Fatigue Delamination Behavior Research Articles

Characterisation of near pure mode II quasi-static and fatigue delamination behaviour of a carbon-glass interface using a pure-moment-loaded DCB test rig

This work covers the characterisation of delamination growth in a carbon-glass interface, representative of the one in a wind turbine blade. A pure-moment-loaded Double Cantilever Beam (DCB) test rig is used under quasi-static and fatigue loading. A new fatigue characterisation procedure is adopted by real-time controlling the prescribed energy release rate G. This procedure enables sweeping any desired G range at a user-defined rate independently of the compliance of the specimen. A finite element simulation using a cohesive zone model confirms the testing was performed at near pure mode II conditions. From the quasi-static tests, the onset and steady-state fracture toughnesses are characterised. Fibre bridging is observed, which is believed to explain the characterised R-curve effect. From the fatigue tests, Paris’ law is characterised in a wide Gmax range in each test. The characterisation of the quasi-static and fatigue fracture behaviour of the carbon-glass interface is thus successfully achieved.

A slope-based J-integral approach and advanced image processing for assessment of the cyclic fatigue delamination behavior of adhesive joints

A fatigue fracture mechanics methodology was developed and established, employing a slope-based J-integral approach combined with advanced image processing techniques. Adhesively bonded double cantilever beam (DCB) specimens were tested under constant displacement amplitude loading. The beam rotation was tracked by affixing a repetitive pattern on the DCB specimens and capturing images at the maximum displacement amplitude. Using a custom-developed image processing procedure, the beam rotation was deduced. To validate the methodology, DCB fatigue experiments were conducted at 23, 60 and 75 °C on aluminum adherends bonded with a structural 2-K epoxy adhesive. The J-based approach was compared with a conventional, compliance-based linear elastic fracture mechanics (LEFM) method. The epoxy was a rather brittle, high-modulus adhesive with a bond line thickness of 0.25 mm, resulting in predominantly linear elastic material behavior. By analyzing the images taken during fatigue testing, a stiffening effect of the steel load blocks was observed. Excluding pattern elements directly below the load block yielded the best agreement between J-integral and LEFM data. Both approaches were in excellent agreement within the investigated temperature range. The investigated adhesive exhibited a highly temperature-dependent behavior, which was associated with higher crack propagation rates and a lower fatigue threshold at 60 and 75 °C.

Mode I fatigue delamination growth behavior and model for multidirectional laminates with the same overall stiffness

In order to isolate the influence of ply orientation on the mode I fatigue delamination propagation behavior of carbon fiber reinforced composite laminates, multidirectional laminates with the same overall stiffness are designed while three different interfaces (0//0, 45//45, and 90//90), which can avoid the coupling effect of remote plies. Test results show that the fatigue delamination behavior is obviously affected by the interface angle. In addition, a novel and simple method for determining the fatigue delamination resistance is proposed. The measured fatigue delamination resistance is lower than the quasi-static one for all interfaces. The specimen with 45//45 interface exhibits more significant delamination resistance, whereas the delamination resistances of specimens with 0/0 and 90//90 interfaces are similar, which is consistent with the phenomenon of more bridging fibers accompanying in the delamination process of 45//45 interface. In order to consider the effect of fiber bridging and reduce the dispersion of the data, experimental data are further processed using a normalized model considering the effect of fiber bridging. The normalized results can be characterized by a single curve, suggesting that the normalized model is effective in describing the fatigue delamination behavior in the presence of fiber bridging.

