Cracking Properties Research Articles

Features of CO2 fracturing deduced from acoustic emission and microscopy in laboratory experiments

AbstractWe conducted hydraulic fracturing (HF) experiments on 170 mm cubic granite specimens with a 20 mm diameter central hole to investigate how fluid viscosity affects HF process and crack properties. In experiments using supercritical carbon dioxide (SC‐CO2), liquid carbon dioxide (L‐CO2), water, and viscous oil with viscosity of 0.051–336.6 mPa · s, we compared the results for breakdown pressure, the distribution and fracturing mechanism of acoustic emission, and the microstructure of induced cracks revealed by using an acrylic resin containing a fluorescent compound. Fracturing with low‐viscosity fluid induced three‐dimensionally sinuous cracks with many secondary branches, which seem to be desirable pathways for enhanced geothermal system, shale gas recovery, and other processes.

Evaluation of crack parameters by a nonlinear frequency-mixing laser ultrasonics method

Nonlinear acoustic methods are commonly used in crack detection because of their high sensitivity in comparison to linear ones. However, the dependence of the nonlinearities on the crack state can not only localize it but also provides information on its characteristics. We present a laser ultrasonic method, based on nonlinear frequency-mixing, to locally evaluate several crack parameters, including some, like the local crack elasticity, which are assessed uniquely by the present technique. Two laser beams, independently intensity modulated at two cyclic frequencies ωH and ωL(ωH>>ωL), excite the sample. For a large sinusoidal thermo-elastic stress generated by the low-frequency modulated laser beam, the crack oscillates between closed and open states. In the presence of this variation, nonlinear frequency-mixing ultrasonic components at frequencies ωH±nωL (n is an integer) are detected. By modifying the intensity of the laser beam modulated at ωL, we can influence the time spent in opened and closed state over a period (2π/ωL). The developed theoretical model demonstrates the dependence of the nonlinear components at ωH±nωL on the time spent by the crack in each state. Comparison between theoretical and experimental results offers a way to characterize some local crack properties, including its width and effective rigidity.

Effect of Fiber Type and Fiber Hybrids on Strain-Hardening and Multiple Cracking Properties of the Ultra-High Performance Cementitious Composites under Uniaxial Loads

Five series of strain hardening ultra-high performance cementitious composites (SHUHPCC) incorporated with different types of fibers and hybrid fibers were produced. Three types of fibers (steel fiber, polyvinyl alcohol fiber and polyethylene fiber) were used as mono or hybrid reinforcement in SHUHPCC with the same volume fraction of 2%. The primary strengths, strain hardening and multiple cracking behaviors of hybrid fiber reinforced SHUHPCC under the uniaxial tensile are investigated. Test results show that the SHUHPCC containing PE fibers exhibited higher strain hardening capacity and lower first cracking strength than composites reinforced with mono PVA fiber or mono steel fiber. The composites containing PVA fibers or steel fibers have higher tensile strength and first cracking strength than the composite reinforced by mono PE fiber. Hybridization reinforcement with different fibers is able to make up defects of mono fiber reinforcement for SHUHPCC. The change laws of tensile strength and uniaxial compression strength of SHUHPCC with mono PE fiber and mono PVA fiber are opposite to each other.

Life Assessment and Crack Growth Properties of Laser Cut Dual-Phase Steel

The effect of laser cut-edges has been studied as a method for producing an optimum fatigue life performance of advanced high-strength steel. During this study, DP600 high-strength steel laser cut-edges have been fatigue tested under S-N and E-N fatigue loading regimes. The cut-edge surface characteristic properties and internal metallurgical alterations have been observed to directly influence fatigue life of the steel. This paper has investigated the crack initiation and growth properties of the initial crack to mode two. It is shown that alterations in the surface properties can be harnessed so that beneficial properties can be produced to retard crack initiation. It was determined that the laser power and cutting speed can be used independently to produce the appropriate balance between microstructure and optimum surface properties. Optimal fatigue lives were attained by minimizing the laser cut-edge surface damage, maintaining the formation of wide area striations and by forming a uniform layer of martensitic material close to the cut-edge. These results suggest that laser cutting can be used to enhance the fatigue life to failure of fracture-sensitive steel grades.

