Delamination Toughness Research Articles

Effect of Crack Length Measurement Technique and Data Reduction Procedures on the Perceived Toughness from Four-Point Bend End-Notched Flexure Tests

Results are presented from a study to determine the most appropriate crack length measurement and data reduction techniques for use with the four-point bend end-notched flexure test. Using two different graphite/epoxy materials, the variability in crack length measurements due to different observers and/or measurement techniques, including visual observation and ultrasonic inspection, and their effects on the perceived values of the mode II delamination toughness, GIIc, were evaluated for two different data reduction techniques. It was found that a technique using pretest compliance calibrations is the only approach that will produce accurate non-precracked and precracked values of GIIc regardless of the material being tested or the nature of crack advance. For those materials where GIIc values, as obtained by the more common approach that utilizes compliance values obtained during the test, agree with those obtained by compliance calibration, the results from the more common approach may be applied to obtain resistance curve toughnesses.

Modelling of inter-laminar toughening from chopped Kevlar fibers

Inter-laminar toughening from sparsely distributed chopped fibers is analyzed in this paper. A detailed micromechanical model describing the special toughening effects of those chopped Kevlar fibers lying within the delamination plane has been developed. To determine bridging forces of the Kevlar fibers lying in the matrix near a delamination-crack face, a simplified model based on coupling of matrix spalling (matrix fracture following by separation of fiber away from the matrix) and fiber pull-out is proposed. The model is then used to study toughening of carbon-fiber/epoxy composite laminates with chopped Kevlar fibers. The model shows that the energy dissipation in the pull-out process of chopped Kevlar fibers is much more than that in the matrix spalling process. The numerical results indicate that interfacial properties, interactions among Kevlar fibers and percentage of Kevlar fibers involved in crack bridging play important roles in delamination toughening. The modelling results are in good agreement with the experimental results available in the literature.

A method for determination of delamination toughness in different positions in the thickness direction of paperboard

A method for determination of delamination toughness in different positions in the thickness direction of paperboard

Mode II delamination toughness of z-pinned laminates

Mode II delamination toughness of z-pin reinforced composite laminates is investigated using finite element (FE) method. The z-pin pullout process is simulated by the deformation and breakage of non-linear springs. A critical shear stress criterion based on linear elastic fracture mechanics is used to simulate crack growth in an end-notched-flexure (ENF) beam made of z-pinned laminates. The mode II toughness is quantified by the potential energy release rate calculated using the contour integral method. This FE model is verified for an unpinned ENF composite beam. Numerical results obtained indicate that z-pins can significantly increase the mode II delamination toughness of composite laminate. The effects of design variables on the toughness enhancement of z-pinned laminates are also studied, which provides an important technological base and useful data to optimize and improve the z-pinning technique.

Interlaminar fracture toughness of CF/PEI composites at elevated temperatures: roles of matrix toughness and fibre/matrix adhesion

An experimental study was conducted to characterise temperature effects on the relationship between the plane-strain fracture toughness of polyetherimide (PEI) matrix and mode-I and mode-II interlaminar fracture toughness of woven carbon fibre/polyetherimide (CF/PEI) thermoplastic composites. The decreasing trend in initiation toughness, GIC,ini and GIIC,ini, for mode-I and mode-II interlaminar fracture was not consistent with the increase in matrix toughness with increasing temperature, but was related more to the decrease in fibre/matrix interfacial shear strength (IFSS) due to the overriding role of the weakened interface at delamination initiation. On the other hand, during delamination propagation, the increase in composite toughness, GIC,prop and GIIC,prop, corresponded to that in matrix toughness. Further, it was found that the fibre bridging promoted by the weakened interface made clear contributions to the translation of resin toughness to delamination toughness at elevated temperatures, especially in mode-I testing.

Interlaminar fracture toughness and the associated fracture behavior for glass fiber reinforced polymers (GFRP)