Experimental characterization and cyclic cohesive zone modeling of mode I delamination growth in glass/epoxy composite laminates under high cycle fatigue

Large-scale fiber bridging makes a significant impact on fatigue delamination growth (FDG) in fiber-reinforced polymer composites. As bilinear cyclic cohesive zone modeling (CCZM) is not a promising modeling tool in this case, another solution is needed. The present study aims to evaluate a new easily-calibrating trilinear CCZM framework for modeling the FDG in glass/epoxy double cantilever beam (DCB) laminates with large-scale fiber bridging. First, mode I delamination growth, characterized by a significant R-curve effect, is experimentally determined under both quasi-static and high cycle fatigue regimes. Next, trilinear forms of the Turon et al. and the Kawashita-Hallett damage models are developed to predict such fatigue delamination behavior. Results show that the accuracy and efficiency of the proposed trilinear CCZM framework are enhanced by increasing the bridging length of the composite structure. Additionally, the Kawashita-Hallett trilinear model yields the most consistent and precise predictions with the least computational time.

Hygrothermal effects on fatigue delamination behavior in composite laminates

Fatigue delamination growth (FDG) is an important failure in composite structures during their long-term operations. Hygrothermal aging can have significant effects on interlaminar resistance. It is therefore really necessary to explore FDG behavior in composite laminates with hygrothermal aging. Dynamic mechanical thermal analysis (DMTA), mode I FDG experiments and fractographic examinations were conducted to fully investigate hygrothermal aging effects and the corresponding mechanisms on FDG behavior. The DMTA results indicated that environmental aging can induce obvious Tg decrease. Mode I experimental fatigue data interpreted via different Paris-type correlations demonstrated that: Bridging has obvious retardation effects on FDG behavior via the Paris interpretations; The modified Paris relation can well characterize the intrinsic FDG behavior around the crack front; The use of the two-parameter Paris-type relation can appropriately account for R-ratio effects, contributing to a master resistance curve in determining mode I FDG behavior. According to these interpretations, it can be concluded that hygrothermal aging can have adverse effects on mode I FDG behavior. SEM examinations demonstrated that moisture absorption can cause fibre/matrix debonding and resin matrix pores/voids in the composite. However, no obvious difference in damage mechanisms was identified in mode I fatigue delamination for composite with/without environmental conditioning. Both fibre/matrix debonding and matrix brittle fracture were identified on fatigue fracture surfaces. Accordingly, it was concluded that fibre/matrix interface and matrix degradation induced by water absorption were the main reasons for a faster mode I fatigue crack growth in environmental aged composite.

Mode II Delamination under Static and Fatigue Loading of Adhesive Joints in Composite Materials Exposed to Saline Environment.

This study investigates the fatigue delamination behavior of adhesive joints in epoxy carbon composite materials under Mode II fracture loading. The joints were characterized using the End-Notched Flexure (ENF) test, comprising adhesive joints formed by bonding two unidirectional carbon fiber epoxy matrix laminates with epoxy adhesive. These joints were subjected to different exposure periods (1, 2, 4, and 12 weeks) in a saline environment. Prior to dynamic fatigue testing, critical Mode II energy release rate values were determined through quasi-static tests, serving as a reference for subsequent fatigue characterization. This study aimed to comprehend how exposure duration to a saline environment affected the initial stage of fatigue delamination growth and employed a probabilistic model based on the Weibull distribution to analyze the experimental data. The results, gathered over a two-year experimental program, revealed varying behaviors in adhesive joint resistance to delamination based on exposure duration. A noteworthy reduction in fatigue strength capacity was observed, with fracture energies for infinite fatigue life reaching approximately 20% of their static loading capacity. This study sheds light on the deterioration of adhesive joints when exposed to a saline environment.

