G. Zhou

Damage detection and assessment in fibre-reinforced composite structures with embedded fibre optic sensors-review

A state-of-the-art review is presented regarding the research and development of in situ fibre optic damage detection and assessment systems (FODDAS) embedded in fibre-reinforced composite structures. Representative individual fibre optic strain sensors and distributed sensor networks are briefly described. A major emphasis is placed on their capabilities for detecting damage, determining damage location and assessing the nature of damage, arising primarily from specific events such as impacts or quasi-static stress overloads. The main features of such systems as custom-built and structure-specific units with minimal human involvement are highlighted. Issues that could affect the validity of the performance of such strain sensors are discussed. Fracture and non-fracture of fibre optic sensors are identified as two fundamentally different approaches for damage detection and their primary features are discussed in relation to location determination and evaluation of the nature of damage. The major advantages and limitations of each approach are discussed. Directions and areas of potential future research in the development of related FODDAS are highlighted.

Impact Behaviour of Fibre-reinforced Composite Materials and Structures

Impact behaviour of fibre-reinforced composites Recent developments in impact damage assessment of fibre composites Modelling impact of composite structures using small specimens Impact damage - tolerant composite structural design Damage resistance and tolerance of thick laminated woven roving GFRP plates subjected to low-velocity impact Elastic impact stress analysis of composite plates and cylinders Impact behaviour and analysis of CFRP laminated plates Perforation of FRP laminates and sandwich panels subjected to missile impact High velocity impact damage to polymer matrix composites.

Impact damage and residual strengths of woven fabric glass/polyester laminates

Thick glass/polyester laminates of four different dimensions subject to low-velocity impact have been investigated using a guided drop-weight test rig with a flat-ended impactor in ascending energy order up to 3100 J. The characteristics of impact response and energy absorption have been determined by impact force and absorbed energy histories, and impact damage incurred was examined by cross-sectioning and ultrasonic C-scanning. Residual compressive strengths were measured, and the damage tolerance of the laminates was assessed by the retaining ability of these strengths. It is found that the salient features in force-time history curves can be related to fracture processes occurring in the laminates, and that the established relationships between impact force and incident kinetic energy (IKE) can be used to identify damage initiation without examining impacted specimens, which is later confirmed by the damage force maps. The constructed damage force and energy maps have shown not only damage initiation in an unstable fashion but also increase of damage size with IKE and force until reaching their load-bearing capabilities. Residual compressive strengths are reduced very rapidly with the increase of impact damage due to extensive delamination.

Numerical modelling of impact damage

A method is described for predicting barely visible impact damage in realistic composite structures. It avoids a three-dimensional finite element analysis by ‘embedding’ coupon tests as a calibrator in the finite element model of the larger actual structure. Predictions are then made for all the dynamic responses including force and strain histories. A theoretical method is devised for predicting threshold fracture loads. This strategy is successfully validated by tests for carbon composite structures, and then extended to glass and Kevlar laminated plates with varying degrees of success.

Impact response of thick glass fibre reinforced polyester laminates

Abstract Thick glass/polyester woven roving laminated plates subject to low-velocity impact have been investigated using a guided drop-weight test rig in ascending energy order up to 1500 J. The impact response and energy-absorbing characteristics have been determined by impact-forces and absorbed-energy histories, and by force-displacement relationships. Impact damage is examined by visual inspection, ultrasonic C-scan and an optical microscope so that a three-staged sequential damage model is proposed to characterize damage growth. Static tests are also conducted to examine strain rate effect. It is demonstrated that the maximum impact forces are increased by 36% for the thin plates and by 22% for the thick plates, although the initial threshold forces are less strain-rate sensitive. Two thicknesses of laminated plates are used to study the thickness effect, and the scaling rules are developed for the delaminated plates. It is shown that both the maximum static and impact forces, and the incident kinetic energy can be scaled by the thickness ratio if these laminates have the same diameter and their behaviour is dominated by shear. The finite element modelling is carried out for relatively low energy cases, the impact structural response is well captured by linear elastic solution.

Damage mechanisms in composite laminates impacted by a flat-ended impactor

Abstract In order to be able to predict impact damage, complex impact behaviour in laminate composites has to be well understood and quantitatively characterized. To achieve this, thick glass-fibre-reinforced laminates of various dimensions have been subjected to low-velocity impact with a flat-ended impactor in the energy range 15–3000 J. Post-impact examination of damage is carried out by visual inspection, diametric cross-sectioning, and ultrasonic C-scanning. A number of damage mechanisms have been observed, and their influence on impact behaviour is found to be generally dependent on the impact force or incident kinetic energy (IKE) level. Although post-impact techniques are very useful, impact force and IKE histories derived by using an instrumented drop-weight impact rig are essential to the quantitative interpretation of damage development. Delamination, fibre shear-out, and fibre fracture are found to be the predominant damage mechanisms in absorbing IKE and controlling the load-bearing capabilities of the laminates. They have been shown to occur in a sequential manner and to be geometry dependent.

