Interlaminar Crack Research Articles

Analysis of bimaterial singular regions for orthotropic and isotropic materials under thermal loading

A method to formulate an asymptotic elasticity solution for bimaterial singular regions in orthotropic materials is presented for the thermal case. In the vicinity of material and geometric singularities, approximate methods such as finite elements fail to converge and procedures to improve these solutions become necessary. The results presented herein can be used in conjunction with approximate numerical techniques for the determination of stresses and stress intensity factors in composite bonded joints, cracked bimaterial interfaces, or a junction of dissimilar materials under thermal loading. The thermal displacement and stress fields at the tip of a bimaterial wedge are developed by means of eigenfunction expansions including both singular and nonsingular terms. The thermal elasticity solution for isotropic materials as the limiting case of the orthotropic solution is also obtained. The formulations obtained are applicable to bonded joints with interfacial or interlaminar cracks, geometric and material discontinuities, and uniform thermal loading. The detailed formulation presented provides an essential component in the application of finite elements for the determination of generalized intensity factors and stresses in composites, bonded joints, or other dissimilar materials under thermal loading. Formulation results for orthotropic materials, including the limiting case of isotropic materials, are compared with published and closed form results that use different formulations, and excellent agreement is observed. Furthermore, the existence of logarithmic stress singularities of the form (k⋅ΔT⋅lnr) is investigated and discussed.

Fracture and fatigue behavior of Bambusa Vulgaris-Schrad Bamboo

This paper presents the results of an experimental study of the fatigue and fracture behavior of Bambusa Vulgaris-Schrad bamboo. Mechanisms of crack initiation and growth are elucidated under monotonic and cyclic compressive failure or flexural loading. The microscopic studies of fatigue and fracture explore the effects of fiber/crack/ply orientation on crack growth and toughening mechanisms in single-edge notched and double -edge notched fracture mechanics bend specimens, which are studied in the “crack-arrestor” and “crack-divider” orientations. The compressive fatigue behavior of Bambusa Vulgaris-Schrad bamboo is also investigated in smooth un-notched specimens. The resulting stress-life behavior is shown to occur in a regime in which the maximum stresses exceed the critical conditions for the onset of progressive fiber buckling and shear band formation. This results ultimately in the nucleation and propagation of interlaminar cracks and shear cracks across the Bambusa Vulgaris-Schrad structures. The implications of the results are discussed for the design of robust bamboo structures.

Free‐edge delamination in composite laminates under tensile loading: An analytical closed‐form approach

AbstractIn the present study, interlaminar crack initiation in symmetric angle‐ply laminates under tensile loading is investigated using the coupled stress and energy criterion, whereby the required stress and energy quantities are determined by means of a new analytical closed‐form model. The identification of unknown interface fracture toughnesses is addressed by a parameter fit within physical reasonable limits. The resulting effective failure loads are compared to experimental data and show the high potential of the coupled criterion on the one hand and of the proposed analytical model on the other hand.

The crack quasi-static growth and analysis of interlaminar crack resistance of layered composite

Using the model of quasi-static crack growth and test result of the double cantilever beam (DCB) sample of layred carbon/epoxy composite, the general regularities of the interlaminar crack resistance of mode 1 were studied. The main attention was focused on the variability of crack resistance associated, on the one hand, with the continuity of the fracture process, and, on the other hand, with the non-homogeneity of the material structure, which causes local random deviations from the average characteristics of the material.The dissipation energy rate (total crack resistance) and crack resistance function (R-curve) were extracted from test results and their properties analyzed.

Propagation characteristic of axial and circumferential interface waves and detection of the interlaminar defect in multilayered pipes

This paper focuses on the dispersion characteristic of interface waves and the detection of interlaminar defects in multilayered pipes. The interface waves propagating along axial and circumferential directions are simplified as strain problems of planes (r, z) and (r, θ), respectively. The axial interface wave is well suited to axial localization of the defect, just as the circumferential interface wave is to circumferential localization. The dispersion curves and wave structures in generalized forms are built based on double- and treble-layered examples. Both interface waves converge to the Stoneley wave velocity at high frequency, while energy is concentrated on the interface. An experimental system is built. Theoretical and experimental group velocities are compared. The results indicate that the average relative errors of these two interface waves are 0.47% and 0.53%, respectively. An interlaminar crack is detected effectively both in axial and circumferential directions, revealing that interface waves hold great potential for non-destructive evaluation.

