Cracked Specimens Research Articles

A machine learning-based approach to predict the fatigue life of three-dimensional cracked specimens

Evaluation of damage tolerance is significant in aerospace structures, while predicting the fatigue life accurately and promptly is greatly in need for complex practiced structures. Although traditional experiment and analysis based on fracture theory have been well established and have accumulated considerable data, time consumption and diseconomy are still encountered. To address these challenges, a new data-driven approach based on machine learning is presented. The back propagation neural network is trained and adopted to predict the basic curve da/dN-ΔKeff, i.e., fatigue crack growth rate versus the crack closure-based effective stress intensity factor range, and the fatigue life of surface cracked specimens under constant amplitude cycle loading. The influence of different number of neural units and layers on effectiveness of the trained neural network are investigated, Moreover, the neural network model is compared with other regression models. The test result shows that the trained neural network model is superior to other models and can obtain material data and fatigue life accurately and rapidly. Finally, through the active learning algorithm of query by committee algorithm, we select a small number of samples for training to obtain a model that meets the accuracy. This model can directly bridge the gap between the preliminary detailed stress analysis and the final fatigue life, and greatly reduce the cost of obtaining data sets. This provides a new way to solve engineering problems.

Should the translaminar fracture toughness of laminated composites be represented by the [formula omitted] or the [formula omitted] curve? A comparison of their consistency and predictive capability

The translaminar fracture toughness of laminated composites can be determined experimentally assuming either linear elastic fracture mechanics (LEFM) or cohesive zone model (CZM) hypotheses. Each theoretical frame is different in terms of complexity and predictive capabilities. To clarify the latter point, we derived the R(Δa) (LEFM) and J(ω) (CZM) curves from a set of published results on over-height compact tension fracture specimens of different sizes. Then, these curves were used to predict the strength of other coupons of the same material (geometrically similar, scaled, open hole and center cracked specimens). Since the idealization of the fracture phenomena in cohesive zone models is more realistic than in LEFM, the fracture property so obtained is more independent from the size of the specimen used to measure it and its predictive capability embraces a wider range of sizes and geometries.

Fractal damage and crack propagation of PMMA in multiple slit charge blasting

To study the formation mechanism and propagation characteristics of explosive cracks of slit charges with different slit numbers, the propagation law was analyzed using PMMA as the test material and a digital laser dynamic caustics explosion loading test system. The ABAQUS explicit dynamic analysis module is used to simulate the generation process of the initial crack in slotted charge blasting, and the propagation law of stress waves under the action of slotted charges with different numbers of slots is revealed. Based on fractal theory, the damage values in the slit and non-slit directions are calculated quantitatively. The results show that when the number of slits of the slit charge is two, three, and four, the directional crack along the slit direction of the charge can be controlled. The Von Mises stress along the slit direction near the blast hole wall is 9.5–15.6 times that in the non-slit direction, and it gradually tends to equalize with the increase in the distance from the center of the blast hole. In the process of crack propagation, the propagation speed and dynamic stress intensity factor of the exploded main crack of the specimen exhibit an oscillatory downward trend. The propagation law is the same for all the main cracks in the same specimen. Increasing the number of slits can increase the overall energy utilization of the specimen. When the number of slits is two, three, and four, the damage value (0.6130–0.638) of the burst crack in the cutting direction of the specimen is significantly greater than that in the non-cutting direction (0.4540–0.4885).

On the Problem of Getting a Correct Crack Shape in Fracture Toughness Specimens of Low-Carbon Steel

Fracture toughness test in full thickness is an essential part of structural steel certification and qualification of welding procedures for critical structures intended for low-temperature service. The material to be tested should be in a stress relieved state that closely relates to a straight shape of the fatigue precrack front. A lot of technological treatment procedures for specimens preparation have been experimentally tried and quantitatively calculated. This paper describes a part of these efforts in respect of applicability of increased cyclic asymmetry during precracking. This method appears useful to get a straight fatigue crack in base metal specimens as well.

