Failure prediction for a glass/epoxy cruciform specimen under static biaxial loading

Material models were developed to predict the mechanical behavior of glass/epoxy multidirectional laminates under complex stress states. An incremental plane stress analysis was performed, taking into account the anisotropic material non-linearity, separate damage onset conditions and distinct post-failure stiffness degradation rules. Theoretical formulations were implemented in a shell element of the 1st order shear deformation theory. Numerical results were validated via comparison with test data from cruciform specimens subjected to static biaxial tensile loading. Local strain gauge and full-field strain measurements, obtained using the Digital Image Correlation (DIC) technique, corroborated numerical predictions. Improved strength and failure mode results were derived when, in addition to stiffness reduction, compressive strength degradation in the fiber direction was also considered.     

Fretting fatigue failure of polyester resin and its glass fibre mat composites

When relative movement can arise between mating parts and one is subjected to cyclic stresses, then the fatigue life can be very much reduced. Such a phenomenon is referred to as fretting fatigue. In this paper, results of fretting fatigue experiments obtained with unsaturated polyester and its glass fibre mat composites are reported. In addition, some fracture and wear properties of the materials are discussed.     

Composites of glass/modified jute fabric and unsaturated polyester resin

Hybrid composites of glass and jute fabric modified by treatment with γ-aminopropyl trimethoxy silane (silane), isopropyl triisostearoyl titanate (titanate) and tolylene diisocyanate (tdi) were fabricated using unsaturated polyester resin (usp). An improvement in mechanical properties of laminates was observed when jute fabric was modified by titanate treatment. Better retention of mechanical properties in humid environments was observed in these laminates.     

Failure envelope under biaxial tensile loading for chopped glass-reinforced polyester composites

In this paper we present the biaxial failure curve of a chopped glass-reinforced polyester composite. The curve has been obtained combining the results of experimental tests with the results of numerical simulations developed by means of the Finite Element Method. The experimental tests have been performed applying perpendicular loads to cruciform specimens. Then, taking into account the numerical results, we have constructed the failure envelope in the tensile–tensile quadrant. Moreover we propose to modify the cruciform geometry to obtain, varying the widths of the loaded arms and the tapered central zone, points of the failure curve in which the two components of the plane stress tensor are different.     

Uniaxial and biaxial flexural fatigue of glass reinforced inter-penetrated network polymer composites

Conclusions Flexural fatigue of uniaxially and biaxially stressed IPN/glass mat composites was investigated using four point bend (4PB) and concentrically loaded (CL) specimen geometries. Regions of nearly constant bending moment between the inner spans of a 4PB beam and within the inner annulus of a CL circular plate yield quasi-uniform uniaxial and biaxial stress, respectively, on the tensile faces. The specimen dimensions were optimized for both loading geometries to give: (1) reduced specimen deflection through maximizing the ratio of the induced tensile stresses to the applied load, (2) minimized contact stresses by maximizing the induced stress with respect to the unit contact load, and (3) a large material volume exposed to the maximum cyclic stress (i.e., statistical fracture initiation).A power model was used to analyze the fatigue data for the 4PB and CL specimens. Both IPN composite materials studied fatigued more rapidly under the more severe loading conditions imposed by the CL specimen geometry.Fractography revealed that debond fracture was the dominant damage process for both geometries. The initial debond cracks were uniformly distributed throughout the stressed regions, confirming the presence of nearly uniform tensile stress. Damage localization followed after further cycling and was characterized by a locally high debond fracture density, fiber fracture, and always occurred where several glass strands crossed near the specimen surface. Final specimen failure resulted from the preferential growth of dominant cracks through the specimen thickness.     

Crack growth in discontinuous glass fibre reinforced polypropylene under dynamic and static loading conditions

Crack propagation in single edge notched tensile specimens of isotactic polypropylene reinforced with short E-glass fibres has been investigated under both fatigue and creep loading conditions. Fatigue crack propagation (FCP) experiments have been performed at three different frequencies (0.1, 1, 10 Hz) and at a mean applied tensile load of 1200 N. Isothermal creep crack propagation (CCP) tests have been conducted under a constant tensile applied load of 1200 N at various temperatures in the range from 32 to 60 °C. Analysis of FCP data allowed an estimation of the pure fatigue and pure creep components of the crack velocity under the adopted cyclic loading conditions. Crack growth at low frequencies (0.1 and 1 Hz) is mainly associated with a non-isothermal creep process. At higher frequency (10 Hz), the pure fatigue contribution appeared more pronounced. Finally, the comparison of FCP and CCP as a function of the mean applied stress intensity factor confirmed the major contribution of creep crack growth during FCP process at low frequencies.     

