Shallow Cracks Research Articles

Brittle Behavior in Aspirin Crystals: Evidence of Spalling Fracture

Single crystals of aspirin form I were cleaved and indented on their dominant face. Upon inspection, it was possible to observe strongly anisotropic shallow lateral cracks due to the extreme low su...

Empirical optimization of corrosion rate for magnesium-chromium composites

In this study, optimization of the corrosion rate (CR) of Mg-composites has been evaluated by varying the concentration, reinforcement percentage, and immersion time. As prime material, pure Mg is preferred for this research and chromium (Cr) consider as a reinforcing material with different percentages. CR (miles/yr) has been optimized by varying parameters such as reinforcement percentage as 3%, 5% and 7% including NaCl immersion medium (%) as 2.4%, 3.5% and 4.7% with immersion time (h) such as 48h, 72h and 96h. By using, DOF, minimal CR has been measured with the assistance of Minitab Software having ANOVA and Taguchi approaches. Optimized results reveal that the percentage of corrosion solution is influenced upto 66.10%, reinforcement percentage contributed to 27.56% and immersion time influenced upto 2.81%. An optimized combination of CR is 7wt. % Cr with 2.4% NaCl for 96h. However, microscopy results illustrate shallow crack boundaries in Mg/Cr composites due to its chemical reaction in alkaline solution.

Quantitative detection of shallow subsurface cracks in pipeline with time-of-flight diffraction technique

When the ultrasonic time-of-flight diffraction (TOFD) technique is applied to the inspection of pipeline, the propagation direction of the direct longitudinal wave is not parallel to the pipe surface, leading to the increment of the depth of dead zone and the measurement errors of shallow subsurface cracks. In this paper, the differences between the dead zones in flat plate and pipeline are compared theoretically. Subsequently, the method based on the mode-converted waves in TOFD B-scan image, which has been used for reducing the dead zone in flat plate, is developed to restrict the dead zone in pipeline and to realize the accurate measurement of shallow subsurface cracks. The simulated and experimental results indicated that the depth of dead zone in the carbon steel pipeline with a radius of 148 mm was reduced from 6.3 mm to 3.0 mm and that the measurement errors of crack length and angle were no more than 0.27 mm and 1.57°, respectively. The depth of the dead zone in pipeline and the measurement errors before modification both increase with the reduction of the ratio of pipe radius to probe center spacing (PCS). The modified method has universality for the detection of the shallow subsurface cracks in pipeline with ultrasonic TOFD technique.

The Comparative Analysis and Verification of the Engineering Methods for Considering the Shallow Crack Effect for the Prediction of the Fracture Toughness of the Materials for Nuclear Reactor Pressure Vessels

The key features of the fracture of the specimens with shallow cracks are considered, and a brief analysis of the main methods making it possible to predict the temperature dependence of the fracture toughness KJc(T) for shallow cracks is given. These methods include the DA method, (J–Q) method, (J–T) method, “local” methods utilizing multiparameter probabilistic approaches, GP method utilizing the deterministic “power” approach, as well as two engineering methods—RMSC (Russian Method for Shallow Crack) and EMSC (European Method for Shallow Crack). Detailed verification and comparative analysis of these two engineering methods are carried out in respect of the materials of the VVER and PWR nuclear reactor pressure vessels considering the shallow crack effect on the basis of 13 sets of experimental data for domestic and imported steels.

