Ermias Gebrekidan Koricho

Design of a composite engine support sub-frame to achieve lightweight vehicles

The main objective of this paper is to analyse the possibility of the replacement of the engine sub-frame, which at present is made of steel or in some advanced cases in lightweight metals, with a new component made of carbon/epoxy composite material. A methodology is developed that helps to point out and solve the existing major problems with respect to the use of composite structures, by taking advantage from composite features such as the effect of variation of stacking sequence, ply angles, and stiffeners on load carrying capacity and stiffness of engine sub-frame. Furthermore, detailed numerical analysis has been performed to determine the natural frequencies and modes shapes of the proposed solutions. Results show competitive performance (with particular attention to the equivalent stiffness and natural frequency) of the new proposed solution based on composite material with respect to the reference steel sub-frame solution.

Bolt tension monitoring with reusable fiber Bragg-grating sensors

Bolted/mechanical fastening is one of the oldest and most widely used joining techniques. While it has many advantages such as ease of assembly and repair, it also has some important limitations. One such concern is bolt clamping load control and monitoring during and after joint assembly. Conventionally used torque wrenches can provide only an approximation of the clamping load and cannot be used for load monitoring. Clamping force transducers are bulky, expensive, and cannot usually be incorporated into the bolted joint for continuous load monitoring in the field. In this work, a novel implementation of a transducer device, called here for convenience the “bolt tension monitor,” is described and tested. It utilizes removable and reusable fiber Bragg-grating sensor(s) embedded in a bolt shaft for preload and retained clamping force measurements. While the inherent small size of the fiber Bragg-grating provides precise monitoring without significant effect on the intrinsic properties of the bolt, the embedding of an fiber Bragg-grating sensor with temporary adhesives allows quick assembly, disassembly, and reassembly of the sensor. Furthermore, these instrumented bolts can be used for structural health monitoring and defect detection in joints and structures. The technique shows great potential in simple adaptation to conventional manufacturing practices, precise clamping load measurement, and structural health monitoring of bolts and resulting joints.

Implementation of Composite and Recyclable Thermoplastic Materials for Automotive Bumper Subsystem

In order to meet the current targets not only in terms of safety, but also in terms of lightweight that means lower polluting gas emissions and fuel consumption, for a newly developed vehicle it is necessary to perform a number of component based tests. This kind of experimental test is time consuming and very expensive. Therefore, it is recommended to develop cost effective design methodology and analysis using existing finite element methods in order to evaluate the performance of different design solutions under various loading, material and environmental conditions, from the earliest stages of the design activity. This paper intends to address such design aspects and method of analysis with particular reference to the application of composite and recyclable thermoplastic materials to automotive front bumper design. Major constraints that have been dealt with are bumper crash resistance, absorbed energy and stiffness with particular reference to the existing bumper standards. Finally, the results predicted by the finite element analysis are evaluated and interpreted to examine the effectiveness of the proposed solution.

Optical transmission scanning for damage quantification in impacted GFRP 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.

High resolution imaging of impacted CFRP composites with a fiber-optic laser-ultrasound scanner

Damage induced in polymer composites by various impacts must be evaluated to predict a component’s post-impact strength and residual lifetime, especially when impacts occur in structures related to human safety (in aircraft, for example). X-ray tomography is the conventional standard to study an internal structure with high resolution. However, it is of little use when the impacted area cannot be extracted from a structure. In addition, X-ray tomography is expensive and time-consuming. Recently, we have demonstrated that a kHz-rate laser-ultrasound (LU) scanner is very efficient both for locating large defects and evaluating the material structure. Here, we show that high-quality images of damage produced by the LU scanner in impacted carbon-fiber reinforced polymer (CFRP) composites are similar to those produced by X-ray tomograms; but they can be obtained with only single-sided access to the object under study. Potentially, the LU method can be applied to large components in-situ.

