P.-Y. B. Jar

Mode I and mode II delamination properties of glass/vinyl-ester composite toughened by particulate modified interlayers

Abstract Various vinyl-ester (VE)/poly(acrylonitrile-butadiene-styrene) (ABS) blends were used for interlayer-toughening of a glass/VE composite to increase delamination resistance of the base material under mode I and mode II loading. Dry ABS powder was mixed with the liquid resin in four weight ratios: 3.5, 7, 11 and 15 phr (parts per hundred parts of resin) while the layer thickness was varied within the range of 150–500 μm. Firstly, mode I fracture toughness and tensile properties of the VE/ABS blends were assessed. By using the Raman Spectroscopy technique a chemical reaction was discovered which occurred during ABS–VE mixing: i.e. butadiene transition from the ABS particles to the VE. A butadiene saturation was discovered to occur in the VE beyond 7% ABS particle content. Both mode I and mode II fracture toughness were significantly improved with application of the interlayers. Mode I fracture toughness was found to be a function of layer thickness and particle content variations. The latter dominated G Ic after the saturation point. On the other hand mode II fracture toughness was found to be independent of the layer thickness (within the used layer thickness range) and only moderately influenced by the particle content. Important Toughening mechanisms were plastic deformation and micro-cracking of the layer materials. Evidence of both mechanisms has been found using optical and scanning electron microscopy (SEM).

Influence of interlaminar fracture toughness on impact resistance of glass fibre reinforced polymers

This work is concerned with delamination resistance of glass fibre reinforced polymers (GFRP) and its influence on GFRPs resistance to transverse, point impact. The study used two GFRP, one with polyurethane matrix and the other isophthalic polyester. The two GFRPs show distinctly different mode I delamination resistance. Under the same impact condition, at speeds from 1.5 to 5 m/s, the polyurethane-based GFRP developed a smaller damage size than the isophthalic polyester-based counterpart, suggesting that the former has higher impact toughness. However, the two GFRPs showed little difference in the total energy absorbed during the impact, which is another measure commonly used for impact toughness evaluation. We conclude from the study that a consistent trend exists between the delamination resistance in mode I and the critical force for the incipient impact damage. Difference of the impact resistance between the two GFRPs is mainly on the impact damage size developed. The total energy absorbed during the impact remains the same, which is independent of mode I delamination resistance of the GFRP.

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.

Characterization of toughness variation due to intrinsic defects in high-thermal-resistant poly(acrylonitrile-butadiene-styrene) (ABS)

A mechanical testing method, named TACL test, is proposed to characterize the effect of gel-like particles on mechanical properties of high-thermal-resistant poly(acrylonitrile-butadiene-styrene) (ABS). The TACL test introduces cracks in the vicinity of gel-like particles by cyclic loading. Mechanical properties are then measured using the cyclically loaded specimens under monotonic tension. Preliminary results show that the mechanical properties, especially the maximum elongation and the total absorbed energy, are very sensitive to the cyclic loading. The study suggests that the TACL test can serve as a means to “semi-quantitatively” characterize number and distribution of the gel-like particles, which is useful for monitoring batch-dependent toughness variation of the ABS.

Comparison of damage development in random fiber-reinforced polymers (FRPs) under cyclic loading

It has been well known that suppression of debonding and matrix cracking could improve fatigue resistance of fiber-reinforced polymers (FRPs). In this study, the roles of these two mechanisms on the damage development in FRPs with in-plane random glass fiber reinforcement have been investigated. Two polymers are used as the matrix - isophthalic polyester and polyurethane. Polyurethane-based FRP shows higher ultimate tensile strength (UTS) and strain to failure, but lower elastic modulus. Under zero-tension fatigue loading (with the maximum stress level equivalent to 50% of their respective UTS), the change in modulus, energy dissipation rate, and the corresponding damage development process are investigated. The damage development is analyzed at the macroscopic and microscopic levels, and found to be closely related to the modulus degradation and change in energy dissipation rate. The study concludes that the two FRPs show significantly different behavior under fatigue loading. The polyurethane-based FRP had better fatigue resistance, in view of the mild modulus change and the capability of absorbing energy through plastic deformation. Results from the study suggest that the excellent fatigue resistance of the polyurethane-based FRP is due to good toughness of the matrix.

