Heat conduction analysis of laser CFRP processing with IR and UV laser light

To investigate carbon fiber reinforced plastic (CFRP) composite processing, cutting experiments are performed using a Nd:YAG laser. Both ultraviolet (λ = 266 nm) and infrared (λ = 1064 nm) lights are examined to optimize the laser conditions for cutting CFRP. The experimental data are compared to the results calculated by heat conduction models. The good agreement between the experimental and calculated results indicates that the cutting quality depends on the wavelength of the cutting laser.     

Characteristics of aluminum/CFRP short square hollow section beam under transverse quasi-static loading

Energy absorption capability and bending collapse behavior of an aluminum (Al)/carbon fiber reinforced plastic (CFRP) short square hollow section (SHS) beam were investigated under transverse quasi-static loading. The Al SHS beam was reinforced by CFRP, and the specimen was co-cured via an autoclave curing process. Three-point bending test was performed with five different lay-up sequences and three different laminate thicknesses. Stable bending collapse accompanying plastic hinge was observed in all specimens. Individual bending collapse behaviors were different depending on the lay-up sequences. The specific energy absorbed (SEA) was improved by up to 29.6% in the Al/CFRP SHS beam specimen with a [0/+45°/90°/−45°]_n lay-up sequence and laminate thickness of 1.168 mm (thickness ratio of Al: CFRP = 1: 0.87, 8 plies of prepreg) compared to the Al SHS beam. The SEA was not related with damage area of the Al/CFRP SHS beam. Finite element analysis and theoretical analysis based on Kecman’s model were performed to investigate the effect of reinforcement by CFRP on the Al SHS beam.     

Effects of pulsed Nd:YAG laser welding parameters and subsequent post-weld heat treatment on microstructure and hardness of AISI 420 stainless steel

Martensitic stainless steels are often used in cases where high strength and medium corrosion resistance are required. In this study, pulsed Nd:YAG laser welding of AISI 420 martensitic stainless steel is considered. Welding of samples were carried out autogenously. The spacing between samples was set to almost zero. All samples were butt welded. The effect of welding parameters such as voltage, laser beam diameter, frequency, pulse duration, and welding speed on the weld dimensions were investigated and the optimum values were obtained for the 450 V voltage, 0.6 mm focal diameter, 6 Hz frequency, 5 ms pulse duration and 1.5 mm/s welding speed. Microstructure of weld pool and heat affected zone (HAZ) were investigated by optical microscopy (OM) and scanning electron microscopy (SEM). Micro-hardness studies were also carried out. The results showed the presence of some remaining delta-ferrite in the martensitic weld structure and coarsening of M₂₃C₆ carbides in HAZ. The magnitude of hardness in the HAZ was higher than that of the weld zone. To reduce the hardness of weld and HAZ and to increase the toughness in these regions, two types of post-weld heat treatments (PWHTs) were carried out. In type 1, samples tempered for 2 h. In type 2, samples austenitizied for 0.5 h at 1010 °C and then tempered for 2 h. In order to achieve high strength and toughness, optimum temper temperatures for type 1 and 2 heat treatments were obtained for 595 and 537 °C, respectively. The results showed higher toughness for type 2 than type 1.     

Investigation of laser pulse fatigue effect on unpainted and painted CFRP structures

Laser ultrasonic based nondestructive evaluation (NDE) techniques are being increasingly used in aerospace industries. Generally, the service lifetime for an aircraft could be more than 25 years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy in the long-term repetitive maintenance inspection. In this paper, the effect of repeat laser pulse scanning on the mechanical characteristics of unpainted and painted CFRP specimens (USN175BX Carbon UD prepreg, Bisphenol A epoxy resin) is investigated to verify the reliability regarding the use of laser ultrasonic scanning based NDE techniques on the inspection of the CFRP structure. A high-speed laser ultrasonic scanning system is setup to perform repeat scanning of 1300 times on both CFRP specimens at the five laser pulse energy levels using the 532-nm and 1064-nm Q-switched continuous wave lasers. Elastic modulus assessment based on the ultrasonic Lamb wave pitch–catch method is used and the surface condition of the scanned area is investigated by a microscope. As a result, the laser pulse fluences that is shown in this paper are suitable for the long-term repetitive maintenance inspection in unpainted and painted CFRP structure even if it demonstrates an embrittlement phenomenon similar to the modulus measurement resolution in the unpainted CFRP specimen. In addition, the laser pulse fluence for maximum signal-to-noise ratio without any damage is investigated in both unpainted and painted CFRP specimens. As a result, both 102.45 mJ/cm² in unpainted CFRP specimen and 51 mJ/cm² in painted CFRP specimen can be the laser pulse energy for the maximum signal-to-noise ratio without any damage.     