A proposal for similitude in characterizing fatigue delamination behavior with fibre bridging of carbon-fibre reinforced polymer composites

Methods based on fracture mechanics have been widely used in fatigue delamination growth (FDG) characterization of composite laminates. These methods are based on the similitude hypothesis. It is therefore important to have appropriate parameters to well represent the similitude, which is useful for fatigue delamination test standard development aimed by Technical Committee 4 of the European Structural Integrity Society (ESIS TC4) and the ISO/TC61/SC13. In the present study, discussions on similitude parameters for fibre-bridged fatigue delamination interpretation have been conducted via fatigue data with fibre bridging at different R-ratios. The results clearly demonstrate that the strain energy release rate (SERR) indeed applied around the crack front, rather than the total applied SERR, should be employed to represent the similitude for FDG interpretation with large-scale fibre bridging. Particularly, the use of Δ√Gtip can well determine fibre-bridged delamination behavior of a given R-ratio, but it is not valid for FDG at different R-ratios in accordance with the similitude principles. A new similitude parameter, in terms of both Δ√Gtip and the maximum SERR Gmax_tip, was therefore proposed to appropriately represent FDG behavior with fibre bridging at different R-ratios. This study can not only provide database, but also give important insights for the development of mode I fatigue delamination test standard of composite laminates.

Influence of curing pressure on the mode I static and fatigue delamination growth behavior of CFRP laminates

Curing conditions affect the mechanical properties of manufactured composite structures. However, the effect of curing pressure on the delamination behavior of laminates is still lack of investigation. This work studies the mode I static and fatigue delamination of carbon/epoxy laminates prepared at curing pressures of 1 MPa and 3 MPa. Delamination tests are conducted using double cantilever beam set-up. Test results show that more bridging fibers present in the laminates cured at 3 MPa, with more significant R-curve behavior, higher values of steady-state fracture toughness and maximum bridging stress than those in the laminates cured at 1 MPa. SEM images show that the laminates cured at 3 MPa mainly occur interlaminar damage while the major failure mechanism in the laminates cured at 1 MPa is the fiber/matrix debonding. A normalized model is proposed to describe the fatigue delamination behavior of laminates with fiber bridging effect. It shows that all fatigue data can be characterized by a single linear curve and curing pressure affects the slope of the linear curve. Furthermore, simulation for the delamination growth is conducted based on a tri-linear cohesive zone model. Good agreements between experimental and predicted load–displacement responses are achieved, illustrating the applicability of the established FE model.

Fatigue delamination behavior in composite laminates at different stress ratios and temperatures

This study provides an investigation on mode I fatigue delamination growth (FDG) with fibre bridging at different R-ratios and temperatures in carbon-fibre reinforced polymer composites. FDG experiments were first conducted at different temperatures of R-ratios 0.1 and 0.5 via unidirectional double cantilever beam (DCB) specimens. A fatigue model, employing both the strain energy release rate (SERR) range and the maximum SERR around crack front as similitude parameter, was proposed to interpret FDG behavior. The use of this model can collapse FDG data with fibre bridging at different R-ratios into one master curve, obeying well with the similitude principles. Accordingly, it was found that FDG can accelerate with elevated temperature, but decrease at sub-zero temperature. Furthermore, there are strong correlations between the fatigue model parameters and temperature using this model in FDG interpretations. Taking these correlations into account can extend the model to accurately predict FDG behavior of other temperatures. Fractographic examinations demonstrated that temperature has effects on the FDG damage mechanisms. Both fibre/matrix interfacial debonding and matrix brittle failure were observed in FDG of −40℃. Fibre/matrix interfacial debonding becomes the dominant failure in FDG of RT and 80℃. No obvious difference on the fracture morphology was identified for FDG at different R-ratios of a given temperature.

Methodology for local ageing and damage development characterization of solar glass/encapsulant interfaces under superimposed fatigue stresses and environmental influences

