Mixed-mode crack growth in bonded composite joints under standard and impact-fatigue loading
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- Ashcroft, I.A., Casas-Rodriguez, J.P. & Silberschmidt, V.V. J Mater Sci (2008) 43: 6704. doi:10.1007/s10853-008-2646-6
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Carbon fibre reinforced polymers (CFRPs) are now well established in many high-performance applications and look set to see increased usage in the future, especially if lower cost manufacturing and solutions to certain technical issues, such as poor out-of-plane strength, can be achieved. A significant question when manufacturing with CFRP is the best joining technique to use, with adhesive bonding and mechanical fastening currently the two most popular methods. It is a common view that mechanical fastening is preferred for thicker sections and adhesive bonding for thinner ones; however, advances in the technology and better understanding of ways to design joints have lead to increasing consideration of adhesive bonding for traditionally mechanically fastened joints. In high-performance applications fatigue loading is likely and in some cases repetitive low-energy impacts, or impact fatigue, can appear in the load spectrum. This article looks at mixed-mode crack growth in epoxy bonded CFRP joints in standard and impact fatigue. It is shown that the back-face strain technique can be used to monitor cracking in lap-strap joints (LSJs) and piezo strain gauges can be used to measure the strain response of impacted samples. It is seen that there is significant variation in the failure modes seen in the samples and that the crack propagation rate is highly dependent on the fracture mode. Furthermore, it is found that the crack propagation rate is higher in impact fatigue than in standard fatigue even when the maximum load is significantly lower.
High-performance fibre-reinforced polymer composites (FRPs) are now well established in many applications such as military aircraft, high-speed marine vessels and sports equipment. Increasing usage is also being found in civil aircraft, automotive and building applications. The original reason for using these materials was the high-specific strength and stiffness; however, other potential advantages include reparability, insulating properties, corrosion resistance, suitability for stealth applications and fatigue resistance. In fact, the good resistance of FRPs to fatigue lead to an early design philosophy based on quasi-static strength alone. However, with further research and increased studies of components after extended periods in service, it is now recognised that fatigue is potentially damaging to composites and hence is worthy of serious study. Furthermore, fatigue is also linked with two of the main drawbacks of these materials, namely, sub-surface damage initiation making it difficult to detect, and, second, possibility of the transfer from stable to unstable crack growth at short crack lengths. Together these two characteristics can mean that the first sign of fatigue damage can be complete failure of the structure. This has lead to research into the fatigue of composites, including the fatigue propagation of sub-surface cracks caused by low-energy impact, such as the classic scenario of the dropped tool during maintenance work [1–3]. Most of this research work has been conducted using simple constant amplitude, sinusoidal waveforms or in some cases simplified versions of load spectra taken from experimental measure using techniques such as the rainflow method. However, the in-service load spectra for structural applications can in some cases contain repetitive low-energy impacts, which are termed impact fatigue. This type of loading has received little attention to date, but has been shown to be damaging to composite materials [4–6].
Composite fabrication methods mean that the number of joints in a structure can often be significantly reduced compared with similar metal structures. However, it is inevitable in all but the simplest applications that composite parts will need to be joined to other parts. In many cases, such as the use of FRP panels in cars, the FRP may have to be joined to a different material, such as aluminium. The most common method of joining is probably mechanical fastening but this is less than ideal for a number of reasons. First, mechanical fastening usually involves drilling a hole in the composite, which will have a detrimental effect on its structural integrity. Furthermore, the site of the mechanical fastener will be a site of high-stress concentration, and potential fretting fatigue can occur between the mechanical fastener and the composite causing further damage. Also, if the structure needs to contain liquid, such as the fuel-holding function of many aircraft wings, then the mechanical fastening is a potential leakage site. Finally, the mechanical fasteners can add significant weight and the fastening process can be expensive, especially if sealing and dressing are required. The obvious alternative to mechanical fasteners is adhesive bonding, and indeed this joining method removes or reduces many of the disadvantages stated above. However, inevitably, certain disadvantages are also associated with adhesive bonding. These include, sensitivity to the manufacturing process (particularly, poor surface preparation), difficulty in detecting poorly bonded areas, environmental sensitivity and a lack of trusted design methods for real in-service conditions. Most of these problems can be overcome, however, and hence there has been a lot of work into the adhesive bonding of composite parts (e.g. [7–12]). The reduction in stress concentration, decrease in damage to the composite and increase in stiffness of bonded joints compared to mechanically fastened joints would indicate improved fatigue resistance, and research has shown that good fatigue resistance can, indeed, be seen in bonded composite joints [13–16]. A significant research effort has also been put into looking at the response of bonded joints to impact loads [17–21], however relatively little work has been published to date on the impact fatigue of bonded composite joints, even though it has been shown that impact fatigue is highly detrimental to bonded joints [22, 23]. In  the impact-fatigue behaviour of bonded epoxy-CFRP lap-strap joints was compared to the behaviour of the same joints subjected to non-impact, constant amplitude sinusoidal loading (i.e. standard fatigue). It is shown that the impact fatigue is significantly more damaging to the joints than the standard fatigue. The fracture surfaces for the two types of loading were seen to be quite different, with the impact fatigued joints showing less uniformity and more signs of brittle fracture. The response of similar joints to fatigue spectra incorporating short blocks of impact fatigue in a standard fatigue spectrum has also been reported . It was seen that the incorporation of the impact-fatigue blocks significantly changed the dynamics and mechanisms of crack growth in the joints, resulting in a greatly decreased fatigue life.
