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Showing papers on "Fractography published in 1994"


Journal ArticleDOI
TL;DR: In this article, the CTOA values during stable tearing were measured by two independent methods, optical microscopy and digital image correlation, for 2.3mm thick sheets of 2024-T3 aluminum alloy.
Abstract: The stable tearing behavior of 2.3-mm thick sheets of 2024-T3 aluminum alloy was experimentally investigated for middle crack tension specimens having initial flaws that were: (a) flat fatigue cracks (low fatigue stress) and (b) 45-deg through-thickness slant cracks (high fatigue stress). The critical CTOA values during stable tearing were measured by two independent methods, optical microscopy and digital-image correlation. Results from the two methods agree well. The CTOA measurements and observations of the fracture surfaces have shown that the initial stable tearing behavior of low and high fatigue stress tests is significantly different. The cracks in the low fatigue stress tests underwent a transition from flat-to-slant crack growth, during which the CTOA values were high and significant crack tunneling occurred. After crack growth equal to about the thickness (Δa>B), CTOA reached a constant value of 6 deg and after crack growth equal to about twice the thickness (Δa>2B), crack tunneling stabilized. The cracks in the high fatigue stress tests reach the same constant CTOA value after crack growth equal to about the thickness, but produced only slightly higher CTOA values during initial crack growth. The amount of tunneling in the high fatigue stress tests was about the same as that in the low fatigue stress tests after the flat-to-slant transition. This study indicates that stress history has an influence on the initial portion of the stable tearing behavior. The initial high CTOA values, in the low fatigue crack tests, coincided with large three-dimensional crack front shape changes due to a variation in the through-thickness crack-tip constraint. The measured CTOA reached a constant value of 6 deg for crack growth of about the specimen thickness. This coincided with the onset of 45-deg slant crack growth and a stabilized, slightly tunneled (about 20 percent of the thickness) crack-front shape. For crack growth on the 45-deg slant, the crack front and local field variables are still highly three dimensional.

142 citations


Journal ArticleDOI
TL;DR: In this article, a full-coverage model is proposed to estimate the critical hydrogen content that makes ZIRCALOY-4 totally brittle, and the effect of microstructure on hydride embrittlement in different metallurgical states is explained according to the modeling.
Abstract: The hydride embrittlement in ZIRCALOY-4 was studied at room temperature and 350 °C. Sheet tensile specimens of two fabrication routes in the stress-relieved, recrystallized, andβ-treated states were hydrided with or without tensile stress. It was found generally that the effect on strength of increasing hydrogen content was not important. However, for the tensile tests at room temperature, there is a ductile-brittle transition when the hydrogen content is higher than a certain threshold. The prior thermomechanical treatment shifts this transition considerably.In situ scanning electron microscopy (SEM) tests, fractography, and fracture profile observations were carried out to determine the fracture micromechanisms and the microscopic processes. At 20 °C, the fracture surfaces are characterized by voids and secondary cracks for low and medium hydrogen contents and by intergranular cracks and decohesion through the continuous hydride network for high hydrogen contents. This phenomenon disappears at 350 °C, and the hydrogen seems to exert no more influence on the fracture micromechanism even for very high hydrogen contents (up to 1500 wt ppm). A full-coverage model is proposed to estimate the critical hydrogen content that makes ZIRCALOY-4 totally brittle. The effect of microstructure on hydride embrittlement in different metallurgical states is thus explained according to the modeling. Special attention is devoted to relating the micromechanisms and the modeling in order to propose the possible measures needed to limit the hydride embrittlement effect.

120 citations


Journal ArticleDOI
TL;DR: In this article, the crack healing induced by ethanol in poly(methyl methacrylate) (PMMA) has been studied at temperatures of 40-60°C and it is found that crack closure rate is constant at a given temperature.
Abstract: The crack healing induced by ethanol in poly(methyl methacrylate) (PMMA) has been studied at temperatures of 40–60°C. Crack healing occurs because the effective glass transition temperature of PMMA is reduced to below the test temperature by ethanol plasticization. It is found that crack closure rate is constant at a given temperature. The fracture strength of healed PMMA is lower than that of the original samples. By comparing the fracture stress with the morphology of the crack edge on the PMMA surface, we found that a high degree of swelling is responsible for the incomplete recovery of mechanical strength. The fractography of the completely healed sample shows a very different fracture morphology from that of virgin PMMA. The transport of ethanol in PMMA also is studied. At lower temperatures, transport is described by ideal Case II behavior. As the temperature increases, the kinetics shift from ideal Case II to anomalous behavior. The first stage of crack healing is controlled by Case I transport. © 1994 John Wiley & Sons, Inc.