Monitoring of fatigue damage in composite lap-joints using guided waves and FBG sensors

Adhesive bonding is being increasingly employed in many applications as it offers possibility of light-weighting and efficient multi-material joining along with reduction in time and cost of manufacturing. However, failure initiation and progression in critical components like joints, specifically in fatigue loading is not well understood, which necessitates reliable NDE and SHM techniques to ensure structural integrity. In this work, concurrent guided wave (GW) and fiber Bragg grating (FBG) sensor measurements were used to monitor fatigue damage in adhesively bonded composite lap-joints. In the present set-up, one FBG sensor was strategically embedded in the adhesive bond-line of a lap-joint, while two other FBGs were bonded on the surface of the adherends. Full spectral responses of FBG sensors were collected and compared at specific intervals of fatigue loading. In parallel, guided waves were actuated and sensed using PZT wafers mounted on the composite adherends. Experimental results demonstrated that...

Monitoring the effect of micro-/nanofillers on curing-induced shrinkage in epoxy resins

Abstract The curing process of polymers/resins in composites introduces volumetric shrinkage, which, based on the component type and boundary conditions, can detriment the strength of the resulting materials and components. In this work, the effect of resin reinforcement (micro- and nanofillers) and curing cycle on curing-induced shrinkage is presented. Dynamic measurements of the pristine and reinforced resin volume changes during the curing process were performed using a pycnometer. A special technique was developed to correct the existing pycnometer measurement methodology to make it independent of the testing chamber temperature, thereby allowing accurate measurements of volumetric changes during exothermic curing of resins. Overall, the study lays a foundation for measuring shrinkage-induced effects on the strengths of resins and resulting composites.

Evaluation of progressive damage of nano-modified composite laminates under repeated impacts

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.

Influence of nano-/microfillers on impact response of glass fiber-reinforced polymer composite

The reinforcement of nano-/microfillers in polymer resin has gained wide acceptance in structural applications for the enhancement of the mechanical, thermal, and electrical properties of resulting composite materials. In this work, impact behavior of glass fiber-reinforced polymer composites consisting of SC-15 epoxy resin reinforced with varying concentrations of nanoclay and hollow glass microspheres was evaluated using drop-weight tests. The time histories of impact-induced dynamic strains and forces were studied. Dye penetration tests were used to evaluate the extent of damage. Experimental results revealed that the energy absorption mechanics and damage extent vary depending on the loading conditions and filler concentration. Understanding of these novel micromodified composites could allow for the design of lightweight structural components for a wide range of applications.

Lightweight solutions for vehicle frontal bumper: Crash design and manufacturing issues

Vehicle impact on the environment is one of the main concerns in recent years and is encountered in several ways throughout vehicle life cycle. On one hand, fossil fuels are still the main energy source for automobiles and this results in a very large amount of global emissions of Green-House-Gasses (GHG) and in particular of CO2. A large contribution to the noxious gas emission reduction can come from vehicle lightweight design, through the adoption of lighter material solutions. On the other hand, the request of material recycling at the vehicle end-of-life is growing and it is clearly not sufficient to recycle only its metallic part. Therefore, lightweight design together with end-of-life recyclability are the major challenges for car manufactures and have leaded law makers to set even stricter rules and legislations to contribute to protect the environment. Vehicle fuel consumption and, consequently, noxious gas exhaust are directly depending on the vehicle weight, automakers are developing advanced technologies to tackle the issue. This includes improvements to engines, drive trains, transmissions and body aerodynamics of the cars but also the utilization of hybrid or full electric power systems or traditional internal combustion engines operated with alternative fuels. However one of the fundamental and effective means to reduce CO2 emission and end life issues comes through the use of novel lightweight and easily recyclable materials such as composite, particularly composite with thermoplastic matrix as the recyclability of thermo-set resin is still relatively complex. In this work, an automobile bumper subsystem is considered for material substitution and innovative design. A number of different composite material types are examined together with two related manufacturing technologies, namely pultrusion and die forming, pointing out the advantages that can come from each alternative. Finally a novel beam-crash box integrated bumper subsystem made from the selected lightweight materials through die forming process is designed and analysed numerically. The comparison made with the reference steel material solution shows that, through proper geometry optimization, the proposed composite material solution can substitute the current steel solution with a significant weight reduction and comparable or even better performance.