PB Specimens Research Articles

Experimental and meso-scale investigation of size effect on fracture properties in three-point concrete beams (TPB)

In this study, the size effect on concrete fracture parameters has been investigated in three distinct groups. While the geometry is similar across each group, the span-to-height ratio varies among the three groups. The study investigates the impact of three different span-to-height ratios (1.85, 2.65, and 4) and five different notch-to-beam height ratios (0.1, 0.2, 0.3, 0.4, and 0.5) by using three-point notched beams (TPB). Beams with identical spans but different heights are used. A new method is proposed to determine the fracture properties of concrete based on the Boundary Effect Method (BEM). A proposed formula for the fictitious cracks is presented based on the results of numerical analyses at the mesoscale and used in the BEM to determine the values of tensile strength and fracture toughness with the effects of span-to-height ratio. The established mesoscale fracture model reproduces the load-displacement curves of 3 PB specimens concrete, which verifies the rationality of the model with the experimental results. Numerical simulation reveals that the fictitious crack length in concrete is highly sensitive to the size of aggregates, their distribution and spacemen’s dimensions. Discussion on BEM based on principles of mathematical fitting and data processing is provided for more objective quasi-brittle fracture experiments and modeling. The size effect law (SEL) is limited by its requirement for sample geometries to be similar, which introduces some additional error when applied to samples with dissimilar geometries. To minimize this error, this study proposes a new formulation within the BEM method. It is found that the fracture parameters predicted by the BEM method using the proposed formulation for different geometries are consistent with the size effect law (SEL) for similar geometries.

Temperature Effect on Short—Term Strength of Lithium Hydride with Tensile and Three—Point Bend Specimens

Strength is one important mechanical property of lithium hydride (LiH) for application as a neutron moderator and absorbent at elevated temperatures. Using the digital image correlation (DIC) method for deformation measurement, the short—term strength of LiH was investigated with both tensile and three—point bend (3PB) specimens in the range of room temperature (RT) to 600 °C. The results show that the temperature dependence of the strength from tensile and 3PB tests shows similarity. As the temperature increases, the strength changes slightly, then rises to the maximum and finally decreases due to the softening effect. The fracture surfaces of specimens show the main characteristics of trans—granular fracture at different temperatures, except for 600 °C. With increasing temperature, the fracture surfaces become rough and numerous tortuous micro—cracks are found. Especially for 3PB specimens, there are dimple—like structures with distorted grains and obvious intergranular cleavage fracture.

Behaviour of retrofited precast UHPC and Ni-Ti SMA column-to-foundation connection with CFRP wrapping layers

Four precast column-to-foundation connection specimens, previously tested under lateral reversed cyclic loading and constant axial load (first-stage tests), were repaired with Carbon Fibre Reinforced Polymer (CFRP) layers in the areas where plastic hinges had formed and were retested under the same loads (second-stage tests). The columns were originally manufactured with ultrahigh-performance concrete (UHPC) with high stee-fibre content (volumetric steel/fibre ratio 1.9%). The steel bars in the connection were replaced with superelastic Ni-Ti SMA bars that crossed the column/foundation interface. Two types of connection between the precast column and the foundation were analysed: protruding bars and smooth pocket bars (SP). The specimens were subjected to the same constant level of relative axial load in both the first and second-stage tests. The objective was to determine whether the initial performance of precast UHPC and SMA reinforced column-to-foundation connections against lateral cyclic loads can be restored after a seismic event by means of simple and economic retrofitting. The effectiveness of the number of strengthening CFRP layers was also studied (two or three CFRP wrapping layers) for each column-to-foundation connection type. The main results for PB and SP connection specimens with respect to the original specimens were respectively: an average strength capacity loss of 8% and 19%, displacement ductility increase of 8% and 42%, dissipated energy 48% and 53% higher and initial stiffness 9.5% and 21% lower. No significant differences were observed in the repairs done with two or three CFRP wrapping layers.

