Corner Cracks Research Articles

Fatigue life prediction for boom structure of concrete pump truck

Frequent premature fatigue failure of construction machinery has spurred the demand for fatigue life prediction of such equipment. In this study, fatigue cracks were investigated in a concrete pump truck boom typically a high-strength steel plate welded box girder structure. The focus was two-tip corner cracks which frequently initiate at the weld toe between the top flange plate and the web plate because these are commonly observed in the field. A fatigue crack growth numerical approach for three-dimensional shell problems was proposed to simulate fatigue crack growth in cracked structures. Fatigue experiments were performed on a full-scale cracked boom to validate the effectiveness of this approach. The influence of the initial crack length was examined using the proposed approach. Numerical results revealed that variation in the initial crack length can affect the remaining fatigue life. For engineering convenience, an interpolation method was employed to determine the remaining fatigue life of a cracked boom with an arbitrary initial crack length. The comparisons between interpolation and numerical results demonstrate that the interpolation method can be used to guide repair decisions with reasonable precision.

Corner fatigue cracking behavior of hybrid laser AA7020 welds by synchrotron X-ray computed microtomography

To elucidate the fatigue corner cracking behaviors related with gas pores of hybrid laser arc welded 7020–T651 alloys, both high-resolution in situ synchrotron X-ray computed microtomography and finite element based predictions containing the closure effect or significant plasticity have been undertaken in the domain of fracture mechanics. It is found that fatigue cracks are preferentially initiated from a pore intersecting with the corner free surface and that it steadily propagates inside the weld with a quarter elliptical shape. Moreover, the fatigue corner crack exhibits a very complex morphology. A quantitative agreement between the experiment and prediction of crack shape under executed loading cycles is observed. By using the critical stress intensity factor along the crack line, the modified fracture toughness of a given hybrid laser weld can also be determined numerically for three-dimensional non-penetration curved fatigue cracks.

Fatigue Life Prediction Based on Crack Closure and Equivalent Initial Flaw Size.

Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. In this paper, a general methodology is proposed to predict fatigue life of smooth and circular-hole specimens, in which the crack closure model and equivalent initial flaw size (EIFS) concept are employed. Different effects of crack closure on small crack growth region and long crack growth region are considered in the proposed method. The EIFS is determined by the fatigue limit and fatigue threshold stress intensity factor △Kth. Fatigue limit is directly obtained from experimental data, and △Kth is calculated by using a back-extrapolation method. Experimental data for smooth and circular-hole specimens in three different alloys (Al2024-T3, Al7075-T6 and Ti-6Al-4V) under multiple stress ratios are used to validate the method. In the validation section, Semi-circular surface crack and quarter-circular corner crack are assumed to be the initial crack shapes for the smooth and circular-hole specimens, respectively. A good agreement is observed between model predictions and experimental data. The detailed analysis and discussion are performed on the proposed model. Some conclusions and future work are given.

Eddy Current Probability of Detection for Mid-Bore and Corner Cracks in Bolt Holes of Service Material

Fatigue cracks are prone to develop around fasteners found in multilayer aluminum structures on aging aircraft. Probability of detection (POD) studies using eddy current techniques within the bolt holes contribute to risk assessments used in evaluating the serviceability of these aircraft. Signal response to corner and mid-bore cracks by eddy current testing using standard split-D differential probes has been examined for in-service material from the horizontal stabilizer of a Hercules aircraft. Results are compared to data obtained from laboratory samples with drilled holes. Based on a logistic regression analysis, the results show no significant difference for a90/95 (90% of the cracks of size a will be detected 95% of the time) for mid-bore crack length of 0.60 mm in this study compared to 0.62 mm in earlier work on drilled holes. However, there was a substantial loss of sensitivity for corner cracks. In this study on in-service material, the a90/95 for corner cracks was 0.82 mm compared to 0.31 mm obtained from drilled holes. Decrease in sensitivity was attributed to damage at the hole’s corners of the in-service material, including rounding, folded metal, and burrs. Hole damage also significantly increased the risk of a false call.

