiny. Then, attention is turned to assessing the merits of the novel design approach. The comparison between the experimental and numerical values obtained in this work and their estimates provided by the design equations shows a very good correlation, perfectly in line with that observed in the recent studies available in the literature−this means that the validation and calibration of the above design approach may be deemed (successfully) completed. Finally, the paper closes with the presentation and assessment of small alterations to the existing design expressions, aimed at improving their accuracy and rationality, thus paving the way towards codification in the near future.

Numerical investigation and direct strength design of cold-formed steel lipped channel columns experiencing local–distortional–global interaction

This paper reports the results of a numerical investigation concerning the relevance and Direct Strength Method (DSM) prediction of the ultimate strength erosion caused by local-distortional-global (LDG) interaction in cold-formed steel fixed-ended lipped channel columns. The geometries of the columns analysed (cross-section dimensions and lengths) were carefully selected to ensure that the three competing critical buckling loads are not more than 20% apart, thus guaranteeing a fairly high level of LDG coupling, and ordered in all the various possible sequences. In order to cover a wide slenderness range, several yield stresses are considered, falling below, in-between and above the lowest and highest critical buckling stresses. After providing a brief description of the column selection procedure, which is based on buckling analyses performed with Generalised Beam Theory (GBT), the methodology adopted to identify the most detrimental initial geometrical imperfection shape (in the sense it minimises the column strength) is addressed – it employs Koiter's asymptotic method along with a Monte Carlo simulation. Then, columns containing those initial geometrical imperfections are compressed up to failure, by means of Abaqus shell finite element analyses (SFEA), making it possible to acquire in-depth knowledge on the behaviour of lipped channel columns undergoing LDG interaction and gather considerable failure load data. Finally, the last part of the paper is devoted to the DSM prediction of those failure loads and uses the obtained “data bank” to assess whether the available design approaches are able to handle adequately the ultimate strength erosion caused by the triple interaction phenomenon under investigation – if this is not the case, new design curves must be developed.

Effect of Different Luting Agents on the Retention of Lithium Disilicate Ceramic Crowns.

No studies are available that evaluate the retention of disilicate crowns according to different cementation procedures. The purpose of this study was to measure the retention of lithium disilicate crowns cemented using two different cementation systems. Twenty extracted mandibular premolars were prepared. Anatomic crowns were waxed and hot pressed using lithium disilicate ceramic. Teeth were divided into two groups (n = 10): (1) self-curing luting composite and (2) glass-ionomer cement (GIC). After cementation, the crowns were embedded in acrylic resin block with a screw base. Each specimen was pulled along the path of insertion in Universal Testing Machine. Failure load in Newtons (N) and failure mode were recorded for each specimen. Failure mode was classified as decementation or fracture. Failure load data were analyzed using one-way analysis of variance (ANOVA). Failure modes were compared using Pearson’s Chi-square test. Mean failure load was 306.6(±193.8) N for composite group and 94.7(±48.2) N for GIC group (p = 0.004). Disilicate crown cemented with luting composite most often failed by fracture; otherwise, crown cemented with glass-ionomer cement most often failed by decementation (p = 0.02). Disilicate full crown cemented with luting composite showed higher failure load compared with conventional cementation with glass-ionomer cement.

Stress field based truss model for shear-critical prestressed concrete beams

Despite more than a century of continuous effort, shear still remains one of the few areas of research into fundamentals of the behavior of concrete structures where dispute remains amongst researchers about the mechanisms that enable the force flow through a concrete member and across cracks. This paper firstly presents the results of 9 full-scale prestressed concrete I-shaped beams subjected to a four-point bending test until failure. Two stereo-vision digital image correlation (DIC) systems were used to discretely measure three-dimensional displacements in both zones where a shear force exists. A numerical technique has been adopted to generate optimized patterns for DIC and the resulting speckle pattern was applied onto each specimen using a stencil printing technique. Using the sectional shear design procedure from Eurocode 2, a severe underestimation of the experimentally observed shear capacity was found. Therefore, a parametric nonlinear finite element (FE) model was constructed to further investigate the mechanical behavior. The ability of the FE model to reconstruct the observed structural behavior was validated using the experimental data concerning failure load, failure mode and (full-field) displacement data. Finally, based on the numerically obtained stress fields, an internally statically indeterminate truss model was proposed which clearly visualizes the flow of forces and accurately describes the observed failure load and failure mode. A simple relationship between the applied shear force and the amount of shear force carried by the shear reinforcement was proposed. The proposed relationship was validated using the reported experimental data.

