Surface Crack Distribution Research Articles

Study on the Relationship between Early Shrinkage Cracking and Mechanical Properties of Nano-Clay Cement Mortar Based on Fractal Theory

In order to study the influence of nano-clay on the crack resistance of cement-based materials, two kinds of nano-metakaolin (NMK) and two kinds of nano-attapulgite clay (NMA) were considered. The early cracking process and mechanical properties of nano-clay cement mortar (NCM) was studied by using a plate knife-edge constraint test. Based on fractal theory, the distribution characteristics of NCM surface cracks were revealed, and the calculation method forNCM maximum crack width was given. The results show that the cracking time of the NMK-3 specimen is 2 and 6 h later than that of NMK-1 and NMA-2, respectively; the smaller the particle size of nano-clay, the earlier the cracking time of the specimen. However, nano-clay effectively inhibited the expansion of mortar cracks, and the cracks on the surface of NCM were thin and sparse. At 28 days, the maximum crack width of NMK-3 was 46.7% and 33.3% lower than that of NMK-1 and NMA-2, respectively. NMK hadthe best improvement effect on the mechanical properties cement mortar. The smaller the particle size, the more pronounced the improvement effect.The flexural strength ratio and compressive strength ratio at 7 and 28 days are 76.7%, 67.4%, and 61.2%, respectively.The distribution of surface cracks on NCM has fractal characteristics, and the fractal dimension of surface cracks is smaller than that of ordinary cement mortar. The larger the particle size of nano-clay, the smaller the fractal dimension of cracks. The quantitative relationship between fracture fractal dimension and NCM elastic modulus and shrinkage tensile stress is established.

Characteristics of Overburden and Ground Failure in Mining of Shallow Buried Thick Coal Seams under Thick Aeolian Sand

Mining can lead to overburden failure and ground damage, which are more severe in mining shallow buried thick coal seams (SBTCS) under thick aeolian sand (TAS). We attempted to discover characteristics of mining in this particular geological condition through theoretical derivation and numerical simulation, and field monitoring. Theoretical methods, combined with numerical simulation and field monitoring methods, reveal the essence of the development and distribution of surface cracks caused by mining SBTCS and depth to thickness ratio (DTR) to be 13.43, less than 15. The findings show that, when mining SBTCS, the overburden breaks down periodically, the initial collapse distance is greater than the collapse step, approximately 55 m on average, and the collapse step is approximately 45 m, on average, in the Daliuta Coal Mine. The collapsed blocks are stacked into goaf and form “masonry beams”, and many cracks and pores are generated between the blocks. The weak stress of the aeolian sand layer causes the movement angle in the aeolian sand layer to be smaller than that in the bedrock, and leads to much sheer, tension and compression failure on the ground, and the main forms of cracks are compression uplift, tensile cracking, shear step.

Investigation on the flexural toughness evaluation method and surface cracks fractal characteristics of polypropylene fiber reinforced cement-based composites

In this study, the compressive and flexural properties of polypropylene fiber reinforced cement-based composites are investigated, and a suitable method for evaluating the flexural toughness is proposed. The compressive strength, flexural strength, deflection, flexural toughness, and equivalent flexural toughness ratio are also measured. The results indicate that the addition of 2 % polypropylene fiber and 60 % fly ash into matrix facilitates an improvement in strength, and such a cement matrix shows better crack resistance and flexural toughness than ordinary concrete. JSCE-SF4 and ASTM C1609 methods are the main methods for calculating the flexural toughness, but their single calculation process limits the application range. Therefore, based on JG/T 472–2015, a simple method which is not limited by deformation and can ignore the influence of load fluctuation, is proposed to evaluate the load carrying capacity and flexural toughness of engineered cementitious composites (ECC) in the strain-hardening stage. Furthermore, it is verified that the surface cracks of specimens under flexural load exhibit fractal characteristics, and the “fractal dimension” can be used to describe the distribution of surface cracks. Besides, a quantitative relationship is established between the fractal dimension of surface cracks and the flexural toughness evaluation parameters obtained by the improved JG/T 472–2015 method, which indicates that the fractal dimension of surface cracks can be used to characterize the flexural properties of ECC.