Laminated fiber reinforced polymers (FRP) have excellent in-plane stiffness and strength, but suffer from transverse impact loading due to the lack of fiber reinforcement in that direction. Under transverse impact, extensive delamination can occur in the interlaminar resin-rich regions, which leads to a reduction of stiffness and strength. Therefore, many researchers have attempted to establish a standard testing method that characterizes the interlaminar fracture toughness for delamination [1]. In addition to the measurement of the delamination toughness, analysis of the fracture surface using optical microscopy or scanning electron microscopy (SEM) can also provide useful information for the delamination resistance characterization. Recently, we have evaluated mechanical properties of two glass-fiber-reinforced polymers (GFRP) [2], using transverse impact, double cantilever beam (DCB) and end-notched flexure (ENF) tests. Results from the transverse impact and DCB tests showed a clear difference in delamination resistance between the two GFRP, but such a difference was not found from the ENF test. Based on the ENF test results, the two GFRP should have similar delamination resistance in the shear mode (Mode II). Although this discrepancy in delamination resistance may be caused by the toughness variation in different modes of loading, we believe that the discrepancy is mainly due to a problem with the ENF test for the characterization of the delamination resistance. This issue has been raised in several studies [3, 4], and is further investigated here through the examination of fracture surfaces to provide additional evidence. Two GFRP used in this study have polymer matrices that are the same as those used previously, that is, isophthalic polyester (TMR300 isopolyester, provided by Viking Plastics, Edmonton) and polyurethane-based resin (PUL-G polyurethane, provided by Resin System Inc., Edmonton). The PUL-G polyurethane resin contains 15% CaCO3 particulates for stiffness enhancement. For convenience, the two GFRP will be named PI-GFRP and PU-GFRP in this paper, for isopolyesterbased GFRP and polyurethane-based GFRP, respectively. The glass fiber used is 9-oz/yd2 warp, unidirectional woven fabric (provided by ZCL Composites, Edmonton) which consists of parallel fiber bundles stitched together with a gap of around 1 mm. The gap between the fiber bundles generated resin-rich zones in the GFRP. Therefore, the GFRP have resin-rich zones in the intra-laminar, inter-fiber-bundle regions and the interlaminar regions. GFRP plates of nominally 6.0 mm thick were fabricated using a resin transfer molding (RTM) technique. The fiber lay-up of the transverse impact specimens is [(0/90)5]s, and that of the DCB and ENF specimens is [(0/90)402]s of which the four central 0-degree layers were to provide a uni-directional fiber environment in the direction of crack growth, in accordance with most of the test coupons used in the past. The specimens for DCB and ENF tests contain an aluminum insert film, 15 μm thick, that is placed between the 2nd and the 3rd of the four central layers, acting as a starting defect for delamination crack growth. Dimensions of the specimens are 93 × 93 mm2 for the transverse impact test, and 120 × 20 mm2 for the DCB and ENF tests. Overall fiber volume fraction of the specimens is around 40%, estimated using the following equation [5]:

Modifying adhesion of linear low-density polyethylene to polypropylene by blending with a homogeneous ethylene copolymer

Adhesion of a Ziegler–Natta catalyzed ethylene copolymer (ZNPE) to polypropylene (PP) was studied by measuring the delamination toughness G of coextruded microlayers by using the T-peel test. Low values of G compared to a homogeneous copolymer with approximately the same short chain branch (SCB) content were attributed to an amorphous interfacial layer of low molecular weight, highly branched ZNPE fractions. Blending ZNPE with a homogeneous metallocene catalyzed copolymer (mPE) increased G. In this regard, mPE with higher SCB content was more effective than mPE with slightly lower SCB content. The ZNPE interface was mimicked by microlayering ZNPE and ZNPE blends with polystyrene from which the ZNPE layers were easily separated without damage to the surface. Examination with atomic force microscopy revealed a soft coating about 8 nm thick on the surface of the ZNPE layer. Blending with mPE reduced or eliminated the amorphous interfacial layer. It was proposed that mPE increased miscibility of low molecular weight, highly branched fractions of ZNPE and prevented their segregation at the interface. After blending with mPE eliminated the interfacial layer, G increased to a value comparable to that of a homogeneous copolymer with about the same SCB content as ZNPE bulk chains. The increase in G was attributed to epitaxial crystallization of the ethylene copolymer in the absence of an amorphous interfacial layer. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 109–115, 2004

Adhesion of polyethylene blends to polypropylene

The effect of chain microstructure on adhesion of ethylene copolymers to polypropylene (PP) was studied using coextruded microlayers. Adhesion was measured by delamination toughness G using the T-peel test, and interfacial morphology was examined by atomic force microscopy. Good adhesion to PP was achieved with homogeneous metallocene catalyzed copolymers (mPE) with density 0.90 g cm −3 or less. Good adhesion was attributed to entanglement bridges. In contrast, a heterogeneous Ziegler-Natta catalyzed copolymer (ZNPE) of density 0.925 g cm −3 exhibited poor adhesion to PP due to an amorphous interfacial layer of low molecular weight, highly branched fractions that prevented effective interaction of ZNPE bulk chains with PP. Blending mPE with ZNPE eliminated the amorphous interfacial layer and resulted in epitaxial crystallization of ZNPE bulk chains with some increase in G. Increasing the mPE content of the blend past the amount required to completely resolve the amorphous interfacial layer of ZNPE resulted in a steady, almost linear, increase in G. Phase separation of mPE and ZNPE during crystallization produced an interface with regions of epitaxially crystallized ZNPE bulk chains and other regions of entangled mPE chains. Entanglement bridges imparted much better adhesion than did epitaxially crystallized lamellae.

