Edge Crack Torsion Specimen Research Articles

Reexamination of the edge crack torsion test for determining the mode III delamination toughness of laminated composites

The edge crack torsion (ECT) test is reexamined to address two primary concerns with this method. The first of these relates to inconsistencies in the data reduction procedure that was originally proposed and is most commonly used to compute the mode III interlaminar fracture toughness, GIIIc, from the experimental data. The second relates to the observation that plies neighboring the preimplanted insert in the standard ECT layup are unable to contain initiation and growth of intralaminar cracks, and that growth of these cracks contributes to errors in the apparent values of GIIIc. To address these issues, an alternative data reduction technique, which uses a polynomial expression to represent the ECT specimen’s compliance versus crack length relation, is proposed. The accuracy and potential drawbacks of this expression are investigated using FE simulations. Next, seven groups of ECT specimens are fabricated and tested. Each of these groups contains specimens with preimplanted inserts that are bounded by plies at different +θ and −θ orientations. The seven groups consist of values of θ that vary from 0° to 90° in 15° increments. Tests on these specimens show that intralaminar cracking occurs in specimens from all batches except those for which θ equals 15° or 30°. However, values of GIIIc measured using specimens with these two ply orientations were not equal and, for θ = 30°, the values of GIIIc that were obtained were not completely independent of initial starter crack length. Potential issues accounting for these observations and other challenges associated with measuring GIIIc using the ECT test are discussed.

Cohesive zone modelling of Mode III delamination using the edge crack torsion test

In the experimental studies of mode III delamination using the edge crack torsion test, the crack initiation and propagation measurement are always difficult. This information could be obtained through numerical modelling. The objective of this study is to propose a guideline to model mode III delamination behaviour using cohesive elements. Finite element models of an edge crack torsion specimen were developed based on the data from the literature. The delamination behaviour of the specimen along the pre-crack, which was located at the mid-Thickness location, was modelled using cohesive elements. Through parametric studies, it was found that for reliable numerical modelling, a mesh size of 0.5 mm was suggested, which provided three elements in the cohesive zone. As for the interface strength, it was recommended to choose 80 MPa. In addition, a viscosity parameter of 110-3 was found to be a good choice for reasonable computational time and converged numerical results. Besides, the interface stiffness was suggested to be 4106 MPa/mm. Furthermore, the fracture process zone contour revealed that the delamination was started at a normalised location of approximately 0.7. Not only that, the fracture energy and strain distribution plots have shown the delamination was mode III dominated within the normalised distance of 0.34-0.86. The results from this study suggested that cohesive zone modelling is a useful method for the detailed analysis of the mode III delamination of an ECT specimen. The numerical modelling approach suggested from this study could be applied to ECT specimens at various different initial crack lengths. It also has the potential to be used to simulate the mode III delamination of other various types of laminated composites.

Crack Length Dependence of Mode III Delamination Using Edge Crack Torsion Test

The objective of this study is to compare the mode III delamination behavior of edge crack torsion (ECT) specimens at different initial crack lengths, ao. Finite element models of ECT specimens at ao = 20 mm and 30 mm were developed based on the data from the literature. Delamination behavior was investigated using cohesive zone modeling, where cohesive elements were placed at the mid-thickness of the specimens. Results showed that the experimental and numerical force-displacement curves were comparable, with less than 10% difference in the slopes and peak loads. In addition, it was found that the cohesive zone in both models contained three elements. Furthermore, the crack front (CF) and fracture process zone (FPZ) contours revealed that the largest crack extensions were found at normalized locations of approximately 0.4 and 0.7 for ao = 20 mm and 30 mm specimens, respectively. Finally, comparison between the fracture energy distributions and phase angle indicated that at least 30% of the crack front was mode III dominant, with phase angle of 85° and above.

Experimental determination of fracture toughness for adhesively bonded composite joints

The characterization of fracture toughness or critical energy release rate of adhesive joint in composite structures is a key parameter. In the present study, fracture toughness of adhesively bonded composite joints is experimentally investigated for the basic fracture modes. Experiments are conducted on the double cantilever beam, end notched flexure and edge crack torsion specimens to determine the fracture toughness of three modes, namely mode-I, mode-II and mode-III. Specimens are bonded with epoxy adherent and cured at room temperature. The fracture toughness values of mode-II and mode-III are 2.6 and 4.6 times respectively of mode-I. Acoustic emission (AE) technique is also employed to aid in determining the fracture toughness of bonded joints. The results indicate that AE signals have good correlation with load–displacement behavior to determine the fracture initiation stage especially in the mode-II loading.

