Fatigue damage accumulation in notched woven-ply thermoplastic and thermoset laminates at high-temperature: Influence of matrix ductility and fatigue life prediction

This work was aimed at investigating the influence of matrix ductility on the high-temperature tensile fatigue behaviour in notched and unnotched C/PPS (thermoplastic) and C/Epoxy (thermoset) laminates. Damage mechanisms and overstress accommodation near the hole have been discussed by means of X-rays observations and fractography analysis. In order to quantitatively evaluate the fatigue damage within notched and unnotched laminates as a function of the cycles, a damage variable based on a mean strain calculated on each cycle from the experimental stress–strain loops has been used. Finally, a simple analytical model was applied to test its predictive capabilities to evaluate the fatigue damage accumulation in both materials. This model proved to be relevant to predict the evolution of fatigue damage in notched C/PPS composites but not in C/Epoxy laminates.     

Dynamic characteristics of thermoplastic composite laminates

Dynamic characteristics of carbon fibre/polyetheretherketone (cf/peek) composites have been investigated by the impact test and the sinusoidal free vibration test. Using a cantilevered beam with rectangular cross-section, the natural frequencies and damping properties were measured. Also, dynamic tests for the cantilevered beam with a torsional bar have been performed to identify the shear modulus and damping in torsional motion. Results for the specific damping capacity given by the impact test are very close to those given by the sinusoidal vibration test. The elastic moduli from the impact test are slightly smaller than those from the tension test. The dynamic characteristics of angle-ply, cross-ply and quasi-isotropic laminates have been computed by the finite element method. For the purpose of verification, experimental tests were carried out by the impact test for these laminates. The experimental values are very close to the numerical values. In addition to cf/peek composites, experiments have been performed for cf/epoxy composites. The damping of cf/epoxy is much larger than that of cf/peek.     

热塑性复合材料层合板的层间应力与损伤机理

使用有限元生成软件FEPG和表征热塑性复合材料AS4/PEEK非线性行为和应变率相关行为的三维粘塑性模型,计算了复合材料角铺设层合板在单向拉伸时的界面层间应力.层间应力的三维分布图表明,AS4/PEEK对称角铺设层合板的层间剪应力在自由边缘处存在很明显的自由边缘效应;层间正应力也存在自由边缘效应,对于轴向拉伸,其在自由边缘处的值为负.随着铺设角的增大,自由边缘处二者的值均减小.层间应力存在端头效应,甚至比边缘效应还明显.随着铺设角的增大,层间应力在两端头处的值降低,层间正应力由压应力变为拉应力.主要由纤维控制的角铺设AS4/PEEK层合板,在自由边缘处较大的层间剪应力是引起其层间分层的主要原因;主要由基体控制的角铺设AS4/PEEK层合板,其首先产生的是面内应力破坏,而不是层间分层.     

复合材料层板开孔拉伸损伤分析

针对纤维增强复合材料层板开孔拉伸, 将复合材料层板的失效分为层内失效和层间失效, 建立了复合材料层板开孔拉伸损伤分析模型。该模型基于逐渐损伤分析, 对不同复合材料开孔层板进行了失效预测, 并与文献试验结果进行了对比, 破坏强度和失效模式均与文献试验结果非常吻合。结果表明本文中所建立的层板开孔拉伸损伤分析模型能够模拟含孔层合板拉伸过程中的损伤起始、 损伤扩展和最终破坏模式, 并最终预测含孔层合板拉伸失效模式和破坏强度。      

Characterization of long fiber thermoplastic/metal laminates

Long fiber thermoplastic (LFT) composite/metal laminate (LML) is a hybrid composite consisting of alternate layers of metals such as aluminum and an LFT composite, which combines advantages from both the constituents. The LFT/Al laminates (LMLs) were processed by compression molding and were characterized for their Young’s modulus, mechanical strength, and low-velocity impact (LVI) properties. The average values of specific elastic modulus and specific tensile strength were approximately 20 GPa/(gcm⁻³) and 108.5 MPa/(gcm⁻³), respectively. Failure mechanisms included delamination between LFT composite and Al, fiber fracture and pullout in LFT composite, and shear fracture of aluminum and LFT composite layers. Rule-of-mixtures (ROM) predictions of laminate properties in tension compared well with the experimental values. Specific perforation energy of the laminates determined by LVI tests was 7.58 J/(kgm⁻²), which is significantly greater than that of the LFT composite alone, 1.72 J/(kgm⁻²). Overall, the LML showed significant improvement in the properties as compared to the LFT composite.     

