Monitoring of Impacted Aramid-Reinforced Composites by Embedded PVDF Acoustic Emission Sensors

Abstract:  An embedded piezoelectric [poly(vinylidene fluoride) (PVDF)] thin film sensors system for acoustic emission (AE) was realized to investigate the possibility of monitoring, in real time, the post-impact damage in aramid woven fabric-reinforced epoxy. The same sensors have been used in a previous work on similar specimens tested in flexure but not previously impacted, with the aim of verifying the suitability of these sensors to be embedded and their ability to detect AE signals under loading. This work is a continuation of the previous one aiming at evaluating the ability of these embedded PVDF sensors to point out the presence of impact damage, issue widely studied in literature. Aramid fibre/epoxy composite specimens with embedded PVDFs, previously impacted at different energies, namely 5, 10 and 15 J, were tested using three-point bending tests. It appeared from mechanical tests that the flexural strength decreased passing from non-impacted specimens to those impacted with the highest energy and that the embedment of PVDFs in the laminates did not markedly affect the structural integrity of the impacted composites. The degree of impact damage, represented by the decrease in mechanical properties, has been correlated with the AE activity by means of a parametric analysis of the AE signals detected during post-impact mechanical tests.     

Residual strength distribution of impacted glass/epoxy laminates with embedded shape memory alloy at low temperature

This paper evaluated the strength reduction and probabilistic behaviors of the residual flexural strength for impacted glass/epoxy laminates with embedded shape memory alloy (SMA) wires at various temperatures. A series of impact tests were performed on base (glass/epoxy laminates without SMA wires) and SMA laminates (glass/epoxy laminates with embedded SMA wires) at temperatures of 293 K, 263 K and 233 K. Three point flexural tests were then carried out so as to investigate the post-impact strength at the aforementioned temperatures. Strength reduction behavior of impacted laminates could be described by Caprino’s residual strength prediction model. A probabilistic model was developed in order to estimate the variation in residual strength of the impacted laminates with temperature. As the temperature decreased, the variation in residual strength increased due to the embrittlement of the constituent materials of the laminates at lower temperatures. When compared to the base laminates, the SMA laminates exhibited a higher variation in residual strength, especially at lower temperatures.     

Cure cycle effect on impact resistance under elevated temperatures in carbon prepreg laminates investigated using acoustic emission

Voids in composites have been a perennial problem, since the amount of porosities is deemed to bear a strong relation with the degradation of service performance of laminates. On the other side, the effect of porosity on impact resistance of laminates is often dependant on their distribution in the material, especially with respect to the location and severity of impact damage in it. In this study, the influence of void content percentage on the residual flexural strength of CFRP laminates impacted at very low energy, in the region of 1 J, at different temperatures was investigated. Laminates were fabricated using 16 layers of Cycom 977-2 prepreg material in a [0/90/90/0]_2S layup with different void contents in the range from 1 to 7% by varying cure conditions. Low velocity impact tests were conducted on three categories of laminates, defined as high pressure cured, low pressure debulked and low pressure non debulked respectively, each of these at ambient temperature (30 °C) and elevated temperatures of 50, 70 and 90 °C. Post-impact residual flexural strength of the laminates was measured by three-point bending tests followed in real time by acoustic emission (AE) monitoring. From the separation of frequency bands and of their amplitude levels, identification of the different failure modes, such as matrix cracking, delamination and fibre failure, was performed. The results indicated that in the case of very low impact energy high porosity laminates, such as non debulked ones, may possess slightly higher residual flexural strength and an enhanced delamination resistance with respect to debulked ones.     

Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour of Steel‐Fibre Reinforced Concrete

Abstract: This work aims in studying the mechanical behaviour of concrete, reinforced with steel fibres of different geometry and volume fraction. Experiments include compression tests and four‐point bending tests. Slump and air content tests were performed on fresh concrete. The flexural toughness, flexural strength and residual strength factors of the beam specimens were evaluated in accordance with ASTM C1609/C1609M‐05 standard. Improvement in the mechanical properties, in particular the toughness, was observed with the increase of the volume fraction of steel‐fibres in the concrete. The fibre geometry was found to be a key factor affecting the mechanical performance of the material.     

