Characterization of crack propagation in mode I delamination of multidirectional CFRP laminates

During the experimental characterization of the mode I interlaminar fracture toughness of multidirectional composite laminates, the crack tends to migrate from the propagation plane (crack jumping) invalidating the tests. In an earlier numerical study [9], we reported that this problem could be eliminated by choosing the appropriate bending stiffness of the beam arms.     

Monitoring of delamination onset and growth during Mode I and Mode II interlaminar fracture tests using guided waves

A delamination monitoring method was proposed to characterize Mode I and Mode II delamination onset in carbon fiber/epoxy (CF/EP) composite laminates through interrogation of guided waves activated and captured using piezoelectric actuators and sensors in a pitch–catch configuration. Mode I and Mode II interlaminar fracture tests were conducted using double cantilever beam (DCB) and end notch flexure (ENF) specimens to evaluate the proposed method. The changes in wave propagation velocity and wave magnitude (or attenuation), and the degree of waveform similarity between excitation and response signals, were calculated as delamination-sensitive wave parameters and plotted versus displacement recorded using a materials testing system. The kink points determined from wave parameter–displacement curves agreed well with the deviation from linearity (NL), visual observation (VIS) and maximum load (Max) points, which are often used in conventional methods for determining interlaminar fracture toughness. The propagation characteristics of the A₀ wave mode in a low frequency range were demonstrated to have high sensitivity to Mode I and in particular Mode II delamination onset in CF/EP composite laminates. It was concluded that the guided waves propagating in the DCB and ENF specimens were capable of determining Mode I and Mode II interlaminar fracture toughness, complementing current practices based on visual inspection or trivial interrogation using load–displacement curve alone.     

Double cantilever beam testing of multidirectional laminates

Several difficulties in the double cantilever beam (DCB) tests of multidirectional laminates often prevent valid measurements of the mode I critical strain energy release rate G_Ic. In this paper, several DCB specimens were analysed with 3D finite element models. The results showed that the undesired effects of residual stresses and of mode-mixity can be minimised. An interlaminar stress based fracture criterion predicts that the G_Ic of multidirectional specimens is typically 10–40% higher than the G_Ic of unidirectional [0°]_n laminates. This agrees with the few valid experimental data available.     

Fracture toughness improvement of CFRP laminates by dispersion of cup-stacked carbon nanotubes

Several techniques are introduced to enhance the interlaminar fracture toughness of CFRP laminates using cup-stacked carbon nanotubes (CSCNTs). Prepared CSCNT-dispersed CFRP laminates are subject to Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests in order to obtain mode-I and mode-II interlaminar fracture toughness. The measured fracture toughnesses are compared to that of CFRP laminates without CSCNT to evaluate the effectiveness of CSCNT dispersion for the improvement of fracture toughness. All CSCNT-dispersed CFRP laminates exhibit higher fracture toughness, and specifically, CSCNT-dispersed CFRP laminates with thin epoxy interlayers containing short CSCNTs have three times higher fracture toughness than CFRP laminates without CSCNT. SEM observation of fracture surfaces is also conducted to investigate the mechanisms of fracture toughness improvement. Crack deflection mechanism is recognized in the CSCNT-dispersed CFRP laminates, which is considered to contribute the enhancement of interlaminar fracture toughness.     

双悬臂梁试件裂纹动态扩展的准静态数值分析

本文采用双悬臂梁（DCB）试件研究了复合材料层合板层间插入韧性胶膜（Interleaf）层的Ⅰ型断裂行为。试验结果表明,含和不含Interleaf层试件分别呈现脆性非稳态和脆性稳态分层扩展特性。针对非稳定裂纹扩展问题,依据动态断裂力学中应变能释放率与动能变化率的关系,提出了以断裂韧性值G_IC变化来抵消动能变化对裂纹扩展过程影响的准静态分析方法,根据试验中裂纹扩展的韧性变化,推导出适用于准静态裂纹扩展模拟的等效韧性G_IC*,利用ABAQUS平台和虚裂纹闭合技术（VCCT）建立了三维有限元计算模型;实现了从起裂到止裂的整个裂纹动态扩展过程的数值模拟,揭示了非稳定裂纹扩展过程中一些复杂的力学现象。      