Experimental study on the CO2 laser cutting of carbon fiber reinforced plastic composite

Promotion of massive application of carbon fiber reinforced plastics (CFRPs) in the industry can be accomplished by using faster and more flexible technologies such as laser cutting. The anisotropic and heterogeneous features of the CFRP make laser processing very challenging.A comprehensive study on the cut performance of a CO₂ laser to process sheets (3 mm thick) of a CFRP composite is presented. A high-beam quality CO₂ laser has been used in order to ascertain the capabilities of CO₂ laser cutting machines, widely used in metalworking applications, on the machining of this material. On the other hand, the influence of processing parameters, in both CW and pulsed mode, on the cut quality was studied.Cuts with a minimum heat affected zone, about 540 μm, were achieved using a high-beam quality CO₂ laser working in pulsed mode. In consequence, the CFRP strength remains practically unaffected compared to more conventional mechanical machining.     

Mechanical behaviour of corroded RC beams strengthened by NSM CFRP rods

Corrosion of steel in reinforced concrete leads to several major defects. Firstly, a reduction in the cross-sectional area of the reinforcement and in its ductility results in premature bar failure. Secondly, the expansion of the corrosion products causes concrete cracking and steel–concrete bond deterioration and also affects the bending stiffness of the reinforced concrete members, causing a reduction in the overall load-bearing capacity of the reinforced concrete beams. This paper investigates the validity of a repair technique using Near Surface Mounted (NSM) carbon-fibre-reinforced polymer (CFRP) rods to restore the mechanical performance of corrosion-damaged RC beams. In the NSM technique, the CFRP rods are placed inside pre-cut grooves and are bonded to the concrete with epoxy adhesive.Experimental results were obtained on two beams: a corroded beam that had been exposed to natural corrosion for 25 years and a control beam, (both are 3 m long) repaired in bending only. Each beam was repaired with one 6-mm-diameter NSM CFRP rod. The beams were tested in a three-point bending test up to failure. Overall stiffness and crack maps were studied before and after the repair. Ultimate capacity, ductility and failure mode were also reviewed. Finally some comparisons were made between repaired and non-repaired beams in order to assess the effectiveness of the NSM technique. The experimental results showed that the NSM technique improved the overall characteristics (ultimate load capacity and stiffness) of the control and corroded beams and allowed sufficient ductility to be restored to the repaired corroded elements, thus restoring the safety margin, despite the non-classical mode of failure that occurred in the corroded beam, with the separation of the concrete cover due to corrosion products.     

The effect of processing temperature and placement rate on the short beam strength of carbon fibre–PEEK manufactured using a laser tape placement process

The ability of a modern near infra-red laser tape placement system to produce high-quality laminates is investigated by performing short beam strength tests on samples manufactured at different process temperatures from 400 °C to 600 °C at placement rates of 100 mm/s and 400 mm/s. The temperature history in tape placement is highly dynamic and the correlation between the process control temperature, laser power and the consolidation temperature is not well understood. The complete temperature history was therefore estimated with a previously developed optical-thermal model and validated using long wave infra-red imaging. Short beam strengths equivalent to conventional manufacturing methods were found for placement rates of 400 mm/s. Failure modes of the samples were elucidated by scanning electron microscopy of the fracture surfaces. Signs of degradation were observed on samples prepared with a 600 °C process temperature at 100 mm/s, however none was evidenced at 400 mm/s for the same process temperature.     

