Leif Asp

Prediction of matrix-initiated transverse failure in polymer composites

A study has been conducted of failure in unidirectionally-reinforced fiber composites loaded in tension normal to the fibers. The case considered is when this failure is governed by failure of the matrix rather than fiber/matrix debonding. Both yielding and cavitation-induced brittle failure of the matrix are considered. The latter mode of failure was suggested previously as the likely mode to occur in epoxies under stress states that are purely or nearly hydrostatic tension. Three fiber packing arrangements (square, hexagonal and square-diagonal) with different fiber volume fractions were studied numerically by a finite element method to determine the local stress states. It is found that cavitation-induced brittle failure occurs much before yielding in all cases. Experimental data taken from the literature support this finding.

A criterion for crack initiation in glassy polymers subjected to a composite-like stress state

Three epoxy systems of interest as composite matrix materials are examined for their yielding and failure behavior under uniaxial, biaxial and triaxial stress states. Yield criteria applicable to glassy polymers, i.e. accounting for the hydrostatic stress effect on the deviatoric stress to yielding, are assessed. It is found that under stress states resembling those in matrix constrained between fibers, e.g. equibiaxial and equitriaxial tension, yielding is suppressed while brittle failure, presumably caused by crack growth from cavitation, occurs. A criterion for this mode of failure is proposed as the critical dilatational strain energy density. Experimental data are found to support this criterion.

Delamination Growth and Thresholds in a Carbon/Epoxy Composite Under Fatigue Loading

This paper presents a study on delamination growth in Mode I, Mode II and mixed mode under fatigue loading in an HTA/6376C composite. The computed slopes of the modified Paris plots were high. Therefore, threshold values of the strain energy release rate for delamination growth were determined. Low fatigue threshold values revealed a significant effect of fatigue loading. The largest effect was found for the ENF test (Mode II) for which the fatigue threshold value was only 10% of the critical strain energy release rate in static tests. Threshold values for MMB (mixed mode) and DCB (Mode I) tests were 15% and 23% of the static values, respectively. Fractographic evaluation revealed identical initial failure mechanisms in fatigue and static loading conditions for the ENF specimen. The ENF specimen failed by formation and coalescence of microcracks. The low fatigue threshold for the ENF specimen was explained by microscopical observations on the specimen edge. It was also shown that the fracture surfaces generated in static and fatigue DCB and MMB tests were similar.

The effects of moisture and temperature on the interlaminar delamination toughness of a carbon/epoxy composite

Abstract HTA/6376C composite has been investigated for influence of temperature and moisture content on the interlaminar delamination toughness in mode I, mode II and mixed mode conditions. Dry and moisture-saturated specimens were tested over the temperature range − 50 °C to 100 °C. Evaluation methods based on load/displacement and load measurements were employed. In pure mode II the critical strain-energy release rate drops with moisture content and increase in temperature. In mixed mode the critical strainenergy release rate also decreases with moisture content, but no general trends in the dependence on temperature is observed. The critical strain-energy release rate in pure mode I is unaffected by changes in moisture content and was found to increase slightly at elevated temperatures. During crack propagation, enhanced fiber bridging due to increases in temperature and moisture content promotes R-curve behavior in the pure mode I tests. The resulting mode mixity of the mixed-mode bending (MMB) tests is found to be severely affected by the evaluation methods. Methods based on load measurements only are considered to give unreliable strain-energy release rates as the measured compliance/displacement relationships were found to be non-linear even prior to crack growth. Further studies are needed to assess the mixed-mode ratio in the MMB test.

Formation of damage and its effects on non-crimp fabric reinforced composites loaded in tension

Abstract Non-crimp fabric (NCF) composites, manufactured by resin infusion techniques are one of the most promising next generation composite materials. They offer large potential for application in primary structures as they give excellent performance at low production costs. However, before NCF composites will be efficiently used in design, detailed understanding of governing micro mechanisms must be accumulated and described by predictive models. In the present study, NCF cross-ply laminates have been tested in tension. Intralaminar cracks caused in the 90° fibre bundle layers and their effect on laminate mechanical properties have been monitored. Occurrence of ‘novel’ type of cracks propagating in the load direction (longitudinal cracks) is explained by a thorough FE analysis using an Representative Volume Element (RVE) approach, revealing stress concentrations caused by 0° fibre bundle waviness. Effects of damage on mechanical properties are modelled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals mechanical degradation to be ruled by the crack opening displacement (COD). However, unlike traditional composites, transverse cracks do not generally extend through the entire thickness of the 90° layer, but are rather contained in single fibre bundles, limiting the COD.

