Intralaminar Fracture Research Articles

Intralaminar fracture mechanism in unidirectional CFRP composites: Part I: Intralaminar toughness and AE characteristics

Three types of representative carbon fiber reinforced unidirectional composite materials were used and their intralaminar fracture behavior was investigated using the double-cantilever beam specimen with a simultaneous acoustic emission measurement. The intralaminar fracture toughness was evaluated by both the compliance method and energy area method. As a result, it was found that the intralaminar fracture toughness without bridging fibers had a constant value during crack propagation but it increased greatly when bridging fibers were present. The effect of bridging fibers on the intralaminar fracture toughness was estimated quantitatively by cutting the bridging fibers. Distinct differences in load–displacement curves, compliance, crack propagating behavior and acoustic emission signal characteristics between these three types of unidirectional composite materials were observed. It was also found that bridging fiber failure generated relatively large power spectra and contributed to the peak frequencies of 600–700 kHz in the spectrum analysis of acoustic emission (AE) signals. This suggested that the bridging fibers were also an important source of AE signals. Furthermore, a linear relationship between crack length and normalized cumulative AE event count rate was obtained.

Intralaminar fracture mechanism in unidirectional CFRP composites — part II: analysis

Three kinds of representative carbon fiber reinforced unidirectional composite materials are used, and their intralaminar fracture behavior is investigated by using the double-cantilever beam (DCB) specimen with a simultaneous acoustic emission measuring. In Part I, the experimental results on the crack propagation, the bridging fibers, the intralaminar fracture toughness acoustic emission characteristics and microscope observations were obtained. Here, we use a bridging fiber model to analyze the debonding force acting on a bridging fiber and try to estimate the number of bridging fibers during the crack propagating process. At the same time, the intralaminar fracture toughness is calculated by both the adhesive force model and the finite element analysis. As a result, it is found that the intralaminar fracture toughness without the bridging fibers will have a constant value during the crack propagation, but it increases greatly when bridging fibers exist. It is clear that the bridging fibers play an important role in the intralaminar fracture toughness. The debonding forces acting on the bridging fibers and the number of bridging fibers are obtained. Furthermore, the quantitative estimation of the increment of the intralaminar fracture toughness contributed by bridging fibers is made according to the adhesive force model and it is comparable with the results obtained by the finite element analysis.

Interlaminar and Intralaminar Fracture Toughness of Unidirectional Graphite/Epoxy Laminates under Mixed-Mode (I+II) Loading.

Mixed-mode I+II fracture toughness of unidirectional graphite/epoxy laminates, T800H/# 3631. was studied. Mixed-mode propagation of interlaminar cracks was examined by mixed-mode bending tests where crack growth was stabilized by the coordinate conversion control method. Intralaminar fracture toughness was measured by tension tests of off-axis laminates with an inclined crack-like notch. The value of interlaminar fracture toughness increased with crack extension for GI/GIIl1.37 because of development of fiber bridging, while was nearly constant during crack growth for GI/GIIg0.57. There was no big difference between the interlaminar and intralaminar fracture conditions in the range of the mixed-mode ratio of GI/GIIl0.11. For GI/GIIl0.57, the mode I component of mixed-mode fracture toughness, GI, was insensitive to the mode II component, GII. while for GI/GIIl0.57, the GI value decreased to zero as the GI/GII ratio decreased. A new criterion for the mixed-mode fracture condition was proposed, showing a good fit to the experimental results.

Dependence of Intralaminar Fracture Toughness on Direction of Crack Propagation in Unidirectional Composites

Mode I intralaminar fracture toughness GIC of graphite fiber-reinforced bismaleimide (X5260/G40-800) has been studied experimentally using double edge notch (DEN) specimens and three-point-bend specimens. The intralaminar fracture toughness was found to depend significantly on the direction of fracture propagation. Examination of the fracture surface showed the fracture surface to be relatively smooth or ragged depending on the direction of the fracture propagation, thus substantiating the measured differences in of graphite fiber-reinforced bismaleimide (X5260/G40-800) has been studied experimentally using double edge notch (DEN) specimens and three-point-bend specimens. The intralaminar fracture toughness was found to depend significantly on the direction of fracture propagation. Examination of the fracture surface showed the fracture surface to be relatively smooth or ragged depending on the direction of the fracture propagation, thus substantiating the measured differences in GIC. Furthermore the material exhibited a substantial. Furthermore the material exhibited a substantial R-curve behavior, which appeared related to fiber pull-out.

