Mechanical Fatigue Performance Research Articles

Advanced lithium disilicate: A comparative evaluation of translucency and fatigue failure load to other ceramics for monolithic restorations

The aim of this in vitro study was to evaluate the surface roughness, translucency, fatigue failure load (FFL), and number of cycles for fatigue failure (CFF) of a recently released lithia-based material called advanced lithium disilicate and three other ceramics indicated for monolithic restorations. First, ALD (advanced lithium disilicate, CEREC Tessera, Dentsply Sirona), LD (lithium disilicate, IPS e. max CAD, Ivoclar), LS (lithium silicate-disilicate, Suprinity, Vita Zahnfabrik), and 4Y-PSZ (Yttria-stabilized zirconia, IPS e.max ZirCAD MT, Ivoclar) discs (n = 15, Ø = 10 mm and thickness = 1.0 mm) were fabricated from CAD/CAM blocks/discs, A2 shade. The discs were sintered/crystallized and subsequently analyzed by a rugosimeter (Mitutoyo SJ-410) to determine Ra and Rz surface roughness parameters. Next, they were evaluated to determine the translucency parameter (TP) using a bench-top spectrophotometer (SP60, EX-Rite). The discs were subsequently cemented to glass fiber epoxy resin discs, and the specimens were tested under cyclic loading (Step-test), immersed in distilled water at a frequency of 20 Hz, with an initial cyclic load of 200 N for 5,000 cycles and increments of 50 N every 10,000 cycles until failure. Fatigue failure load (FFL) and number of cycles for fatigue failure (CFF) were recorded for subsequent Kaplan Meier analysis, with post-hoc Mantel-Cox and Weibull analysis (α = 0.05). Complementary fractographic, topographic and energy dispersive spectroscopy analyses (EDS) were performed. 4Y-PSZ showed higher survival (p < 0.05), with higher FFL and CFF (1077 N; 180,333 cycles), followed by LD (980 N; 161,000 cycles), LS (937 N; 152,333 cycles) and ALD (910 N; 147,000 cycles). No differences were observed between the tested groups regarding Weibull modulus. ALD presented TP (28.14) equal to DL (28.27) and higher than LS (25.51). All lithia-based materials had higher translucency than 4Y-PSZ (TP = 8.62) (p < 0.05). ALD appears to have a similar elemental composition to LD and LSD for oxygen and silicon. ALD and LSD have a similar zirconium content. Fractures originated on the cemented surface of the ceramic discs. Lithia-based ceramics showed lower surface roughness, with ALD (Ra = 0.04 μm; Rz = 0.66 μm) showing the lowest values (p < 0.05). Despite showing lower FFL when compared to LD and 4Y-PSZ, ALD has compatible translucency and mechanical fatigue performance with its indication for fabricating monolithic, anterior and posterior adhesively cemented single-unit restorations. However, further studies are needed to substantiate its clinical performance.

Influence of height discrepancy between pulp chamber floor and crestal bone in the mechanical fatigue performance of endodontically-treated teeth restored with resin composite endocrowns

ObjectiveTo explore and characterize the effect of the discrepancy between crestal bone height (CB) and pulp chamber floor (PCF) in the fatigue performance of endodontically-treated teeth rehabilitated with an endocrown restoration. Materials and methodsA total of 75 human molars free of defects, caries history or cracks were selected, then endodontically treated and randomly allocated into 5 groups (N = 15) according to the difference between PCF and CB, as follows: PCF 2 mm above, PCF 1 mm above, PCF leveled, PCF 1 mm below and PCF 2 mm below. Endocrown restorations were made with composite resin (Tetric N-Ceram, shade B3, Ivoclar) in 1.5 mm thickness and luted with a resin cement (Multilink N, Ivoclar) onto the dental elements. Monotonic testing was performed to define the fatigue parameters, and a cyclic fatigue test was used until failure of the assembly. The collected data were submitted to statistical survival analysis (Kaplan-Meier followed by Mantel-Cox and Weibull), fractographic analysis and finite element analysis (FEA) were performed as complementary analyzes. ResultsThe PCF 2 mm below and PCF 1 mm below groups presented the best results regarding fatigue failure load (FFL) and number of cycles for failure (CFF) (p < 0.05), but presented no difference between each other (p > 0.05). The PCF leveled and PCF 1 mm above groups presented no statistical difference between them (p > 0.05), but performed better than the PCF 2 mm above group (p < 0.05). The rate of favorable failures of PCF 2 mm above, PCF 1 mm above, PCF leveled, PCF 1 mm below and PCF 2 mm below groups were 91.7%, 100%, 75%, 66.7% and 41.7%, respectively. FEA showed different stress magnitudes according to the pulp-chamber design. ConclusionThe insertion level of the dental element to be rehabilitated with an endocrown interferes in the mechanical fatigue performance of the set. The discrepancy between the CB height and the PCF has a direct effect, where the higher the PCF in relation to the CB, the greater the risk of mechanical failure of the restored dental element.