Loading mode effect on brittle fracture resistance of cracked component under large-scale yielding

This study assesses the applicability of the Weibull stress for the assessment of the crack-tip plastic constraint effect as well as the mixed mode I and II loading effect on brittle fracture resistance of a ferritic steel. Brittle fracture toughness under different crack-tip plastic constraint conditions and different loading mode conditions was respectively obtained by conducting 3-point bend (3PB) test for single edge cracked specimens with different crack depth subjected to mode I load, and 4-point shear (4PS) test for a single edge cracked specimen with deep crack depth subjected to mixed mode I and II load. By using the results of 3PB tests, the critical Weibull stresses distribution independent of crack-tip plastic constraint was identified, and the critical Weibull stresses obtained by 4PS tests provided significantly smaller distribution than that for 3PB specimen. The mixed mode loading provides the different combined stress field around the crack-tip from that under mode I loading, where the 4PS specimen showed the lower distribution of σ2/σ1 and σ3/σ1 than the 3PB specimens. The lower principal stress ratio to be enlarge the fracture driving force was found to have no or much less influence to linear energy release rate for micro-crack, that was employed in the conventional derivation of the Weibull stress. Consequently, the reason why the conventional Weibull stress could not evaluate the effect of loading mode on fracture resistance could be that the linear energy release rate could not exactly take into account the local fracture driving force enlarged due to the combined stress field under mixed mode loading.

Determination of concrete strength and toughness from notched 3 PB specimens of same depth but various span-depth ratios

Notched three-point-bending (3 PB) tests of small concrete samples with a fixed depth/size but various span/depth ratios can be performed in almost every laboratory. Additional usefulness is added to the flexibility of those fixed-depth 3 PB tests in this study by providing a simple closed-form solution for determination of the depth and span independent tensile strength ft and fracture toughness KIC measurable from 3 PB samples with any span-depth ratio. Material constants, ft and KIC, only existed asymptotically for large concrete structures are determined by the simple fracture mechanics model regardless the span-depth ratio. Recent results of notched 3 PB tests with 6 span-depth ratios (2, 2.5, 3, 4, 5 and 6) were analysed again by the closed-form model and compared with the effective fracture toughness in the previous study (without ft). A good agreement in KIC values was obtained. That is the two different fracture models have affirmed the usefulness of small notched 3 PB tests of a fixed depth/size but with flexible choices of the span/depth ratio. Moreover, the weight of specimens is suggested to be considered when the span/depth ratio is greater than 4.0 for 3 PB tests. Finally, fracture statistics analysis, not considered by the previous method, was included in this study, which provided the 95% reliability band besides the mean ft and KIC values.

Employing a hybrid GA-ANN method for simulating fracture toughness of RCC mixture containing waste materials

The present study is conducted to investigate the efficiency of evolutionary algorithms such as genetic algorithm (GA)-evolved neural network in estimating fracture properties of roller compacted concrete pavement (RCCP) mixtures with different compositions. The effect of waste materials, including reclaimed asphalt pavement (RAP) and crumb rubber on fracture toughness in both pure mode I and pure mod II, were investigated using a real coded GA and an evolution with a back-propagation algorithm. Moreover, the geometry effect of SCB and 4 PB specimens on predicting fracture toughness was evaluated using GA. To evaluate the GA-based neural network's performance, the NSE criterion was applied for fitness function, a different approach for fitting in this area. Many researchers have studied the fracture behavior of RCC in diverse modes. Still, their studies have been restricted to the materials' fracture behavior, which has rarely come to a model. As pure mode II fracture toughness experiment is a complicated procedure with high uncertainties, the introduced model provides a powerful tool for predicting mode II fracture toughness via mix composition and pure mode I fracture toughness of RCC based on GA. The proposed model outperforms the traditional artificial neural network (ANN) models. The geometry effect survey on predicting fracture toughness shows that the 4 PB fracture model has been better fitted to observed data. Also, the MSE results show that prismatic specimens present relatively reliable results than SCB specimens.