Synergistic effect of cyclic mechanical loading and moisture absorption on the bending fatigue performance of carbon/epoxy composites

This study investigates the effects of hygrothermal condition on the static bending strength, the bending fatigue and the residual bending strength of carbon/epoxy composite laminates. Displacement-controlled three-point bending fatigue tests were conducted on carbon/epoxy composite laminates of immersion for 0, 7 and 14 days, respectively. After 40000 cycles the fatigue test was stopped and the residual properties were measured on the tested specimens. The effects of hygrothermal condition and fatigue on the micrographs of the specimens have been studied by the metallurgical microscope and scanning electron microscope. Experimental results reveal that moisture absorption can accelerate damage propagation of the composite; the accumulation of irreversible structural damage under the cyclic loading leads to a change in the macroscopic mechanical properties of the composites; the bending strength and the residual strength retention decreased with increased immersion time; hygrothermal aging lowered the threshold level for the onset of fatigue.     

Effect of temperature on low velocity impact damage and post-impact flexural strength of CFRP assessed using ultrasonic C-scan and micro-focus computed tomography

The effect of temperature on the low velocity impact resistance properties and on the post-impact flexural performance of CFRP laminates were studied. With this aim, 150 × 75 mm cross-ply carbon fibre/epoxy laminates with a [0/90/90/0]_2s layup, therefore with a total of sixteen layers, were impacted at ambient temperature (30 °C) and at elevated temperatures (55, 75 and 90 °C) at a velocity of 2 m/s using a drop weight impact tower. This was followed by flexural tests carried out at ambient temperature using a three-point bending rig. Damage assessment of impact and post-impact behaviour were carried out using ultrasonic C-scan and microfocus X-ray computed tomography (μCT). Interrupted flexural tests using μCT allowed delamination propagation to be observed. In general, lower projected damage was observed at elevated temperatures, which resulted also in a possible hindrance to delamination and shear cracks propagation during impact and in a greater amount of retained flexural strength after impact.     

Influence of pressurization on flexural strength distributions of PMMA-based bone cements

Flexural strength distributions of standard viscosity and low viscosity bone cements based on Polymethylmethacrylate were obtained by testing the materials in four-point bending according to the ISO 5833 protocol. The cement dough was poured into a mold and was allowed to cure at atmospheric pressure. An additional set of specimens of the standard viscosity cement was prepared under pressure while the cement dough was polymerizing in the mold. Following preparation, test specimens were stored in a 37 °C water bath for 48 h. The two-parameter Weibull model, which was used to analyze the data, gave a good representation of the fracture loads distribution. Low viscosity cement displayed a higher mean flexural strength and a slightly lower data scatter than standard viscosity cement. The mean flexural strength of the cement increased about 60% when pressure was applied compared with the same material cured at atmospheric pressure. The Weibull modulus, m, characterizes the scattering in the measured values of strength. For the cement prepared at atmospheric pressure the m value was 8.6 while for the cement cured under pressure it was 12.3, which reveals a reduction in the data scatter. The cement tested in four-point bending displayed lower mean flexural strength compared with the cement tested in three-point bending. The influence of the load type upon the mean flexural strength was satisfactory predicted by Weibull model.     

Qualitative separation of the effect of voids on the bending fatigue performance of hygrothermal conditioned carbon/epoxy composites

This study investigates the effect of voids on the static bending and bending fatigue properties of T300/914 composite laminates that are exposed to room temperature, hygrothermal and drying environment, respectively. Displacement-controlled three-point bending fatigue tests were conducted on specimens, while damage in the composite was continuously recorded with a metallurgical microscope. After 40,000 cycles the fatigue test was stopped and residual properties were measured on the tested specimens. Reduction in material strength was found to depend on the level of the specimen’s void content. The changes in weight and size gradually increased with an increase in porosity from 0.33% to 1.50% in the hygrothermal environment. It is found that the maximum rate of dimensional change occurred in the thickness direction. Both bending strength and fatigue performance were reduced with increasing porosity. The damage evaluation of aged specimens was more severe than non-aged and drying specimens.     