84 citations


Journal ArticleDOI
TL;DR: In this paper, thigh-buttuohead specimens of an advanced silicon nitride were tested in uniaxial tension at temperatures between 1422 and 1673 K. In the range 1477 to 1673 k, creep deformation was reliably measured using high-temperature contact probe extensometry.
Abstract: Cylindrical buttuohead specimens of an advanced silicon nitride were tested in uniaxial tension at temperatures between 1422 and 1673 K. In the range 1477 to 1673 K, creep deformation was reliably measured using high-temperature contact probe extensometry. Extensive scanning and transmission electron microscopy has revealed the formation of lenticular cavities at two-grain junctions at all temperatures (1422–1673 K) and extensive triple-junction cavitation occurring at the higher temperatures (1644–1673 K). Cavitation is believed to be part of the net creep process. The stress rupture data show stratification of the Monkman–Grant lines with respect to temperature. Failure strain increased with increase in rupture time or temperature, or decrease in stress. Fractography showed that final failure occurred by subcritical crack growth in all specimens.

83 citations


Journal ArticleDOI
TL;DR: The experimental design, computational analysis, and fractography were applied to two generically different bonding systems and showed largely interfacial adhesive failure, while SBM showed adhesive-cohesive failure with occasional dentin adhesions attached to the composite interface and vice versa.

69 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of fiber composition, size, and surface treatment on the mechanical behavior of slip-cast fused-silica composites were investigated, and the ambient and 1000°C stiffness and strength, fracture toughness, GR -curve behavior, and fiber-matrix interface bond strength were determined.
Abstract: The effects of fiber composition, size, and surface treatment on the mechanical behavior of slip-cast fused-silica composites were investigated. The ambient and 1000°C stiffness and strength, fracture toughness, GR -curve behavior, and fiber-matrix interface bond strength were determined. Quantitative fractography and scanning electron microscopy were used to ascertain the mechanisms of toughening and strengthening. Composites with weak interface bonding exhibited good strength retention and rising GR -curve behavior. The fracture resistance was improved primarily through crack deflection.

58 citations


Journal ArticleDOI
TL;DR: In this paper, preliminary experiments were conducted to examine the process of environmentally enhanced creep crack growth in Inconel 718 alloy in terms of possible mechanisms and rate controlling processes, and the results showed that crack growth was intergranular in air and in vacuum with brittle appearing grain boundary separation in air, and extensive cavity formation in vacuum.
Abstract: Inconel 718 alloy is widely used in high temperature applications. Because of its sensitivity to environmentally enhanced crack growth at high temperatures, its use has been limited to modest temperatures (i.e., below 973 K). To improve its performance and to better predict its service life, it is important to develop a better understanding of the processes of crack growth at high temperatures in this alloy. It has been shown that the creep crack growth rates (CCGR) in air are at least two orders of magnitude faster than those in vacuum or inert environments. CCGR were also found to depend strongly on temperature. Fractographic studies showed that crack growth was intergranular in air and in vacuum with brittle appearing grain boundary separation in air and extensive cavity formation in vacuum. The increased CCGR in air has been attributed to the enhancement by oxygen; principally through enhanced cavity nucleation and growth by high-pressure carbon monoxide/dioxide formed by the reactions of oxygen that diffused into the material with the grain boundary carbides. The appropriateness of this mechanism, however, may be questioned by the absence of cavitation on the crack surfaces produced in air. As such the mechanism for crack growth needs to bemore » re-examined. Because of the presence of moisture in air, the possible influence of hydrogen needs to be considered as well. In this study, preliminary experiments were conducted to examine the process of environmentally enhanced creep crack growth in Inconel 718 alloy in terms of possible mechanisms and rate controlling processes. Creep crack growth experiments were carried out in air, oxygen (from 2.67 to 100 kPa), moist argon (water vapor) and pure argon at temperatures from 873 to 973 K.« less

