Investigations of interlaminar fracture toughness of laminated polymeric composites

The behavior of interlaminar fracture of fiber reinforced laminated polymeric composites has been investigated in modes I, II, and different mixed mode I/II ratios. The experimental investigations were carried out by using conventional beam specimens and the compound version of the CTS (compact tension shear) specimen. In this study, a compound version of the CTS specimen is used for the first time to determine the interlaminar fracture toughness of composites. In order to verify the results obtained by the CTS tests, conventional beam tests were also carried out. In the beam tests, specimens of double cantilever beam (DCB) and end notched flexure (ENF) were used to obtain the critical rates of the energy release for failure modes I and II. The CTS specimen is used to obtain different mixed mode ratios, from pure mode I to pure mode II, by varying the loading conditions. The highest mixed mode ratio obtained in the experiment was G _I /G _II =60. The data obtained from these tests were analyzed by the finite element method. The separated critical rates G _I and G _II of the energy release were calculated by using the modified virtual crack closure integral (MVCCI) method. The experimental investigations were performed on a unidirectional glass/epoxy composite. The results obtained by the beam and CTS tests were compared. It was found that the interlaminar fracture toughness G _IC ^init of mode I at crack initiation and the corresponding value G _II ^Cinit of mode II obtained by the conventional beam and the CTS tests were in rather good agreement. The experimental results of interlaminar fracture of mixed mode were used to obtain the parameters required for the failure criterion. The two different failure criteria were compared. The best correlation with the experimental data was obtained by using the failure criterion proposed by Wu in 1967 containing linear and quadratic terms of the rates of the energy release.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 3, pp. 307–322, May–June, 1998.     

Mode I,mode II,and mixed-mode I/II interlaminar fracture toughness of GFRP influenced by fiber surface treatment

The interlaminar fracture behavior of unidirectional glass fiber reinforced composites with fiber surface treatment has been investigated in modes I and II and for fixed mode I to mode II ratio of 1.33. The data obtained from these tests have been analyzed by using different analytical approaches. The present investigation is focused on the influence of the glass fiber surface treatment on the interlaminar fracture toughness of unidirectional laminates. Glass fibers with two different fiber surface treatments have been investigated. fiber surface treatment was carried out by using a polyethylene or silane coupling agent in combination with modifying agents. The glass fibers were embedded in the brittle epoxy matrix. Mode I, mode II, and mixed-mode I/II tests were performed in order to determine critical strain energy release rates. Double cantilever beam (DCB), end-notched flexure (ENF), and mixed-mode flexure (MMF) specimens were used. For both types of fiber surface treatment about the same values of mode I initiation fracture toughness G_IC ^init were obtained. It was observed that in mode I interlaminar crack growth in the DCB test for the composite sized by polyethylene, the crack propagation is accompanied by extensive fiber bridging. For both fiber surface treatments interlaminar fracture toughness increases considerably with increasing of crack length. For the fiber surface treatment with the silane coupling agent, the value of mode II initiation fracture toughness G_IIC ^init was about 2.5-times higher in comparison with that of a composite sized by polyethylene. For both types of fiber surface treatments the mixed-mode I/II test has shown a similar behavior to the mode I DCB test.     

Fracture toughness characteristics of laminated composites

Parameters characterizing the resistance of laminated composites to interlaminar fracture are discussed. The properties of the specific interlaminar fracture work, i.e., the amount of work spent on the formation of a unit of new surface of interlaminar crack, were examined. Taking account of the anisotropy of the material, this work may be characterized using a matrix. Upon change in the direction of crack growth, the matrix elements are transformed similarly to the components of a symmetrical second rank tensor. An interpretation is offered for the matrix elements. The proposed theoretical model was in accord with our experimental results.Moscow Engineering Institute. Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, No. 1, pp. 21–31, January–February, 1996.     

Interlaminar fracture and low-velocity impact of carbon/epoxy composite materials

The interlaminar fracture and the low-velocity impact behavior of carbon/epoxy composite materials have been studied using width-tapered double cantilever beam (WTDCB), end-notched flexure (ENF), and Boeing impact specimens. The objectives of this research are to determine the essential parameters governing interlaminar fracture and damage of realistic laminated composites and to characterize a correlation between the critical strain energy release rates measured by interlaminar fracture and by low-velocity impact tests. The geometry and the lay-up sequence of specimens are designed to probe various conditions such as the skewness parameter, beam volume, and test fixture. The effect of interfacial ply orientations and crack propagation directions on interlaminar fracture toughness and the effect of ply orientations and thickness on impact behavior are examined. The critical strain energy release rate was calculated from the respective tests: in the interlaminar fracture test, the compliance method and linear beam theory are used; the residual energy calculated from the impact test and the total delamination area estimated by ultrasonic inspection are used in the low-velocity impact test. Results show that the critical strain energy release rate is affected mainly by ply orientations. The critical strain energy release rate measured by the low-velocity impact test lies between the mode I and mode II critical strain energy release rates obtained by the interlaminar fracture test. Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Published in Mekhanika Kompozitnykh Materialov, Vol. 36, No. 2, pp. 195–214, March–April, 2000.     

