Crack Measurement Method Research Articles

Full-field deformation measurement and cracks detection in speckle scene using the deep learning-aided digital image correlation method

An enhanced full-field deformation and crack measurement method for oblique optical-axis conditions was proposed. First, the thickness of the calibration plate causes an error in the homography conversion (H matrix) and the final displacement results. To solve this problem, a Perspective-n-Point (PnP) based thickness compensation method for H calibration was proposed. Second, for better measurement of the full-field deformation, the measured surface is mapped with random speckles and markers, and the complex background and covering problems create obstacles for crack skeleton detection. To overcome these interferences, a strain-guided two-phase detection method based on the deep learning model was innovatively designed. Third, the traditional crack width calculation method is susceptible to the binarization threshold and the connection to the speckles, and a crack width calculation method using the displacement of control points was suggested. During a bending test, the above methods were applied to a basalt fiber-reinforced polymer (BFRP) beam. The enhancement in the displacement measurement accuracy and the robustness of crack detection, particularly the potential of detecting hidden cracks, were verified by comparing the performance of the proposed method to that of traditional contact sensors.

A Subpixel Concrete Crack Measurement Method Based on the Partial Area Effect

To improve the accuracy of concrete crack measurement with a machine vision method in structural health monitoring and in technical status evaluation, a subpixel crack measurement method based on the partial area effect was proposed. (1) First, a pixelwise crack image segmentation method was established through a multi-step process of multi-threshold fusion and morphology operation, and a novel pixel degree crack width calculation method was developed with the extraction of the middle points, the center line and its normal, and the intersection of the center line normal and crack edges. (2) Then, a subpixel algorithm based on the partial area effect was introduced to locate vertical, horizontal, and oblique cracks in subpixel crack edges, and the subpixel crack width could be calculated along the crack center line pixelwise. (3) Finally, the proposed method was verified by indoor concrete beam crack measurement tests with a digital microscope, and the results show that the maximum relative errors of the subpixel width of the horizontal, vertical, and oblique straight cracks measured by the proposed method were 3.06%, 8.97%, and 5.16%, respectively. The absolute error of the crack length was less than 0.30 mm, and the measurement accuracy could reach 0.01 pixels. The subpixel crack measurement method provides a novel possible solution for structural health monitoring.

REAL TIME MEASUREMENT AND ANALYSIS OF FULL SURFACE CRACKING CHARACTERISTICS OF CONCRETE BASED ON PRINCIPAL STRAIN FIELD

The measurement of the crack propagation process is significant to reveal the failure mechanism and evaluate the mechanical properties of concrete structure. This paper presents a method of crack location and width measurement based on the deformation field of concrete surface. The high-resolution deformation field of the concrete specimens is obtained by using multi-camera digital image correlation method at first. It is found that the virtual principal strain field around the crack obviously differs from that in uncracked zone because of the gradient of displacement field caused by crack, and the principal strain field can be regarded as approximately Gaussian distributed in the normal direction of the crack. A new method of crack location based on the principle strain field is proposed based on the Steger algorithm which is widely applied in laser stripe center extracting, and the difference of in-plane displacement vectors on the normal direction between two sides of the crack is obtained. The projection along the normal direction is taken as the width of mode I crack, while the projection along the normal vertical direction is taken as the width of mode II crack. The experiment of simulating crack propagation is performed by using a high precision translation table to verify the measurement accuracy of crack width measurent. The results of the experiments show that the measurement error of crack width is between 0.010 pixel and 0.017 pixel, which is consistent with the theoretical prediction. The standard deviation is between 0.006 pixel and 0.008 pixel, which illustrates that the measurement accuracy of the proposed method is high. The accuracy of the proposed method is better than that of image-based crack measurement method at the same image resolution. The method proposed can automatically measure crack propagation in real time, which provides a reliable and accurate method for the concrete experiment.

A cracking approach to inventing new tough materials: fracture stranger than friction.

A cracking approach to inventing new tough materials: fracture stranger than friction.

Mechanical and thermal properties of multiwalled carbon-nanotube-reinforced Al2O3 nanocomposites

Multiwalled carbon nanotube (MWCNT)-reinforced Al2O3-matrix nanocomposites were obtained by wet ball milling composite powders and spark plasma sintering. The mechanical and thermal properties of the nanocomposites with five different MWCNT concentrations from 0 to 2.0 wt% were studied. Results show that the matrix grain sizes of the composites decreased after MWCNT addition. The hardness, bending strength, and fracture toughness of 0.5 wt% MWCNT nanocomposite are the highest, which are 35.7%, 55%, and 182% higher than those of monolithic alumina, respectively. Errors of fracture toughness values obtained by the single-edge notch beam method are significantly smaller than those measured by the direct crack measurement method. With the addition of MWCNTs, the thermal conductivity of the nanocomposites decreases gradually, reaching the minimum of 12 W/mK in 2.0 wt% MWCNT composite at 300 °C. The thermal conductivity decreased owing to the agglomeration of MWCNTs and changes in the porosity and grain size of the composite matrix. No distinct difference is observed in thermal expansion behaviours between the nanocomposites and monolithic Al2O3 owing to the low content of MWCNTs.