Crack Propagation and Burst Pressure of Pipeline with Restrained and Unrestrained Concentric Dent-Crack Defects Using Extended Finite Element Method

Mechanical damage in form of dents, cracks, gouges, and scratches are common in pipelines. Sometimes, these damages form in proximity of each other and act as one defect in the pipe wall. The combined defects have been found to be more injurious than individual defects. One of the combined defects in pipeline comprises of a crack in a dent, also known as dent-crack defect. This paper discusses the development of finite element models using extended finite element criterion (XFEM) in Abaqus to predict burst pressure of specimens of API X70 pipeline with restrained and unrestrained concentric dent-crack defects. The models are calibrated and validated using results of full-scale burst tests. The effects of crack length, crack depth, dent depth, and denting pressure on burst pressure are investigated. The results show that restrained dent-crack defects with shallow cracks (depth less than 50% wall thickness) inside dents do not affect pipeline operations at maximum allowable operating pressure if crack lengths are less than 200 mm. Releasing restrained dent-cracks when the pressure is at maximum allowable operating pressure can cause propagation of deep cracks (depth of 50% wall thickness or more) longer than 60 mm. However, only very long cracks (200 mm and higher) propagate to burst the pipe. Cracks of depth less than 20% of wall thickness inside dents formed at zero pressure are not propagated by the maximum allowable operating pressure. Dent-crack defects having dents of depth less than 2% outside diameter of pipe behave as plain cracks if the dents are formed at zero denting pressure but are more injurious than plain cracks if the dents are formed in pressurized pipes.

Coupling Geometric and Microstructure Gradients in Fatigue Crack Formation

Scribe marks on aircraft structures are micron size surface scratches that usually result from removing sealant prior repainting. Some efforts revealed that these scratches can nucleate multiple shallow cracks through thickness and consequently reduce the fatigue life of aircraft components. Since the size of scribe marks and microstructure (e.g., grains) are of comparable dimensions, they both play a role in fatigue crack initiation in metals. This presentation introduces an approach that enables the quantification of microstructural effects in fatigue crack growth from notches with a low computational cost.

A simplified estimation procedure for the Weibull stress parameter, m, and applications to predict the specimen geometry dependence of cleavage fracture toughness

This study describes a simplified procedure to estimate the Weibull stress parameter, m, using cleavage fracture toughness values measured from testing high constraint fracture specimens. The estimation procedure builds upon the conventional toughness scaling methodology (TSM) based on the Weibull stress coupled with the weakest link model to correct measured toughness distributions for high constraint fracture specimens with identical in-plane dimensions but varying thickness. The approach thus requires a correspondence between predictions based upon the Weibull stress methodology and the weakest link model for measured toughness values under high constraint conditions. Applications of the proposed procedure to analyze specimen geometry effects on fracture toughness values in a typical structural steel provide predictions that agree remarkably well with experimental data for shallow crack bend specimens with different thickness. These exploratory studies demonstrate the potential capability of the proposed estimation procedure which can greatly simplify engineering integrity assessments based upon the Weibull stress model.

UV Aging Behavior of Functionalized Mullite Nanofiber-Reinforced Polypropylene.

In this study, the effect of accelerated ultraviolet (UV) aging on the properties of polypropylene (PP) as well as its blend with PP-graft-maleic anhydride (PP-g-MA) and composite with amine-functionalized mullite nanofibers (AMNF) was compared. Solid-state NMR exhibited some changes in the macromolecular chain structure after aging, whereas the formation of degradation products was confirmed through Fourier transform infrared (FTIR) spectroscopy. The aged composite was observed to exhibit the least increment in the crystallinity from X-ray and differential scanning calorimetry (DSC) analyses (0.3 and 0.5%, compared to 9.7 and 10.4%, respectively, for PP) owing to the stability of its amorphous phase against degradation. Similar resistance toward degradation was also confirmed by thermogravimetric analysis (TGA). The surface morphology of the materials also exhibited the lowest extent of surface embrittlement as well as a small number of shallow cracks in the case of a-PP/PP-g-MA/AMNF composite. The aged composite had a much higher impact strength of 14.9 kJ m–2 compared to 2.5 kJ m–2 for aged PP, thus exhibiting its stability against degradation owing to a synergistic combination of the filler and compatibilizer. The optimal performance of the composite was further confirmed through the least extent of reduction in the tensile strength and elongation at break. These findings demonstrate the superior performance of AMNF-reinforced PP composite over PP for outdoor applications.

