L. Reis

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.

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.

Mechanical Behavior of Sandwich Structures using Natural Cork Agglomerates as Core Materials

Cork is a material of great value to the Portuguese economy. Unfortunately, its use is still restricted to traditional areas, with the agglomerate form in particular not being used to its full potential. The objective of this article is to analyze the viability of using cork-based material as core materials in sandwich structures in aeronautical and aerospace applications. The use of cork-based material is proposed because of its isolation properties (both thermal and acoustic) and there is no significant performance loss, when compared with the currently used materials. It presents other advantages, as well as, less wastage of energy in manufacturing and a better environmental integration, both in the transformation stage and in the end of life recycling stage. The objective of this work is to study the mechanical behavior of different sandwich specimens, with carbon/epoxy faces, and cores of different cork agglomerates and their comparison with the results obtained with similar specimens using current material cores. Experimental shear tests and three-point bending tests were carried out and the evolutions of the load— displacement curves of the different cork agglomerates/sandwiches were analyzed and discussed. The obtained results show that significant room for improvement still exists in use of cork-based core materials.

GFRP sandwich panels with PU foam and PP honeycomb cores for civil engineering structural applications

Purpose – The purpose of this paper is to present experimental investigations on the structural behaviour of composite sandwich panels for civil engineering applications. The performance of two different core materials – rigid plastic polyurethane (PU) foam and polypropylene (PP) honeycomb – combined with glass fibre reinforced polymer (GFRP) skins, and the effect of using GFRP ribs along the longitudinal edges of the panels were investigated.Design/methodology/approach – The experimental campaign first included flatwise tensile tests on the GFRP skins; edgewise and flatwise compressive tests; flatwise tensile tests on small‐scale sandwich specimens; and shear tests on the core materials. Subsequently, flexural static and dynamic tests were carried out in full‐scale sandwich panels (2.50×0.50×0.10 m3) in order to evaluate their service and failure behaviour. Linear elastic analytical and numerical models of the tested sandwich panels were developed in order to confirm the effects of varying the core mater...

Eco-composite: the effects of the jute fiber treatments on the mechanical and environmental performance of the composite materials

In this study, untreated and treated jute fiber composites were investigated as candidates to replace glass fibers as reinforcement to produce structural composites with better environmental performance. The surface of the jute fibers was modified by drying and bleaching/drying treatments to improve the wetting behavior of the apolar polyester, improving the mechanical properties of the composites. The mechanical characterization of the composites was obtained according to the ASTM standards (D-3039/D-790) and dynamic mechanical analysis. The environmental characterization was obtained by life-cycle assessment method. The treatment characterization was obtained by horizontal attenuated total reflectance infrared spectroscopy and thermogravimetry. Finally, jute composites were compared with glass composites and results show that the jute fiber treatments imply a significant increase of the mechanical properties of the composites without damaging their environmental performances.

A New Criterion for Evaluating Multiaxial Fatigue Damage under Multiaxial Random Loading Conditions

Generally, mechanical components or structures are subjected to random and a three-dimensional stress state; there are very few field loading paths which can be experimentally fully simulated in laboratory. Loading path parameters such as load sequence, stress level or proportionality/non-proportionality presences are unknown variables with unknown levels under random loading conditions which are impossible to modulate in laboratory because the load spectra is unknown. The load spectrum depends on numerous factors such as environmental, mechanical or user behavior. At design stages the fatigue life estimation is based on typical loading paths or typical loading spectra, however that assumption may be very different from the usage regime. From here it can be concluded that the random multiaxial fatigue issue is of utmost importance to monitoring the in-field damage accumulation. This work presents a proposal to estimate the accumulated damage resulted from multiaxial random loadings based on the SSF equivalent stress and SSF virtual cycle counting concept.

Automation in Strain and Temperature Control on VHCF with an Ultrasonic Testing Facility

Increased safety and reliability in mechanical components has become a subject of prime importance in recent years. Therefore, a proper understanding of damage and fracture mechanics in materials and components designed to withstand very high cycle fatigue (VHCF) loadings is extremely important nowadays. However, the use of conventional machines for fatigue testing is very time consuming and costly for VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to IE8 to IE10 cycles within a considerably reduced amount of time. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain rate, temperature, and frequency) is still a matter of concern and ongoing development. Because of the high frequencies involved in VHCF testing, a huge amount of heat is generated over the specimen, which greatly affects the variables determining the fatigue behavior. This paper describes the design and instrumentation of an ultrasonic fatigue testing machine that operates at a working frequency of 20 kHz. Among other features, it incorporates automated strain and temperature control. In order to run automated tests, a closed-loop monitoring and control system was developed based on the measured temperature and displacement amplitudes. Temperature readings are made with a pyrometer and thermography camera, and displacement is monitored at the free end of the specimen with a high-resolution laser. The machines power output is continuously adjusted from the displacement readings, so that the stress variations within the specimen are as flat as possible. When the temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data are acquired, managed, and processed with a data acquisition device working at a 400 kHz sampling frequency. The advantages and limitations of metal fatigue testing at very high frequencies are discussed in this paper, with special emphasis on strain and temperature-control issues. Comparisons are made of tests carried out with and without both displacement and temperature control on two metallic alloys, copper 99 % and carbon steel, with the determination of strength-life (S-N) curves.

Push-to-Talk in IMS Mobile Environment

The IP Multimedia Subsystem (IMS), originally designed by 3GPP and later updated by 3GPP, 3GPP2 and TISPAN, presents itself as an architectural framework that allows the convergence of the Internet, wireless and wireline networks and a global platform for the delivery of IP multimedia applications in Next Generation Networks (NGN). One of the most used services to test IMS capabilities is Push-To-Talk (PTT). This service follows the IMS specifications and has motivated standardization work by the Open Mobile Alliance to assure interoperability between different operators. This paper describes a PTT over IMS solution designed with a Talk Burst Control Protocol based on SIP messages for call session control. The implementation was deployed and tested both with high-bandwidth LAN and CDMA2000 wireless network, demonstrating that IMS is a powerful and flexible architecture allowing the quick and easy development of multimedia applications, like PTT, for convergent networks

Mechanical Behaviour of Sandwich Beams Manufactured with Glass or Jute Fiber in Facings and Cork Agglomerates as Core

The environment is a prominent issue today. Designing environmentally sustainable products is an attempt to address this question. In many cases, natural materials are environmentally friendly for product design manufacturing. The goal of this work is to study the mechanical behaviour of NL10 and NL30 cork agglomerates. Compression, shear and bending tests in sandwich specimens made of glass or jute fiber in facings and cork agglomerates as core were carried out. The sandwich specimens were manufactured by Resin Transfer Moulding (RTM) process. Results show that NL30 has a higher compression strength and shear resistance than NL10 agglomerate due to its manufacturing process, which originates superior density, but the NL30 agglomerate superior density is undesirable. Sandwich test specimens that presented failure by rupture of the core in both types of tests, core shear tests and three point bending tests, showed that the failure is mainly adhesive occurring between the adhesively joined cork grains. Since grains are unaffected and remain intact, it is possible to improve these materials by using better agglutinants and new bonding techniques with the intent of getting cork agglomerates with higher shear and flexural strength.