Crack Propagation Parameters Research Articles

A rapid testing method for assessing mode I fatigue delamination of carbon fibre-reinforced polymer

Characterising fatigue delamination in composite materials following standardised testing protocols is often associated with high costs and significant time investment. Therefore, it can be helpful to have a testing strategy to reduce the testing time and easily assess the fatigue delamination of multiple materials and conditions. This work shows how to apply one of the new rapid testing techniques known as the stiffness method on composite materials. The method has been developed to predict the fatigue delamination of composite materials by monitoring the fatigue damage through the compliance of the specimen. The obtained data in terms of displacement and force is used to obtain the fracture toughness, fatigue crack onset, fatigue crack threshold, and crack propagation parameters by using a reduced number of specimens and a few testing hours. The testing procedure has been developed on carbon fibre-reinforced polymer used in aerospace structures. The obtained results in a short time are promising, reporting a deviation below 10% compared to the values obtained in the standardised fatigue delamination tests.

Wmic-GMTS and Wmic-GMERR criteria for micron-scale crack propagation in red-bed soft rocks under hydraulic action

Micron-scale crack propagation in red-bed soft rocks under hydraulic action is a common cause of engineering disasters due to damage to the hard rock–soft rock–water interface. Previous studies have not provided a theoretical analysis of the length, inclination angle, and propagation angle of micron-scale cracks, nor have they established appropriate criteria to describe the crack propagation process. The propagation mechanism of micron-scale cracks in red-bed soft rocks under hydraulic action is not yet fully understood, which makes it challenging to prevent engineering disasters in these types of rocks. To address this issue, we have used the existing generalized maximum tangential stress (GMTS) and generalized maximum energy release rate (GMERR) criteria as the basis and introduced parameters related to micron-scale crack propagation and water action. The GMTS and GMERR criteria for micron-scale crack propagation in red-bed soft rocks under hydraulic action (abbreviated as the Wmic-GMTS and Wmic-GMERR criteria, respectively) were established to evaluate micron-scale crack propagation in red-bed soft rocks under hydraulic action. The influence of the parameters was also described. The process of micron-scale crack propagation under hydraulic action was monitored using uniaxial compression tests (UCTs) based on digital image correlation (DIC) technology. The study analyzed the length, propagation and inclination angles, and mechanical parameters of micron-scale crack propagation to confirm the reliability of the established criteria. The findings suggest that the Wmic-GMTS and Wmic-GMERR criteria are effective in describing the micron-scale crack propagation in red-bed soft rocks under hydraulic action. This study discusses the mechanism of micron-scale crack propagation and its effect on engineering disasters under hydraulic action. It covers topics such as the internal-external weakening of nano-scale particles, lateral propagation of micron-scale cracks, weakening of the mechanical properties of millimeter-scale soft rocks, and resulting interface damage at the engineering scale. The study provides a theoretical basis for the mechanism of disasters in red-bed soft-rock engineering under hydraulic action.

Investigation on rock breakage by high-velocity water jet impact under different stress loading conditions: Fracture characteristic, stress and damage evolution laws

Numerical simulations were conducted based on SPH-FEM coupling method to investigate the damage evolutions and failure mechanism of rock impacted by a high-velocity water jet under different stress loading conditions. The influences of stress loading magnitudes and stress differences on the rock fracture characteristics, including parameters of broken pit morphology and crack propagation, were obtained. The development of damage and crack inner rock will be inhibited due to stress loadings. The rock failure mechanism in different breakage zones were revealed by exploring the damage and stress evolutions inner rock. The results show that the rock surface elements near jet impact center suffers from the instantaneous brittle failure, which is scarcely affected by stress loadings. The elements inner rock below jet impact and sidewall elements surrounding broken pit are destroyed due to the cumulative plastic damage, and the stress loadings delay the initial damage time and prolong the cumulative damage time.

