Applied Composite Materials (v.15, #3)

Detecting Damage in Composite Material Using Nonlinear Elastic Wave Spectroscopy Methods by Michele Meo; Umberto Polimeno; Giuseppe Zumpano (115-126).
Modern aerospace structures make increasing use of fibre reinforced plastic composites, due to their high specific mechanical properties. However, due to their brittleness, low velocity impact can cause delaminations beneath the surface, while the surface may appear to be undamaged upon visual inspection. Such damage is called barely visible impact damage (BVID). Such internal damages lead to significant reduction in local strengths and ultimately could lead to catastrophic failures. It is therefore important to detect and monitor damages in high loaded composite components to receive an early warning for a well timed maintenance of the aircraft. Non-linear ultrasonic spectroscopy methods are promising damage detection and material characterization tools. In this paper, two different non-linear elastic wave spectroscopy (NEWS) methods are presented: single mode nonlinear resonance ultrasound (NRUS) and nonlinear wave modulation technique (NWMS). The NEWS methods were applied to detect delamination damage due to low velocity impact (<12 J) on various composite plates. The results showed that the proposed methodology appear to be highly sensitive to the presence of damage with very promising future NDT and structural health monitoring applications.
Keywords: Nonlinear elastic wave spectroscopy; Delamination; Damage detection

Current applications require an understanding of the relationships that exist between the functional characteristics of composite material systems and the long-term behavior of those materials under mechanical loading. In the present work, Electrochemical Impedance Spectroscopy was used to characterize damage initiation and progression in woven glass/epoxy composites that were fatigued loaded out of plane. A systematic approach was taken to establish a proof of concept that electrochemical impedance at different frequencies of excitation in woven glass/epoxy composites is related to the development of damage in that material under cyclic end-loaded bending. Impedance spectroscopy measurements were found to show large, consistent, and clear distinctions as damage developed, and also proved capable of detecting non-visual localized damage in thin specimens.
Keywords: Electrochemical impedance spectroscopy; Woven composites; Out of plane bending; End-loaded fatigue

The objective of the study was to characterise the energy absorption of composite panels with tied cores, subjected to a drop weight impact test. Numerical simulations based on explicit finite element analysis have successfully modelled low velocity impact tests carried out on sandwich panels with web-core structure and plastic foam. The numerical model has been validated in terms of the failure behaviour of the panel and the variation of the contact force after the initial peak load corresponding to flexural failure. The numerical model is used for a better interpretation of the test results and of the failure mechanisms within the structure. The contribution to the overall energy absorption of the different parts composing the panels has been studied, with the aim of evaluating the feasibility of using low density foam in combination with web-core reinforcement in structural applications.
Keywords: Web-core; Sandwich; Impact; FEA

Thermal Residual Stress Distribution in Carbon Fiber/Novel Thermal Plastic Composite by Chun Lu; Ping Chen; Qi Yu; Julong Gao; Baijie Yu (157-169).
Thermal residual stress is one of the major factors affecting composite mechanical performance. In this paper, a 3-D FEA technique was utilized to analyze the thermal residual stress distribution in Carbon fiber/PPESK composite. Parabolic failure criterion was used to predict composite potential failure zone. Results indicate that, thermal residual stress distributions in different parts of the composite are different. At composite free end zone, the maximum thermal residual stress is located at fiber surface; in composite inner zone, the maximum stress is located in the matrix; at composite defect zone, stress concentration is located at defect surface. Thermal residual stress at composite free end zone will lead to fiber–matrix interfacial de-bonding. Thermal residual stress concentration at composite defect zone will decrease composite mechanical performance.
Keywords: Thermal residual stress; Thermoplastic composite; Poly (phthal azione ether sulfone ketone); FEM

A model for predicting the transverse coefficients of thermal expansion (CTE) for carbon fibre composites is presented in this paper. The transverse CTE were calculated by finite element analysis using a representative unit cell. The analytical micromechanical models from literature were reviewed by comparing with the FEA data. It shows that overall Hashin model provides the best accuracy. However, the calculating process of Hashin model is very complicated and inconvenient for practical applications. By using FEA, Design of Experiments (DOE), and Response Surface Method (RSM), the transverse CTE of unidirectional carbon fibre composites were studied and a regression-based model was developed. The model was validated against the FEA and experimental data. It shows that the developed model offers excellent accuracy while reduces complicated computation process. The advantage of this model is that it provides a simple and accurate method for predicting the transverse CTE of composites, which helps effective and efficient design of composite structures.
Keywords: Coefficient of thermal expansion; Composites; Carbon fibre