Interlaminar and intralaminar fracture toughness of uniaxial continuous and discontinuous carbon fibre/epoxy composites

The interlaminar and intralaminar fracture toughness of a continuous carbon fibre/epoxy composite and an aligned discontinuous carbon fibre (DISCO)/epoxy composite have been measured using the compact tension and double cantilever beam test geometries. The DISCO composite was found to have a slightly greater misalignment of the fibres from the average fibre direction and this misalignment was found to increase both the initiation fracture toughness and to a greater extent the propagation fracture toughness. The increase in fracture toughness in the DISCO samples was due to fibres bridging the crack and this has been modelled as if the bridging fibres provide an increase in compressive stress across the crack analogous to a craze at the crack tip.

The interlaminar and intralaminar fracture toughness of carbon-fibre/polymer composites: The effect of temperature

The mode I interlaminar toughness and intralaminar fracture toughness, and mode II interlaminar shear fracture toughness of a thermoplastic and a toughened thermosetting polymer reinforced with aligned and cross-ply carbon fibres are investigated over a wide range of temperature. The nature of the fracture process and fracture resistance are explained qualitatively on the basis of measurements of toughness, crack-tip and crack-wake events, and post-mortem fractographic analysis.

Moisture effects on the behavior of graphite/polyimide composites

AbstractThe effect of moisture on the material behavior of Magnamite® IM7 Graphite/Avimid® K3B thermoplastic polyimide composite laminates has been investigated. Laminates consisting of a 62 vol% fiber were laid up with several different stacking sequences: [90]10or unidirectional, [02/90]Sor thin cross‐ply, [02/902/02]Sor thick cross‐ply, [45/0/‐45/90]Sor quasi‐isotropic, and [0/90] laminates. In this study, the glass‐transition temperature,Tg, and the intralaminar fracture toughness,GIC, were measured for dry and moisture‐saturated unidirectional samples. When laminates were saturated with moisture, the value ofTgwas found to decrease from its baseline (dry) value, but recovered upon redrying the samples. This observation is consistent with the effects of moisture on theTgof other polymer composites. Permanent toughness losses have not been observed in samples conditioned in room‐temperature 75% and 100% relative humidity environments. However, during long‐term conditioning of cross‐ply and quasi‐isotropic samples in liquid water, transverse cracks initiated in the absence of an applied mechanical load. Moisture uptake curves for conditioning in room‐temperature liquid water, 80°C (176°F) liquid water, and at room temperature and 75% relative humidity were used to calculate Henry's Law constants and diffusion coefficients. Non‐Fickian behavior, consisting of a postsaturation increase in moisture uptake, was observed in crossply and quasi‐isotropic laminates and might be due to the observed transverse cracking.

INTRALAMINAR FRACTURE TOUGHNESS OF 90° LAYER IN CROSSPLY COMPOSITE LAMINATES

A new scheme for estimating the intralaminar fracture toughness is proposed. Recent research on the transverse cracking damage in crossply composite laminates suggests the use of the energy-release rate (ERR) of the 90° layer as the fracture toughness of the 90° layer. A brief survey of the authors' formulation of transverse cracking damage is presented, which shows that the ERR of the 90° layer is constant during damage progress in spite of the fact that the transverse cracking is a stochastic process. The ERR of the 90° layer is a constant which completely characterizes the damage evolution process. Some benefits of using the ERR of the 90° layer as a measure of the intralaminar fracture toughness are discussed in comparison with the other parameters such as the transverse strength, the crack resistance and the ERR of the whole laminate.

The effect of isothermal aging on transverse crack development in carbon fiber reinforced cross‐ply laminates

AbstractAn investigation into the effect of isothermal aging on the development of transverse cracks in cross‐ply laminates of two high temperature composite systems was performed. The composite materials investigated were BASF X5260/640–800 and DuPont Avimid K/IM6. Changes in the glass transition temperature, composite weight loss, crack density, and mode I intralaminar fracture toughness were monitored during isothermal aging in air at 177°C for up to 2232 h. The two laminate configurations used in this study include two variations of the generic cross‐ply configuration [02/90n]s, in whichnequals 1 and 2. The results of this investigation show that a layer of degraded material forms at the surface of the X5260/640–800 bismaleimide laminates and that the thickness of the degraded layer increases with aging time. After 744 h of aging, transverse cracks form in the surface plies and an increasing crack density evolves as aging time is increased; however, transverse cracks do not form in the inner 90° ply groups with aging during the time period investigated. The Avimid K/IM6 thermoplastic polyimide laminates, which show evidence of cracking prior to aging, do not exhibit any significant change in crack density with aging. The results of the aging experiments also show that the bismaleimide system exhibits a weight loss of 1.5% and an increase in glass transition temperature from 250°C to 300°C after 2232 h of aging at 177°C, while the thermoplastic polyimide system shows a weight loss of only 0.05% and an increase in glass transition temperature from 280 to 285°C after 2232 h. Changes in the resistance to crack formation are also seen in these materials during aging. The mode I intralaminar fracture toughness, a measure of resistance to transverse crack formation, shows a 50% decrease after aging for 2232 h for the bismaleimide system, while the behavior exhibited by the thermoplastic polyimide shows little evidence of a reduction.

