Notch Crack Research Articles

Hydrogen enhanced fatigue in full scale metallic vessel tests – Results from the MATHRYCE project

This study investigates the fatigue life of CrMo pressure vessels for hydrogen storage by hydraulic and hydrogen pressure cycle tests. Two different sized cylinders have been tested; 35 L inner volume and 28 MPa working pressure (WP) and 198 L volume and 41 MPa WP. On the inner surface of the cylinders U-shaped notches of different depths were machined by electro discharge machining technique. The initial notch sizes were designed based on a two stage fatigue predictive model based on fracture mechanics to develop through wall cracks in the deepest notches after about 50,000 hydraulic cycles together with crack propagation of the intermediate notches and crack initiation in the smallest. The cylinders were cycled between the nominal pressure of 2 MPa and the WP until leak before break (LBB). Strain gauges were placed at the external surface of the cylinders in correspondence of the internally machined notches. On the notches which developed through wall, the strain showed a progressive decrease followed by an increase of the hoop strain during the final stage of crack propagation until LBB failure. Hydrogen effect was clearly identified by the reduction in the number of cycles to failure comparing tests in hydrogen and in oil. Subsequent failure analysis at the end of each test revealed a typical trans-granular fatigue crack surface morphology (with fatigue striations) for tests in oil, while quasi cleavage and intergranular fracture appearance were found for hydrogen tests.

Notch Sensitivity and Crack Initiation Position Influence on Life Evaluation of Notched Specimens under Non-proportional Multiaxial Low Cycle Fatigue

Notch Sensitivity and Crack Initiation Position Influence on Life Evaluation of Notched Specimens under Non-proportional Multiaxial Low Cycle Fatigue

Stress intensity factors of double edge cracks in large groove plates under mode I tension

The solutions of stress intensity factors (SIFs) of double edge cracks in large groove plates are investigated. The plate is assumed to fulfil the plain strain condition and it is axially stressed. According to the literature, the SIFs of the cracks in circular notches are available. However, the roles of cracks in large groove are difficult to obtain. Therefore, different grooves and crack geometries are modelled numerically using the ANSYS finite element software. The present model is first validated using an existing model of double edge cracks without notches and it is found that both models are in close agreement. The large grooves are then introduced and the cracks are positioned in the mid-point. It is found numerically that the SIFs are strongly affected by the relative crack depth and the groove geometries. It is also found that the large groove is capable of reducing the SIFs in comparison with the circular notched due to lower stress concentration factors.

Microstructural investigation of subcritical crack propagation and Fracture Process Zone (FPZ) by the reduction of rock fracture toughness under cyclic loading

This study presents the results of laboratory diametrical compression tests performed on Brisbane tuff disc specimens to develop much-needed understanding of the micro-mechanical and micro-structural dynamics of sub-critical crack propagation. The strength variation in rocks under mechanical loading without corrosive chemical environment was investigated by investigating the mode-I (tensile) fracture toughness (KIC) response to static and cyclic loading. In some cases, cracks can grow at a lower load level compared to the static case. This phenomenon is called subcritical crack propagation and depends on the behaviour of the Fracture Process Zone (FPZ). The KIC response to cyclic loading was found to be different from that under static loading in terms of the ultimate load and the damage mechanisms in front of the chevron crack. A maximum reduction of the static KIC of 43% was obtained for the highest amplitude increasing cyclic loading test. Detailed Scanning Electron Microscope (SEM) examinations were performed on the surfaces of the tips of the chevron notch cracks, revealing that both loading methods cause fatigue in the disc rock specimens. When compared with static rupture, the main difference with the cyclically loaded specimens was that intergranular cracks were formed due to particle failure under cyclic loading, while smooth and bright cracks along cleavage planes were formed under static loading. It is believed in this study that the rock texture characteristics such as interlocked and cemented grains, cement volume, cement mineralogy play very important role on damage behaviour of rocks and development of FPZ and subcritical cracking.

