Mechanisms for rate effects on interlaminar fracture toughness of carbon/epoxy and carbon/PEEK composites

The objective of this study was to investigate strain-rate dependent energy absorption mechanisms during interlaminar fracture of thermosetting (epoxy) and thermoplastic (PEEK) uni directional carbon fibre (CF) composites. A simple model addressing the translation of matrix toughness to mode I and mode II interlaminar toughness of the composite is presented, in conjunction with a fractographic examination of the fracture surfaces and the fracture process. The observed rate dependency of composite fracture toughness is attributed to the rate dependent toughness of the viscoelastic matrix and the size of the process zone around the crack tip. Other important factors identified are the roughness of the fracture surface and fibre bridging.     

The influence of microstructure on the failure behaviour of PEEK

In this study, different molecular weight PEEK materials were used to determine the effect of spherulite size on fracture. Melt processing of the PEEK at different temperatures produced samples of different average spherulite size. A permanganic etching technique was used to reveal the spherulites. It was found that for low molecular weight 150P PEEK, the spherulite size increased with melt processing temperature; but, for the higher molecular weight 450G PEEK, the spherulite size remained approximately constant. Also, the average spherulite size was markedly lower for the material of higher molecular weight. The failure behaviour of these samples was studied using a compact tension test. It was found that the fracture toughness of PEEK varied with processing temperature. Also, the average spherulite size of a PEEK material had a profound influence on the fracture mechanism.     

The influence of microstructure on the failure behaviour of PEEK

In this study, different molecular weight PEEK materials were used to determine the effect of spherulite size on fracture. Melt processing of the PEEK at different temperatures produced samples of different average spherulite size. A permanganic etching technique was used to reveal the spherulites. It was found that for low molecular weight 150P PEEK, the spherulite size increased with melt processing temperature; but, for the higher molecular weight 450G PEEK, the spherulite size remained approximately constant. Also, the average spherulite size was markedly lower for the material of higher molecular weight. The failure behaviour of these samples was studied using a compact tension test. It was found that the fracture toughness of PEEK varied with processing temperature. Also, the average spherulite size of this material had a profound influence on the fracture mechanism.     

Cooling rate influences in carbon fibre/PEEK composites. Part II: interlaminar fracture toughness

A study has been made of the effect of cooling rate on interlaminar fracture toughness of unidirectional carbon fibre–polyetheretherketone (PEEK) matrix composites. It is shown that the propagation values of both mode I and II propagation interlaminar fracture toughness increased with increasing cooling rate towards a saturation level for the range of cooling rate studied. The cooling rate dependency of the composite interlaminar fracture toughness is the result of complex interactions between two important properties, namely the matrix ductility and fibre–matrix interface bond strength. These two properties varied totally in an opposite manner against cooling rate through its effect on crystallinity: matrix ductility varied directly proportional to cooling rate while the converse is true for interface bond strength. The extent of plastic deformation of PEEK matrix contributed a predominant part to composite toughness, while an adequate interface bond is required to allow matrix deformation to take place to a full degree. A practical implication is that these two properties need to be optimised using an appropriate cooling rate to produce composites possessing high interlaminar fracture resistance.     

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.     

Influence of cooling rate on interlaminar fracture properties of unidirectional commingled CF/PEEK composites

The influence of cooling rates on the mechanical property profile (transverse flexure properties and modes-I and -II interlaminar fracture toughness) has been investigated for unidirectional commingled CF/PEEK composites. A laboratory hot press with a steel mould was used to process the composites at 400°C for 60 min, at an applied pressure of 1 MPa. Cooling rates from fast (quenching in oil) to slow (hot press cooling) were achieved at ambient pressure. The results indicate that different matrix morphology was found at different cooling conditions, although deconsolidation occurred in the CF/PEEK composites during cooling. When the cooling rate was shifted from slow to fast, consolidation quality of the CF/PEEK composites was improved. The resulting effect of the consolidation quality and cooling rates on the mechanical property profile of commingled CF/PEEK composites is presented. It was found that the effect of the cooling rate on the mechanical property profile of the commingled CF/PEEK composites could not be isolated from the consolidation quality.     

PEEK复合材料用碳纤维上浆剂研究进展EI北大核心CSCD

热塑性聚醚醚酮(PEEK)复合材料具有优异的断裂韧性、抗冲击性能、耐疲劳性能,广泛应用于航空航天领域。上浆剂作为碳纤维的核心配套产品,对复合材料界面有重要影响。受分解温度限制,传统热固性碳纤维上浆剂难以满足PEEK复合材料使用,制约高性能PEEK复合材料的研制和应用,因此研制匹配PEEK复合材料的碳纤维上浆剂具有重要意义。本文分析了PEEK复合材料界面特性及上浆剂作用机理;重点介绍了改性PEEK、聚酰亚胺前驱体、聚醚酰亚胺等类型上浆剂的研究进展和成果,并对不同体系上浆剂进行分析总结;最后对PEEK复合材料用碳纤维上浆剂的研制提出建议,对上浆剂绿色环保多功能化趋势进行了展望。     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.     

