Determining the Stages of Deformation and Destruction of Composite Materials in a Static Tensile Test by Acoustic Emission

Composite materials are used in many industries. They are construction materials that are being used more and more often, which makes it necessary to accurately identify the process of their destruction. Recent decades have resulted in an intensive increase in diagnostic tests of structures and mechanical elements. Non-destructive testing (NDT) represents a group of test methods (surface and volumetric) that provide information about the properties of the tested element without changing its structure. The method of acoustic emission (AE) is also being used more frequently. Thanks to the ability to detect and locate signal sources, as well as to perform tests during operation, it is a method that is increasingly used in industry. In this article, the acoustic emission was used to analyze the changes occurring in composite materials. Obtained parameters helped to determine the signals originating from fibre delamination, fibre cracking, etc., as well as the starting point of these changes and the stress values at which these changes occurred. The analysis of acoustic emission signals recorded during the tests helped to determine the values of amplitudes characteristic for the destruction mechanisms of considered composite materials. Signals with an amplitude in the range of 30–41 dB may indicate elastic–plastic deformation of the matrix. Signals with an amplitude in the range of 42–50 dB indicate matrix cracks with the accompanying phenomenon of fibre delamination. Signals with amplitudes greater than 50 dB indicate fibre breakage. Based on the test results, the permissible stress was determined; when exceeded, the mechanisms of damage to the structure of composite materials accumulate. This stress limit for the tested material is 70 MPa. The use of the acoustic emission method in mechanical tests may contribute to a greater knowledge of composite materials used as a construction material, as well as determine the stresses allowable for a given structure.

Modeling Stiffness Degradation of Fiber-Reinforced Polymers Based on Crack Densities Observed in Off-Axis Plies

A model that predicts the stiffness degradation in multidirectional reinforced laminates due to off-axis matrix cracks is proposed and evaluated using data from fatigue experiments. Off-axis cracks are detected in images from the fatigue tests with automated crack detection to compute the crack density of the off-axis cracks which is used as the damage parameter for the degradation model. The purpose of this study is to test the effect of off-axis cracks on laminate stiffness for different laminate configurations. The hypothesis is that off-axis cracks have the same effect on the stiffness of a ply regardless of the acting stress components as long as the transverse stress is positive. This hypothesis proves to be wrong. The model is able to predict the stiffness degradation well for laminates with a ply orientation similar to the one used for calibration but deviates for plies with different in-plane shear stress. This behavior can be explained by the theory that off-axis cracks develop by two different micro damage modes depending on the level of in-plane shear stress. It is found that besides influencing the initiation and growth of off-axis cracks, the stiffness degradation is also mode dependent.

Mechanical and Microstructural Assessment of Inhomogeneities in Oxide Ceramic Matrix Composites Detected by Air-Coupled Ultrasound Inspection

Ceramic Matrix Composites (CMC) are promising materials for high-temperature applications where damage tolerant failure behavior is required. Non-destructive testing is essential for process development, monitoring, and quality assessment of CMC parts. Air-coupled ultrasound (ACU) is a fast and cost-efficient tool for non-destructive inspections of large components with respect to the detection of material inhomogeneities. Even though ACU inspection is usually used for visual inspection, the interpretation of C-scan images is often ambiguous with regard to critical defects and their impact on local material properties. This paper reports on a new approach to link the local acoustic damping of an oxide CMC plate obtained from ACU analysis with subsequent destructive mechanical testing and microstructural analyses. Local damping values of bending bars are extracted from ACU maps and compared with the results of subsequent resonant frequency damping analysis and 3-point bending tests. To support data interpretation, the homogeneous and inhomogeneous CMC areas detected in the ACU map are further analyzed by X-ray computed tomography and scanning electron microscopy. The results provide strong evidence that specific material properties such as Young’s modulus are not predictable from ACU damping maps. However, ACU shows a high, beneficial sensitivity for narrow but large area matrix cracks or delaminations, i.e., local damping is significantly correlated with specific properties such as shear moduli and bending strengths.

Experimental Investigation on the Low Velocity Impact Response of Fibre Foam Metal Laminates

The combination of fibre metal laminates (FML) and sandwich structures can significantly increase the performance under impact of FMLs. The goal of this work was to create a material that will combine the superior properties of FMLs and foam sandwich structures in terms of the impact resistance and simultaneously have lower density and fewer disadvantages related to the manufacturing. An extensive impact testing campaign has been done using conventional fibre metal laminates (carbon- and glass-based) and in the proposed fibre foam metal laminates to assess and compare their behaviour. The main difference was observed in the energy absorption mechanisms. The dominant failure mechanism for fibre foam laminates is the formation of delaminations and matrix cracks while in the conventional fibre metal laminate the main failure mode is fibre cracking due to high local stress concentrations. The reduction in the fibre cracking leads to a better after-impact resistance of this type of structure improving the safety of the structures manufactured with these materials.