In this paper, a novel fracture mechanics test-setup was developed, implemented and applied for assessment of the fatigue delamination behavior of glass/encapsulant laminates under superimposed mechanical stresses and environmental influences. Double cantilever beam (DCB) specimens with glass substrates were prepared using ethylene vinyl-acetate copolymer (EVA) or polyolefin elastomer (POE) encapsulants. Displacement-controlled fatigue tests were conducted at 80 °C under dry (5% rh) and humid (80% rh) conditions. The damage zone ahead of the delamination crack tip was monitored with a topside mounted camera system. A superior delamination resistance of POE based double glass laminates was ascertained. Under less service-relevant, hot-dry conditions, the glass laminates exhibited a mm-sized damage process zone ahead of the crack tip, in which void nucleation and fibril formation took place. While the POE laminate failed also in a cohesive manner within the encapsulant at 80 °C and 80% rh (i.e., hot-humid), the crack was propagating in the glass/EVA laminate close to the interface in a brittle manner. Local ageing mechanisms were confirmed for the fractured EVA laminates by XPS. The superposition of mechanical and thermal stresses along with humidity resulted in sodium ion migration presumably promoting deacetylation of EVA. Interestingly, the analyzed degradation products were in good agreement with data obtained for damp heat aged glass/foil PV modules characterized subsequently by monotonic delamination testing. The main advantage of the novel environmental fracture mechanics methodology is the high acceleration factor (50 to 100x faster) compared to conventional ageing and delamination testing of PV modules. • Novel concept for highly accelerated delamination testing of PV module interfaces. • Superimposed mechanical fatigue stresses and environmental influences. • Service-relevance confirmed for glass/EVA interface under hot-moist conditions. • Potential acceleration factor of 50–100 due to local ageing effects. • High-throughput screening of novel material formulations & interfaces.

Study of the fatigue delamination behaviour of adhesive joints in carbon fibre reinforced epoxy composites, influence of the period of exposure to saline environment

This work analyses the fatigue delamination and fracture stress behaviour under mode I of adhesive joints made on an epoxy matrix composite material with unidirectional carbon fibre reinforcement and a commercial epoxy-based adhesive. DCB type tests (for mode I) were used with the aim to quantify the influence of the period of exposure to a degradation process in a salt spray chamber, to which the tested samples were subjected, on their fatigue behaviour. For this purpose and after a previous static characterisation of the material in which the critical values of the energy release rate for different exposure periods were determined, the levels of the energy release rate to be applied in the fatigue tests and the exposure periods to be considered (no exposure, exposure during one week and twelve weeks) and a ratio of fatigue stress levels of R = Gmin/Gmax = 0.1 were defined. From this experimental data, the G-N fatigue initiation curves and the G-da/dN growth curves were obtained. The experimental data obtained, in the fatigue initiation phase of the delamination process, have been treated by means of a probabilistic model based on a Weibull distribution, the application of models of these characteristics has allowed a better interpretation of the experimental results obtained. The most relevant result of the work is that, in general, the fatigue limits obtained for the adhesive joint, under mode I fracture, when subjected to a degradation process in a saline environment, do not translate into a relevant loss of its resistance capacity against this fatigue delamination phenomenon, in its initiation phase. On the other hand, the crack growth rates of the material subjected to different periods of exposure to a saline environment are similar and higher than those obtained for the material without exposure.

Mode II fatigue delamination behaviour of composite multidirectional laminates and the stress ratio effect

Shear-dominated delamination is among one of the most common damages types. Deep understanding of the mode II fatigue delamination growth (FDG) behaviour is critically important for the damage tolerance design of composite laminates. Researchers have paid much attention to mode II FDG behaviour in unidirectional laminates and its stress ratio effect. However, the FDG behaviour in the widely used multidirectional laminates with effects of fiber bridging is still lack of studies. This work investigates FDG behaviour of multidirectional laminates under three kinds of stress ratio by using end-notch flexure set-up. Corrected beam theory with effective crack length is applied for calculating strain energy release rate (SERR), which can avoid visual observation for the delamination length. The R-curve behaviour of fatigue delamination has been observed. In addition, a FDG model has been proposed to characterize the combined effects of fiber bridging and stress ratio on the mode II FDG behaviour. It is found that all fatigue data collapse into one single master curve. The proposed model is also validated by data from public literature results.

