The fatigue damage mechanics of notched carbon fibre/PEEK laminates

A model is presented for the strength, post-fatigue residual strength and damage propagation in notched, cross-ply carbon fibre/polyetheretherketone (PEEK) laminates. Fracture mechanics principles are used to predict quasi-static damage growth, and the application of a Paris law permits extension to fatigue damage. Strength is predicted by applying a failure criterion based on the tensile stress distribution in the 0° plies, as modified by damage (either quasi-static or fatigue). The volume dependence of strength is included by using a simple Weibull distribution. The parameters of the model are determined from independent experiments. Good agreement with experimental results is obtained. Comparisons are made with previous results from carbon fibre/epoxy laminates. The behaviour of the carbon fibre/PEEK is similar, although the extent of delamination and matrix cracking is reduced owing to the higher inherent toughness of the matrix.     

On the impact performance of carbon fibre laminates with epoxy and PEEK matrices

A study was made of the mechanical properties and impact performance of carbon fibre/PEEK (0,90), ($\text{±45}$) and ($\text{±45,0}$) laminates and comparisons were made with similar carbon fibre/epoxy laminatesFibre dominated properties such as plain tensile strength were similar to those of epoxy laminates with similar fibres and volume fractions. Because of the increased toughness of PEEK there was less extensive matrix cracking, even though there was fibre debonding, and this gave increased transverse and shear cracking strains, increased shear strengths but decreased notched tensile strengths. The lower modulus and yield stress of PEEK caused lower compressive strengths, but PEEK absorbed little moisture and at 120°C moisture had little effect on mechanical properties.Dropweight impact produced less extensive damage in carbon fibre/PEEK laminates. Residual tensile strengths were similar but, because of the less extensive damage and greater delamination fracture energy, the residual compressive strengths were significantly greater with a PEEK matrix.Microscopic examination showed less matrix cracking and more fibre buckling in the carbon fibre/PEEK and this is discussed in relation to mechanical properties.     

Fatigue damage mechanisms and residual properties of graphite/epoxy laminates

Damage mechanisms and accumulation, and associated stiffness and residual strength reductions were studied in cross-ply graphite/epoxy laminates under cyclic tensile loading. Stress-life data were fitted by a two-parameter wearout model and by a second-degree polynomial on a log-log scale. The fatigue sensitivity is highest for the unidirectional laminates and it decreases for the crossply laminates with increasing number of contiguous 90° plies. Five different damage mechanisms were observed: transverse matrix cracking, dispersed longitudinal cracking, localized longitudinal cracking, delaminations along transverse cracks, and local delaminations at the intersection of longitudinal and transverse cracks. Failure patterns vary with cyclic stress level and number of cycles to failure. Under monotonie loading, failure is brittle-like and concentrated. At high stress amplitudes and short fatigue lives failure results from few localized flaws, whereas at lower stress amplitudes and longer fatigue lives failure results from more dispersed flaws. The residual modulus shows a sharp reduction initially, followed by a more gradual decrease up to failure. The residual strength showed a sharp reduction initially, followed by a plateau or even some increase in the middle part of the fatigue life, and a rapid decrease in the last part of the fatigue life. A tentative cumulative damage model is proposed based on residual strength and the concept of equal damage curves.     

Compression failure of carbon fibre-reinforced coupons containing central delaminations

As part of a study on the tolerance of carbon fibre-reinforced composites to impact, the effect of delaminations between the plies of laminates was investigated. Experiments were carried out on carbon fibre/PEEK and carbon fibre/epoxy coupons with artificially-introduced central delaminations to determine the effect on compressive strength. Delaminations in carbon fibre/epoxy grew prior to failure, those in carbon fibre/PEEK did not. A finite element method was developed to predict the strength reduction and delamination growth. It was found that the predictions matched experimental results, provided large displacement effects were included.     

Mechanical properties,fatigue damage and microstructure of carbon/epoxy laminates depending on fibre volume content

This paper investigates the effect of fibre volume fraction on the fatigue behaviour and damage mechanisms of carbon/epoxy laminates. Epoxy resin and unidirectional carbon/epoxy specimens with two different fibre volume fractions are tested under quasi-static tensile and tension–tension fatigue loads at angles of 0°, 45° and 90°. Fracture surfaces are studied with scanning electron microscopy. The results show that stiffness and strength increase with increasing fibre volume fractions. The damage behaviour of off-axis specimens changes with increasing fibre volume content and the height of the applied cyclic load. While matrix cracking and interfacial debonding are dominating damage mechanisms in specimens with low fibre content, fibre bridging and pull out are monitored with increasing fibre content. The higher the applied load in fatigue tests transverse to fibre direction, the more similar behave specimens with different fibre volume fractions.     