Effects of matrix cracks on delamination of composite laminates subjected to low-velocity impact

This paper investigates delamination mechanisms of composite laminates subjected to low-velocity impact. To obtain delamination data between plies with different fiber orientations, thermal-deply tests were conducted on quasi-isotropic laminates after low-velocity impact. Regularities of delamination positions in different interfaces as well as the matrix cracks distribution through the laminate thickness were analyzed. It is found that delamination positions are influenced by matrix cracks and the distribution of matrix cracks is determined by the out-of-plane shear stress distribution. To illustrate the formation process of delamination, a novel micro-block finite element model was established. It is revealed that due to the interaction between adjacent cracks, inter-laminar cracks growing towards each other show higher stress intensity factors and are thus more likely to form delamination. This crack propagation pattern eventually leads to the formation of two centrosymmetric fan-shaped delaminations in each interface.

Tufting thread and density controls the mode-I fracture toughness in carbon/epoxy composite

Herein, interlaminar crack initiation and its growth in tufted carbon/epoxy composite under Mode-I loading are investigated for different thread materials and tuft densities. Two configurations of Double Cantilever Beam (DCB) test specimens are fabricated – one with 2 rows of tuft (hereafter referred to as lower tuft density) and another with 3 rows of tuft (will be referred to as higher tuft density). The crack front is arrested and delamination growth is delayed by tufting, which increases interlaminar fracture toughness(GIC). Higher enhancement in fracture toughness is observed for carbon thread tufted specimens followed by Kevlar and then glass thread tufted specimens. Fracture toughness of tufted specimens is about 4.5 to 10 times of untufted specimen depending on the thread material and tuft density. An increase in tuft density increases fracture toughness from 175% for Kevlar to 272% for glass threads. Fiber bridging from the parent laminate layer is observed in the untufted specimen, whereas in the tufted specimen, this phenomenon is insignificant except for bridging due to tufting thread. Fracture analysis shows that the failure is mainly due to the rupture of thread at the interface. Thread pull-out or slippage is absent exhibiting good adhesion with epoxy matrix.

Characterization of Adhesives Bonding in Aircraft Structures.

Structural adhesives play an important role in aerospace manufacturing, since they provide fewer points of stress concentration compared to faster joints. The importance of adhesives in aerospace is increasing significantly because composites are being adopted to reduce weight and manufacturing costs. Furthermore, adhesive joints are also studied to determine the crashworthiness of airframe structure, where the main task for the adhesive is not to dissipate the impact energy, but to keep joint integrity so that the impact energy can be consumed by plastic work. Starting from an extensive campaign of experimental tests, a finite element model and a methodology are implemented to develop an accurate adhesive model in a single lap shear configuration. A single lap joint finite element model is built by MSC Apex, defining two specimens of composite material connected to each other by means of an adhesive; by the Digimat multi-scale modeling solution, the composite material is treated; and finally, by MSC’s Marc, the adhesive material is characterized as a cohesive applying the Cohesive Zone Modeling theory. The objective was to determine an appropriate methodology to predict interlaminar crack growth in composite laminates, defining the mixed mode traction separation law variability in function of the cohesive energy (Gc), the ratio between the shear strength τ and the tensile strength σ (), and the critical opening displacement υc.

Improved mode Ⅰ interlaminar fracture toughness of random polypropylene composite laminate via multiscale reinforcing formed by introducing functional nanofibrillated cellulose

Interface engineering and phase engineering of composites offer a promising route to rationally tune the interlaminar toughness of laminates, as high toughness plays an important role in preventing interlaminar failure and ensuring the safety of components. Herein, unlike the previous melt-mixing process, we coated the second reinforcement TONFC (TEMPO-oxidized nanofibrillated cellulose) on the surface of the main reinforcement (continuous glass fiber) by sol impregnation, and successfully prepared the commingled yarn random polypropylene (PPR) composite laminate with high interlaminar fracture toughness. The surface groups of each raw material of the composite system before and after the reaction were characterized and thereby obtaining the interface strengthening mechanism. Besides, the heterogeneous nucleation effect of TONFC and the mechanism of inducing ꞵ-phase formation were revealed by analysis of crystallization behavior and crystal structure. Further, through a double cantilever beam (DCB) test and morphology analysis, the effect mechanism of the addition of TONFC on the interlaminar crack propagation of PPR composite laminate was explored. This work provides a rapid and efficient avenue for the interlaminar modification of commingled yarn composites.