Thermal destruction of polymeric fibers in the theory of temporary dependence of strength

Objectives. This study mathematically describes the mutual influence of micro- and macrostages of the process of destruction of polymer materials and determines its main parameters and limiting characteristics. In addition, a relationship is established between molecular constants characterizing the structure of a material and those characterizing its macroscopic characteristics of strength. Finally, theoretical representations of the thermokinetics of the process of thermal destruction of polymer fibers from the standpoint of the kinetic thermofluctuation concept are developed, which makes it possible to predict the thermal durability of a sample under thermal loading.Methods. The structural–kinetic thermofluctuation theory was used to describe the initial stages of the fracture process and to derive a generalized formula for the rate of crack growth. The mathematical theory of cracks is used to describe the thermally stressed state of a material in the vicinity of an internal circular crack under mechanical and thermal loadings of the sample.Results. A theoretical formula for the full isotherm of durability in the range of mechanical stresses from safe to critical, as well as a theoretical relationship for the time dependence of the strength of polymer fibers under purely thermal loading in the full range of heat loads from safe to critical and at the stage of nonthermal crack growth, is given. The main parameters and limiting characteristics of durability under thermal loading are also indicated.Conclusions. A generalized structural–kinetic theory of the fracture of polymer fibers under purely thermal action on cracked specimens is presented. The developed theory combines three independent approaches: structural–kinetic (thermofluctuation theory), mechanical, and thermodynamic. The obtained theoretical relations are of practical interest for the development of methods for localization, intensification, and control of the crack growth kinetics.

Effects of Fixture Configurations and Weld Strength Mismatch on J-Integral Calculation Procedure for SE(B) Specimens.

This work presents the development of a J-integral estimation procedure for deep and shallow cracked bend specimens based upon plastic ηpl factors for a butt weld made in an S690 QL high strength low alloyed steel. Experimental procedures include the characterization of average material properties by tensile testing and evaluation of base and weld metal resistance to stable tearing by fracture testing of square SE(B) specimens containing a weld centerline notch. J-integral has been estimated from plastic work using a single specimen approach and the normalization data reduction technique. A comprehensive parametric finite element study has been conducted to calibrate plastic factor ηpl and geometry factor λ for various fixture and weld configurations, while a corresponding plastic factor γpl was computed on the basis of the former two. The modified ηpl and γpl factors were then incorporated in the J computation procedure given by the ASTM E1820 standard, for evaluation of the plastic component of J and its corresponding correction due to crack growth, respectively. Two kinds of J-R curves were computed on the basis of modified and standard ηpl and γpl factors, where the latter are given by ASTM E1820. A comparison of produced J-R curves for the base material revealed that variations in specimen fixtures can lead to ≈10% overestimation of computed fracture toughness JIc. Furthermore, a comparison of J-R curves for overmatched single-material idealized welds revealed that the application of standard ηpl and γpl factors can lead to the overestimation of computed fracture toughness JIc by more than 10%. Similar observations are made for undermatched single material idealized welds, where fracture toughness JIc is overestimated by ≈5%.

Computational and experimental identification of coefficients of the Williams series expansion by considering higher order terms in the cracked specimens through digital image analysis

In the study, digital photoelasticity method is used for finding of the linear elastic fracture mechanics parameters for a plate of finite width with two inclined interacting cracks. The digital image processing tool for the whole set of experimental data namely isochromatic fringes and isoclinic phase maps obtained from the photoelasticity experiments is developed and exploited. The digital image processing tool is based on the well-known Ramesh technique but allows us to scan the image in any direction and to analyse the image after any number of logical operations. The performed digital image analysis allows us to extract photoelastic data from the whole field isochromatic and isoclinic fringe patterns. In the digital image processing fringe analysis, the optical data contained in the transmission photoelastic isochromatic fringe patterns near the crack tips were converted into text file and then the points of isochromatic fringes with minimum light intensity were used for evaluating fracture mechanics parameters. The multi-point stress field description is used. The mixed mode fracture parameters, especially stress intensity factors, T-stresses and higher-order coefficients of the Williams series expansion are obtained for specimen configurations like plates with two inclined interacting cracks using 1) the algorithm based on the classical over-deterministic method and 2) the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm for minimization of the objective function. It is shown that the latter method has undeniable advantages over the classical over-deterministic method. The effects of higher-order terms in the Williams expansion were analysed for different the considered cracked specimen. It is shown that the higher order terms are needed for accurate characterization of the stress field in the vicinity of the crack tip. The experimental SIF values, T-stresses and higher-order coefficients estimated using the proposed method are compared with finite element analysis (FEA) results, and are found to be in good agreement.