Defect detection in CFRP by infrared thermography with CO2 Laser excitation compared to conventional lock-in infrared thermography

This paper presents a NDT by a CO₂ Laser infrared thermography applied to defect detection in CFRP. The CO₂ Laser is an infrared laser with the wavelength of 10.6 μm. This excitation has a controllable heating beam by a geometric relation D = 0.01575·d, which allows to heat the samples at a specific position (placed at the distance “d”) and area (of a diameter “D”). The PPT interpretation principle was used to reduce the non-uniformity’s effect of the excitation causing inhomogeneous heat. The test with this excitation is much faster than the tests with conventional lock-in thermography method.     

Microstructure of selective laser melted nickel–titanium

In selective laser melting, the layer-wise local melting of metallic powder by means of a scanning focused laser beam leads to anisotropic microstructures, which reflect the pathway of the laser beam. We studied the impact of laser power, scanning speed, and laser path onto the microstructure of NiTi cylinders. Here, we varied the laser power from 56 to 100 W and the scanning speed from about 100 to 300 mm/s. In increasing the laser power, the grain width and length increased from (33 ± 7) to (90 ± 15) μm and from (60 ± 20) to (600 ± 200) μm, respectively. Also, the grain size distribution changed from uni- to bimodal. Ostwald-ripening of the crystallites explains the distinct bimodal size distributions. Decreasing the scanning speed did not alter the microstructure but led to increased phase transformation temperatures of up to 40 K. This was experimentally determined using differential scanning calorimetry and explained as a result of preferential nickel evaporation during the fabrication process. During selective laser melting of the NiTi shape memory alloy, the control of scanning speed allows restricted changes of the transformation temperatures, whereas controlling the laser power and scanning path enables us to tailor the microstructure, i.e. the crystallite shapes and arrangement, the extent of the preferred crystallographic orientation and the grain size distribution.     

Laser welding of AZ31B magnesium alloy to Zn-coated steel

The characteristics of laser lap welding of AZ31B magnesium alloy to Zn-coated steel were investigated. Welding was difficult when the laser beam was irradiated onto the AZ31B alloy and the processing parameters were set to obtain a keyhole welding mode. The difference in the physical properties between the two materials resulted in unstable welding process particularly when the laser beam penetrated into the steel specimen and a keyhole was formed therein. By switching to a conduction mode, the process stability was improved and successful welding could be achieved because the liquid metal film remained unbroken and the laser beam did not penetrate into the material. A 25 mm wide joint failed in tensile shear testing at loads exceeding 6000 N. This high joint strength was attributed to the formation of a 450 nm thick layer of Fe₃Al intermetallic compound on the steel surface as a result of the interaction between Al from the AZ31B alloy and Fe. The presence of Zn-coating layer was essential to eliminate the negative effects of oxides on the joining process.     

An investigation of contact resistance between carbon fiber/epoxy composite laminate and printed silver electrode for damage monitoring

An addressable conducting network (ACN) enables the structural condition to be monitored by the electrical resistance between electrodes on surface of CFRP (carbon fiber reinforced polymer) structure. To improve the reliability of ACN for damage detection, the contact resistance between the electrodes and CFRP laminates needs to be minimized. In this paper, the silver nanoparticles electrodes were fabricated via printed electronics techniques on CFRP composite. The contact resistance between the silver electrodes and CFRP was measured with respect to various fabrication conditions such as the sintering temperature of silver nanoink and the surface roughness of CFRP laminates. The interfaces between silver electrode and carbon fibers were observed using scanning electron microscope (SEM). From the study, it was found that the lowest contact resistance of 0.3664 Ω could be achieved when the sintering temperature of the silver nanoink and surface roughness were 120 °C and 230 nm, respectively.     