In this article, crack growth in standard and impact fatigue of bonded epoxy-CFRP lap-strap joints is investigated. The back-face strain technique is explored as a means of in-situ monitoring of damage in the joints as well as measuring transient strains in the impact fatigued lap-strap joints (LSJs) for the first time. An effort is also made to correlate crack growth with various fracture parameters as a means of developing predictive fatigue crack growth laws.
Samples were manufactured by adhesively bonding cured carbon fibre reinforced polymer (CFRP) panels. This is known as secondary bonding and is distinguished from co-bonding and co-curing, in which the adhesive and CFRP are cured together. The advantage of secondary bonding is that different (optimum) curing cycles can be used for the adhesive and CFRP and that distortion of the CFRP in the joint area during curing can be avoided. There is also potentially greater freedom in the manufacturing process as well as cost savings due to a possibility to make parts in smaller assemblies. However, the obvious disadvantages are the time and cost penalties of replacing a single process with two.
Elastic properties of T800/5245C composite at room temperature
Quasi-static and standard fatigue testing
A servo-hydraulic fatigue testing machine with digital control and computer data logging was used in the quasi-static and standard fatigue testing. The quasi-static failure load was calculated as the average of the maximum force reached by two specimens tested at a displacement rate of 0.05 mm/s. Standard fatigue testing was in load control with a maximum load of 7.8 kN, which was approximately 60% of the average quasi-static failure load. A sinusoidal waveform was used with an R-ratio (minimum-to-maximum load) of 0.1 and frequency of 5 Hz. All testing was in ambient laboratory environmental conditions where temperature and relative humidity varied between 18–25 °C and 50–60%, respectively. Thermocouples were placed at various points on the surfaces of the samples in order to investigate any thermo-elastic heating during testing, however, no change in temperature was observed.
In-situ crack growth in the samples was measured by means of a portable optical microscope and digital camera. The edges of the samples were painted white and marked with a scale prior to testing in order to increase the contrast between cracked and non-cracked material and, hence, increase the accuracy of the crack measurements. Back-face strain was also investigated as a means of in-situ measurement of crack length: this is described further in section “Fractography”.
Back-face strain measurement
After testing, the edges and fracture surfaces of each sample were examined with an optical microscope. This was primarily to locate the macro fracture path in the joint and to look for different areas of fracture for further study. Scanning electron microscopy was then used for higher magnification examination of selected fracture surfaces. Specimens were extracted using a diamond saw and gold-coated prior to examination to prevent charging under the electron beam.
Finite element analysis
The LSJ was modelled in 2-D with the commercial FEA software package MARC-MENTAT (2007-R1) from MSC. The aims of the models were to (a) simulate back-face strain signals for various gauge locations and (b) determine fracture parameters, such as strain energy release rate (G) as a function of crack length and crack path. Four-noded plane strain isoparametric elements with assumed strain interpolation were used as these provided the better calculation of fracture mechanics parameters. However, this meant that a high degree of mesh refinement was necessary to remove a high-mesh dependency in the fracture mechanics parameters determined from the FEA models. The simulated back-face strain measurements were less mesh-sensitive.
Determination of fracture mechanics parameters
Results and discussion
Back-face strain simulations
The results from the FEA simulations with the strain gauge on the strap adherend back face are shown in Fig. 11. Again it can be seen that strain gauge location has a strong effect on crack monitoring. The first thing to note is that the strain levels and the difference between maximum and minimum strains are greater than for the gauge on the lap adherend, which is potentially useful in decreasing experimental scatter, depending on the noise in the experimental strain gauge system. On this adherend the trend is a steady increase in strain as the crack progresses, followed by a large decrease in strain as the crack passes the location of the gauge, after which strain increases again. The big advantage of siting the gauge on the strap adherend is that the gauge can be placed to be most accurate at the site of most interest but is still able to monitor crack growth along the whole length.
Standard fatigue test results
It can be concluded from this work that the back-face strain technique can be used to monitor crack growth in LSJs in both standard and impact fatigue. However, the location of the gauge is critical, with the best location being on the strap adherend and placed along the length at the position, in which the greatest accuracy is required. Ideally, a series of crack gauges along the length of the strap should be used. It is also found that in impact a piezo strain gauge should be used rather than a standard electrical resistance gauge, both for noise suppression and to achieve the high sampling rates needed to characterise the strain response under high-rate conditions.
In both standard and impact-fatigue testing, it was found that complex crack paths could develop introducing variability in the crack growth behaviour of similar joints subjected to the same loading. This can be attributed to the complex and variable microstructure of both the adhesive and CFRP, the many and complex micro mechanisms of damage and failure in the joints and the significance of small geometrical, material and flaw variations in determining the crack path. Since the crack growth rate is highly dependent on the mode of fracture this means that a high degree of scatter in crack growth behaviour is inevitable, and this seems to be particularly the case in the dynamic conditions of fracture in impact fatigue.
The authors are very grateful for a partial financial support by the Royal Society within the framework of its International Joint Projects scheme.