Paul Compston

The effect of matrix toughness and loading rate on the mode-II interlaminar fracture toughness of glass-fibre/vinyl-ester composites

Abstract Glass-fibre-reinforced composites are increasingly used for structural applications. However, like high-performance carbon-fibre composites, they are susceptible to mode-II-dominated delamination. In response to this problem, this paper investigates the effect of matrix toughness and loading rate on the mode-II interlaminar fracture toughness (GIIc) of unidirectional glass-fibre composites with brittle and rubber-toughened vinyl ester matrices. Mode II tests were conducted σn end-notch-flexure (ENF) specimens at test rates ranging from 1 mm/min to 3 m/s. The GIIc results were compared to the order of matrix GIc. There was no significant effect of loading rate or matrix toughness on GIIc. The absence of a loading rate effect is consistent with the bulk of the experimental data in the literature, but the absence of a matrix effect is not. Microscopic examination of fracture surfaces shows similar matrix deformation in each composite. The through-thickness matrix deformation zone size is also similar. These observations suggest similar energy absorption in each composite and hence support the GIIc test results. It is concluded that failure is interface controlled, whereby unstable fracture is initiated after a similarly short period of crack growth in each composite, and before an increase in GIIc as a result of increased matrix toughness becomes apparent. The GIIc results indicate that the use of rubber-toughened vinyl ester matrices in glass-fibre composites will not improve resistance to impact-induced mode II-delamination. However, through-thickness impact damage in composite structures is likely to result from mixed-mode (I/II) loading. Therefore, suggestions for future work include investigation of the matrix effect on mixed-mode (I/II) interlaminar fracture toughness, and on delamination resistance of plate structures subjected to transverse low-velocity impact.

Matrix effect on the static and dynamic interlaminar fracture toughness of glass-fibre marine composites

The interlaminar fracture toughness of four marine composites has been investigated. The matrix material was different in each composite. Mode I and Mode II tests were performed under static loading and the focus was placed on the transfer of matrix toughness to the composite. A coefficient of toughness transfer was used to quantify the synergy experienced in the Mode I results. Mode II tests were also performed under dynamic loading with the focus placed on the order of results in relation to the order of matrix toughness. The study concludes that synergy in Mode I results for crack initiation and steady-state propagation is greatest in the composites with superior interfacial bonding. The Mode I results do not correspond directly to matrix toughness owing to interfacial strength limitations, relative to matrix strength, in the composites with the toughest matrices. A similar pattern is evident in the results for crack initiation in static mode II tests. At the maximum load point in static Mode II tests, where fast fracture occurred, and in dynamic Mode II tests, the results are consistent with the order of matrix toughness. For these results, differences in interfacial bonding seem to have an insignificant effect.

A system dynamics approach in LCA to account for temporal effects-a consequential energy LCI of car body-in-whites

PurposeThe purpose of this paper is to take steps towards a life cycle assessment that is able to account for changes over time in resource flows and environmental impacts. The majority of life cycle inventory (LCI) studies assume that computation parameters are constants or fixed functions of time. This assumption limits the opportunities to account for temporal effects because it precludes consideration of the dynamics of the product system.MethodsSystem dynamics methods are used in a consequential, fleet-based LCI that accounts for some aspects of the dynamics of the wider system. The LCI model compares the life-cycle energy consumption of car body-in-whites (BIWs) in Australia made from steel and aluminium. It incorporates two dynamic processes: the flow of BIWs into and out of the fleet, and the recycling of aluminium from end-of-life BIWs back into new BIW production. The dynamical model computes both product-based and fleet-based estimates.Results and discussionThe product-based computations suggest that an aluminium BIW consumes less energy than a steel BIW over a single life cycle. The fleet-based computations suggest that the energy benefits of aluminium BIWs do not begin to emerge for some time. The substitution of aluminium for steel is a low-leverage intervention that changes the values of a few parameters of the system. The system has a delayed, damped response to this intervention because the large stock of BIWs is a source of high inertia, and the long useful life leads to a slow decay of steel BIWs out of the fleet. The recycling of aluminium back into BIW production is a moderate-leverage intervention that initially strengthens a reinforcing feedback loop, driving a rapid accumulation of energy benefits. Dominance then shifts to a balancing loop, slowing the accumulation of energy benefits. Both interventions result in a measureable reduction in life-cycle energy consumption, but only temporarily divert the underlying growth trend.ConclusionsThe results suggest that product-based LCIs overestimate the short-term energy benefits of aluminium by not accounting for the time required for the stock of preexisting steel components to decay out of the fleet, and underestimate the long-term energy benefits of aluminium components by not accounting for changes in the availability of recycled aluminium. The results also suggest that interventions such as lightweighting and other efficiency measures alone can slow the growth of energy consumption, but are probably inadequate to achieve sustainable energy consumption levels if the fleet is large.