Some experimental findings in compression-after-impact (CAI) tests of CF/PEEK (APC-2) and conventional CF/epoxy flat plates

Compression-after-impact (CAI) tests have been conducted for quasi-isotropic thick plates with 48 plies by using the NASA method and on plates with 32 plies by using the SACMA method. Specimens are made of CF/PEEK (APC-2) and conventional CF/epoxy for the NASA plates and CF/epoxy for the SACMA plates. In the NASA CAI tests, the sequence of delamination buckling and its propagation is clearly revealed through various experimental techniques. One major technique is moiré topography, and the other is thermo-mechanical stress analysis with a high-accuracy infrared sensor. The arrest of delamination propagation just before catastrophic failure due to high fracture toughness is clearly captured by the moiré camera. This behavior provides good CAI values of CF/PEEK. The initial buckling properties of the delaminated region by the impact are then extensively discussed. Numerical predictions of initial buckling stress have been obtained by modelled geometry of the delaminated region simplified from its precise structure clarified by ultrasonic C-scanning. They agree fairly well with the experimental results. The in-plane stress distribution in the delaminated region before initial buckling is measured by an infrared stress graphic system. This compared favorably with finite element predictions. Two types of symmetric buckling modes with respect to the central plate surface, twin and single peak ones, are experimentally captured.     

高温热塑性树脂基复合材料的断裂行为

本文研究了碳纤维增强高温热塑性塑料(PEEK)基复合材料的断裂行为.发现其强度极限没缺口断裂应力均高于同种纤维增强的热固性(环氧)树脂基材料,并且随0°叠层含量的增加,二者之间的差距加大.另外,前者比后者缺口敏感性低,断裂韧性高.      

聚醚醚酮增韧改性环氧树脂

采用共混法用聚醚醚酮(PEEK)改性环氧树脂(EP),借助差示扫描量热分析(DSC)确定了环氧树脂的固化工艺,测试了共混体系的工艺性能,研究了聚醚醚酮含量对环氧树脂力学性能的影响.借助扫描电子显微镜(SEM)对材料断裂面的形态结构进行了分析,探讨了体系的形态结构与冲击性能之间的关系.结果表明,在改性材料的韧性有所提高的同时,压缩强度、马丁耐热都没有降低.从断裂面的形态来看,是属于韧性断裂.当PEEK的加入量为6%时,韧性最好,达到19.1 kJ/m2,比纯的环氧树脂增加了107.6%.     

聚醚醚酮增韧改性环氧树脂

采用共混法用聚醚醚酮(PEEK)改性环氧树脂(EP),借助差示扫描量热分析(DSC)确定了环氧树脂的固化工艺,测试了共混体系的工艺性能,研究了聚醚醚酮含量对环氧树脂力学性能的影响。借助扫描电子显微镜(SEM)对材料断裂面的形态结构进行了分析,探讨了体系的形态结构与冲击性能之间的关系。结果表明,在改性材料的韧性有所提高的同时,压缩强度、马丁耐热都没有降低。从断裂面的形态来看,是属于韧性断裂。当PEEK的加入量为6%时,韧性最好,达到19．1kJ/m~2,比纯的环氧树脂增加了107．6%。     

Measuring the fracture toughness of ultra-thin films with application to AlTa coatings

An experimental technique is presented for measuring the fracture toughness of brittle thin films. In this technique, long rectangular membranes are fabricated from the film of interest using standard silicon micromachining techniques. A focused ion beam is then used to introduce pre-cracks of different lengths along the centerline of the membranes and the membranes are pressurized until rupture. The fracture stress of the membrane is measured as a function of pre-crack length and the fracture toughness of the film is determined from a simple fracture mechanics analysis. The technique is applicable to a wide range of materials and is especially suited for ultra-thin films. We have demonstrated the experimental procedure for a 150 nm AlTa intermetallic film and obtained a room-temperature fracture toughness of K _1c = 4.44 ± 0.21 MPam^1/2.     

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Abstract

Recently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of ≥1400 MN m^?2 but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.