Experimental Investigation on Shear Connectors in Steel-concrete Composite Deck Slabs

Background/Objectives: The steel-concrete composite construction has become an effective construction practice in the recent years. The cold form profiled sheet acts a platform for during the construction phase and acts as an external reinforcement after the construction. In steel-concrete composite construction, the shear connectors transfer the longitudinal shear force across the steel flange/concrete interface. The ability to transfer longitudinal shear force by shear connectors mainly depends on the strength of concrete against longitudinal cracking and mechanical properties of the shear connectors. This research investigates shear connector's capacity; the experimental investigation is involved using push-out test for two sets with different shear connectors framing to the profile steel sheet. Methods/Statistical Analysis: Set-W consists of three composite slab specimens with welded stud shear connectors, while set-B consists of three composite slab specimens with bolted shear connectors. Trapezoidal profile steel sheet of thickness 1 mm with rectangular embossments is used. Several key parameters of the composite slab are examined and measured, including slippage capacity, load carrying capacity, composite slab ductility and failure mechanism. Findings: the welded shear connectors provided a better performance in resisting slippage and providing a welded set higher load carrying capacity, however, the bolted shear connector specimens set showed more ductile behavior as compared to of specimens.

Investigation of the rheological properties of elastomeric polymer-modified bitumen using warm-mix asphalt additives

In recent years, warm-mix asphalt (WMA) is extensively used in the hot-mix asphalt industry for reducing energy requirements and emissions by lowering mixing and compaction temperatures of bitumen. In addition, the use of polymer-modified bitumen (PMB) has become a very important part of pavement construction due to its superior performance, including less ageing, enhanced rutting resistance and lower fatigue cracking properties. This paper presents an experimental evaluation of elastomeric PMB involving WMA additives. In scope of the study, base bitumen has been modified with three different concentrations of styrene–butadiene–styrene (SBS) copolymer. The prepared PMB samples have been mixed with organic and chemical types WMA additives. Following the determination of the conventional properties as well as the rheological properties of the samples using dynamic shear rheometer, rutting behaviour of bitumen samples has been evaluated by zero shear viscosity test. The results of conventional bitumen tests showed a decrease in temperature susceptibility in PMB involving WMA additives. Based on the rheological test results, it was observed that the utilisation of organic WMA additive into PMB improves rutting performance of the modified bitumen especially in the 4% concentration of SBS copolymer.

Tolerance of polymer-zeolite composite membranes to mechanical strain

Numerous studies have been reported for zeolite membranes grown on inflexible supports, such as alumina and metals, and examined for gas transport measurements. Polymer membranes are also used for gas separations, and have a far greater practical impact, primarily because of lower cost. Polymer membranes are made by rapid roll to roll methods, and large quantities can be generated, whereas zeolite membranes are typically made in a batch fashion at much smaller sizes. Because the mechanism of gas transport between zeolites and polymers is different, zeolite membranes have certain performance advantages. Combining the attributes of both zeolites and polymers in a membrane is of significant interest. In this paper, we examine two types of zeolite polymer membranes based on faujasitic zeolites, one with zeolite grown on top of porous polyethersulfone (Type I), and the second where the zeolite is grown within the pores of the porous polymer (Type II). We have previously reported that both Type I and II membranes perform satisfactorily for CO2/N2 separation. However, Type I membranes’ performance is less reproducible and more sensitive to handling. In this study, we explore the elastic properties of these two types of membranes using Peak Force Quantitative Nanomechanical Measurement (PFQNM) technique by Atomic Force Microscopy. It is found that Type II membranes have lower elastic modulus even though the zeolite thickness in these membranes exceeds those of Type I membranes by an order of magnitude. Mechanical stress leads to cracks within the zeolite layer for both types of membranes, though Type II membranes tolerate higher stress. We propose that the higher stress-resistance of the Type II membranes arises because the polymer surrounding the zeolite acts to arrest the crack propagation. The genesis of cracking and elastic property measurements by AFM will be relevant for other polymer-inorganic composites, where an inorganic film exists on/within a polymer.

