Fatigue Delamination Propagation Research Articles

The effect of fiber bridging on mode I fatigue delamination propagation—Part II: Cohesive zone model

AbstractThis is Part II of a series of two papers in which the effect of fiber bridging on fatigue delamination propagation is assessed. In Part I, unidirectional double cantilever beam specimens composed of the carbon fiber‐reinforced polymer prepreg AS4/8552 were tested by means of fatigue cycling. Fiber bridging in beam specimens composed of unidirectional plies causes the apparent fatigue delamination curves to exhibit growth which is slower than that for the case when fiber bridging does not occur. Generally, fiber bridging does not occur in laminate structures. In Part II of this study, a cohesive zone model (CZM) is developed and used to carry out finite element analyses to simulate the experiments. The CZM is employed to quantify and eliminate the contribution of fiber bridging to the fatigue delamination growth curves. In this way, more realistic results are obtained. These results are compared to an upper bound curve determined in Part I.

The effect of fiber bridging on mode I fatigue delamination propagation—part I: Testing

AbstractThe purpose of this investigation is to assess the effect of fiber bridging on fatigue delamination propagation. Fiber bridging occurs when testing beam type laminates specimens consisting of unidirectional plies. Unidirectional double cantilever beam specimens composed of the carbon fiber reinforced polymer prepreg AS4/8552 were tested by means of fatigue cycling. A Paris relation was determined based on these tests. Fiber bridging in beam specimens composed of unidirectional plies causes the apparent fatigue delamination curves to exhibit slower growth predicting overly conservative results. In Part I of this study, the effect of fiber bridging was eliminated experimentally from the results. In Part II of this study, a cohesive zone model is developed and used to carry out finite element analyses to simulate the experiments, as well as to eliminate the influence of fiber bridging.

A Numerical Assessment of the Influence of Local Stress Ratio in the Fatigue Analysis of Post-Buckled Composite Single-Stringer Specimen

This paper presents a numerical approach for investigating fatigue delamination propagation in composite stiffened panels loaded in compression in the post-buckling field. These components are widely utilized in aerospace structures due to their lightweight and high-strength properties. However, fatigue-induced damage, particularly delamination at the skin–stringer interface, poses a significant challenge. The proposed numerical approach, called the “Min–Max Load Approach”, allows for the calculation of the local stress ratio in a single finite element analysis. It represents the ratio between the minimum and maximum values of the stress along the delamination front, enabling accurate evaluation of the crack growth rate. The methodology is applied here in conjunction with the cohesive zone model technique to evaluate the post-buckling fatigue behavior of a composite single-stringer specimen with an initial delamination. Comparisons with experimental data validate the predictive capabilities of the proposed approach.

Effect of number of fatigue cycles on fatigue data: CFRP (prepreg and wet‐layup)

AbstractIn carrying out fatigue delamination propagation tests to assess the propagation rate versus a function of the energy release rate, the question arises as to the necessary number of cycles required to properly characterize the behavior. Tests were performed elsewhere on two different carbon/epoxy, multidirectional woven composite laminates containing a delamination using constant amplitude cycling by means of displacement control for various cycle ratios. The aim was to carry out the tests for cycles. Most specimens complied with this requirement. In this study, the fatigue data are analyzed including the first cycles, the first cycles, the first cycles, and cycles. It was seen that although the delamination length versus the cycle number could be quite similar between different cycle ranges, the Paris relation constants could be significantly different. It is suggested to carry out tests for as many cycles as possible.

Fatigue delamination propagation: Various effects on results

AbstractIn this investigation, three subjects are considered. First, the effect of the human factor is examined. In carrying out analyses of fatigue delamination propagation tests on laminate composites, the delamination length must be measured. The human effect on measurement of the delamination length by different investigators is discussed. A limited influence of the human factor on the measurement of the delamination length , as well as on delamination propagation rate , is observed in this study. Secondly, the paper proceeds to discuss the influence of the ratio on the fatigue delamination growth rate. It is found that a higher rate of crack/delamination propagation is associated with lower ratios which appears to contradict conventional wisdom. This behavior is confirmed in tests. Thirdly, a comparison between the fatigue propagation rates of two material systems is considered. It is concluded that the energy release rate used to assess these materials should not be normalized.