Torsional fatigue mechanisms of an A357-T6 cast aluminium alloy

The mechanisms controlling the fatigue response of an A357-T6 cast aluminium alloy under cyclic torsional loading are investigated. Surface crack monitoring coupled with Electron BackScattering Diffraction (EBSD) analysis is used to study crack initiation. Determination of S-N curve combined with interrupted in situ fatigue testing using synchrotron tomography allows the study of the propagation behaviour. It is observed that fractographic morphologies depend on the stress level. At intermediate-low stress levels (τmax < 100 MPa), the grain structure controls the crack initiation and propagation periods. Cracks are usually nucleated in mode II from slip planes close to the specimen axis or perpendicular to it. Mode II crack growth dominates the early stages of crack propagation as mode III inward crack growth is rapidly decelerated. This behaviour leads to the formation of characteristic shallow surface cracks. Once the crack is long enough for the mode II driving force to be overtaken by mode III, propagation branches into ~ 45∘ mode I.

Breakage mechanism of hard-rock penetration by TBM disc cutter after high pressure water jet precutting

To achieve greater efficiency of TBM driving and lower cutter maintenance costs in hard rock formations, the TBM integrated high pressure water jet system was developed. By examining the working modes of water jets and disc cutters, this paper investigates the mechanical response and failure mechanisms for the simple case in which a water jet is first used to cut, followed by disc cutter rock breaking. As disc cutters use penetration-type rock breaking, laboratory quasi-static penetration tests and numerical simulations were carried out for kerf specimen precutting with a high pressure water jet. The influence of kerf parameters on the mechanical behavior of rock breaking was studied, and the mechanisms of surface and internal cracks and fractures were revealed and discussed. The results show that peak penetration force decreases with the increase of kerf depth, and slightly increases with the increase of kerf spacing. The specific energy decreases significantly with the increase of kerf depth and gradually becomes stable once the kerf depth becomes greater than 18.14 mm. The internal cracks formed in shallower kerf specimens mainly include shallow tensile cracks and deep vertical split cracks. However, the internal cracks of deeper kerf specimens incline and bend, extending towards the bottom of the kerf. The theoretically obtained minimum shear load, optimal failure depth, and fragment area all increase with kerf spacing, but the angle of the shear plane remains approximately 45°. These findings may provide some reference for the design and construction of TBMs integrated with high-pressure water jets.

Deep learning–based roadway crack classification with heterogeneous image data fusion

By providing accurate and efficient crack detection and localization, image-based crack detection methodologies can facilitate the decision-making and rehabilitation of the roadway infrastructure. Deep convolutional neural network, as one of the most prevailing image-based methodologies on object recognition, has been extensively adopted for crack classification tasks in the recent decade. For most of the current deep convolutional neural network–based techniques, either intensity or range image data are utilized to interpret the crack presence. However, the complexities in real-world data may impair the robustness of deep convolutional neural network architecture in its ability to analyze image data with various types of disturbances, such as low contrast in intensity images and shallow cracks in range images. The detection performance under these disturbances is important to protect the investment in infrastructure, as it can reveal the trend of crack evolution and provide information at an early stage to promote precautionary measures. This article proposes novel deep convolutional neural network–based roadway classification tools and investigates their performance from the perspective of using heterogeneous image fusion. A vehicle-mounted laser imaging system is adopted for data acquisition (DAQ) on concrete roadways with a depth resolution of 0.1 mm and an accuracy of 0.4 mm. In total, four types of image data including raw intensity, raw range, filtered range, and fused raw image data are utilized to train and test the deep convolutional neural network architectures proposed in this study. The experimental cases demonstrate that the proposed data fusion approach can reduce false detections and thus results in an improvement of 4.5%, 1.2%, and 0.7% in the F-measure value, respectively, compared to utilizing the raw intensity, raw range, and filtered range image data for analysis. Furthermore, in another experimental case, two novel deep convolutional neural network architectures proposed in this study are compared to exploit the fused raw image data, and the one leading to better classification performance is determined.