Effects of Crack Tip Constraint on the Fracture Toughness Assessment of 9% Ni Steel for Cryogenic Application in Liquefied Natural Gas Storage Tanks.

Recently, increasing demand for the accurate assessment of the structural integrity and fitness-for-service (FFS) analysis of engineering structures has elevated constraint effects to one of the most important issues in fracture mechanics and structural integrity research. In this paper, the effect of crack tip constraints are investigated on the fracture toughness assessment of 9% Ni steel for application in liquefied natural gas storage tanks. Crack tip opening displacement (CTOD) tests were conducted using both conventional standard three-point bending (3PB) and wide plate (WP) specimens at a cryogenic temperature of −196 °C. The distribution of the stress and strain fields near the crack tip in the 3PB and WP specimens were then obtained by FE (Finite Elements) analysis. Based on both the experimental and numerical results, the parameters of the Weibull distribution were obtained to evaluate the critical Weibull stress at brittle fracture. The equivalent CTOD ratio β is defined as the ratio of the CTOD of the 3PB specimen to the CTOD of the WP specimen at the same Weibull stress. The application of the proposed CTOD toughness correction method to the WP results was then demonstrated in the context of a failure assessment diagram (FAD). It was determined that the conventional evaluation yields an excessively conservative result for WP specimens, but can be reasonably reduced by applying β.

Fracture toughness determination from load-line displacement of 3-point bend specimen using 3D digital image correlation method for CLF-1 steel

Load-line displacement (LLD) is essential for determining fracture toughness in terms of J-integral for nuclear structure materials. To obtain the accurate LLD, using a three-dimensional (3D) digital image correlation (DIC) method, the different LLD corresponding to different load points were measured on the miniature 3-point bend (3 PB) specimens of Chinese low activation ferritic/martensitic (CLF-1) steel in unloading compliance testing, and the load-displacement curves and J-integral determination were analyzed. The results show a higher LLD value generally derives a higher J-integral value. The J-integral values increase and then decrease with the distance from the top of specimen along crack direction, which is similar to the variation of LLD. It is interesting that the LLD of the load point almost at the neutral axis derive the peak J0.2(B) value (447 and 471 kJ/m2 at 250°C, 270 and 331 kJ/m2 at 450°C), close to the values (441 and 470 at 250°C, 269 and 332 kJ/m2 at 450°C) from the corresponding crack mouth opening displacement (CMOD). And the corresponding J0.2(B) values from LLD of points near the notch tip are much lower. The LLD of the load point at the neutral axis was recommended the accurate LLD to determine J-integral of the 3 PB specimens.

Micromechanical modelling of ductile fracture – local approach

Micromechanical modelling of ductile fracture by using the local approach has been presented as the review of previously obtained results for welded joints made of low alloyed high strength steel. Experimental work was performed on 3PB specimens and tensile panels, which were then modelled by finite element method, using two- and three-dimensional meshes, respectively. The local approach was used to simulate both for crack initiation and growth during ductile fracture process.

Crack propagation in 3PB specimen made from welded joint

Crack propagation in 3PB specimen has been investigated experimentally by using Digital Image Correlation (DIC) technique. For this purpose, specimens were taken from butt welded plates, made of P460NL1 micro-alloyed steel. Fatigue pre-crack of 3PB specimens was introduced in the weld metal. Specimens were then statically loaded and analyzed using digital image correlation software ARAMIS to assess strain fields and crack propagation paths. Bifurcation of cracks was noticed as the main characteristics of crack prop-agation in the analyzed case.