Delamination buckling and growth for delaminations at different depths in a slender composite panel

A numerical and experimental investigation for delamination buckling and growth for slender composite panels loaded in compression is presented. The investigated panels consisted of 35 plies in a cross-ply layup with artificially embedded delaminations inserted after three, five or seven plies from the upper surface. The tests clearly and consistently showed that for all delamination depths, delaminated panels failed by delamination growth slightly below the global buckling load of the undamaged panel, whereas the undelaminated panels failed in compression at global buckling. The analysis was done with a finite element based computational model that accounts for contact between delaminated members and fracture mode separation and where crack propagation was simulated with a moving mesh scheme. For all delamination depths, the analysis showed a dramatic increase in the energy release rate when global buckling takes place. Features seen in the tests were captured in the computational analysis. Excellent agreement with tests was found for loads at which delaminated members buckle, the load for onset of delamination growth and the evolution of delamination, e.g., delamination shape and out-of-plane displacements.

Effects of a composite-like stress state on the fracture of epoxies

The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of composites suggest the triaxial matrix stress state as one of the explanations. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfully applied to four epoxies in the glassy state. The chosen specimen geometry mimicked the most severe stress state in the matrix as determined by finite element analysis of a transversely loaded glass-fiber/epoxy composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8-7%. The triaxial stress state in composite matrices may therefore by itself be a sufficient explanation for low values of transverse composite strains to failure.

Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites

Abstract Failure initiation in polymer-matrix composites loaded transverse to the fibers is investigated by a numerical parametric study where the effects of constituent properties, interphase properties and thickness are examined. Failure initiation in the matrix only is studied, interfacial debonding not being considered. Two modes of failure—yielding and cavitation-induced brittle failure—are examined. A criterion for the cavitation-induced brittle failure has been proposed previously and failure prediction based on this criterion was found to agree with experimental data for a glass-fiber-reinforced epoxy. The present study shows that the elastic modulus of fibers has a large effect on the stress and strain to failure initiation. A rubbery interphase material is found in most cases to have a beneficial effect. The site at which failure initiates and the governing mode of failure initiation are also affected by the fiber modulus and the interphase properties.

Effects of temperature on delamination growth in a carbon/epoxy composite under fatigue loading

Abstract This paper presents a study of delamination growth in HTA/6376C carbon fibre/epoxy laminates. Tests were conducted under Mode I, Mode II and mixed-mode static and fatigue loading at both ambient conditions and elevated temperature. The results show that the strain energy release rate threshold values for delamination growth under fatigue loading are significantly lower than the critical energy release rates in static tests. At elevated temperature, the threshold values in the fatigue loading were only about 10% of the critical values in the static tests. A fractographic analysis of the delamination growth revealed that the fracture surfaces generated at elevated temperature generally were similar to the fracture surfaces generated at room temperature. Nevertheless, some differences in morphology of the fracture surfaces were observed, and their effect on the static and fatigue delamination growth is discussed in detail.

Assessment of Evaluation Methods for the Mixed-Mode Bending Test

The evaluation procedure for the mixed mode bend (MMB) delamination test is assessed with focus on analytically equivalent evaluation models, expressed in load-displacement or load-only parameters. In particular, the assessment concerns the sensitivity of the interlaminar toughness to the test rig forces as well as material and geometrical properties of the specimen. For a typical example, neglect of test rig forces causes a 10% relative error in the calculated mixed mode ratio when using methods based on load only. When all additional forces were considered, both evaluation methods produced almost identical results. However, evaluation based on load only is sensitive to variations in specimen flexural modulus and dimensions and had a larger scatter. The application of crack length corrections for calculating the Mode I component is discussed in detail. The findings of the study are summarized in recommendations for the MMB test procedure and its subsequent evaluation.