How does the occlusal contact region influence the mechanical fatigue performance and fracture region of monolithic lithium disilicate ceramic crowns?

ObjectiveTo characterize the effect of the occlusal contact region on the mechanical fatigue performance and on the fracture region of monolithic lithium disilicate ceramic crowns. Materials and methodsMonolithic lithium disilicate ceramic crowns were machined in a CAD/CAM system and adhesively luted onto glass-fiber reinforced epoxy resin preparations with resin cement. The crowns were divided into three groups (n = 16) according to load application region (cusp tip: restricted to cusp tips; cusp plane: restricted to cuspal inclined plane; or mixed: associating tip cusp and cuspal inclined plane). The specimens were submitted to a cyclic fatigue test (initial load: 200 N; step-size: 100 N; cycles/step: 20,000; loading frequency: 20 Hz; load applicator: 6 mm or 40 mm diameter stainless steel) until observing cracks (1st outcome) and fracture (2nd outcome). The data were analyzed by the Kaplan-Meier + Mantel-Cox post-hoc tests for both outcomes (cracks and fracture). Finite element analysis (FEA), occlusal contact region, contact radii measurements, and fractographic analyzes were performed. ResultsThe mixed group presented worse fatigue mechanical behavior (550 N / 85.000 cycles) compared to the cuspal inclined plane group (656 N / 111,250 cycles) (p < 0.05) for the first crack outcome, while the cusp tip group was similar to both groups (588 N / 97,500 cycles) (p > 0.05). The mixed group had the worst fatigue behavior (1413 N / 253,029 cycles) in relation to the other groups (Cusp tip: 1644 N / 293,312 cycles; Cuspal inclined plane: 1631 N / 295,174 cycles) considering the crown fracture outcome (p < 0.05). FEA showed higher tensile stress concentration areas just below the load application region. In addition, loading on the cuspal inclined plane induced a higher tensile stress concentration in the groove region. The most prevalent type of crown fracture was the wall fracture. Groove fracture was observed in 50% of the loading specimens exclusively on the cuspal inclined plane. ConclusionLoad application on distinct occlusal contact regions affects the stress distribution pattern and consequently the mechanical fatigue performance and fracture region of the monolithic lithium disilicate ceramic crowns. A combination of loading at distinct regions is recommended to promote better evaluation of the fatigue behavior of a restored set.

Influence of connector cross-sectional geometry on the load-bearing capacity under fatigue of implant-supported zirconia fixed partial prosthesis

Statement of problemZirconia has been reported to be suitable for multiunit monolithic restorations. However, as the connector region is the weakest part of the system, studies are needed to determine the optimal connector geometry. PurposeThe purpose of this in vitro study was to evaluate the load-bearing capacity under fatigue of implant-supported fixed partial prostheses made of monolithic zirconia with different connector cross-sectional geometries. Material and methodsThree-unit monolithic fixed partial prostheses (from mandibular second premolar to the second molar) were fabricated in zirconia (Zenostar T) by computer-aided design and computer-aided manufacture (CAD-CAM). Different connector cross-sectional geometries, with an area of 9 mm2, were tested (n=10): round, square with rounded angles, or trapezoid with rounded angles. The prostheses were screwed over 2 implants and inserted into acrylic resin bases. The specimens were submitted to a mechanical fatigue test until failure, with load applied to the pontic using the stepwise stress methodology (initial load of 100 N for 5000 cycles, followed by an increase to 200 N for 20 000 cycles, with a subsequent increase of 50 N each step). For data analysis, fatigue failure load (FFL) and cycles for fatigue failure (CFF) were recorded. Failed structures were analyzed by fractography. The Kaplan-Meier test followed by a log-rank Mantel-Cox post hoc test was used to analyze FFL and CFF (α=.05). Weibull analysis was also performed. ResultsThe round (1065 N) and trapezoid (1010 N) groups presented higher FFL values than the square one (870 N) (P≤.05). For CFF, the round connector (358 777) was better than the square (280 310) (P=.006), and the trapezoid (337 773) was statistically similar to both (P>.05). No difference among groups was observed in Weibull modulus either for CFF or FFL data. All failures were catastrophic, originating at the base of the connector and propagating toward the occlusal surface of the pontic (region under loading). ConclusionsThe connector cross-sectional geometry significantly influenced the mechanical fatigue performance of implant-supported fixed partial prostheses made of monolithic zirconia.

