Finite element analysis of composite T-joints used in wind turbine blades

Abstract

This paper presents a finite element (FE) analysis of the fracture behaviour of composite T-joints with various fibre reinforcement architectures subjected to pull-out loading. The FE model accounts for the effect of interface strength and interlaminar fracture energy on the ultimate load to failure; a linear softening fracture based law is adopted to describe crack growth in the form of delamination. The numerical simulation shows that the failure load increases with increasing interlaminar strength, which controls delamination initiation. The FE also demonstrates that the failure load increases with increasing interface fracture energy and the delamination propagation depends largely upon the fracture energy, which is enhanced by introducing interlaminar veils or through-thickness tuft yarns (stitching). Predictions were validated using experimental data for E-glass fibre/epoxy T-joints subjected to a tensile pull-out loading. The load–displacement response from the FE analysis is in a good agreement with measurements, illustrating the effectiveness of through thickness tufting that results to progressive, a more ‘ductile’, rather than abrupt catastrophic failure.     

Anticipating paint cracking: The application of fracture mechanics to the study of paint weathering

The relationships between the fracture energy (G_c), film thickness, stress level, and weathering time were investigated for a number of automotive clearcoats. As weathering progressed, most clearcoats embrittled due to the combined effects of photooxidation, hydrolysis, and other degradation mechanisms. This embrittlement was measured and related to the chemical composition changes that take place in the clearcoat during weathering. When the fracture energy dropped below the driving energy for cracking, G, brittle cracking occurred. The driving force for cracking was shown to depend on stress level and film thickness. The importance of fatigue loading was also qualitatively investigated. These techniques were successfully used to anticipate the long-term weathering behavior of automotive clearcoats. Materials Science Department, Dearborn, MI 48121.     

Delamination toughness of fiber-reinforced composites containing a carbon nanotube/polyamide-12 epoxy thin film interlayer

We present a simple, out-of-autoclave approach to improve the delamination toughness of fiber-reinforced composites using epoxy interlayers containing 20 wt.% polyamide-12 (PA) particles and 1 wt.% multi-walled carbon nanotubes (MWCNTs). Composites were prepared by integrating partially cured thin films at the laminate mid-plane using vacuum-assisted resin transfer molding. The introduction of epoxy/PA interlayers increased fracture toughness due to the ductile deformation and crack bridging of PA particles within an interlaminar damage zone with uniform thickness of about 20 μm. Composites interlayered with epoxy/PA/MWCNT exhibited nearly 2.5 and 1.5 times higher fracture toughness than composites containing neat epoxy and epoxy/PA interlayers, respectively, without an observable increase in interlaminar thickness. The fracture surface was analyzed to identify failure modes responsible for the fracture toughness improvement. The MWCNTs are proposed to inhibit critical loading of defects by minimizing stress concentration within the interlaminar region, thereby enabling greater deformation of the PA particles during fracture.     

The application of the essential work of fracture methodology to the plane strain fracture of ABS 3-point bend specimens

The applicability of the EWF methodology to 3-point bend (SEB) specimens under conditions other than plane stress has been assessed experimentally. Different fracture conditions, pure plane strain and plane strain/plane stress transition, were obtained by varying the specimen thickness and testing temperature (20 and 80 °C). Post-mortem fracture surfaces appeared always completely stress-whitened, indicating ductile fracture. The load-line displacement plots are similar over a well-defined range of ligament lengths for which the application of the EWF methodology was in principle possible. Nevertheless, in experiments conducted at room temperature, crack growth was observed to initiate before maximum load and complete ligament yielding. This behaviour was confirmed through plastic collapse analyses. A critical ligament length was found, over which the total specific work of fracture was dominated by edge effects. Below this critical ligament length, EWF methodology was still applicable and it was possible to extrapolate reliable w_Ie values.     

