Delamination Fracture Toughness Research Articles

Delamination fracture toughness of woven-fabric composites under mixed-mode loading

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Dynamic Mode-I crack-propagation in a carbon/epoxy composite

A modified DCB specimen has been used to study the dynamic Mode-I delamination fracture toughness of an AS4/3501-6 carbon/epoxy composite at high crack-propagation speeds. A strip of FM-73 adhesive film was placed at the tip of a pre-crack created during laminate lay-up. The adhesive strip substantially increased the crack-initiation toughness so that after initiation an unstable crack-propagation was produced and high crack-propagation speeds were achieved. Specimens were loaded at a constant displacement rate of 0·1mms−1 on an MTS machine. Crack extension was monitored by means of a conductive circuit placed along the path of the propagating crack. Continuous records were obtained for the load, deflection and crack extension. The strain-energy release rate was calculated by using plate finite elements based on the crack-propagation data. It was found that the critical dynamic strain-energy release rate for crack speeds up to 200ms−1 is a constant and is equal to the critical static strain-energy release rate.

Long-Term Thermo-Oxidative Aging in Composite Materials: Experimental Methods

The combination of this work, an earlier study [1] concerning the effects of thermo-oxidative aging on the thermal, fracture and compressive behavior of carbon-fiber reinforced epoxy composites, and a companion paper [2] discussing the failure mechanisms due to thermo-oxidative aging provide the first published comprehensive study of the effects of long-term thermo-oxidative aging on such a wide range of properties for the same materials systems. In this phase of the work, tensile tests (both unidirectional 0° and ±450 layups), edge delamination, and compression-after-impact (CAI) were examined for G30-500/R922-1 and G30-500/R6376 carbon/epoxy specimens aged in air at 177°C (350°F) up to 10,000 hours. Additionally, 10,000 hour data not included in the previous study are also included for comparison. The purpose of the present study is not to identify suitable candidate materials, but to evaluate the effect of test methods and failure modes in characterizing thermo-oxidative stability. The materials systems chosen were selected as model materials for the study of degradation phenomena and not for performance evaluation at 177°C which is clearly beyond the useful range of these materials. Nevertheless, unidirectional tensile properties were found to be relatively unaffected up to 5,000 hours of aging. By contrast ±45° tensile strengths and unidirectional compression strengths decreased between 10 and 35% after only 1,000 hours exposure. Mode I fracture toughness increased with aging due to increased fiber bridging and Mode II fracture toughness was, by contrast, relatively unaffected. Edge delamination fracture toughness was severely affected by thermo-oxidative aging decreasing to between 5 and 15% of the unaged value after 10,000 hours aging suggesting a strong effect of interfacial degradation. Because even small weight losses lead to correspondingly large reductions in certain mechanical properties, even small errors in the determination of weight loss lead to large errors in estimates of mechanical properties. Indeed, it is likely that weight loss, even with the presence of corresponding mechanical data is not a good indicator of property degradation. Weight loss is only useful as a negative indicator: it indicates an unacceptable material; it does not yield useful information as to useful lifetimes.

Modified Mixed-Mode Bending Test Apparatus for Measuring Delamination Fracture Toughness of Laminated Composites

A modified mixed-mode bending test apparatus was developed to measure delamination fracture toughness and fatigue delamination growth rates of laminated composites. The test apparatus is compact, lightweight, and has very few attachments. The use of needle bearings and a lightweight loading lever makes the apparatus particularly suitable for fatigue tests. The design parameters of the apparatus were selected to minimize the geometric nonlinear effects. The total weight of the aluminum lever and the saddle is about 1.02 lb (4.5 N). The lever is graduated to directly read the load position and calculate G I/ G II ratio. For this apparatus, simple linear equations for mode I and II strain energy release rates ( G) were derived and were found to be accurate. The test apparatus was used to measure mixed-mode fracture toughness of unidirectional IM7/5260 graphite/Bismaleimide composites. The delamination growth was stable for mode I loading and mixed-mode loading with G I/ G II = 4.55/1 and 1.16/1. Whereas, it was unstable for mode II and mixed-mode loading with G I/ G II = 1/3.4. The average mode I and mode II fracture toughnesses were 1.53 lb/in (268 N/m) and 13.53 lb/in (2,370 N/m), respectively. The average fracture toughness for mixed-mode loading with ( G I/ G II) = 4.55/1, 1.16/1, and 1/3.4 was 2.18 lb/in (382 N/m), 3.15 lb/in (552 N/m), and 7.44 lb/in (552 N/m), respectively. The linear mixed-mode delamination fracture criteria ( G Ic m/ G IIc m) found to give a lower bound solution. The delamination growth resistance and the mixed-mode ratio were practically constant.