Damage Indexing Method for Shear Critical Tubular Reinforced Concrete Structures based on Crack Image Analysis.

Image analysis techniques have been applied to measure the displacements, strain field, and crack distribution of structures in the laboratory environment, and present strong potential for use in structural health monitoring applications. Compared with accelerometers, image analysis is good at monitoring area-based responses, such as crack patterns at critical regions of reinforced concrete (RC) structures. While the quantitative relationship between cracks and structural damage depends on many factors, cracks need to be detected and quantified in an automatic manner for further investigation into structural health monitoring. This work proposes a damage-indexing method by integrating an image-based crack measurement method and a crack quantification method. The image-based crack measurement method identifies cracks locations, opening widths, and orientations. Fractal dimension analysis gives the flexural cracks and shear cracks an overall damage index ranging between 0 and 1. According to the orientations of the cracks analyzed by image analysis, the cracks can be classified as either shear or flexural, and the overall damage index can be separated into shear and flexural damage indices. These damage indices not only quantify the damage of an RC structure, but also the contents of shear and flexural failures. While the engineering significance of the damage indices is structure dependent, when the damage indexing method is used for structural health monitoring, the damage indices safety thresholds can further be defined based on the structure type under consideration. Finally, this paper demonstrates this method by using the results of two experiments on RC tubular containment vessel structures.

A Research on Static and Dynamic Fracture-Resistance of Welding Parts of Coolant Piping Material

Tensile and fracture toughness tests of coolant piping materials were performed at various loading rates and temperatures. The crack measurement method for coolant piping materials consisted of the unloading compliance method for quasi-static loading conditions and condition, while the electric potential drop method and the normalization method for quasi-dynamic loading conditions. Tensile and fracture toughness tests of coolant piping materials were performed at the design temperatures of the primary piping system of nuclear power plants, which are 449K (176°C) and 589K (316°C). Also, the plastic η factor considering strength mismatch of welding part was applied to the fracture toughness test results and the J-R curves by the plastic η factor considering strength mismatch of welding part was compared with the J-R curves by the plastic η factor introduced in ASTM E1820-01.

Assessing of Bendability of Aluminum Alloy Sheets for Autobodies

In recent years, reducing carbon dioxide is being demanded in terms of preventing global warming. Lightening autobodies is one of the most practical ways to conduct it, for which converting the body sheets from conventional steel to Al-Mg-Si alloy is effective. Although the Al-Mg-Si alloys have the advantage that they have hardenability during paint baking and do not cause stretcher strain patterns, poor formability is a crucial drawback. Bendability is one of the most important properties related to formability. However, there has been no assessing method having both speediness and quantitative reproducibility. In this study, we have developed a assessing method based on the electric resistance decrease arising from the decrease in conducting section when cracks are formed by bending. Bendability was assessed by electrical resistance change as well as by crack density on the tension surface measured with an SEM. It was found that the new method have far greater speediness with the same quantitative reproducibility than the crack measurement method.

Characterization and wear behavior of modified silicon nitride

Silicon nitride-based materials are applied to many tribological components because of their superior thermal and mechanical properties and corrosion resistance. The purpose of this study is to investigate the wear performance of modified silicon nitride which contained 3 wt.% La 2O 3 and 3 wt.% Y 2O 3. The relationships between microstructure and mechanical properties have been also addressed. Vickers microhardness and toughness were measured, the later being determined by means of the direct crack measurement method (DCM). Scanning electron microscopy technique (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) was used for the morphological characterization of the samples as well as for the characterization of the wear scars. Wear properties were studied under a load of 10 N by using the ball-on-disk tribometer. AISI 52100 steel balls and balls of WC + 6% Co (6 mm diameter) were used as static counterparts, respectively. Steady state friction coefficients of 0.66 and 0.62 were measured for the tribological pairs WC + 6 % Co/Si 3N 4 and AISI 52100/Si 3N 4, respectively. The wear mechanism was determined in each case, and comparison between the wear performance of the traditional and modified silicon nitride is also presented.

Transformation Driving Force for Indentation Cracking in Zirconia Ceramics

The transformation driving force for indentation cracking in phase-transformation-toughened ZrO[sub 2] ceramics is confirmed with the direct crack measurement method. This driving force is produced by expansion of the plastic zone beneath the indenter due to stress-induced transformation, and it promotes the extension of the indentation cracks. The driving force cannot be neglected when evaluating the indentation fracture toughness of the materials.

Fracture toughness of pressureless sintered silicon carbide: A comparison of Klc measurement methods

Four different K lc measurement methods, all of them using small bars, were compared. The materials studied in this paper were pressureless sintered α-and ß-SiC. K lc values obtained by the direct crack measurement method depended on the indentation load. The single edge notched beam method yielded a higher value than others. Indentation strength bending and chevron notched beam methods showed almost the same K lc values: both methods measured K lc with a sharp crack as controlled defect. Effect of notch width was observed on single edge notched beam specimens as the strain rate and span length dependence on chevron notched beam specimens; chevron notched beam K lc values were also strain rates and span length dependent. Influence of the temperature was also studied. The fracture toughness of sintered SiC increases at high temperature (1350°C, in air).