Flow through a convergence. Part 1: Critical conditions for unstable flow

The critical conditions leading to fracture in elongation and different types of flow instabilities were examined in uniaxial elongation and in a capillary rheometer equipped with dies having different entry profiles. Either ductile or brittle fracture may be observed, ductile being related to necking of material. The critical stress approach was used to predict fracture in elongation. All linear polymers studied in this work exhibited ductile fracture in uniaxial elongation, but the transition to brittle fracture is discussed in relation to existing experiments with other materials. In a ductile fracture regime, critical stress and work both increase with an increasing rate of deformation, whereas in a brittle regime the critical values remain constant. The converging flow studies indicated that two types of flow instability that have been previously related to each other, namely, pressure oscillations and voltions distortions, are of different origins. The critical flow rate for pressure oscillations is independent of entry profile, and the origin for this type of instability lies along the wall of the capillary. On the other hand, the critical flow rate for volume distortions increased with a decreasing entry angle, indicating that volume distortions are not a consequence of pressure oscillation, nor are their origin at the capillary wall. Numerical simulations were used to determine the stress profiles within the flow, and it was shown that the onset of volume distortions is directly related to the magnitude of elongational stress and work, and may therefore be considered to be caused by fracture in elongation. In dies with 90° entry profile, volume distortions were observed simultaneously with pressure oscillations, making it difficult to distinguish between the two phenomena.     

Fractography of injection molded polycarbonate acrylonitrile-butadiene-styrene terpolymer blends

Fractography has been used in the post-failure analysis of single edge notched specimens of injection molded blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene terpolymer (ABS). The mode of ductile tensile fracture of single edge notched specimens depended on comosition. Plane stress shear tearing was observed in the composition range PC/ABS 90/10 to 70/30 by weight where PC was the continuous phase. Intermediate compositions, PC/ABS 60/40 to 40/60, had a co-continuous or almot co-continuous phase morphology; these blends fractured by mixed mode pop-in, where a tunneling center crack relieved the triaxiality and permitted plane stress shear lips to form near the edges. Herringbone fracture, a plane strain mode characterized by discontinuous crack growth, was observed when ABS was the continuous phase, PC/ABS 30/70 to 10/90. An S-shaped relationship was observed between the ductile-to-brittle transition temperature and the composition. Addition of ABS to PC increased ductility up to PC/ABS 70/30 and 60/40, which were the most ductile compositions. Further addition of ABS decreased the ductility, and the least ductile compositions were PC/ABS 30/70 and 10/90.     

Characterization of material ductility of PP/EPR/talc blend under wide range of stress triaxiality at intermediate and high strain rates

A ductile fracture locus formulated in the space of the effective plastic strain to fracture and the stress triaxiality for the polypropylene (PP) blended with ethylene‐propylene rubber (EPR) and talc fillers is obtained at the intermediate and high strain rates by using a combined experimental‐numerical approach. Biaxial loading tests on the flat butterfly specimens are carried out to characterize fracture behaviors under pure shear, combined shear and tension, pure tensile loading conditions at various loading velocities. Corresponding finite element analysis is performed to determine the evolution of stress, strain, and strain rate states. It is found that the material ductility strongly depends on the stress triaxiality for the present PP/EPR/talc blend. Meanwhile, the fracture surfaces are observed by scanning electron microscopy, revealing that there exist two competing failure mechanisms: the multiple crazing at high positive stress triaxialities and void‐sheeting like fracture mode at the low stress triaxiality. The transition of the failure modes occurs in the intermediate range of stress triaxialities. The obtained fracture locus covers a wide range of the stress triaxiality. It would be applicable to the fracture analysis of real automotive components such as interior or exterior polymeric components under various impact loading conditions. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Strain rate dependence of fracture in a rubber-toughened epoxy system

The toughening mechanisms in rubber-modified epoxies appear to be viscoelastic in nature since their fracture behavior is dependent on loading rate. This behavior has been studied in detail and modeled for only one system, a model toughened epoxy often used in research work. The present study examines the loading rate effect for a new material based on acrylic rubber by measuring the fracture energy in constant cross-head speed tests conducted over a wide range of speeds. As expected, decreasing the loading rate produced an increase in toughness. Just as in the previous studies, the fracture energies could be modeled with a power law relationship when the loading rate was characterized by the time of failure. Moreover, the parameters involved in the model are quite consistent with the earlier results. For most rates, the behavior was approximately linear elastic with little or no r-curve behavior. Below a critical rate, however, there was a transition to ductile failure with a large r-curve and very high fracture energies. The transition is very sudden which may help explain why some previous studies have observed this effect while others have not.     