Experimental study on the interaction between two cracks by digital photoelasticity method: construction of the Williams series expansion

The multi-point Williams series expansions for the near-crack-tip fields in the vicinity of two interacting cracks in an isotropic linear elastic material are experimentally and numerically determined. The experimental method used to study the interaction of two cracks is the digital photoelasticity method which is applied for determination of coefficients of the higher-order terms of the Williams series expansion for a plate of finite width with two interacting cracks. The digital image processing tool for handling of the entire set of experimental data isochromatic fringes and isoclinic phase maps obtained from the photoelasticity experiments is developed and exploited. The digital image processing tool is relied on the classical Ramesh technique but enables to scan the experimental image in any direction and to analyse the image after any number of logical operations. The realized digital image analysis allows us to extract photoelastic data from the entire field isochromatic and isoclinic fringe patterns. In the digital image processing fringe analysis, the optical data contained in the transmission photoelastic isochromatic fringe patterns in the vicinity of the crack tips are converted into text file and then the points of isochromatic fringes with minimum light intensity are used for estimating linear elastic fracture mechanics parameters. The multi-parameter stress field description with the first fifteen terms is used. The mixed mode fracture parameters, especially stress intensity factors, T-stresses and higher-order coefficients of the Williams series expansion are obtained for specimen configurations like plates with two interacting cracks using 1) the classical over-deterministic method in the form based on the stress field and 2) the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm for minimization of the objective function. It is demonstrated that the BFGS method has indisputable advantages over the classical over-deterministic method. The significance of higher-order terms in the Williams expansion is proved for the considered cracked specimen. It is shown that the higher order terms are needed for accurate characterization of the stress field in the vicinity of the crack tip. The experimental SIF values, T-stresses and higher-order coefficients estimated using the proposed method are compared with finite element analysis (FEA) results, and are found to be in good agreement. This verifies the experimental analysis

Importance of the Higher Order Terms of the Williams series expansion: Experimental Aspects and Finite Element Simulations

The study is devoted to the multi-point Williams series expansion for the crack-tip fields in isotropic linear elastic materials and methods of estimating the coefficients of higher order terms in the power expansion. It is shown that higher order terms are very important in processing of the experimental data obtained from interference – optic methods. It is demonstrated that the accurate interpretation of the experimental data requires keeping, considering and evaluating the higher order terms of the Williams expansion. A large cohort of experiments on cracked specimens from optically sensitive materials using digital photoelasticity method was performed. The interference fringe patterns obtained show that the correct description of the stress fields in the vicinity can be given if we take into account higher order terms in the asymptotic presentation of the near crack tip fields. Two interference-optical methods were used. Digital photoelasticity method and digital holography interferometry technique are applied for determination of the coefficients of the Williams series expansion. It is shown that we can restore theoretically the stress field in the vicinity of the crack tip if at least fifteen terms in the power expansion are hold. The classical over-deterministic method is used for the evaluation of the coefficients.

Analysis of Crack Expansion and Morphology of Cross-Laminated Timber Planar Shear Test