Observation of intralaminar cracking in the edge crack torsion specimen

The edge crack torsion (ECT) test is evaluated to determine its suitability for measuring fracture toughness associated with the onset of mode III delamination in laminated composites. ECT specimens with preimplanted midplane inserts of different lengths are tested and examined using nondestructive and destructive techniques. Ultrasonic inspections of all tested specimens reveal that delamination growth occurs one ply interface beneath the midplane. This is confirmed by sectioning and optical microscopy, which also show that macroscopic delamination advance results from a coalescence of angled intralaminar matrix cracks that form in-between and then extend through the midplane plies. The relative orientation of these intralaminar cracks is approximately 45° with respect to the midplane, suggesting that their formation is caused by resolved principal tensile stresses arising due to the global mode III shear loading. Examination of ECT specimens tested to loads below the level corresponding to the onset of delamination growth reveals that initiation of intralaminar cracking occurs prior to or concurrently with the onset of nonlinearity in the specimen’s force–displacement response. The existence of intralaminar cracking prior to delamination growth and the resulting delamination extension at an unintended interface render the ECT test, in its current form, unsuitable for characterizing the onset of mode III delamination growth. The broader implications of the mechanisms observed in this study are also discussed with respect to the current understanding of shear-driven delamination in tape-laminate composites.

Fractographic Observation of Various Loading Modes of Fibre Reinforced Laminates

One of the major disadvantages of laminated composites is their tendency to delaminate. Unidirectional glass/epoxy laminates have been tested under static conditions by the use of fracture mechanics. Mode I, mode II, mixed mode I-II, mode III and mixed mode II-III tests were performed. Double cantilever beam (DCB), end-notched flexure (ENF), mixed-mode bending (MMB) and edge crack torsion (ECT) specimens were used. Scanning electron microscopy technique was used to identify distinguishing fractographic features and to establish the differences between the various modes of fracture after specimens testing. The propagated orientation of the delamination could be specified from the patterns of fracture surface. Scanning electron micrographs of fractured surfaces showed that the most predominant fractographic features in mode I and mode II are the large amount of fibre pull-out and the cusps markings respectively. In the MMB specimen the fracture surfaces are characterized by fibre breakage under shearing with fractures localized in the resin with cusps having an orientation of 90º (mode II) and also fractures localized in the resin and along the resin/fibre interface (mode I). Mode III characterization concluded that some limited mixed mode II-III seems to be present for ECT specimen on delamination initiation and growth, but a large majority of mode III delamination is present.

Analysis of pure mode III and mixed mode (III + II) interlaminar crack growth in polymeric woven fabrics

This paper deals with mode III and mixed mode (III+II) delamination behavior of woven glass fiber reinforced polymer laminates; numerically and experimentally. An improved test method was employed to study fracture under pure out of plane shear condition (mode III) using edge crack torsion (ECT) specimen. Load–displacement curves indicated that deviation from linearity, PC (NL) and maximum failure loads, PC (Max) ranged about 35% with the change of crack length. The actual R-curve obtained by the compliance combination method showed fracture toughness raised with the increase of crack length. In addition, the finite element analyses suggested that the pure mode III fracture state existed in the mid-section of the ECT specimen in spite of the existence of a small mode II component near the free edges of the delamination front. The six-point bending plate (6PBP) test was proposed and developed to obtain mixed mode ratios (III+II). In spite of pure mode III, experimental results showed a quasi-linear load–displacement curve to failure. Dependently of the fracture tests, three-dimensional finite element analyses were undertaken, and the distribution of mode I, mode II, and mode III strain energy release rates (SERRs) at the delamination front were evaluated by the stress intensity factor (SIF) approach and showed non-uniform crack advance behavior in 6PBP test.

Fractographic analysis of delamination in glass/fibre epoxy composites

Unidirectional laminates of reinforced glass/epoxy composites UE400/ET443 were tested by the use of fracture mechanics tests. Mode I, Mode II, Mode III and Mixed-Mode I-II were, respectively, obtained from specimens submitted to double cantilever beam, end notch flexure, Edge Crack Torsion and mixed-mode bending tests. Two different loadings were applied to the edge crack torsion specimens: the original and the modified edge crack torsion test. Scanning electron microscopy, analytical and numerical analysis were used to identify and separate fractographic features and to establish the differences between the various modes of fracture. Scanning electron micrographs of fractured surfaces showed that the most predominant fractographic features in Mode I and Mode II are the large amount of fibre pull-out and the cusps markings, respectively. Mode III characterisation shows that some limited Mixed-Mode II-III appear in both ends of the edge crack torsion specimen on delamination initiation and growth, but a predominant Mode III delamination exists in the middle of the specimens.

Modification of the edge crack torsion specimen for mode III delamination testing. Part II – experimental study

Experimental studies of carbon/epoxy edge crack torsion specimen have been conducted using a specially designed twist test fixture. Of particular concern was verification of the recommendations expressed in the analytical part of this study (Part 1), where it was suggested that overhang (sections of specimen laying outside of the loading and support pins) in the x- and y-directions should be minimized, and fracture testing at longer delamination lengths should be avoided. The experimental test results verified that the specimens with the smallest overhang produced the most consistent delamination toughness data, GIIIc. Specimens with large overhangs exhibited high apparent GIIIc values at long delamination lengths. This was most likely due to nonuniform loading and associated nonuniform delamination extension.