Experimental study of creep-damage coupling in concrete by acoustic emission technique

In order to design reliable concrete structures, prediction of long term behaviour of concrete is important by considering a coupling between creep and damage. An experimental investigation on the fracture properties of concrete beams submitted to creep bending tests with high levels of sustained load is reported. The influence of creep on the residual capacity and the fracture energy of concrete is studied. The progression of fracture is followed by the measurement of the crack mouth opening displacement during a three-point bending test. The sustained loading seems to increase the flexural strength of concrete, probably because of the consolidation of the hardened cement paste. The acoustic emission (AE) technique is used to perform the characterization of the influence of creep on the crack development. Results give wealth information on the fracture process zone (FPZ) and the propagation of the crack. A decrease in the amplitude distribution of AE hits is observed in the post-peak region for creep specimens. The width of the FPZ also decreases in this later indicating that the material has a more brittle behaviour which may be due to the development of microcracking under creep and the prestressing of the upper zone of the beam.     

Temperature Dependence of Flexural Strength of a CF-SMC Composite

This paper presents the temperature dependence of predicting the flexural strength of a carbon fiber-reinforced sheet molding compound (CF-SMC). First, three-point flexural tests were performed to measure strength and elastic modulus at a variety of temperatures for CF-SMC specimens. Next, simple equations for predicting flexural strength were derived based on the fracture mechanics approach. The predicted flexural strength was in reasonably good agreement with the experiment results. The scatter of flexural strength was ascribed to the variation of location and size of the initial damage. In addition, the effect of temperature on flexural strength and delamination behavior was explained in association with the temperature dependence of the elastic modulus and fracture toughness.     

SIZE EFFECTS ON IMPACT RESPONSE OF COMPOSITE LAMINATES

Delamination was known to be one of the most important damage modes in composite laminates subjected to impact loading. In an effort to further understand the impact response of composite laminates, various degrees of impact ranging from subperforation to perforation were introduced to glass/epoxy laminates through an instrumented drop-weight impactor. In addition, composite laminates of various in-plane dimensions and thicknesses were examined for in-plane dimensional and thickness effects, respectively. Experimental results showed that in-plane dimensional effect was not as significant as thickness effect. The impacted composite laminates were then subjected to compression after impact (CAI) tests for characterizations of residual mechanical properties. Experimental results showed that perforation was the most important damage stage in composite laminates subjected to impact loading since impact characteristics (peak force, contact duration and absorbed energy) and mechanical properties degradation (residual compressive maximum force and residual compressive absorbed energy) of composite laminates became stable once perforation took place. However, it was also found that delamination played a very important role in the characterizations of mechanical properties degradation. Since the impact response of composite laminates is due to plate bending to some extent, bending analysis was used to explain the greater influence of thickness effect to in-plane dimensional effect. It was also found that bending analysis was feasible for interpretation of delamination in mechanical properties degradation.     

A comparative study of the fatigue and post-fatigue behavior of carbon–glass/epoxy hybrid RTM and hand lay-up composites

The paper presents a study of the fatigue and post-fatigue behavior of a hybrid carbon–glass biaxial fabric reinforced epoxy composite manufactured by the resin transfer molding (RTM) and the hand lay-up (HL) processes, with the main objective of assessing whether a material characterization run at the prototype level of a handicraft technology could be significant for a mass production technology and whether a comparison on static properties (a viable task at an industrial level) could ensure the same level of agreement for the fatigue life and residual properties. Tensile and flexural static tests as well as displacement-controlled bending fatigue tests (R ratio of 0.10) were conducted on two sets of standard specimens, having fiber orientation parallel to the loading direction (on-axis specimens) and at 45° to the loading direction (off-axis specimens). Specimens were subjected to different fatigue loading, with the maximum load level up to 60% of the average ultimate flexural strength, and damage in the laminate was continuously monitored through the loss of bending moment during cycling. After 10⁶ cycles, the fatigue test was stopped and residual properties were measured. Micrographs of sample sections revealed some voidage for HL specimens while resin rich areas were observed for RTM specimens. Results of the static tensile and flexural tests pointed out lower mechanical properties for the RTM specimens when tested on-axis and slightly higher properties when tested off-axis. Regardless of specimen fiber orientation, the fatigue and post-fatigue performance of RTM samples was inferior to that of HL specimens with the gap increasing for increasing fatigue load levels. The result was ascribed to the presence in RTM samples of resin-rich areas, which are reported to have limited influence on the laminate static properties but which may act as initiation sites for fatigue cracks.     