To describe the dynamic cracking process of the CLT vertical layer, the correlation between a load-displacement curve, specimen cracking, and planar shear failure mechanism of the CLT were explored. A three-point bending test and an improved planar shear test are used to evaluate the shear performance of the CLT. In this study, the load-displacement curve is recorded, the experimental part is synchronized with the video, the dynamic process of cracking of the vertical layer is observed and analyzed throughout the test. From the load-displacement curve, the image characteristics of the initial cracking and the sudden increase of the cracking of the specimen are summarized. The description results of the whole dynamic process of the CLT vertical layer cracking are analyzed by planar shear strength value, cracking phenomenon, and azimuth angle of cracking surface. The main conclusions show that the three-point bending test and the improved plain shear test can be used to test the plain shear strength of the CLT, with a difference of only 5.7%. The original crack and the new crack expansion account for 18.9% and 81.1% of the main cracking surface, respectively. And the vertical layer of the CLT specimen under three-point bending has three cracking morphologies, such as radial shake, ring shake, neither along with the radial shake nor along with the ring shake. The azimuth angle of the cracking surface of the CLT vertical layer under planar shear is quite consistent with the first main plane azimuth of the vertical layer of the CLT specimens under the three-point bending test and the shearing test. The shape in the cracking direction of the left half-span or the right half-span of the vertical layer of the specimen is similar to the Chinese character eight.

Mixed mode surface crack growth in aluminium alloys under complex stress state

In this study fatigue crack growth tests were carried out on hollow cylindrical specimens, made of D16T and B95AT aluminum alloys (analogue of 2024 and 7075 aluminum), with initial external semi-elliptical surface cracks and undergoing complex stress state. The crack growth behavior of aforementioned materials was studied under cyclic axial tension, pure torsion and combined tension+torsion fatigue loading. Optical microscope measurements and the crack mouth opening displacement (CMOD) method were respectively used to monitor crack length and calculate crack depth. The experimental crack front positions were highlighted by using a beach mark procedure during the tests. The stress strain field along the crack front of semi-elliptical cracks in the cylindrical hollow specimens was assessed by Finite Element Method (FEM) analysis. The stress intensity factors (SIFs) were calculated by the 3D M-integral approach and the distributions of equivalent elastic SIFs along the crack front were used for crack growth rate assessment under mixed mode conditions. As a result, the fracture resistance parameters of aluminum alloys under complex stress state were presented. The simulation of cracks propagation turned out to be consistent with experimental results.

Influence of Acicular Ferrite Microstructure on Toughness of Ti-Rare Earth Metal (REM)-Zr Killed Steel

Novel oxide particles composed of Ti, rare-earth metal and Zr was found by the present authors to promote the formation of acicular ferrite (AF), but their effect on toughness has not yet been clarified. Therefore, influence of AF formation on toughness of low carbon steel has been investigated using Ti-Rare earth metal-Zr-killed (TRZ) steel and Al-killed (AL) steels in this study. Dominant microstructure of the AL steel is bainite forming block/packet structure elongated from prior austenite grain boundaries. In contrast, a large amount of AF which grows from intragranular inclusions is formed in the TRZ steel. Toughness of the TRZ steel is better than the AL steel. In addition, at higher austenitizing temperature, AF formation is promoted in the TRZ steel, leading to lowering ductile-brittle transition temperature. Cracks in Charpy impact specimens propagate along {001}α plane and deflect at Bain unit boundary across which {001}α planes are largely misoriented, which means the effective grain size for crack propagation is Bain unit size. Bain unit size is smaller in the TRZ steel than in the AL steel since bainite structure is subdivided by pre-formed AFs belonging to different Bain groups from bainite. Martensite-austenite constituent (MA) in AF is less-elongated in comparison to those in bainite and thus the amount of elongated MA in the TRZ steel is less than in the AL steel. These results clarify that AF formation in the TRZ steel improves toughness due to the refinement of Bain unit size and reduction of the amount of elongated MA.

Transverse Crack Detection of Rail Base Considering Wedge-Like Structure and Using a Bulk-Wave Electromagnetic Acoustic Transducer

In this article, the contributions are to further study and verify a bulk-wave electromagnetic acoustic transducer (EMAT) for the transverse crack detection of rail base and to propose an auxiliary method for achieving efficient detection. There is no verification for rail treated as a plate in the previous publication. For the verification, the wedge-like rail base is reasonably simplified into the front view and the side view to replace the 3-D finite-element model (FEM) simulation. In addition, the experiment to detect an artificial transverse crack of the standard rail specimen is carried out to verify the treatment of the wedge-like rail base. Both simulation and experimental results demonstrate that the wedge-like rail base can be inspected as a steel plate using a spiral-coil EMAT, but the only time of flight is hard to detect the transverse crack of the wedge-like rail base due to low SNR. For solving the problem of low SNR, this article proposes an auxiliary method of watching for the disturbances of the back-wall echo. Also, the limit condition of the auxiliary method is analyzed for practical applications.