Interlaminar and intralaminar fracture modes in 0/90 cross-ply glass/epoxy laminate

Delamination of a cross-ply 0/90 glass fibre-reinforced composite laminate with an epoxy-phenol matrix was studied using a double cantilever beam test. Fracture toughness was determined by measurement of bend angle of the cantilever beams. Results obtained with this method were in agreement with those from conventional compliance and area methods. Two different fracture modes were observed: interlaminar and intralaminar. In the interlaminar fracture mode, crack jumps in the space between two neighbouring 0° and 90° plies were observed. With the interlaminar fracture mode, during crack initiation G Ic decreased with crack length. Intralaminar fracture mode consisted of the gradual growth of a crack through a 0° ply. Fibres bridging the opposite sides of the crack were observed in this case, and fracture toughness G Ic did not change with crack length. G Ic (420 J m −2) at intralaminar fracture mode was approximately twice that at interlaminar fracture mode (220 J m −2). The difference in fracture toughness was explained by the dissipation of energy by fibres bridging the opposite sides of the crack at intralaminar fracture mode.

Effect of the Interfacial Interleaf to the Interlaminar Fracture and Intralaminar Fracture of a New BMI Matrix Composites System

Systematic studies were conducted to investigate the effect of the introduc tion of a soft thermoset adhesive layer at appropriate interface to the interlaminar fracture and intralaminar fracture of the BMI matrix based composite laminates. Tests include DCB and ENF tests for measuring interlaminar fracture toughnesses, quasi static tension test on the crossply laminates and edge delamination test for measuring matrix crack initi ation, edge delamination onset, mixed-mode interlaminar fracture toughness and the ulti mate laminate strength. The improvements of the interlaminar fracture toughnesses of the GFRP and CFRP laminates are significant. An elastic foundation beam model is intro duced to analyze the interleaved DCB specimen. Results show stress concentration at the crack-tip of the interleaved DCB specimen is reduced. Besides the factor of the possible formed plastic zone in interleaf, the modified interlaminar bond strength of the adhesive/ composites interface may lead to higher interlaminar fracture toughness. The edge delamination strength and the mixed-mode interlaminar fracture toughness in interleaved laminate are raised significantly. However, the interleaf has no significant effect on the laminate strength because the interfacial interleaf only improves the interlaminar fracture feature while the final laminate fracture is controlled by the intralaminar properties of the lamina. On the matrix cracking in interleaved laminate, both the matrix crack growth rate and its final saturation crack density decrease in the interleaved multidirectional laminate. The matrix crack was found to initiate early in the interleaved crossply laminate. This*Author to whom correspondence should be addressed Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 13-June 1994 0731-6844/94/06 0509-32 $6.00/0 @ 1994 Technomic Publishing Co., Inc.result agrees with the analysis of the one-dimensional shear-lag model. But in the edge delamination specimen, matrix crack initiation is raised by the interleaf.

Investigation of the transverse properties of a unidirectional carbon/epoxy laminate: Part 2—Laminate properties

The transverse mechanical behaviour of a unidirectional continuous carbon-fibre-reinforced/epoxy (CFRP) composite has been studied in detail. Carbon fibres at six different levels of an oxidative fibre surface treatment were converted into prepreg with a high-performance TGDDM/DDS epoxy matrix which has been the subject of an earlier study (Part 1). Laminates of [90] 16 configuration were used to measure elastic properties and failure strengths in tensile and flexural loading modes. Intra-laminar fracture toughness values were measured by using compact tension specimens. The distribution of tensile transverse strengths within the CFRP laminates have been studied using hybrid sandwich laminates consisting of 0° GRP outer plies encompassing a core of 90° CFRP. CFRP cores consisting of both 8 and 16 plies of 0% (untreated) and 100% (standard treatment) fibre treatment levels have been studied. These laminates were tested in tension leading to the formation of multiple transverse cracks within the CFRP core, enabling Weibull strength distribution charts to be constructed. These data have been used in conjunction with similar bulk resin sandwich laminate data to compare strength and failure mechanisms. Finally, scanning electron microscopy has been used for post-morten examination of the fracture surfaces. The results are used to draw conclusions about the operative failure mechanisms and it is suggested that embedded defects only have a secondary role in formation of the critical failure sites.