On the Analyses of Cure Cycle Effects on Peel Strength Characteristics in Carbon High-Tg Epoxy/Plasma-Activated Carbon PEEK Composite Interfaces: A Preliminary Inquiry

In this study, a high-Tg aerospace-grade epoxy composite plate was co-curing welded using a unidirectional PEEK thermoplastic carbon fibre tape to develop advanced composite joints. To account for the surface roughness and the weldability of carbon–epoxy/carbon–PEEK composites, plasma treatments were performed. The co-curing was conducted by the following steps: each treated thermoplastic tape was first placed in the mould, and followed by nine layers of dry-woven carbon fabrics. The mould was sealed using a vacuum bag, and a bi-component thermoset (RTM6) impregnated the preform. To understand the role of curing kinetics, post-curing, curing temperature, and dwell time on the quality of joints, five cure cycles were programmed. The strengths of the welded joints were investigated via the interlayer peeling test. Furthermore, cross-sections of welded zones were assessed using scanning electron microscopy in terms of the morphology of the PEEK/epoxy interphase after co-curing. The preliminary results showed that the cure cycle is an important controlling parameter for crack propagation. A noticeable distinction was evident between the samples cured first at 140 °C for 2 h and then at 180 °C for 2 h, and those cured initially at 150 °C for 2 h followed by 180 °C for 2 h. In other words, the samples subjected to the latter curing conditions exhibited consistently reproducible results with minimal errors compared to different samples. The reduced errors confirmed the reproducibility of these samples, indicating that the adhesion between CF/PEEK and CF/RTM6 tends to be more stable in this curing scenario.

Quantitative assessment for tooth crack of the sun gear in a planetary gearbox

The sun gear in a planetary gearbox is prone to generate tooth crack due to its much more severe load conditions than other gears. The cracked sun gear will evolve into distinct tooth breakages and thus may deteriorate the operation status and even cause catastrophic loss of the whole transmission system. To avoid tooth crack induced malfunction of the planetary gearbox, the tooth crack of sun gear should be accurately identified during the early stage. Therefore, a novelty quantitative assessment methodology for tooth crack damage is proposed, which is a mathematical model for crack propagation of the sun gear in planetary gearboxes based on vibration acceleration signal. Firstly, the quantitative relationship between the crack propagation parameters along the gear thickness and width directions and the time-varying mesh stiffness is analytically formulated. The analytical time-varying mesh stiffness formulation is incorporated into a translational–torsional dynamic model to reveal the effects of crack propagation on the vibration signal of internal and external meshing pairs in the planetary gearbox. Secondly, a quantitative mapping function is established by taking the cracked section area as the variable and the residual of the summed envelope spectrum amplitude of the vibration acceleration signal as the assessment index. Finally, a numerical simulation as well as an experimental test is conducted and compared to verify the correctness of the proposed assessment methodology. We believe that the established model and the corresponding index provide a physically intuitive yet mathematically quantitative reference for diagnosing tooth crack in planetary gearboxes.

Rigorous SIF-based prediction of fatigue life improvement for prestressed-CFRP-repaired cracked steel plates

Carbon fiber reinforced polymer (CFRP) can be widely used to repair damaged plates in marine engineering equipment. To predict the fatigue life improvement by this repair, the stress intensity factor (SIF) of the cracked plate without/with repair is analyzed rigorously. Specifically, the SIF of a cracked plate without repair is analyzed and the error due to the finite element model (FEM) mesh is evaluated by comparing with the exact solution. By applying the same mesh having the same error, the exact SIF of the repaired plate is obtained in a convenient form. Fatigue experiments reveal that the same crack propagation parameters (C and m) can be applied for the unrepaired/repaired plates. Based on those, the prediction equation of fatigue life is obtained for cracked steel plates repaired with prestressed CFRP. The results show that this prediction method has the advantages of simplicity and efficiency.

An Interval Prediction of Chloroprene Rubber Crack Propagation Characteristics Based on Thermal Accelerated Aging.

Thermo-oxidative aging plays an important role in changing the properties of rubber materials; it significantly decreases the fatigue life of air spring bags and further causes safety hazards. However, due to the great uncertainty of rubber material properties, an effective interval prediction model has not been established considering the effect of aging on airbag rubber properties. To solve the problem, this study proposes an interval parameter correlation model that can more accurately describe rubber crack propagation characteristics by considering material uncertainty. Furthermore, an aging prediction model of the rubber crack propagation characteristic region is established based on the Arrhenius equation. The effectiveness and accuracy of the method are verified by comparing the test and prediction results under the temperature spectrum. The method can be used to determine the variations in the interval change of the fatigue crack propagation parameters during rubber aging and can guide fatigue reliability analyses of air spring bags.