Investigation of machining performance in die-sinking electrical discharge machining of pentagonal micro-cavities using cylindrical electrode

The aim of the present article is to fabricate pentagonal micro-cavities and describe the influence of current on the machining performances. Machining of EN-24 alloy steel samples was performed in die-sinking electrical discharge machining (EDM) machine with varying values of current using polygon cycle approach. The machined pentagonal cavities were examined under optical microscope and scanning electron microscope (SEM) to evaluate machining performances in terms of corner error, white layer formation, surface crack distribution, and globule formation. It is found that as the value of current increases, there is more formation of white layer with non-uniform distribution of cracks and the thickness of white layer increased from 6.21 to 8.20 µm with increase in current. On the other hands, surface finish deteriorates when the current value rises. In addition to this, there is an enhancement in tool wear rate with increasing current. At the higher values of current, the spark energy increases which leads to greater melting and evaporation and production of smoke and bubbles on the dielectric surface. This study revealed that die-sinking EDM coupled with short electronic pulses and precise electrode movement is capable of producing microstructures under appropriate operating conditions.

The influence of restraint on the expansion of concrete due to delayed ettringite formation

The influence of restraint on expansion, expansive pressure, and cracking patterns due to delayed ettringite formation (DEF) in concrete was experimentally evaluated. Especially, the expansive pressure was estimated with two approaches: calculation from the strain of the steel and direct measurement using load cells. The expansive behaviors were strongly affected by the restraint, especially in the restraint direction. The expansive pressure measured by the load cell was 1.9–3.9 MPa, which is nearly consistent with those calculated from the steel bar strain. The expansive pressure of DEF was almost the same order of magnitude as for ASR expansion, despite larger free DEF expansion than ASR. From a simplified calculation, it is estimated that the imposed DEF expansion was reduced from the stress-free expansion by approximately 80%. Although the total length of surface cracking was independent of the degree of the restraint, the distribution of surface cracks was significantly modified by the degree of the restraint. On the contrary, the inner crack pattern was similar for the restraint case while large gap formation was observed for the stress-free case.

Remaining Life Assessment for Steel After Low-Cycle Fatigue by Surface Crack Image.

After pre-fatigue cycles at different strain amplitudes with different N/Nf values (33.3%, 50%, and 75%), specimens of HRB335 steel were subjected to uniaxial tension until failure. By this method the mechanical properties of the specimens after pre-fatigue testing were measured, and the fracture morphology and microscopic morphology in the vicinity of the specimen’s neck surface near the fracture were observed. The verification of the characteristics to be used to estimate the damage caused during the loading cycles was conducted. By observing optical microscope images of the surface area near the neck of the specimens, it was found that the images of surface cracks were significantly different and strongly depended on the number of pre-fatigue cycles the specimen had undergone. In response to this phenomenon, both the microscopic images taken directly from the photos of the surface crack distribution and the binary images based on them were statistically analyzed, and then a parameter, S, denoted as the “unit crack area”, characterizing the cumulative fatigue damage was suggested. Furthermore, the test procedure and the calculation formula for determining the image parameters were summarized, and a method for evaluating the remaining life of steel after low-cycles of reversed tension and compression was proposed.

Using tapered specimens to study the effect of hydrogen and surface finish on SCC initiation in Alloy 182 under boiling water reactor conditions

ABSTRACTThe influence of dissolved hydrogen content and surface finish on the stress corrosion cracking (SCC) initiation behaviour in Alloy 182 weld metal was studied with flat tapered tensile specimens in simulated boiling water reactor water at 274°C using constant extension rate tensile tests. The surface crack distribution, crack depth and stress threshold for SCC initiation were analysed by detailed post-test quantitative characterisation. Intergranular SCC initiation has been found in all specimens tested with dissolved hydrogen contents varying from 10 to 2000 ppb. The highest crack density and depth and lowest stress threshold for crack initiation were found to be at the Ni/NiO phase transition line with 253 ppb dissolved hydrogen content. In addition, ground surfaces showed a tendency to higher SCC initiation susceptibility compared to electro-polished surfaces.

Slip distribution of the 2014 Mw = 8.1 Pisagua, northern Chile, earthquake sequence estimated from coseismic fore‐arc surface cracks

AbstractThe 2014 Mw = 8.1 Iquique (Pisagua), Chile, earthquake sequence ruptured a segment of the Nazca‐South America subduction zone that last hosted a great earthquake in 1877. The sequence opened >3700 surface cracks in the fore arc of decameter‐scale length and millimeter‐to centimeter‐scale aperture. We use the strikes of measured cracks, inferred to be perpendicular to coseismically applied tension, to estimate the slip distribution of the main shock and largest aftershock. The slip estimates are compatible with those based on seismic, geodetic, and tsunami data, indicating that geologic observations can also place quantitative constraints on rupture properties. The earthquake sequence ruptured between two asperities inferred from a regional‐scale distribution of surface cracks, interpreted to represent a modal or most common rupture scenario for the northern Chile subduction zone. We suggest that past events, including the 1877 earthquake, broke the 2014 Pisagua source area together with adjacent sections in a throughgoing rupture.

