A method to measure fracture toughness of the fiber/matrix interface using the single-fiber fragmentation test

A fracture mechanics model was developed for determining the fracture toughness of the fiber/matrix (F/M) interface based on a modified test procedure for the single fiber fragmentation test (SFFT). After loading the specimen until the first fiber fracture and instantaneous debonding events occur, the specimen is unloaded and loaded again until the debond propagates. The critical load for debond propagation is measured and is used with a fracture mechanics analysis to determine the interface fracture toughness. The analysis considers also friction between the fiber and matrix in the debonded region. To obtain the necessary data for calculation of residual radial stress at the F/M interface due to matrix cure shrinkage, simultaneous measurements of dynamic modulus and cure shrinkage were conducted on the matrix (vinylester) during cure. Tests employing E-glass/vinylester SFFT specimens provided fracture toughness values of G_cd = 62 J/m² (frictionless) and 48 J/m² (friction).     

Multiscale considerations for interface engineering to improve fracture toughness of ductile fiber/thermoset matrix composites

Mechanical interlocking at both large-scale and fine-scale as well as continuous and intermittent chemical bonding were employed to engineer the friction during fiber pullout to improve toughness. Excellent toughening results have been achieved over a large range of typical fracture feature size in the large micro-/small meso-scale of 1–100 μm. Interphase engineered features for size scales below the microscale were smoothed over or subsumed during fracture. For mechanical interlocking at the large scale, the overall fiber shape is critical and there is little interfacial adhesion. For mechanical interlocking at the fine scale, a protruding “break-away” feature performs better than a concave, cavity-like feature. For chemical bonding, intermittent coating gives superior toughness. In all of the interface or interphase designs, it is found that there is an intermediate anchoring of the fiber to best utilize fiber plasticity to improve composite toughness.     

Microstructural efficiency and fracture toughness of short fiber/thermoplastic matrix composites

This paper outlines the fracture behavior of composites with thermoplastic matrices of different fracture toughness K_cm (increasing in the order PPS → PET (I) → PET (II) → ETFE → PC). In particular, the way in which the fracture toughness of these composite systems, K_cc, is affected by the volume fraction, orientation and distribution of short glass fibers across the plaque thickness (fiber length ≈ 200 μm, fiber diameter ≈ 10 μm), and by the quality of their interfacial bonding to the matrix is discussed. SEM studies were carried out to define the microstructural details and the dominant mechanisms of energy adsorption during breakdown of the composites.In general, an increase in composite toughness can be expected with increasing extent of reinforcement if the matrix is in a brittle condition (here also verified by K_c-tests at lower temperatures) and if the fibers are well bonded and mostly oriented perpendicular to the crack front. An opposite tendency may occur for matrices which behave in a highly ductile manner even in the presence of fibers. The probability of this behavior is favored in poorly bonded systems. The results are discussed in terms of a ‘microstructural efficiency factor’ M, which mainly considers the relative contributions of fiber and matrix related mechanisms to energy dissipation during breakdown of a composite (‘energy absorption ratio’ n) as well as the reinforcement content and its arrangement in the matrix (‘reinforcing effectiveness parameter’ R).     

Influence of fiber/matrix interfacial adhesion on composite fracture behavior

A systematic study has been conducted to identify the effect of fiber/matrix interface strength on various composite properties. A new fiber treatment technique was developed to allow fibers to be treated and then made into prepregs and composites of acceptable quality. T500 carbon fibers were treated with release agent to establish the extreme case of poor fiber/matrix interface. Composite systems made of toughened epoxy R6376 and T500 fibers with and without such a treatment were subjected to a number of fracture and impact tests. For tests involving propagating pre-existing delamination cracks, such as double cantilever beam (DCB), end notched flexural (ENF) and crack lap shear (CLS) methods, the material properties were not appreciably affected by the release agent-treated fiber surfaces. For tests that had to initiate cracks in specimens without pre-introduced cracks, such as impact and edge delamination, the material variables and failure modes were highly sensitive to the fiber/matrix interface. The critical role of the fiber/matrix interface in crack initiation was demonstrated in this study.     