Influence of intralaminar cracking on the apparent interlaminar mode I fracture toughness of cross‐ply laminates

One of the major difficulties in interlaminar fracture tests of multidirectional laminates is the high tendency for intralaminar cracking and the resulting wavy crack propagation. Experimental work showed that this occurred in double cantilever beam (DCB) tests of cross‐ply laminates having a starter crack on a 0°/90° interface. Moreover, under steady‐state propagation conditions, the apparent values of the critical strain energy release rate G_Ic were two times higher than those of 0°/0° specimens. In this paper, a finite‐element‐based progressive damage model was used to simulate crack propagation in cross‐ply specimens. The results showed that transverse cracking alone cannot be responsible for the above difference of G_Ic values. Therefore, the higher propagation G_Ic values for cross‐plies must be attributed to the more extensive fibre bridging observed and to plastic deformations of the 90° interfacial ply.     

Interpreting the stress ratio effect on delamination growth in composite laminates using the concept of fatigue fracture toughness

This paper provides a study on fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be interface configuration independent but significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface roughness is observed to be similar in different interfaces at the same stress ratio. But it is obviously more rough for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.     

Prestandardization study on mode I interlaminar fracture toughness test for CFRP in Japan

This paper summarizes results from a series of interlaboratory round robin tests (RRTs) performed in order to establish a JIS standard for mode I interlaminar fracture toughness test using double cantilever beam (DCB) specimens. For the case of unidirectional laminates, brittle and toughened CF/epoxy, and CF/PEEK systems were used. Only a brittle CF/epoxy system was used for woven laminates. The round robin tests were conducted with two main aims: first, to examine the influence of starter films and the precracking condition on the initial mode I fracture toughness values; and second, to establish the definition of initial fracture toughness. Polyimide starter films stuck to the epoxy matrix, and caused unstable crack growth from starter films. Comparison of the tests with and without mode I precracks from starter films indicated that tests with precracks gave lower values of initial fracture toughness. The definition of initial fracture toughness values was discussed, based on the reproducibility. A 5% offset point was recommended as the initial fracture toughness from the RRT results. The influence of loading apparatus, data reduction methods, etc. was also discussed.     

Interlaminar fracture (model II) of commingled yarn-based GF/PP composites

A 5050 wt % mixture of commingled glass/polypropylene fibre system was selected to study the correlations between the morphological details, mode II interlaminar fracture toughness and corresponding failure mechanisms. Mode II interlaminar fracture tests were performed by using the end-notched flexure test procedure. Compared to conventional composite laminates, mode II interlaminar crack extension in these commingled yarn-based composites was very stable, and extensive fibre nesting occurred along the main crack plane. Crack jumping and non-broken matrix links were observed.R-curve behaviour for these materials was identified and the toughness for initiation was much lower than that for propagation. Compared to mode I interlaminar fracture toughness, similar trends in effects of cooling rates and isothermal crystallizations on mode II interlaminar fracture toughness were observed. However, the effects were not as significant as those found for mode I interlaminar fracture toughness.Alexander von Humboldt Fellow.     

Influence of silane coupling agents on interlaminar fracture in glass fibre fabric reinforced unsaturated polyester laminates

The relationship between the adhesive properties of the interphase of glass fibre/resin and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate (GFFL) was studied. The Mode I interlaminar fracture toughness of GFFL was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of GFFLs which have two silane coupling agents and three concentration finishes is discussed on the basis of interlaminar fracture toughness. The crack propagation behaviour of DCB testing was mainly divided into stable and unstable manners. The fracture toughness and the crack propagation behaviour were dependent on the types and concentration of silane coupling agents.     