Statistical Inference of Equivalent Initial Flaw Size Distribution for Fatigue Analysis of an Anisotropic Material

A novel methodology for the fatigue life uncertainty quantification of anisotropic structures is presented in this work. The concept of the equivalent initial flaw size distribution (EIFSD) is employed to overcome the difficulties in small cracks detection and fatigue prediction. This EIFSD concept is combined with the dual boundary element method (DBEM) to provide an efficient methodology for modelling the fatigue crack growth. Bayesian inference is used to infer the EIFSD based on inspection data from the routine maintenance of the structure, simulated with the DBEM. A large amount of DBEM simulations were required for the Bayesian inference. Therefore, surrogate models are used as part of the inference to further improve computational efficiency. A numerical example featuring an anisotropic plate is investigated for demonstrating the proposed methodology. When considering a low level of uncertainty in the crack propagation parameters, an error of 0.12% was found between the estimated fatigue life obtained using the proposed method compared to actual fatigue life, and only 0.35% error when considering high level of uncertainty. The application of the estimated fatigue life can be used to determine an appropriate inspection interval for aircraft maintenance.

Micromechanical transverse tensile crack propagation of unidirectional fiber reinforced epoxy SMC slice imbedded in a TDCB specimen

A mode-I type of crack propagation and branching on account of fracture energy failure mechanisms methodology is investigated in unidirectional (UD) fiber reinforced epoxy sheet molding compounds (SMC). In order to eliminate the influence of structural boundary conditions with various crack lengths on tip K (stress intensity factor) value to a greatest extent, a tapered double cantilever beam (TDCB) tensile sample is selected as the far-field boundary, the SMC slices with pre-cracks are embedded into TDCB samples, which are referred to as the embedded TDCB (E-TDCB) geometries. By applying the transvers tensile load to E-TDCB, embedded SMC is provided with far-field driving force for crack propagation, so as to obtained a great agreement between stable crack propagation and minor tip K oscillation. Both E-TDCB and dummy TDCB (d-TDCB) samples were selected for experimental test with comparation. The dynamic crack propagation parameters (work components of far-field driving force as well as crack length and Crack tip open displacement et al.) were measured and analyzed with progressive damage criterion. Although in disparate principle, the numerical prediction with proposed constitutive law correlates well with experimental results.

Characterising the stress ratio effect for fatigue crack propagation parameters of SAE 1045 steel based on magnetic flux leakage

The aim of this study was to determine the relationship between fatigue crack length, a, the number of cycles, N, the magnetic flux gradient intensity, dH/dx and the stress ratio, R. Fatigue crack growth tests were performed using a constant tensile load amplitude on SAE 1045 steel in the form of single-edge cracks. Stress ratio values of 0, 0.1, 0.2, 0.3 and 0.4 were used to study the characteristics of the magnetic flux gradient. A crack opening displacement device was used to detect the parameters of the stress intensity factor range, ΔK, fatigue crack growth rate, da/dN, and N. Meanwhile, a metal magnetic memory sensor device was used to detect the H value at the stress concentration zone when a 1 mm-increase in the crack occurred. The experimental results showed that as the fatigue cycle and crack length increased for each R, dH/dx also increased exponentially. The governing equations were derived based on the magnetic flux gradient for the fatigue crack length equation and the fatigue cycle equation, with the stress ratio. Based on a statistical analysis of the magnetic flux leakage signals, it was found that all the data were within a range based on a 90 % confidence level. Thus, the magnetic flux leakage parameters could be used to construct fatigue crack growth behaviour models for ferromagnetic materials and potentially as an alternative method for predicting the fatigue life.

Improvement of shear capacity of RAC beams by adopting identical mortar volume method of mix-design along with dual-stage mixing approach

The shear performance of concrete beams produced from construction and demolition (C&D) waste is evaluated by considering two different shear-span to depth ratios (a/d). A total of twelve beams are cast, in which four beams of natural aggregate concrete (NAC) of conventional mix-design procedure (denoted as NAC), four beams of recycled aggregate concrete (RAC) of identical mortar volume (IMV) mix-design procedure in conjunction with normal mixing procedure (NMP) (denoted as RAC/NMP), and rest four beams of RAC of IMV mix-design procedure in conjunction with proposed dual-stage mixing procedure (DSMPSiC) (symbolized as RAC/DSMPSiC). Out of four beams of each of the above categories, two beams are tested by considering a/d = 2.5 and the other two are tested by considering a/d = 3. The load-deflection curve, ultimate load carrying capacity, and crack propagation parameters are also evaluated. The results are compared with different empirical relations proposed in codes and published literature. Analysing critically, it is observed that the shear strengths of RAC/NMP beams are reduced by 19% for a/d = 2.5 and 13.5% for a/d = 3 as compared to corresponding NAC beams. However, after following the proposed DSMPSiC, the above reduction percentages are reduced by 11.2% and 5.2%, respectively.