Cold‐formed steel columns under L‐D‐G interaction

AbstractThe paper addresses an experimental programme that is part of an ongoing investigation on the behaviour, strength and design of cold‐formed steel lipped channel columns undergoing mode interaction involving local, distortional and global buckling. The 16 column specimens tested were selected to exhibit either (i) close local, distortional and critical buckling loads (PcrL ≈︁ PcrD ≈︁ PcrG) or (ii) PcrL ≈︁ PcrD (condition ensured via the cross‐sectional dimensions) and PcrG up to around 20 % below or above that value (difference controlled via the column length). The test set‐up, procedure and results are presented and discussed, and the failure load data obtained are used to explore the possibility of developing a design approach based on the direct strength method (DSM).

Analysis and Comparison of the Biomechanical Properties of Univalved and Bivalved Cast Models

Fiberglass casts are frequently valved to relieve the pressure associated with upper extremity swelling after a surgical procedure or when applied after reduction of a displaced fracture in a child. Although different opinions exist regarding the valving of casts, no research to date has explored the biomechanical effects of this commonly used technique. As cast integrity is essential for the maintenance of fracture reduction, it is important to understand whether casts are structurally compromised after valving. Understanding the effects of valving on cast integrity may help guide clinicians in the technique of valving while minimizing the potential for a loss of fracture reduction. Thirty standardized cylindrical fiberglass cast models were created. Ten models were left intact, 10 were univalved, and 10 were bivalved. All the models were mechanically tested by a 3-point bending apparatus secured to a biaxial materials testing system. Load to failure and bending stiffness were recorded for each sample. Differences in load of failure and bending stiffness were compared among the groups. Unvalved cast models had the highest failure load and bending stiffness, whereas bivalved casts showed the lowest value for both failure load and bending stiffness. Univalved casts had a failure load measured to be between those of unvalved and bivalved cast models. Analysis of variance showed significance when failure load and bending stiffness data among all the groups were compared. A post hoc Bonferroni statistical analysis showed significance in bending stiffness between intact and bivalved models (P < 0.01), intact and univalved models (P < 0.01), but no significant difference in bending stiffness between univalved and bivalved models (P > 0.01). Differences in measured failure load values were found to be statistically significant among all cast models (P < 0.01). Valving significantly decreases the bending stiffness and load to failure of fiberglass casts. Univalved casts have a higher load to failure than bivalved casts. Valving adversely alters the structural integrity of fiberglass casts. This may impair a cast's ability to effectively immobilize an extremity or maintain a fracture reduction.

Effect of connector design on the fracture resistance of all-ceramic fixed partial dentures

Statement of Problem. Fracture of all-ceramic fixed partial dentures (FPDs) tends to occur in the connector area. Purpose. The objective of this study was to test the hypothesis that the radii of curvature at the connector affects the fracture resistance of 3-unit FPDs. Material and Methods. With the use of a standardized silicone mold, 40 three-unit FPD wax patterns were fabricated with the same dimensions and divided into 4 groups of 10 specimens per group. Each pattern was modified at the connector areas of the occlusal embrasure (OE) and the gingival embrasure (GE); 2 wax carvers with radii of curvature at their tips of 0.90 mm and 0.25 mm were used. The dimensions of the connectors were standardized with an electronic caliper to 4 ± 0.12 mm in height and 5 ± 0.13 mm in width. Connector designs were as follows: Design I: OE and GE 0.90 mm; Design II: OE 0.90 mm and GE 0.25 mm; Design III: OE 0.25 mm and GE 0.90 mm; and Design IV (control): OE and GE 0.25 mm. An experimental hot-pressed core ceramic was used to make the FPD frameworks, which were consequently cemented on epoxy dies with dual-polymerizing composite (Variolink II) and loaded to fracture in a universal testing machine at a crosshead speed of 0.5 mm/min. The failure load data were analyzed with analysis of variance (ANOVA; P=.05) and Duncan's test (α=.01). Results. The mean failure loads and standard deviations were as follows: 943 ± 151 N for Design I; 746 ± 106 N for Design II; 944 ± 144 N for Design III; and 673 ± 55 N for Design IV. ANOVA revealed a significant difference (P≤.0001) between the mean failure loads of different connector designs. The mean loads to failure for Designs I and III were significantly higher than those for Designs II and IV (Duncan's test). Conclusion. Within the limitations of this study and for the experimental ceramic tested, as the radius at the gingival embrasure increased from 0.25 to 0.90 mm, the mean failure load increased by 140%. The radius of curvature at the occlusal embrasure had only a minor effect on the fracture susceptibility of 3-unit FPDs. (J Prosthet Dent 2002;87:536-42.)