Miniature specimens for fracture toughness evaluation of dental resin composites

ObjectiveMillimeter-scale (“miniature”) specimens enable in-situ evaluation of mechanical properties of engineering materials at reduced cost. Here three such specimens for measuring fracture toughness (KC) are developed and implemented to new dental materials. The latter include concurrent methacrylate-based and new ether-based resin composites designed to reduce polymerization stress and enhance service life in restored teeth. MethodsFracture toughness of four experimental and one commercial dental resin composites are evaluated using three-point bending (3PB), wedge double-cantilever-beam (WDCD) and edge chipping miniature test specimens. The values of KC were compared with those obtained following ISO standard method ISO6872: 2014. The stress intensity factor (K) for the 3PB and WDCB specimens was determined using linear fracture mechanics analyses made in conjunction with the Finite Element technique, with due consideration given to the finite width of pre-crack. ResultsAnalytic expressions for predicting KC were developed for all three miniature specimens. The width of pre-crack, generally neglected for conventional specimens, significantly affect K. Measured KC conclusively agree with those of commercial or well-studied materials as obtained using conventional specimens, with error bounded by 5–10 percent. SignificanceThe edge chipping test was successfully applied for the first time to non-brittle materials like dental resin composites. The miniature specimens developed will expedite the evaluation of fracture toughness of dental resin composites by saving materials and provide needed in-situ assessment capability. The chipping test which requires no introduction of initial crack and involves no use of elastic constants is especially suitable to functionally graded materials and in-situ study of restored teeth. The WDCB specimen enables stable crack growth, a useful trait in fatigue studies.

Flow cytometry minimal residual disease assessment in peripheral blood of adult acute lymphoblastic leukemia patients.

e18517 Background: Multiparametric flow cytometry (FC) of bone marrow aspirate (BM) is a widely used method of minimal residual disease (MRD) assessment in acute lymphoblastic leukemia (ALL). In practice, ALL with hematogone-like phenotype and patients with recurrence of isolated extramedullary disease present challenges in accurate determination of MRD status. We hypothesized that addition of FC MRD of peripheral blood would aid in the interpretation of MRD status. Methods: 76 matched BM and PB specimens were analyzed independently for presence of ALL MRD by 6-color FC. Results: The overall rate of BM MRD-positivity was 24% (18/76) and PB was also MRD-positive in 22% (4/18) of BM-positive cases. PB MRD sensitivity and specificity relative to BM MRD was 13% [95% confident interval (CI) 2%-42%] and 98% [88-100%] for B-ALL samples (n = 65) and 67% [13%-98%] and 75% [36%-95%] for T-ALL samples (n = 11), respectively. We identified 2 cases with evidence of leukemic cells in PB at the time of the extramedullary relapse that were interpreted as MRD-negative in BM. Conclusions: PB MRD demonstrates high specificity with relatively low sensitivity, especially in B-ALL when compared to T-ALL. The use of combined PB and BM samples in MRD assessment by current FC methods may have added clinical and diagnostic value in patients with high risk of extramedullary relapse, including PB MRD as a non-invasive method for monitoring of systemic relapse.

Experimental Study on Extremely Low-Cycle Material Fatigue Resistance for Two Types of Welded Structural Details

Extremely low-cycle fatigue and fracture is one of the most critical damage types of the welded beam-to-column connections when the steel frame subjected to the strong earthquake excitation. The investigation on extremely low cycle fatigue behavior of welded structural details is useful for the fatigue design of welded structure. In the present study, extremely low cycle material fatigue tests were carried out to examine the fatigue failure mechanism of two types of welded structural details, which were the plate butt weld (PB) specimen and cruciform load-carrying groove weld (CLG) specimen. Fatigue tests were performed on 12 welded specimens under strain-controlled constant loading in the plastic range. The fitting S-N curves of the two types were presented. The fatigue S-N curves of the similar welded structural details in IIW Fatigue Recommendation was adopted to correlate the fatigue resistance of the two types of welded structural details. The results demonstrated that the failure mode of PB specimens was not the same as that of CLG specimens. The extremely low-cycle material fatigue strength of the investigated welded structural details was lower than expected high cycle fatigue S-N curves provided by IIW, showing shorter fatigue lives, which indicated that the cumulative damage of welded details involved ductile damage besides cyclic fatigue damage in extremely low-cycle fatigue situation.