Evaluation of interlaminar‐toughened poly(etherlmide)‐modified epoxy/carbon fiber composites

AbstractAmorphous poly(ether imide) has been used as interlaminar toughening particulate agent in laminated carbon fiber/epoxy composites. Mode I and Mode II delamination fracture toughness was characterized using the double cantilever beam (DCB) and end‐notched flexure (ENF) specimens. The delamination surface was examined using a scanning electron microscopy (SEM) to investigate relationships between the morphology and properties. The results revealed that the PEI‐modified composites exhibited a significantly increased fracture toughness, which increased with the PEI content. GIC was improved from 165 to 540 J/m2 (at 1 mm/min crosshead speed). GIIC was improved more significantly from 290 to 1300 J/m2. It is believed that these values could be further improved if the processing cycle were to be optimized.

The Effects of Starter Film Thickness, Residual Stresses and Layup on GIc of a 0°/0° Interface

This paper reports on an investigation of the mode I delamination fracture toughness of 0°/0° interfaces in a carbon-epoxy system (T800-924). It has been found that thicker starter films give a lower GIc. The toughness of a natural interface (ie unaffected by the presence of a starter film) has been found to be closest to the toughness measured using the thinnest starter films. Tests on 0°/0° interfaces in multidirectional laminates have shown that residual stresses have a significant effect on GIc. Finally, it has been shown that bend-twist coupling in the arms of the Mode I specimen can lead to considerable changes in the apparent toughness.

Interfacial mobility and its effect on interlaminar fracture toughness in glass-fibre-reinforced epoxy laminates

The effects of interfacial treatment of glass fibres in glass/epoxy composites were studied through Mode I delamination fracture toughness tests using a double cantilever beam specimen. The treatment of glass fibres with two similar silane coupling agents has been shown to improve the mechanical properties of the composite as a function of the type of coupling agent, γ-aminopropyltriethoxysilane (APS) and γ-aminobutyltriethoxysilane (ABS) have similar chemistry, but differ in mobility (molecular motion) at the coupling agent-epoxy interface. The critical energy release rate, G 1c, for the APS-treated composites (0.59±0.05 kJ m−2) was shown to be higher than that of the ABS-treated one (0.37±0.01 kJ m−2) and also the untreated one (0.31±0.02 kJ m−2). In this case, the bulk structural property appears to be a function of the microscopic interfacial properties including the dynamics of the coupling agent layer. Optical characterization of the fracture surfaces reveal delamination at the epoxy-glass interface for the untreated samples, while the ABS- and APS-treated samples showed less interfacial delamination, respectively.

Measurement of mixed-mode delamination fracture toughness of unidirectional glass/epoxy composites with mixed-mode bending apparatus

Initiation of cracking and delamination growth in a unidirectional glass/epoxy composite were evaluated under mode I, mode II, and mixed mode I + II static loading. They have been expressed in terms of the total critical strain energy release rate, G TC, and the total fracture resistance, G TR. For the mixed mode I + II, a mixed-mode bending apparatus was used. The loading in this test is a simple combination of the double cantilever beam mode I and the end notch flexure mode II tests. In addition to characterisation of delamination initiation, whatever the value of the G II G T modal ratio, this apparatus allows the plotting of an R curve, from which we obtain the total fracture resistance, G TR. Experimental results were correlated with computations of a semi-empirical criterion through the plotting of the total critical strain energy release rate, G TC, versus the G II G T modal ratio, and of the total fracture resistance, G TR, versus the G II G T modal ratio.

Interlaminar morphology effects on fracture resistance of amorphous polymer-modified epoxy/carbon fibre composites

Effects of three amorphous thermoplastic polymers used as interlaminar toughening particles in carbon fibre/epoxy laminated composites have been compared. Mode I and mode II delamination fracture toughness were characterized using double cantilever beam and end-notched flexure specimens, respectively, and the delaminated surfaces of specimens were examined using scanning electron microscopy to investigate the interlaminar morphology after fracture. This study showed that the poly(ether imide)-modified and phenoxy-modified systems exhibited similar improvement. The interlaminar morphology in these two systems is compared and discussed. Surprisingly, incorporation of bisphenol-A polycarbonate (PC), with its solvent-induced crystallinity during particulate preparation, induced only very little improvement in the modified composite system. The microstructural morphology at the PC/epoxy interlaminar region showed insoluble chunks of PC domains with crystals, the brittleness and lack of wetting with the epoxy phase adversely affected the macroscopic mechanical fracture behaviour.