49 citations


Journal ArticleDOI
TL;DR: In this article, the effect of varying volume fraction of the (Mo) phase on the microstructure, bend strength, and ambient temperature fracture behavior of selected NiAI-Mo two-phase alloys was evaluated.
Abstract: The phase relationship in the NiAI-Mo system is characterized by a eutectic equilibrium between binary NiAl and the terminal (Mo) solid solution, thereby offering the potential for development of ductile-phase-toughened composites. A study was conducted to evaluate the effect of varying volume fraction of the (Mo) phase on the microstructure, bend strength, and ambient temperature fracture behavior of selected NiAI-Mo two-phase alloys. Above room temperature, the NiAI-Mo alloys showed an increase in bend strength compared to monolithic NiAl, with reasonable strength retention up to ≈800 °C. The results demonstrated moderate improvements in toughness in the NiAI-Mo alloys in comparison to monolithic NiAl. A further enhancement in toughness was realized through hot working. Fractography studies showed evidence for substantial decohesion between the (Mo) phase and the NiAl matrix, thereby suggesting the presence of a weak interface. This weak interface between the (Mo) phase and the NiAl matrix, in conjunction with modulus mismatch stresses, causes the crack to deflect from the (Mo) rein-forcement and propagate preferentially along the (Mo)/NiAl interface. These attributes limit the potential for significant ductile-phase toughening in the NiAI-Mo system. An addition of 0.2 at. pct Ti resulted in a marked improvement in the room-temperature fracture toughness of NiAI-Mo. Fractography observations show some evidence for (Mo)/NiAl interface strengthening with the Ti addition.

43 citations


BookDOI
01 Jan 1994
TL;DR: In this paper, the authors present a quantitative analysis of the origin of fracture in flat glass and fractal analysis of flat glass containers with and without fiberglass fiberglass, and they propose a method for the analysis of fracture origins in glass.
Abstract: Identation Fractography (B.R. Lawn, D.B. Marshall). Quantitative Fractographic Analysis of Fracture Origins in Glass (J.J. Mecholsky). Stress Wave Fractography (H. Richter, F. Kerkhof). Fractography of Stress Corrosion Cracking in Glass (T.A. Michalske). Fractography of Optical Fibers (H.C. Chandan et al.). Fractography of Fiberglass (P.K. Gupta). Fracture of Glass Containers (J.B. Kepple, J.S. Wasylyk). Fracture and Fractography of Flat Glass (N. Shinkai). Index.

42 citations



Journal ArticleDOI
TL;DR: In this article, the right-hand trailing-edge flap (TEF) from an RAAF F/A-18 separated during a flight over sea, and the mode of failure suggested that the outboard hinge lug was first to fail.

Journal ArticleDOI
TL;DR: In this article, the mode-l-plane-strain fracture toughness at initiation of various engineering plastics was determined for test speeds between 10−4ms−1 and 1.0 ms−1, using a high-speed, servohydraulic testing apparatus.
Abstract: The mode-l-plane-strain fracture toughness at initiation of various engineering plastics was determined for test speeds between 10−4ms−1 and 1.0 ms−1, using a high-speed, servohydraulic testing apparatus. At high rates of testing, the transient acceleration of the specimen was reduced by the use of a damping technique aimed at overcoming dynamic effects. The results obtained are correlated with fractographic analysis performed by scanning electron microscopy (SEM) and by digital image analysis of macroscopic fracture surfaces. The results show that for some materials the high values of K lc measured at low testing rates are associated with deformation detectable by either method. The extent of such deformation tends to be reduced as the testing velocity is increased and the fracture toughness drops.

Journal ArticleDOI
TL;DR: In this article, a single specimen fractographic technique based on critical stretch zone width measurements is suggested for the estimation of fracture toughness (JIc) for highly ductile materials, and the salient feature of this technique is that it overcomes the problem of fatigue precracking and is able to predict the fracture toughness of a material using a blunt notch specimen.
Abstract: A single specimen fractographic technique based on critical stretch zone width measurements is suggested for the estimation of fracture toughness (JIc) for highly ductile materials. The salient feature of this technique is that it overcomes the problem of fatigue precracking and is able to predict the fracture toughness of a material using a blunt notch specimen. Fracture toughness tests on commercially pure Armco iron, nickel, and aluminum as well as Al-Mn based austenitic stainless steel and En28 steel were carried out to validate the method.