Extremely low cycle fatigue assessment of corner crack in concrete-filled steel piers based on effective notch strain approach

Extremely low cycle fatigue assessment of corner crack in concrete-filled steel piers based on effective notch strain approach

Fatigue life prediction of cracked attachment lugs using XFEM

In the present paper, a three dimensional finite element method (FEM) is used to compute the stress intensity factor (SIF) in straight lugs of Aluminum 7075-T6. Extended finite element method (XFEM) capability available in ABAQUS is used to calculate the stress intensity factor. Crack growth and fatigue life of single through-thickness and single quarter elliptical corner cracks in attachment lug are estimated and then compared with the available experimental data for two different load ratios equal to 0.1 and 0.5. The SIF calculated from XFEM shows that the introduction of different loading boundary conditions significantly affect the estimated fatigue life.

Measurement and Analysis of Concrete Curling Slab Using Imbedded Sensors

A series of accelerated full-scale tests on rigid pavement were conducted to evaluate the concrete slab curling effects on the pavement design. Data from deflection sensors and embedded strain gages were acquired and analyzed along with the observed performance data. The collected data can be used for identifying the slab corner cracking and potentially giving the slab curling amount. Traffic tests were simulated using the finite element method in three dimensions and compared with sensor readings. A comparison of maximum tensile stresses between the curling slab and flat slab was performed. It was found that the critical stresses are located near the longitudinal joint in the curling model, the same as the distress observation of concrete slab.

Prediction of shape change of corner crack by fatigue crack growth circles

As a fatigue crack grows, its shape changes. Conventional method for predicting the shape change typically requires the computation of several hundred increments to get accurate results. In this paper, a new finite element simulation technique that uses fatigue crack growth circles is developed. Since the circles are perpendicular to the new crack front as well as to the current crack front, they can represent the real path of the fatigue crack well and yield more accurate results. The new technique is validated by applying it to the case of a quarter-elliptical corner crack in a plate with an open hole subjected to tension loading. The effect of the Paris–Erdogan exponent is also investigated.

Technology Development for Controlling Slab Transverse Corner Crack of Typical Micro-alloyed Steels

A mathematical model for simulating the fluid flow, heat transfer and solidification in the conventional mold and the chamfer mold, together with a finite element stress-strain model in the straightening process of both molds, were established for the typical niobium, vanadium, and titanium micro-alloyed steels. On the basis of both numerical analysis, the mold copper plate with an optimum chamfered shape was designed and applied in industrial tests. The predicted results from numerical simulation of fluid flow, heat transfer and solidification in the conventional mold and the chamfer mold show that the increased chamfered angle leads to an approximately linear increase of the slab surface temperature, but it also causes strong flow near the slab corner. Very small chamfered length can lead to a significant increase of the temperature near the slab corner. However, with further increasing the chamfered length, the temperature of the slab corner increased slightly. The calculated results from the finite element analysis of stress-strain during the straightening process show that at the same slope width, the tangential strain on the slab edges and corners is minimum when the chamfered angle is 30° and 45°, which is only 40% to 46% of rectangular slabs with the same cross-section area. At the same chamfered angle of 30°, when the chamfered length is controlled between 65–85 mm, the tangential strain on the part of the slab edges and corners is relatively smaller. Industrial test results show that the slab corner temperature at straightening segment increases about 100 °C by using chamfer mold compared to the conventional molds. The slab transverse corner cracks have been reduced more than 95% in comparison with those in the conventional mold.

Research of edge defect formation in plate rolling by finite element method

For hot-rolled flat products major losses of steel are due to surface defects, which are caused by the quality of an incoming continuous cast slab. Each typical surface defect of a continuous cast slab if not detected and eliminated transforms into a surface defect of a rolled plate. One of the most critical problems of hot rolled plates is connected with the longitudinal surface cracks located at the distance up to 80mm from the edges. This study covers the research and simulation of transformation of surface cracks into the edge defects of the hot-rolled plate by finite element method. It is shown that the mechanism of crack movement during plate rolling comes from inclination of front and tail faces of slab due to temperature gradient between top and bottom surfaces. To reduce the possibility of corner crack formation it is proposed to increase the temperature of the edge zone of the slab due to the rounding of edges or by induction heating. The results of the study will be useful for material saving during plate rolling.