Influences of thermal cycling and low-energy impact on the fatigue behavior of carbon/PEEK laminates

Effects of low-energy impact and cyclic thermal loading on fatigue behavior of carbon fiber reinforced polyetheretherketone (carbon/PEEK) laminates have been examined. The fatigue behavior of the virginal composites, low-energy impacted composites, and low-energy-impacted and thermally exposed composites were investigated. Cyclic thermal loading was performed in the temperature range between 60 and −60°C. Residual tensile strength was measured to aid in understanding the influence of low-energy impact on the retention of tensile strength. Fatigue testing involved a stress ratio of 0.1, with a frequency of 3 Hz. The Weibull distribution function was used to evaluate the ultimate tensile strength and fatigue life. S–N curves were plotted and the influence of thermal cycling and the low-energy impact on the fatigue sensitivity of the carbon/PEEK laminates was investigated. Stiffness variation during fatigue testing was monitored and differences in stiffness reduction for three test conditions were compared. C-scan was used to investigate the damage zone under different low-energy impacts and to understand damage propagation during fatigue testing. Moreover, scanning electron microscopy (SEM) was used to examine the fracture morphologies of carbon/PEEK composites in both tensile failure and fatigue failure conditions.     

Effect of fibre length on fatigue of short carbon fibre/epoxy composite

The effect of fibre length on the fatigue of a random short carbon fibre/epoxy composite containing 1, 5 or 15 mm length fibres has been studied. All laminates gave a sloping S-N curve with longer fatigue lives obtained at decreasing peak stresses. The fatigue life was independent of fibre length at any peak strain, within experimental variation. Damage accumulation during fatigue cycling is studied in terms of residual strength and modulus reduction. Both techniques suggest that fatigue failure is the result of a ‘sudden death’ mode of failure. Finally, the effect of matrix type on the fatigue life of laminates containing 5 mm length fibres was investigated by adding a greater quantity of flexibilizer to the epoxy matrix. Shorter fatigue lives were obtained for laminates having the more flexible matrix.     

FATIGUE OF A PARTICULATE REINFORCED ALUMINIUM METAL MATRIX COMPOSITE SUBJECTED TO AXIAL, TORSIONAL AND COMBINED AXIAL/TORSIONAL LOADING CONDITIONS

Abstract— The goals in this research were to analytically and experimentally investigate the fatigue behavior of a particulate reinforced metal matrix composite subjected to axial, torsional and combined axial/torsional loadings. A series of fully-reversed uniaxial, torsional and combined axial/torsional fatigue tests were performed on a 6061/Al₂O₃/20p-T6 metal matrix composite material. This research investigated the ability of the Fatemi-Kurath and the Smith-Watson-Topper (SWT) damage parameters to correlate the experimentally obtained fatigue life data and also to represent the fatigue life using uniaxial strain-life constants. The Fatemi-Kurath damage parameter correlated the experimental fatigue data from all loading cases better than the SWT damage parameter. Using uniaxial strain-life constants, both damage parameters predicted fairly reasonable fatigue life calculations for the intermediate fatigue lives (10³ to 10⁴ cycles to failure), while producing non-conservative results for the shorter fatigue lives (< 10³ cycles to failure).     

Tension fatigue analysis and life prediction for composite laminates

A tension fatigue life prediction methodology for composite laminates is presented. Tension fatigue tests were conducted on quasi-isotropic and orthotropic glass epoxy, graphite epoxy, and glass/graphite epoxy hybrid laminates. Edge delamination onset data were used to generate plots of strain energy release rate as a function of cycles to delamination onset. These plots were then used along with strain energy release rate analyses of delaminations initiating at matrix cracks to predict local delamination onset. Stiffness loss was measured experimentally to account for the accumulation of matrix cracks and for delimination growth. Fatigue failure was predicted by comparing the increase in global strain resulting from stiffness loss to the decrease in laminate failure strain resulting from delaminations forming at matrix cracks through the laminate thickness. Good agreement between measured and predicted lives indicated that the through-thickness damage accumulation model can accurately describe fatigue failure for laminates where the delamination onset behaviour in fatigue is well characterized, and stiffness loss can be monitored in real time to account for damage growth.     

