A ply-level electrical resistance approach to monitor crack evolution in a laminate SiC/SiC composites

In the aim of providing a reliable technique to monitor the development of damage in 0°/90° melt-infiltrated SiC-fiber reinforced prepreg laminate ceramic-matrix composites, it was hypothesized that the electrical resistivities of different layers of this material were significantly different due to their free Si content and morphology. Three distinct layers: a 0° fiber ply, a 90° fiber ply and a matrix only ply, were distinguished in the composite architecture. Free silicon is the most conductive phase in this composite system; however, the Si content and morphology were different in each of the three types of plies. Unidirectional and [0°/90°]_2s specimens enabled quantification of ply-level resistivities. An electric circuit model was constructed; it consists of parallel resistors where each resistor represents a ply in the composite system. This ply-level electrical model was validated using composites of different vintages which contained different silicon contents. A room temperature stepped fatigue test was conducted and the ply level circuit model was used to discern crack morphology with the support of acoustic emission and digital image correlation.     

Modelling complex progressive failure in notched composite laminates with varying sizes and stacking sequences

The emergence of advanced computational methods and theoretical models for damage progression in composites has heralded the promise of virtual testing of composite structures with orthotropic lay-ups, complex geometries and multiple material systems. Recent studies have revealed that specimen size and material orthotropy has a major effect on the open hole tension (OHT) strength of composite laminates. The aim of this investigation is develop a progressive failure model for orthotropic composite laminates, employing stepwise discretization of the traction–separation relationship, to predict the effect of specimen size and laminate orthotropy on the OHT strength. The results show that a significant interaction exists between delamination and in-plane damage, so that models without considering delamination would over-predict strength. Furthermore, it is found that the increase in fracture toughness of blocked plies must be incorporated in the model to achieve good correlation with experimental results.     

Use of 2D Graphene Nanoplatelets (GNP) in cement composites for structural health evaluation

This study investigates the use of Graphene Nanoplatelets (GNP) in cement composite to quantify the material damage extent. The damage sensing capability of this new cement composite is demonstrated by experimentally measuring the electric potential across prisms with a known notch depth and comparing it with the finite element simulations. Meanwhile, the damage extent is directly related to the change in electric potential based on a mathematical analogy between the electrostatic field and the elastostatic field under anti-plane shear loading. It is shown that the fractional change in electric potential arising from damage is equivalent to the fractional change in elastic compliance, which can be exploited for structural health evaluation.     

Monitoring the damage evolution of flexural fatigue in unidirectional carbon/carbon composites by electrical resistance change method

This paper reported simultaneous monitoring damage evolution of flexural fatigue in unidirectional carbon-fiber-reinforced carbon composites (C/C composites) by electrical resistance change (ERC) methods. The degree of irregularity in electrical resistance changes increased with stress levels increasing. The shapes of electrical resistance change rate–fatigue cycle curves can reflect stress levels and damage types of tested samples: sawtooth shapes reflected delamination at a higher stress level; and “peak” shapes reflected inner damages in one fiber bundle at the fatigue limit stress level. In addition, the similarity of initial electrical resistance–fatigue life curve and S–N curve was observed clearly. In summary, ERC methods can monitor the damage evolution and qualitatively estimate the fatigue life of unidirectional C/C composites.     

Z-fiber influence on high speed penetration of 3D orthogonal woven fiber composites

