Thermal shock effects on the impact resistance,tolerance and flexural strength of alumina based oxide/oxide ceramic matrix composites

In this study the effects of thermal shock on the impact damage resistance, damage tolerance and flexural strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates with balanced and symmetric layup were gradually heated to 1200°C in an air-based furnace and held for at least 30 min before being removed and immersed in water at room temperature. The laminates were then subjected to low velocity impacts via a hemispherical steel impactor. The resultant damage was characterized non-destructively, following which the laminates were subjected to compression tests. Three-point bend tests were also performed to evaluate the effect of thermal shock on the flexural strength and related failure modes of the laminates. Thermally shocked laminates showed smaller internal damage and larger external damage areas in comparison to their pristine counterparts. For the impact energy and resultant damage size considered, the residual compressive strengths for the thermally shocked and pristine laminates were similar.     

Tensile and fatigue behavior of oxide/oxide ceramic matrix composite with simulated foreign object damage in combustion environment

In this study, oxide/oxide ceramic matrix composite test coupons were quasi‐statically indented and tested for tensile strength and fatigue life in a combustion environment. The combustion environment simulated the gas turbine engine environment in an aircraft. Two different dent sizes were created on two different sets of test coupons with a blunt conical indentor. During mechanical testing, the combustion flame simultaneously impinged on the dent region resulting in a maximum test coupon surface temperature of 1250 ± 50°C. For a life of 90 000 cycles, the fatigue limit in the combustion environment was 85% of the postindentation degraded tensile strength. Microscopy images of the failed test coupons showed damage modes of fiber fracture and matrix cracking at the dent site. The run‐out test coupons which did not fail within 90 000 cycles showed residual strength that was not significantly different from that of their virgin counterparts.     

Effect of active rubber powder on structural two-component epoxy resin and its mechanical properties

The aim of the research was to investigate a possibility to use active rubber powder (ARP) from a process of tyres recyclation in an area of a filler into a reactoplastics matrix applied in structural adhesive bonds. This study focuses on an analysis of a tensile strength, a shear impact strength and a hardness of the composite mixture itself and further on the interaction with an adhesive bonded material, i.e. it evaluates an influence of different ratio of ARP on the adhesive bond strength. Effects of the cyclic degradation environment combining the decreased temperature ?40 °C and the increased temperature 90 °C at the simultaneous acting of the increased moisture up 90% is a part of the research. The tensile strength and the hardness were decreased by adding ARP. The experiment results proved a positive effect of ARP in the area of adhesive bonds exposed to the cyclic degradation at increased and decreased temperatures. Elastic ARP is able to absorb an inner tension in the layer of the adhesive bond.     

Study on the damage behavior of carbon fiber composite after low-velocity impact under hygrothermal aging

In this article, T800 carbon fiber/epoxy resin composite was subjected to hygrothermal aging. By analyzing the mass change, surface morphology before and after aging, infrared spectra, and dynamic mechanical properties, the effect of hygrothermal aging on the composite properties was studied. The hygrothermal aging of the composite after low-velocity impact, the effects of environmental factors on the damaged area, and the post-impact compression properties of composites were studied. The results showed that the saturation moisture absorption rate of the composite after aging (71°C constant temperature) was 0.88%. Upon increasing the impact energy, an indentation appeared before the inflection point at 35 J. When the impact energy was less than 15 J, aging did not affect invisible damage. Above this, the damaged area and number of internal cracks and defects in the composite were increased. After aging, the compressive strength of composite laminates with impact damage decreased obviously. During the aging stage, the residual compressive strength of the sample was the lowest in the moisture saturated state, and hygrothermal aging had little effect on the compression failure mode after impact.     

复合材料层合板损伤区域扩展全过程分析

对碳纤维NCF层合板进行了2组准静态压痕试验。第1组采用不同的压头尺寸探究不同压头直径对层合板损伤区域和面积的影响;第2组进行固定压头尺寸的重复试验,使用非接触空气耦合超声波扫描试样损伤情况,研究静压痕力、凹坑深度、损伤面积三者之间的联系。利用Abaqus有限元软件,根据Hashin准则和内聚力单元的结合,对第2组层合板准静态压痕试验进行全过程的损伤扩展分析。分析结果表明,基体拉伸的破坏是层合板最容易出现的破坏,而且其损伤区域涵盖了其他4种损伤区域。凹坑深度与损伤面积存在一定联系,试验开始阶段凹坑增加速度缓慢,损伤面积增加速度较快;接近极限载荷时,凹坑深度增加速度加快,损伤面积增加速度减缓。     