Modeling fiber bridging effects on fatigue delamination propagation in composites using the interface thick level set method

Modeling fiber bridging effects on fatigue delamination propagation in composites using the interface thick level set method

An efficient and versatile use of the VCCT for composites delamination growth under fatigue loadings in 3D numerical analysis: the Sequential Static Fatigue algorithm

Composite materials are susceptible to delamination when undergoing fatigue loads. Being able to accurately and efficiently predict the fatigue propagation of a delamination defect would increase the safety and the adoption of lightweight composites within industry. In this work, a new VCCT-based 3D fatigue propagation algorithm, called Sequential Static Fatigue (SSF), is presented. The algorithm is able to accurately simulate the fatigue propagation of delamination defects by performing several sequential static simulations. Compared to the benchmark (Abaqus direct cyclic algorithm), the SSF reduced the computational time of several cases by three orders of magnitude. A better accuracy was also achieved. This work thus sets the basis for a new drastically more efficient modelling technique for fatigue delamination propagation.

Neural cohesive model for delamination simulation in composite laminates under cyclic loadings

This work proposes a novel fatigue cohesive model that utilizes a single-hidden-layer neural network as its fatigue constitutive law for simulation of fatigue delamination under mixed-mode loadings. The neural network takes variables of the cohesive zone as inputs and outputs the accumulation rates for a fatigue damage variable to enforce fatigue delamination propagation, which is then embedded in Abaqus to verify the model under mode-I, mode-II and mixed-mode loadings with Paris law and Hartman-Schijve equation as references. On top of the good correlations, it is found that the proposed model is also insensitive to mesh sizes and very computationally efficient.

An incremental-onset model for fatigue delamination propagation in composite laminates

We propose to treat propagation of a fatigue-driven delamination in composite laminates as a sequence of incremental delamination onsets and develop a cohesive zone model (CZM) to simulate it. The model employs a fatigue damage evolution law that is based on the reduction of critical strain energy release rate. Unlike existing fatigue CZMs, which assume and use Paris' law derived from propagation tests as input, the proposed model relies solely on the experimental data from fatigue onset tests (namely the G-N curve, where G is the supplied strain energy release rate and N is the number of cycles to onset). The model is implemented within an implicit finite element code using three-dimensional cohesive elements and is demonstrated to predict fatigue delamination propagation in unidirectional carbon fibre/epoxy composites under Mode I loading. This incremental onset model predicts the Paris’ law, which compares well with experimental data for this case. Composites with toughened inter-laminar interface and under other modes of deformation and their combinations may exhibit a different fatigue behaviour.

Digital image correlation assisted characterization of Mode I fatigue delamination in composites

A digital image correlation (DIC) based method to characterize the Mode I fatigue delamination onset and propagation in composite laminates is proposed. The characterization is performed using the double cantilever beam test (DCB) under fatigue loading. The DIC is used to analyse the displacement field on the edge of a DCB specimen, from which the crack tip position is determined as a place where opening displacement converges to zero. The method was successfully applied to both the crack onset detection (fatigue onset life) and propagation monitoring under fatigue cycling. The DIC-based method, which relies on the actual crack tip position, gives conservative values for the fatigue onset life in comparison with the compliance based method recommended in the ASTM standard. The DIC assisted method reduces operator and structure dependent errors and shows promise for real-time delamination detection under fatigue loadings.

Correction to: Multi-directional composite laminates: fatigue delamination propagation in mode I—a comparison

Due to an unfortunate turn of events this article which is part of the “GEF-2019” special issue was mistakenly published.