EXPERIMENTALLY VALIDATED SIMULATIONS OF BLIND HOLE DRILLING IN 3D WOVEN CARBON/EPOXY COMPOSITE WITH PROCESSING-INDUCED RESIDUAL STRESSES

Manufacturing-induced residual stresses in carbon/epoxy 3D woven composites arise during cooling after curing due to a large difference in the coefficients of thermal expansion between the carbon fibers and the epoxy matrix. The magnitudes of these stresses appear to be higher in composites with high throughthickness reinforcement and in some cases are sufficient to lead to matrix cracking. This paper presents a numerical approach to simulation of development of manufacturing-induced residual stresses in an orthogonal 3D woven composite unit cell using finite element analysis. The proposed mesoscale modeling combines viscoelastic stress relaxation of the epoxy matrix and realistic reinforcement geometry (based on microtomography and fabric mechanics simulations) and includes imaginginformed interfacial (tow/matrix) cracks. Sensitivity of the numerical predictions to reinforcement geometry and presence of defects is discussed. To validate the predictions, blind hole drilling is simulated, and the predicted resulting surface displacements are compared to the experimentally measured values. The validated model provides an insight into the volumetric distribution of residual stresses in 3D woven composites. The presented approach can be used for studies of residual stress effects on mechanical performance of composites and strategies directed at their mitigation.

STRENGTH SIZE EFFECT IN FIBER COMPOSITES FAILING UNDER LONGITUDINAL AND TRANSVERSE COMPRESSION

The size effect in the structural strength of fiber reinforced composites has been typically analyzed for tensile failures. However, this is not true for the equally important compressive failures, primarily due to the difficulties in conducting compression tests on specimens of multiple sizes. These size effects are analyzed here numerically for two important compressive failure mechanisms in composites, viz. (i) fiber kink bands forming under longitudinal compression (typically accompanied by axial splitting matrix cracks) and (ii) inclined shear cracks forming under transverse compression. The former mechanism is modeled by a semi-multiscale microplane model, while the latter by the fixed crack model. Both models are calibrated and verified using available test data on carbon fiber composites and then used to predict the failure and load bearing capacities of geometrically scaled pre-cracked specimens of different sizes. In all cases, the predicted failure is found to be of a propagating nature, accompanied by release of strain energy from the specimen causing a distinct size effect in the nominal strength. For the composite considered here, under longitudinal compression, the fracture process zone (FPZ) is found to be fairly small (<1 mm) and the strength size effect is seen to follow linear elastic fracture mechanics (LEFM). The size effect deviates from LEFM for smaller specimen sizes due to increased flaw size insensitivity but cannot be fitted by Bažant's size effect law since the geometric similarity of the failure mode is lost. On the other hand, under transverse compression the FPZ is found to be much larger (34 to 42 mm) and the size effect is found to obey Bažant's size effect law, deviating from LEFM. The failure is geometrically similar despite being inclined to the pre-crack. These findings provide evidence of the general applicability of fracture mechanics-based size effect laws to compressive failure in fiber composites, and prompt suitable experimental investigations.

PREDICTION OF NUMBER OF CYCLES FROM FATIGUE TEST CONDITIONS AND TRANSVERSE CRACK DENSITY OF CFRP CROSS-PLY LAMINATES

In the present paper, we proposed a methodology that can predict the number of applied load cycles in tension-tension fatigue test of CFRP laminates from microscopic damages and test conditions. It is difficult to predict the fracture of CFRP laminates and to estimate the remaining life of CFRP laminates for ensuring the long-term reliability of the CFRP components because the fracture process of CFRP laminates is quite complex. The damage process of CFRP consists of various microscopic damage such as matrix cracks, fiber/matrix interfacial debondings, delamination and so on. In order to quantitatively estimate the remaining life of CFRPs, we focused on the degree of the microscopic damages and relate that to the remaining life of them. The tension-tension fatigue tests of CFRP cross-ply laminates were carried out, and we suspended the tests at arbitrary cycles. When the tests were suspended, we counted the number of transverse cracks occurred on the specimens by a replica method, and measured the stiffness degradation of the specimens. We formulated an equation that can predict the stiffness degradation using fatigue test conditions. The predicted stiffness degradation to the number of cycles using the formula agreed well with the experimental results. The result demonstrated that the formula can predict the number of subjected cycles from fatigue test conditions and transverse crack density.

IMPACT DAMAGE AND INJECTION REPAIR STRENGTH RESTORATION

Impact damage to composite structures can lead to a range of damage modes. Of interest is modest damage composed of delaminations less than 50 mm in size, and no visible impact-side fiber breakage. While resin injection is a current-practice repair technique that can be used to address these damage modes in a manner that is much less invasive than bonded scarf patch repair, the injection technique is not currently credited as one restoring strength back to laminate. Issues of quantifying the removal of any internal contamination, assessing degree of resin fill, and demonstration of how much strength is restored are being addressed within the scope of this research activity. Resin injection will be conducted and the resulting strength restoration assessed in local fracture tests (end-notch flexure). The formation of actual impact damage morphologies, namely multiple planes of delamination interconnected with matrix cracks, is a critical aspect of this problem. Three 25-ply composite panel types having varying percentage of 0o fiber content have been impacted under low velocity at a range of energy levels. Resulting force vs. time and ultrasonic mapping of damage extent. Damage produced by such impacts will be used in subsequent injection repair studies. Intentional contamination will be introduced, and then removal will be achieved via injected solvents and atmospheric plasma, with monitoring of contaminant presence achieved by in-line quantitative chemical analysis.