Static Fracture Tests Research Articles

Fracture energy of concrete after sustained loading

The fracture energy of concrete after sustained loading is a necessary parameter to calculate the residual carrying capacity of concrete structures in service. This study investigated the fracture energy of concrete after sustained loading by combining experimental observations and theoretical analyses. Firstly, a series of creep fracture tests were conducted on 100 three-point bending beams under various sustained loads, i.e., 75 %, 80 %, 85 %, and 90 % of peak loads. After different loading time, i.e., 10 %, 30 %, 50 %, 70 %, and 90 % of creep fracture lifetimes, the static fracture tests were performed until failure. Then, based on the relationship between the work done by external forces and the energy consumed by the crack propagation of specimens, an analytical model for the fracture energy of concrete after sustained loading was developed, describing the quantitative relationship between the fracture energy of concrete and the crack opening displacement, and the loading time. Finally, the impact of the loading time and sustained load levels on the fracture energy was investigated. The results indicated that the fracture energy of concrete after sustained loading decayed rapidly, with the decay rate gradually decreasing until stability over the loading time. Additionally, specimens subjected to lower sustained load levels experienced a longer loading time for the same crack opening displacement, indicating a greater decay degree of the fracture energy. The analytical model proposed in this study can be employed for predicting and evaluating the crack propagation process and residual carrying capacity of concrete structures during their service life.

Simulation method of asphalt mixture based on fracturable digital aggregate model

This study proposed an approach to construct a fracturable aggregate model to make the fracture patterns consistent with the real situation. Firstly, this study utilized three-dimensional structured light technology to obtain the geometric shell of aggregate and divide them into four categories of spherical, angular, elongated and flaky aggregates according to the minimum bounding box algorithm. Secondly, this study proposed a discrete-spheres filling algorithm based on the Bubble Pack Filling algorithm to fill the shell accurately. Thirdly, this study established the conversion equations between the macro and micro parameters of aggregate by using the linear parallel bonding model, and used it to establish the fracturable aggregate model. Through the indoor and virtual static fracture tests, this study found that the fracture patterns of the fracturable aggregate models are similar to the real situation and characterized the fracture patterns of four types of aggregates. Further analyzing the virtual static fracture tests, this study found that the bearing capacity of aggregates is determined by the compressive strength of host rocks and the shape of aggregate. Through the indoor and virtual uniaxial compression creep tests, this study investigated the behavior of the mixture specimens consisted by the fracturable aggregate model. The stress-strain curves of the fracturable and unfracturable aggregate model in this test were plotted in this study, and the results showed that the relative error between the results obtained by the fracturable model and those of the indoor test is much smaller than that of the unfracturable model. Considering that existing simulation tests on asphalt mixture often ignore aggregate deformation and fracture, so this study used the method mentioned above to obtain the fracturable aggregate model, which can improve the confidence of asphalt mixture simulation results.

Evaluation of asphalt concrete’s fatigue behavior using cyclic semi-circular bending test

Fatigue cracking in asphalt concrete (AC) is one of the major distresses that cause premature failure of flexible pavements. Fatigue tests are often lengthy, cumbersome, and expensive. Therefore, in the performance-related mix-design specifications, some static tests are recommended due to their simplicity. The monotonic semi-circular bending (SCB) test is among the most popular. Even though the SCB test is practical, the actual number of cycles to failure due to cyclic loads cannot be obtained. Implementing SCB testing under cyclic loading conditions for AC performance evaluation could be promising. This study aims to develop a cyclic SCB fatigue testing method. To this end, SCB specimens were produced, and static SCB fracture testing was first conducted to determine the loading levels of the cyclic SCB fatigue test. Then, cyclic SCB tests were conducted by varying load levels to obtain the S-N (stress-fatigue life) curves. To demonstrate the applicability of the proposed framework to differentiate fatigue behavior of mixtures, two AC mixtures were designed and subjected to the proposed protocol, varying the type of aggregate (gneiss and mica schist). Additionally, the S-N curves from cyclic SCB tests were compared to the ones from indirect tensile (IDT) fatigue tests. A considerable similarity in the results between the two approaches (SCB vs. IDT) under cyclic loading was observed, while SCB showed good test repeatability. The SCB configuration with a pre-notch allows fracture mechanics-based testing to monitor the cracking mechanisms and promote rapid cracking evolution, which can reduce testing time.