Effect of hydrothermal environment on moisture absorption and mechanical properties of wood flour–filled polypropylene composites

The moisture absorption and mechanical properties of wood flour–filled polypropylene composites in a hydrothermal environment have been studied by immersing the composites in water at 23, 60, and 100°C. The degree of moisture absorption was found to be dependent on the modification of matrix, the weight percentage, mesh size, and surface treatment of wood flours. It increased with increasing the immersion temperature. The tensile strength of all composites with wood flours of different contents, mesh sizes, and surface treatments increased after immersion in water baths of various temperatures, to either greater or lesser extents. The flexural strength and modulus followed a similar trend when immersed in water at ambient temperature. However, the contrary was true for composites when immersed in 60 and 100°C water baths. The impact strength increased after immersion in water at each immersion temperature, and the extent of such increment decreased with increasing the immersion temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2824–2832, 2002     

热疲劳作用对混凝土力学性能及微观结构的影响

我国西北地区日、年温差大,混凝土经历着温差产生的热疲劳劣化。保持环境湿度恒定,在20 ℃、30 ℃、40 ℃温差下开展两种强度等级的混凝土热疲劳试验,测定其抗压强度、劈裂抗拉强度等宏观性能变化规律;通过超声无损检测技术和压汞试验测定微观结构。结果表明:热疲劳劣化效应明显,随循环温差的增大和循环次数增加,混凝土强度下降明显,C40混凝土下降幅度大于C25混凝土,且劈裂抗拉强度较抗压强度对热疲劳作用更敏感;超声波速呈减小趋势,说明混凝土内部裂隙缺陷增多;同一循环温差下,混凝土的孔隙率、孔隙总体积、平均孔径、中值孔径、最可几孔径随温差循环次数增加而增大,孔隙总表面积减小,孔隙结构表现出粗化的特征且呈劣化的趋势,C40混凝土的孔隙率小于C25混凝土,但其孔隙率相对变化值更大,从微观层面揭示了混凝土在热疲劳作用下强度损伤的内在原因。     

Open hole flexural and izod impact strength of unidirectional flax yarn reinforced polypropylene composites as a function of laminate lay‐up

An open hole flexural strength and impact energy of flax yarn‐reinforced polypropylene (PP) composites were studied in this work. Highest flexural strength and strength retention were observed for axial (0₆) and cross‐ply (0/90/0)_s laminates, respectively, while also examining the influence of laminate lay‐up and open hole size on flexural strength. It was found that maleic anhydride‐grafted polypropylene (MAPP)‐treated composite laminates achieved marginal improvement on flexural strength for all kinds of laminate lay‐up. Off‐axial laminates (±45₆) showed a good strength retention for open hole laminates after MAPP treatment. The fractography study confirmed microbuckling and matrix crack propagation over the compressive and tensile side of the laminate, respectively. Furthermore, severe surface damage was detected over the tensile side of 8‐mm hole size laminates. Impact test of the flax/PP laminates showed slight improvement by MAPP treatment. High‐ and low‐impact energy was experienced for axial and off‐axial laminates. The damaged impact sample shows evidence of fiber pull‐out for untreated flax yarn reinforced laminates. POLYM. COMPOS., 34:1912–1920, 2013. © 2013 Society of Plastics Engineers     

Effects of moisture absorption on mechanical and viscoelastic properties in liquid thermoplastic resin/carbon fiber composites

This article investigated the effect of moisture on the tensile strength and in‐plane shear of laminated composites. For this, the results of a composite system based on a new thermoplastic Elium® 150 resin were compared to a traditional epoxy resin result. Both composites were fabricated via VARTM using a 0/90° plain weave carbon fiber fabric. For the non‐conditioned specimens, the thermoplastic composites presented 30% more tensile resistance in comparison to epoxy composites. For conditioned specimens, this difference was 14%. These results were related to plasticization, which tends to favor the polymer softening providing a greater matrix plastic deformation, promoting a ductile fracture of the composite. On the other hand, the in‐plane shear properties were 30% higher for the thermosetting laminates for both conditions. In this case, moisture may have favored the formation of surface cracks and weakened the fiber/matrix interfacial adhesion. Additional analysis based on design of experiments has shown that the Elium® 150 resin significantly affects all responses and presented in fact a better behavior in comparison to Epoxy resin. While the conditioning effects have featured a statistically noticeable contribution to the tensile strength, the presence of the moisture did not provide a significant enhancement to the in‐plane shear strength. Besides that, the unknown fractographic aspects of the fracture surfaces of both composites were used as a complementary tool for the mechanical characterization. POLYM. ENG. SCI., 59:2185–2194, 2019. © 2019 Society of Plastics Engineers     