A fracture mechanics-based approach to estimating the endurance limit of notched components

The fatigue notch effect can be estimated by using fracture mechanics-based support factors. To do so, the SIFs for cracks in notches must be calculated. There is a problem of transferability when 2D reference geometries are used. This can be avoided by modelling 3D cracks in the part being assessed. As demonstrated in this study, this approach is more laborious but leads to better results. The proposed approach was validated using a broad database of different sintered steels. The statistical evaluation showed that the mean value was almost exactly predicted with a relatively small scatter compared to alternative approaches.

Numerical Observation of Three-Dimensional Wing Cracking of Cracked Chevron Notched Brazilian Disc Rock Specimen Subjected to Mixed Mode Loading

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by International Society for Rock Mechanics for measuring mode I fracture toughness of rocks. Subsequently, this specimen geometry has been widely extended to conduct mixed mode fracture tests on rocks as well. A straight through crack front during the fracturing process upon the root of the chevron notch is assumed in the testing principle, but has never been thoroughly evaluated before. In this study, for the first time, the progressive rock fracture mechanism of the CCNBD rock specimen under mixed mode loading is numerically simulated. Specimens under representative mixed mode loading angles are modelled; and the assumption of the straight through crack front growth is critically assessed. The results show that not only the notch tip but also the saw-cut chevron notch cracks during the experiments, yielding a prominent twisted front, far from being straight. The crack front never grows up to the root of the notch ligament and the straight through crack front assumption is never satisfied in the realistic rock fracture progress of this chevron notched specimen subjected to mixed mode loads. In contrast, the fracture progress features typical three-dimensional wing cracking towards the loading ends. The numerically observed progressive fracture mechanism reveals that the measuring principle of mixed mode fracture tests employing CCNBD specimens is significantly violated and the measures of both modes I and II fracture toughness are uncertain.

Determination of Stress Intensity Factors and Fatigue Characteristics of Al-SiC Composite Material and Al-SiC Functionally Graded Material by Simulation

Crack propagation studies on Functionally Graded Material (FGM) have been done using simulation techniques. Simulation of crack propagation has been done on an Isotropic Aluminium Single Edge Notch(Crack) Bend Test Specimen under Three Point Bend Test Conditions and also on an Aluminium-Silicon Carbide composite specimen with single edge crack under Three Point Bend Test Conditions to estimate the KI Stress Intensity Factor (SIF) for different initial crack length to depth ratios. Crack propagation studies on an Aluminium plate and Aluminium-Silicon Carbide composite plate under three point bend test conditions were done experimentally and the peak loads carried by these specimens have been used in simulation techniques. Simulation of crack propagation under three point bend test conditions has also been done on an Aluminium- Silicon Carbide Functionally Graded Material (FGM). Estimation of KI Stress Intensity Factors by simulation techniques has been done for Isotropic Aluminum, Aluminium-Silicon Carbide composite and Aluminium- Silicon Carbide FGM and the estimated KI Stress Intensity Factor been compared between these three materials. Also compared are the induced von-mises stress and the maximum deflection of the specimens of these three materials obtained using simulation. Also the stress at the crack edge and the stress at the crack tip for the specimens of Isotropic Aluminum, Aluminium-Silicon Carbide composite and Aluminium- Silicon Carbide FGM have been obtained using simulation techniques and compared between these three materials.

The influence of short-term strength on the notch sensitivity and crack growth resistance of the metal of rotors under creep conditions

Comparative assessment of the effect the yield strength has on the characteristics of EI-415 steel under creep conditions is carried out based on the results from long-term (for more than 70000 h) tests of notched samples at an elevated temperature. The metal of three rotors (EI-415 steel) (one after it had been in operation for 137000 h and the other two in the delivery state with relatively increased and decreased levels of yield strength) was used as material under study. The notch sensitivity was evaluated from the ratio between the long-term strength of smooth samples and that of notched samples tested for long-term strength with recording the crack opening width. The resistances to crack incipience and to crack growth with a preset rate were determined from the lengths of cracks appeared at the end of two auxiliary notches of a smaller depth, and the creep fracture toughness was calculated from the break conditions. It is shown that the metal the properties of which are at the lower boundary of yield strength requirements is slightly more advantageous in the level of the above-mentioned characteristics.