Fatigue delamination behaviour of carbon fibre/epoxy composites interleaved with thermoplastic veils

Interleaving thermoplastic veils has proved to enhance the interlaminar fracture toughness of carbon fibre/epoxy composites under static loading conditions. However, the fatigue delamination behaviour has yet to be investigated. Herein, meltable Polyamide-12 (PA) veils and non-meltable Polyphenylene-sulphide (PPS) veils were used for interlay toughening of unidirectional (UD) and non-crimp fabric (NCF) laminates that were manufactured using a prepreg process and resin transfer moulding process, respectively. The results of Mode-I fatigue delamination tests demonstrated a significant improvement in the fatigue life of the laminates due to interleaving. Additionally, the fatigue resistance energy has been maximumly increased by 143% and 190% for the UD and NCF laminates, respectively. The microscopy analysis revealed that the toughening mechanisms of thermoplastic veils were affected by the form of the thermoplastic veils in the laminates (melted or non-melted), the fracture mechanisms of the reference laminates and the adhesion/miscibility between the thermoplastic veils and the epoxy.

A new model characterizing the fatigue delamination growth in DCB laminates with combined effects of fiber bridging and stress ratio

For composite multidirectional laminates widely used in engineering, there is still no effective model to characterize the fatigue delamination behavior with the combined effects of fiber bridging and stress ratio. A new fatigue delamination model was thus proposed in this study to deal with this problem. The model was established based on the fatigue driving force and fatigue delamination resistance, which characterized the effects of stress ratio and fiber bridging, respectively. A series of mode I fatigue delamination tests on composite multidirectional laminates were conducted under various stress ratios (0.1, 0.3 and 0.5) and different pre-crack lengths. Both the experimental results of this study and public fatigue data from laminates with other materials and various interfaces showed that a single master curve with a narrow band can be obtained by using the new model for all fatigue data under different stress ratios and fiber bridging effects. In addition, the determined model parameters obtained from fatigue data under specific stress ratios were used for predicting the fatigue delamination behavior under other stress ratios. Good agreements between the predictions and experimental results can be achieved, which further indicated the model’s applicability.

Influence of the Test Method on the Characterization of the Fatigue Delamination Behavior of a Composite Material under Mixed Mode I/II Fracture.

Composite materials manufactured by overlapping plies with certain specific geometries are likely to lose part of their strength due to the presence of internally delaminated regions. The aim of this paper is to experimentally evaluate the generation and propagation of these interlaminar cracks in a carbon-epoxy composite material subjected to fatigue loading under mixed mode I/II fracture. Two different test methods were used for this purpose: The standardized mixed-mode bending (MMB) test and the asymmetric double cantilever beam (ADCB) test, with the goal of exploring the viability of the ADCB test as a simpler alternative to perform than the MMB test, especially in fatigue testing. With this aim in mind and after prior static characterization of the material in which the critical values of the energy release rate were determined under both test methods, the levels of the energy release rate to be applied in fatigue tests were defined for two mode mixity ratios, GII/Gc = 0.2 and 0.4 (0.34 ADCB), and a fatigue loading ratio, R = Gmin/Gmax = 0.1. The G-N fatigue onset curves were subsequently obtained from these experimental data. The most relevant result of the study is that the fatigue limits obtained using the MMB method are generally more conservative than those obtained via the ADCB method.

A novel understanding of the normalized fatigue delamination model for composite multidirectional laminates

Normalized fatigue delamination models have been widely applied by researchers in the characterization of the fatigue delamination behavior of composite laminates. However, the inherent mechanism of this normalization method has not been explored. This study aims to present a physical understanding on the normalized fatigue delamination model from a viewpoint of energy. The fatigue delamination behavior is considered to be governed by the driving force and delamination resistance, and based on this principle the physical mechanism of the fatigue delamination is studied. A new physics-based normalized fatigue delamination model is proposed in this paper. In order to experimentally validate the proposed fatigue delamination model, mode I fatigue delamination tests are performed on double cantilever beam specimens to obtain the experimental data with different amounts of the fiber bridging. The results show that the normalized model is suitable to accurately characterize the fatigue delamination behavior of the composite laminates by using a single master curve. The master curve is finally employed as a standard approach to predict the fatigue results. Good agreement between the predicted and the experimental results is achieved, therefore it approves the applicability of the proposed fatigue delamination model in characterizing the fatigue delamination growth behavior.