Multi-directional composite laminates: fatigue delamination propagation in mode I—a comparison

Double cantilever beam (DCB) specimens composed of carbon fiber reinforced polymer laminate composites were tested. Two material systems were investigated. One consisted of plies from a woven prepreg alternating with tows in the $$0^{\circ }/90^{\circ }$$-directions and the $$+45^{\circ }/-45^{\circ }$$-directions. The second was fabricated by means of a wet-layup process with the same multi-directions as the prepreg. In addition, for the second material system, a unidirectional (UD) fabric ply was added. The delamination for this laminate was between the UD fabric and the woven ply with tows in the $$+45^{\circ }/-45^{\circ }$$-directions. Both fracture resistance R-curve and fatigue delamination propagation tests were carried out. It is found that the initiation value of the interface energy release rate is substantially lower for the wet-layup; whereas, their steady state values are quite similar. The fatigue delamination propagation tests were performed at various cyclic R-ratios. The delamination propagation rate da/dN was calculated from the experimental data and plotted using a modified Paris equation with different functions of the mode I energy release rate. As expected, the da/dN curves depend upon the R-ratio. By using another parameter based on the Hartman–Schijve equation for metals, it is possible to obtain a master-curve for all R-ratios. It is seen that the propagation rate for the prepreg is faster than that of the wet-layup.

Nearly Mode I Fracture Toughness and Fatigue Delamination Propagation in a Multidirectional Laminate Fabricated by a Wet-Layup

Five double cantilever beam specimens were tested quasi-statically to obtain a GIR resistance curve. In addition, nine double cantilever beam specimens were tested in fatigue to obtain a Paris-type relation to describe the delamination propagation rate da/dN where a is delamination length and N is the cycle number. Displacement ratios of Rd = 0.10 and 0.48 were used for five and four specimens, respectively. The specimens were fabricated by means of a wet-layup process from carbon fiber reinforced polymer plies. The interface containing the delamination was between a unidirectional fabric and a woven ply. The fracture toughness and fatigue delamination propagation protocols are outlined. The mechanical and thermal residual stress intensity factors were obtained by means of finite element analyses and the conservative M-integral along the delamination front. They were superposed to determine the total stress intensity factors. It was found that the total mode I stress intensity factor dominates the other two stress intensity factors. Thus, nearly mode I deformation was achieved. Interpolation expressions for the mechanical and thermal residual stress intensity factors were determined using three and two-dimensional fittings, respectively. Results are presented with an expression for GIR determined. Moreover, the fatigue data is described including threshold values and master-curves. These results shed light on the behavior of delamination propagation in multidirectional laminate composites.

Compliance based method for testing fatigue delamination propagation in laminates

The paper proposes testing methods for experimental characterization of fatigue delamination growth under Mode I and Mode II loading conditions. Their major advantage is eliminating visual inspection of the delamination front propagation. Instead, the delamination length is calculated from specimen compliance based on the results of compliance calibration performed at the end of the test. The testing procedure for Mode I uses the double cantilever beam specimen with the additional clamping fixture for compliance calibration. Delamination growth under Mode II loading conditions is investigated with the end notch flexure set-up.Both methods presented in the paper were used in experimental investigations of delamination resistance properties of laminates made of unidirectional prepreg Cytec MTM 46. The calculations of delamination length, performed according to the proposed procedure, were compared against the effective delamination length approach and, for Mode I, with visual observations with the use of a digital camera. The experiments produced consistent results with the coefficient of variation below 7% for Parameters of Paris’ law. The ability to conduct tests overnight considerably reduced the duration of testing. The methods can be implemented in most laboratories as it does not require special equipment other than a testing frame able to apply cyclic displacement.

Prediction of mode II delamination propagation life under a standard spectrum loading in a carbon fiber composite

Mode II constant amplitude fatigue delamination propagation tests were conducted at three different stress ratios, viz. R = 0.0, 0.5, and −1.0 using modified three-point bend test fixture and end notched flexure (ENF) test specimens of unidirectional IMA/M21 carbon fiber composite. Delamination length was measured by compliance technique. The delamination propagation rate (da/dN) plots were constructed for various stress ratios. Using an equivalent energy release rate concept, Geq, all the curves were merged into a single curve in the form of Paris equation, da/dN = C(Geq)k. The Paris constants, C and k, determined from the experimental results were subsequently used in predicting the delamination propagation life under a standard mini FALSTAFF spectrum load sequence. Experiments were also conducted under the same spectrum load sequence with various reference loads to determine the propagation lives. A reasonably good correlation was observed between the predicted and experimental mode II delamination propagation life under spectrum loads.