Compression strength of aligned carbon fibre-reinforced thermoplastic laminates

Compression strength measurements have been made on a number of unidirectional carbon fibre-reinforced thermoplastics fabricated by film stacking or by melt impregnation and comparisons made with a standard aerospace epoxide system. The results indicate that polyetheretherketone based laminates, despite having a lower matrix shear modulus, have similar compression strengths to the epoxide system.     

Elasto-plastic behavior of thermoplastic composite laminates under cyclic loading

An experimental and numerical study of the elasto-plastic behavior of thermoplastic matrix composite laminates under static and cyclic loads is presented. Off-axis and angle ply specimens cut from laminates of poly(ether ether ketone) (PEEK) reinforced with continuous carbon fibers have been tested under cyclic sinusoidal tensile loads and the hysteresis loops have been monitored. A micro mechanical model, which includes a parabolic criteria based on the plastic behavior of the matrix, has been adopted to study the composite non-linear behavior and a correlation between plastic deformation and a strong rise of damping and temperature at high stresses is outlined. Good agreement is shown between theory and experimental results. The mathematical mdoel presented here can be used to predict the visco-elastic-plastic response of the material at high stresses and its influence in the fatigue damage.     

Modelling delamination onset and growth in pin loaded composite laminates

A three-dimensional (3D) finite element (FE) model is created with cohesive zone elements (CZE) to simulate a mechanically fastened [0°/90°]_s pin-loaded joint in a composite laminate. The model incorporates fully integrated solid elements in the pin-loaded area to accurately capture the high stress gradients. Contact based cohesive elements with a bilinear traction–separation law are inserted between the layers to capture the onset and growth of delamination. The stress distribution around the pin-loaded hole was verified with the widely used cosine stress distribution model. Results from the FE model show that delamination damage initiated at the point of maximum average shear stress at the 0°/90° interface. The delaminated area develops an elliptical shape which grows in a non-self similar manner with increasing pin displacement. It is concluded that a progressive damage model should be included to provide a full understanding of the failure sequence, work that the authors are currently engaged with.     

Enhanced delamination initiation stress and monitoring sensitivity of quasi-isotropic laminates under in-plane tension by interleaving with CNT buckypaper

Delamination initiation and the corresponding in-situ monitoring method have been investigated for a T300/epoxy quasi-isotropic laminate. Interfaces of the laminate, in which the delamination tends to occur under in-plane tensile load, have been interleaved with porous carbon nanotube (CNT) buckypapers. Both sectional loading to the delamination initiation and full tension to the fracture of specimens were performed to evaluate the reinforced effect and self-sensing properties of the CNT buckypapers on the laminates. As expected, enhanced delamination initiation stress level was obtained, improved by 7.7% compared with that of the base laminate. Simultaneously, electrical resistance and acoustic emission (AE) responses of the laminates were also measured and used to determine the initiation of delamination. The tests have exhibited that the CNT buckypapers have significant influence on the resistance change of the laminate, showing potential to be used as a detector. This study has preliminarily demonstrated that the CNT buckypapers can serve as both sensing and strengthening constituent.     

Edge effects of uniformly loaded cross-ply composite laminates

Interlaminar stresses resulting from bending of rectangular cross-ply composite laminates are determined using a layer wise laminate theory. Two types of laminates are considered. First a fully simply supported laminate subjected to bi-directional bending is analyzed. The results obtained from this theory are compared with those of the published three-dimensional elasticity solutions to verify the validity and accuracy of the present theory. Then laminates with two edges simply supported and the other two edges free are examined. The results indicate the presence of significant interlaminar stresses near the free edges.     

