Seawater ageing of low styrene emission resins for marine composites: Mechanical behaviour and nano-indentation studies

Polyester resins are widely employed for pleasure boat construction. In order to satisfy new environmental legislation on styrene emissions resin suppliers have proposed modified formulations, but these show lower failure strains under tensile loading. This paper examines the influence of wet ageing on mechanical properties of low styrene polyester, together with standard polyester and vinyl ester resins, and their glass reinforced composites. Results are presented from accelerated ageing in natural sea water for nine months, at temperatures of 20, 40 and 60 °C. Nano-indentation was used to study local changes in the resin after ageing. The diffusion kinetics of the low styrene emission resins and their composites are similar to those for standard polyester resins. The low styrene resins appear to lose strength more slowly than the standard resins but after nine months in seawater at 40 °C similar property losses are noted.     

Progressive crushing of stitched glass/polyester composite cylindrical shells

This paper examines the effect of mode I interlaminar fracture toughness (G_Ic) on the specific energy absorption of stitched glass/polyester composite cylindrical shells under axial compression. The laminated composite cylindrical shells used as energy absorbers, absorb large amount of impact energy during collision. Since mode I delamination in the thin wall of axially collapsed shell is one of the major energy absorbing modes, contribution of G_Ic to specific energy absorption (SEA) of tubes is significant during collision. The G_Ic values are determined through double cantilever beam (DCB) test with stitched and unstitched planar specimens. The four and six-layered cylindrical tubes of D/t ratios 29.27 and 20, respectively, with G_Ic values ranging from 1.68 to 8.09 kJ/m² are prepared by stitching and are subjected to quasi-static axial compression. Increasing G_Ic up to certain value leads to controlled progressive crushing, which is a good energy absorbing mechanism, beyond which failure is uncontrolled. Cylindrical tubes having G_Ic up to 6.34 kJ/m² leads to 40% increase in SEA for four-layered tubes and 6.6% for six-layered tubes comparing with the corresponding unstitched tubes. When the tubes have G_Ic of 8.09 kJ/m², four-layered tubes undergo unstable failure, but six-layered tubes undergo stable progressive crushing with 22% increase in SEA. Transition from stable to unstable failure depends upon the thickness of tubes. An analytical model is developed based on energy approach to predetermine the steady state mean crush load of cylindrical composite shells under axial compression. The model results are validated by experimental results, and show good agreement.     

Mechanical Properties of Composites Based on Low Styrene Emission Polyester Resins for Marine Applications

Glass fibre reinforced polyester composites are used extensively for hulls and decks of pleasure boats. Boat-builders must optimise manufacturing technology, not only with respect to mechanical properties but also limiting volatile organic compounds (VOC) emissions. One way to achieve this is through modified polyester resin formulations such as low styrene content, low styrene emission or combinations of these. The resin matrix selection procedure is based on design specification (mechanical behaviour) but also manufacturing requirements and cost considerations. For this application post-cure is rarely used so it is important to optimise curing conditions. In this study the influence of the curing cycle on mechanical properties was examined first for two polyester resins. Then for one cycle (16 h at 40°C) the properties of eight resins have been determined. Significant differences in failure strain are observed, from 0.9% to 3.3%. The resins with improved VOC performance are the most brittle. The transverse tensile behaviour of these resins in composites with unidirectional glass fibre reinforcement and the limit of linearity for composites with glass mat both depend on these failure strains. These results are discussed in terms of admissible composite strains for boat design.     

The fracture toughness of bast fibre reinforced polyester composites Part 1 Evaluation and analysis

Hemp and jute fibre reinforced polyester composites were fabricated to various fibre volume fractions (V _f) up to 0.45. Laminates reinforced with a chopped strand mat (CSM) glass fibre were also manufactured. The tensile properties of these materials were evaluated. Fracture toughness was assessed, using linear elastic fracture mechanics (LEFM) principles, under quasi-static loading conditions. At equivalent V _f (0.2) it was found that the fracture toughness (K _Ic) of the CSM glass fibre reinforced material was approximately 3 times greater than that of the natural fibre reinforced laminates and an order of magnitude greater than the unreinforced polymer alone. Critical strain energy release rates (G _c) and plastic zone radii were computed. The G _c of the natural fibre reinforced laminates was approximately an order of magnitude lower than that of the CSM reinforced material at the same V _f. It was hypothesised that the size of the crack-tip plastic zone influences the energy absorbing capacity of the material. By comparing the relative volumes of the plastic zones, implications regarding the toughening mechanisms operative in natural fibre reinforced composites have been made. The applicability of LEFM to characterise toughness in these materials is discussed.     