An Experimental investigation of Sustainable Concrete by incorporating E-waste and Fly Ash

Nowadays, recycling of electronic waste (E-waste) is a tremendous issue due to rapid growth of IT sector. This E-waste can cause several hazards to human health. In this investigation, fly ash was substituted for cement and E-waste fibres were added to concrete, which was tested for its mechanical properties. E-waste fibres have been incorporated to increase the mechanical strength and to reduce the cracks in the concrete specimens. Concrete samples of M30 grade had made by replacing 5, 10, and 15 percentages of cement with fly ash. Various experimental tests have been conducted with different proportion of fly ash and E-waste fibre. There was a percent increase in mechanical strength after E-waste replacement. The flexural strength of the concrete beams improved by some amount with the largest increase in flexural strength obtained for the FA10%. The results of the study demonstrated that the employment of EW and FA in the manufacture of concrete as a partial replacement for cement can result in the development of more durable concrete with high strength.

On the statistical nature of fatigue crack-growth through Monte Carlo simulations and experimental data

Understanding factors that contribute to scatter in crack-growth rates data is fundamental to reduce risk of unexpected structural failures. This document discusses the statistical nature of material scattering observed in fatigue crack propagation tests through a comparison between Monte Carlo simulations and experimental test data. The case study consists of constant amplitude tests on central cracked 2024-T3 aluminium alloy plate specimens loaded in pure Mode-I with stress ratio R equals to 0.2. It was found that results of simulations matched the experimental fatigue lives in terms of both mean value and dispersion around it. Based on these outcomes, the crack-growth behaviour can be described in a statistical manner with the expectation to reduce the inaccuracy present in life predictions.

Durability of concrete with Belite-Gehlenite clinker as fine aggregate

The Japanese cement industry uses large quantities of industrial waste and by-products as raw materials in the production of cement clinker. Although the amount of industrial waste generated annually has remained almost constant, domestic demand for cement has been falling. In order to maintain the amount of waste re-used by the cement industry, there is a need to explore new ways of utilizing clinker besides in cement production. The proportion of waste used in the production of Belite-Gehlenite clinker featured in this study is about twice as much as in normal clinker. Previous studies have shown that when clinker is used as aggregate in mortar and concrete, clinker hydration products fill cracks as they occur for added self-healing performance. In this study, in addition to the basic characteristics of concrete containing Belite-Gehlenite clinker as fine aggregate, the resistance to cracking of specimens made with the concrete is investigated. Compared with concrete using natural sand, it is confirmed that compressive strength is improved, drying shrinkage is reduced, carbonation is suppressed, and freeze-thaw resistance is maintained. It is also demonstrated that resistance to cracking is improved.

On the cleavage stress promoting crack path deflection and anisotropic fracture in cold drawn pearlitic steel

Crack path deflection is a phenomenon appearing in cold-drawn pearlitic steel wires as a consequence of the pearlite microstructure orientation induced by the manufacturing procedure, the deflection angle being approximately 90º, thereby representing a crack propagation step (fracture wall), a signal of splitting, delamination or debonding in the form of axial cracking (or micro-cracking) by means of a microscopic fracture mode in the form of enlarged and oriented cleavage in the direction of the wire axis (cold drawing direction). The described phenomenon is a signal of anisotropic fracture behaviour (strength anisotropy) associated with mixed-mode propagation. This paper demonstrates that, in addition to the oriented microstructure of the cold drawn pearlitic steels (at the two microstructural levels of pearlite colonies and lamellae), a sufficient level of stress triaxiality (constraint) is required, as in the case of cracked specimens in which a cleavage stress appears as a consequence of the singular stress distribution in the close vicinity of the crack tip (crack tip stress-strain field).