HIGHWAY AND AIRPORT RUNWAY PAVEMENT INSPECTION USING MOBILE LiDAR

Abstract. With an ever-increasing network of thousands of miles of pavement laid out over highways, road networks, and airport runways, their continuous monitoring is a task of utmost importance to public agencies responsible for their maintenance. The existing approaches mostly rely on a manual detection of pavement distress based on acquired image or video data – an approach that is time-consuming, costly, and whose results are subjective to the designated rater. This necessitates the need for a system that is capable of a quick data acquisition along with an efficient algorithm for the detection and quantification of pavement distress based on the acquired data. This paper proposes a LiDAR-based pavement distress detection and quantification using a mobile mapping system (MMS). Starting with a comparison of a medium-grade and high-grade MMS in terms of their accuracy and captured level of detail, this paper proves the ability of the high-grade MMS to allow the detection of shallow potholes and cracks in the pavement. Next, a fully automated algorithm is proposed to detect pavement distress from 3D point cloud followed by a quantification of the severity (in terms of the depth and volume) of the detected potholes/cracks. Finally, an experimental verification conducted over a 10 mile highway segment and two airport runway strips indicates the efficient performance of the proposed data acquisition system as well as the algorithm to report the pavement distress ranging from shallow cracks over airport runways to deeper potholes along highway segments.

Shallow crack effect on evaluation of residual tensile strength: Harmless and stable cracks in finite-sized structure made of ductile metals

Residual tensile strength (hereinafter abbreviated as ‘residual strength’) is a critical fail-safe consideration but has rarely been reported in the case of shallow pre-cracked structures. When the pre-crack depth is shallower than a critical value, the residual strength of cylindrical specimens made of interstitial-free steel exhibits a plateau identical to the tensile strength of smooth specimens, and with a rupture occurred far away from the pre-cracked cross-section. According to the damage characteristics by microscopic observation and the plastic strain distribution by kernel average misorientation measurement, such a situation exists when the physical meaning of residual strength changes from fracture instability to plastic instability. The corresponding mechanism, with the assistance of finite element analysis, is ascribed to the following: 1. The plastic strain localization induced by shallow cracks preferentially hinders plastic instability in the pre-cracked cross-section rather than promotes fracture instability at the crack tip because the high fracture toughness makes shallow cracks remain non-propagating or stable-propagating at least until plastic instability occurs; 2. Whether a shallow crack strengthens or weakens the pre-cracked cross-section depends on the integrated factors of the effective area of cross-section for bearing load narrowed by the crack depth (negative effect), the strain hardening and damage initiation in near crack-tip region affected by gradient plasticity (positive or negative effect), and uniform plasticity in remote crack-tip region alleviated by plastic strain localized near the crack tip (positive effect). Then, the above features are defined as the ‘shallow crack effect’ in the residual strength issue. To incorporate this problem in design, a residual strength diagram considering physical meanings of residual strength and corresponding damage characteristics is suggested. It helps to estimate the trend of residual strength from the perspective of plastic strain localization. Finally, influential factors on the ‘shallow crack effect’ in the residual strength issue are discussed.

Factors affecting the agreement between unloading compliance method and normalization method

Unloading compliance method (UC) and normalization method (NM) are two of the most commonly used methods for determining the fracture toughness of steels. Since the NM has been recognized as the most practical method there is a need to evaluate its accuracy against the classic UC method. This paper intends to investigate the factors that affect the agreement between UC and NM, including material mechanical properties, specimen thickness, the ratio of initial crack length to width (a0/W), specimen configuration, final crack extension, side-groove and the standards used. Both the literature survey and experiments are conducted to produce data for comparison. Based on a thorough analysis of test results, it is found in the paper that the agreement between UC and NM can be greatly affected by material mechanical properties, the ratio of initial crack length to width and the specimen configuration whilst the specimen thickness, the final crack extension and the side-groove only slightly influence the agreement. NM is recommended for specimens with shallow initial crack length, i.e., small a0/W. The results presented in the paper can assist engineers and researchers with accurate use of the normalization method for determining fracture toughness of steels.