STRAIN RATE DEPENDENCE OF FRACTURE IN A RUBBER-TOUGHENED EPOXY SYSTEM

The toughening mechanisms in rubber-modified epoxies appear to be viscoelastic in nature since their fracture behavior is dependent on loading rate. This behavior has been studied in detail and modeled for only one system, a model toughened epoxy often used in research work. The present study examines the loading rate effect for a new material based on acrylic rubber by measuring the fracture energy in constant cross-head speed tests conducted over a wide range of speeds. As expected, decreasing the loading rate produced an increase in toughness. Just as in the previous studies, the fracture energies could be modeled with a power law relationship when the loading rate was characterized by the time of failure. Moreover, the parameters involved in the model are quite consistent with the earlier results. For most rates, the behavior was approximately linear elastic with little or no r-curve behavior. Below a critical rate, however, there was a transition to ductile failure with a large r-curve and very high fracture energies. The transition is very sudden which may help explain why some previous studies have observed this effect while others have not.     

Experimental investigation and numerical prediction of static strength and fracture behaviour of notched epoxy-based structural adhesives

The influence of stress concentrations on the tensile static strength and fracture behaviour of notched bulk specimens was investigated by comparing the response of two epoxy-based structural adhesives (a rubber-toughened and a polyurethane-toughened). Numerical predictions of failure stress were carried out using a 3D-FEA model with a hydrostatic dependent elastoplastic material behaviour and the equivalent plastic strain for failure assessment. The ductile adhesive, which plastically deforms more under high stresses, provided experimental evidence of a notch strengthening effect. Conversely, the less ductile adhesive has shown a reduction in tensile strength compared to un-notched samples. For both adhesives fracture surface analysis showed the presence of stress whitening and voids close to the notch regions. These regions could be correlated to higher values of stress triaxiality using numerical simulation. The more ductile adhesive underwent widespread stress whitening prior to failure, whereas the response of the less ductile adhesive was more localised. Numerically based predictions showed excellent agreement with experimental results with average error of 5.1% for different notch types in both adhesives.     

基于Matlab的聚乙烯管韧性破坏的分析计算

韧性破坏是聚乙烯管的主要破坏形式之一,这是因为聚乙烯为粘弹性材料,其弹性模量随时间的增大而降低,在内压作用下,聚乙烯管壁厚不断减薄,应力不断增大。且聚乙烯的屈服应力随应变率的减小而减小。当内压增加到一定值时,von mises等效应力与屈服应力相等,聚乙烯管材发生韧性失效。采用prony级数模拟聚乙烯管的粘弹性力学性能,采用Matlab进行编程分析,从而得到韧性失效寿命与所受内压载荷的关系和屈服应力与所受内压载荷的关系。     

A fatigue crack initiation map for polycarbonate

The mechanisms of fatigue crack initiation for various stress levels and thicknesses have been determined for single-edge notched specimens of polycarbonate and used to assemble a map. Three basic fatigue crack initiation mechanisms were identified and named as cooperative ductile (the damage zone formed ahead of crack consisting of yielded material), solo-crack brittle (very little damage zone development), and cooperative brittle (identified as a cloud of microcracks or crazes that developed at the notch tip). With a given applied stress and within the same failure mechanism, the values of the number of cycles to crack initiation decrease with increase in thickness. The transition from cooperative ductile to solo-crack brittle initiation mechanisms is sudden with increasing thickness. Transition from cooperative ductile to cooperative brittle with decreasing stress was less well defined. Regions where combinations of mechanisms were observed are also identified in the map. © 1993 John Wiley & Sons, Inc.     