Combined tension and bending testing of tapered composite laminates

A simple beam element used at Bell Helicopter was incorporated in the Computational Mechanics Testbed (COMET) finite element code at the Langley Research Center (LaRC) to analyze the responce of tappered laminates typical of flexbeams in composite rotor hubs. This beam element incorporated the influence of membrane loads on the flexural response of the tapered laminate configurations modeled and tested in a combined axial tension and bending (ATB) hydraulic load frame designed and built at LaRC. The moments generated from the finite element model were used in a tapered laminated plate theory analysis to estimate axial stresses on the surface of the tapered laminates due to combined bending and tension loads. Surfaces strains were calculated and compared to surface strains measured using strain gages mounted along the laminate length. The strain distributions correlated reasonably well with the analysis. The analysis was then used to examine the surface strain distribution in a non-linear tapered laminate where a similarly good correlation was obtained. Results indicate that simple finite element beam models may be used to identify tapered laminate configurations best suited for simulating the response of a composite flexbeam in a full scale rotor hub.The U.S. Government right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Shear strength of polymers and fibre composites: 1. thermoplastic and thermoset polymers

The shear strengths of eight thermoplastics and three DGEBA-based epoxies in sheet form have been tested by the punch and Iosipescu tests. The testing temperatures ranged from 20 to 120°C, and the glass transition temperatures were measured as well. The shear strengths of the epoxies were also estimated from compressive tests on short cylinders. The Iosipescu test gave very unreliable results for polymers in the rubbery state because large deformations were induced before failure, and this caused high tensile stresses to develop instead of high shear stresses. With the punch test the force often had two maxima before failure, and the discs punched out did not have straight sides, so there was also much doubt about the purity of the shear stresses developed. The two methods were often in sharp disagreement. However, they gave comparable results with epoxies tested at room temperature. Comparison with compressive tests indicated that the ratio of compressive yield strength to shear yield strength varied from 1.5 to 2.4. In view of the uncertainties in the tests, compressive testing may be a good method to obtain an approximate value for the shear strength. The Tresca criterion (i.e. divide the compressive yield strength by two to get the shear yield strength) is probably good enough in view of the uncertainties in the shear tests. It works equally well for the ultimate shear strength. A new and better test is clearly needed for the estimation of the shear strength of polymers in sheet form.     

Modelling the tension and compression strengths of polymer laminates in fire

Thermo-mechanical models are presented for predicting the time-to-failure of polymer laminates loaded in tension or compression and exposed to one-sided radiant heating by fire. Time-to-failure is defined as the time duration that a polymer laminate can support an externally applied load in a fire without failing. The models predict the temperature rise and through-thickness temperature profile in a hot decomposing laminate exposed to fire. Using this thermal data, mechanics-based models based on residual strength analysis are used to calculate the time-to-failure. A preliminary evaluation of the accuracy of the models is presented using failure times measured in fire-under-load tests on a woven glass/vinyl ester laminate. The model was evaluated at temperatures between ∼250 and 800 °C by testing the laminate at heat flux levels between 10 and 75 kW/m². It was found that the time-to-failure of the laminate decreased with increasing heat flux and increasing applied stress for both the compression and tension load conditions. The tests also revealed that the failure times were much shorter (by about one order of magnitude) when the laminate was loaded in compression. The models can predict the time-to-failure with good accuracy for both compression and tension loading for certain heat flux levels. However, because the models have only been evaluated for one type of laminate (woven glass/vinyl ester), further evaluation is necessary for other laminate systems. The paper also presents new experimental insights into the strengthening mechanisms of laminates at high temperature.     

Failure behavior of quasi-isotropic laminates with three-pin loaded holes

This paper presents the effect of hole positions on the failure behavior of glass–epoxy laminated composite plates fabricated from stacking sequence [0/90/±45]_S subjected to a traction force by three-pins. Three different hole distance parameters, namely the ratio of free edge distance to the outer holes/pin diameter (E/D = 1, 2, 3, 4, 5), the ratio of longitudinal distance between the holes/pin diameter (F/D = 2, 4, 6) and the ratio of transverse distance between the parallel holes/pin diameter (G/D = 3, 4, 5) were used to investigate the effects of hole positions on failure load and failure mode. Shear out failure mode which is not the desired mode for a structure occurs around the first and second pin holes very close to the free edge of the specimen (E/D = 1). The other specimens were damaged in bearing mode which is the most desired mode.     