In the current work we present a computational investigation of high speed penetration response of 3D orthogonal woven fiber composites (3D OWC) utilizing sub-unit cell, meso-level partitioned damage mechanics with the specific aim of understanding the role of Z-fibers in the mechanical response. In our model, two primary sources of nonlinearities have been addressed – one resulting from the strain rate dependence and large deformation of the composite constituents and the other from evolving failure. We reduce a number of arbitrary parameters typically present in high speed models by taking advantage of specific geometrical properties of 3D OWC which prevent extensive delamination. This property allows us to partition the structure into resin impregnated fibers assumed to be wholly responsible for the progressive damage behavior and bulk resin which is identified as the source of visco-plasticity and strain rate dependence. The fibers are modeled as anisotropic linear elastic with strain rate dependent progressive damage evolution. The resin is modeled using an advanced high strain rate large deformation Mulliken–Boyce polymer model (Mulliken and Boyce 2006) together with a terminal thermo-mechanical failure criterion. The projectile is assumed to be cylindrical, isothermal, rigid and impacting at right angles to the plate. The shape of the damaged area and the extent of penetrative damage compares favorably with experiments. We find that Z-fibers aid in improving penetration and impact resistance by both energy absorption and structural engagement. However, we also find that they are susceptible to localized de-bonding especially around the winding crowns. In addition, we found crucial differences in mechanical response in wave propagation brought about by the interplay of fiber architecture and damage with respect to simplified membrane models.Finally, the Z-fibers were found to influence the shape and nature of the damaged area in the fibers compared to layered composites where the matrix damage is spread more evenly while the fiber damage is restricted towards the fiber axes directions.     

Using thin-plies to improve the damage resistance and tolerance of aeronautical CFRP composites

Thin-ply composites are currently receiving specific attention from researchers due to their capabilities to delay matrix cracking. In this paper, the aim is to design a hybrid laminate that contains both thin- and normal plies. The objective is to improve the tolerance of normal plies by adding thin-plies to the composite in different configurations. Two alternatives were designed, tested, and compared to the specimens made of traditional plies. Impact and compression after impact tests were conducted on each configuration at different impact energies. After being impacted, the specimens were c-scanned to define the delamination pattern. Results showed that surrounding each normal ply with two thin-plies improved the delamination threshold by 15% as compared to the specimens made all of normal plies. Under compression, 15% improvements in the compression after impact strength were obtained. By using thin-plies, the size of each individual delamination was reduced, resulting in small threads instead of peanut delaminations.     

Damage in biocomposites: Stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading

The non-linear stress–strain behaviour of plant fibre composites is well-known in the scientific community. Yet, the important consequences of this, in terms of the evolution of stiffness as a function of applied strain and cycles to failure, are not well-studied in literature. This is despite the fact that stiffness degradation is a well-accepted indicator of damage in a composite material, and is regularly used as a component failure criterion. This article systematically explores the evolution of stiffness of various aligned plant fibre composites, subjected to (i) monotonic loading, (ii) low-cycle, repeated progressive loading, and (iii) fatigue loading. The evolution in stiffness in plant fibre composites is found to be complex: structural changes in the elementary fibre cell wall and damage development in the composite have often competing effects on stress–strain behaviour. Indeed, the evolution in stiffness of plant fibre composites is found to be unlike that typically observed in traditional composites, and therefore needs to be taken into account in the design of structural components.     

Characterising the loading direction sensitivity of 3D woven composites: Effect of z-binder architecture

Three different architectures of 3D carbon fibre woven composites (orthogonal, ORT; layer-to-layer, LTL; angle interlock, AI) were tested in quasi-static uniaxial tension. Mechanical tests (tensile in on-axis of warp and weft directions as well as 45° off-axis) were carried out with the aim to study the loading direction sensitivity of these 3D woven composites. The z-binder architecture (the through-thickness reinforcement) has an effect on void content, directional fibre volume fraction, mechanical properties (on-axis and off-axis), failure mechanisms, energy absorption and fibre rotation angle in off-axis tested specimens. Out of all the examined architectures, 3D orthogonal woven composites (ORT) demonstrated a superior behaviour, especially when they were tested in 45° off-axis direction, indicated by high strain to failure (∼23%) and high translaminar energy absorption (∼40 MJ/m³). The z-binder yarns in ORT architecture suppress the localised damage and allow larger fibre rotation during the fibre “scissoring motion” that enables further strain to be sustained by the in-plane fabric layers during off-axis loading.