The effect of notches on the fatigue behavior in NiTi shape memory alloys

Fatigue experiments have been performed on superelastic NiTi double-notched plate form specimens with different notch types and sizes. The fatigue loading was controlled by the average stress at the minimum cross-section. The fatigue life as well as the fatigue fracture surface has been investigated in detail. The fatigue life decreases from specimens without notches, to specimens with semi-circular notches followed by v-type notched specimens. Specimens with crack notches show the shortest fatigue lives. The fatigue life decreases with increasing the notch acuity. Not only notch type but also notch size affects the fatigue life under the same macro-average stress. The fatigue fracture surfaces are quite similar to those of the conventional ductile materials with crack initiation, propagation and final fracture area. Typical characteristics of low cycle fatigue surfaces such as multi-fatigue crack initiation sites, weak striations and tyre-like patterns were found. Finite element (FE) simulations were performed to investigate the stress and phase transformation distributions in order to explain and understand the experimental results.

Numerical Assessment of the Progressive Rock Fracture Mechanism of Cracked Chevron Notched Brazilian Disc Specimens

The International Society of Rock Mechanics (ISRM) suggested cracked chevron notched Brazilian disc method falls into a major testing category of rock fracture toughness measurement by virtue of chevron notched rock samples. A straight through crack front during the whole fracturing process is assumed in the testing principle but is never assessed. In this study, the progressive rock fracture mechanism of cracked chevron notched Brazilian disc rock specimens is numerically simulated for the first time. Two representative sample types with distinct geometry of notch ligaments are modelled. The assumption of a straight through crack front for chevron notched fracture samples is critically assessed. The results show that not only the notch tip but also the saw-cut chevron notch cracks during the experiments. The straight through crack front assumption is never satisfied in the realistic rock fracture progress of chevron notched disc samples. In addition, the crack features prominent curved front, far from being straight. In contrast to the sample type with narrow notch ligament, the acoustic emission (AE) of the simulation on the sample with wide notch ligament depicts obvious biased fracturing of the prescript fracturing route of the notch. The numerically observed progressive fracture mechanism calls for more attention on how to accurately calibrate the critical dimensionless stress intensity factor for a better measurement of Mode I fracture toughness via chevron notched samples.

The Energy Based Approach to Fatigue

This paper presents the energy based approaches developed to describe different aspects of fatigue. Different topics covered include fatigue crack initiation, crack initiation at a notch, multiaxial fatigue and fatigue crack propagation. Specific examples treated include, crack initiation at a notch, cracking at solder joint in electronic application, fatigue life estimation in a synthetic rubber and fatigue crack propagation in a metallic material.

Concepts for estimating the fatigue strength of sintered steel components

The fatigue notch effect can be estimated using fracture mechanics-based support factors. Stress intensity factors for cracks in notches must therefore be calculated. There is a problem of transferability when 2D reference geometries are used for this. This can be avoided when modelling 3D cracks in the notch of the actual part to be assessed. This is more laborious, but leads to better results, which will be shown in this presentation. The questions of aspect ratio and to point on the crack front, where stresses are evaluated, will be discussed. The approach is validated using a broad database of different sintered steels. The statistical evaluation shows an almost exact prediction of the mean value with a relatively small scatter.

Experimental Investigation of Modes II and III Fatigue Crack Growth in Unalloyed Titanium

The study is focused on finding and describing microscopic paths of modes II and III cracks in commercially pure α-titanium (hcp structure). Experiments were done on cylindrical bars with a circumferential cracked notch loaded in shear (modes II, III and II + III) and torsion (mode III). Fractography analysis showed that the crack growth was crystallography-controlled and the deflection and twisting angles of crack propagation direction (with respect to the shear plane) were relatively high. This behavior is a mixture of previously observed morphology in ARMCO iron (bcc) and austenitic steel (fcc). It can be understood in terms of a number of available slip systems in bcc, fcc and hcp lattices.