A comparison of soft-tissue anchors in tendo achilles reattachment

This prospective study evaluated four soft-tissue fixation modalities, used in seven different combinations, to reattach the tendo Achilles in 34 cadaveric specimens. Ultimate loads, elastic moduli, and modes of failure were evaluated by loading the specimen in a cantilevered fashion on an Instron. Mann-Whitney U tests were performed to compare the failure load data for statistical significance. Although the use of two Mitek SuperAnchors showed better load resistance than one anchor (p < .01), there was no significant improvement between using two or three anchors (one anchor 116 +/- 24 N, two anchors 234 +/- 21 N, three anchors 277 +/- 80 N). Two Bionx Bankart Tacks demonstrated no significant difference over using a single tack (one tack 178 +/- 57 N, two tacks 214 +/- 86 N). No statistical difference was observed between the screw and washer systems (screw with polyacetal resin washer 307 +/- 80 N, screw with metal washer 290 +/- 81 N). Both screw and washer systems did show greater stability when compared with a single Mitek SuperAnchor (p < .01) or a single Bionx Bankart Tack (p < .05). Similar analyses using the Mann-Whitney U tests were performed on the elastic modulus data. Analysis of the displacement data among all groups showed no statistical difference. Observations of the mode of failure exhibited 86% of Mitek SuperAnchor failed secondary to suture, and 70% of the Bionx Bankart Tack and 90% of the screw and washer systems failed because of the tendon shearing around the fixation. The comparisons of cost-effectiveness among the fixations showed the Synthes screw and polyacetal resin spiked washer to have the lowest cost to load ratio ($0.15/N).

Analysis of resin-transfer-moulded single-lap joints

This paper presents a fracture-mechanics-based approach to the prediction of the failure of single-lap joints manufactured by resin-transfer moulding. Since single-lap joints can undergo large deformation, a higher-order large-deformation theory is used to analyse the joint. Mode I and mode II components of energy release rate are determined from the loads and moments in the adherend and the overlap region at the end of the overlap. A linear failure criterion is used to predict the failure load. Comparison with experimental failure load data showed excellent agreement with theoretical prediction.

The Performance of Epoxy-Coated Shear Reinforcement

This paper examines the performance of epoxy-coated stirrups in reinforced concrete beams under bending and shear. A review of previous studies in bond and detailing of stirrups is provided. The results of an experimental program are described. From these results, several conclusions are reached, supported by ultimate failure load, deflection, stirrup strain, crack pattern, and crack width data. All beams reinforced with epoxy-coated stirrups and those with uncoated stirrups failed at higher than calculated shear capacities from those in American Concrete Institute (ACI) 318-89 and the proposed American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) 1993 Bridge Specifications.

Failure of All-ceramic Fixed Partial Dentures in vitro and in vivo: Analysis and Modeling

Hertzian cone cracks visible at the loading site of 20 all-ceramic fixed partial dentures (FPDs), tested in vitro, led to the hypotheses that failure was due to the propagation of localized contact damage crack systems (Hertzian stress state) and that such damage was an unlikely clinical failure mode. Fractographic analysis of the 20 laboratory-failed and nine clinically-failed all-ceramic FPDs allowed for definitive testing of these hypotheses and a comparison between in vitro and in vivo failure behavior. In all cases, failure occurred in the FPD connectors (none from contact damage), with approximately 70 to 78% originating from the interface between the core and veneer ceramics. The coincidence between failure origins provides strong evidence that the in vitro test modeled aspects of structural behavior having clinical importance. The fractographic observations, coupled with the in vitro failure load data, furnished very specific boundary conditions which were applied to constrain mathematical models of FPD connector failure. Finite element analysis (FEA) of the laboratory FPDs found that maximum principal tensile stresses would occur at locations consistent with the fractographic observations only if: (1) there were appropriate elastic moduli differences between the ceramics; and (2) a small amount of abutment rotation was allowed. Weibull failure probability (Pf) calculations, incorporating FEA stress profiles, very closely replicated the laboratory failure distribution only when: (1) the veneer ceramic was much weaker than the core ceramic; and (2) the Weibull modulus of the core-veneer interface was much lower than that for the free veneer surface (i.e., the interface is of lower quality with regard to defects).(ABSTRACT TRUNCATED AT 250 WORDS)

The Weibull distribution applied to post and core failure

In this study, data on initial failure loads of direct post and core-restored premolar teeth were analyzed using the Weibull distribution. Restorations consisted of a prefabricated titanium alloy post, and an amalgam, composite or glass cermet core buildup in human upper premolar teeth. The specimens were subjected to compressive forces until failure at angles of 10, 45 and 90° to their long axis. The two- and three-parameter Weibull distributions were compared for applicability to the failure load data. For estimation of the parameters of the two-parameter distribution: σ 0 (reference stress) and m (Weibull modulus), linear regression was used. In this distribution, it is assumed that the third parameter, σ u (cut-off stress), equals 0. The Maximum Likelihood (MLH) method was used to estimate all three parameters. It was found that the choice of distribution has a strong influence on the estimated values and that the three-parameter distribution is best fitted for the failure loads in this study. Comparisons were made between the failure probability curves as found by MLH estimation for the different core materials and loading angles. The results indicated that the influence of loading angle on the failure mechanism was stronger than that of core material.