The behaviour of boron-aluminium composites during and after impact

The behaviour of two types of aluminium alloy (1200 and 2024) reinforced unidirectionally by boron fibres has been studied and the effect of an impact on the residual properties and under fatigue conditions determined. Spherical and cylindrical indenters have been used over a range of speeds covering six orders of magnitude. It is found that the fibres fail due to bending induced by the impact, but that the matrix surrounding the breaks is not necessarily cracked. The tensile strengths of impacted specimens have been compared to strengths of similar specimens containing a partial surface notch, simulating the impact damage, and good agreement found. Impacted specimens have been tested in circular bending fatigue on a machine which has been developed so as to maintain the maximum applied load constant despite any change of specimen compliance. The matrix bridge is quickly broken under cyclic loading and the B-2024 specimens show signs of progressive fibre damage. TheS-N curve for the B-1200 specimen is much flatter due to the inability of the softer matrix to transmit high stresses to fibres neighbouring fibre breaks. The residual tensile strength of impacted boronaluminium is found to depend on the remaining intact fibres and not on the matrix. In fatigue the notch effect produced by the damage zone is reduced at long lifetimes as cracking of the matrix parallel to the fibres isolate the damaged region.     

复合材料夹芯板低速冲击后弯曲及横向静压特性

对低速冲击后的复合材料Nomex 蜂窝夹芯板进行了纯弯曲和准静态横向压缩实验, 用X 光技术、热揭层技术和外观检测等对板内的损伤进行测量, 分析了被冲击面在受压情况下蜂窝夹芯板的弯曲破坏特点, 对比了横向静压与低速冲击所造成的板内损伤, 讨论了不同横向压缩速度时接触力P-压入位移$h 的变化规律和损伤情况。结果表明: 低速冲击可使蜂窝夹芯板的弯曲强度大幅度降低; Nomex 蜂窝夹芯板对低速冲击不敏感。      

Comparison between “standard” fracture toughness results and surface flaw data for silicon carbide

Stress intensity factors were measured for single-edge notched silicon carbide specimens subjected to three-point bending. These data are compared with experimental results from surface flawed specimens subjected to three-point bending. Surface flawed specimens were tested since they resemble defects in structures more closely than any other specimen. The ability to estimate failure conditions for a structural component using measurements of fracture toughness and appropriate equations was evaluated. In many of these tests, acoustic emission and moire interferometry techniques provided valuable information on the crack growth process, constitutive damage zone propagation, and the critical stress intensity factors.     

Determination of mechanical behavior of natural fibre based hybrid composites experimentally

This paper deals with the production and the mechanical testing of natural fibre‐based hybrid composites. Hybrid composite is produced by vacuum assisted resin infusion method of woven jute fabric and nonwoven wool felt along with glass fabric. Tensile, flexure and impact properties have been evaluated and compared. The tensile strength and the Young's modulus were found to be 70.66 MPa and is 5.63 GPa, respectively. Flexure and impact tests were performed on both woven jute fabric surface and on nonwoven wool felt surface of the composite specimens. The highest flexural strength was obtained in the specimen loaded at the woven jute fabric surface. Impact test results show that the specimens impacted at the nonwoven wool felt surface were carrying higher loads than the specimens impacted at the woven jute fabric surface.     