Interlaminar fracture toughness of graphite fiber-reinforced polyimide composites

Conclusion The present study has proved the effectiveness of the application of viscoelastic polymers with increased fracture toughness to graphite/polyimide composites interlaminar fracture toughness improvement. Thermoplastic polysulphone film and thermoresistant structural adhesive have proved to be inherently more effective for composites' delamination resistance growth than maleimide resin toughening and structural modification. The former inevitably results in increase of the honeycomb delamination resistance (Fig. 1) and its durability.Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 6, pp. 848–852, November–December, 1994.     

Fibre stability in anisotropic matrix at small strains

As acknowledged in almost all monographs on the fracture of composite materials, one of the major fracture mechanism in unidirectional fibrous composites under uniaxial compression along the reinforcing elements is the stability loss of the material structure (the structural instability). According to this mechanism, theoretical investigations of the fracture along the fibres are reduced to those of the stability loss in the material structure, and the value of external critical forces is accepted as the value of failure forces. At present, numerous theoretical investigations have been carried out in this field with the use of the three-dimensional linearized theory of stability in the framework of the piecewise-homogeneous body model. However, in all the investigations it is assumed that the matrix and the fibre material are isotropic. It is evident that in many cases it is necessary to take into account the anisotropy of the matrix material when investigating the stability loss of fibres. In view of the above, in the framework of the piecewise-homogeneous body model using the three-dimensional linearized theory of stability, the present paper considers the stability loss of the fibre in the anisotropic (transversally isotropic) matrix. The effect of the properties of the matrix material on the critical values of the external loading is examined.Submitted to the 10th International Conference of Mechanics of Composite Materials (Riga, April 20–23, 1998).Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 603–611. September–October, 1997.     

Thermal deformation of oriented irradiated polyethylene and ethylene-propylene copolymer under uniaxial tensile load

The deformation of preoriented irradiated polyethylene and ethylenepropylene copolymer has been investigated under conditions of multiple heating above the melting point of the crystals and subsequent cooling with application of a constant uniaxial tensile load. The specimens are found to shorten during heating and to lengthen during cooling. However, this process is not reversible and irreversible elongation of the specimen occurs during each temperature cycle (heating and cooling).Mekhanika Polimerov. Vol. 3, No. 3, pp. 392–394, 1967     

Effect of thermal treatment temperature on the structure and properties of carbon fibers

Conclusions 1. It has been shown that the presence of a maximum in the dependence of strength on Young's modulus for carbon fibers made from PAN fiber may be explained by an effect of the process of temperature stress accumulation which takes place under the conditions of isometric heating. The start of this process, which causes a rearrangement of the internal structure of the high-modulus fiber, coincides with the start of the anomalous rise in fiber density.2. The interconnection between surface and internal defects and the elastic-strength properties of carbon fibers made in the temperature treatment range 600–3000°C has been studied.3. Original data on the elastic-strength properties of borided carbon fibers have been obtained; the structure of these is marked by a high degree of perfection. It has been shown that in boriding, which facilitates graphitization of the carbon, the process of regular reduction in fiber strength which is reached in the precrystallization stage is somewhat retarded.All-Union Scientific-Research Institute of Aviation Materials, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1036–1042, November–December, 1976.     

Time and temperature dependent deformation of poly(ether ether ketone) (PEEK)