Formation of voids in composite laminates: Coupled effect of moisture content and processing pressure

Void formation as a result of prepreg moisture content and processing pressure during cure was experimentally investigated in thermosetting composite laminates. This was achieved by determining the void contents of eight‐ply laminates fabricated from TenCate^® BT250/7781 E‐glass/epoxy prepreg at processing pressures of 1.7, 3.0, 4.4, and 5.8 atm. At each processing pressure, three types of laminates were fabricated using: (i) unconditioned prepregs (direct from the storage bag); (ii) prepregs conditioned at 25% relative humidity; and (iii) 99% relative humidity. Dynamics of prepreg moisture uptake during conditioning was measured using a moisture analyzer and was shown to exhibit Fickian diffusion behavior. The void contents of the cured laminates were found to vary from 1.6% to 5.0% depending on humidity environment the prepregs were exposed and the pressure applied during fabrication. The void contents of all laminates were observed to approach an asymptotic value of ∼1.6% as pressure was increased. The experimental results indicated the processing pressure applied during fabrication was increasingly carried by the fiber bed, reducing resin pressure during cure. Therefore, an enhanced void formation model was proposed through the addition of a pressure reduction factor and an asymptotic void content term. The proposed model was found to accurately predict the void content of laminates made of prepregs exposed to constant/varying humidity environments and fabricated at a wide range of processing pressures. POLYM. COMPOS., 36:376–384, 2015. © 2014 Society of Plastics Engineers     

Effect of relative humidity on the mechanical properties of poly(1,4-butylene terephthalate)

Three grades of poly(1,4-butylene terephthalate) (PBT) were aged up to three years at 100, 75, 50, and 11% relative humidity and temperatures of 66–93°C. The decrease in mechanical properties, caused by hydrolysis, occurs rapidly at the higher temperatures and relative humidities and progressively slows as the temperature and/or humidity are decreased. Equations for making life-cycle predictions at any combination of temperature and humidity were derived from Arrhenius plots. If a 50% loss in tensile strength constitutes failure, then the PBT examined should be expected to last only three to four years at 50°C and 100% relative humidity. Reducing the humidity level to 50% extends the useful life at this temperature to 10–20 years. Predictions based on the tensile strength half-life should not be used where toughness or impact properties are important because PBT embrittles long before the tensile strength half-life is reached.     

T700/LT-03A层合板在不同环境下的拉伸性能研究

在室温、低温和湿热三种环境下,对碳纤维层合板分别开展了静力和拉-拉疲劳试验。得到了T700/LT-03A层合板的拉伸性能和破坏机理。在抗拉强度和抗疲劳性能方面,室温环境优于其它两种环境。试验和模拟结果表明:T700/LT-03A层合板在三种环境下的应力结果较为接近;与室温环境下的结果相比,低温和湿热环境下层合板的应力分别减少了3.37%和4.3%,而湿热环境下层合板的应力增大了5.69%。环境对该层合板的疲劳性能影响较大。研究成果对碳纤维复合材料的工程应用提供一定的参考。     

Effects of alkaline conditioning and temperature on the properties of glass fiber polymer composite rebar