Ultimate torsional strength of cracked stiffened box girders with a large deck opening

The present paper studies the ultimate torsional strength of stiffened box girders with large deck opening due to the influence of cracks. Three types of hull girders with different spans are provided for comparison. Potential parameters which may have effects on the torsional strength including the mesh refinement, initial deflection, material strain hardening, geometric properties of crack and stiffener are discussed. Two new concepts that play an significant role in the ultimate strength research of damaged box girders are introduced, one of which is the effective residual section (ERS), the other is the initial damage of the failure zone (IDFZ) for intact structures. New simple formulas for predicting the residual ultimate torsional strength of cracked stiffened box girders are derived on the basis of the two new concepts.

A comparative study on the compatibility of PVA and HTPP fibers with various cementitious matrices under flexural loads

The compatibility of polyvinyl alcohol (PVA) and high tenacity polypropylene (HTPP) fibers with various cementitious matrices in terms of bending performance has been investigated. Matrices were prepared at three water to binder (W/B) ratios. In order to compare the flaw tolerancy of matrices, natural flaw structure of half of the specimens were modified by using poly-ethylene beads (at 3mm diameter). The effects of fiber type, water/binder (W/B) ratio and flaw structure modification on the multiple-cracking behavior of bending composites were determined under flexural loads. The matrix phase of composites is mainly composed of ordinary Portland cement (OPC), Metakaolin (M), and limestone powder, respectively. Prismatic specimens (25×60×300mm) have been prepared and subjected to 4-point loading test 28days after specimen preparation. Flexural properties, multiple cracking potentials, crack properties (by means of number and width) of composites were compared. Results showed that, HTPP fibers seem more advantageous compared to PVA fibers in terms of flaw tolerancy with metakaolin based matrices.

Laser powder deposition of carbon nanotube reinforced nickel-based superalloy Inconel 718

Electroless Ni-P coating was plated on the surface of multi-walled carbon nanotubes (MWCNTs). The Ni-P coated carbon nanotubes (NiPCNTs) were dispersed into Inconel 718 (IN718) powder by ball milling. Then the powder mixtures were deposited into the IN718/NiPCNTs composite coatings by laser powder deposition process. The survivability of MWCNTs, susceptibility to heat affected zone (HAZ) liquation cracking and tensile mechanical properties of the coatings were studied. The results showed that the MWCNTs were successfully incorporated into the IN718 coatings but most of them were transformed into the porous carbon nano ribbons (CNRs), graphene nano sheets (GNSs) and diamond-like nano particles (DNPs). The CNRs were resulted by the inter-welding of the MWCNTs and GNSs. Large thin CNRs bridging with the Laves particles and interdendritically bonded regions improved the stress transfer across the interdendritic regions. The stress localized on the last remaining liquation film could be depressed and the susceptibility to HAZ liquation cracking was suppressed. Furthermore, the tensile strength of the IN718/NiPCNTs coatings was increased significantly whereas the ductility of the coatings was somewhat reduced, which was attributed not only to the addition of the carbon nano allotropes but also the increased formation of the hard but brittle Laves phase.

Relating asphalt binder elastic recovery properties to HMA cracking and fracture properties

Cracking in HMA pavements reduces the long-term durability of HMA pavements and undesirably increases the costs of road maintenance. Asphalt binder as one of major constituents of HMA plays an important role in the HMA cracking performance. Previous studies have indicated that higher elastic recovery of asphalt binder was favored to resist fatigue cracking. This study was undertaken to further investigate the relationships between the asphalt binder elastic recovery (ER) properties and the HMA cracking resistance and fracture properties, measured in the laboratory. The ER test (or ductility test) and MSCR test were used to measure and quantify the asphalt binder ER properties. Similarly, the OT and IDT tests were used to measure and quantify the HMA cracking resistance (cycles to crack failure) and fracture properties (tensile strength, fracture energy [FE], and FE Index), respectively. The test results indicated that HMA mixes with high ER properties (⩾59%) showed superior fracture properties and better laboratory cracking resistance performance, i.e., higher FE Index values (>1.0) and more load cycles to crack failure (>500 OT cycles), respectively. By contrast, no definitive performance trends were found for the HMA mixes with low ER properties (<59%).