Behavior of unreinforced masonry prisms and beams retrofitted with engineered cementitious composites

The impact of a thin layer of a ductile fiber-reinforced concrete referred to as engineered cementitious composites (ECC) on unreinforced masonry (URM) prisms and beams has been evaluated. The objective of the research was to characterize the performance and potential benefits of using ECC to retrofit URM with eventual application to masonry infill walls in non-ductile reinforced concrete frames. Compression tests of masonry prisms and flexural tests of masonry beams with different ECC retrofit schemes were conducted. The variables studied were the use of wall anchors to improve the ECC-masonry bond and alternate steel reinforcement ratios within the ECC layer in the form of welded wire fabric. The ECC retrofit was found to increase the strength and stiffness of URM prisms by 45 and 53 %, respectively compared to those of a plain specimen. When wall anchors were installed on the masonry specimens, the bond between the ECC layer and the masonry surface was improved. Four-point bending tests indicated that the strength and more importantly the ductility of an ECC retrofitted brick beam are increased significantly, especially when light reinforcement is added to the ECC layer, relative to an URM beam. Analytical models for estimating the strength and stiffness of ECC retrofitted masonry specimens are proposed and evaluated.     

Post-Impact Mechanical Characterisation of Glass and Basalt Woven Fabric Laminates

Two woven fabric laminates, one based on basalt fibres, the other on E-glass fibres, as a reinforcement for vinylester matrix, were compared in terms of their post-impact performance. With this aim, first the non-impacted specimens were subjected to interlaminar shear stress and flexural tests, then flexural tests were repeated on laminates impacted using a falling weight tower at three impact energies (7.5, 15 and 22.5J). Tests were monitored using acoustic emission analysis of signal distribution with load and with distance from the impact point. The results show that the materials have a similar damage tolerance to impact and also their post-impact residual properties after impact do not differ much, with a slight superiority for basalt fibre reinforced laminates. The principal difference is represented by the presence of a more extended delamination area on E-glass fibre reinforced laminates than on basalt fibre reinforced ones.     

Effect of excessive bending on residual tensile strength of hybrid composite rods

Excessive bending has been identified as a concern for the hybrid composite core that is currently being used as the structural member for the Aluminum Conductor Composite Core Trapezoidal Wire (ACCC/TW™) transmission line. In this work the flexure strength of the ACCC core was measured in a series of four point bend tests while monitoring acoustic emissions. To quantify the stress state within the rods and to evaluate its flexure strength, an analytic solution for the bending stress was derived and numerically verified using the finite element method. In the second part of the study several specimens that had been subjected to excessive bending were subsequently tested for their residual tensile strength. It was found that wrapping the ACCC core around a 1 m mandrel, which is a common loading condition in practice, will not generate significant structural damage in the composite core. It was determined that the diameter of the mandrel that would cause failure of the composite core is 467 mm. From this work it was found that excessive bending, up to 90% of the flexural strength of the ACCC core, had no detrimental effect on the residual tensile strength of the hybrid composite. It was observed that the majority of the micro-structural damage that was accrued during the excessive bending of the cores presented itself in the form of matrix damage without any significant fiber kinking.     

The compression-after-impact strength of woven and non-crimp fabric reinforced composites subjected to long-term water immersion ageing

The current work examines the durability of composites reinforced with glass fibre woven fabric as well as non-crimp fabrics (NCF) immersed in water at 43, 65 and 93 °C for up to 2.5 years. Low velocity normal impact has been induced at various time intervals before and after water immersion at energy levels of 2.5, 5 and 10 J. Following impact the plates were tested statically in compression to determine the residual strength for assessment of damage tolerance. The compression strength suffered significant reductions from the water absorption and the low velocity impact with values being dependent to the time of immersion and the water temperature. A parallel behaviour was monitored, in terms of strength reduction over time, of plates impacted prior to water immersion with the plates that contain no damage. For specimens where impact damage introduced after water immersion lower compression-after-impact (CAI) strength was observed at the same energy levels. An increase in damage diameter was evident, regardless the reinforcement type, though the gradually produced greater density of through thickness damage was responsible for the significant lower compression strength values. The presence of 0° fibres for the NCF composites as the main load bearing element dictated the sensitivity to impact as well as the corresponding residual strength. For composites with woven reinforcement, damage was contained and localized by the fabric weave and effective stress redistribution seemed to be the mechanism for the relatively higher residual strengths obtained. A semi-empirical model has been used with high accuracy in fitting the given experimental data and draw conclusions from the comparisons.