However, studies on the effect of nano-reinforcements in repeated impact scenarios are relatively limited. This work investigates the effect of resin nanoclay modification on the impact resistance of glass-fiber reinforced polymer (GFRP) composites subjected to repeated impacts. Three impact energy levels were used in experiments with a minimum of four specimens per case for statistical significance. Each sample was subjected to 40 repeated impacts or was tested up to perforation, whichever happened first. The impact response was evaluated in terms of evolution of the peak force, bending stiffness, visual damage inspection and optical transmission scanning (OTS) at critical stages as a function of number of impacts. Also, the damage degree (DD) was calculated to monitor the evolution of damage in the laminates. As expected, the impact response of the GFRP composites varied based on the presence of nano-clay and the applied impact energy. The modification of the resin with nano-clay introduced novel phenomena that changed the damage progression mechanism under repetitive impacts, which was verified by visual observation and optical transmission scanning. A better understanding of these phenomena (e.g. crack-bridging, tortuosity) and their contributions to enhancements in the impact behavior and modifications of the types of damage propagation can lead to better design of novel structural composites.
Glass fiber reinforced polymer (GFRP) composites constitute nearly 90% of the global composites market and are extensively used in aerospace, marine, automotive and construction industries. While their advantages of lightweight and superior mechanical properties are well explored, non-destructive evaluation (NDE) techniques that allow for damage/defect detection and assessment of its extent and severity are not fully developed. Some of the conventional NDE techniques for GFRPs include ultrasonics, X-ray, IR thermography, and a variety of optical techniques. Optical methods, specifically measuring the transmission properties (e.g. ballistic optical imaging) of specimens, provide noninvasive, safe, inexpensive, and compact solutions and are commonly used in biomedical applications. In this work, this technique is adapted for rapid NDE of GFRP composites. In its basic form, the system for optical transmission scanning (OTS) consists of a light source (laser diode), a photo detector and a 2D translation stage. The proposed technique provides high-resolution, rapid and non-contact OT (optical transmittance)-scans, and does not require any coupling. The OTS system was used for inspection of pristine and low-velocity impacted (damaged) GFRP samples. The OT-scans were compared with conventional ultrasonic C-scans and showed excellent agreement but with better resolution. Overall, the work presented lays the groundwork for cost-effective, non-contact, and rapid NDE of GFRP composite structures.
Multilayered transparent composites having laminates with polymer interlayers and backing sheets are commonly used in a wide range of applications where visibility, transparency, impact resistance, and safety are essential. Manufacturing flaws or damage during operation can seriously compromise both safety and performance. Most fabrication defects are not discernible until after the entire multilayered transparent composite assembly has been completed, and in-the-field inspection for damage is a problem not yet solved. A robust and reliable nondestructive evaluation (NDE) technique is needed to evaluate structural integrity and identify defects that result from manufacturing issues as well as in-service damage arising from extreme environmental conditions in addition to normal mechanical and thermal loads. Current optical techniques have limited applicability for NDE of such structures. This work presents a technique that employs a modified interferometer utilizing a laser diode or femtosecond fiber laser source to acquire in situ defect depth location inside a thin or thick multilayered transparent composite, respectively. The technique successfully located various defects inside examined composites. The results show great potential of the technique for defect detection, location, and identification in multilayered transparent composites.
In general, inherent susceptibility to environmental noise has tended to retard adoption of interference methods by industry. Phase shifting is a key step in optical interference techniques of measurement. An improved max-min scanning (IMMS) method for phase determination was reported in 2004, but the environmental noise tolerance of this approach relative to other methods has been an open question. We discuss the noise tolerance of the IMMS method. Numerical simulations and simple experiments show that the method has good tolerance to some specific environmental vibration inputs, including high-frequency noise and small-amplitude low-frequency noise. The simplified data acquisition and improved noise tolerance of IMMS broadens potential applications of interferometry, particularly digital speckle pattern interferometry in the factory and the field. Another benefit of the IMMS method is an online technique to calibrate and test the phase shifter in real time. This idea is described and demonstrated.
Many advances in optics theory and technology have been stimulated by the demands of engineering research. From another viewpoint, the science of optics unifies diverse important areas of experimental research. Fundamental problems in fluid mechanics, rock mechanics, biomechanics, fracture mechanics, materials science, manufacturing technology, nondestructive inspection, and glacier mechanics are being solved by methods of optical metrology. The development of expertise and facilities to have in hand the most appropriate of available techniques is a forbidding task, but it is necessary in order to solve problems in the most efficient way. This paper describes in elementary terms some examples of applications which are relatively uncommon, including biomechanical contouring, glacier strain and flow measurement, 3-dimensional fracture mechanics, and nondestructive inspection of composites. In each case, one or more techniques, some fashionable and some old-fashioned, were adapted, extended or combined to perform the necessary measurements. The rationales for choosing the methods to solve each problem are outlined. The techniques include electronic speckle, electronic shearography, moire interferometry, moire photography with optical filtering, differential moire contouring, Doppler velocimetry, and noncoherent-light speckle photography.
This paper examines the application of Electronic Speckle Pattern Interferometry (ESPI), Electronic Shearography (ES), and Laser Doppler Vibrometry (LDV) to the detection of damage in fiber reinforced composite materials. Each of these techniques is sensitive to a different displacement parameter. ESPI detects changes in the out-of-plane displacement, ES indicates the change in the surface slope and LDV provides dynamic information on the structure (dynamic stiffness and loss factor). Experiments were done to evaluate the potential of each of these techniques for detecting impact damage in fiber reinforced composite materials. The results indicated that the three methods provide complimentary information. LDV is a fast screening procedure while ESPI and ES can provide information on the extent and location of the damage.