Closed-form analysis of interlaminar crack initiation in angle-ply laminates

The layerwise discontinuous stiffness properties in composite laminates may cause highly localized, singular concentrations of interlaminar stresses. These stress singularities occur in the vicinity of free edges and may induce interfacial cracks, often referred to as delaminations. Since conventional design processes, based on classical laminate plate theory, only consider inplane stress components, they do not cover the so-called free-edge effect and the associated delamination onset and, therefore, disregard premature laminate failure. In the present study, interlaminar crack initiation in symmetric angle-ply laminates under tensile loading is investigated assuming the spontaneous formation of delaminations of finite length. A coupled stress and energy criterion is used requiring only two fundamental material parameters, the strength and the fracture toughness. The mechanical behaviour of the laminate is modeled by a highly efficient analytical closed-form approach providing results for the interlaminar shear stress in the interface and the energy release rates of potential cracks. The challenge of unknown interface fracture toughness is addressed by a parameter fit within physically reasonable limits. The resulting effective failure loads are compared to experimental data and show the high potential of the coupled criterion on the one hand and of the proposed analytical model on the other hand.

A comparative study of a quasi 3D woven composite with UD and 2D woven laminates

To develop a composite with high in-plane properties and sufficient delamination resistance, a quasi-three-dimensional (Q3D) woven composite has been designed. In Q3D, some yarns in each layer are woven into those in the adjacent layers and form a 3D interlocking network. This study examines the performance of a carbon fiber/SC-15 epoxy triaxial Q3D [(0°/60°/−60°)4] composite. For comparison, a [0°/60°/−60°]4 composite laminate with unidirectional (UD) plies and a [0°/60°/−60°]4 laminate with two-dimensional woven (2DW) layers were fabricated using the same carbon fiber and epoxy system. The in-plane tensile, interlaminar fracture, and low-velocity impact tests were performed for all three types of composites. The results show that the Q3D composite has the in-plane tensile modulus and strength comparable to that of UD and 2DW composites, but offers a higher resistance to interlaminar crack propagation and impact damage. This work provides insights into the influences of fiber structures on composite properties and performances.

Damage accumulation analysis of cfrp cross-ply laminates under different tensile loading rates

This paper investigates the loading rate effect on both mechanical properties and damage accumulation process of [0°2/90°4]S carbon fiber-polymer laminates under tensile loading. In-situ edge observations, Acoustic Emission and Digital Image Correlation techniques were utilized simultaneously to monitor the state of damage in real time. Results showed that the axial modulus and strength were less sensitive to loading rates than failure strain, which increased with the decrease of the loading rate. In the viewpoint of damage accumulation process, high density and uniform distribution of transverse matrix cracks, and H-shape crack patterns, incorporating inter-laminar cracks, were more likely to occur at low loading rates while variable crack spacing occurred at higher rates. When loading rates were lower than a certain level, maximum transverse matrix crack density decreased slightly due to the restriction of relatively widely generated inter-laminar cracks. Furthermore, the cumulative acoustic emission energy of low-frequency signals was linearly correlated to transverse matrix crack density, providing a promising way to quantify crack accumulation in real time. Finally, spatial consistence was observed between transverse matrix cracks at edges and stress concentrations at the exterior 0° ply, and the peaks of axial strain at local concentration regions locate either near the newest cracks or at the place with minimum crack spacing.