Effect of matrix toughness on the shear strength of brittle and rubber-modified glass-fiber/vinyl ester composites

Due to its superior mechanical properties and corrosion resistance, vinyl ester resin is increasingly used as the matrix for fiber-reinforced composites instead of polyester [1]. Corrosion resistance in particular is an important attribute for composites used in applications such as marine structures, where the environment is highly corrosive. However, while vinyl ester is also tougher than polyester, with a mode I strain energy release rate (GIc) of 0.13 kJ/m2 compared to 0.06 kJ/m2 for isophthalic polyester [2], it is still relatively brittle. This brittleness makes the composite susceptibel to delamination through interlaminar fracture. In response to this delamination problem, recent work investigated interlaminar fracture toughness for unidirectionally reinforced glass-fiber composites with brittle and rubber-modified vinyl ester matrices [3, 4]. There was a considerable increase in toughness with a GIc of nearly 2 kJ/m2 reported for the toughest neat resin and, under mode I tensile loading, a GIc of 2.4 kJ/m2 for steady-state crack propagation in the composite counterpart [3]. However, under mode II shear loading, there was no significant matrix effect on composite interlaminar fracture toughness, GIIc [4]. The absence of a matrix effect on shear induced interlaminar fracture toughness is surprising, which led to the next step in the characterization of these composites; to study the matrix effect on the shear strength properties. This letter presents the results from short beam shear and compression induced shear tests, and discusses them in relation to the previously obtained fracture toughness results. The influence of loading pin diameter on short beam shear strength is also addressed. The composite laminates were made by hand lay-up using 24 plies of unidirectional E-glass fiber (Owens

Fracture initiation in emulsion-polymerized poly(acrylonitrile-butadiene-styrene) (ABS)

Rubber particles have long been used to improve polymer toughness. Many glassy polymers, that are strong but brittle, show excellent improvement in toughness by the addition of rubber particles [1, 2]. For poly(acrylonitrile-butadiene-styrene) (ABS), our previous work has shown that the efficacy of rubber toughening varies with both rubber content and the bond strength between the particle and matrix [3, 4]. Other parameters such as particle size, particle size distribution, and particle morphology are also known to affect the toughness [5, 6]. However, these parameters could not fully account for the toughness variation that we recently observed in ABS. Tensile toughness for the ABS was found to differ more than 50% between two batches of specimens that have the same above-mentioned materials parameters. While results from a full investigation on the causes of the toughness variation will be published in a separate article, this paper will point out that in addition to the above material parameters, an intrinsic defect, named large particles here, should be considered for the toughness variation. Evidence for the origin of the large particles and their role as a fracture initiator in the deformation process will be presented in this paper. The ABS used in our study is a mixture of an ordinary emulsion-polymerized ABS (named ABS-g1), poly(styrene-co-acrylonitrile) (SANadd), and poly(styrene-N -phenyl-male-imide) (SMI), in a weight ratio of 36 : 44 : 20 (named “blended ABS” in the rest of the text). The ABS-g1 contains 50 wt% polybutadiene rubber particles with a bimodal size distribution of 0.1 μm and 0.5 μm. Matrix of the ABS-g1 is poly(styrene-co-acrylonitrile) that is denoted SANABS to distinguish it from SANadd. The SANadd was added to the blend during the extrusion process. SMI is a random copolymer with a Tg of 196 ◦C [7]. As SMI forms a miscible blend with SANABS and SANadd, the blended ABS shows single Tg which is higher than that of ABS-g1 [8]. Details of the mixing ratio, monomer composition, and constituent molecular weight for the blended ABS are given in Table I. The blended ABS was prepared in the following way. ABS-g1, SANadd, and SMI were mixed using a twinscrew extruder with the highest barrel temperature set at 280 ◦C. The resulting pellets were then injectionmolded at 260 ◦C to form dumb-bell specimens 3.3 mm thick, 12.3 mm wide, and 65 mm long. The high shear rate in the processes produced a resin temperature of ∼300 ◦C in the twin-screw extruder and ∼275 ◦C in the injection molding machine. The mold temperature for injection molding was kept at 60 ◦C. To identify the large particles in the dumb-bell specimens, deformation was introduced using cyclically loading with the minimum stress set at 0 MPa and the maximum at 34 MPa (approximately 70% of the yield strength) for 1000 times at a frequency of 0.1 Hz. The loading was applied using an Instron 4505 equipped with an advanced function panel. The cyclic loading introduced damage zones at the interface of the large particles and the matrix to facilitate the identification of the large particles. Thin sections were prepared after cyclic loading by petrographic thin sectioning [9] and polished to approximately 5 μm thick for optical microscopy and transmission infrared spectroscopy. Optical micrographs were taken using a Zeiss Axioskop with transmitted light. Infrared spectra were recorded using a Bruker A590 infrared microscope and IFS28 FTIR spectrometer. Infrared spectra for the large particles were collected from individual particles exposed on both the top and bottom surfaces of a section, ensuring