Brittle fracture assessment of embedded flaw in heat-affected zone based on Weibull stress criterion

This paper presents a method for estimating the condition for the brittle fracture limit of a welded joint with an embedded flaw in the heat-affected zone (HAZ) based on the Weibull stress criterion. A welded joint with an embedded flaw generated by cold cracking was subjected to a wide plate (WP) tension test, and a welded joint with a fatigue crack was subjected to three-point bending (3PB) tests; both tests were conducted at −30 °C. Brittle fractures occurred in all specimens, and the microstructure of the fracture initiation point for the WP specimen in the coarse-grained HAZ (CGHAZ) was upper bainite with M-A constituents. The critical Weibull stress distribution of the CGHAZ was calculated by finite element analyses for the 3PB specimens. The predicted fracture load and global displacement of the WP specimen at the brittle fracture obtained by the critical Weibull stress distribution were in agreement with experimental results from the WP tensile test. This confirmed that the brittle fracture limit of a welded joint with an embedded flaw could be predicted from the Weibull stress criterion.

High Frequency of Circulating CD8+ Memory Stem T Cells in Acquired Aplastic Anemia

Background. Aplastic anemia (AA), the prototypical bone marrow (BM) failure syndrome, is caused by immune-mediated destruction of hematopoietic stem/progenitor cells (HSPCs). CD8+ cytotoxic T cells with restricted TCR diversity (oligoclonal T cells) are expanded in AA, leading to production of proinflammatory cytokines, such as IFN-γ, which induce apoptosis of HSPCs. Recent studies have identified a new subset of memory T cells with stem cell-like properties, TSCM, which are the least differentiated cells of all distinct memory populations. Functionally, TSCM possess an enhanced capacity for self-renewal and can generate multiple memory T cell populations, and they likely have an important role in controlling immunity. In autoimmune diseases, there is abnormal CD4+ and CD8+ T cell activation. We evaluated TSCM frequency in AA and its association with severity, treatment response, relapse, and changes after immunosuppressive therapy (IST). Further, to evaluate the TSCM in other autoimmune diseases, we examined CD4+ and CD8+ TSCM frequencies in uveitis, systemic lupus erythematosus (SLE), and sickle cell disease (SCD), as compared with healthy controls.Method. We retrospectively analyzed CD4+ and CD8+ TSCM populations by flow cytometry. PB specimens were collected from 55 AA samples and 41 age-matched healthy donor samples. Among 55 AA samples, 21 samples were analyzed at diagnosis and 34 after IST. For comparison, blood samples were obtained from 34 uveitis patients (27 inactive or 7 active cases), 43 SLE patients who met the American College of rheumatology (ACR) criteria for the disease [19 inactive SLE (SLE disease activity index-2K (SLEDAI-2K) score < 3; and 24 active SLE (SLEDAI-2K score > 3)], and 5 SCD patients who were receiving frequent transfusions. TSCM was defined as CD3+ CD4 (CD8)+ CD45RO- CD45RA+ CCR7+ CD27+ CD95+ population.Results and Discussion. In healthy controls, TSCM represented a relatively small percentage of circulating CD4+ or CD8+ T cells (median 2.4% CD4+ TSCM and 2.1% CD8+ TSCM, Fig. 1A). A significantly higher CD8+ TSCM frequency was detected in AA patients (4.2% vs. 2.1%, p < 0.05) while there was no difference in the CD4+ TSCM frequency (p > 0.05), compared to controls (Fig. 1B-C). In AA, high CD8+ TSCM frequency at diagnosis correlated with complete (CR) or partial response (PR) to IST [5.0 % in CR and PR vs 2.8 % in non-responders (NR), p < 0.05). In AA patients prior to IST (n=21), CD8+ TSCM frequency was not correlated with age, sex, absolute neutrophil count, platelet count, time from diagnosis to therapy, and serum ferritin levels. CD8+ TSCM were significantly increased in the two AA cohorts (with or without IST), relative to controls (p < 0.05, respectively). Higher CD8+ TSCM frequency after IST associated with treatment-failure (3.5 % in responders vs 5.5 % in NR or relapse, p < 0.05). Stimulation with anti-CD3/CD28 beads successfully induced cytokine production in CD4+ and CD8+ T cells from AA and healthy controls. Elevated IFN- γ and IL-2 levels were seen in CD4+ and CD8+ TSCM in AA compared to healthy controls. We next compared CD4+ or CD8+ TSCM frequency between each patient group (AA, uveitis, SLE, or SCD) and a healthy control group. Among the four patient groups, the uveitis group alone displayed a reduction in CD4+ TSCM frequency (1.8%) relative to the healthy controls (2.4 %; p < 0.05). An elevated CD8+ TSCM frequency was observed in AA (4.2 %), uveitis (3.6 %), and SCD (4.3 %), but not in SLE, compared to controls (2.1%; p < 0.05) (Fig. 2A). Positive correlation between CD4+ and CD8+ TSCM frequencies was found in AA, autoimmune uveitis, and SLE (Fig. 2B). Evaluation of PD-1 expression revealed that TSCM were the least exhausted T cell compartment, as compared to other types of memory T cells. Immune therapies appeared to have negative effects on the TSCM population both in uveitis and SLE patient cohorts, as well as in AA.Conclusion. We provide evidence for increased circulating CD8+ TSCM in AA, underscoring the importance of this novel subset in regulation of immune responses and pathogenesis of autoimmunity. Our work described previously unknown potential roles of TSCM in AA, such as cytokine secretion correlated with effector functions. Understanding the CD8+ TSCM population may offer new therapeutic strategies and novel mechanistic insight into the various autoimmune diseases. [Display omitted] [Display omitted] DisclosuresTownsley:Novartis: Research Funding; GSK: Research Funding. Dumitriu:Novartis: Research Funding; GSK: Research Funding. Young:Novartis: Research Funding; GSK: Research Funding.

