Ian A. Ashcroft

Mode I fracture of epoxy bonded composite joints: 1. Quasi-static loading

Abstract Mode I constant displacement rate tests were conducted on epoxy-bonded CFRP joints at –50, 22 and 90°C. A comparison of experimental compliance and different beam theory approaches indicated that care needs to be taken when applying beam theory approaches across a wide temperature range. Temperature was seen to influence the mode of fracture which progressed from stable, brittle fracture at low temperatures to slip-stick fracture at room temperature and finally to stable ductile behaviour at elevated temperatures. This behaviour has been attributed to the dependence of critical strain energy release rate on crack velocity for epoxy adhesives and a model for the fracture behaviour of viscoelastic materials has been used to explain these results. The critical strain energy release rate was seen to increase with temperature and the failure locus transferred from predominantly in the composite substrate to predominantly in the adhesive.

Mode I fracture of epoxy bonded composite joints 2. Fatigue loading

Abstract The main aims of this work were to investigate the effect of temperature on fatigue crack propagation (FCP) in bonded joints and to compare this with fracture under quasi-static loading and fatigue failure in uncracked lap joints. The fatigue tests were conducted on epoxy bonded carbon fibre reinforced polymer joints at −50°C, 22°C and 90°C and a number of techniques for determining strain energy release rate and crack propagation rate were evaluated. It was seen that temperature had a significant effect on the locus of failure and FCP, indicating that service temperature must be taken into account when designing bonded composite joints. The applicability of fracture mechanics data to the prediction of fatigue failure in uncracked lap joints was assessed by attempting to predict fatigue thresholds in two types of lap joints at three different temperatures. In most cases reasonable predictions were made, the notable exception being the overprediction of the fatigue threshold load in double lap joints tested at 90°C. This was attributed to creep in the double lap joints, which accelerated fatigue failure. It was recommended that in order to improve current prediction techniques, efforts should be made to base predictive methods on accurate physical models of the degradation and failure processes in the joints.

The effect of environment on the fatigue of bonded composite joints, part 1: testing and fractography

In this work, the effect that test environment and pre-conditioning had on the fatigue behaviour of CFRP/epoxy lap–strap joints was investigated. It was shown that the fatigue resistance of the lap–strap joints did not vary significantly until the glass transition temperature, Tg, was approached, at which point a considerable reduction in the fatigue threshold load was observed. It was also noted that absorbed moisture resulted in a significant reduction in the Tg of the adhesive. This must be taken into account when selecting an adhesive to operate at elevated temperatures. The locus of failure of the joints was seen to be highly temperature dependent, transferring from primarily in the composite adherend at low temperatures to primarily in the adhesive at elevated temperatures. It was also seen that as the crack propagated along the lap–strap joint, the resolution of the forces at the crack tip tended to drive it into the strap adherend, which could result in complex mixed mode fracture surfaces.

Prediction of fatigue thresholds in adhesively bonded joints using damage mechanics and fracture mechanics

The prediction of fatigue threshold in composite adhesively bonded joints using continuum damage mechanics (CDM) and fracture mechanics (FM) approaches has been investigated. Two joint types were considered in this study: double lap (DL) and lap strap (LS) joints. The substrates, which were made of uni-directional (UD) or multi-directional (MD) composite laminates, were bonded together using an epoxy film adhesive. The joints were tested under fatigue loading with a load amplitude ratio of 0.1 at various test temperatures. Damage evolution laws were derived using thermodynamics principles. The number of cycles to failure was then expressed in terms of the stresses in the adhesive layer and material constants. The stresses were calculated from non-linear finite element analyses, considering both geometrical and material non-linearities. The damage laws generated for the UD/DL joint data were then used to predict the fatigue crack initiation thresholds for the MD/DL, UD/LS, and MD/LS joints. The FM approach uses the crack closure integral method to compute the strain energy release rate at the threshold load (G th) from the results of geometrical non-linear finite element analysis. The G th value for an inherent crack size at the centre of the bondline in the UD/LS joint is used as the failure criterion in order to predict the fatigue threshold for the MD/LS, UD/DL, and MD/DL joints. It was found that the predictions using CDM were slightly more accurate than those obtained using the FM approach. In general, when predicting the fatigue thresholds of the LS joints using the DL joints data, or vice versa, good agreement was obtained between the measured and predicted thresholds at ambient and low temperatures, but poor agreement was seen at the high test temperature. This was attributed to the deleterious effect of creep, which was greater in the DL joints than in the LS joints.