The fatigue properties of weft-knit fabric reinforced epoxy resin composites

Carbon and glass tows were fabricated into Interlock, Full-Cardigan, Milano and Rib fabric stitches by weft-knitting. The fatigue strength of composites made from these fabrics and epoxy resin was studied in terms of the relationship of the knitting stitch and the applied direction, the stress number of the fatigue distribution, hysteresis heating, lost strength, and fatigue damage propagation rate. In addition, by using the above composites, we compared the fatigue strength with plain-weave fabric composites.

The weft-knit composites loaded by fixed cantilever bending showed that the wale direction had a greater stiffness than the course direction. The Interlock fabric composite provides the highest fatigue resistance in these weft-knit stitches. However, the fatigue strength of a plain-weave fabric composite is higher than that of the weft-knit fabric composites. In a fatigue test, the hysteresis heating is below 60°C. On the other hand, the fatigue resistance ability can be increased about 40–60% by a sandwich lamination method.     

A static fatigue model for the durability of glass fibre reinforced cement

This paper presents a model for predicting service lives for glass-fibre reinforced cement (grc) components using hot-water accelerated ageing. It improves on previous models, being derived from consideration of a specific proposed micro-mechanical strength loss mechanism based on static fatigue principles and can be applied from time = 0. The model fitted well to all available strength vs. time data pertaining to various grc formulations. The activation energies thus derived for the strength loss process (80–90 kJ mol^–1) were consistent with those derived previously and those proposed for general glass dissolution mechanisms. Updated acceleration factors for predicting service lives for grc are advanced. The model was also applied to grc made with modified cement matrices. For metakaolin modified matrices, the activation energy appeared similar to that for OPC-grc, thus the use of similar acceleration factors appears justified. There is some evidence that calcium sulphoaluminate modified grc degrades according to a different activation energy. More data are required for modified matrix grcs if the model is to be applied thereto with confidence.     

Damage tolerance of glass mat/epoxy laminates hybridized with flexible resin under static and impact loading

This study deals with the impact property and damage tolerance of matrix hybrid composite laminates with different laminate constitution. The matrix hybrid composite laminates consisted of the laminae with a conventional epoxy resin and the laminae with a flexible epoxy resin modified from the conventional resin to avoid the interlaminar delamination. The impact energy absorption ratio greatly depended on the matrix resin placed at the impact face. The energy absorption was almost constant if the conventional resin was placed at the impact surface layer, while it increased exponentially with the increasing fraction of the flexible resin if the flexible resin was placed at the impact face. The impact energy was absorbed by the damage development and propagation in the laminate with conventional resin laminae as the impacted face, while it was absorbed by both the recoverable deformation of the flexible resin and the damage propagation in the laminate with flexible resin laminae as the impacted face.     

Internal reinforcement joints in grp under static and fatigue loading

Several forms of glass reinforcement for polyester resins are supplied as rolls of finite width and thus inevitably give rise to reinforcement joints in large laminates. Internal joints in flat and tubular specimens reinforced with chopped strand mat and a plain weave fabric were studied under static and fatigue loading. Under static loading a butt joint in chopped strand mat reinforcement had a small effect on the strength compared with a similar specimen without a joint. Under fatigue loading the presence of a joint produced serious reduction in strength. However with fabric reinforcement a lap joint produced a reduction in static strength and a negligible reduction in strength at long fatigue lives.     