Redesign of the ECT Test for Mode III Delamination Testing. Part I: Finite Element Analysis

A series of finite element analyses (Part I) and tests (Part II) on various modifications of the edge crack torsion (ECT) test have been conducted in an effort to render the test more suitable for characterizing mode III delamination in laminated composites. To this end, two ECT specimen configurations were considered. The first configuration involves loading the specimen at a single location, and the second configuration involves a symmetric double load point application. Investigations were conducted on the effect of specimen overhang, along the length and width direction, on the inferred strain energy release rate distributions across the delamination front. Stress distributions in the vicinity of the delamination front were used to infer the corresponding modes I, II, and III strain energy release rate distributions. Results indicate the single and double loading configurations exhibiting similar stress distributions along the delamination front. Specimens with short crack lengths and small amounts of overhang yielded the most uniform distribution of mode III loading along the delamination front.

The Influence of Loading Rate on the Mode III Interlaminar Fracture Toughness of Composite/Steel Bi-material Systems

In this article, the Mode III interlaminar fracture properties of a glass/ epoxy-steel bi-material system are investigated as a function of crosshead displacement rates using the edge crack torsion (ECT) test geometry. For purposes of comparison, tests were also undertaken on the plain glass/epoxy composite. For a given crack length, it has been shown that the interlaminar fracture toughness of the bi-material system was inferior to that offered by the plain composite, an effect that is attributed to the reduced toughness of the interface between the two materials. The Mode III fracture toughness of both the plain composite and the bi-material samples exhibited a crack length dependency, with the measured value of GIIIc increasing with crack length. Verification of the trends in the experimental data was achieved by conducting a finite element analysis of the ECT specimen and good agreement was achieved. Furthermore, the interlaminar fracture toughness of the plain composite remained roughly constant over the range of crosshead displacement rates considered here, suggesting that it does not exhibit a rate-sensitive fracture behavior. In contrast, the bi-material samples exhibited very low values of Mode III fracture toughness at impact rates, associated with extensive debonding of the steel from the composite material.

Characterisation of the edge crack torsion (ECT) test for the measurement of the mode III interlaminar fracture toughness

An experimental and numerical investigation were carried out on a glass/epoxy laminate to evaluate the edge crack torsion (ECT) test to determine its suitability for inducing mode III delamination growth. Two fixtures were used: an original and a modified ECT test which differ from each other in the constraints. The finite element analysis of the original ECT specimens without constraints in the guide pins showed that the propagation of mode III appears in the middle of the sample away from both ends, while with constraints, the propagation in mode III appears in the side of the loading point. For the modified ECT specimens with constraints the mode III distribution is similar to that occurring in the original ECT specimens without constrains. A C’scan analysis of the original and modified ECT specimens showed that delamination grows near the loading point for a load just after initial delamination.

The Influence of Loading Rate on the Mode III Fracture Properties of Adhesively Bonded Composites

The Mode III interlaminar fracture properties, GIIIc, of an adhesively-bonded glass/epoxy composite are investigated over a wide range of crosshead displacement rates, using the edge crack torsion (ECT) test geometry. The ECT test fixture has been modified to conduct impact testing on these bonded materials. Tests on all of the samples highlighted a significant crack length dependency with the value of GIIIcincreasing rapidly with increasing crack length. Tests were also undertaken on the plain glass reinforced epoxy over an equally wide range of crosshead displacement rates.For a given crack length, the interlaminar fracture toughness of the adhesively-bonded system was superior to that offered by the plain composite, an effect that is attributed to the presence of significant crack-tip blunting within the adhesive layer. The interlaminar fracture toughness of the composite and the adhesively-bonded system remained roughly constant over the range of crosshead displacement rates considered here, suggesting that these systems do not exhibit any significant rate-sensitive fracture behavior. Finally, the crack tip loading conditions are verified by conducting an FEA analysis of the ECT specimen. Here, it was shown that Mode III loading predominates at the center of the test specimen, whereas regions of Mode II loading were observed close to the test supports.