Application of Weibull Stress Criterion to Brittle Fracture Assessment of Heat-Affected Zone-Notched Welds With Residual Stress

A constraint loss correction procedure using the Weibull stress criterion is specified in ISO 27306. However, this standard is applicable only to structural steel components with defects, not to welded joints. Therefore, we propose a method for estimating the brittle fracture limit of a weld with a notch in the heat-affected zone (HAZ) and residual stress based on the Weibull stress criterion. Three-point bending (3PB) tests and wide-plate (WP) tension tests of HAZ-notched welds made of 780-MPa class high-strength steel were conducted at −40 °C. The minimum critical crack tip opening displacement (CTOD) of the WP specimen fracturing at the coarse-grained region of the HAZ (CGHAZ) was approximately four times that of the 3PB specimen. Then, the effects of specimen geometry, residual stress, crack-front shape, and HAZ microstructure classification on the Weibull stress were investigated by using a finite element analysis (FEA). The results of these analyses showed that the specimen geometry, the welding residual stress, and HAZ microstructure affect the Weibull stress of HAZ-notched welds as crack driving force. Based on above results, the CTOD–Weibull stress curves for 3PB and WP specimens fracturing at CGHAZ were calculated by using an FEA. It was confirmed that the brittle fracture limit of an HAZ-notched weld with residual stress could be predicted from the Weibull stress criterion because predicted critical CTOD of WP specimens obtained by Weibull stress included experimental critical CTOD of WP specimens.