보강재 본딩접합 복합재 적층판구조 피로손상 균열진전 수명예측에 대한 연구

본 연구에서는 적층판 시편의 피로손상 균열진전 시험결과와 적층보강판 구조의 응력강도 해석결과를 기초로 충격손상을 모사한 원공과 노치손상을 내재한 보강재 본딩접합 적층보강판 구조의 피로손상 균열진전 수명예측에 대하여 고찰하였다. 그리고 적층보강판 구조시편에 대한 손상허용 시험결과와 손상진전 수명예측 해석결과를 비교분석한 결과 손상균열 길이 변화에 따라 최종파단에 대한 잔여강도를 예측하고 손상허용성 평가를 할 수 있었다. The prediction and analysis procedure of fatigue damage crack growth life for a stiffener bonded composite laminate panel including center hole and edge notch damage, was studied. It was performed on the basis of fatigue damage growth test results on a laminated skin panel specimens and the analysis results of stress intensity factor for the stiffener bonded composite panel. According to the comparison between experimental test and prediction results of fatigue damage growth life, it was concluded that the residual strength and damage tolerance assessment can be carried out along to the edge notch crack growth.

A Study on the Evaluation of the Fracture Process Zone in CCNBD Rock Samples

The safety of many civil and mining concrete and rock structures including pre-existing crack networks is fundamentally affected by the mechanical behaviour of the material under static and cyclic loading. In cyclic loading case, cracks can grow at a lower load level compared to the monotonic case. This phenomenon is called fatigue due to subcritical crack propagation and depends on the behaviour of the fracture process zone (FPZ). This study presents the results of laboratory diametrical compression tests performed on Brisbane tuff disc specimens to investigate their mode-I (tensile) fracture toughness response to static and cyclic loading and relevant FPZ. The FPZ and fracture toughness response to cyclic loading was found to be different from that under static loading in terms of the ultimate load and the damage mechanisms in front of the chevron crack. A maximum reduction of the static fracture toughness (K IC ) of 42 % was obtained for the highest amplitude increasing cyclic loading test. Detailed scanning electron microscope (SEM) examinations were performed on the surfaces of the tips of the chevron notch cracks, revealing that both loading methods cause FPZ development in the CCNBD specimens. When compared with monotonic FPZ development, the main difference with the cyclically loaded specimens was that intergranular cracks were formed due to particle breakage under cyclic loading, while smooth and bright cracks along cleavage planes were formed under static loading. Further, the SEM images showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix because of both intergranular and transgranular subcritical fracturing.

Observation and quantification of three-dimensional crack propagation in poly-granular graphite

Observations of fracture are generally restricted to the surface of test specimens; yet the fracture process occurs within the material. X-ray computed tomography (XCT) provides valuable insights into the structures within materials: when XCT is combined with digital volume correlation (DVC) the response to applied loads can be measured with high precision in the form of the three-dimensional displacement field within the material. This paper reports a study of the fracture behaviour of a short-bar chevron notch crack propagation specimen fabricated from polygranular nuclear graphite – a quasi-brittle material. A three-dimensional linear elastic finite element simulation of the specimen obtained the relations between crack length, opening displacement and stress intensity factor along the crack front. Tomographic absorption contrast images were obtained from the specimen before and after crack propagation, whilst loaded. The DVC-measured displacement field was used to measure and map the crack opening displacements in 3D; the experimentally calculated crack opening displacements were consistent with the FE-predicted values. The measurements demonstrate the existence of a cohesive fracture process zone ahead of the crack tip, which is a characteristic of quasi-brittle materials. This suggests that simulation of the fracture of non-irradiated polygranular nuclear graphite requires a material model capable of showing softening behaviour.

