M. de Freitas

Failure mechanisms on composite specimens subjected to compression after impact

Abstract Composite panels are widely used in aeronautic and aerospace structures due to their high strength/weight ratio. The stiffness and the strength in the thickness direction of laminated composite panels is poor since no fibres are present in that direction and out-of-plane impact loading is considered potentially dangerous, mainly because the damage may be left undetected. Impact loading in composite panels leads to damage with matrix cracking, inter-laminar failure and eventually fibre breakage for higher impact energies. Even when no visible impact damage is observed at the surface on the point of impact, matrix cracking and inter-laminar failure can occur, and the carrying load of the composite laminates is considerably reduced. The greatest reduction in loading is observed in compression due to laminae buckling in the delaminated areas. The objective of this study is to determine the mechanisms of the damage growth of impacted composite laminates when subjected to compression after impact loading. For this purpose a series of impact and compression after impact tests were carried out on composite laminates made of carbon fibre reinforced epoxy resin matrix. An instrumented drop-weight-testing machine and modified compression after impact testing equipment were used together with a C-scan ultrasonic device for the damage identification. Four stacking sequences of two different epoxy resins in carbon fibres representative of four different elastic behaviours and with a different number of interfaces were used. Results showed that the delaminated area due to impact loading depends on the number of interfaces between plies. Two buckling failure mechanisms were identified during compression after impact, which are influenced more by the delamination area than by the stacking sequence.

A Unified Numerical Approach for Multiaxial Fatigue Limit Evaluation

A unified numerical approach to evaluate the endurance limit for general multiaxial fatigue loading, under proportional or nonproportional loading, is presented. A minimum circumscribed ellipsoid approach is proposed for computing the amplitude and mean value of the equivalent shear stress, and an efficient numerical algorithm is developed for solving the min–max problem by using a constrained optimization method. The approach is general and efficient, especially suitable for computer-aided design and optimization. With this approach, nonproportional loading effects can be taken into account, overcoming limitations of many other multiaxial fatigue criteria. Experimental results collected from the literature are compared with results predicted by the numerical approach proposed in this paper. The comparison shows that the predictions agree well with experiments for both proportional and nonproportional loading situations.

Numerical evaluation of failure mechanisms on composite specimens subjected to impact loading

Abstract Composite panels are in common use, especially in aeronautic and aerospace structures due to their high strength/weight and stiffness/weight ratio. The out-of-plane impact loading is considered potentially dangerous mainly because the damage may be left undetected and because the loading itself acts in the through-the-thickness direction of the laminated composite panel. This direction is the weakest in the composite since no fibres are present in that direction. The impact loading can lead to damage involving three modes of failure: matrix cracking, delamination and eventually fibre breakage for higher impact energies. Even when no visible impact damage is observed at the surface on the point of impact, matrix cracking and delamination can occur, and the residual strength of the composite is considerably reduced. The objective of this study is to determine the mechanisms of the damage growth of impacted composite laminates when subjected to impact loading. For this purpose a series of impact tests were carried out on composite laminates made of carbon fibre reinforced epoxy resin matrix. An instrumented drop-weight-testing was used together with a C-scan ultrasonic device for the damage identification. Two stacking sequences of two different epoxy resins and carbon fibres, representative of four different elastic behaviours with a different number of interfaces were used. A numerical evaluation of these specimens was also carried out, using static analysis only. Results showed that the delaminated area due to impact loading depends on the number of interfaces between plies. Two failure mechanisms due to impact were identified, which are influenced by the stacking sequence and by the thickness of the panels.

Damage growth analysis of low velocity impacted composite panels

Abstract Low velocity impact loading in aircraft composite panels is a matter of concern in modern aircraft and can be caused either by maintenance accidents with tools or by in-flight impacts with debris. The consequences of impact loading in composite panels are matrix cracking, inter laminar failure and, eventually, fiber breakage for higher impact energies. Even when no visible impact damage is observed on the surface at the point of impact, matrix cracking and inter laminar failure can occur, and the carrying load of the composite laminates is considerably reduced. The greatest reduction in loading is observed in compression due to laminae buckling in the delaminated areas. The objective of this study is to determine the limit loading capacity and the damage growth mechanisms of impacted composite laminates when subjected to compression after impact loading. For this purpose a series of impact and compression after impact tests were carried out on composite laminates made of carbon fiber reinforced epoxy resin matrix. Four stacking sequences representative of four different elastic behaviours were used. Results show that the compressive, after impact, failure stress is influenced by the stacking sequence but a relatively independent strain to failure is observed.