Journal ArticleDOI
TL;DR: In this paper, two types of high-modulus short-carbon-fibre-reinforced commercially pure aluminium-matrix composites were fabricated in-house using a home-made squeeze caster.
Abstract: Two types of high-modulus short-carbon-fibre-reinforced commercially pure aluminium-matrix composites were fabricated in-house using a home-made squeeze caster. The type-I composites were fabricated from short-fibre preforms in which fibres exist as dispersed bundles. The type-II composites were fabricated from preforms in which individual fibres were uniformly dispersed. The detailed processes are described in the text. A three-point-bending strength of higher than 200 M Pa was obtained for the type-1 composite with 17 vol% of fibre. When more fibre was incorporated, both the strength and the ductility decreased due to inadequate infiltration. However, a bending strength of greater than 240 MPa was recorded on a hot-rolled type-I composite with a fibre content as high as 28 vol%. This significant improvement in the mechanical properties is explained by a hot-rolling-inducedvoid-healing effect. The type-II composites, with lower fibre volume fractions than those of the type-I due to their different preforms, exhibited bending strengths up to 166 MPa. Scanning electron microscopy fractography shows that the two types of composites fracture in distinctive manners. Transmission electron microscopy results featured thermal-stress-induced dislocations at carbon-aluminium interfaces as well as submicrometre-sized aluminium carbide, the reaction product, which nucleated from the interface and grew into the matrix interior.

Journal ArticleDOI
TL;DR: In this article, the cyclic strain energy release rate range, ΔG, was calculated and correlated with fatigue crack growth rate, da/dN, for double cantilever beam points bonded with two commercial adhesives (filled and filled and toughened) at a load ratio of 0.2 and frequencies of 2 and 20 Hz.

Journal ArticleDOI
TL;DR: In this paper, the authors used fractrographic features for post-failure analysis of a filled thermoplastic polyester (PETG) and showed that these features were characteristic of the fracture mode and did not depend on filler type or filler content.
Abstract: Fractography has been used for the postfailure analysis of a filled thermoplastic polyester. The five fracture modes that were previously defined on the basis of macroscopic stress–strain behavior were distinguished by certain fractrographic features. These features were characteristic of the fracture mode and did not depend on filler type or filler content. The Mode A ductile fracture surface consisted of two regions: a pullout region of slower crack growth and a rosette region of faster crack growth. The Mode B ductile fracture surface contained only a ductile pull out texture. The Mode C quasi-brittle fracture surface exhibited secondary fracture features that sometimes included the herringbone pattern. The Mode D quasi-brittle fracture surface consisted of a stress-whitened dimple region and a brittle fracture region. The Mode E Fracture surface exhibited primarily the rough texture characteristic of brittle fracture. The failure mechanisms inferred from analysis of the fracture surfaces confirmed a microscopic failure model of the ductile-to-quasi-brittle transition in filled PETG that is based on the strain-hardening strength of the polymer ligaments between debonded filler particles. © 1994 John Wiley & Sons, Inc.

Book ChapterDOI
Norihiko Shinkai1
01 Jan 1994
TL;DR: In this article, a review of the fracture characteristics of flat glass is presented, focusing on the characteristic fracture marks formed on the crack surfaces under local stress conditions, and analyzing the crack patterns affected by the total stress system over a flat glass plate.
Abstract: This review deals with the fracture characteristics of flat glass. Important points in the fractography of flat glass are to observe the characteristic fracture marks formed on the crack surfaces under local stress conditions, and to analyze the crack patterns affected by the total stress system over a flat glass plate.

Journal ArticleDOI
TL;DR: In this article, the anisotropy in room-temperature flexural strength of ceramics as a result of machining test bar tensile surfaces parallel vs perpendicular to the bar axis was studied for various porous bodies.
Abstract: The anisotropy in room-temperature flexural strength of ceramics as a result of machining test bar tensile surfaces parallel vs perpendicular to the bar axis was studied for various porous bodies. This shows that fine, relatively homogeneous porosity has no significant effect on such strength anisotropy, implying that such porosity has no significant effect on flaw sizes or shapes, which was also shown by fractography. However, as the size of pores or pore clusters (due to pore heterogeneity) increases, the strength anisotropy diminishes, becoming zero when the pores or pore clusters dominate failure. Logarithm of strength vs porosity (P) plots for the two machining directions followed nearly parallel lines for fine, homogeneous porosity, but have less separation and intersect at lower porosity as pore size or heterogeneity increases. Fracture toughnesses calculated from fractography data for Al[sub 2]O[sub 3] and B of varying porosity levels could be normalized to values at P [approximately] 0. Thus, extrapolation of strengths to P = 0 is clearly justified for bodies with fine, homogeneous porosity, but may be uncertain in bodies with coarser, or heterogeneous porosity.