The effect of injection molding process parameters on mechanical and fracture behavior of polycarbonate polymer

In this work, the mechanical and failure behavior of injection molded aviation standard optical grade polycarbonate (PC) was investigated through uniaxial tensile testing. The effect of different injection molding process parameters including injection velocity, packing pressure, cooling time, mold temperature, and melt temperature were determined to observe their effect on yield and postyield behavior of PC. Out of these examined parameters, the mold and melt temperature show significant effect on mechanical behavior of studied polymer. The yield and flow stresses in polymer increase with the increase in mold and melt temperature during injection molding. However, other process parameters i.e., packing pressure, injection velocity, and cooling time showed little effect on mechanical performance of the polymer. The molded specimens were annealed at different temperatures and residence time to evaluate its effect on mechanical behavior and fracture morphology. The yield stress increases gradually with the increase in annealing temperature and time. The annealing treatment also changed the failure mode of PC specimens from ductile to brittle. In addition to process parameters, the effect of increased loading rate was also undertaken which shows substantial effect on mechanical and failure behavior of PC. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44474.     

Deformation Mechanisms in Compression-Loaded, Stand-Alone Plasma-Sprayed Alumina Coatings

Cylindrical, stand-alone tubes of plasma-sprayed alumina were tested in compression in the axial direction at room temperature, using strain gauges to monitor axial and circumferential strains. The primary compression-loading profile used was cyclic loading, with monotonically increased peak stresses. Hysteresis was observed in the stress–strain response on unloading, beginning at a peak stress of 50 MPa. The modulus decreased as the maximum applied stress increased. The stress–strain response was only linear at low stresses; the degree of nonlinearity at high stresses scaled with the stress applied. One-hour dwells at constant stress at room temperature revealed a time-dependent strain response. Using transmission electron microscopy and acoustic emission to investigate deformation mechanisms, the stress–strain response was correlated with crack pop-in, growth, and arrest. It is proposed that the numerous defects in plasma-sprayed coatings, including porosity and microcracks, serve as sites for crack nucleation and/or propagation. As these small, nucleated cracks extend under the applied stress, they propagate nearly parallel to the loading direction along interlamellae boundaries. With increasing stress, these cracks ultimately link, resulting in catastrophic failure.     

Critical stress intensity factor of epoxy mortar

Fracture behavior of epoxy mortar was investigated in Mode I fracture using single edge notched beams with varying notch depth and beam thickness. The beams were loaded in both 3-point and 4-point bending. Influence of polymer content and temperature on the fracture behavior of epoxy mortar was studied using uniform Ottawa 20–30 sand. The polymer content was varied between 10 percent and 18 percent of the total weight of the composite. The temperature was varied between 22°C and 120°C. The flexural strength of the polymer mortar increases with increase in polymer content while the flexural modulus goes through a maximum. The critical stress intensity factor (K_IC) was determined by several methods including compliance method (based on crack mouth opening displacement) and finite element analysis. The K_IC for epoxy mortar increases with increase in polymer content and epoxy mortar strength but decreases with increase in temperature. The critical stress intensity factor of epoxy mortar is represented in terms of polymer content and polymer strength or stiffness. Numerical tests based on random sampling and stratified sampling procedures were performed to substantiate the experimentally observed fracture toughness values of epoxy mortar.     

Crazing,yielding, and fracture of polymers. I. Ductile brittle transition in polycarbonate

Most thermoplastics far below their glass transition give a brittle fracture when de-formed in uniaxial tension. Bisphenol-A polycarbonates are an exception and deform in a ductile manner. However, it has been observed in Izod impact studies of notched samples that the mode of failure changes from a ductile to a brittle fracture on annealing samples below T_g. It has been found that, when notched samples are stressed, a Griffith type flaw is formed under the notch. The criterion for the ductile brittle transition is evaluated in terms of σG (the stress required to propagate the Griffith flaw), and σ_y, the yield stress for the polymer. It has been found that the density and yield stress for the samples annealed at various temperatures are dependent upon previous thermal history and in particular on the molecular weíAght. On the basis of these measurements, it is concluded that many of the so-called anomalous effects observed with polycarbonate can be explained.     