Fracture Analysis and Distribution of Surface Cracks in Multicrystalline Silicon Wafers

Solar silicon wafers are mainly produced through multiwire sawing. The sawing process induces micro cracks on the wafer surface, which are responsible for brittle fracture. Hence, it is important to scrutinize the crack geometries most commonly generated in silicon wafer sawing or handling process and link the surface crack to the fracture of wafers. The fracture of a large number of multicrystalline silicon wafers has been investigated by means of 4-point bending and twisting tests and a failure probability function is presented. By neglecting the material property variation and assuming that one surface crack is dominating the wafer breakage, 3D finite element models with various crack sizes (depth, length, and orientation) have been analyzed to identify the distribution of surface crack geometries by fitting the failure probability from the experiments. With respect to the 63% probability, the existing surface cracks in the wafers studied appear to have depth and length ratios less than 0.042 and 0.19, respectively. Furthermore, it has been shown that the surface cracks with depth in the range from 10 to 20 μm, length up to 10 mm and angles in the range of 30 deg–60 deg, can be considered as the most common crack geometries in wafers we tested. Finally, it has been found that the mechanical strength of the wafers tested parallel to the sawing direction is approximately 15 MPa smaller than those tested perpendicular to the sawing direction.

Thermal Fatigue Material Degradation of Caster Rolls' Surface Layers

Degradation of surface layers of continuous caster rolls under service conditions is studied. The roll material is steel 25 Kh1M1F. A statistical analysis of size, orientation and distribution of surface cracks is carried out. Microhardness measurements are performed and structural degradation of the material surface layers is evaluated as a function of the distance to the roll surface. It is found that the damage of the surface layers, estimated by means of a homogeneity coefficient, is 60–72% greater than the damage at the depth h = 9–32 mm. The evolution of the roll material microstructure under tensile deformation is studied as well.

Fractal Behavior of Concrete Crack and Its Application to Damage Assessment

In this study, the fractal behavior of crack development in damaged concrete materials and its application to structural damage assessment are experimentally investigated. The experimental results demonstrate that the crack evolving is closely related to the characteristics of fractal behavior, and the surface-crack distribution of the damaged concrete structures possesses fractal characteristics. It thus indicates that the fractal may be capable of performing quantitative characterization of crack development, which cannot be comprehended by Euclidian geometry. The application of fractal characteristics of surface-crack distribution in structural damage assessment is then investigated. By correlating the fractal dimension of surface-crack distribution with the model parameters (e.g., natural frequency) of different magnitudes of damage, a linear relationship exists between the fractal dimension and natural frequency. Since the natural frequency is one of the most fundamental damage characteristic factors, the above linear relationship shows that the fractal dimension of surface-crack distribution is able to effectively act as an effective damage characteristic factor. Different from the conventional damage characteristic factors generated from vibration tests, this factor reflects structural damage from a new perspective of fractal analysis, and provides a viable and novel approach for health monitoring and safety evaluation for concrete structures.

Relationship between electrode size and surface cracking in the EDM machining process

This paper presents a study of the EDM machining of H13 and D2 tool steels using electrodes of different diameters. Scanning electron microscopy is employed to analyze the machined surface, and the concept of a Crack Critical Line (CCL) is introduced to explore the influence of electrode size, EDM parameters and material thermal conductivity on surface cracking. The current results reveal that the surface crack distribution is influenced by the machining parameters, the electrode diameter and the material conductivity. It is noted that cracks tend not to appear when the machining is performed with a decreased pulse current and an increased pulse-on duration. Furthermore, it is observed that changing the electrode diameter causes a parallel shift of the CCL location within the crack distribution map. The intercept of the line depends on the electrode size. When small diameter electrodes are employed in the machining process, the location of the CCL shifts upwards. This causes the no-crack zone to enlarge, and therefore permits a wider choice of machining parameters to be adopted.