The Influence of Fibre Volume Fraction on the Mode I Interlaminar Fracture Toughness of a Glass-Fibre/Vinyl Ester Composite

This paper investigates the influence of fibre volume fraction on the mode I interlaminar fracture toughness G _Ic of a glass-fibre/vinyl ester composite. Two fibre volume fraction parameters are defined; a global value for the composite specimen and a value for the fibre-dense intralaminar regions. The range of global fibre volume fraction studied was 32–52 %. Results show that G _Ic values for crack initiation are independent of fibre volume fraction and similar to matrix resin G _Ic . Variations in the G _Ic for steady-state crack propagation, and the amount of fibre bridging, are not completely explained by changes in global fibre volume fraction. Instead they are consistent with fibre volume fraction in the fibre-dense intralaminar regions, through which the crack preferred to grow. It is concluded that this latter parameter is more relevant for G _Ic characterisation as a function of fibre volume fraction.     

Predicting evolution of ply cracks in composite laminates subjected to biaxial loading

An energy-based model is developed to predict the evolution of sub-critical matrix crack density in symmetric multidirectional composite laminates for the case of multiaxial loading. A finite element-based numerical scheme is also developed to evaluate the critical strain energy release rate, G_Ic, associated with matrix micro-cracking, a parameter that previously required fitting with experimental data. Furthermore, the prediction scheme is improved to account for the statistical variation of G_Ic within the material volume by using a two-parameter Weibull distribution. The variation of G_Ic with increasing crack density is also accounted for based on reported experimental evidence. The simulated results for carbon/epoxy and glass/epoxy cross-ply laminates demonstrate the ability of the improved model to predict the evolution of multidirectional ply cracking. By integrating this damage evolution model with the synergistic damage mechanics approach for stiffness degradation, the stress-strain response of the studied laminates is predicted. Finally, biaxial stress envelopes for ply crack initiation and pre-determined stiffness degradation levels are predicted to serve as representative examples of stiffness-based design and failure criterion.     

Macroscopic analysis of interfacial properties of flax/PLLA biocomposites

This study presents results from a study of the mechanical behaviour of flax reinforced Poly(l-Lactic Acid) (PLLA) under in-plane shear and mode I interlaminar fracture testing. Slow cooling of the unreinforced polymer has been shown to develop crystalline structure, causing improvement in matrix strength and modulus but a drop in toughness. The in-plane shear properties of the composite also drop for the slowest cooling rate, the best combination of in-plane shear performance and delamination resistance is noted for an intermediate cooling rate, (15.5 °C/min). The values of G_Ic obtained at this cooling rate are higher than those for equivalent glass/polyester composites. These macro-scale results have been correlated with microdroplet interface debonding and matrix characterization measurements from a previous study. The composite performance is dominated by the matrix rather than the interface.     

Round Robin analysis of G Ic interlaminar fracture test

This note presents the results of an exercise to evaluate the variation in values of G _Ic at initiation, determined independently by 36 researchers interpreting the same load-displacement curve from a mode I double cantilever beam (DCB) test on unidirectional carbon fibre reinforced polymer composite. The results indicate a significantly larger coefficient of variation for values corresponding to a definition of initiation at non-linearity on the load-displacement curve than for a 5% compliance offset criterion.     

Comparison of Interlaminar Fracture Toughness in Unidirectional and Woven Roving Marine Composites

This paper investigates the effect of fibre lay-up and matrix toughness on mode I and mode II interlaminar fracture toughness (G_Ic and G_IIc) of marine composites. Unidirectional and woven roving fibres were used as reinforcements. Two vinyl ester resins with different toughness were used as matrices. Results from both modes showed toughness variation that is consistent with matrix toughness. Values of G_Ic were not significantly influenced by fibre lay-up except at peak load points in the woven roving/brittle-matrix composite. Each peak load point, caused by interlocked bridging fibres, signified the onset of unstable crack growth. For unidirectional specimens, crack growth was stable and G_Ic statistically more reliable than woven roving specimens, which gave fewer G_Ic values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, G_IIc was higher in woven roving composites. This was due to fibre bridging, perpendicular to the crack growth direction, which encouraged stable crack growth and increased energy absorption. Mode II R-curves were obtained for the woven roving specimens. These R-curves provide additional information useful for characterising delamination resistance. The paper concludes that composites with woven roving fibres show similar mode I delamination characteristics to the unidirectional composites; but their mode II delamination characteristics, after crack initiation, are quite different.     