This article investigated long term alkaline conditioning and temperature on the physical and mechanical properties of glass fiber‐reinforced polymer (GFRP) composite rebar for structural applications. The GFRP rebar was immersed in alkaline solution (pH ≈ 13) for 23 months at 23°C, and for 24 months at 60°C. The moisture absorption was found to be 0.34% at 23°C after 23 months, and 0.76% at 60°C after 24 months. At both temperatures, moisture absorption did not reach equilibrium which was attributed to two stages non‐Fickian behavior. Glass transition temperature (T_g) of the polymer matrix of rebar that conditioned at 23°C was found to be decreased because of plasticization, whereas T_g of the rebar that conditioned at 60°C was remained greater than the T_g of control rebar due to nonplasticization effect. Shear strength was retained by 83.5% at 23°C and 80.5% at 60°C, flexural strength was retained by 81% at 23°C and 69% at 60°C, and tensile strength was retained by 91.2% at 23°C and 74.3% at 60°C. It was revealed that durability of GFRP rebar in alkaline environment was controlled by the absorbed moisture; this was because the load transfer efficiency of fiber/matrix interface is vulnerable to moisture. POLYM. COMPOS., 37:3181–3190, 2016. © 2015 Society of Plastics Engineers     

Effect of the talc filler content on the mechanical properties of polypropylene composites

This research examines the effect of a microsize/nanosize talc filler on the physicochemical and mechanical properties of filled polypropylene (108MF10 and 33MBTU from Saudi Basic Industries Corp. and HE125MO grade from Borealis) composite matrices. A range of mechanical properties were measured [tensile properties, bending properties, fracture toughness, notched impact strength (at the ambient temperature and ?20°C), strain at break, and impact strength] along with microhardness testing and thermal stability testing from 40 to 600°C as measured by differential thermal analysis and thermogravimetric analysis. Increasing filler content lead to an increase in the mechanical strength of the composite material with a simultaneous decrease in the fracture toughness. The observed increase in tensile strength ranged from 15 to 25% (the maximum tensile strength at break was found to be 22 MPa). The increase in mechanical strength simultaneously led to a higher brittleness, which was reflected in a decrease in the mean impact strength from the initial 18 kJ/m² (for the virgin polypropylene sample) to 14 kJ/m², that is, a 23% decrease. A similar dependency was also obtained for the samples conditioned at ?20°C (a decrease of 12.5%). With increasing degree of filling of the talc–polypropylene composite matrix, the thermooxidative stability increased; the highest magnitude was obtained for the 20 wt % sample (decomposition temperature = 482°C, cf. 392°C for the virgin polymer). © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Evaluation of Thermal Stability and Glass Transition Temperature of Different Aeronautical Polymeric Composites

In this work, the thermal stability of different polymeric composite laminates used in the aeronautical field was investigated by thermogravimetric (TGA) and dynamic mechanical thermal (DMTA) analyses. Four different types of laminates manufactured by combining two epoxy resin systems (F584 and F155) and two different types of carbon fiber fabric reinforcements were examined. The mass losses of the laminates were evaluated by TGA and the glass transition temperatures (T_g) were determined by DMTA. After being conditioned at 80°C and 90% humidity for a period of 8 weeks, the samples were analyzed by DMTA technique to evaluate the influence of the moisture and the elevated temperature on the T_g of the laminates. TGA results showed that the F584 and F155 epoxy systems are thermally stable until 200°C in nitrogen and in air atmospheres. The DMTA results showed the decreasing of T_g after conditioning for both resin systems. Despite the decrease of T_g, the values found are within the acceptable limits for use in aeronautical components.     

Development and evaluation of a novel pH indicator biodegradable film based on cassava starch

A pH indicator film based on cassava starch plasticized with sucrose and inverted sugar and incorporated with grape and spinach extracts as pH indicator sources (anthocyanin and chlorophyll) has been developed, and its packaging properties have been assessed. A second‐order central composite design (2²) with three central points and four star points was used to evaluate the mechanical properties (tensile strength, tensile strength at break, and elongation at break percentage), moisture barrier, and microstructure of the films, and its potential as a pH indicator packaging. The films were prepared by the casting technique and conditioned under controlled conditions (75% relative humidity and 23°C), at least 4 days before the analyses. The materials were exposed to different pH solutions (0, 2, 7, 10, and 14) and their color parameters (L*, a*, b*, and haze) were measured by transmittance. Grape and spinach extracts have affected the material characterization. Film properties (mechanical properties and moisture barrier) were strongly influenced by extract concentration presenting lower results than for the control. Films containing a higher concentration of grape extract presented a greater color change at different pH's suggesting that anthocyanins are more effective as pH indicators than chlorophyll or the mixture of both extracts. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A Method of Predicting the Stresses in Adhesive Joints after Cyclic Moisture Conditioning