Effect of stitch distribution on mode I delamination toughness of laminated DCB specimens

A new nonlinear finite stitch element is proposed for parametric analysis on the effect of distribution of through-thickness stitching reinforcement on improving the delamination crack growth resistance in double cantilever beam (DCB) specimens. Both the classical and Timoshenko beam theories are used to analyze the DCB specimens. A novel two-node rod element is developed to model the stitches along the crack plane and the micromechanical stitching models available in the literature are adopted to simulate the mechanical behavior of the stitches. The bridging traction of each discrete stitch is simulated as a nonlinear spring. The numerical examples show that stitch distribution can significantly affects the improvement on delamination crack growth resistance in DCB specimens. When keeping the stitch density or stitch volume fraction as a constant, the stress intensity factor at the delamination crack tip can be significantly affected by altering the stitch distribution along the crack plane.

Mechanical property improvement of carbon fiber reinforced polybenzoxazine by rubber interlayer

AbstractThe rubber interlayer method was chosen in order to improve the properties of carbon fiber‐reinforced polybenzoxazine composites. The resin used is benzoxazine based on bisphenol‐A, formaldehyde and 3,5‐xylidine. The effect of rubber concentration on the flexural properties of the composites is investigated. Sized and desized carbon fiber woven fabrics are used to study the effect of the sizing materials on the mechanical properties. The delamination toughness of the composites is increased by the ATBN rubber interlayer with increasing ATBN concentration. The strength of the composite also increased, but an anomalous concentration effect has been observed.

Crack blunting and the strength of soft elastic solids

When a material is so soft that the cohesive strength (or adhesive strength, in the case of interfacial fracture) exceeds the elastic modulus of the material, we show that a crack will blunt instead of propagating. Large–deformation finite–element model (FEM) simulations of crack initiation, in which the debonding processes are quantified using a cohesive zone model, are used to support this hypothesis. An approximate analytic solution, which agrees well with the FEM simulation, gives additional insight into the blunting process. The consequence of this result on the strength of soft, rubbery materials is the main topic of this paper. We propose two mechanisms by which crack growth can occur in such blunted regions. We have also performed experiments on two different elastomers to demonstrate elastic blunting. In one system, we present some details on a void growth mechanism for ultimate failure, post–blunting. Finally, we demonstrate how crack blunting can shed light on some long–standing problems in the area of adhesion and fracture of elastomers.

Numerical study on the mode I delamination toughness of z-pinned laminates

A finite element (FE) model is developed to investigate mode I delamination toughness of z-pin reinforced composite laminates. The z-pin pullout process is simulated by the deformation of a set of non-linear springs. A critical crack opening displacement (COD) criterion is used to simulate crack growth in a double-cantilever-beam (DCB) made of z-pinned laminates. The toughness of the structure is quantified by the energy release rate, which is calculated using the contour integral method. The FE model is verified for both unpinned and z-pinned laminates. Predicted loading forces from FE analysis are compared to available test data. Good agreement is achieved. Our numerical results indicate that z-pins can greatly increase the mode I delamination toughness of the composite laminates. The influence of design parameters on the toughness enhancement of z-pinned laminates is also investigated, which provides important information to optimise and improve the z-pinning technique.

Adhesion of ethylene–styrene copolymers to polyethylene in microlayers

AbstractThe effects of styrene content, layer thickness, and temperature on the adhesion of ethylene–styrene copolymers (ES) to low‐density polyethylene (LDPE) were examined by measuring the delamination toughness of LDPE/ES microlayers in the T‐peel test. Delamination toughness at ambient temperature decreased with increasing styrene content. A linear correlation between delamination toughness and styrene content in ES was observed. Extrapolation predicted that a copolymer with 72.5 wt % styrene would have no adhesion to LDPE. Experiments on microlayers with relatively thin (8–18 μm) ES layers demonstrated that delamination toughness was proportional to ES layer thickness. In situ observations of the damage zone showed stretching throughout the entire thickness of the ES layer. For higher adhesion systems, ES deformation occurred concurrently with localized stretching and crazing of the LDPE layer at the interface. For two representative systems, the effect of temperature on delamination toughness was studied. Major transitions in delamination toughness and fracture mode were observed at the primary thermal transitions of the ES copolymer. The contribution by stretching of the ES layer to measured delamination toughness was approximated by relating the damage zone to the ES engineering stress–strain curve and was estimated to be about half. The remaining contribution to delamination toughness was assumed to be due to LDPE deformation in the damage zone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 153–162, 2003

Refinements to the Mixed-Mode Bending Test for Delamination Toughness

The mixed-mode bending (MMB) test for delamination toughness was first introduced in 1988. This simple test is a combination of the standard Mode I (opening) test and a Mode II (sliding) test. This MMB test has become widely used in the United States and around the world for mixed-mode toughness measurements. Because of the widespread use of this test method, it is being considered for standardization by ASTM Committee D30. This paper discusses several improvements to the original test method. The improvements to the MMB test procedure include an improved method for calculating toughness from the measured test quantities, a more accurate way of setting the mixed-mode ratio to be tested, and the inclusion of a new alignment criterion for improved consistency in measured values.