The stress-strain behavior in tension and the effect of temperature on the creep of poly(ether ether ketone) (PEEK) have been studied. At room temperature, 130° below the glass-transition temperature, the material does not become brittle, and the specimens show necking in tension over a wide range of elongation rates. The stress and strain at yield and the strain at break are almost linear functions of the logarithmic elongation rate. The values of stress and strain at yield increase slightly with increasing elongation rate, while the strain at break decreases markedly. The short-term creep tests were conducted at temperatures extending from 20 to 200°C. The glass-transition temperature was found to be about 155°C. The creep of PEEK is greatest at temperatures above 130°C. In the glass region the time dependence of the deformation is much weaker. It has been found that the time-temperature relation for PEEK corresponds well with its thermorheological simplicity in the temperature range investigated. The data on the temperature shift factor below and above the glass-transition temperature may be fitted separately to the Arrhenius and Williams-Landel-Ferry (WLF) equations, respectively. The long-term creep tests show that PEEK has excellent creep resistance at room temperature. After 14-month tests at a stress level of 30 MPa the total strain exceeds the instantaneous elastic strain only by a factor of 1.15.Institute of Polymer Mechanics. Latvian Academy of Sciences, 23 Aizkraukles St., LV-1006 Riga. Latvia. Department of Polymeric Materials, Chalmers University of Technology. S-412 96 Gothenburg, Sweden. Published in Mekhanika Kompozitnykh Materialov, No. 6, pp. 734–746, November–December, 1997.     

Analysis of methods of determining the interlaminar shear strength of composite materials

Conclusions The range of geometrical dimensions of a sample in which the values of the interlaminar shear strength determined by four of the five methods discussed are practically constant has been established. Good agreement is shown in the values of the characteristic being determined which are obtained by each of the four methods. It has been established that the experimental data obtained by the method of three-point deflection of short beams does not characterize their interlaminar shear strength. It is shown that the interlaminar shear strength of glass-plastic and the shear strength of the adhesive with which it is prepared are very similar in value.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 640–648, July–August, 1976.     

Morphological characteristics of carbon fibers

Scanning electron microscopy has been used to study the surface, before and after thermochemical treatment, and the fracture sites of PAN carbon fibers heat-treated at 2100°C. The fractographs are analyzed in an attempt to estimate the effect of various defects on the strength of PAN carbon fibers.All-Union Scientific-Research and Design-Technological Institute of Electro-Carbon Products, Moscow Region. Translated from Mekhanika Polimerov, No. 1, pp. 158–160, January–February, 1976.     

Accumulation of submicroscopic damage in polymers under simple loading

The accumulation of microdamage in specimens of polystyrene, Ftorlon, and high-pressure polyethylene loaded continuously to failure has been investigated. It is shown that at small specimen lifetimes the residual damage accumulates primarily in the fracture zone. Outside the fracture zone appreciable damage accumulation is observed at specimen lifetimes close to the long-term strength. The ultimate strain at various lifetimes is determined. It is shown that the strain corresponding to failure depends on the fraction of irreversible damage accumulated and is largely determined by the resistance of the polymers to fracture.Plastopolimer Scientific-Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 519–524, May–June, 1972.     

Effect of temperature on the fracture surfaces of filled systems based on SKS-85 butadiene-styrene copolymer

The tear fracture surfaces of mixtures of SKS-85 butadiene — styrene copolymer (85% styrene) with ordinary (carbon black, chalk) and polymeric (Kapron and cellophane powder) fillers have been investigated on the interval from –60 to +40°C. As the temperature varies within the limits of the glassy state (T_g SKS-85=+24°C) of the filled polymer, the nature of the fracture surface of specimens of filled mixtures, like that of the unfilled polymer, changes; in the region of the temperature transition of the copolymer associated with the mobility of the phenyl groups (–10±5°C) there is a slowing of the fracture process. At temperatures below the T_g of the copolymer the tear fracture surfaces of specimens of mixtures containing ordinary and polymeric fillers differ sharply. The introduction of fillers (20 vol. %) with a coefficient of thermal expansion different from that of the filled polymer considerably reduces the resistance of the material to fracture and leads to a sharp increase in the rate of crack propagation; the introduction of polymeric fillers with coefficients of thermal expansion similar to that of the filled polymer leads to an increase in the resistance of the material to fracture and to a decrease in the rate of crack propagation.Moscow Technological Institute of the Meat and Dairy Industry; State Institute of Polymer Adhesives, Kirovakan. Translated from Mekhanika Polimerov, No. 5, pp. 819–826, September–October, 1969.     

Fracture in Ceramic-Matrix Composites Reinforced with Strongly Bonded Metal Particles

The resistance of a ceramic matrix composite to the cleavage cracking across a field of strongly bonded, uniformly distributed metal particles is studied. The crack trapping and bridging effects of the metal particles are analyzed by means of calculating the strain energy and the traction work. An explicit expression for the critical energy release rate as a function of particle volume fraction has been obtained. The fracture resistance is independent of elastic properties of the matrix and the sample geometry and is predominantly determined by the size/spacing ratio of the particles. It is shown that the theoretical curves agree with experimental data quite well. The methodology developed in this article can be used in studying the fracture resistances of composites with high filler contents and irregular filler geometries.__________Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 41, No. 3, pp. 303–318, May–June, 2005.     