The effect of microstructure and environment on fatigue crack growth in 7049 aluminium alloy at negative stress ratios

Abstract The influence of environment and microstructure on fatigue crack growth has been investigated on a high strength 7049 aluminium alloy. This aluminium alloy was artificially aged to underaged (UA) and overaged (OA) microstructures. The heat treatment procedure was performed in order to obtain an UA and OA microstructure having the same yield strength properties, but differing in the mode of slip deformation: the UA alloy deforms by planar slip and that of the OA alloy by wavy slip. The crack growth measurements were performed in MT specimens at constant load ratios for R =0, −1, −2, −3 near-threshold and Paris regime in ambient air and vacuum conditions. Crack closure loads were measured in order to determine the P open for each R ratio. Micromechanisms of near-threshold crack growth are briefly discussed for several concurrent processes involving environmentally assisted cracking with intrinsic microstructural effects. The results showed that the presence of humid air leads to a larger reduction in Δ K th for both the ageing conditions, but the UA specimens were superior probably because of crack branching. The role of environmental effect and microstructures near-threshold regime seems to be more significant than any mechanical contributions to the crack closure, such as plasticity, roughness, oxide, etc.

Analysis of residual stresses induced by laser processing

Abstract A model for the analysis of residual stresses induced by laser coatings, involving the transition to the liquid state, is presented. The residual stresses due to the thermal gradient associated with laser hardening are analysed using a thermo-elastic-plastic finite element computer code, with isoparametric bidimensional elements in plane strain conditions. The heating and cooling cycle describing the thermal history was obtained from a previous finite difference thermal model. Since the complex composite alloys created with the coating have unknown thermomechanical properties, the model was checked for specimens treated without powder injection, for which the residual stresses were measured by X-ray diffraction. The experimental stress results are consistent with the calculations.

Fractographic analysis of delamination in glass/fibre epoxy composites

Unidirectional laminates of reinforced glass/epoxy composites UE400/ET443 were tested by the use of fracture mechanics tests. Mode I, Mode II, Mode III and Mixed-Mode I-II were, respectively, obtained from specimens submitted to double cantilever beam, end notch flexure, Edge Crack Torsion and mixed-mode bending tests. Two different loadings were applied to the edge crack torsion specimens: the original and the modified edge crack torsion test. Scanning electron microscopy, analytical and numerical analysis were used to identify and separate fractographic features and to establish the differences between the various modes of fracture. Scanning electron micrographs of fractured surfaces showed that the most predominant fractographic features in Mode I and Mode II are the large amount of fibre pull-out and the cusps markings, respectively. Mode III characterisation shows that some limited Mixed-Mode II-III appear in both ends of the edge crack torsion specimen on delamination initiation and growth, but a predominant Mode III delamination exists in the middle of the specimens.

Wear behaviour of laser surface hardfaced steels with tungsten carbide powder injection

Abstract The use of a laser beam for surface alloying with carbide powder injection gives thick coatings (750–800 μm) without defects such as porosity and cracks. A precise correlation between the treatment parameters and the phenomena induced during the laser-material interaction is established. Optimum treatment conditions are found and applied to laser alloying of 4140 and high speed M2 tool steels with tungsten carbide injection. The laser surface treated zones, for both steels, are strongly alloyed and present a very fine microstructure with various morphologies and very high average values of the microhardness (900 HV 50 gf and 1200 HV 50 gf) compared with those of the substrates (300 HV 50 gf and 250 HV 50 gf respectively). Friction and wear tests (using a plane-ring experimental device), revealed that the laser surface coatings on both steels present wear resistances considerably higher than that of a conventional plasma-sprayed coating.

Development of a Very High Cycle Fatigue (VHCF) multiaxial testing device

The very high cycle region of the S-N fatigue curve has been the subject of intensive research on the last years, with special focus on axial, bending, torsional and fretting fatigue tests. Very high cycle fatigue can be achieved using ultrasonic exciters which allow for frequency testing of up to 30 kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a device designed to produce biaxial fatigue testing using a single piezoelectric axial exciter is presented, as well as the preliminary testing of this device. The device is comprised of a horn and a specimen, which are both attached to the piezoelectric exciter. The steps taken towards the final geometry of the device are presented. Preliminary experimental testing of the developed device is made using thermographic imaging, strain measurements and vibration speeds and indicates good behaviour of the tested specimen.

Mechanical bending behaviour of composite T-beams

Abstract A study of the design and mechanical behaviour of co-cured T-beams subjected to very high loading is presented. The T-beams were made by press moulding from pre-pregs of uni-directional glass or carbon fibre and glass fabric reinforced high performant epoxy matrix. Each type of beam was instrumented with strain gauges in the web and flange in order to carry out experimental four point bending tests. Analytical and numerical studies were also performed to compare experimental versus numerical and analytical results and to establish the suitability of a simplified bending theory for statically determinate composite beams constructed from laminated composite panels. The maximum carrying loads in the beam layers were evaluated experimentally and analytically using the Tsai-Wu failure criterion. Results showing the suitability of the simplified beam theory are presented and discussed.