MST/1413     

The performance of isoparametric finite elements in stress intensity factor determination

Analysis of a compact compression specimen used for fracture toughness evaluation of cementitious materials is carried out by the finite element method using isoparametric elements. Both triangular and rectangular elements were used with those surrounding the crack tip being of the quarter point type. Solutions were obtained for different mesh subdivisions and convergenece curves for the stress intensity factor were obtained by several methods based on extrapolation and energy techniques. It is found that monotonic convergence was obtained for all cases considered. Employing uniformly graded rectangular element representations converged solutions for the stress intensity factor (assuming a 1 percent convergence criterion) were obtained by the energy methods using a total of 720 degrees of freedom for solving half the structure.Tests on modified 100 mm cubes with symmetrical notches were conducted to determine the fracture toughness. The fracture toughness was calculated from the stress intensity factor and the maximum load obtained from the tests which were conducted in a stiff Instron testing machine. The fracture toughness is found to be independent of the size of the notch.     

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.     

Probabilistic treatment of fracture using two failure models

A probabilistic methodology for brittle fracture based on two local failure models is presented. Probabilistic fracture parameters are obtained using a weakest link and a chain-of-bundles formulation. Both models define limiting distributions for the fracture stress described by a two-parameter Weibull distribution. Numerical procedures employing measured toughness data and finite element solutions are also described to calibrate the Weibull parameters. An application of the methodology then follows to predict geometry and stable crack growth effects on the distribution of macroscopic fracture toughness (J_c) for a high-strength steel. Measured fracture toughness values for a high-constraint geometry that exhibit no prior ductile tearing are effectively ‘transferred' to a different geometry having much lower constraint and in which tearing precedes cleavage. The inherent difficulty in predicting the scatter of experimental fracture toughness, as well as constraint and ductile tearing effects, within the scope of conventional procedures appears greatly reduced in the framework presented in this work.     

The Effective Fracture Toughness in Hydraulic Fracturing

This paper examines the effective fracture toughness approach which is used in hydraulic fracturing in order to explain the high net-pressures that are often observed in field operations. The effective fracture toughness is calculated using a fully deterministic elasto-plastic hydraulic fracturing model. Rock is modelled by Mohr–Coulomb flow theory of plasticity for cohesive-frictional dilatant material. Fluid flow is modelled by lubrication theory. A cohesive crack model which takes into account the softening behaviour of rocks is employed as the propagation criterion. The fully coupled model is solved numerically by the finite element method and the effective fracture toughness is calculated using the path independent J-integral. The results show that plastic yielding near the tip of a propagating fracture provides an effective shielding, resulting in an increase in the rock effective fracture toughness by more than an order of magnitude. It is demonstrated that an elastic model based on the concept of effective fracture toughness matches the results of plasticity quite well. The effective fracture toughness increases with formation yielding, which is influenced by the deviator of the in-situ stresses, the rock strength, the elastic modulus and the pumping parameters. Tables of effective fracture toughness for a representative set of physical parameters are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental results of fracture energy and fracture toughness of Radiata Pine laminated veneer lumber (LVL) in mode I (opening)

The load-carrying capacity of notched timber beams can be predicted using linear elastic fracture mechanics (LEFM). Material properties such as fracture toughness and energy are needed for the analysis. The micro and macroscopic complexity of wood and its anisotropic nature give different fracture properties in the longitudinal, radial and tangential grain directions. This complexity and the infrequent use of LEFM mean there is little data available. While wood is highly anisotropic, fracture analysis can use a subset of the possible material properties because wood normally cracks parallel to its grain due to its low tensile strength perpendicular to grain. This allows a significant reduction in the number of tests required to measure fracture properties, with considerable saving of resources. This paper presents the results of an experimental study investigating the fracture energy and fracture toughness of Radiata Pine laminated veneer lumber in mode I (opening). A more efficient test apparatus is proposed and shown to produce identical results to the test apparatus used by others. Results are presented for the fracture toughness properties in the grain direction, and include fifth percentiles and coefficients of variation. The influence that the specimen size has on the fracture toughness is also presented. Numerical analyses using the ABAQUS software package show good agreement with the experimental test results. The experimental results are within the range of experimental values reported in the literature for solid wood.     

Fracture and fracture toughness of stoichiometric MgAl2O4 crystals at room temperature

The fracture toughness and path of stoichiometric spinel (MgAl₂O₄) crystals were determined at 22 °C for key low-index planes by double cantilever beam, as well as fractography of flexure specimens failing from either machining or indentation flaws. These results are compared with other single and polycrystalline MgAl₂O₄ fracture toughness values measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental and theoretical results shows (1) the fracture toughness of the spinel {110} plane is only a limited amount (e.g. 6%) higher than for the {100} plane (1.2 MPa m^1/2), (2) fractography of machining flaw fracture origins was the most effective source of K _IC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of elastic anisotropy (especially for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especially for notch-beam tests), and the frequent lack of symmetric plastic deformation and fracture (especially for indentation techniques).Retired.