Investigation of the acoustic emission associated with the fracture of cylindrical and flat specimens

The problem of the brittle tensile fracture of an infinite cylinder is examined theoretically. Relations are derived between the parameters of the nonstationary ultrasonic pulse generated at fracture and the fracture characteristics. The conclusions obtained are extended to the tensile fracture of an infinite plate and of two face-bonded plates with a bond zone in the form of an infinite strip. Recommendations are given concerning the choice of specimen geometry. The theoretical conclusions have been checked experimentally. The agreement between the theoretical and experimental data is good.Scientific-Research Institute of Standard and Experimental Design of Residential and Communal Buildings, Leningrad Zone. Translated from Mekhanika Polimerov, No. 1, pp. 141–148, Janaury–February, 1976.     

Calculation of shrinkage stresses in fibrous composites with regular structure

The effect of the chemical and thermal shrinkage of the epoxy matrix on the structural residual stresses in plastics reinforced with anisotropic fibers has been investigated. The process of residual stress formation in all stages of a given curing regime is examined with reference to a regular triangular fiber distribution. The model of a hardening hypoviscoelastic medium [8] is used to describe the variation of the rheological properties of the matrix. The effect of the cooling rate on the residual stresses is investigated with reference to a single inclusion.Moscow Power Engineering Institute. Translated from Mekhanika Polimerov, No. 3, pp. 409–415, May–June, 1976.     

Comparison of the long-term strengths of polymers under uniaxial alternating and pulsating loads

The laws of fracture of PMMA and PS under uniaxial alternating and pulsating cyclic loading have been investigated at several loading frequencies and two test temperatures. An analysis of the dependence of the long-term strength on stress, heating kinetics and the form of the specimen fracture surface has shown that transition from cyclic tension to more complex alternating loading in the tension-compression regime does not change the basic nature of the cyclic fatigue effect and is reflected only in an increase in the local stress concentration and heating.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 483–488, May–June, 1972.     

Electron diffraction investigation of the fracture surfaces of filled mixtures of SKS-85 butadiene-styrene copolymer

As the result of an electron microscope study of replicas from the fracture surface during the disintegration of samples of mixtures of SKS-85 butadiene-styrene copolymer with 20 vol.% DG-100 carbon black and chalk, in the temperature range from +60 to –60°C, it is shown that, in the region of vitrification of the copolymer and of the loss of the reinforcing effect of the carbon black, there is a sharp change in the character of the fracture micro-surface, and the particles of carbon black become the sites of additional fracture. In the region of the temperature transition of the copolymer, connected with the mobility of the phenyl groups in the same way as in the vitrification region, there is a sharp decrease in the number of particles of carbon black at the fracture surface, while the fracture surface of a mixture with chalk passes selectively along the contact boundary of a polymer with larger particles. The microstructure of the Wallner lines on the surface of a mixture with carbon black, at Tst, is made up of traces of secondary fracture, arising around the particles of carbon black.Moscow Institute of Technology for the Meat and Dairy Industry. M. V. Lomonosov Institute for the Technology of Fine Chemicals, Moscow. É. L. Ter-Gazaryan State Scientific-Research and Design Institute, Kirovakan. Translated from Mekhanika Polimerov, No. 3, pp. 437–444, May–June, 1970.     

Predicting certain structural characteristics and physicomechanical properties of glass-reinforced plastics from their thermal activity

A procedure and apparatus for determining the thermal activity of glass-reinforced plastics with one-sided access have been developed. A correlation is established between the interlaminar shear modulus and interlaminar shear strength and the thermal activity of a glass laminate. The effect of structural inhomogeneities and bonding flaws on the thermal activity of a glass-reinforced plastic is investigated and it is shown that the location of an inhomogeneity within the article can be determined.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 128–132, January–February, 1976.     

Creep and long-time strength of oriented glass-reinforced plastics in interlaminar shear

The creep and long-time strength in interlaminar shear and the creep in compression in the direction of the reinforcement have been experimentally investigated for certain types of oriented glass-reinforced plastics. The specimens in the interlaminar creep tests took the form of short beams loaded in bending. The experimental creep data for shear and compression are well described by the hereditary theory with a kernel of the Abel type (shear) or in the form of a Rabotnov function (compression). If the stresses are constant in time, good agreement with experiment is also given by Findley's form of the aging theory. A deformation criterion of interlaminar shear strength is also obtained. The experimental curves and values of the creep and long-time strength constants are presented.Translated from Mekhanika Polimerov, No. 6, pp. 1003–1012, November–December, 1971.