Rate dependence of ultra high toughness cementitious composite under direct tension

Ultra high toughness cementitious composite (UHTCC) usually shows strain hardening and multiple cracking under static tension loads. In practice, structures could be exposed to high strain rates during an earthquake. Whether UHTCC can maintain its unique properties and provide high structural performance under seismic loading rates largely determines whether it can successfully fulfil its intended function. To determine the rate dependence of UHTCC, uniaxial tensile tests with strain rates ranging from 4×10−6 s−1 to 1×10−1 s−1 were conducted with thin plates. The experimental results showed that UHTCC had significant strain hardening and excellent multiple cracking properties under all the rates tested. The ultimate tensile strain lay in the range of 3.7% to 4.1% and was almost immune to the change in strain rates. The rate of 1×10−3 s−1 seemed to be a threshold for dynamic increase effects of the first crack tensile strength, elastic modulus, ultimate tensile strength, and energy absorption capability. When the strain rate was higher than the threshold, the dynamic increase effects became more pronounced. The energy absorption capability was much higher than that of concrete, and the average ultimate crack widths were controlled below 0.1 mm under all rates. Several fitting formulas were obtained based on the experimental results.

Modelling and estimation of water infiltration into cracked asphalt pavement

The present method used to estimate water infiltrating into the pavement of highway is regional and less representative. Based on the flow theory in porous media and Poiseuille cubic law in cracked media, a model was established to quantify the water balance between surface course and the drainage layer in the pavement of highway to estimate the pavement infiltration rate (PIR). Through mathematical derivation of governing equation of water balance in the pavement system, an analytical solution was obtained to express the PIR, which comprehensively takes crack length, crack open width, crack interval, thickness and permeability of structure layers into consideration. The quantitative analysis results showed that the PIR is proportional to crack open width, crack length, drainage layer thickness and permeability of surface course; on the contrary, the PIR is inversely proportional to crack interval distance, surface course thickness and permeability of the drainage layer. The crack property affects the PIR in terms of improving the water entry, while various thicknesses of pavement structure layers and hydraulic conductivities result in a different hydraulic gradient between the top and bottom of surface course thereby leading to a different PIR.

Optimized inspection design for the thermographic characterization of sub-pixel sized through cracks

Non-destructive evaluation of structural components is critical for reducing costs from unnecessary replacements and maintenance. We study the utility of a non-contact modality for the inspection of thin metal plates for the presence of through cracks. Sensitivity to early stages of deterioration allows for simpler and less expensive repair than if a flaw propagates and becomes more damaging. Hence, we focus on the characterization of very small cracks with a thermal imaging technique. Through cracks interact with the flow of heat within a component, so that the characterization of cracks from a thermal image amounts to solving an inverse problem to discover unknown parameters that describe the crack.We consider cracks with length of less than a millimeter, falling under the pixel resolution of the recording thermal camera. Although these flaws are not directly visible from imaging data, the well-understood theory of heat conduction can be used in inference of crack properties. Herein we present a method to design an inspection modality that yields optimal data for such inference. Numerical experiments are performed to compare our optimized inspection setup to previous thermographic inspection scenarios found in the literature. Our design is found to produce the same quality of inference as these previous experiments which require much more expensive equipment (e.g. more powerful lasers and more sensitive IR cameras).

Crack Formation in Cement-Based Composites

The cracking properties in cement-based composites widely influences mechanical behavior of construction structures. The challenge of present investigation is to evaluate the crack propagation near the crack tip. During experiments the tension strength and crack mouth opening displacement of several types of concrete compositions was determined. For each composition the Compact Tension (CT) specimens were prepared with dimensions 150×150×12 mm. Specimens were subjected to a tensile load. Deformations and crack mouth opening displacement were measured with extensometers. Cracks initiation and propagation were analyzed using a digital image analysis technique. The formation and propagation of the tensile cracks was traced on the surface of the specimens using a high resolution digital camera with 60 mm focal length. Images were captured during testing with a time interval of one second. The obtained experimental curve shows the stages of crack development.