Influence of intralaminar cracking on the apparent interlaminar mode I fracture toughness of cross‐ply laminates

One of the major difficulties in interlaminar fracture tests of multidirectional laminates is the high tendency for intralaminar cracking and the resulting wavy crack propagation. Experimental work showed that this occurred in double cantilever beam (DCB) tests of cross‐ply laminates having a starter crack on a 0°/90° interface. Moreover, under steady‐state propagation conditions, the apparent values of the critical strain energy release rate G_Ic were two times higher than those of 0°/0° specimens. In this paper, a finite‐element‐based progressive damage model was used to simulate crack propagation in cross‐ply specimens. The results showed that transverse cracking alone cannot be responsible for the above difference of G_Ic values. Therefore, the higher propagation G_Ic values for cross‐plies must be attributed to the more extensive fibre bridging observed and to plastic deformations of the 90° interfacial ply.     

Damping of dynamic effects with elastomers in instrumented impact testing

The efficiency of four silicon elastomers as damping materials was studied in high rate (2.9 m/s) instrumented impact testing. The measurements were done on injection molded PP specimens. Dynamic effects could be efficiently reduced by all four silicon rubbers. Mechanical damping leads to smooth force versus deflection correlations, which considerably facilitates the determination of valid fracture mechanics characteristics. Damping does not influence the maximum force measured during fracture, K _Ic is independent of rubber type and thickness. Since the damper consumes considerable energy, G _Ic is significantly modified by damping, the effect depends both on the viscoelastic properties and the thickness of the damper. The approach proposed earlier for the correction of energy could be applied in all cases where a load versus deflection trace void of oscillations was registered. Similarly to K _Ic, corrected G _Ic values proved to be completely independent of the conditions of damping, i.e. the type and thickness of the damper. The parameters of the non-linear constitutive equation which was used to describe the deformation behavior of the damper could not be related to properties determined by simple measurements (hardness, modulus, rebound elasticity, etc.).     

Crack propagation in a glass particle-filled epoxy resin

Crack propagation in an epoxy resin reinforced with spherical glass particles has been followed using a double-torsion test. In particular the effect of strain rate, volume fraction and particle size upon the stability of propagation, the Young's modulus, the critical stress intensity factor,K _Ic and the fracture energy,G _Ic has been studied. It has been shown that the crack propagation behaviour can be explained principally in terms of crack pinning, although it has been found that propagation is also affected by blunting the breakdown of the particle—matrix interface. It has been demonstrated that crack-front pinning is consistent with a critical crack opening displacement criterion.     

Interlaminar Tests for Marine Applications. Evaluation of the Influence of Peel Plies and Fabrication Delays

This paper presents results from a study of the influence of surface preparation on the mechanical performance of overlaminated polyester composites. Panels of 16 woven glass plies have been prepared in two halves by hand lay-up. After the first 8 plies were laminated the surface was either protected by a peel ply or left in air. The overlamination of the second half of the composite thickness was completed after different periods and interlaminar shear, flexure and mode I fracture specimens were tested. The results enabled the influence of the delay and the surface condition to be related to mechanical performance. Surfaces protected with peel plies show very low G_Ic propagation energy release rates. Results are compared to those from continuous lamination of the 16 ply composite. Interlaminar fracture tests are shown to be much more sensitive to overlamination conditions than the traditional short beam shear test. Results are interpreted in terms of fracture surface features.     

The efficiency of various toughening agents in novel phenolic type thermoset resin systems

Two novel phenolic type thermosetting resin systems are investigated regarding the effectiveness of different toughness modifiers. These modifiers derive from different groups such as elastomers, thermoplastics, and core–shell polymers. Measurements are accomplished by mechanical, thermal, and microscopical studies. Toughness improvement is determined by increasing K _Ic and G _Ic values while glass transition temperature, flexural strength, and modulus must not suffer greatly. Suggestions on the mechanisms of toughness modification in the novel resins are made based on images from scanning electron microscopy.     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.     