Effect of matrix on the dry friction coefficient of unidirectional fiber-reinforced composite

The coefficients of dry rest (μ₀) and sliding (μ _s ) friction on a polished disk made of quenched steel have been measured for various polymer matrices and fibers and for a composite unidirectionally reinforced with poly(amidobenzimidazole) (PABI) fibers. It is established that μ₀ > μ _s for matrices and fibers with glass transition temperatures T _g below room temperature T _R , otherwise μ₀ ≈ μ _s . This effect is explained by a sharp growth in the plasticity of polymers at T _g , which leads to an increase in the polymer-steel contact area. For a composite with T _g > T _R , the dry friction coefficients obey the relation μ₀ ≈ μ _s ≈ μ _e /C, where μ _e is the coefficients of sliding friction of PABI fibers and C is their concentration. For T _g < T _R (plastic matrix), the friction coefficients of the composite and matrix are close because the latter cannot hold the fiber ends during friction. As a result, they are bent and aligned along the matrix surface or embedded in the surface layer.     

树脂基复合材料增韧研究进展

经过几十年的发展,国内高韧性树脂基复合材料的研究有了很大进展,但其在工程上的应用才刚刚开始,仍存在一些问题。基于国内外树脂基复合材料的增韧研究现状,总结了不同的增韧方法,从基体增韧、层间增韧和Z向增韧3个方面出发,归纳出不同增韧方法的机理以及优缺点。     

Interlaminar fracture toughness for composite materials

A new equation for the energy release rate for a double cantilever beam specimen is proposed within the framework of the higher order shear deformable plate theory. The interlaminar fracture toughnesses (IFTs) found using the present theory, the ASTM round robin test method and the acoustic emission method are compared for thermoset graphite/epoxy and thermoplastic AS4/PEEK composites. As a result, IFTs by the present theory show agreement within 5% when compared with those found using the ASTM method and it is shown that IFTs found by the acoustic emission method are lower than those found by the ASTM method. It is observed that the IFT of the thermoplastic AS4/PEEK composite is about 10 times larger than that of the thermoset graphite/epoxy composite.     

Tensile fracture behavior of notched fiber reinforced titanium metal matrix composite

The static fracture behavior of a titanium based metal matrix composite (MMC) with a central hole or a straight notch was investigated. The MMC used was SCS-6/Ti-β21-S with a quasi-isotropic lay-up. Different sizes of hole or notch were used which provided cut-out size to specimen width ratios from 0·1 to 0·4. Two test temperatures were used: ambient and 650°C. At both temperatures, the tested MMC showed a mild hole size effect or notch sensitivity. The failure mechanisms involved the debonding of fibers followed by failure of fibers, and then by failure of the matrix.     

Evaluation and analysis of interlaminar fracture toughness of bead filled matrix hybrid composite materials using orthotropic fracture mechanics

Evaluation of Mode I interlaminar fracture toughness for unidirectional hybrid composites fabricated with a bead filled epoxies was carried out. The two important fracture toughness parameters, G_IC and K_IC values of hybrid composites, were reviewed in accordance with the orthotropic fracture model. The deviation of measured G_IC and K_IC values from predicted values were explained based on the critical review of the basic assumption of orthotropic fracture model and characteristic material properties of hybrid composites. It can be said that, basically, the orthotropic fracture model can be used for evaluation of hybrid composite materials. However, careful analysis for G_IC and K_IC values which were derived from different source and some correction factor for K_IC values are necessary.     

On the fracture toughness of ceramic matrix composites

Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed.     

Modified single fiber fragmentation test procedure to study water degradation of the fiber/matrix interface toughness of glass/vinylester

A modified single fiber fragmentation test (SFFT) procedure, that permits separation of the fiber break and fiber/matrix (F/M) debond propagation events, was employed to characterize the (F/M) interface toughness of dry and water saturated E-glass/vinylester. By focusing solely on the debond propagation event, and by measuring the critical load for debond propagation, fracture mechanic analysis enabled determination of the fracture toughness of the fiber/matrix interface. After immersion in seawater, the interface was substantially degraded. The fracture toughness was reduced by approximately a factor of two.     

Effect of reinforcement size and matrix microstructure on the fracture properties of an aluminum metal matrix composite

The effects of systematic changes in reinforcement size and matrix microstructure on the crack initiation and growth toughness of a 7091 aluminum alloy reinforced with SiC particulates were studied. It is shown that changes in matrix microstructure have a significant effect on both initiation and growth toughness. The effect of reinforcement size on these properties is far less marked. These observations have been related to local microstructural parameters and the nature of the distribution of the reinforcement.     

Effects of carbon nanotubes on the interlaminar shear strength and fracture toughness of carbon fiber composite laminates: a review

Journal of Materials Science - The interlaminar mechanical properties of composites are important parameters for the application of laminates, and many scholars have applied carbon nanotubes (CNTs)...