Cohesive element analysis of fatigue delamination propagation in composite materials with improved crack tip tracking algorism

A novel approach for accurate delamination propagation simulation under high-cycle fatigue loading with cohesive element is proposed. Traditional bilinear traction–separation relationship of cohesive zone model is associated with Paris-law related fatigue damage evaluation. An improved crack tip tracking technique is proposed containing a non-local algorism for propagation direction and effective propagation length calculation. The major advantage of the non-local algorism is its applicability to two-dimensional propagation problems with non-orthogonal meshes used. Test cases are investigated for method verification and illustration including fatigue delamination of (i) Double Cantilever Beam, 4-point End Notch Flexure and Mixed Mode Bending specimen with orthogonal meshes, (ii) Double Cantilever Beam specimen with non-orthogonal meshes, and (iii) centrally-loaded circular plate.

Low‐cycle fatigue delamination initiation and propagation in fibre metal laminates

AbstractThe aim of this paper is to develop an elastic–plastic‐damage constitutive law and a tool for simulation of delamination initiation and propagation in fibre metal laminates (FMLs) under low‐cycle fatigue loading regime. In the previous studies, the significance of plasticity in delamination growth and modelling of FMLs was not considered. Hence, cohesive zone law that combines the damage evolution with plasticity is developed. The new fatigue damage model is implemented as user‐written subroutines that links with ansys based on the cohesive finite element method. The cohesive zone model constitutive law has been verified by modelling of the delaminated adhesively bonded aluminium joint under normal and shear loadings and compared with the available results in the literature. The developed procedure and tool have been used for the analyses of DCB and ENF specimens under uniform and variable loadings. The obtained results for progressive damage and delamination and stress–strain curves are discussed in this paper.

Mode I delamination fatigue crack growth in unidirectional fiber reinforced composites: Results from ESIS TC4 round-robins

Two round robins on mode I fatigue delamination propagation organized by Technical Committee 4 of the European Structural Integrity Society compared three unidirectional carbon fiber reinforced composites, one with thermoplastic (poly-ether-ether-ketone) and two with thermoset (epoxy) matrix tested at five laboratories. Different approaches for data evaluation and their effect on the in- and inter-laboratory scatter are discussed and compared. Calculated delamination rates da/dN and applied GImax from displacement controlled tests are sensitive to small scatter in the load signal, and, therefore, a new route to evaluate the crack growth rate from pairs of load and displacement data is presented.

Effect of data reduction and fiber-bridging on Mode I delamination characterization of unidirectional composites

Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam specimen is used to measure fracture toughness, GIc, and strain energy release rate, GImax, for delamination onset and growth in laminated composites under Mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy double-cantilevered beam specimens to evaluate the draft standard “Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional Fiber-Reinforced Polymer Matrix Composites.” Static results were used to generate a delamination resistance curve, GIR, for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic GImax level equal to 90% of the fracture toughness, GIc, to determine the delamination growth rate. Two different data reduction methods, a two-point and a seven-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the non-normalized results. Paris Law exponents were found to decrease by 5.7 to 47.6% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at three lower cyclic GImax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material.

Fatigue delamination behaviour of unidirectional carbon fibre/epoxy laminates reinforced by Z-Fiber® pinning

Z-Pin reinforced carbon-fibre epoxy laminates were tested under Mode I and Mode II conditions, both quasi-statically and in fatigue. Test procedures were adapted from existing standard or pre-standard tests. Samples containing 2% and 4% areal densities of carbon-fibre Z-pins (0.28 mm diameter) were compared with unpinned laminates. Quasi-static tests under displacement control yielded a dramatic increase of the apparent delamination resistance. Specimens with 2% pin density failed in Mode I at loads 170 N, equivalent to an apparent G IC of 2 kJ/m 2. Fatigue testing under load control showed that the presence of the through-thickness reinforcement slowed down fatigue delamination propagation.