Investigating fatigue damage evolution in adhesively bonded structures using backface strain measurement

Predicting the service life of adhesive joints under fatigue loading remains a major challenge. A significant part of this task is to develop laws that govern the crack initiation phase. This paper contributes to this area through the development and application of the backface strain technique. A numerical study was carried out to investigate the effect of key parameters on the technique and to determine optimum gauge specification and location. Calibration curves were then produced relating the change in strain to the extent of damage. These numerical studies were then validated by undertaking a series of fatigue tests on both aluminium and GRP (glass-reinforced polymer)-bonded joints. Following various degrees of predicted damage the joints were carefully sectioned, polished, and studied using optical microscopy. The predicted and observed damage showed close correlation. The fatigue tests have also indicated that, for unmodified joints (intact fillets), even at high loads (50% static failure load) there was an initiation phase that accounted for about half the fatigue life of the joint. Removal of the adhesive fillet has been found to eliminate the initiation phase and consequently reduce fatigue life.

Environmental degradation of the interfacial fracture energy in an adhesively bonded joint

The mixed mode flexure and notched coating adhesion tests have been carried out in order to characterise interfacial fracture for a range of environmental exposure conditions and to find a meaningful interfacial strength parameter using a fracture mechanics approach. The moisture uptake of the adhesive was accelerated using an open-faced configuration. The critical loading to cause interfacial fracture was measured and was used in conjunction with finite element analysis (FEA) to determine the fracture energy under various exposure conditions. Moisture dependent material properties were incorporated in the FEA. Scanning electron microscopy was used to characterise the nature of the failure surface. Significant degradation of the fracture energy of the interface was found and this was matched by observed changes to the failure surface. The fracture energies were found to be largely independent of test method, exposure environment and time and was primarily related only to the moisture concentration.

Numerical prediction of fatigue crack propagation lifetime in adhesively bonded structures

Abstract In this short paper, a generalised numerical procedure using finite element (FE) analysis for prediction of the fatigue lifetime of adhesively bonded structures is proposed. The number of cycles to failure (Nf) is calculated by integrating a fatigue crack growth law between initial and final crack lengths. This crack growth law is formulated in terms of the strain energy release rate (SERR), which is determined, at any crack length, from an FE analysis. This complete process is implemented within the FE code, enabling automated calculation of the fatigue life for a given set of boundary conditions. This is a development of the approach outlined for single-lap joints [Int. J. Fract., 103 (2000) 41]. However, being fully implemented within an FE code it is not limited by the approximations of the simplified analytical expressions and furthermore can be applied to any structural configuration. The procedure was evaluated by application to a single-lap joint and good results were obtained in comparison with those using other methods. Furthermore, the use of the total SERR (GT) and mode I SERR (GI) as crack-propagation-controlling parameters are investigated and briefly discussed.

A Study on the Laser Spatter and the Oxidation Reactions During Selective Laser Melting of 316L Stainless Steel, Al-Si10-Mg, and Ti-6Al-4V

Abstract The creation of an object by selective laser melting (SLM) occurs by melting contiguous areas of a powder bed according to a corresponding digital model. It is therefore clear that the success of this metal Additive Manufacturing (AM) technology relies on the comprehension of the events that take place during the melting and solidification of the powder bed. This study was designed to understand the generation of the laser spatter that is commonly observed during SLM and the potential effects that the spatter has on the processing of 316L stainless steel, Al-Si10-Mg, and Ti-6Al-4V. With the exception of Ti-6Al-4V, the characterization of the laser spatter revealed the presence of surface oxides enriched in the most volatile alloying elements of the materials. The study will discuss the implication of this finding on the material quality of the built parts.

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Abstract Fibre reinforced polymer composites (FRPs) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.

Transparency Built‐in

The supply chains found in modern manufacturing are often complex and long. The resulting opacity poses a significant barrier to the measurement and minimization of energy consumption and therefore to the implementation of sustainable manufacturing. The current article investigates whether the adoption of additive manufacturing (AM) technology can be used to reach transparency in terms of energy and financial inputs to manufacturing operations. AM refers to the use of a group of electricity‐driven technologies capable of combining materials to manufacture geometrically complex products in a single digitally controlled process step, entirely without molds, dies, or other tooling. The single‐step nature affords full measurability with respect to process energy inputs and production costs. However, the parallel character of AM (allowing the contemporaneous production of multiple parts) poses previously unconsidered problems in the estimation of manufacturing resource consumption. This research discusses the implementation of a tool for the estimation of process energy flows and costs occurring in the AM technology variant direct metal laser sintering. It is demonstrated that accurate predictions can be made for the production of a basket of sample parts. Further, it is shown that, unlike conventional processes, the quantity and variety of parts demanded and the resulting ability to fully utilize the available machine capacity have an impact on process efficiency. It is also demonstrated that cost minimization in additive manufacturing may lead to the minimization of process energy consumption, thereby motivating sustainability improvements.