Fracture Resistance and Microleakage around Direct Restorations in High C-Factor Cavities.

The aim of this research was to evaluate the mechanical impact of different direct restorations in terms of fracture resistance, and subsequent fracture pattern, in occlusal high C-factor cavities. Furthermore, the adaptation of different direct restorations in the form of gap formation was also evaluated. Seventy-two intact mandibular molars were collected and randomly distributed into three groups (n = 24). Class I occlusal cavities with standardized dimensions were prepared in all specimens. After adhesive treatment, the cavities were restored with direct restorations utilizing three different materials. Group 1: layered conventional packable resin composite (Filtek Ultimate), Group 2: bulk-fill resin composite (SDR), Group 3: bulk-fill short fibre-reinforced composite (SFRC; everX Posterior) covered with packable composite occlusally. Half of the restored specimens underwent static load-to fracture testing (n = 12/group), while the rest underwent sectioning and staining for microleakage evaluation and gap formation analysis. Fracture patterns were evaluated visually among the mechanically tested specimens. The layered composite restoration (Group 1) showed significantly lower fracture resistance compared to the bulk fill groups (Group 2, p = 0.005, Group 3, p = 0.008), while there was no difference in fracture resistance between the other groups. In terms of gap formation values, the layered composite restoration (Group 1) produced significantly higher gap formation compared to the bulk-fill groups (Group 2, p = 0.000, Group 3, p = 0.000). Regarding the fracture pattern, SFRC (Group 3) produced the highest number, while SDR (Group 2) produced the lowest number of repairable fractures. The use of bulk-fill resin composite (fibre or non-fibre-reinforced) for occlusal direct restorations in high C-factor cavities showed promising achievements regarding both fracture resistance and microleakage. Furthermore, the use of short fibre-reinforced bulk-fill composite can also improve the fracture pattern of the restoration-tooth unit. Bulk-fill materials provide a simple and effective solution for restoring and reinforcing high C-factor occlusal cavities.

Influence of one-wall remaining coronal tooth with resin abutment and fiber post on static and dynamic fracture resistance.

The objective of this study was to determine the influence of height and thickness of the one wall remaining coronal tooth structure on the fracture resistance of an endodontically treated root with resin abutment build-up using resin composite and fiber-reinforced resin composite post. Static and dynamic fracture tests were performed by placing the remaining tooth wall on the tensile side and applying loads at an angle of 30° from the tooth axis. Superior static fracture resistance was observed when the wall remaining on the tooth had a height and thickness greater than 1.0 mm. The dynamic fatigue test showed high loading capacity or fracture resistance in specimens with large height and thickness. The dynamic fatigue test showed the influence of the remaining tooth structure on fracture resistance clearly. In conclusion, the static and dynamic fracture resistances increased with the height and thickness of the one wall remaining tooth structure.

Fatigue properties of plasma nitriding for dental implant application

Statement of problemFatigue failure of implant components is a common clinical problem. Plasma nitriding, an in situ surface-strengthening method, may improve fatigue properties of dental implants. PurposeThe purpose of this in vitro study was to evaluate the effect of plasma nitriding on the fatigue behavior of implant systems. Material and methodsThe preload and friction coefficient of plasma nitrided abutment screws, as well as settlement of the implant-abutment interface, were measured. Then, the reverse torque values and pullout force were evaluated after cyclic loading. Finally, the fatigue properties of the implant system were investigated with static fracture and dynamic fatigue life tests, and the morphology of the fracture on the surface of the implant system was observed. ResultsThe plasma nitriding treatment reduced the friction coefficient; increased the preload, settlement value, reverse torque values, pullout force, and static fracture load; and prolonged fatigue life. Furthermore, abutment screws with plasma nitriding treatment showed a different fatigue fracture mode. ConclusionsPlasma nitriding improved mechanical performance and may be a suitable way to optimize the fatigue behavior of dental implants.