Size distribution of surface cracks and crack pattern in austenitic SUS316 steel plates fatigued by cyclic bending

The size distribution of surface cracks and the crack pattern were examined on the specimens of the SUS316 steel plates fatigued by cyclic bending. The size distribution of the cracks could be approximated to a logarithmic normal distribution, irrespective of the maximum total strain range or the number of fatigue cycles. The number of the cracks (Nu) of the length (x′) equal to or larger than a given size (X) could be approximated to a power law, Nu∝ X−a, with a scaling exponent a at the larger crack sizes in the fatigued specimens of the SUS316 steel. The value of a decreased with increasing the number of fatigue cycles because of the increase in the number and size of fatigue cracks, and was larger in the specimens tested at the smaller total strain range. Effects of experimental variables on the scaling exponent (a) were also shown in this study. The fractal dimension of spatial crack distribution (the fractal dimension of crack pattern) (D) increased in the range from about 0.9 to about 1.2 with increasing the number of fatigue cycles, and was larger in the specimens fatigued at the larger total strain range. There was a negative correlation between the value of a and the value of D on fatigue cracks, although there was no unique relationship between these two values.

Fibreoptic reflectometric technique for the automatic detection and measurement of surface cracks

A new fibreoptic reflectometric technique for the automatic detection and measurement of surface cracks is proposed, described and analysed. A relationship between the system output image and the surface crack distribution is obtained from theory and a crack measurement principle is derived. Analysis of the experimental data obtained using a first laboratory prototype has validated the proposed measurement principle. The results of a second generation prototype confirm that the technique promises to be a valuable tool in practical inspection tasks.

Surface Damage Modeling of Oxidized Metal Matrix Composite Laminates under Axial and Transverse Tension

The oxidation effects on surface crack development of oxidized Ti-15-3 and fl-21S Metal Matrix Composite (MMC) unidirectional laminates under applied axial or transverse tension loadings are investigated in the present study. An approximate mechanical model is established and a stress analysis of an oxidized laminate with surface cracks is performed. To simplify the analysis, motivated by experimental observations, a periodic distribution of surface cracks in the oxide layer is assumed. The critical value of the applied mechanical load, which is necessary for the information of new cracks as a function of crack density, is calculated for given oxide layer thickness. The effective stiffness reduction, caused by the formation of the surface cracks, is also evaluated. Finally, the influence of gradient effective properties of the metal matrix underneath the oxide layer, caused by the diffusion of oxygen, on the stress field is considered.

FATIGUE DAMAGE IN TWO‐STEP LOADING OF 316L STEEL II. SHORT CRACK GROWTH

Abstract— The evolution of the surface crack distribution in single‐step and two‐step plastic strain amplitude controlled cycling was studied. The frequency of crack occurrence and total crack density were evaluated together with crack growth rates of the largest cracks. The crack density increases, reaches a maximum, and then decreases during fatigue life. The maximum value of crack density partitions the whole life into a “crack generation period” and a “crack propagation period”. In the first period the crack growth rate is approximately constant and is determined by the applied plastic strain amplitude. The crack growth rate increases with crack length in the second period, but is basically determined by the crack growth rate in the “crack generation” period. Hence the crack growth rate in the “crack generation” period is the life determining factor.

Modeling and evaluating the scattering of Rayleigh waves by a linear distribution of surface cracks

A numerical evaluation model of the reflection coefficient of Rayleigh waves for a linear distribution of surface cracks is developed using the weight function estimation method. The numerical evaluation of the reflection coefficients varying with respect to the crack depth ratios, frequencies, and the number of the cracks for several commonly used engineering materials is performed. The results show that the model can effectively be used to evaluate the reflection coefficient for a linear distribution of the cracks. The crack depth which can be evaluated has been extended substantially and the evaluation results have been improved.

Ultrasonic scattering models for distribution of cracks

To evaluate the initiation and growth behaviour of a distribution of cracks, an acoustic scattering model for a planar distribution of cracks is developed based on generalized reciprocity theory and Kino's scattering formalism. Subsequently, an acoustic scattering model for a linear distribution of surface cracks in the form of the local stress intensity factors is derived by using the elastostatic energy balance of crack formation. The numerical evaluation of the reflection coefficient of Rayleigh waves for a linear distribution of semi-elliptic surface cracks is subsequently performed. The results show that the reflection coefficient, varying with respect to the crack depth ratio for a linear distribution of cracks, can effectively be evaluated by using the model.

Water Jet Impact Damage at Convex, Concave, and Flat-Inclined Surfaces

Results showing the damage arising from water jet impact on convex, concave, and flat-inclined polymethylmethacrylate targets are presented and discussed. The direction and distribution of surface cracks and pits within the zone of impact are related to liquid flow during the impact and the essential features of the damage are analyzed in terms of stress wave phenomena.