Notch sensitivity of fiber-reinforced ceramics

Crack bridging from an elliptical hole in fiber-reinforced ceramic composites is studied. For some fiber-reinforced ceramic composites, matrix toughness is much less than the toughness gained in the bridging zone, i.e. the bridging zone runs across the entire width of a specimen at a small load. In such case, load-carrying capacity of the specimen only depends on one parameter which is the measure of notch sensitivity. Solving the crack bridging problem for various aspect ratios of the elliptical hole and various bridging law shapes, the role of crack bridging from the hole is determined. The results presented may be used to guide design in addition to providing an improved understanding of the mechanism of fiber-matrix failure.     

Theoretical and experimental response of composite laminates with delaminations loaded in compression

Experimental and analytical approaches for studying the response of delaminations to in-plane compressive loads are presented. Laminates were manufactured with delaminations via an implanted thin. Teflon film. The out-of-plane displacements were monitored via shadow moire. A nonlinear, sublaminate analysis was developed using an assumed modes Rayleigh-Ritz, minimum potential energy procedure. Sublaminate growth was predicted using previously published critical Mode I interlaminar strain energy release rates (G_1c) and compared to the experimental results. Good agreement was found between the analytical model and the experimental results. An important conclusion is that simple, linear sublaminate buckling models may not be adequate for predicting delimination propagation and laminate failure.     

On avoiding thermal degradation during welding of high-performance thermoplastic composites to thermoset composites

One of the major constraints in welding thermoplastic and thermoset composites is thermal degradation of the thermoset resin under the high temperatures required to achieve fusion bonding of the thermoplastic resin. This paper presents a procedure to successfully prevent thermal degradation of the thermoset resin during high-temperature welding of thermoplastic to thermoset composites. The procedure is based on reducing the heating time to fractions of a second during the welding process. In order to achieve such short heating times, which are much too short for commercial welding techniques such as resistance or induction welding, ultrasonic welding is used in this work. A particularly challenging scenario is analysed by considering welding of carbon-fibre reinforced poly-ether-ether-ketone, with a melting temperature of 340 °C, to carbon-fibre reinforced epoxy with a glass transition temperature of 157 °C.     

Anisotropy in hysteresis loss and tension set in short-carbon-fibre-filled thermoplastic elastomers

The effect of short-carbon fibre on the anisotropy in hysteresis loss and tension set of two thermoplastic elastomers, based on natural rubber, high-density polyethylene blend and styrene-isoprene-styrene block copolymer, were studied. The composites based on natural rubber-high density polyethylene blend show anisotropy both in hysteresis loss and tension set, whereas styrene-isoprene-styrene shows lack of anisotropy in both cases. An empirical relationship has been established relating ( w)_lt, (the difference between the hysteresis loss between systems with longitudinal fibre orientation and transverse orientation) and the strain per cent and volume per cent of fibres.     

On transverse matrix cracking in cross-ply laminates loaded in simple bending

A one-dimensional analysis of a cross-ply laminate, containing cracked transverse plies, loaded in flexure is presented. Simple bending theory is used in conjunction with a shear-lag analysis, to calculate the degraded longitudinal modulus of a cracked transverse ply, enabling the flexural modulus of the laminate to be determined. The solution is shown to agree well with a more sophisticated stress transfer model in the literature. The analysis is then extended to calculate the applied bending moment at transverse crack onset under flexural loading using a fracture mechanics approach. The results suggest that the in situ transverse ply stress at which matrix cracking commences for the beam loaded in flexure is very close to the stress level at which the same ply would crack if the laminate were loaded in tension.     

Post-buckling behaviour of thermoplastic matrix composite laminates subjected to pure shear

The present study focuses on the determination of the buckling load and post-buckling behaviour of simply supported laminated composite rectangular panels loaded in shear. The nonlinear structural response is studied with a non-linear finite element approach. In order to determine the accuracy of the procedure, several tests have been performed comparing the finite element solutions for isotropic and laminated composite rectangular panels with existing ones, adopting different sequences of lamination and different length to width ratios. The analysis then considers the behaviour of laminates produced with innovative thermoplastic matrix composites developed in the frame of a national research program.