Effect of Treatment Conditions on the Mode I Delamination Fracture Toughness of Sandwich Structures

The mode I interlaminar fracture behaviour of aluminum-composite sandwich structures has been investigated. Double cantilever beam (DCB) specimens have been used for the determination of G Ic-values. Various different skin treatments were used. The data obtained from these tests have been analysed using several different analytical approaches found in literature and compared with respective G Ic-values derived from a simple analytical method developed by the authors. The original contribution of the present method when compared to all the existing methods is that a variable correction factor for G Ic as well as for G I(prop) describing crack front propagation history is defined. Most important is the fact that the method applies to all types of mechanical behaviour such as linear elastic, nonlinear elastic and inelastic. The results obtained can be used for the optimum design of sandwich structures since suggestions for optimal skin treatment are presented.

Delamination Fracture Criteria for Composite Laminates

Due to the singularity nature of delamination, the fundamental concept of fracture mechanics was applied. However, the oscillatory characteristics near the delamination tip prohibited the use of conventional definition assuming the crack embedded into a homogeneous solid. A proper definition for the stress intensity factors and energy release rates of bimaterial interface cracks was introduced in this paper to study the fracture criterion for delamination. To measure the delamination fracture toughness, the test specimen usually used for the unidirectional composites was redesigned to suit for the cases that the delamination lies between two laminae with different fiber orientations. The test results show that the double cantilever beam (DCB) test is near to pure Mode I and the end-notched flexural (ENF) test is near to pure Mode II. Therefore, like the cracks in homogeneous materials, the test methods DCB and ENF are useful for the measurement of delamination fracture toughness GC and G11,. Based upon this conclusion, a series of cracked-lap shear (CLS) and modified end-notched flexural (MENF) tests and finite element simulation are conducted in this paper to establish the mixed-mode delamination fracture criterion. To validate the proposed fracture criterion, two commonly encountered problems are predicted and tested. One is the prediction of the most probable interface for the onset of delamination and its possible minimum load, in which no delamination was embedded prior to loading. The other is the prediction for the test with delamination existing prior to loading. The results show that the prediction by using this proposed criterion is well matched with the experiment.

Mixed-Mode Delamination Behavior of Carbon/Epoxy Composites

The mixed-mode bending (MMB) test was employed in an investigation of delamination fracture behavior in static and fatigue loading for carbon/epoxy composites 1300/M10 and HTA/6376. As specimens were loaded at different ratios of alL for different mixed-mode ratios, both investigated composites exhibited brittle unstable or brittle stable behavior of crack growth. Mixed-mode delamination fracture toughness was determined at a IL = 0.5. Fracture surfaces of MMB specimens were examined using scanning electron microscopy to distinguish fracture features. Stable crack growth was observed over the entire range of crack lengths investigated in the cyclic tests. A consistent relation of crack growth per cycle to the cyclic strain energy release rate was obtained. The trend of reduction in crack growth rate data with diminution in cyclic strain energy release rate suggests the presence of a threshold strain energy release rate.

Energy Release Rates for the ENF Specimen Using a Beam on an Elastic Foundation

A new modified beam theory analysis is presented for the ENF specimen, which is used to evaluate the mode II delamination fracture toughness of fiber reinforced composite materials. The analysis combines the solution of a beam on a generalized elastic foundation to incorporate the effect of crack tip deformation, and a Timoshenko Beam Theory solution of the ENF to incorporate transverse shear on the predicted energy release rates. A distinctive feature of this approach is that crack tip deformation and shear deformations are treated separately and explicitly in accounting for deviations from simple beam theory. Two unknown parameters are introduced, however, that must be determined by comparison with finite element solutions. The resulting solution nevertheless demonstrates considerable accuracy over a wide range of material properties (e.g., axial to shear modulus ratios) and crack lengths. In addition, the current analysis compares very favorably with other analyses of the ENF that incorporate the effect of crack tip deformations on the energy release rate.

Mode I Interlaminar Fracture Toughness in Carbon Fiber Reinforced Thermoplastic Laminate.

Delamination fracture toughness and fracture mechanism in carbon fiber reinforced Nylon laminates have been investigated. The mode I interlaminar fracture toughness tests are carried out on double cantilever beam (DCB) specimens made from four kinds of angle-ply laminates. These laminates exhibit the unstable behavior on the crack propagation. In order to obtain the continuous GI-R curve describing the stick-slip behavior, one modification is made to Kageyama's modified compliance method. From the continuous GI-R curves, it is found that the stable delamination is characterized by a flat region of GIstable a 1200 J/m2 for all lay-ups except for a 60°//-60°specimen which shows the rising GI-R curve to final failure. The interlaminar fracture toughness of these composite laminates is considerably higher than that of carbon/epoxy laminates. Based on the continuous GI-R curves and the observation of the fracture surfaces, a mechanism is proposed in which the unstable stick-slip behavior is explained by the development of the large-scale bridging of the fiber bundle or layer and its breakage.