Recycling of carbon fibers from carbon fiber reinforced polymer using electrochemical method

In this research, we proposed an electrochemical method for the recycling of carbon fibers from carbon fiber reinforced polymer (CFRP). Experiments were designed with different solution concentrations (3%, 10%, and 20% NaCl) and various levels of applied current (4 mA, 10 mA, 20 mA, and 25 mA) so as to identify the significant parameters that affect carbon fiber recycling efficiency. The recycled carbon fibers were characterized by using the single fiber tensile strength test, SEM, XRD, and XPS techniques. Test results showed that the maximum tensile strength of the reclaimed carbon fibers was 80% of the virgin carbon fibers (VCF). The increase in electrolyte concentration did not improve the recycling efficiency but resulted in severe oxidation and chlorination on the surface of recycled carbon fibers. From the experimental results, it can be concluded that the recycling of carbon fibers with electrochemical method is simple, effective, and economical.     

Fabrication,process simulation and testing of a thick CFRP component using the RTM process

The present article introduces the case of a CFRP con-rod beam, and describes many aspects regarding its production with the Resin Transfer Moulding (RTM) process.The objective was to find the best process parameters of the injection and curing stages in order to manufacture the 20 mm thick CFRP part. The results are analysed in terms of the aesthetic aspect, the porosity and the mechanical properties of the final component.For the resin injection stage, results obtained from production experiences are presented, which have been performed with different set-ups, and simulations of the resin flow are used to analyse them. The results show that the resin flow during injection could be rather unpredictable, probably because of the fibre rearrangement and race tracking effects. Improvements in terms of aesthetic aspect and porosity of the part could be achieved by a process which included final compaction of the cavity by means of compressed air.Regarding the curing stage, the article presents the simulation results of a curing cycle, and it’s validation through DSC analysis of specimens obtained from the finished component.Finally, results of tensile mechanical tests are provided, performed on finished components produced by RTM and compared to others produced with the method of hand lay-up of pre-impregnated plies and curing in autoclave (Prepreg + Autoclave). The results confirm that it is possible to achieve components through RTM with comparable mechanical performance to those produced with the Prepreg + Autoclave process.     

Microstructures and mechanical property of laser butt welding of titanium alloy to stainless steel

Laser butt welding of titanium alloy to stainless steel was performed. The effect of laser-beam offsetting on microstructural characteristics and fracture behavior of the joint was investigated. It was found that when the laser beam is offset toward the stainless steel side, it results in a more durable joint. The intermetallic compounds have a uniform thickness along the interface and can be divided into two layers. One consists of FeTi + α-Ti, and other consists of FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅. When laser beam is offset by 0 mm and 0.3 mm toward the titanium alloy side, the joints fracture spontaneously after welding. Durable joining is achieved only when the laser beam is offset by 0.6 mm toward the titanium alloy. From the top to the bottom of the joint, the thickness of intermetallic compounds continuously decreases and the following interfacial structures are found: FeAl + α-Ti/Fe₂Ti + Ti₅Fe₁₇Cr₅, FeAl + α-Ti/FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅ and FeAl + α-Ti, in that order. The tensile strength of the joint is higher when the laser beam is offset toward the stainless steel than toward the titanium alloy, the highest observed value being 150 MPa. The fracture of the joint occurs along the interface between two adjacent intermetallic layers.     

Improvement of mechanical properties of stainless maraging steel laser weldments by post-weld ageing treatments

The response of stainless maraging steel weldments to post-weld ageing treatment has been investigated. Post-weld ageing was performed at five different temperatures, viz., 420 °C, 460 °C, 500 °C, 540 °C, and 580 °C. Metallographic characterization of weldment revealed three zones, namely fusion zone, heat-affected zone (HAZ) and unaffected parent metal zone. Hardness and tensile properties were evaluated after ageing at different temperatures. Hardness in HAZ and fusion zone varied with ageing temperature differently from that of the parent metal; it became higher in HAZ and fusion zone than in parent metal zone above 420 °C. Among the applied ageing treatments, ageing at 460 °C achieved the highest tensile strength. A graph was constructed for determination of fracture location and post-weld heat treatment efficiency based on experimental results, using hardness ratio of HAZ to the treated parent material and hardness ratio of HAZ to the as-received parent material.     