Mode I Fracture Toughness Testing of Composite Pipes

A common industrial production process for axially symmetric composites is filament winding. For this type of material, interlayer properties cannot be evaluated using the available ISO and ASTM standards because the curved surfaces and partially interwoven fibers of cylindrical parts can affect the results. This paper presents a special test geometry for measuring the critical energy release rate G_Ic directly from actual filament wound products. Different specimen geometries were investigated numerically with respect to their stress state at the crack tip and tested experimentally. The results were compared against those of flat specimens tested according to ISO/ASTM standards and made from the same constituent materials as the original test pipe. Testing specimens taken from an actual filament wound pipe yields more realistic results of G_Ic?=?1,220 J/m² than testing flat specimens (G_Ic?=?330 J/m²), especially made for fulfilling the test standard’s requirements.     

Mode I interlaminar fracture toughness properties of advanced textile fibreglass composites

The Mode I interlaminar fracture toughness properties of vinyl ester-based composites reinforced with fibreglass manufactured by the advanced textile technologies of braiding, knitting, stitching and through-the-thickness weaving are assessed in comparison to a variety of traditional composites made from fibreglass such as unidirectional or woven rovings. The interlaminar fracture toughness (G_Ic) of braided and knitted composites are higher than traditional composites by factors of more than two and four, respectively. Toughening in these textile composites was caused by extensive crack branching as the interlaminar crack was forced to follow a tortuous path through the complex fibre architectures. The G_Ic values of the composites reinforced in the through-thickness direction by weaving or stitching were higher than traditional composites by factors of nearly two and three, respectively, with the main toughening mechanism being crack bridging by the through-thickness binder yarns/stitches. A review of Mode I interlaminar fracture data collected from papers shows that advanced textile techniques are capable of manufacturing composites with substantially improved delamination resistance.     

Fracture toughness and molecular structure of unfilled epoxy adhesives

The cohesive, mode I (tensile cleavage) fracture energy (or fracture toughness), G _Ic, of bulk tapered double cantilevered beam (TDCB) samples of a series of three epoxy thermoset networks was determined using a linear elastic fracture mechanics (LEFM) analysis. Networks of different crosslink density were obtained by mixing various amounts of an aromatic epoxy novolac and an aliphatic epoxy and crosslinking with an imidazole catalyst. Brittle, stick-slip fracture was observed for all formulations, with G _Ic increasing as the amount of aliphatic epoxy increased. However, fracture surface morphologies exhibited evidence of increasing plastic deformation as G _Ic increased. In the investigation of structure-property relationships for this series of thermoset networks, G _Ic was found to be inversely related to both network crosslink density and glass transition temperature (T _g). It was also found that the room temperature frequency of the glassy state transition (-transition) increased as fracture toughness increased.     

The influence of notch depth on the fracture mechanics properties of polymer concrete

This research deals with unreinforced and fiber-reinforced Polymer Concrete (PC). Polymer Concrete is usually composed of natural aggregates such as silica sand, binded together with a thermoset resin, such as unsaturated polyester. In this paper it is reported the use of a direct method to calculate two size independent fracture parameters, the critical stress intensity factor, K _Ic, and the critical crack tip opening displacement, CTOD_C, from experimental results of two different size single edge notched beams, 10 and 20 mm, subjected to three point bending under quasi-static loading condition. This method is called the Two Parameter Fracture Model (TPFM). Also the Fracture Energy, G _f, of the specimens were measured to verify its size dependency. Epoxy and polyester resin specimens were studied and epoxy resin specimens reinforced specimens with short carbon and glass fibers were considered.     

Temperature dependence of fracture toughness of epoxy resins cured with diamines

The fracture toughness (critical stress intensity factor, K _Ic) of epoxy resins cured with four diamines has been measured as a function of temperature over the range from –35° C to above T _g. It was found that K _Ic for each epoxy-diamine system did not vary below room temperature, and in the higher temperature range K _Ic increased with increasing temperature to a maximum and then decreased. The temperature which maximized K _Ic, agreed with the temperature at which the flexural modulus of the epoxy resins abruptly dropped. This temperature was therefore considered as T _g. This temperature was found to be about 20° C lower than the heat deflection temperature under load (1.82 M Pa) of the resins.