FAILURE OF CFRP AND TITANIUM TUBULAR ADHESIVE LAP JOINTS AT EXTREME TEMPERATURES

In this work, carbon fiber reinforced polymer and titanium adhesive tubular lap- joints (TLJs) at cold-temperature, room-temperature, and elevated-temperature are studied. Finite element analysis is employed to investigate the competing damage mechanisms within the TLJs. The relationship between the joint strength and adhesive bondline length is also determined. X-ray micro-computed tomography (μCT) technique is utilized to analyze composite damage characteristics; whilst statistical design of experiment (DoE) approach is used to understand the interaction between bondline length and temperature on the mechanical performance of TLJs. Results show a mixed-mode failure region at elevated-temperatures, causing the TLJ to fail by either interlaminar shear or titanium net-section fracture. Results further reveal that deformation within the titanium creates shear stresses on the composite tube, leading to the initiation, propagation, and coalescence of delamination. Employing numerical, experimental, and statistical techniques, this works provides insights into the parameters and mechanisms affecting the behavior and failure of adhesive TLJs at extreme temperature conditions.

A Novel Finite Element Method Approach in the Modelling of Edge Trimming of CFRP Laminates

Nowadays, the development of robust finite element models is vital to research cost-effectively the optimal cutting parameters of a composite machining process. However, various factors, such as the high computational cost or the complicated nature of the interaction between the workpiece and the cutting tool significantly hinder the modelling of these types of processes. For these reasons, the numerical study of common machining operations, especially in composite machining, is still minimal. This paper presents a novel approach comprising a mixed multidirectional composite damage mode with composite edge trimming operation. An ingenious finite element framework which infer the cutting edge tool wear assessing the incremental change of the machining forces is developed. This information is essential to replace tool inserts before the tool wear could cause severe damage in the machined parts. Two unidirectional carbon fibre specimens with fibre orientations of 45∘ and 90∘ manufactured by pre-preg layup and cured in an autoclave were tested. Excellent machining force predictions were obtained with errors below 10% from the experimental trials. A consistent 2D FE composite damage model previously performed in composite machining was implemented to mimic the material failure during the machining process. The simulation of the spring back effect was shown to notably increase the accuracy of the numerical predictions in comparison to similar investigations. Global cutting forces simulated were analysed together with the cutting tool tooth forces to extract interesting conclusions regarding the forces received by the spindle axis and the cutting tool tooth, respectively. In general terms, vertical and normal forces steadily increase with tool wear, while tangential to the cutting tool, tooth and horizontal machining forces do not undergo a notable variation.

Fatigue Damage Monitoring of CFRP Elements by Thermographic Procedure under Bending Loads

For structural health of mechanical structures, non-destructive detection and material defect characterization represent the main useful tools for mechanical decay prediction caused by local composite damage phenomena. In this work, internal delamination due to alternate bending were characterized in flat specimens, performing fatigue and static tests, coupled with thermographic, optical, and ultrasonic analysis for damage detection and evolution purposes. Damage to rupture behavior of CFRP material through mechanical tensile tests is performed on several samples and non-destructive inspection procedures are optimized during successive HCF tests to detect in real time local compliance variations and damage initiation. Thermographic continuous monitoring and occasional ultrasonic analysis are implemented to analyze composite anomalies during fatigue life and to elaborate a procedure for identification of delamination induced damage before failure. IRT and UT results are computed with MATLAB analysis for damage evaluation with strain and compliance data acquired during tests.

Damage mode and load distribution of countersunk bolted composite joints

Tensile experiments were conducted with single-bolted and multi-bolted countersunk composite-metal joints. The finite element models with Hashin criteria and modified Camanho degradation law were developed to predict the bearing load and to simulate the failure behavior. The models were validated with the experimental results, including load-displacement curves and composite damage profiles. Based on the verified models, bolt load distributions of multi-bolted joints were calculated and analyzed before and after damage occurs. Results show that composite damages can change the bolt load distributions to a certain extent, which needs to be considered during the structure design. To adjust the bolt load distribution, the effects of joint variables, including bolt-hole clearance, bolt spacing, countersunk height ratio, and bolt stiffness combination, have been studied. It is found that the increase of the pin clearance around the largest bearing bolt would decrease its corresponding bearing load. The countersunk height ratio affects the bolt load distribution difference that a greater ratio leads to a less difference. The bolt spacing and bolt stiffness can also be used to reduce the bolt load distribution difference with elaborately designed combinations.

Textile Composite Damage Analysis Taking into Account the Forming Process

The internal structure of composite materials is modified during manufacturing. The formation of woven prepregs or dry preforms changes the angle between the warp and weft yarns. The damage behaviour of the consolidated composite is modified by these changes of angle. It is important when designing a composite part to consider this modification when calculating the damage in order to achieve a correct dimensioning. In this paper, a damage calculation approach of the consolidated textile composite that takes into account the change in orientation of the yarns due to forming is proposed. The angles after forming are determined by a simulation of the draping based on a hypoelastic behaviour of the woven fabric reinforcement. Two orthogonal frames based on the warp and weft directions of the textile reinforcement are used for the objective integration of stresses. Damage analysis of the cured woven composite with non-perpendicular warp and weft directions is achieved by replacing it with two equivalent Unidirectional (UD) plies representing the yarn directions. For each ply, a model based on Continuum Damage Mechanics (CDM) describes the progressive damage. Two examples are presented, a bias extension specimen and the hemispherical forming coupon. In both cases, the angles between the warp and weft yarns are changed. It is shown that the damage calculated by taking into account these angle changes is greatly modified.

A Bayesian fusion method for composite damage identification using Lamb wave

This study presents a novel method for composite damage identification using Lamb wave. A probabilistic integration of the elliptical loci method and the RAPID (reconstruction algorithm for probabilistic inspection of defects) in a Bayesian framework is proposed. The proposed method allows for the incorporation of multiple damage sensitive features in a rational manner to improve the reliability and robustness for a given array of sensors. Numerical studies are performed to verify the effectiveness of the proposed method and to compare its accuracy with existing methods. Experimental investigation using a realistic composite plate is made to further validate the proposed method. The influence of damage location and the number of participating sensors on the performance of the proposed method is discussed. Results indicate that the proposed method yields more accurate and reliable results comparing with existing methods.