The durability of adhesive joints is of special concern in structural applications and moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying environmental conditions throughout their life. This paper presents a methodology to predict the stresses in adhesive joints under cyclic moisture conditioning. The single lap joints were manufactured from aluminium alloy 2024 T3 and the FM73®-BR127® adhesive-primer system. Experimental determination of the mechanical properties of the adhesive was carried out to measure the effect of moisture uptake on the strength of the adhesive. The experimental results revealed that the tensile strength of the adhesive decreased with increasing moisture content. The failure strength of the single lap joints also progressively degraded with time when conditioned at 50°C, immersed in water; however, most of the joint strength recovered after drying the joints. A novel finite element based methodology, which incorporated moisture history effects, was adopted to determine the stresses in the single lap joints after curing, conditioning, and tensile testing. A significant amount of thermal residual stress was present in the adhesive layer after curing the joints; however, hygroscopic expansion after the absorption of moisture provided some relief from the curing stresses. The finite element model used moisture history dependent mechanical properties to predict the stresses after application of tensile load on the joints. The maximum stresses were observed in the fillet areas in both the conditioned and the dried joints. Study of the stresses revealed that degradation in the strength of the adhesive was the major contributor in the strength loss of the adhesive joints and adhesive strength recovery also resulted in recovered joint strength. The presented methodology is generic in nature and may be used for various joint configurations as well as for other polymers and polymer matrix composites.     

Hygrothermal and Stacking Sequence Effects on Carbon Epoxy Composites with Molded Edges

Moisture and high temperature may cause a variety of effects on the mechanical and thermophysical properties of polymeric composites. The diffusion of moisture in the laminate occurs primarily by capillarity until a state of equilibrium is reached. Moisture affects the matrix and the fiber/matrix interface. The stacking sequence and the free edge finishing may affect the laminate tensile strength causing a free edge delamination; they can also affect the moisture absorption process. Laminates produced with molded edges do not require machining operations for the trimming and finishing of the free edges causing a gain in productivity. However, the mechanical behavior of laminates with molded edges saturated with moisture has not been characterized. This work presents a comparative study of the tensile strength of cross ply laminates with molded and machined edges under dry condition and saturated with moisture.     

Effects of thermal oxidising exposure on the tensile strength of Hi-Nicalon fibres

ABSTRACT

Hi-Nicalon^TM silicon carbide fibre tows were respectively exposed to argon, pure oxygen, and wet oxygen atmosphere at 1300–1500 °C for 1?h. The tensile strength of the Hi-Nicalon fibres decreased dramatically with increasing temperature under all exposure conditions. The most severe deterioration of the tensile strength of the fibres, which occurred under exposure to pure oxygen, because of the synergistic effects of microcracking of silica formed by oxidation and the growth of SiC-nanocrystals at high temperature. The relatively high effective load-bearing area and uniform loading resulting from rapid formation of viscous silica under the wet oxygen environments were considered as reasons for the intermediate deterioration of the tensile strength of the fibres.     

Effects of alternating temperatures and humidity on the moisture absorption and mechanical properties of ramie fiber reinforced phenolic plates

In this article, ramie fiber reinforced phenolic (RFRP) plates were prepared with compression molding process, and the plates were subjected to 98% humidity environment and alternating temperatures (from 25°C to 55°C in 24 h for a cycle) for 4 weeks. The resulted moisture absorption and the variation of the mechanical properties of RFRPs were studied. As found, compared to constant exposure temperatures (25°C or 60°C), alternating temperatures brought in higher moisture uptake and more serious degradation in the flexural strength, flexural modulus and short beam shear strength of the RFRP samples under the same humidity condition. The deteriorated effects of alternating temperatures is attributed to more remarkable degradation of the bonding between the fiber and resin, due to the moisture uptake and the internal cyclic stress around the ramie fibers with alternating temperatures. The flexural modulus of RFRP plates was much more susceptible to the moisture uptake than the flexural strength. After fully drying, the mechanical properties of the RFRP samples were recovered to some extent, but still less than the original values, indicating permanent damages occurred. Fiber Bragg grating sensors embedded in the RFRP plate was applied to monitor the variation of the internal strain during the exposure. As indicated, the moisture absorption and alternating temperatures bring in relaxation of the internal tension stress formed during compressing process, and decrease in the coefficient of thermal expansion of the RFRP samples. POLYM. COMPOS., 36:1590–1596, 2015. © 2014 Society of Plastics Engineers