Fatigue and post-fatigue behaviour of carbon/epoxy non-crimp fabric composites

This paper focuses on the static, fatigue and post-fatigue tensile properties of a biaxial carbon/epoxy non-crimp fabric composite. In a series of quasi-static tensile tests, the stress–strain level where damage initiates was determined. This stress level was then used as the maximum stress level in tensile–tensile fatigue tests in the fibre direction. It was found that in fibre direction, this load level can be considered safe for fatigue up to very high cycle numbers. The damage evolution during the tests was monitored at certain cycle times with X-ray radiography. The post-fatigue residual static tensile properties were determined after different numbers of cycles. A series of tensile–tensile fatigue tests at various higher stress levels allowed for the fatigue life curves to be constructed in each of the four testing directions. This revealed that the damage initiation load level is well below the practical fatigue limit of the material.     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—II. impact behaviour

Quasi-static, low-hanging and high-velocity impact tests have been conducted in order to study the effect of fibre/matrix adhesion on the impact properties of fibre-reinforced metal laminates. Differences in fibre/matrix adhesion were achieved by using treated or untreated carbon fibres in an epoxy resin system. Chemical removal of the aluminium layers and a sectioning technique were applied to examine and characterize the impact damage in the laminates. The results show that the laminates with the weaker fibre/matrix adhesion exhibit larger damage zones, although the back face crack length and permanent indentation after impact are smaller for a given impact energy. Residual tensile strength after impact is also higher for the untreated fibre laminates due to increased fibre/matrix splitting in the composite layer.     

LONG LIFE SPECTRUM FATIGUE OF CARBON AND STAINLESS STEEL WELDS

Abstract— Fatigue tests of non-load carrying carbon and stainless steel fillet welds have been performed using spectrum loading typical for rail vehicles. The proportion of spectrum cycles exceeding the constant amplitude fatigue limit ranged between 0.86% and 100% and cycles to failure ranged from 4.2 ± 10⁵ to 2.1 ± 10⁷. For the longest tests, the majority of fatigue damage was contributed by cycles with stress ranges less than the constant amplitude fatigue limit. For the carbon steel welds a significant portion of fatigue damage was produced by cycles with stress ranges less than 50% of the fatigue limit but only a small fraction of damage was produced by cycles of this size for the stainless steel welds. The carbon steel welds had slightly better fatigue strength at lives less than 10⁷ cycles but results suggest that stainless steels may have superior long-life variable amplitude fatigue strength when a greater portion of life is spent in the early stages of crack nucleation and growth.     

The environmental fatigue behaviour of carbon fibre reinforced polyether ether ketone

The fatigue behaviour of carbon fibre/PEEK composite is compared with that of carbon/ epoxy material of similar construction, particularly in respect of the effect of hygrothermal conditioning treatments. Laminates of both materials were of 0/90 lay-up, and they were tested in repeated tension at 0° and at 45° to the major fibre axis. The superior toughness of the polyether ether ketone and its better adhesion to the carbon fibres results in composites of substantially greater toughness than that of the carbon/epoxy material, and this is reflected in the fatigue behaviour of the carbon fibre/PEEK. The tougher PEEK matrix inhibits the development of local fibre damage and fatigue crack growth, permitting a 0/90 composite with compliant XAS fibres to perform as well in fatigue as an epoxy laminate with stiffer HTS fibres. Hygrothermal treatments have no effect on the fatigue response of either material in the 0/90 orientation. The fatigue response of a cross-plied carbon/PEEK laminate in the ±45° orientation is much better than that of equivalent carbon/epoxy composites, again because the superior properties of the thermoplastic matrix.     

Residual properties of notched [0/90]4S AS4/PEEK composite laminates after fatigue and re-consolidation

Notch fatigue strengthening under different cyclic stress levels and elapsed number of cycles has been studied in [0/90]_4S AS4/PEEK laminates. Quick and extensive 0° fiber splitting and the corresponding 90° fiber shear off were found to be the underlying causes of stress concentration alleviation. This effectively raised the residual strength of the notched laminates and increased their fatigue lives to beyond one million cycles. On the other hand, re-consolidation of fatigued specimens removed most of the internal damages and in the meantime reverses the above strengthening. Detailed study of the residual strength changes and damage development history using re-consolidation lent support to the above deductions on the notch fatigue strengthening phenomenon in [0/90]_4S AS/PEEK laminate.     