ＣＦ／エポキシ複合材０°層間はく離に及ぼす負荷保持および積層構成の影響

Unidirectional CFRP has superior in-plane mechanical properties. Its low delamination resistance, however, is a week point. Thus the delamination strength has been evaluated based on the fracture mechanics as delamination resistance or delamination fracture toughness in these days. In the previous paper, the authors have shown the effects of loading rate and load holding on the delamination resistance and the occurrence of stick-slip phenomena. In this paper, the effect of load holding was examined in detail by using a scanning acoustic microscope and the effect of stacking sequence of 0° laminae on the delamination resistance was evaluated by conducting mode I delamination tests on two types of stacking sequence. Based on these results, the stick-slip phenomena during the load holding was explained by heterogeneous crack growth with visco-elasticity in the resin rich region and the micromechanism of delamination between 0° laminae of the multidirectional laminates was proved to be the same as the unidirectional reinforced lamina except for the effect of residual stress caused by curing.

Interaction between matrix cracking and edge delamination in composite laminates

Matrix cracking and edge delamination are two main damage modes in continuous-fibre composite laminates. They are often investigated separately, and so the interaction between two damage modes has not yet been revealed. In this paper, a simple parallel-spring model is introduced to model the longitudinal stiffness reduction due to matrix cracking and edge delamination together. The energy release rate of edge delamination eliminating the matrix crack effect and the energy release rate of matrix cracking in the presence of edge delamination are then obtained. Experimental materials include carbon- and glass-fibre-reinforced bismaleimide composite laminates under static tension. The growth of matrix cracks and edge delaminations was recorded by means of NDT techniques. Results show that matrix cracks may initiate before or after edge lamination. This depends on the laminate layup, and especially on the thickness of the 90° plies. Edge delamination may also induce matrix cracking. Matrix cracking has a significant effect on the stiffness reduction in GRP laminates. The present model can predict the stiffness reduction in a laminate containing both matrix cracks and edge delaminations. The mixed-mode delamination fracture toughness obtained from the present model shows up to 50% differences compared with O'Brien's model for GRP laminates. However, matrix cracking has a small effect on the mixed-mode interlaminar fracture toughness of the CFRP laminates.

Fracture behaviour of model toughened composites under Mode I and Mode II delaminations

The fracture behaviour of two toughened epoxy composite systems was investigated using various microscopy techniques. The Mode I delamination fracture toughness,GIC, Mode II delamination fracture toughness;GIIC, and compression after impact (CAI) strength of these model composite systems were also measured. Under Mode I fracture, it was found that these composites exhibit nearly identical toughening mechanisms to those of the rubber-modified neat resins. The composites differ primarily in having smaller damage zones than the neat resin equivalents. Under Mode II fracture, the typical hackles were found to initiate from inside the resin-rich interlaminar region due to the presence of the toughener particles. The CAI strength, based on the present study as well as the work conducted by others, appeared to be related to, but not necessarily strongly dependent on, the interlaminarGIC andGIIC, the thickness of the interlaminar resin-rich region, and the type of the interlaminar toughener particles. Approaches for improving theGIC,GIIC, and CAI strength of high-performance toughened composites are discussed.

The effects of sorbed moisture on resin-matrix composites

The effects of water absorption have been investigated on the diffusion behavior, moisture-induced expansion, and fracture properties of thermoset and thermoplastic resin-matrix composites. The composite materials were submerged in distilled water at temperatures from 45°C to 90°C for more than 8,000 h. Moisture saturation levels were considerably higher in the thermoset composites than in the thermoplastic composite. Moisture sorption in the graphite/epoxy composite exhibited both classical Fickian and non-Fickian diffusion behavior. Deviant, non-Fickian behavior was related to chemical modification and physical damage in the composite. Moisture sorption in thermoplastic resin composites follows Fickian diffusion rather closely. Anomalous moisture expansion behavior in graphite-epoxy composites was attributed to environmental-induced damage of the laminate. Sorbed moisture reduced the delamination fracture toughness (DFT) of a thermoset composite, whereas moisture sorption had virtually no effect on the DFT of a thermoplastic composite.

Prediction of Delamination in Composite Laminates Subjected to Low Velocity Impact

A method for predicting impact-induced delamination in composite lami nates was proposed. This method is suitable for low velocity impact with heavy impactors. Static delamination fracture toughness was used to predict delamination crack growth under impact conditions. Curing stresses were also considered and found to play a signifi cant role in evaluating the fracture toughness of some laminates such as [905/05/90 5]. Ex periments were performed to obtain the impact force history from which the peak force was used to determine the extent of delamination crack length. The prediction of delami nation size using static fracture toughness was found to agree very well with the experi mental result.

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.