DCB test simulation of stitched CFRP laminates using interlaminar tension test results

A simulation model for the delamination extension of stitched CFRP laminates and 3-D orthogonal interlocked fabric composites (3-D OIFC) has been developed using a 2-D finite element method incorporating interlaminar tension test results to simulate the experimental results of their DCB tests. The mechanical properties of through-the-thickness fiber were determined from the results of interlaminar tension tests in which the specimen included only one through-the-thickness yarn. The fracture phenomena around the through-the-thickness thread, such as debonding from the in-plane layer, slack absorption, fiber bridging, and the pull-out of broken threads from the in-plane layers, are also introduced into the FEM model. The present FEM simulation results were compared to DCB test results for certain stitched laminates and a 3-D OIFC, and the simulation results showed good agreement with the experimental results of DCB tests, including the load–displacement curve and Mode I strain energy release rate (GI). While it was difficult to estimate GI accurately when the DCB test specimen included different types of z-fiber fracture modes, the present model of FEM analysis can simulate the experimental results of DCB tests of stitched laminates and 3-D OIFC. It is suggested that the GI of CFRP with arbitrary z-fiber densities can be predicted by using this FEM analysis model together with interlaminar tension test results.     

Improvement of interlaminar mechanical properties of CFRP laminates using VGCF

In order to improve the interlaminar mechanical properties of CFRP laminates, hybrid CFRP/VGCF laminates have been fabricated by using a newly-developed method, i.e., powder method, where the powder of vapor grown carbon fiber (VGCF) is added at the mid-plane of [0°/0°]₁₄ CFRP laminates. Experimental results of double cantilever beam (DCB) tests indicate the improvement on the interlaminar mechanical properties of Mode-I fracture behavior with much higher critical load P_C and fracture toughness G_IC with VGCF interlayer. Crack propagation and fracture surface have also been observed to interpret this improvement mechanism. Moreover, based on experimental G_IC, numerical simulations using finite element method (FEM) with cohesive elements have been carried out to analyze the delamination propagation. The interlaminar tensile strength of hybrid CFRP/VGCF laminates, which is obtained by matching the numerical load–COD (crack opening displacement) curves to experimental ones, is higher than that of base CFRP laminates.     

碳纤维增强树脂复合材料-热成型钢超混杂层合板层间力学性能

通过双悬臂梁试验(DCB)研究了金属表面处理和界面插层协同作用对碳纤维增强树脂复合材料(CFRP)-热成型钢超混杂层合板层间力学性能的影响。试验结果表明,采用金属表面处理与界面插层协同增韧方案,可以极大地提升层合板的I型层间断裂韧性。其中,喷砂/界面胶膜插层试件(GB36#/AF)的I型层间断裂韧性相比于脱脂试件提高了343%;喷砂/界面纯树脂插层试件(GB36#/EP)相比于脱脂试件,其Ⅰ型层间断裂韧性提高了129%。并基于内聚区模型对CFRP-热成型钢超混杂层合板分层失效进行了有限元模拟。最后借助激光共聚焦扫描显微镜(LSM)、接触角测量仪(CAG)、扫描电子显微镜(SEM)等对热成型钢表面形貌和试件的断裂面进行了表征并揭示了层间增韧的机制。      

A parametric study on the design of stitched laminated DCB specimens

Through-the-thickness stitching has been shown to have the capacity of improving the poor interlaminar toughness of laminated composites for many years. Double cantilever beam (DCB) testing has been used to assess the enhanced delamination toughness of stitched laminates. To avoid the premature flexure failure of delaminated substrate beams, thick tabs are bonded to either side of the standard DCB specimens. Consequently, higher opening load can be applied to the stitched tabbed DCB (TDCB) specimens to propagate the delamination crack due to the enlarged bending stiffness and strength. This creates a significant bending of the loading pin, which needs to be accounted for in the numerical analysis. In this paper, a parametric analysis was conducted to investigate the effect of reinforcing tab thickness on the measured and predicted delamination toughness.     