Journal ArticleDOI
TL;DR: In this article, the effect of particulates on the failure mechanism of an Al-Mg-Si alloy 6061 with 20% angular alumina particles was studied, and the interaction of the reinforcement phase with the crack was investigated by optical microscopy and scanning electron microscopy.
Abstract: The effect of particulates on the failure mechanism of an Al-Mg-Si alloy 6061 with 20% angular alumina particles was studied. Fracture toughness tests were conducted on compact tension peak-aged specimens. The interaction of the reinforcement phase with the crack was investigated by optical microscopy and scanning electron microscopy, both on the surface and in the mid-thickness of the fractured specimen. It is shown that the fractured particles ahead of the crack tip, in particular the larger particles, play an important role in the void-initiation phase of the fracture process. Particle size and aspect ratio determine the likelihood of fracture. Some differences in the failure mechanisms have been observed between the mid-thickness and the surface of the specimen because of the difference between plane strain and plane stress fractures.

Journal ArticleDOI
TL;DR: In this paper, the effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009-SiC-15w composites are analyzed.
Abstract: The effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009/SiC/15w composites are analyzed in this study Ductility and fracture toughness of the 2009/SiC/15w, in which dispersoid-forming elements such as manganese and iron were nearly absent, were greater than in the 2124/SiC/15w, while its tensile and yield strengths were somewhat less Microstructural examination and fracture parameter analysis revealed that the improved fracture toughness of the 2009/SiC/15w compared to the 2124/SiC/15w was due to the increase in the critical microstructural distance,l* when manganese-containing particles are absent 2009/SiC/15w was also heat-treated in T4P and overaged (OA) conditions The OA 2009 composite showed lower fracture toughness than the 2009-T4P composite and the critical fracture strain of the OA condition was much lower, too Detailed fractographic analyses indicated that interface precipitates facilitate premature SiC whisker failure in the OA condition

Book ChapterDOI
01 Jan 1994
TL;DR: It is a guiding principle of brittle fracture that a crack always propagates essentially perpendicularly to the direction of the maximum principal tensile stress as mentioned in this paper, which implies that the crack will change its direction of propagation if the actual MPS changes.
Abstract: It is a guiding principle of-brittle fracture that a crack always propagates essentially perpendicularly to the direction of the maximum principal tensile stress. This implies that a crack will change its direction of propagation if the direction of the actual maximum principal tensile stress changes.

Journal ArticleDOI
TL;DR: In this article, a qualitative treatment of the interaction of electrons with a sample and the effect of the scanning electron microscopy operating parameters on image formation, quality, and X-ray analysis is presented.

Journal ArticleDOI
TL;DR: The tensile fracture strain, stress and fracture mode for a discontinuously reinforced aluminum matrix composite, 2014Al/15vol.%Al 2 O 3, were determined and compared with those of the unreinforced matrix material, 2014A1, at various temperatures as discussed by the authors.
Abstract: The tensile fracture strain, stress and fracture mode for a discontinuously reinforced aluminum matrix composite, 2014Al/15vol.%Al 2 O 3 , were determined and compared with those of the unreinforced matrix material, 2014A1, at various temperatures. Tests were conducted under uniaxial tension at elevated temperatures with a strain rate of 0.1 s − . It was found that the tensile fracture strain as well as fracture stress of the composite were lower than those of the matrix material. The tensile fracture mode changed from transgranular fracture to intergranular fracture between 400 °C and 500 °C for both materials. For the composite, at temperatures below 400 °C the growth and coalescence of voids occurred via a dislocation creep process primarily along the AlAl 2 O 3 interface. Above 400 °C voids initiated and grew at the AlAl 2 O 3 interface and grain boundaries via a diffusion creep process. The void growth was found not along the tensile direction but along the AlAl 2 O 3 interface and grain boundaries, and this resulted in a low fracture strain. A method for determining quantitatively the characteristics of the void initiation and growth is discussed.