The Influence of Strain Rate on the Mode III Interlaminar Fracture of Composite Materials

The Mode III interlaminar fracture toughness, GIIIc, of composite materials based on both thermoplastic and thermosetting-matrices have been investigated using the edge crack torsion (ECT) test geometry. Tests were undertaken at room temperature and over a range of crosshead displacement rates to study the influence of strain rate on the interlaminar fracture properties of these materials. Further information concerning the crack tip loading conditions was obtained by undertaking a finite element analysis of the ECT specimen geometry. The experimental results show that the value of GIIIc depends on initial crack length, increasing steadily with increasing crack length for both types of material. It has been shown that the interlaminar fracture toughness of the glass fiber/epoxy-based system was superior to that offered by its thermoplastic counterpart, an effect that may be due to the fact that the glass fiber-reinforced polypropylene composite was slow-cooled from its processing temperature. The interlaminar fracture toughness of both types of composite remained roughly constant over the range of crosshead displacement rates considered here suggesting that they do not exhibit any rate-sensitive fracture behavior. The finite element analysis of the ECT specimens showed that the specimen is subjected to pure Mode III loading over the central part of the test specimen whereas regions of locally-high Mode II loading were observed over the region in which the load was applied. The Mode III strain energy release rate profile does not depend on specimen thickness or the displacement of the ECT test geometry.

Effect of Stress Ratio on Growth Behavior of Mode II Interlaminar Fatigue Cracks in High Strength GFRP

The mode III crack growth tests were carried out by edge crack torsion specimens under cyclic and monotonic loadings with UD laminates (made of unidirectional laminates) and C laminates (made of unidirectional laminates and cloth laminates) of high strength glass fiber reinforced plastics (GFRP). The test results of mode III crack growth were compared with those of mode I and II. The fracture toughness, GIIIR, was nearly constant during the crack extension of about 2 mm for UD and C laminates. The fracture toughness value for UD laminates was much lower than that of C laminates. The fracture toughness increased in the order of mode I, III, and II. Fatigue crack growth tests were carried out at the stress ratios of R =0.1, -0.5, and -1. In the low growth rate region, the crack growth behavior was controlled by the range of the energy release rate, ΔGIII, in UD and C laminates. The threshold value of the range, ΔGIIIth, was independent of the stness ratio in UD lanninate, while, in C lanninates, the value of ΔGIIth at the negative stress ratio was about 1.6 times larger than at positive stress ratio. In the high growth rate region of UD and C laminates, the crack growth behavior was controlled by the maximum value of the energy release rate, GIIImax, as in the case of mode II.

Mode III fracture behavior of laminated composite with edge crack in torsion

A three-dimensional (3-D) finite element analysis was performed on a [90,(+45/−45) n ,(−45/+45) n ,90] s class of laminated composites under the edge crack torsion (ECT) test configuration. Finite element delamination models were established and formulas for calculating the Mode III fracture toughness from 3-D finite element models were developed. The relations between the interlaminar fracture behavior and various configuration parameters were investigated and the effects of point loads, ends, geometry, Mode II interference, and friction were evaluated. Results showed that with proper selection of ECT specimen configuration and layup, the delamination could grow in pure Mode III in the middle region of the specimen. Specimen end effect played an important role in the ECT test. A Mode II component occurred in the end regions but it did not interfere significantly with the Mode III delamination state. Specimen dimension ratio, layup, and crack length exhibited significant effect on the interlaminar fracture behavior and the calculated strain energy release rates. However, friction between crackfaces was found to have negligible effect on the interlaminar properties.

Simplified Data Reduction Methods for the ECT Test for Mode III Interlaminar Fracture Toughness

Simplified expressions for the parameter controlling the load point compliance and strain energy release rate were obtained for the Edge Crack Torsion (ECT) specimen for mode 3 interlaminar fracture toughness. Data reduction methods for mode 3 toughness based on the present analysis are proposed. The effect of the transverse shear modulus, G(sub 23), on mode 3 interlaminar fracture toughness characterization was evaluated. Parameters influenced by the transverse shear modulus were identified. Analytical results indicate that a higher value of G(sub 23) results in a low load point compliance and lower mode 3 toughness estimation. The effect of G(sub 23) on the mode 3 toughness using the ECT specimen is negligible when an appropriate initial delamination length is chosen. A conservative estimation of mode 3 toughness can be obtained by assuming G(sub 23) = G(sub 12) for any initial delamination length.

Analysis of a symmetric laminate with mid-plane free edge delamination under torsion: theory and application to the edge crack torsion (ECT) specimen for mode III toughness characterization

A shear deformation theory is developed to analyze a symmetric laminate with mid-plane edge delamination under torsion loading. The theory is based on an assumed displacement field which includes shear deformation. The governing equations and boundary conditions are obtained from the principle of virtual work. Closed-form solutions for the compliance and strain energy release rate are obtained. The theory is used to analyze a mid-plane Edge Crack Torsion (ECT) specimen used for characterizing the mode III delamination behavior of composites. The prediction of the present theory for a [90, (±45)n, (±45)n, 90]s class of laminates is compared with existing test data, and good agreement is observed. In addition, the Classical Lamination Theory (CLT) is found to be less accurate for the analysis of the edge crack torsion specimen. The layup effect on the behavior of some ECT specimens is analyzed using the present theory.