Low Energy Impact Damage Modes in Aluminum Foam and Polymer Foam Sandwich Structures

The energy absorption of an aluminum foam sandwich structure and a conventional polymer foam sandwich structure is similar for impacts ranging from 5 to 25 J. The polymer foam-based samples exhibit localized damage in the form of skin fracture and core crushing, but with negligible permanent out-of-plane deformation. In contrast, the aluminum foam-based samples show little fracture but exhibit extensive out-of-plane deformation radiating from the impact point. This deformation suggests that the impact damage could be more easily detectable in the aluminum foam sandwich structure. Surface strains are lower in the aluminum foam sandwich samples during post-impact loading in a single cantilever beam test, suggesting improved damage tolerance.

The Influence of Strain Rate on the Mode III Interlaminar Fracture of Composite Materials

The Mode III interlaminar fracture toughness, GIIIc, of composite materials based on both thermoplastic and thermosetting-matrices have been investigated using the edge crack torsion (ECT) test geometry. Tests were undertaken at room temperature and over a range of crosshead displacement rates to study the influence of strain rate on the interlaminar fracture properties of these materials. Further information concerning the crack tip loading conditions was obtained by undertaking a finite element analysis of the ECT specimen geometry. The experimental results show that the value of GIIIc depends on initial crack length, increasing steadily with increasing crack length for both types of material. It has been shown that the interlaminar fracture toughness of the glass fiber/epoxy-based system was superior to that offered by its thermoplastic counterpart, an effect that may be due to the fact that the glass fiber-reinforced polypropylene composite was slow-cooled from its processing temperature. The interlaminar fracture toughness of both types of composite remained roughly constant over the range of crosshead displacement rates considered here suggesting that they do not exhibit any rate-sensitive fracture behavior. The finite element analysis of the ECT specimens showed that the specimen is subjected to pure Mode III loading over the central part of the test specimen whereas regions of locally-high Mode II loading were observed over the region in which the load was applied. The Mode III strain energy release rate profile does not depend on specimen thickness or the displacement of the ECT test geometry.

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 (GIc and GIIc) 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 GIc 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 GIc statistically more reliable than woven roving specimens, which gave fewer GIc values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, GIIc 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.

Comparison of surface strain for stamp formed aluminum and an aluminum-polypropylene laminate