Book ChapterDOI
01 Jan 1994
TL;DR: In this article, the fracture mirror radius is related to fundamental fracture parameters, such as fracture stress, elastic modulus, fracture surface energy and fracture toughness, and the size and shape of the mirror region contains information regarding the nature and magnitude of the fracture stress.
Abstract: When a glass optical fiber breaks, a characteristic morphology develops on the fracture surface. There is a smooth region adjacent to the fracture origin which is called the mirror. The fracture mirror radius is related to fundamental fracture parameters, such as fracture stress, elastic modulus, fracture surface energy and fracture toughness. The size and shape of the mirror region contains information regarding the nature and magnitude of the fracture stress. Such information is used for quality control in the manufacture of long lengths of optical fiber. Fractography can also be used to analyze any failures related to cable manufacturing, field installation or service.

Journal ArticleDOI
TL;DR: In this article, the effect of gas nitriding on fatigue behavior has been studied in commercial pure titanium under rotating bending, and it was concluded that the improved fatigue strength of the nitrided materials can be attributed to enhanced resistance to both crack initiation and growth.


Journal ArticleDOI
TL;DR: In this paper, a quantitative comparison is made between tensile and bending results from (circularly) cylindrical glass rods of borosilicate glass and of fused silica.
Abstract: To fully characterize the mechanical behavior of ceramics, it is useful to study their response to both tensile and bending tests. In this investigation, a quantitative comparison is made between tensile and bending results from (circularly) cylindrical glass rods of borosilicate glass and of fused silica. These experimental results show that in each material, the Weibull exponentm is approximately the same for both types of test. The fractured samples clearly showed that all failures initiated at surface-located flaws. The stress level at which the probability of fracture in tension equals 50 Pct was determined; the maximum tensile stress for 50 Pct probability of fracture in bending was also determined. The ratio of these two stresses was predicted previously from simple, theoretical considerations based upon the applicability of the Weibull function and on the distribution of defects in the specimens. This theoretical result is inconsistent with the experimental observations in view of the fractography. Therefore, it is concluded that failure in the materials tested is not satisfactorily described by Weibull statistics.

Journal ArticleDOI
TL;DR: The results of a fractographic study carried out on automotive components by Gene Wistehuff and associates at GM's Central Foundry Division are presented and discussed in this article, where several types of casting alloys were fractured using various loading modes (uniaxial tension, bending, impact, torsion, and cyclic stressing), and corresponding mechanical properties were determined.

Journal ArticleDOI
TL;DR: In this article, it has been confirmed that transgranular stress-corrosion cracking (T-SCC) can be discontinuous under slow strain-rate testing, at least for materials which deform by planar slip.
Abstract: It has been confirmed that transgranular stress-corrosion cracking (T-SCC) can be discontinuous under slow strain-rate testing, at least for materials which deform by planar slip(ie, those which have low stacking-fault energy) Interpretation of the load and current transients shows that the crack velocity is on the order of 100 μm/s, depending on the environment — too slow to be explained by a running brittle crack and too fast to be explained by Faradaic dissolution Support of such an interpretation is given by the agreement between predictions of crack area by mechanical analysis of the load transients (taking into account the elastic displacement of the load train and of the specimen because of both the changing load and the crack advance) and the predictions from analysis of the current transients, as well as the agreement of such predictions with the resulting crack-advance distance determined from fractography Such agreement follows if the assumption is made that cracking is fully plastic, that is, if deformation accompanies cracking such that the nominal stress on the uncracked cross section is maintained at the flow stress The significance of this finding with respect to a corrosion-assisted microcleavage model is discussed

Journal ArticleDOI
TL;DR: In this paper, the authors examined the fracture surfaces and found relationships between features of the fracture surface and the corresponding mechanical data, and the characteristic features of failure have been pointed out and the changes of the features with variations in matrix material, testing rate and loading mode have been shown.
Abstract: Interlaminar fracture is recognized as an important mode of failure of composite materials and structures. In order to characterize twobismaleinimide-matrix (BMI) composites and two polyimide-matrix (PI) composites regarding their delamination behaviour, interlaminar fracture tests in mode I, mode II and mixed-mode loading conditions were carried out. The aim of this study was to examine the fracture surfaces and to find relationships between features of the fracture surface and the corresponding mechanical data. The characteristic features of failure have been pointed out and the changes of the features with variations in matrix material, testing rate and loading mode have been shown. The results of the mechanical testing can be explained by means of SEM images.