Papers by Keyword: Impact Damage (ID)

Abstract: The ultimate objective of the current programme of work is to detect and quantify low-velocity impact damage in structures made from composite materials. There are many situations in the use of composites where an impact does not result in perforation of the material but causes damage that may not be visible, yet still causes a substantial reduction in structural properties. Impacts that do not cause perforation are usually termed low-velocity. When a composite structure undergoes such impacts, it is important to know the type and level of damage and assess the residual strength. In this study, following a systematic series of experiments on the induction of impact damage in composite specimens, Scanning Electron Microscopy (SEM) was used to inspect the topographies of the specimens at high magnification. Matrix cracking, fibre fracture, fibre pullout and delamination were the types of damage observed in the composite laminates after the low-velocity impacts. The study also conducted a (very) preliminary correlation between the damage modes and the impact energy.

Abstract: Approaches to detect, assess, monitor and repair damage in critical aircraft components fabricated from composite materials are essential for safe and cost effective operation. In metallic aircraft structures, it is common to leave some fractures in situ for a prescribed period until it is convenient to repair, provided strict inspection and verification processes are in place. Under current military aircraft structural management guidelines, visible damage to critical composite components requires either immediate repair or replacement. Much has been learnt about the behaviour of damaged composite structures, but further investigation is required to develop validated residual strength and life prediction tools. A preliminary review of an early physically based, residual strength prediction method was conducted. The accuracy of this method for use in predicting the strength of composite following a complex damage was tested by comparing the results with compression-after-impact test data for a composite laminate representative of F/A 18 fracture critical structure.

Abstract: The influence of nanoclay on the impact damage resistance of carbon fiber-epoxy (CFRP) composites has been investigated using the low-velocity impact and compression after impact tests. The load-energy vs time relations were analyzed to gain insight into the damage behaviors of the materials. Compression-after-impact (CAI) test was performed to measure the residual compressive strength. The CFRPs containing organoclay brought about a significant improvement in impact damage resistance and damage tolerance. The composites containing organoclay exhibited an enhanced energy absorption capability with less damage areas and higher CAI strengths compared to those made from neat epoxy. A 3wt% phr was shown to be an optimal content with the highest damage resistance.

Abstract: The finite-element simulation models of the projectile and the discrete rod impacting to the aircraft panel structure in high velocity are established according to some experiment projects. Based on dynamic finite-element Program, the forming of impact damage in the panel structure is simulated. Through comparing the simulation results of damage pattern and size in the panel to the experiment results, the reliability of the material models and equations of state and contact algorithm used in the simulations is testified. Take the simulation of projectile vertically impacting to the panel as example, the aircraft panel structure response characteristics are analyzed briefly based on the results including the displacement of typical node in the panel, the stress course of one element and the energy change of the panel.

Abstract: The concept of light weight design will be very helpful for the tilting train to travel around a curve at a faster speed because the light weight car body makes the center of gravity lower. The impact characteristics of composite materials should be investigated because the impact property of composites is very important. This paper investigates the low-velocity impact characteristics and residual strength conducted on honeycomb sandwich panels. Low-velocity impact tests were carried out at different energy levels on honeycomb sandwich panels using a drop-weight apparatus. The impact-induced damages such as fiber breakage, matrix crack and delamination are examined with an optical microscope apparatus. Residual strength was evaluated by applying three point bending load to honeycomb sandwich panels. Experimental results show that the residual strength is greatly reduced as the impact energy increases. From the microscopic observation data, matrix crack, fiber breakage and delamination are the main impact damages of honeycomb sandwich panels.

Abstract: Recently, the use of composite material to railway vehicles is gradually increasing due to its light weight. The concept of light weight design will be very helpful for the tilting train to travel around a curve at a faster speed because the light weight car body makes the center of gravity lower. The impact characteristics of composite materials should be investigated because the impact property of composites is very important for the design and repair of a tilting train. This paper investigates the low-velocity impact and damage characteristics conducted on woven carbon/epoxy laminates for a tilting train. Low-velocity impact tests were carried out at different energy levels on woven type of laminates using a drop-weight apparatus. The impact-induced delaminations are examined by ultrasonic technique such as C-Scan. Those delaminations are also examined by laser technique such as ESPI (Electronic Speckle Pattern Interferometry). Experimental results show that the type of damage is dependent on the impact energy level and the delamination area becomes larger as the impact energy increases.

Abstract: The laminated composite structures applied to the wing and the speed brake of an aircraft or the turbine blade of a compressor. These structures may be impacted by birds and hails during operation. They may also be impacted by drop of a tool during manufacture or repair. Unlike high velocity impact damage, which can be easily found by the naked eye, the damage due to low velocity impact may be difficult to detect. Damage which is not detected may cause failure of a structure and result in damage propagation. Growth of damage means reduction of stiffness on the structure. So, exact prediction of damage caused by a low velocity impact is very important in order to guard against sudden failure of the structure. In this study, modified delamination failure criterion has suggested in order to predict the failure behavior of a composite plate subjected to low-velocity impact. The criterion includes the assumption which is matrix cracking mode causes delamination failure. Predicted damage using supposed delamination criterion is similar to experiment results.

Abstract: The goals of the paper are to identify the impact damage and strength reduction behavior of sandwich structure, composed of carbon/epoxy laminates skin and Nomex core with two kinds of thickness (10 and 20mm). For these, low velocity impact tests were conducted using the instrumented impact-testing machine and damages are inspected by SAM. And then, subsequent static tests are conducted under flexural loading to identify the strength reduction behavior of the impacted sandwich structures. The impact damages are mainly delamination in carbon/epoxy skin and their behavior is mostly independent of core thickness. Also, their energy absorbing behavior is identified through calculating the energy absorbed by impact damage. Finally, the strength reduction behavior is evaluated through Caprino’s model, which was proposed on the unidirectional laminates.

Abstract: The paper presents the fatigue behavior of carbon/epoxy laminates with impact-induced damage under 2-stage block loading. The impact damage parameter is proposed to evaluate the effect of impact damage on fatigue life. Based on this parameter, the model is developed to predict the residual life at second block. Also, the model for equivalent stress is proposed to estimate the fatigue life under 2-stage block loading based on the S-N curves under constant amplitude loading. For these models, the 2-stage block loading fatigue tests were performed on the impacted composite laminates. The effect of impact damage on fatigue life under 2-stage block loading can be characterized by the impact damage parameter. Additionally, the results by the present residual life prediction model agree with experimental results regardless of applied impact energy. Also, the equivalent stress and corresponding fatigue life by the present model agrees well with the experimental results.

Abstract: Ultra-high grinding speed and point contact are the salient natures of Quick-point grinding. Based on the damage dynamics theory, there are impact damages with a lot of micro-cracks and micro-defects and adiabatic shearing caused by ultra-high strain rate in the contact layer in such grinding manner. The micro-damages are relation to the strain rate and the materials properties, and increase with raise of strain rate. Because of the weakening effect of strain rate, the dynamic strength of the contact layer in Quick-point grinding is decreased and the grinding performance of ductile and brittle materials is also changed. The dynamic strength model of the contact layer in Quick-point grinding was built in this paper, and the weakening effects of strain rate caused by impact micro-damages on the dynamic strength of the layer and the materials removal mechanism in grinding layer were analyzed through the study on the theory and the experiments.