A new simple specimen for mixed-mode (I/II) fracture and fatigue tests: Numerical and experimental studies

In the present investigation, a simple and efficient specimen geometry for conducting mixed mode (I/II) static fracture tests and fatigue crack growth studies has been proposed. No additional complicated fixtures are needed for loading the specimen and it can be used for both metallic and non-metallic materials. Large number of finite element analyses and mixed mode fracture experiments have been conducted. Present results of the experimental studies have been compared with widely used mixed mode (I/II) fracture criteria in terms of fracture toughness. The present experimental results are in very good agreement with the selected mixed mode (I/II) fracture criteria. Results of the present investigation clearly recommend the proposed specimen geometry as a useful mixed mode (I/II) specimen for fracture and fatigue studies.

Determination of mode I fracture toughness of rocks with field fitting and J-integral methods

Static fracture testing was performed on ISRM-suggested semi-circular bend (SCB) specimens with a constant CMOD increment rate of 0.0002 mm·s−1, and the digital image correlation (DIC) technique, as a practical and effective non-contact tool, was adopted for full-field deformation measurement and fracture process zone (FPZ) size determination. Two displacement-based methods, i.e. field fitting method and J-integral method, were employed to measure the fracture toughness of three types of rocks, i.e. red sandstone, marble, and granite. The effectiveness and robustness of two displacement-based methods were verified with synthetic images containing a mode I crack. Moreover, the sensitivity of the affecting factors in displacement-based methods, such as crack tip location, area for field fitting, the number of solution terms, and the integral paths were studied for more reliable fracture toughness of SCB specimens. Both displacement-based methods give the effective fracture toughness inherently considering the existence of the FPZ above the notch tip, without knowledge of the geometry of the specimen and applied loads. Experimental results indicated that the crack tip location estimated by the field fitting method is close to the merged point of the opening displacement contours, i.e. the tip of the FPZ. Both displacement-based methods generate comparable fracture toughness of three rocks, and an increase of 20%∼35% in fracture toughness is observed compared to the standard load-based method. Moreover, the effectiveness and applicability of the modified load-based method that takes the FPZ length into account are also verified with the two displacement-based methods.

Influence of bedding anisotropy on the dynamic fracture behavior of layered phyllite

Layered phyllite, a typical anisotropic rock caused by the existence of beddings, is common in rock engineering. To evaluate the influence of beddings and loading rates on the dynamic fracture behavior of layered phyllite, notched semicircular bending (NSCB) specimens were measured using a split-Hopkinson pressure bar (SHPB) system. Static fracture testing using a servo-controlled material testing system was also conducted for comparison. The results of this study indicate that three typical fracture paths can be found for NSCB specimens under both static loading and dynamic loading. The fracture path evidently exhibits dependence on the loading rate. For specimens under static loading, dominated fractures are more likely to propagate along the bedding planes while the dominated fractures tend to ignore bedding planes as the loading velocity increases. Consequently, the fracture length ratio along the bedding plane is considerably lower under dynamic loading than under static loading. Anisotropic effects on static and dynamic fracture toughness are evident; however, the anisotropy of fracture toughness decreases as impact velocity increases. The results of this study confirm that a reduced fracture length ratio along the bedding plane induced by an increase in loading rate weakens the dependence of dynamic fracture toughness on anisotropy.

Energy Requirement for Rock Breakage in Laboratory Experiments and Engineering Operations: A Review

Based on the review of a wide range of literature, this paper finds that: (1) the average specific surface energy of various single crystals is only 0.8 J/m2. (2) The average specific fracture energy of the rocks with a pre-crack under static cleavage tests is 4.6 J/m2. (3) The average specific fracture energy of the rocks with a pre-cut notch but with no pre-crack under static tensile fracture (mode I) tests is 4.6 J/m2. (4) The average specific fracture energies of regular rock specimens with neither pre-made crack nor pre-cut notch are 26.6, 13.9 and 25.7 J/m2 under uniaxial compression, tension and shear tests, respectively. (5) The average specific fracture energy of irregular single quartz particles under uniaxial compression is 13.8 J/m2. (6) The average specific fracture energy of particle beds under drop weight tests is 74.0 J/m2. (7) The average specific fracture energy of multi-particles in milling tests is 72.5 J/m2. (8) The average specific energy of rocks in percussive drilling is 399 J/m3, that in full-scale cutting is 131 J/m3, and that in rotary drilling is 157 J/m3. (9) The average energy efficiency of milling is only 1.10%. (10) The accurate measurements of specific fracture energy in blasting are too few to draw reliable conclusions. In the last part of the paper, the effects of inter-granular displacement, loading rate, confining pressure, surface area measurement, premade crack, attrition and thermal energy on the specific fracture energy of rock are discussed.