Experimental and Numerical Study of Strength and Failure Behavior of Precracked Marble under True Triaxial Compression

Cracks play an important role in evaluating the strength and failure behavior of engineering rock mass. In order to increase the understanding of strength and failure mechanism of precracked rock, crack propagation and coalescence from preexisting cracks under true triaxial compression are investigated using true triaxial compression tests and Cellular Automata Software for engineering Rockmass fracturing process (CASRock). Three types of specimens were studied experimentally and numerically. Experimental and numerical results show that both the preferential angle and areal intensity of preexisting cracks can affect the compressive strength and failure behavior of the specimens. The peak strength firstly decreases and then increases with increase of the preferential angle. Also, the peak strength nonlinearly decreases with the increase of cracks’ areal intensity. The numerical results show that the crack initiation and coalescence are observed and characterized from the inner and outer tips of preexisting cracks in specimens containing single crack and multiple parallel cracks. The main shear failure in the specimen containing multiple unparallel preexisting cracks initiate and propagate from one of the macroscopic preexisting cracks, and other preexisting cracks do not initiate, propagate, and coalesce until reaching the peak strength.

Mechanical and visco elastic analysis of Sal tree gum incorporated epoxy bio composite

The impact of Sal tree (Shorea Robusta) gum powder on the mechanical, visco elastic, and thermal characteristics of epoxy resin was studied. Using the hand layup technique, composite specimens were created by adding bio filler (Sal tree gum powder) at various volume percentages in epoxy resin. Experiments were carried out to assess the epoxy composite's mechanical, thermogravimetric, and viscoelastic characteristics. Results showed that the bio filler had a substantial impact on the mechanical, thermal, and dynamic mechanical characteristics of the samples. Better mechanical and dynamic mechanical properties were obtained at 16% v/v of Sal Tree gum powder. The inclusion of the bio filler, on the other hand, had no appreciable impact on the flexural and tensile strengths of the composite. The morphological features of a tensile cracked specimen were investigated using scanning electron microscopy (SEM) images of the specimen. The homogeneity of the bio composite was also examined by using the Cole Cole plot. According to the findings, the composite specimens containing the bio filler showed uniform features. It was observed that the composite specimens incorporated with the bio filler exhibited homogeneous characteristics. The thermal stability of the composite was investigated using thermogravimetric (TGA) test, and it was found that the inclusion of filler enhanced the thermal stability of the composite, with the composite remaining stable up to 431 °C after the addition of filler. The results of dynamic mechanical analysis (DMA) tests showed that the inclusion of bio filler resulted in an increase in storage modulus, suggesting that the composite is capable of greater energy absorption within the elastic limit. The glass transition temperature was calculated to be 110 °C.

Nonlinear Resonant Ultrasonic Spectroscopy (NRUS) for Monitoring Fatigue Crack Growth in Aluminum

ABSTRACT Nonlinear resonant ultrasonic spectroscopy (NRUS) is a resonance-based acoustic testing technique that yields the hysteretic elastic nonlinearity parameter by measuring the resonance frequency shift with increasing driving amplitude. NRUS offers great potential for nondestructive evaluation since it is relatively simple to implement and can detect incipient damage thanks to the high sensitivity of hysteretic nonlinearity parameter to micro-damage. Previous research has shown that NRUS can monitor distributed damage in a wide variety of materials, but the application of NRUS for detection of local defects in metals is less explored. In this study, the feasibility of using NRUS to assess local progressive damage in aluminum is investigated. We use three-point bending fatigue test to initiate a single fatigue crack in a large aluminum specimen. The cyclic loading is interrupted at several stages in order to image the crack using Scanning Electron Microscopy (SEM) and to perform NRUS and other tests. As the crack grows in length, NRUS records a gradual increase in the resonance frequency shift. However, the trend for the amplitude dependency of resonance frequency shift is different from what has been previously reported in materials with distributed damage; the resonance frequency changes are larger at low strain than at high strains. In addition, the utility of multi-modal NRUS for locating the fatigue crack is demonstrated. Finally, we compare the results of NRUS and those from an impact-based NRUS (INRUS) that uses an automated impact hammer as the excitation source in differentiating the fatigue-damaged from the intact specimen. Our findings suggest the potential of NRUS and INRUS in detecting local damage in metals.