FATIGUE ASSESSMENT OF CORRODED DECK LONGITUDINALS OF TANKERS


 
 
 Fatigue life of deck longitudinals of oil tankers is analysed based on linear elastic fracture mechanics. A parametric formulation for the estimation of stress intensity factors and the Paris-Erdogan law are applied. Long-term effects of corrosion are modelled based on regression equations fitted to thickness measurements made during inspections of two tankers. Parametric studies are performed in order to investigate the importance of the governing parameters of crack propagation. A comparison of the fatigue analyses performed by linear fracture mechanics and S-N approaches is presented.
 
 

Effect of Loading Rate and Water on the Dynamic Fracture Behavior of Sandstone under Impact Loadings

The moisture content of rocks in nature often changes with the natural hydrological environment, and generally natural rock may be classified into three types, that is, dry, natural, and saturated rock. The corresponding responses to dynamic loads may have considerable difference, and therefore, impact tests in this paper were conducted on single cleavage triangle sandstone specimens with three types of conditions, respectively, under various loading rates. The ingredient of sandstone was determined by X-ray diffraction and optical microscopic image. The crack propagation parameters were recorded with the aid of a crack propagation gauge. The dynamic stress intensity factors were calculated using ABAQUS code. The results show that the fracture toughness of all the three types of specimens increase with loading rate, which could be expressed by power functions. Compared with dry and natural sandstone, the crack propagation speed of saturated sandstone specimens is more sensitive to the loading rate. Both the rate effect and the water effect, including water-weakening effect and water-enhancing effect, play a significant part in rock dynamic fracture behavior.

Size-Induced Constraint Effects on Crack Initiation and Propagation Parameters in Ductile Polymers.

Fracture mechanics are of high interest for the engineering design and structural integrity assessment of polymeric materials; however, regarding highly ductile polymers, many open questions still remain in terms of fully understanding deformation and fracture behaviors. For example, the influence of the constraint and specimen size on the fracture behavior of polymeric materials is still not clear. In this study, a polymeric material with an elastic plastic deformation behavior (ABS, acrylonitrile butadiene styrene) is investigated with regard to the influence of constraint and specimen size. Different single-edge notched bending (SENB) specimen sizes with constant geometrical ratios were tested. The material key curve was used to investigate differences in the constraint, where changes for small and large specimen sizes were found. Based on a size-independent crack resistance curve (J–R curve), two apparent initiation parameters (J0.2 and Jbl) were determined, namely, the initiation parameter Jini (based on the crack propagation kinetics curve) and the initiation parameter JI,lim (based on an ESIS TC 4 draft protocol). It was found that J0.2 and Jbl could be used as crack initiation parameters whereby Jini and JI,lim are indicative of the onset of stable crack growth.

Three-dimensional fatigue crack propagation simulation using extended finite element methods for steel grades S355 and S690 considering mean stress effects

The assessment of fatigue crack propagation of steel structures is essential and important especially to improve the application of high strength steel in construction. The load ratio R, reflecting mean stress effects, will be changed with crack extension in the steel structures with complicated geometry. In this paper, the Walker equation is employed to fit the fatigue crack propagation rate of steel grades S355 and S690 based on experimental data in the literature to incorporate the mean stress effects. The material fatigue crack propagation parameters with 95%, 97.7%, and 99% guarantee of Walker equation were obtained by a stochastic analysis using the Monte Carlo method. The fatigue life was firstly predicted by the analytical method and was used as a baseline for numerical fatigue crack propagation simulation. A user-defined fatigue crack propagation subroutine based on the Walker equation was developed using phantom nodes-based extended finite element method (PN-XFEM) and Virtual Crack Closure Technique (VCCT) to consider the mean stress effects. The proposed three-dimensional fatigue crack propagation simulation subroutine is successfully validated of both steel grades, S355 and S690.

Numerical residual strength prediction of stationary shoulder friction stir welding structures

The residual strength of a structure made of dissimilar Aluminium panels joined by a stationary shoulder friction stir welding (SSFSW) process is predicted by numerical simulation using finite elements including cohesive elements for crack propagation. The yield strength and strain hardening parameters within the stirred zone and adjacent thermo-mechanically affected and heat affected zones are derived from a tensile specimen cut out from the panel perpendicular to the weld seam. The identification is conducted by a hybrid numerical/experimental procedure with an inverse search by help of digital image correlation in order to obtain the strain field at the welded surface and to compare them to the numerical calculation. The crack propagation parameters are retrieved from specimens with crack in the stir zone and heat affected zone on either side. After this identification procedure, the fracture behaviour of a coupon specimen with a crack crossing the weld is predicted.