织物增强复合材料层合板I型层间断裂特性

采用双悬臂梁(DCB)试验测试和研究了织物增强复合材料层合板的层间断裂韧性与断裂行为。为了评价测试温度和试样几何尺寸的变化对层间断裂韧性的影响,分别在室温(RT)和液氮温度(77K)条件下对不同尺寸的试样进行了双悬臂梁试验。采用扫描电镜对分层断裂面进行了观察,分析和验证了层间断裂特性。      

Mixed Bending-Tension (MBT) test for mode I interlaminar and intralaminar fracture

A new Mixed Bending-Tension (MBT) test is proposed for mode I fracture of laminated composites. The MBT specimen consists of a relatively small pre-cracked laminate adhesively bonded to pin-loaded steel beams. This design reduces significantly the bending stresses that prevent successful application of DCB tests to certain laminates. The MBT was here applied to carbon/epoxy unidirectional [0°]₂₆ and [90°]₂₆ laminates with starter delaminations. Interlaminar initiation G_IC values of [0°]₂₆ laminates agreed well with previous DCB test results, while [90°]₂₆ laminates exhibited 50% higher values. Significant lengths of fairly planar intralaminar crack propagation were seen in the latter laminates. The results showed a fibre bridging related R-curve, which was more pronounced in [0°]₂₆ laminates. The consistency of the present results indicates that the MBT opens new possibilities for the interlaminar and intralaminar mode I fracture.     

The Climbing Drum Peel Test: An alternative to the Double Cantilever Beam for the determination of fracture toughness of monolithic laminates

The experimental determination of the fracture toughness of inter-ply interfaces in monolithic composite specimens is far from trivial: even in standard test methods such as the Double Cantilever Beam (DCB), some precautions must be taken in the choice of the test configurations and in the post-treatment of the experimental results. Furthermore, non standard measurements such as the crack tip position during propagation are generally required. In this paper, we investigate an alternative test configuration, the Climbing Drum Peel (CDP) test, classically used in the ‘adhesives’ community. The adaptation of the CDP specimen configurations to the testing of monolithic composites is discussed and a systematic comparison is carried out between the CDP and the DCB tests in terms of global and local indicators of the crack propagation behavior.     

缝合复合材料II型层间断裂特性研究

分别采用测量ENF试样加载点位移与测量其端部剪切位移CSD(Crack Shear Displacement)的试验方法,研究了缝合复合材料层合板的II型层间断裂韧性以及缝合密度,缝合线的直径等缝合参数对于缝合复合材料层合板II型层间断裂韧性和分层模式的影响。结果表明,缝合降低了层合板初始分层韧性G_IIi,但对于分层的扩展有良好的抑制作用。缝合参数对此有较大影响。      

Composite toughness enhancement with interlaminar reinforcement

An experimental investigation of a newly proposed through-thickness reinforcement approach aimed to increase interlaminar toughness of laminated composites is presented. The approach alters conventional methods of creating three-dimensional fiber-reinforced polymer composites in that the reinforcing element is embedded into the host laminate after it has been cured. The resulting composite is shown to possess the benefits of a uniform surface quality and consolidation of the original unreinforced laminate. This technique was found to be highly effective in suppressing the damage propagation in delamination double-cantilever beam (DCB) test samples under mode I loading conditions. Pullout testing of a single reinforcing element was carried out to understand the bridging mechanics responsible for the improved interlaminar strength of reinforced laminate and stabilization and/or arrest of delamination crack propagation. The mode I interlaminar fracture of reinforced DCB samples was modeled using two-dimensional cohesive finite-element scheme to support interpretation of the experiments.     

Mode I delamination behaviour of short fibre reinforced carbon fibre/epoxy composites following environmental conditioning

Carbon fibre laminates based on Newport Adhesives NCT-301 uniaxial pre preg were prepared for Mode I DCB testing. Polyamide and Polyolefin web materials supplied by Spunfab Ltd and Toyobos’ Zylon HM fibres were applied to the interlaminar region to enhance fracture toughness. Three of each specimen type were “treated”, subjected to immersion in 95 °C water for a period of 600 h. Two “untreated” control specimens of each type were aged at 21 °C and 40-95% relative humidity. Specimen weight gains in the order of 2% were found to occur in all immersed samples. Fracture toughness increases were evidenced in all treated samples with the exception of the Polyamide Web sample. Average G_Ic increases in treated samples were 250–300%. Untreated samples were found to have exclusively interlaminar crack propagation. Untreated samples containing Polyamide or Polyolefin Web reinforcement displayed increased fracture toughnesses. These toughness increases were attributed to improved interlaminar bonding.