Effect of carbon fibres on the static and fatigue mechanical properties of fibre metal laminates

It is crucial to understand the characteristic fatigue crack initiation and its growth mechanisms, as well as the relationship between the mechanical properties and the fatigue damage evolution in fibre metal laminates (FMLs). Two types of FML were studied in this work: a polyacrylonitrile‐based carbon fibre epoxy matrix composite sandwiched by Ti‐6Al‐4V (Ti‐alloy) sheets (IMS60‐Ti) and a pitch‐based carbon fibre epoxy matrix composite sandwiched by Ti‐alloy sheets (K13D‐Ti). The static and fatigue mechanical properties of IMS60‐Ti and K13D‐Ti were investigated. The increased failure strain of the FML was greater than that of carbon fibre‐reinforced polymer (CFRP) matrix composites. The fatigue life of IMS60‐Ti was much longer than that of K13D‐Ti. The fatigue damage process in IMS60‐Ti was related to the fatigue creep behaviour of the Ti‐alloy face sheet and mode II cracking at the CFRP/Ti‐alloy interface, and the damage in K13D‐Ti was related to the K13D CFRP laminate.     

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.      

Improvement of fatigue life by incorporation of nanoparticles in glass fibre reinforced epoxy

The fatigue properties of glass fibre reinforced epoxy laminates modified with small amounts (0.3 wt.%) of nanoparticles (fumed silica SiO₂ and multi-wall carbon nanotubes (MWCNT)) were evaluated by means of static (90°-tensile and stepped tensile) and dynamic fatigue tests. For the MWCNT-modified matrix, the electrical conductivity was measured in situ. The addition of nanoparticles lead to increases in inter fibre fracture strength of up to 16%. More significantly, the high cycle fatigue life is increased by several orders of magnitude in number of load cycles. The increased inter fibre fracture strength could be correlated to the improved fatigue behaviour, as final failure in high cycle fatigue is strongly correlated to matrix cracks. For the MWCNT-modified composites, the state of load and damage state was monitored by conductivity measurements. A correlation between the onset of matrix cracking and increase in electrical resistivity could be drawn enabling self sensing capabilities.     

Cooling rate influences in carbon fibre/PEEK composites. Part III: impact damage performance

The effect of cooling rate on impact damage performance of carbon fibre/polyether ether ketone (PEEK) matrix composite is characterised based on the instrumented drop-weight impact test, scanning acoustic microscopy (SAM) damage evaluation and compression-after-impact (CAI) test. Judging from the incipient impact load, incipient impact energy and total damage area, the ability to resist damage initiation upon impact was higher in the order of fast-cooled carbon/PEEK, slow-cooled carbon/PEEK and carbon/epoxy laminates. Furthermore, the threshold impact energy was higher and the CAI strength reduction rate was lower for the fast-cooled specimen than the slow-cooled counterpart, strongly indicating higher impact damage tolerance of the former system. The present study demonstrates that the impact damage performance and other important properties of carbon/PEEK composites can be optimised, if not maximised, by proper control of processing conditions, especially the cooling rate.     

Experimental determination of elastic properties of impact damage in carbon fibre/epoxy laminates

This paper describes an investigation of in-plane elastic properties of impact damaged regions in composite laminates. Quasi-isotropic carbon fibre/epoxy laminates were impacted and the impact damage examined by ultrasonic C-scanning, optical microscopy and thermal deplying. After impact damage observations, specimens were cut from the laminates and tested in tension and compression. The elastic modulus of the impact damage was, in both tension and compression, mainly controlled by the amount of fibre breakage. Interestingly, layers with broken fibres could sustain some load in compression, which led to higher elastic modulus in compression than in tension. The effect of delaminations on the elastic modulus was quite small in both tension and compression. The through-the-thickness variation of in-plane stiffness was studied by successively removing plies. The variation in stiffness was negligible, probably as a result of the very uniform distribution of delaminations and fibre breakage through the thickness of the laminates.     

Damage evolution of angle-ply SCS-6/Ti composites under static and fatigue loading

The effect of fibre orientation and laminate stacking sequence on the tensile and fatigue behaviour of SCS-6/Ti 15-3 composites were investigated. The laminates used in this study were: (90)₆, (0/ ± 45)_s, (0/90)_s, and (90/ +-45)_s. The initiation and progression of microstructural damage at various stress levels was thoroughly characterized. It was found that fatigue life at high applied stresses were controlled by fibre fracture; progressive damage involving fibre fracture, interfacial debonding and matrix cracking became dominant at low applied stresses. Observation of the damage mechanisms in the angle-ply laminates under cyclic loading suggests that increasing the fibre-matrix bonding strength may improve the load carrying capability and fatigue life of laminates containing off-axis plies.