Numerical Evaluation and Experimental Comparison of Elasto-plastic Stress-strain Distribution Around the Corner Cracks of a Notched Specimen

This paper concerns the numerical simulations, by a commercial FEM code, related to notched specimens experimentally tested by neutron diffraction and synchrotron radiation diffraction, in order to correlate the deformation states around the crack tip. Two cracked specimens were analyzed. In one case a long crack has been induced into the specimen, which has been investigated subsequently by neutron diffraction technique. In the other case a smaller crack was induced, which has been investigated by synchrotron radiation diffraction technique. The numerical and experimental results are substantially in good agreement. However some differences can be observed; the reason can be found in the microscopic structure and particular history of the material under load.

Characterization of Load Transfer Behavior for Bonded Concrete Overlays on Asphalt

A bonded concrete overlay on asphalt (BCOA) is a rehabilitation method for moderately distressed asphalt pavements by relatively thin plain cement concrete or fiber-reinforced concrete slabs. The joint load transfer behavior for BCOAs plays a significant role in the long-term performance. Poor load transfer across the joints of the concrete slabs initiates debonding of the asphalt layer from the concrete slabs, which promotes the development of corner cracks or longitudinal cracks. However, because of the tediousness involved in characterizing the joint load transfer behavior of BCOAs, this important aspect is not accounted for in many available mechanistic–empirical BCOA design procedures. The influences of joint load transfer behavior on the performance of BCOA are discussed. The joint load transfer behavior for BCOAs with 1.52- × 1.83-m (5- × 6-ft) slabs and 1.22- × 1.22-m (4- × 4-ft) slabs is analyzed with the finite element method. The load transfer contributed by the asphalt layer, as well as the concrete slab, is characterized as a function of the BCOA design features. Finally, a method is proposed to determine the nondimensional joint stiffness (AGG*) for BCOAs as a function of the structural design features of the pavement section. The AGG* is significant in that it is the factor commonly used to characterize joint load transfer behavior when pavements are designed with a mechanistic-based design approach.

Application of a chamfered mold to improve corner defects of HSLA during slab continuous casting

Through the study of the slab continuous casting production process and laboratory simulation, the mechanism of transverse corner cracks on a micro-alloyed high-strength steel slab was explored. Then, a chamfered mold was developed and tested on the caster. The results show that on one hand, the chamfered mold could raise the temperature of the corner in the mold, and on the other hand, the higher temperature of the corner of the slab was an effective way to decrease the corner stress at the straightening point. The straightening deformation rate on the chamfered slab was lower than that on the square slab. Using the large chamfer mold technology, the optimization design of the foot roll structure and second cooling, the transverse corner crack was improved effectively. The hot transporting ratio was increased without the cutting angle.

The effects of thermal stresses on the elliptical surface cracks in PWR reactor pressure vessel

In this study, the effects of thermal stresses on the stress intensity factors (SIFs) of the elliptical corner surface cracks postulated at the nozzle-cylinder intersection of a reactor pressure vessel (RPV) were investigated. A typical RPV of a Westinghouse pressurized water reactor and its set-in nozzle were considered for the analysis. The selected set-in nozzle-cylinder intersection for fracture mechanics analysis is also the highest stress concentration point of the RPV. The numerically computed SIFs for a wide range of corner cracks under pressure and combined (pressure plus thermal) loadings are provided as a reference tool for the fracture mechanics design of the RPV. It was also demonstrated that the operational thermal stresses caused by the provision of the annular chamber for the external reactor vessel cooling, actually reduce the SIF of the corner cracks and they do not endanger the safety of the RPV in normal operating conditions.

Study on Cement Concrete Pavement Treatment Methods of Common Damage Diseases

This paper analyzed for common diseases treatment methods for cement pavement in Jilin province seasonal freeze-thaw area, based on the analyses, disease prevention and repair methods have been put forward for water deposit, fracture, dislocation, potholes, arch, plate corner crack and uneven settlement, the research results have certain reference meanings for cement pavement maintenance.