Unified constraint-based FAD assessments for ductile fracture in cracked pipes

In this work, the unified constraint-based FAD (failure assessment diagram) assessment methodology has been investigated and developed for ductile fracture in cracked pipes. Two unified constraint parameters Ap and Ad are utilized to incorporate both in-plane and out-of-plane constraints. The results show that the FAD approaches based on two-parameter K-Ap and K-Ad can predict similar fracture initiation loads. With decreasing crack length and depth, the constraint level decreases and fracture toughness increases, thus the predicted fracture load increases. For some shallow and short pipe cracks with lower constraints, the unified constraint-based FAD assessment can reduce conservatism degree of the conventional FAD assessment. For some deep and long cracks, their constraint levels may be higher than the standard plane strain specimen, and the application of the unified constraint-based FAD assessment approaches may eliminate non-conservative assessment in the conventional FAD approach. The unified constraint-based FAD approaches and CDF (crack driving farce) approaches using parameters Ap and Ad can produce similar fracture prediction results, thus they are basically equivalent.

Geometric and Material Variability Influences Stress States Relevant to Coastal Permafrost Bluff Failure

Scientific knowledge and engineering tools for predicting coastal erosion are largely confined to temperate climate zones that are dominated by non-cohesive sediments. The pattern of erosion exhibited by the ice-bonded permafrost bluffs in Arctic Alaska, however, is not well explained by these tools. Investigation of the oceanographic, thermal, and mechanical processes that are relevant to permafrost bluff failure along Arctic coastlines is needed. We conducted physics-based numerical simulations of mechanical response that focus on the impact of geometric and material variability on permafrost bluff stress states for a coastal setting in Arctic Alaska that is prone to toppling mode block failure. Our three-dimensional geomechanical boundary-value problems output static realizations of compressive and tensile stresses. We use these results to quantify variability in the loci of potential instability. We observe that niche dimension affects the location and magnitude of the simulated maximum tensile stress more strongly than the bluff height, ice wedge polygon size, ice wedge geometry, bulk density, Young’s Modulus, and Poisson’s Ratio. Our simulations indicate that variations in niche dimension can produce radically different potential failure areas and that even relatively shallow vertical cracks can concentrate displacement within ice-bonded permafrost bluffs. These findings suggest that stability assessment approaches, for which the geometry of the failure plane is delineated a priori, may not be ideal for coastlines similar to our study area and could hamper predictions of erosion rates and nearshore sediment/biogeochemical loading.

Crack propagation and burst pressure of longitudinally cracked pipelines using extended finite element method

This paper presents an investigation of the potential of extended finite element method (XFEM) implemented in Abaqus standard software, with maximum principal strain and fracture energy as damage parameters for analysis of crack propagation and prediction of burst pressure in pipelines. Models of API 5L X60 pipe were calibrated first, using data from single edge notched tension test and then using burst pressures from 3 full scale tests. Both calibrations produced low fracture energy and marginally varied strains as XFEM damage parameters. The models were used to predict burst pressures of pipe specimens having varying sizes of longitudinal cracks that were rectangular in profile and located on the external surface of the pipes. Burst pressures were also predicted using analytical methods and the results compared with tests and XFEM predictions. All analytical methods accurately predicted burst pressures of specimens with shallow crack depth but were inaccurate for deeper cracks. XFEM predictions were satisfactory for all specimens. Although few cracks were analysed, the results show XFEM can be effective for analysis of crack propagation and prediction of burst pressure in pipelines, but investigation using more crack sizes, pipe grades and pipe geometries is recommended for a firm conclusion.