Evaluation of rubber influence on cracking resistance of crumb rubber modified binders with wax additives

Cracking properties of crumb rubber modified (CRM) asphalt binders containing wax additive were evaluated through the dynamic shear rheometer test at 25 °C and the bending beam rheometer test at −12 °C. The CRM binders were produced using three rubber contents of 5%, 10%, and 15% by the binder weight, and then mixed with two commercial wax additives of LEADCAP and Sasobit. Three states of original, short-term and long-term aging were applied to evaluate the cracking properties using rolling thin film oven and pressure aging vessel. From the results, it is concluded that (1) the increase of rubber content significantly decreases the binder stiffness at lower temperature, (2) the higher the rubber content, the lower the G*sinδ of CRM binders with wax additives at intermediate temperature, and (3) the rubber can be used to improve the cracking resistance of asphalt binders modified with wax.

Enhancing the flexural performance of ultra-high-performance concrete using long steel fibers

In this study, the flexural performance and fiber distribution characteristics of ultra-high-performance concrete (UHPC) were investigated according to the fiber length. To do this, three different fiber lengths having an identical diameter were used. Enhancements in flexural strength and energy absorption capacity were observed when longer fibers (or higher aspect ratios of fiber) were used, whereas insignificant effect of fiber length on the first cracking properties (i.e., first cracking strength and corresponding deflection) was obtained. Fiber length had a little influence on the degree of fiber dispersion, but a significant influence on the fiber orientation. A higher fiber orientation coefficient along the flow distance was obtained when shorter fibers were used. A finite element analysis incorporating previously suggested material models was performed and verified by comparing the analytical results with the present experimental data.

Influence of recycled coarse aggregates on normal and high performance concrete subjected to elevated temperatures

Recycled concrete aggregates (RCA) can be a promising solution for sustainable development. For buildings, the high temperature performance is critical to estimate fire resistance. Thus, this paper investigates RCA concretes after exposure to temperatures up to 750°C by considering laboratory and industrial RCA, and normal and high performance concretes. Tests were conducted on cylindrical specimens to assess cracking, mass loss, porosity, and thermal and mechanical properties. The residual performances for the recycled concretes were generally similar to but slightly worse than those observed for the reference concretes. The presence of non-cementitious impurities accelerates the damage of concretes with temperature.

Effects of crack properties and water-cement ratio on the chloride proofing performance of cracked SHCC suffering from chloride attack

This study aims at developing an SHCC mixture that offers improved workability and high rebar corrosion proofing performance while ensuring moderate tensile ductility. Specimens were made from several mixtures with part of the cement replaced with limestone powder, and with a reduced fiber content. Mechanical properties and corrosion proofing performance under tensile stress were examined to determine the effects of chloride in a simulated situation under uniaxial tensile strain. The results indicated that the fiber content in the mixture had no influence on the crack properties of steel reinforced SHCC under tensile load. It was found that the chloride proofing performance of SHCC with multiple cracks was affected by the crack properties such as the number of cracks and the accumulated crack width. Reducing the water-cement ratio was effective to enhance the chloride proofing performance of SHCC.

Microstructures of hot-pressed Cr–Ti–Cu targets and their film properties for perpendicular magnetic recording application

In this study, the effect of Cr50−0.5xTi50−0.5xCux (x = 0, 10, 20, 30, 40, 50) hot-pressed target on film performance was investigated. By increasing the ratio of Cu, the target strength and anti crack properties showed significant improvements due to the decrease of brittle intermetallic compound. In addition, the target properties were found to affect the performance of sputtering process and its film. During sputtering test, the arcing behavior was significantly suppressed by reducing the incoherent defects of the target. The results of film analysis also showed the advantages with increasing Cu ratio in Cr–Ti–Cu alloy systems, including the decrease on sheet resistivity, intrinsic stress and surface roughness. Moreover, all as-deposited films were clarified as amorphous structures to meet the requirement for perpendicular recording media application, and further analyzed with Ω and δ indices, which were subjected to quasi-chemical energy and the atomic size difference, respectively. Unfortunately, the excessive Cu addition makes a side effect on the corrosion resistance in acid environment. Related results mentioned above were supported by bending strength measurement, X-ray diffraction, scanning electron microscope, atomic force microscope, Twyman–Green interferometer, and four-point probes method.