Energy absorption and failure pattern of hybrid composite tubes under quasi-static axial compression

Novel fiber-metal hybrid tubes with overlapped fiber reinforced plastics (FRP) and aluminum layers were proposed in this paper. By combining progressive failure behavior of composites and large plastic deformation of metal materials, the structure was hybridized at mesoscopic scale. To this end, circular fiber/aluminum hybrid tubes with various inner diameters (40, 60, and 80 mm) combined with parent materials (Al-1060, carbon fiber, and glass fiber) were fabricated using vacuum bag molding process. Quasi-static uniaxial compressive tests were conducted to comprehensively explore the effects of geometric factors and failure patterns on energy absorption capability. The experimental results revealed that the pristine FRP and CF/Al (carbon fiber and aluminum) hybrid tubes collapsed in a progressive failure mode and generated abundant intra- and inter-laminar cracks during crushing. The GF/Al (glass fiber and aluminum) hybrid tubes with large diameters collapsed in an unstable and inefficient mode due to local buckling and delamination. Compared with bare aluminum tubes (Al 6061-T6), the specific energy absorption (SEA) and crushing force efficiency (CFE) of CF/Al hybrid structures were improved significantly by respectively 54.3% and 40.4% for tubes with 40 mm inner diameter. Furthermore, the CFE of CF/Al hybrid structures improved by more than 40% when compared to their bare CFRP counterparts, though the SEA reduced by more than 5.5%. In sum, the proposed hybrid design efficiently reduced the peak crushing force with desirable SEA, which has great potential for low-cost and lightweight energy absorber applications.

Real-time Detection of CMAS Corrosion Failure in APS Thermal Barrier Coatings Under Thermal Shock

Calcium-magnesium-alumina-silicate (CMAS) corrosion has been regarded as the most important factor that leads to the degradation of thermal barrier coatings (TBCs). The failure mechanism of TBCs attacked by CMAS corrosion in the actual service conditions is still not clear due to the lack of an environmental simulator and nondestructive testing techniques. To solve the above problems, a real-time acoustic emission method combined with infrared thermography are developed to investigate the failure mechanism of TBCs attacked by CMAS corrosion. The results show that the acoustic emission signal spectrum only depends on the failure mode of the TBCs, and five failure modes are identified: surface vertical cracks, sliding interfacial cracks, opening interfacial cracks, substrate deformation and noise. The lifetime of TBCs attacked by CMAS corrosion is 40 thermal shock cycles, which is nearly six times lower than that of TBCs without CMAS corrosion (350 cycles). Conclusions: The failure mechanism of the former is interlaminar cracking and delamination in the ceramic coating; while that for the latter is interfacial delamination in the vicinity of thermal growth oxide.

Improving delamination resistance through tailored defects

Delamination is a well-known damage mode exhibited by laminated composites which has been extensively studied in unidirectional laminates (UD). In this work, a novel strategy consisting of the introduction of a small interlaminar defect to enhance the fracture resistance to delamination of UD composites is presented. This toughening concept is based on the simultaneous propagation of multiple interlaminar cracks along different interfaces. A preliminary numerical study showed that the minimum size required by the initial defect to trigger dual propagation was 4 times the ply thickness for intermediate and thin plies for any applied mixed mode, except for pure mode II. An experimental campaign to prove the toughening concept with 1 and 3 defects at adjacent interfaces was carried out and compared against a reference configuration under three different applied mode mixities (ϕglob=0.4,0.2 and 0). The single defect configuration reached an improvement in the fracture resistance of +300% under pure applied mode I, and +80% and +100% for applied mixed modes of 0.2 and 0.4, respectively. The configuration with 3 defects reached even higher fracture toughness values: +430% in pure applied mode I, and +115% and +100% for applied mixed modes of 0.2 and 0.4, respectively. A finite element model employing cohesive zones successfully predicted both, the phenomenon of multiple crack propagation, and the effective fracture resistance of the process. Interestingly, among the dissipation mechanisms reported experimentally, the extensive promotion of fiber bridging under applied pure mode I was observed, which was not reported in the baseline configuration.

Mode I interlaminar crack length prediction by the resistance signal of the integrated MWCNT sensor in WGF/epoxy composites during DCB test

The interlaminar peformance of composites palys an important role in the damage process of fiber reinforced composites. In-situ monitoring by an embedded sensor could avoid the sudden failure. This paper investigates the in-situ monitoring of mode I interlaminar crack propagationg behavior in woven glass fiber reinforced epoxy (WGF/epoxy) composite laminates. The resistance signal of multi-walled carbon nanotube (MWCNT) located in the mid-thickness of WGF/epoxy during the double cantilever beam (DCB) test is recorded. Relationship between the relative resistance and the interlaminar crack length is theoretically built and experimentally verified. Results show that the mode I crack propagating length can be successfully predicted by the relative resistance. A linear relationship between the interlaminar crack propagating length and the total relative resistance is found by the experimental and theoretical results.