Effect of Loading Angles and Implant Lengths on the Static and Fatigue Fractures of Dental Implants.

Mechanical properties play a key role in the failure of dental implants. Dental implants require fatigue life testing before clinical application, but this process takes a lot of time. This study investigated the effect of various loading angles and implant lengths on the static fracture and fatigue life of dental implants. Implants with lengths of 9 mm and 11 mm were prepared. Static fracture tests and dynamic fatigue life tests were performed under three loading angles (30°, 40°, and 50°), and the level arm and bending moment were measured. After that, the fracture morphology and fracture mode of the implant were observed. The results showed that 9 mm length implants have a higher static failure load and can withstand greater bending moments, while 11 mm length implants have a longer fatigue life. In addition, as the loading angle increases, the static strength and bending moment decrease linearly, and the fatigue life shows an exponential decrease at a rate of three times. Increasing the loading angle reduces the time of the implant fatigue test, which may be an effective method to improve the efficiency of the experiment.

A comprehensive study of the effects of long-term thermal aging on the fracture resistance of cast austenitic stainless steels

Loss of fracture resistance due to thermal aging degradation is a potential limiting factor affecting the long-term (80+ year) viability of nuclear reactors. To evaluate the effects of decades of aging in a practical time frame, accelerated aging must be employed prior to mechanical characterization. In this study, a variety of chemically and microstructurally diverse austenitic stainless steels were aged between 0 and 30,000 h at 290–400 °C to simulate 0–80+ years of operation. Over 600 static fracture tests were carried out between room temperature and 400 °C. The results presented include selected J-R curves of each material as well as K0.2mm fracture toughness values mapped against aging condition and ferrite content in order to display any trends related to those variables. Results regarding differences in processing, optimal ferrite content under light aging, and the relationship between test temperature and Mo content were observed. Overall, it was found that both the ferrite volume fraction and molybdenum content had significant effects on thermal degradation susceptibility. It was determined that materials with >25 vol% ferrite are unlikely to be viable for 80 years, particularly if they have high Mo contents (>2 wt%), while materials less than 15 vol% ferrite are viable regardless of Mo content.

Fatigue properties of hollow zirconia implants.

The objective of this study was to clarify the fatigue behavior of hollow yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) specimens assuming its use for two-piece implants. The fatigue properties of a solid specimen (which simulated a one-piece implant) and 3 types of hollow specimens (which simulated two-piece implants) were evaluated. Specimens were either solid with a diameter of 4.0 mm (S) or hollow with an inner diameter of 3.0 mm and outer diameters of 4.0 mm (H0.5), 4.5 mm (H0.75), or 5.0 mm (H1.0). For each group, 25 specimens were prepared followed by blast and acid etch treatment. Static fracture and cyclic fatigue tests were conducted by modifying the methods provided in ISO6872. Fracture modes were determined by observing the surfaces under a scanning electron microscope. As a result, the cyclic fatigue load of S and H1.0 were similar, and hollow specimens with outer diameters greater than 0.75 mm displayed the ability to withstand molar occlusal forces.