Design considerations for piezoelectrically powered electrical stimulation: The balance between power generation and fatigue resistance

Quality and timing of bone healing from orthopedic surgeries, especially lumbar spinal fusion procedures, is problematic for many patients. To address this issue, clinicians often use electrical stimulation to improve surgery success rates and decrease healing time in patients with increased risk of pseudarthrosis, including smokers and diabetics. Current invasive electrical stimulation devices require an implantable battery and a second surgery for removal. Piezoelectric composites within an interbody implant generate sufficient power under physiologic loads to deliver pulsed electrical stimulation without a battery and have demonstrated promising preclinical bone growth and fusion success. The objective of the current study was to assess the power generation and fatigue resistance of three commercially manufactured piezocomposite configurations in a modified implant design to demonstrate efficacy as a robust biomaterial within osteogenic implants. The three configurations were electromechanically assessed under physiological lumbar loading conditions, and all configurations produced sufficient power to promote bone healing. Additionally, electrical and mechanical fatigue performance was assessed under high load, low cycle conditions. All configurations demonstrated runout with no gross mechanical failure and two configurations demonstrated electrical fatigue resistance. Future piezoelectric implant design decisions should be based on power generation needs to stimulate bone growth, as mechanical fatigue efficacy was proven for all piezocomposite configurations tested.

Experimental study on CRTS Ⅱ ballastless track-bridge structural system mechanical fatigue performance

With the large-scale application of high-speed railways (HSR) in China, the ballastless track-bridge will inevitably experience fatigue damage. As a result, the maintenance and repair of the HSR track-bridge are becoming extremely urgent. However, the research on the track-bridge is very lacking presently and its mechanical fatigue performance and service life are still uncertain. Therefore, it is of great necessity to carry out a fatigue test on track-bridge to reveal the mechanical properties evolution of the structure and predict its service life under fatigue train loads. In this paper, a quarter-scale specimen of a prestressed simply supported concrete box-beam with CRTS Ⅱ (The China Railway Track System Type Ⅱ) ballastless track was constructed. A series of fatigue tests were carried out on the structural system and the following conclusions were noted. First, there are no visible cracks on the surface of the track-bridge after a fatigue loading test of 18 million cycles, and its mechanical properties of collaborative work perform well. Second, the mechanical performance of the box-beam remain elastic and its stiffness is basically stable under the long-term fatigue train load. Third, the calculated fatigue life of the track-bridge shows that the box-beam, track slab, base slab and cement asphalt mortar (CA mortar) all had extremely long fatigue lives, which demonstrates that it is difficult for them to suffer failure under fatigue train loads. Moreover, after the fatigue test, the results of cumulative fatigue damage show that the most damaged component of the track-bridge is the box-beam, followed by the CA mortar, the base slab and the track slab, and their predicted residual life are expected to be 73.67%, 92.90%, 99.99% and 99.99% of the total fatigue life, respectively. Therefore, more attention should be paid to the damage of box-beam in the repair and maintenance of the HSR track-bridge.