Morphological characteristics and formation mechanisms of the UD-CFRP drill exit-damages

Carbon fiber-reinforced plastic (CFRP) is used widely in many industries. During drilling CFRP, exit drill-induced defects such as burrs and delamination are easy to occur. In the existing model, the crack zone is assumed as a circle or an ellipse, and concrete forming process of the delamination and the burrs does not be truly involved. In this study, the formation mechanisms of the delamination and the burrs are analyzed based on the analytical model and the experimental observation. The results indicate that the critical thrust force only reflects the vulnerability of the crack, but cannot determine the size of the crack. The critical deflection of the fibers tip corresponding to the interlaminar crack extension point determines the size of crack. The burrs appear in four fundamental types.

Application of the Optoacoustic Method to Assess the Effect of Voids on the Crack Resistance of Structural Carbon Plastics

The results of an experimental analysis of the effect of the volume content of voids in the range of 0.25–5% on the crack resistance of structural carbon plastics are presented. To obtain a variation of the volume content of voids, samples were manufactured using the vacuum-infusion method with variations in the vacuum strength from –760 to –150 mm Hg. The total volume content of voids was determined by methods of optoacoustic structuroscopy, scanning electron microscopy, and chemical etching to obtain comparative data. The experimental dependences of the characteristics of the interlaminar crack resistance on the volume content of voids under static and cyclic loading of samples in the normal-separation mode were obtained.

Second-order moiré method for accurate deformation measurement with a large field of view.

In this study, we propose a second-order moiré method by performing digital sampling at two stages to realize high-accuracy deformation measurement in a wide field of view, where a grid image is recorded at a low magnification. Simulations have verified that this method has high strain measurement accuracy when the grid pitch is close to or even smaller than two pixels for both parallel and oblique grids with random noise. As an application, the two-dimensional microscale strain distributions of a carbon fiber reinforced plastic specimen when the grid pitch was about 2.1 pixels were presented. Shear strain concentration was detected before an interlaminar crack emerged, and tensile strain concentration was found prior to the occurrence of a transverse crack. The proposed method makes the two-step phase-shifting technique achieved indirectly, not only enlarging the field of view, but also maintaining the measurement accuracy.

Interlaminar to intralaminar mode I and II crack bifurcation due to aligned carbon nanotube reinforcement of aerospace-grade advanced composites

Aerospace-grade unidirectional carbon microfiber reinforced epoxy prepreg composite laminates were reinforced in the relatively weak interlaminar regions with high densities (>10 billion nanofibers per cm2) of uniformly-distributed vertically aligned carbon nanotubes (A-CNTs), creating a hierarchical architecture termed “nanostitch”. Such nanostitched laminates have been shown to increase laminate in-plane and interlaminar shear strengths. Here, the Mode I and Mode II fracture behavior and associated toughening mechanisms are investigated experimentally by performing double cantilever beam and end-notched flexure tests, respectively, of unidirectional 0° laminates following the ASTM Standards. Investigation of the crack surfaces via microscopy and micro-computed tomography (μCT) show that in both Mode I and II, the interlaminar crack bifurcates into the intralaminar region from the interlaminar precrack, and then propagates within the intralaminar region parallel to the nanostitched interlaminar region as an “intralaminar delamination” in steady state. This before unobserved phenomenon is attributed to the A-CNTs adding interlaminar toughness to a level that causes the interlaminar crack to bifurcate into the less tough intralaminar region. Microscopy and μCT analyses reveal that the A-CNTs do not increase the interlaminar thickness, and drive the crack into the intralaminar region within 1–2 mm of crack initiation in both Mode I and II, with the distance of the “intralaminar delamination” crack from the interlaminar region (at the laminate centerline) being greater in Mode II than in Mode I (~30 μm vs. ~15 μm, respectively). Finite element simulation of the crack bifurcation in Mode I predicts a minimum of 10% increase in interlaminar toughness due to the A-CNTs to propagate the crack in the intralaminar region in steady state, as observed experimentally. This unique crack behavior in advanced composites provides new insights into the magnitude and effects of reinforcement induced by A-CNTs that influence the macroscopic fracture and failure behavior of laminates, and suggests new opportunities for toughening laminates.