Damage performance of particles filled quasi-isotropic glass–fibre reinforced polyester resin composites

The purpose of this work was to determine the toughening mechanisms in interlayered quasi-isotropic glass–fibre reinforced polyester resin (GFRP) composites. Particles of polyethylene and aluminium tri-hydrate, Al(OH)3, were mixed with the polyester resin prior to laminating with woven E-glass-fibre cloth. Mode-I, mode-II, and impact tests were performed to determine critical strain energy-release rates (GIc and GIIc), absorbed energy and residual compressive strength for the laminates with and without particulate additions. Mode-I and mode-II delamination toughness were characterized using double cantilever beam (DCB) and end-notched flexure (ENF) specimens, respectively, and the delaminated surfaces of specimens were examined using scanning electron microscopy (SEM) to investigate the interlaminar morphology after fracture. The results indicate that the interlaminar toughness (GIc and GIIc), absorbed energy and residual compressive strength values of the GFRP composite increases with increase of particle content. The improved behaviour of particle containing GFRP is linked to stress-concentration induced plastic deformation and crack bridging. Polyethylene particles increase the toughness of the matrix material, which results in composites with higher values of mode-I, mode-II and impact than the composites with aluminium tri-hydrate particles. © 1998 Chapman & Hall     

Creep characteristics of glass reinforced cement under flexural loading

Tests are reported on the flexural creep behaviour and flexural strength characteristics of mortar specimens reinforced with 5.0% by weight glass fibres. The specimens were cut off from flat sheet materials produced by premixing and spray suction casting processes. It is shown that fibre reinforcement reduced the deflections under sustained flexural load. The results show unmistakably a very significant influence of the fibre reinforcement in reducing creep strains. Fibres were generally more effective in controlling compression creep than tensile creep. Strength reductions were observed with time which were insignificant in specimens produced with the spray suction method.     

Extrinsic size effects on performance of Zr-based metallic glass under biaxial loading

The size-dependent plastic deformation of a Zr-based metallic glass under biaxial loading was investigated by using small punch (SP) test. Unlike under uniaxial tension or compression, under biaxial loading, both the critical shear offset and the density of shear bands decrease with the reduction of the sample size. However, it was found that the normalized critical shear offset keeps constant, which can well explain the worsened plastic deformation behaviors of the thin sample under biaxial loading. This finding indicates that metallic glass possesses good ductility in nature, yet it is influenced significantly by the loading mode and sample size.     

Prediction of the influence of flaws on the fatigue strength under biaxial loading

A method for the prediction of the fatigue strength of flaw-containing components subjected to in-phase biaxial loading is presented based upon a knowledge of the materials fatigue strength in bending. A key aspect of the method is the relationship between stress concentration factors and stress intensity factors. The fatigue strength is taken to be that stress needed to overcome a materials resistance to crack propagation, and a material constant, r_e, is used to relate fatigue strength and the threshold for crack propagation. Continuous crack growth in components containing flaws occurs when the driving force for crack propagation exceeds the resistance to crack growth which is created by the need to exceed the threshold level as well as to overcome the effects of crack closure. Good agreement between the predictions and experimental results was obtained.     

Lifetime and young's modulus changes of glass/phenolic and glass/polyester composites under fatigue

Fatigue of composites causes a loss of stiffness and development of damage before ultimate failure. These effects were compared for phenolic and polyester resins reinforced by five layers of woven roving/chopped strand combination glass mat. Such polyester laminates are widely used, e.g. in marine applications such as high speed craft. The phenolic laminates are promising candidates for applications where fire resistance is important. S-N and ε-N curves were obtained. In addition, stress versus strain curves were measured continuously under reverse loading (R = -1) up to 10⁶ cycles. Analysis of these curves allows further insight into the fatigue process. Tensile moduli drop dramatically (25%) with increasing number of cycles, but remain fairly constant in the compressive region. This behaviour can be explained by effects of matrix cracking. It also indicates the need to define different moduli for non-linear stress-strain curves. Absorbed energy (damping) can also be determined from the stress-strain curve and can be used as an indicator of the onset of rapid degradation of the material leading to ultimate failure. All data are compared for polyester and phenolic matrix laminates.     

Failure process and mechanism of sandstone under combined equal biaxial static compression and impact loading

The process and mechanism of impact fractures in sandstone were investigated under equal biaxial static compression. The cracking process was captured by a high‐speed video camera. The results indicate that the main crack propagates along the circumference and finally forms a crater‐shaped failure zone. The size of the crater‐shaped failure zone increases as the static stress and impact velocity increase. In addition, microscopic features of the fracture surface were observed using a scanning electron microscope. It was found that the microcracks expand after combination loading, making the rock more susceptible to damage. Finally, the influence mechanism of static loading on dynamic failure of the rock was revealed by theoretical analysis and numerical simulation.