Evaluation of the Influence of Residual Stresses on Ductile Fracture

In this work, the significance of residual stresses on ductile fracture is investigated by a set of experiments that are analyzed by finite element simulations. The treatment of residual stresses as expressed in fracture assessment procedures such as R6 is believed to be very conservative for ductile materials, when fracture occurs at high primary loads. Earlier numerical studies have reinforced this belief. This is supported in the current study. Tests on notched 3PB specimens with and without residual stresses were conducted on two ferritic steels. The residual stresses were introduced by applying a compressive preload on notched specimens. The tests were designed to achieve crack initiation at load levels around the plastic limit load. The crack growth in the tests was measured by a compliance method and by color marking of the crack surface. The crack tip driving force J was evaluated numerically for specimens with and without residual stresses. The experimental results show that the residual stresses clearly contribute to J at low primary loads. However, this contribution diminishes as the primary loads increase. The experimental results were also compared with results evaluated using the R6 procedure. These comparisons revealed overly high conservatism in R6 for cases with residual stresses compared to the ones for cases without residual stresses where less conservatism was evident.

Spatial patterns of geochemical elements measured on rock surfaces by portable X-ray fluorescence: application to hand specimens and rock outcrops

Spatial distribution of geochemical elements at hand-specimen and outcrop scale provides significant evidence for the processes of formation and alteration of rocks. Portable X-ray fluorescence (pXRF) analyses is a non-destructive and cost-effective methodology to rapidly measure multi-elemental concentrations in-situ , and its application makes the study of the distribution of geochemical elements efficient. In this paper two pXRF instruments, the Tianrui EDX-P730S and the Niton XL3t 950, were employed for measuring geochemical data for two samples of Pb and Zn ore hand specimens and at outcrop scale in an area of skarn formation. The data are processed by GIS and principal component analysis (PCA) for interpreting element associations involved in mineralization processes. The first three principal components obtained by PCA for the data measured on the rock surface of the two hand specimens may represent early stage Zn-dominated mineralization, superimposed Zn and Pb mineralization and late stage Pb-dominated mineralization. These three zones of mineralization can be delineated by the scores of multiple elements on the first three principal components. The first four principal components obtained from the outcrop surface data were found to be related to the marble, diorite, and two stages of skarn-type mineralization (Cu and Zn mineralization and Pb, Zn and Fe mineralization).

Three-dimensional T-stresses for three-point-bend specimens with large thickness variation

Three-point-bend (3PB) test specimens are useful for the systematic investigation of the influence of statistical and constraint loss size effects on the cleavage fracture toughness of a material in the ductile-to-brittle transition temperature range. Because the in- and out-of-plane elastic T-stresses (T11 and T33) are a measure of the crack-tip constraint and even the in-plane T11 exhibits three-dimensional (3D) effects, the 3D T-stresses solutions were obtained by running finite element analyses (FEA) for 3PB specimens with wide ranges of the crack depth-to-width ratio (a/W=0.2–0.8) and the specimen thickness-to-width ratio (B/W=0.1–40). The results show that the 3D T11 at the specimen mid-plane tended to deviate from the 2D T11 as B/W increased, with the deviation saturating for B/W⩾2. The mid-plane T33 increased with B/W and was close to the plane strain value νT11 for B/W⩾2.

Effect of pine bark extract on bond strength of brackets bonded to bleached human tooth enamel

Aim. The purpose was to investigate the effect of pine bark (proanthocyanidin, natural antioxidant) solution on the shear bond strength (SBS) of metal brackets bonded with composite resin to human enamel after bleaching with hydrogen peroxide (HP). Materials and methods. Sixty recently extracted premolars were divided into an experimental group (n = 45), which was bleached with 40% HP, and a control group (n = 15), which was not bleached. The experimental group was further divided into three sub-groups. Specimens in group IB (n = 15) were bonded immediately after bleaching; specimens in group SA (n = 15) were bleached, then treated with 10% sodium ascorbate and then bonded; group PB specimens (n = 15) were bleached, then treated with 5% pine bark solution and bonded. The specimens were debonded with Universal testing machine. The adhesive remnant index was calculated. Results. No significant differences in shear bond strength were noted when the pine bark treated group was compared with the control group and sodium ascorbate group (p > 0.05). Treating the bleached enamel surface with 10% sodium ascorbate or 5% pine bark solution reverses the SBS reduction. Conclusion. So, as a natural antioxidant and less hazardous, clinicians can choose pine bark solution instead of sodium ascorbate.