The fracture of brittle thin films on compliant substrates in flexible displays

One mechanical issue in flexible organic light emitting displays (OLED) is the fracture of extremely thin brittle conducting transparent oxide films deposited on thin flexible substrates. Understanding the behaviour of these films under flexed condition is essential for designer of flexible OLED. Controlled buckling experiments on the film and substrate have been designed to study the fracture of the films under both tension and compression. Fracture of the film is superficially similar in both tension and compression. However, under tension a channelling crack is formed, while under compression, the film delaminates, buckles and cracks in a tunnelling motion. The fracture toughness of the film and the delamination toughness have been estimated from these experiments. Design to maximise the flexibility of the device is discussed.

Delamination characteristics of multi‐directional carbon fiber/epoxy composites under high pressure

AbstractIt was shown in a previous study that for unidirectional (0‐deg) graphite/epoxy composites, the fracture toughness under hydrostatic pressure increased 38% as hydrostatic pressure increased from 0.1 MPa to 200 MPa. This work investigates the compressive delamination behavior of multi‐directional graphite/epoxy laminated composites subjected to various hydrostatic pressures. Compressive delamination tests were performed under four hydrostatic pressure levels: 0.1, 100, 200, and 300 MPa Eighty‐eight‐ply dog‐bone type specimens with a single delamination at the center of the specimen were used. The stacking sequence applied was [0°/±45°/90°]lls. The compliance and fracture load were determined from load‐displacement curves as a function of hydrostatic pressure.The results show that the compliance decreases with increasing pressure while fracture load increases with increasing pressure. The compressive delamination toughness, Gc, was determined from the compliance method as a function of applied hydrostatic pressure. The results also show that Gc is significantly affected by hydrostatic pressure and increases from 2.11 kJ/m2 to 3.04 kJ/m2 (44% increase) as hydrostatic pressure increased from 0.1 MPa to 300 MPa. Visual examination of the fractured surface revealed that the increase of Gc is due to the suppression of micro‐cracks With increasing pressure. It was also found from SEM examination of delaminated surface that the Gc increase is due to more epoxy adhering to the fibers and more plastic deformation of epoxy material as applied hydrostatic pressure increases.

Adhesion and fracture of polymeric coatings

There are many important aspects of the performance of polymeric coatings, adhesion and fracture are included amongst them. Industry has devised numerous tests for the measurement of adhesive strength and coating toughness, but, all too often, these are ad-hoc and based on end-use. This limits severely any overall value from use of these procedures. In this paper an attempt will be made to consider a more fundamental approach to such matters and, in particular, to establish a technique for measuring procedures for adhesive strength and coating toughness that are independent of shape. In terms of adhesive strength, flexible laminates can be accommodated as well as rigid joints, and as such method of measuring either the interfacial work of fracture or the delamination toughness can be developed. In terms of coating toughness, ductilebrittle characteristics of the coating or surface scratch resistance can investigated. All of these methods are illustrated in the paper.

A mechanical assessment of flexible optoelectronic devices

This work has demonstrated novel experimental methods and their relevant analysis to evaluate the fracture properties of thin brittle films on compliant substrates for flexible optoelectronic devices. Based on the understanding of the failure mechanisms, mechanical calculation has been provided to estimate the thin film fracture toughness and film delamination toughness. These values serve as design parameters on the device flexibility and reliability.

Rate Effects on Mode-I Delamination Toughness of a Graphite/Epoxy Laminated Composite

Rate Effects on Mode-I Delamination Toughness of a Graphite/Epoxy Laminated Composite

Evaluation of the accuracy of the four-point bend end-notched flexure test for mode II delamination toughness determination

Results are presented from an experimental study to investigate the accuracy of the four-point bend end-notched flexure (4ENF) test for the determination of mode II delamination toughness. In previous studies, 4ENF tests have yielded higher delamination toughnesses than the more commonly used three-point bend end-notched flexure (ENF) test. In a recent finite-element study, it was found that the higher toughnesses found by the 4ENF test cannot fully be attributed to frictional effects. This current study was performed in order to examine if the difference between ENF and 4ENF results is due to current methods used to determine crack length and compliance during a 4ENF test. It was found that if these two parameters are measured accurately then the 4ENF test yields toughnesses similar to those obtained from ENF testing.