Use of a LHFB Device for Testing Mode III in a Composite Laminate.

The present paper studies the fatigue delamination behaviour of an epoxy/carbon composite material under mode III loading using a longitudinal half fixed beam (LHFB) device initially designed for mode III static tests of composite materials formed by the stacking of plies. For this purpose, a series of tests was carried out at different levels of loading representative of the fatigue behaviour of the material, from the crack onset phase through the delamination phase to final fracture. The experimental results were treated statistically, obtaining the values of the fatigue limit for probabilities of fracture of 5% and 50%. Finally, a fractographic analysis of the fracture surfaces was performed which allowed us to identify the same characteristic patterns of static mode III fracture, namely broken fibres, cusps and saw-teeth, in addition to a new morphology consisting of the formation of agglomerations of resin produced by the friction between the lips of the specimen in the fracture zone that point to dynamic mode III fracture. These agglomerations eventually crack and become detached from the fibres, leaving these free of resin.

An insight into three approaches for determining fatigue delamination resistance in DCB tests on composite laminates

Fatigue delamination resistance Gcf, defined as the strain energy release rate during the fatigue crack growth at a certain crack length, was proposed in literature to characterize the real changing resistance of composite laminates against the fatigue delamination growth. Determining Gcf is important for an accurate interpretation of the fatigue delamination behavior when the normalization method is used to evaluate the fatigue delamination growth rate. In this paper, a re-fatigue loading approach is firstly proposed for judging the similarity/difference of the damage states between the fatigue and static delaminations. Combining with other two approaches established in previous studies, a systematic experimental investigation on these approaches is performed by designed tests. The researches show the damage state of the fatigue and static delaminations in composite laminates can be the same or different, depending on the pre-crack location and first-applied fatigue loading, which is the fatigue loading applied to achieve the desired pre-crack length. The re-fatigue approach is suitable for judging the similarity/difference of the damage states between the fatigue and static delaminations. The Gcf can be further effectively determined by the compliance approach. However, the re-static loading approach is not applicable due to the difficulty in identifying the re-growing point.

A new testing device to simultaneously measure the mode I fatigue delamination behavior of a batch of specimens

Determining fatigue onset and crack propagation curves for delamination in composites requires time-consuming test campaigns. This work presents a new test rig and its associated methodology to test several mode I specimens simultaneously in a universal testing machine. The novelty of the solution relies on being able to test several specimens at different levels of load ratio and energy release rates in a single run. The performance of the system is exemplified by measuring the complete onset and crack growth rate curves of a batch of 12 carbon fibre reinforced specimens in only two test runs.

Examining the application of the Hartman-Schijve equation to the analysis of cyclic fatigue fracture of polymer-matrix composites

Use of fiber-reinforced polymer-matrix (FRP) composites in structural design requires material data, in particular on their cyclic fatigue delamination behavior under various modes of loading, e.g., tensile opening (Mode I) or in-plane shear (Mode II) and the associated threshold values. There are no standard test methods for characterizing delamination propagation yet, except one procedure defining the determination of fatigue delamination onset. Data analysis and interpretation is often based on the expertise obtained from cyclic fatigue testing of metals and alloys and remains empirical. In the present contribution, the applicability of the modified Hartman-Schijve equation (originally developed for metals and alloys) to the characterization of fatigue delamination behavior of FRP under cyclic mode I loading will be discussed. The main focus is on interpreting information on the fatigue threshold and its scatter in FRP composites.