Influence of shading technique on mechanical fatigue performance and optical properties of a 4Y-TZP ceramic for monolithic restorations

This study aimed to evaluate the effect of shading procedures on the fatigue performance and optical properties of an yttria-stabilized tetragonal zirconia polycrystal ceramic (4Y-TZP – stabilized by 4 mol% Y2O3). Disc-shaped specimens were produced at pre-sintered stage and randomly allocated into 5 groups (n=15) considering the shading technique: Ctrl (no pigmentation); Brush 1× and Brush 3× (brushing on 1 or 3 applications, respectively); Immer 2min and Immer 4min (immersion for 2 or 4 min, respectively). Following each pigment application, the specimens were dried in accordance with the manufacturers’ guidelines (15 min in a stove at 70 °C) and sintered in a specific furnace. Biaxial flexural fatigue test followed a step-stress approach (initial strength: 200 MPa; step-size: 25 MPa; 10.000 cycles/step; frequency: 20 Hz). Optical measurements were conducted in a spectrophotometer following the CIEDE2000 parameters. Roughness, topography, crystalline grain size, crystalline phase content and fractography analysis were also performed. The flexural fatigue strength (FFS) and the number of cycles to fracture (CFF) were statistically affected by pigmentation techniques (mean FFS in MPa/mean CFF): Ctrl (446.7/105,619) > Brush 1 × (436.7/102,854) ≥ Brush 3 × (405.0/89,962) ≥ Immer 2 min (395.0/85,103) ≥ Immer 4 min (383.3/81,382), although the exposure intensity (increase in number of applications or in application time) to the pigment had no effect (Brush 1 × = Brush 3 × ; Immer 2 min = Immer 4 min). Regarding optical properties, shading techniques similarly affected translucency and opalescence parameters, regardless of the exposure intensity to the pigment (Brush 1 × = Brush 3 × = Immer 2 min = Immer 4 min > Ctrl), whereas the exposure intensity to the pigment only promoted relevant alterations in the color change parameter (ΔE00) for the brushing technique (Brush 3 × > Brush 1 × ). No difference was observed in crystalline phase content, topographic pattern or roughness, although an increase in the zirconia grain size could be detected. Thus, the shading technique might affect the fatigue behavior and promote optical changes in a 4Y-TZP ceramic.

Mechanical fatigue performance of PCL-chondroprogenitor constructs after cell culture under bioreactor mechanical stimulus.

In tissue engineering of cartilage, polymeric scaffolds are implanted in the damaged tissue and subjected to repeated compression loading cycles. The possibility of failure due to mechanical fatigue has not been properly addressed in these scaffolds. Nevertheless, the macroporous scaffold is susceptible to failure after repeated loading-unloading cycles. This is related to inherent discontinuities in the material due to the micropore structure of the macro-pore walls that act as stress concentration points. In this work, chondrogenic precursor cells have been seeded in poly-ε-caprolactone (PCL) scaffolds with fibrin and some were submitted to free swelling culture and others to cyclic loading in a bioreactor. After cell culture, all the samples were analyzed for fatigue behavior under repeated loading-unloading cycles. Moreover, some components of the extracellular matrix (ECM) were identified. No differences were observed between samples undergoing free swelling or bioreactor loading conditions, neither respect to matrix components nor to mechanical performance to fatigue. The ECM did not achieve the desired preponderance of collagen type II over collagen type I which is considered the main characteristic of hyaline cartilage ECM. However, prediction in PCL with ECM constructs was possible up to 600 cycles, an enhanced performance when compared to previous works. PCL after cell culture presents an improved fatigue resistance, despite the fact that the measured elastic modulus at the first cycle was similar to PCL with poly(vinyl alcohol) samples. This finding suggests that fatigue analysis in tissue engineering constructs can provide additional information missed with traditional mechanical measurements.

Cyclic Deformation of Advanced High-Strength Steels: Mechanical Behavior and Microstructural Analysis

The fatigue properties of multiphase steels are an important consideration in the automotive industry. The different microstructural phases present in these steels can influence the strain life and cyclic stabilized strength of the material due to the way in which these phases accommodate the applied cyclic strain. Fully reversed strain-controlled low-cycle fatigue tests have been used to determine the mechanical fatigue performance of a dual-phase (DP) 590 and transformation-induced plasticity (TRIP) 780 steel, with transmission electron microscopy (TEM) used to examine the deformed microstructures. It is shown that the higher strain life and cyclic stabilized strength of the TRIP steel can be attributed to an increased yield strength. Despite the presence of significant levels of retained austenite in the TRIP steel, both steels exhibited similar cyclic softening behavior at a range of strain amplitudes due to comparable ferrite volume fractions and yielding characteristics. Both steels formed low-energy dislocation structures in the ferrite during cyclic straining.