Fatigue Life Assessment and Reliability Analysis of Cope-Hole Details in Steel Bridges

Cope-hole details are widely applied to steel bridges. However, the safety of steel bridges is influenced by the fatigue performance of welded details. So, cope-hole details with flange and web subjected to axial loads were selected as the research object. Based on the basic theory of linear elastic fracture mechanics and the Finite Element Method, the stress intensity factors of cope-holes details were calculated. The influences of geometry size and crack size of the detail on the stress intensity factors were then investigated. The Paris model of fatigue crack propagation predicted the crack propagation life of cope-hole details. Besides, the fatigue limit-state equation was also established to analyse the effect of random variables (such as initial crack size, critical crack size, crack propagation parameter) on the fatigue reliability index. Finally, the recommended value of the detection period was present. The results show that the stress intensity factor gradually increases with the increase of the cope-hole radius, the weld size, the flange plate thickness, the crack length and the web thickness. However, it gradually decreases with the increase of the ratio of the long and short axle to the crack. The predicted number of fatigue cyclic loading required by the fatigue crack depth propagating from 0.5 mm to 16 mm under nominal stress amplitude of 63 MPa is 122.22 million times. The fatigue reliability index decreases with the fatigue growth parameter, the crack shape ratio and the mean of initial crack size increasing, which is relatively sensitive. However, the variation coefficient of the initial crack size has little effect on it. The detection period of cope-hole details is the service time corresponding to the fatigue accumulated cyclic loading of 198.3 million times.

Fatigue Crack Growth Model Incorporating Surface Waviness For Wire+Arc Additively Manufactured Components

Wire+Arc additive manufacturing, due to its high deposition rate, is particularly suited for the construction and repair of large-scale components. For the very same reason, however, an important drawback of this process is the pronounced surface waviness. This waviness introduces multiple adjacent peaks and valleys at the component surface, acting as stress concentration sites which can undermine the fatigue performance. In the present work, the authors propose a framework to model fatigue crack propagation considering the effect of waviness at the component surface. The growth model takes the stress concentration factor as input and incorporates it in a two-fold crack propagation model of a planar flaw. The fracture mechanics based growth models are developed based on El Haddad’s short crack model and the Paris equation for long crack propagation. The effects of stress concentration sites are firstly considered by modifying the short crack propagation parameters. Secondly by introducing a new crack length dependent stress intensity factor threshold, surface waviness is incorporated in the transition from short crack into long crack regime. The results show that short cracks originating from an as-built WAAM surface grow faster as compared to their counterparts originating from machined surfaces.

Fatigue crack growth in railway axle specimens – Transferability and model validation

The issue of transferability of crack propagation parameters from small-scale specimens up to real-scale components is investigated. Crack growth material parameters are determined based on single edge notched bending SE(B) specimen tests and the crack growth model is validated by 1:3 scaled railway axle experiments as well as for 1:1 test axles focusing on the steel grade EA4T. Crack propagation assessment is performed with the aid of the software tool INARA demonstrating the importance of considering secondary mean stress states due to residual stresses and press fits. In addition, short crack behavior and the build-up of the fatigue crack growth threshold is considered by means of an enhanced NASGRO fatigue crack growth equation, which is of great importance at loads with stress intensity factors in the threshold region. A final comparison with results of a crack growth assessment model based on a previous research project shows the potential of the presented improved approach and highlights the importance to include secondary stress states as well as short crack behavior in order to ensure a proper transferability from small-scale specimens to real-scale components in practice.

Quantitative fractography analysis of a chip crack in a Si power MOSFET

When a material breaks and a crack forms, the resulting fracture surface will, in most cases, exhibit characteristic fracture marks. Their examination and interpretation, known as fractography analysis, can deliver useful insights about crack formation and propagation. This in turn helps to identify root causes, clarify the distribution of mechanical stress in the sample, or obtain information about material properties. In most cases, fractography analysis is used qualitatively to identify the origin of crack propagation. We report on the quantitative analysis of a specific type of fracture marks, namely Wallner lines, which allows us to calculate the exact coordinates of the crack origin, along with some further characteristic parameters of crack propagation. The approach, first proposed and applied by Rabinovitch et al. [1], is extended and applied to the fragment of a silicon power MOSFET. The approach can be transferred easily to many kinds of fractures and materials where Wallner lines are observed, as it is based on the fundamental mechanism of how they are created.