Long fiber reinforced polyamide and polycarbonate composites. II: Fatigue behavior and morphological property

Fatigue behavior and morphology of long glass fiber reinforced semicrystalline polyamide (nylon 6,6) and amorphous polycarbonate (PC) composites were investigated. The fiber length distribution in the molded samples was calculated by image analyzer. The tension-tension fatigue loading tests at various levels of stress amplitudes were studied. The two-parameter Weibull distribution function were applied to obtain the statistical probability distribution of experimental data. A good correlation existed between the experimental data and the Weibull distribution curves. Straight line S? N curves of long glass fiber reinforced semicrystalline polyamide and amorphous polycarbonate composites at various probabilities were established. The stiffness of the composite under tension-tension fatigue loading was measured. The thermal stress history was also investigated by thermo-imaging techniques during fatigue life testing. Further, failure morphology was examined by scanning electron microscopy (SEM). The results showed that the fracture behavior of the ductile damage in polyamide is different from the brittle damage in polycarbonate.     

The spinnability of polymer fluid filaments

A theoretical study of the stability of extending liquid filaments has been carried out. The interaction of surface tension and different fluid rheological properties is investigated. It is also hypothesized that cohesive failure or fracture will occur if a critical stress level is reached. It is predicted that viscosity and viscoelasticity tend to stabilize the filaments. However, even extremely high viscosity filaments will neck and exhibit ductile failure. In highly viscoelastic fluids, defects tend to heal during stretch. Highly viscoelastic fluid filaments fail by fracture. The theory is used to predict the failure of molten polymer filaments as a function of molecular weight. The extensibility or spinnability of filaments is predicted to exhibit a maximum at intermediate molecular weights with capillarity-ductile failure occurring at low molecular weights and cohesive fracture, at high molecular weights. The results are compared to experiments on polyethylenes. There is general qualitative agreement especially with the behavior of low and high molecular weights where capillarity and fracture occur. The tendency to necking and ductile failure differs considerably among melts and is more pronounced in high-density than in low-density polyethylenes. The application to continuous spinline behavior is discussed, and draw resonance is suggested to be the continuous process analogue of ductile failure.     

Fracture and impact properties of polycarbonates and MBS elastomer-modified polycarbonates

Mechanical properties of polycarbonates (PCs) and elastomer-modified polycarbonates with various molecular weights (MW) are investigated. Higher MW PCs show slightly lower density, yield stress, and modulus. The ductile–brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of PC MW and with the increase of elastomer content. The elastomer-modified PC has higher impact strength than does the unmodified counterpart if the failure is in the brittle mode, but has lower impact strength if the failure is in the ductile mode. The critical strain energy release rate (G_c) measured at ?30°C decreases with the decrease of PC MW. The extrapolated zero fracture energy was found at M_n = 6800 or MFR = 135. The G_c of the elastomer-modified PC (MFR = 15, 5% elastomer) is about twice that of thee unmodified one. The presence of elastomer in the PC matrix promotes the plane–strain localized shear yielding to greater extents and thus increases the impact strength and G_c in a typically brittle fracture. Two separate modes, localized and mass shear yielding, work simultaneously in the elastomer-toughening mechanism. The plane–strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane–stress mass shear yielding dominates at higher temperatures and ductile failure. For the elastomer-modified PC (10% elastomer), the estimated extension ratio of the yielding zone of the fractured surface is 2 for the ductile failure and 5 for the brittle crack. A criterion for shifting from brittle to ductile failure based on precrack critical plastic-zone volume is proposed.