The effects of interfacial adhesion strength on the characteristics of an aluminum/CFRP hybrid beam under transverse quasi-static loading

The effects of interfacial adhesion strength on the damage behavior and energy absorption characteristics of an aluminum (Al)/carbon fiber reinforced plastic (CFRP) short square hollow section (SHS) beam under three point bending loading was investigated. An Al SHS beam was wrapped by CFRP with a [0°/+45°/90°/−45°]_n (n = 1 or 2) stacking sequence, and four gradations of interfacial adhesion strength were caused by physical or chemical changes of the Al adherend with different mechanical abrasion and optimal Argon plasma treatment. A different level of appropriate interfacial adhesion strength existed for each hybrid specimen depending on the CFRP laminate thickness to obtain the highest energy absorption capability, and this was verified by detailed observation of the failure mechanism of the hybrid specimen. The specific energy absorbed (SEA) was improved by up to 57.2% in the Al/CFRP [0°/+45°/90°/−45°]₂ SHS beam compared to the Al SHS beam without compromising the crush force efficiency (CFE).     

Repair of shear-deficient normal weight concrete beams damaged by thermal shock using advanced composite materials

The use of advanced composite materials such as Fiber Reinforced Polymers (FRPs) in repairing and strengthening reinforced concrete structural elements has been increased in the last two decades. Repairing and strengthening damage structures is a relatively new technique. The aims of this study was to investigate the efficiency and effectiveness of using Carbon Fiber Reinforced Polymer (CFRP) to regain shear capacity of shear-deficient normal weight high strength RC beams after being damaged by thermal shock. Sixteen high strength normal weight RC beams (100 × 150 × 1400 mm) were cast, heated at 500 °C for 2 h and then cooled rapidly by immersion in water, repaired, and then tested under four-point loading until failure. The composite materials used are carbon fiber reinforced polymer plates and sheets. The experimental results indicated that upon heating then cooling rapidly, the reinforced concrete (RC) beams exhibited extensive map cracking without spalling. Load carrying capacity and stiffness of RC beams decreased about 68% and 64%, respectively, as compared with reference beams. Repairing the thermal damaged RC beams allowed recovering the original load carrying without achieving the original stiffness. Repaired beams with CFRP plates with 90° and 45° regained from 90% to 99% of the original load capacity with a corresponding stiffness from 79% to 95%, whereas those repaired with CFRP sheet on the web sides and a combination of CFRP plates and sheet regained from 102% to 107% of the original load capacity with a corresponding stiffness from 81% to 93%, respectively. Finally, finite element analysis model is developed and validated with the experimental results. The finite element analysis showed good agreement as compared with the experimental results in terms of load–deflection and load–CFRP strain curves.     

Optimization of Nd:YAG laser welding parameters for sealing small titanium tube ends

The purpose of the present study is to optimize Nd:YAG laser welding parameters to seal an iodine-125 radioisotope seed into a titanium capsule. If the end of a small titanium tube is irradiated to a Nd:YAG laser beam and melted down to the adequate length, it will be coalesced and sealed. The accurate control of the melted length of the tube end was the most important to obtain a sound sealed state. The effects of the laser welding parameters on the melted length were analyzed and optimized by the Taguchi and regression analysis method. The laser pulse width and focal position among the welding parameters had the greatest effects on the S/N ratio of the melted length. Optimal welding conditions were obtained at 0.86 ms of the pulse width and 3.18–3.35 mm of the focal position in the scope of the experiments. Confirmation experiments were conducted at the optimal welding conditions, and showed that both of the titanium tube ends were sealed soundly.     

Nd:YAG laser drilling in epoxy resin/AlN composites material

The Nd:YAG laser drilling performance of blind vias in epoxy/aluminium nitride (AlN) composite and virgin epoxy is systemically studied through varying the average laser power, the repetition times, and the pulse repetition rate from 0.2 W to 1.2 W, 12 to 48 and 3 KHz to 20 KHz, respectively. The results show that compared with pure epoxy, the drilling conditions of blind vias in epoxy/AlN composite can be changed in a wide range, with no residue found significantly at the surface of the entrance of the blind vias. The influence of the repetition rate is more evident among the parameters of the Nd:YAG laser, such as the average laser power, laser repetition rate, and repetition times, and should be carefully controlled.     

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.