Fatigue behaviour of dental crowns made from a novel high-performance polymer PEKK

ObjectivesThe aim of this study was, firstly, to analyse the long-time fatigue behaviour of crowns constructed from a novel polyetherketoneketone (PEKK) polymer, using artificial prepared teeth. Secondly, to determine the effect of the material’s stiffness that used as an artificial prepared tooth on the fatigue life of the PEKK crowns in comparison to human prepared teeth.MethodsVeneered crowns with a PEKK framework were constructed on three different prepared teeth: artificial polymethyl methacrylate (PMMA) teeth, artificial CoCr teeth and extracted human teeth. As far as applicable, the loading protocol was based on EN ISO 14801:2007 for fatigue testing of dental implants. After initial static fracture tests on three specimens from each group, the remaining crowns were loaded with different force levels until fracture or until 2 × 106 loading cycles were reached. The number of loading cycles until failure was recorded. Wöhler curves were created to display the fatigue limits.ResultsStatic fracture limits as well as fatigue limits differed for all three core materials. The static fracture tests resulted in fracture limits of 1200 (± 293) N for the PMMA group, 1330 (± 219) N for the CoCr group and 899 (± 96) N for the human tooth group. Fatigue limits of 770 N, 840 N and 720 N were determined for the PMMA group, CoCr group and human tooth group, respectively.ConclusionsThe determined fatigue limit of above 720 N (depending on the core material) is sufficiently high and a good performance of this crown material is expected in the clinical loading life. The results showed that using artificial teeth instead of natural teeth for fatigue testing of crowns might result in an overestimation of the fatigue limits of the crown material.Clinical relevancePEKK-made crowns offer a stable and priceworthy treatment for patients, in particular those that suffer from metal allergy.

Effect of Temperature on the Tear Fracture and Fatigue Life of Carbon-Black-Filled Rubber.

The mechanical behaviour of carbon-black (CB)-filled rubber is temperature-dependent. It is assumed that temperature affects the fatigue life of rubber products by changing the tear energy of the material. The static tearing behaviour and fatigue crack propagation behavior of CB-filled rubber at different temperatures were investigated in this study. The critical tear energy of the material was measured through static tear fracture tests at different temperatures; it is shown that the critical tear energy decreases exponentially with increasing temperature. A fatigue crack growth test of a constrained precracked planar tension specimen was conducted at room temperature; the measurements verify that the fatigue crack growth follows a Paris–Erdogan power law. Considering the temperature dependence of the critical tear energy, the temperature dependent fatigue crack growth kinetics of CB-filled rubber was established, and the fatigue life of the material at high temperatures was predicted based on the kinetics. The predictions are in good agreement with experimental measurements.

Filler size effect on fracture behavior of milled-fiber composites

Quasi static flexural and fracture tests are conducted on anhydride rich epoxy system reinforced with three different sizes of milled glass fibers, 1/32”, 1/8” and 1/4”. The matrix is prepared by mixing DGEBA resin and MTHPA hardener in equal ratio by weight whereas the 16 μm diameter slender fillers are reinforced at 3% volume fraction. It is observed that the flexural modulus and fracture toughness increase monotonically with increase in the slender filler size whereas the flexural strength of composites decreases with increasing filler lengths. The largest fillers enhance the flexural modulus and fracture toughness of neat epoxy system by ~ 1.3 and ~ 3 times, respectively. It is suggested that in the presence of large slender fillers, fiber fracture is the dominant toughening mechanism whereas the smaller size fibers are subjected to pull-out in the fracture process

Relationships of bone characteristics in MYO9B deficient femurs.

The objective of this study was to examine relationships among a variety of bone characteristics, including volumetric, mineral density, geometric, dynamic mechanical analysis, and static fracture mechanical properties. As MYO9B is an unconventional myosin in bone cells responsible for normal skeletal growth, bone characteristics of wild-type (WT), heterozygous (HET), and MYO9B knockout (KO) mice groups were compared as an animal model to express different bone quantity and quality. Forty-five sex-matched 12-week-old mice were used in this study. After euthanization, femurs were isolated and scanned using microcomputed tomography (micro-CT) to assess bone volumetric, tissue mineral density (TMD), and geometric parameters. Then, a non-destructive dynamic mechanical analysis (DMA) was performed by applying oscillatory bending displacement on the femur. Finally, the same femur was subject to static fracture testing. KO group had significantly lower length, bone mineral density (BMD), bone mass and volume, dynamic and static stiffness, and strength than WT and HET groups (p < 0.019). On the other hand, TMD parameters of KO group were comparable with those of WT group. HET group showed volumetric, geometric, and mechanical properties similar to WT group, but had lower TMD (p < 0.014). Non-destructive micro-CT and DMA parameters had significant positive correlations with strength (p < 0.015) without combined effect of groups and sex on the correlations (p > 0.077). This comprehensive characterization provides a better understanding of interactive behavior between the tissue- and organ-level of the same femur. The current findings elucidate that MYO9B is responsible for controlling bone volume to determine the growth rate and fracture risk of bone.