The use of experimentally-determined impact force as a damage measure in impact damage resistance and tolerance of composite structures

An instrumented drop-weight impact rig, ultrasonic C-scan and CAI test rig form a basic experimental system needed for the quantitative investigation of impact damage resistance and damage tolerance assessment. It is shown that it is essential to have an impact rig designed with certain traits along with other selected impact conditions so that dominant damage mechanisms occurring during impact can be identified with the threshold forces of recorded impact force-time curves. The so-determined impact force data are used to study impact damage resistance. It is demonstrated that the ratio of measured threshold impact forces is a useful alternative to residual compressive strength usually used for damage tolerance assessment. This provides a fast and cheap technique without resort to complex and expensive CAI tests.

Prediction of impact damage thresholds of glass fibre reinforced laminates

Thick glass fibre reinforced laminates subject to low velocity impact have been investigated using an instrumented drop-weight test rig with a flat-ended impactor. Two sets (small and large) of circular specimens of four different thicknesses are used with incident kinetic energy (IKE) ranging from 15 J to 3000 J. Static tests of the small plates are also conducted. It is demonstrated that using impact response to predict the threshold values of damage initiation is significantly faster and cheaper because it does not need to examine impacted specimens. The ultimate thresholds are found to be dependent on the in-plane dimension of the laminates, two sets of specimens provide two bounds which become close as the thickness of the laminates increases. It is shown that the damage energy and force maps are effective in monitoring damage growth in addition to identifying the thresholds of damage initiation, which are close to the predicted values based on impact response. It is also shown that damage initiation when dominated by delamination can be predicted by a simple analytical model. It is found that strain rate effect is insignificant with moderate damage and becomes appreciable when the laminate load bearing capability is approached.

Damage Characteristics of Composite Honeycomb Sandwich Panels in Bending under Quasi-static Loading

Damage characteristics of composite-skinned honeycomb sandwich panels in bending are investigated with both hemispherical (HS) and flat-ended (FE) indenters. The thickness of the cross-ply skins varies from 8 to 16 plies, whereas the density of the 12.7-mm thick aluminum honeycomb core varies from 50 to 70 kg/m3. Clamped panels with a 100-mm testing area are loaded quasi-statically either in bending or on a rigid base. The effects of varying these parameters on damage mechanisms are examined through response curves as well as cross sections of selected specimens. Special emphasis is placed on their potential change induced by the variation of skin thickness and core density with a specific indenter. Damage mechanisms are identified as core crush, top-skin delamination, and fracture or shear-out. The threshold and ultimate loads as well as the initial slope increase significantly either on increase of skin thickness or change of the nose shape of indenter from a hemisphere to a flat-end. The increase in the post-initial-damage slope is small and can be attributed to membrane stretching of the damaged top skin. Increasing the core density affects substantially not only the threshold load, but also the initial slope associated with the FE indenter. Changing the nose shape of the indenter has an overriding effect on the nature of damage mechanisms. In particular, top-skin delaminations occur after core crush. The panel deflection contributes to 20-53% sandwich deformation. The bottom skin in all the tests remains intact.

Investigation of Parameters Governing the Damage and Energy Absorption Characteristics of Honeycomb Sandwich Panels

Honeycomb sandwich panels of various skin thicknesses and core densities have been investigated under quasi-static loading in bending and indentation with both hemispherical (HS) and flat-ended (FE) indenters. Core crushing, top skin delamination, and top skin fracture are identified as major damage mechanisms. Their characteristics and energy-absorbing capabilities are established using load—displacement and load—strain curves and inspections of cross-sectioned specimens. The effects of varying skin thickness, core density and type, indenter nose shape, and boundary conditions on the damage and energy-absorbing characteristics are examined. The variation of the indenter nose shape is shown to induce a change in the damage mechanisms and have the most significant effect on energy absorption, especially for panels with relatively thicker skins. Increasing the skin thickness significantly increases not only the initial threshold and ultimate loads but also the absorbed energy (AE) of the panels. Increasing the core density has a very small effect on either the ultimate loads or the energy-absorbing capacity, while the effect of the support conditions on the damage and energy-absorbing characteristics is small. The larger 220 mm diameter panels absorb significantly more energy than the 100 mm diameter panels because of the much greater ultimate displacement. Different core materials with a similar density show little difference in either the damage or the energy-absorbing characteristics due to the limited contribution of transverse shear resistance. Panels with a delaminated top skin have lower threshold loads under both indenters and lower ultimate load for the HS indenter.