Stress intensity factors for part-elliptical cracks emanating from dimpled rivet holes

Detailed investigations on the stress intensity factors (SIFs) for corner cracks emanated from interference fitted dimpled rivet holes are conducted using three-dimensional finite element method. The influences of the crack length a, elliptical shape factor t, far-end stress S and interference magnitude δ on the stress intensity factors are systematically studied. The SIFs for corner cracks emanated from open holes are also investigated for comparisons. An empirical formula of the normalized SIF is proposed by use of the least square method for convenience of the engineering application, which is a function of the crack length a, elliptical shape factor t, far-end stress S, interference magnitude δ and the normalized elliptical centrifugal angle φn. Based on the empirical formula, a crack growth simulation for a rivet filled hole is conducted, which shows a good agreement with the test data.

Improved stress intensity factors for a single corner crack at a loaded fastener hole

Improved Mode I stress intensity factors for a large plate containing a single corner crack at a filled fastener hole are determined. Loads are applied separately to the fastener and remotely to the plate, with out-of-plane bending unrestrained or restrained. Contact between the steel fastener and aluminium plate is modelled thus the analysis accounts for the hole propping effect. The results are determined using three-dimensional p-version finite element methods, and presented as compact equations at key crack front locations. The results show some significant differences to those in existing literature, which are due to the simplifying assumptions used previously.

Three-Dimensional SGBEM-FEM Alternating Method for Analyzing Fatigue-Crack Growth in and the Life of Attachment Lugs

AbstractIn the present paper, stress intensity factor (SIF) analyses and fatigue-crack-growth simulations of corner cracks emanating from loaded pinholes of attachment lugs in structural assemblies are carried out for different load cases. A three-dimensional (3D) symmetric Galerkin boundary-element method (SGBEM) and FEM alternating method is developed to analyze the nonplanar growth of these surface cracks under general fatigue. The 3D SGBEM-FEM alternating method involves two very simple and coarse meshes that are independent of each other: (1) a very coarse FEM mesh to analyze the uncracked lug, and (2) a very coarse SGBEM mesh to model only the growing crack surface. By using the SGBEM-FEM alternating method, the nonplanar growth of cracks in 3D (surfaces of discontinuity) up to the failure of structures are efficiently simulated, and accurate estimations of fatigue lives are made. The accuracy and reliability of the SGBEM-FEM alternating method are verified by comparing them to other FEM solutions, ...

Analysis of fatigue delamination growth in flip-chip package

An incremental crack extension procedure is implemented for simulating the growth of an interface defect in an electronic flip-chip device subjected to fatigue temperature cycling. The distributions of fracture mechanics parameters including the strain energy release rate, the stress intensity factors and phase angles along the curvilinear front of an embedded corner defect on the interface of silicon die and underfill are estimated and substituted into a subcritical crack growth model to predict the evolution of the defect under cyclic loading condition. It is observed from the analysis that the corner defect is under crack faces contact condition during temperature cycling, and consequently, the delamination growth is under mode-II and mode-III driving forces. In addition, the crack growth rate is highest in the middle of the crack front, and the corner crack evolves from an initially concave front to a quarter-circular front under temperature cycling.

Field performance of concrete pavements with short slabs and design procedure calibrated for Chilean conditions

Three test sections of concrete pavements with short slabs were constructed between 2004 and 2006.The slab sizes ranged from 0.88 to 3.5 m. The thicknesses varied between 8 and 20 cm. The general performance of thinner slabs (8–10 cm) was poor. The thicker slabs (14 cm and above) showed promising results. The main distress observed in most cases was longitudinal cracking, followed by transverse and corner cracking. The test sections were used for calibrating a new mechanistic empirical pavement design method. The three cracking distresses observed in the field were included in the design method. Design examples are presented. The calculated slab thicknesses are compared with the thicknesses obtained for conventional rigid pavements. Important reductions in thicknesses are achieved as a result of using the shorter slabs.