Partitioning of energy loss in glass-fiber-reinforced polymers under transverse loading

Energy loss for (a) contact indentation and (b) friction between delaminating surfaces was measured experimentally to determine the energy required for matrix cracking and delamination in a glass-fiber-reinforced polymer (GFRP) under several levels of out-of-plane (transverse) quasistatic loading without fiber fracture. The results suggest that the friction between delaminating surfaces and the contact indentation contributed to 30% of the total energy loss. The delamination and matrix cracking were responsible for the remaining 70% of the total energy loss. Due to the significant portion of the total energy loss for the mechanisms unrelated to delamination, we conclude that correcting the measured energy loss is necessary to accurately quantify the GFRPs’ delamination resistance.

Energy absorption for indentation damage in glass fiber reinforced polymers (GFRP)

Although GFRP has a much higher strength-to-weight ratio than conventional materials such as steels and concrete, it suffers from poor “local” resistance to the indentation damage, often introduced by transverse point loading. This is due to the inherent in-plane reinforcement of fiber, which does not provide any strengthening of the GFRP in the out-of-plane direction. Most of the indentation damages generated by transverse loading appear to be small, with a size similar to the contact area, thus being deemed to have little effect on the over-all properties of the GFRP. Consequently, little attention has been paid in the past to understand how material parameters of the GFRP affect its resistance to the indentation. This concept, however, is being challenged by applications such as boat hulls and bridge decks that use thick GFRP laminates. For these types of applications, indentation damage is the most common mechanism that initiates damages such as delamination under the contact surface [1–3], causing significant loss of structural integrity and possibly catastrophic failure. Most research work on indentation of fiber composites [4–6] dealt with damages that were a combination of local indentation and sub-surface delamination. This was because specimens used were not stiff enough to prevent delamination under the contact point. Therefore, the results failed to isolate the indentation damage from other fracture mechanisms, producing information that was not applicable when knowledge of pure indentation damage was required. As the first step to characterize fiber composite’s resistance to indentation damage, a series of experiments were conducted to measure energy absorbed by fiber composites that were subjected to pure indentation damage. This paper summarizes results from the indentation tests, and discusses its significance in over-all energy absorption of GFRP under transverse loading. The two GFRP used in the study have the same fiber volume fraction and lay-up, but different resins for the matrix. One resin was pure isophthalic polyester (TMR300 iso-polyester, provided by Viking Plastics, Edmonton) and the other polyurethane resin with 15% CaCO3 particles (PUL-G resin, provided by Resin Systems Inc., Edmonton). The fiber used was warp unidirectional glass fiber fabric of 9 oz/yd2, which consists of unidirectional fiber bundles that are held in parallel at a distance of approximately 1 mm apart by stitching thread [7]. The fiber lay-up is [(0/90)5]s that forms a nominal specimen thickness of 6 mm with the maximum variation of ±0.05 mm. A resin transfer molding technique was used for the GFRP fabrication to ensure consistent thickness of the test coupons. Due to the inter-fiber-bundle gap, resinrich zones exist in the laminates, in the inter-laminar regions and the intra-laminar, inter-fiber-bundle regions. Overall fiber volume fraction of the GFRP was estimated to be around 40%, based on the following equation [8]