Neutron diffraction bulk texture study with impact property correlation of electron beam welded dissimilar Fe-7%Al alloy to steel joints

Present work focused on the crystallographic bulk texture study of dissimilar Fe-7percentage Al alloy to mild steel joints by electron beam welding using time-of-flight neutron diffraction technique. Textural properties directly correlated with V-notch Charpy impact study. Different processing conditions like the use of beam oscillation and by changing welding speed on texture change and its impact on the performance of the joint was evaluated. Neutron diffraction was performed using general material diffractometer beamline at ISIS neutron radiation source. Texture analysis and their variation obtained in terms of pole-figures, inverse pole figures and orientation distribution functions (ODFs). Beam oscillation shows a uniform and ordered textured grains in both fusion zone (FZ) and heat-affected zone (HAZ) due to more heat mixing that reduced unidirectional solidification gradient. ODF study reveals more gamma fibre components in both FZ and HAZ compared to its non-oscillating counterparts and higher welding speed joints, which predominantly shows more Goss, rotated Goss and cube/alpha fibre components. Charpy impact strength shows approximately 60% improvement in value for the joints, which was prepared by using beam oscillation whereas 20% reduction for higher welding speed condition. Also, X-ray tomography study of Charpy broke sample showed very lesser and shallow cracks and in smaller volume percentage of 0.45 compared to joints produced by the non-oscillating beam showing maximum value of 2.99%.

Research on Surface Crack Detection Technology Based on Digital Image Processing

Digital image technology is used to monitor the fatigue and fracture of components under the operating conditions of mechanical parts, which is beneficial to monitor the causes of shallow cracks and whether the test results affect the life and safety of parts. The detection technology based on the digital image processing mechanism belongs to the category of non-destructive testing. It can quickly detect the surface cracks of mechanical parts and achieve accurate and effective evaluation of the surface defects of the parts. Based on the application of actual digital image processing technology, this paper presents an effective evaluation system for the defect detection of component surface cracks in image processing technology for programming tools used in simulation experiments, digital image processing algorithms, and design experimental software features, calculation methods and steps. After the above argument, the digital image processing method for surface cracks of parts is proposed. We can properly process the acquired images on the spot, obtain the values of piecewise linear stretching mean, median filtering, etc., and prove that digital image processing technology has the advantages of image detail processing the advantage is that it can realize rapid judgment and accurate detection of surface crack defects.

Modeling ductile fracture using critical strain locus and softening law for a typical pressure vessel steel

This work describes the framework to model ductile damage based on phenomenological stress-modified critical strain criterion (SMCS) coupled with a softening law to predict the onset of ductile initiation and ductile propagation in typical fracture specimens, extracted from a flat plate made of ASTM A285 Gr. C steel. A very detailed and well-illustrated methodology has been developed for the identification of material parameters. Laboratory testing of cylindrical tensile bars and SE(B) specimens, at room temperature, provides necessary and sufficient information to calibrate the numerical parameters in the proposed model. For either geometry, the applied loading is measured by a continuous record of the load (P) and displacement (Δ). After the model parameters have been set, verification studies are carried out for SE(B) specimens having shallow cracks with and without side-grooves. Consequently, parameter transferability, outside small-scale yielding condition between specimens having different crack tip conditions, can be addressed and the constraint influence on the driving force better understood. An additional check is performed by comparing the final crack front profile measured on the fracture surface of the SE(B) specimens with the numerical calculated in the finite element analyses. The phenomenological model adopted herein can reproduce and predict reasonably well the experimental data obtained for specimens with different levels of stress triaxiality (constraint). Overall, it is shown that the SMCS criterion combined with a softening law can be used to study and to predict the influence of stress state on ductile failure initiation and ductile crack growth by identifying nine model parameters through testing notched round bar geometries and SE(B) specimens. The proposed methodology shows great potential as an engineering tool for assessing the integrity of complex structures such as welded pipelines and pressure vessels.