The Application of Double Torsion Testing to Unidirectionally Reinforced Composite Materials

To characterize the intralaminar fracture behavior of a unidirectional car bon fiber/epoxy resin composite material, two distinct sets of experiments were per formed. First, the tensile mechanical properties of the lamina were determined by testing specimens with different fiber orientation at varying rates of deformation. Then, the Double Torsion technique was applied to characterize the fracture resistance of the material. The strain energy release rate vs. crack speed curve so derived is in agreement with measurements conducted by using the Double Cantilever Beam and Compact Tension techniques. The results of the research indicate that Double Torsion is an attractive tech nique for studying the time-dependent fracture of composite materials.

The Formation and Propagation of Matrix Microcracks in Cross-Ply Laminates during Static Loading

Recently, a variational mechanics analysis approach has been used to de termine the thermoelastic stress state in cracked, cross-ply laminates (Nairn, J. A. 1989. J. Comp. Mat. , 23:1106). The analysis included a calculation of the energy release rate due to the formation of a microcrack in the 90° plies. This paper describes a series of ex perimental results on a wide variety of composite material systems and of cross-ply layups of generic type [0m,/90n]s. The variational mechanics energy release rate analysis can be used to predict all features of the experimental results and to draw some new conclusions about the progression of damage in cross-ply laminates. The recommended experiments are to measure the density of microcracks as a function of applied stress. Such results can be fit with the energy release rate expression and used to measure the microcracking or intralaminar fracture toughness. The measured microcracking fracture toughnesses are: Hercules AS4/3501-6-240 J/m2, Fiberite 934/T300—690 J/m2, DuPont Avimid® K Polymer/IM6-960 J/m2, Fiberite 977-2/T300-1800 to 2400 J/m2, and ICI PEEK/AS4- 3000 J/m2. Experiments that measure only the stress to initiate microcracking are specifically not recommended because they do not give an accurate measure of the micro cracking fracture toughness.

Resistance to transverse tension of a commingled carbon fiber/poly(ether ether ketone) composite.

Resistance to transverse tension of the unidirectional carbon fiber/Poly(ether ether ketone) (PEEK) composites was examined by uniaxial tensile and cleavage tests. Three types of unidirectional carbon fiber/PEEK composites were fabricated from: (a) prepreg sheet, (b) plain weave cloth (warp: commingled yarn of 1800d of carbon fiber with PEEK fiber, weft: PEEK yarn of 70d), or (c) 5-shaft sateen weave cloth (warp: carbon yarn of 1800d, weft: PEEK yarn of 900d). The initial Young's modulus and the breaking strength measured by the transverse tensile tests were the highest on the specimen (a). Those of the specimen (c) were the lowest because of the lack of the carbon fiber/PEEK interfacial adhesion. On the other hand, the mode I intralaminar fracture toughness estimated from the cleavage test was the highest on the specimen (b). The interfacial adhesion between the carbon fiber and PEEK resin was also investigated by scanning electron microscopic observation of the fractured surface after transverse tensioning. For specimen (b) the fracture surface was rough and irregular, and the fibers were twisted with one another. The fracture surface of specimen (a) was flat and smooth, and the fibers were parallel to one another. It was concluded that the undulation and twisting of the carbon fibers enhanced the fracture toughness of specimen (b).

The Strain Energy Release Rate of Composite Microcracking: A Variational Approach

A variational analysis approach has been used to determine the two- dimensional thermoelastic stress state in cross-ply laminates of type [0 m/90 n] s and [90 m/ 0 n] s. The stress analysis was used to calculate the energy release rate due to formation of a new microcrack. The analysis accurately includes the effect of residual thermal stresses. When compared with experiments, the new energy release rate expressions are found to predict typical data using a single value for the critical energy release rate for microcrack ing. This critical energy release rate has a physical interpretation as a microcracking fracture toughness or as an intralaminar fracture toughness. We also used the variational solution to get an analytical expression for the longitudinal expansion coefficient of the microcracked cross-ply laminate as a function of microcrack density. Finally, variational theorems are used to show that this new microcracking fracture analysis is rigorously more accurate than previous attempts at the same problem.

Interlaminar and intralaminar fracture surface morphology in graphite/epoxy laminates

Fracture surface morphological studies of graphite/epoxy (T300/934) laminates have been performed to distinguish between mode I and mode II interlaminar and intralaminar fractures. Additionally, the effect of a few hygrothermal conditions on mode I fracture surface is also assessed. Centre notched tension, compact tension, double cantilever beam and cracked lap shear specimens have been used to study these fracture modes. While mode I fracture is characterized by branched cracks between fibers, river pattern and chevron markings on resin rich zones, the epoxy hackles dominate mode II fracture surface. The interlaminar and intralaminar fractures can be distinguished by fiber spacings and some matrix features. The effect of increase in temperature and/or moisture is seen to cause reduction in matrix microcracking and increase in fiber pullout in mode I.