Laminate structures incorporating thin layers of metal and polymer, or polymer composite, can offer significant weight savings for engineering structures, while retaining excellent mechanical and impact performance. Laminates based on thin layers of aluminum and glassfiber/polypropylene thermoplastic have been the subject of recent study [1, 2], and have exhibited excellent specific mechanical properties and superior specific impact behavior compared to monolithic aluminum. Such materials, therefore, have great potential for widespread application in engineering structures. One such potential area is the automotive industry where weight reduction and impact performance are pertinent issues. Lighter vehicles will result in improved fuel efficiency, and greater energy absorption capability may contribute to improved crash performance. However, for the automotive industry it is necessary to produce components using a high-volume manufacturing process such as stamping. Thermoplastic-based materials and sandwich structures are good candidates for stamp forming as they can be heated to conform to the mold, and then rapidly cooled for removal from the mold. Mosse et al. [3, 4] investigated the effects of blankholder force, laminate preheat temperature, tooling temperature, and tool radii on FML formability. It was found that significantly lower levels of springback could be achieved over aluminum, and forming defects could be eliminated by restricting process variables to a given range. In particular, it was found that delamination at the bimaterial interface and within the composite layer was eliminated when the laminate was pre-heated to 160 ◦C then formed in a heated die. This is significant as delamination would adversely affect the mechanical performance of a formed component. Further, Kim and Thomson [5] found that high forming speed increased the transverse stiffness of polymer-metal laminates, in turn reducing the inter-laminar shear and the degree of springback. They also found that laminates forming at elevated temperatures decreased the rigidity but improved the springback characteristics. This letter presents some preliminary results from research into stamp-forming aluminum-thermoplastic sandwich materials. Here, the permanent strain on the surface of a channel-formed aluminum-polypropylene laminate is compared to monolithic aluminum. Characterization of the strain is significant as it provides insight into the behavior of the material during formation and assists in the production of parameters for subsequent formation methodologies. The materials used in this study were 5005-H34 aluminum and a self-reinforced polypropylene (Curv, BP). An aluminum-Curv laminate was made in a 2/1 configuration in a 200 × 200 mm picture frame mold. A 0.9 mm thick layer of Curv was sandwiched between two layers of 0.5 mm thick aluminum cleaned with a solvent (isopropanol). A 50 μm thick layer of a hot-melt polypropylene adhesive (Gluco Ltd., UK) was placed at each bi-material interface. The laminate was consolidated by heating to 160 ◦C in a platen press followed by rapid water cooling under a pressure of approximately 1 MPa. The nominal laminate thickness was 2.2 mm. Samples of 19 mm width were sectioned from the laminate and from a plain sheet of 2 mm thick aluminum. A 3 mm circular grid etched onto the surfaces enabled post-forming major strain measurements, that is in the direction of the sample length, to be made. Channel sections were stamped in an open die. Plain aluminum was stamped cold whereas the aluminumCurv laminates were pre-heated to 160 ◦C then immediately transferred to the die, which was pre-heated to 80 ◦C. This enabled a temperature window of 125– 140 ◦C to be maintained during the stamping operation. The channel sections were stamped in an Enerpac 30 tonne press using two tool radii of 3 and 7 mm. The blank holder force was 3.5 kN. Surface strain measurements were taken from ten grids around the mid-point of the sidewall area of the channel section, shown in Fig. 1, using an optical microscope with a graticule scale of 20 μm resolution. Measurements were taken from the sidewall area as it is likely to undergo significant tensile strain during formation. Microscope examination of the sidewall edge, prior to taking the strain measurements, confirmed the absence of delamination. The average major surface strain for the aluminum and aluminum-Curv samples is plotted in Fig. 2. (The

The Transfer of Matrix Toughness to Composite Mode I Interlaminar Fracture Toughness in Glass-Fibre/vinyl Ester Composites

The transfer of matrix toughness to composite mode I interlaminar fracture toughness (GIc) has been investigated in unidirectional glass-fibre reinforced composites with brittle and rubber-toughened vinyl ester matrices. Single-edge-notch bend (SENB) and double cantilever beam (DCB) specimens were used for matrix and composite GIc characteristion, respectively. The initial crack opening displacement rate was used as the parameter for comparison of GIc results. Matrix GIc was completely transferred to composite GIc for crack initiation (GIc -init) in the brittle-matrix composites, but in the toughened composites transfer was only partial due to the presence of fibres. The conclusion is that the maximum contribution to energy absorption by the matrix is more accurately reflected by GIc -init, and should be used for further assessment of the enhancing effect of fibre bridging during steady-state crack propagation, instead of matrix GIc. A plot of composite GIc for steady-state crack propagation, GIc -prop versus GIc -init indicates that the enhancing effect of fibre bridging is greater in the toughened composites. This enhancement is related to a larger deformation zone size in the toughened matrices.

Thermal modelling of the laser-assisted thermoplastic tape placement process

Thermoplastic tape placement opens the possibility of a fully automated composite production. The resulting quality is highly dependent on the thermal history during consolidation. This article focuses on the thermal modelling of a tape placement system employing a near-infrared laser. A nonlinear two-dimensional finite element model is presented for a carbon fibre reinforced thermoplastic (AS4/PEEK) composite placement process using a conformable roller. The relative influence of roller geometry, roller temperature and thermal contact resistance was studied. Temperature measurements were performed using thermocouples welded to the substrate. The model predictions show good correlation in terms of timing of the irradiation, shadow and consolidation regions. The roller temperature was found to have the most significant impact on the bond line temperature distribution.

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 GIc 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 GIc values for crack initiation are independent of fibre volume fraction and similar to matrix resin GIc. Variations in the GIc 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 GIc characterisation as a function of fibre volume fraction.