Influence of hygrothermal effects in the fracture process in wood under creep loading

The knowledge of crack driving forces such as energy release rate is very important in the assessment of the reliability of timber structures. This work deals with both static and creep fracture tests in opening mode crack growth under hygro-thermal and mechanical loadings. The experimental tests combining creep and hygro-thermal loadings are performed in a climatic chamber. The Double Cantilever Beam specimen with variable inertia machined in Douglas Fir and White Fir species is used to investigate the effects of these loadings on fracture processes. Two experimental protocols are presented. First, instantaneous tests are carried out in order to identify the moisture content effects on fracture properties. R-curves are studied using a finite element approach. Secondly, creep tests are performed by imposing high speed humidity variations in a climatic chamber. During these tests, the evolutions of the crack length are recorded.

The use of a fiber sleeve to improve fracture strength of pulpless teeth with flared root canals

ObjectivesThis study aimed to investigate how use of a fiber sleeve may reduce interfacial debonding and improve fracture strength of pulpless teeth with flared root canals. MethodsPulpless premolars with flared root canals were restored either with a fiber-reinforced post (FRP) alone or with an FRP wrapped in a hollow tubular fiber sleeve. A normal root restored with an FRP alone served as a control. The integrity of resin–dentin and resin–fiber interfaces in the restored roots was evaluated by a stereoscopic system after penetrating a dye. Four roots were tested for each experimental group. Fracture resistance in pulpless premolars with flared root canals restored with an FRP alone or with an FRP/sleeve combination were investigated under bonded and non-bonded conditions with static fracture testing (n=8), and stress distribution in these restored premolars were tested by finite element analysis (FEA). ResultsFlared root canals restored with an FRP/sleeve combination demonstrated superior integrity at the cervical resin–dentin interface to root canals with an FRP alone. Premolars with a flared root canal restored with an FRP/sleeve combination showed significantly greater fracture resistance compared with premolars restored with an FRP alone. FEA showed that once interfacial de-bonding extended to the cervical region of the root, stress concentration in the root dentin dramatically increased. SignificanceThe FRP/sleeve combination was effective in reducing debonding and, hence, improving the fracture strength of pulpless premolars with flared root canals.

Dynamic delamination of fire insulation applied on steel structures under impact loading

This paper presents an experimental-numerical approach for evaluating dynamic fracture and delamination of fire insulation from steel structures during impact loading. The experiments encompass drop mass impact tests on steel beams insulated with three types of sprayed applied fire resistive material (SFRM), namely Portland cement-based, gypsum-based and mineral fiber-based, commonly utilized in steel construction. The impact tests are conducted at two kinetic energy levels to evaluate the strain rate-dependency of fracture energy and extent of delamination at steel-SFRM interface. Results from experiments show that the cracking and delamination of SFRM is mainly localized on the bottom flange with slight extension into lower part of web of beam at the mid span. Further, Portland cement-based SFRM can withstand the applied impact energy and no delamination or substantial cracking in SFRM occurs, whereas two other types of SFRM experienced significant fracture and delamination on the bottom flange. A fracture mechanics-based numerical approach is subsequently employed to simulate the conducted experiments using LS-DYNA finite element code. In the explicit numerical model, cohesive zone approach is adopted to model fracture process zone at the interface of steel and SFRM. By quantifying and calibrating the extent of delamination on the bottom flange, the dynamic increase factor of fracture energy and stress–displacement relationships, determined through previous static fracture tests, is estimated. According to numerical simulations, extent of delamination in mineral fiber-based SFRM is not dependent on strain rate, whereas in the case of gypsum-based and Portland cement-based SFRM extent of delamination is a function of strain rate.