The Research on the Method of Simulating Crack Size Based on Reducing Local Structural Stiffness

When conducting modal analysis to the structure with crack defect using ansys finite element, we usually adopt the method of cutting a rectangular notch to simulate the crack on the model structure. As a result, the different crack simulation requires repeated modeling, which is a waste of time, and we cannot guarantee the accuracy of the calculation. In this paper, mathematical model between the local stiffness and crack size is established based on the finite element simulation of a diameter of 8mm steel bar. Thus, we can simulate varisized crack defects through a fixed crack notch by the method of modifying local stiffness. The method we used is more simple and accurate than the traditionary one by cutting a rectangular notch to form cracks, and reduces the workload, and has certain research significances.

Crack Detection in a Rotor Dynamic System by Vibration Monitoring—Part II: Extended Analysis and Experimental Results

Abstract An increase in the power-to-weight ratio demand on rotordynamic systems causes increased susceptibility to transverse fatigue cracking of the shaft. The ability to detect cracks at an early stage of progression is imperative for minimizing off-line repair time and cost. The vibration monitoring system initially proposed in Part I is employed herein, using the 2X harmonic response component of the rotor tilt as a signature indicating a transverse shaft crack. In addition, the analytic work presented in Part I is expanded to include a new notch crack model to better approximate experimental results. To effectively capture the 2X response, the crack model must include the local nature of the crack, the depth of the crack, and the stiffness asymmetry inducing the gravity-forced 2X harmonic response. The transfer matrix technique is well suited to incorporate these crack attributes due to its modular nature. Two transfer matrix models are proposed to predict the 2X harmonic response. The first model applies local crack flexibility coefficients determined using the strain energy release rate, while the second incorporates the crack as a rectangular notch to emulate a manufactured crack used in the experiments. Analytic results are compared to experimental measurement of the rotor tilt gleaned from an overhung rotor test rig originally designed to monitor seal face dynamics. The test rig is discussed, and experimental angular response orbits and 2X harmonic amplitudes of the rotor tilt are provided for shafts containing manufactured cracks of depths between 0% and 40%. Feasibility of simultaneous multiple-fault detection of transverse shaft cracks and seal face contact is discussed.

Evaluation on estimation in characteristics of fatigue crack using micro-encapsulated dye mixing paint

There are various ways to detect the fatigue crack in structural members, but it is difficult to find the initiation, characteristics of fatigue cracks, its length, and propagations due to the cost and effectiveness from the structural members under service condition. This study quantitatively examined the efficiency of estimating the characteristics of fatigue crack of notch crack and gusset welded joint specimens with micro-encapsulated dye mixing paint according to its layers, protective coating, surface conditions. Initial fatigue crack length and propagations were measured by digital images and compared with colored fatigue cracks, which appeared on micro-encapsulated dye mixing paint. And also, fatigue tests were carried out on structural member with perpendicular welding joint to verify its applicability for actual structural members. It was shown to be a good inspection in visual inspection perspective, and the inspection errors were lowered when fatigue crack length increased. The micro-encapsulated dye mixing paint can be considered as an easily applicable inspection method of fatigue crack and propagations through a close inspections around colored crack line after detecting structural defects, despite of difficulty in detecting the initial crack and short fatigue crack.

Mechanism of rock fatigue damage in terms of fracturing modes

A novel outcome of the research described is the observation of the effect of indirect tensile cyclic loading on the fracture toughness (KIC) of rocks. A maximum reduction of the static KIC of 46% was obtained for the highest amplitude dynamic cyclic loading test. A second series of cyclic tests showed that the 45° and 70° inclined chevron notch cracks opened from the beginning of the cyclic loading test, whereas under monotonic loading, the chevron notch cracks closed up to failure. When compared with static rupture, the detailed scanning electron microscope (SEM) examinations showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix, due to both intergranular fracturing and transgranular fracturing, whereas smooth and bright cracks were formed along cleavage planes under static loading.