On the calculation of bolted joints for GRP plastics

The design of joints appropriate to specific stress-strain behaviour is of utmost importance for all load-bearing plastics, especially glass fibre reinforced plastics (GRP). The usual dimensioning rules, developed for traditional materials (steel, concrete, wood), cannot be applied when using plastic materials. In addition to the different short-term behaviour of plastics, the mechanical properties, e.g. strength and deformation, are changed by the time-dependent influences of loading, temperature and ageing.

The following contribution, based upon research done at the University of Aachen, see for example References 1 and 2, deals with the problem of the short-term strength of machine screw joints and GRP elements (mat- and SMC-laminates) and their dependence upon the different geometrical values and quantities of glass fibre.     

The toughness of laser welded joints in the ductile-brittle transition

An important feature of laser welded joints is the high level of strength and toughness mismatch developed between the weld and the parent plate. As a result, defects are frequently located in regions with high strength and toughness gradients. To address this issue, toughness tests have been performed on laser welded joints with strength mismatches in excess of 2. Toughness tests have been performed on high and low constraint mode I geometries and mixed-mode I/II configurations. In highly constrained geometries, the local toughness dominates the failure process causing the crack to deviate into harder and more brittle weld metal, while in low constraint configurations, the size of the plastic zone promotes crack deviation into the softer and tougher parent plate. In Charpy tests the crack also deviates into the tougher parent plate giving potentially misleading indications of the behaviour of defects in highly constrained joints. The experiments are modelled by a local approach technique for functionally graded materials in which the local yield strength and toughness are allowed to vary spatially.     

Fatigue performance of metallic reverse-bent joints

Adhesively bonded lap shear joints have been investigated widely and several ideas have been proposed for improving joint strength by reducing bondline stress concentrations. These include application of adhesive fillets at the overlap ends and use of adhesive with graded properties in the overlap area. Another, less common, approach is to deform the substrates in the overlap area in order to obtain a more desirable bondline stress distribution.
Previous work carried out by the authors on a number of different substrate materials indicated that a reverse-bent joint geometry is useful for increasing joint strength. Results from static stress analysis and experimental testing demonstrated that significant improvements could be achieved. This paper presents results of further work carried out to assess the fatigue performance of reverse-bent joints. Substrates with different yield and plastic deformation characteristics were used and the effects of different overlap lengths on strength were examined. The results of this research show that the improvements obtained under static tests conditions translate to even higher benefits in fatigue. The paper also explains the failure mechanism of the joints under fatigue loading.     

Experimental and analytical strength analysis of double-lap joints for marine applications

Weight-critical marine structures, such as high-speed craft, are often made of high-strength aluminium, and the usual joining method is welding. To improve the performance of high-speed craft there is a tendency to seeking lighter weight materials, especially in parts of the structure where weight-saving is particularly beneficial, such as in the superstructure. Light-weight materials can be fibre-reinforced composites. When joining structures made of composites to components of aluminium or steel, welding is no longer an option. On the other hand, adhesive bonding becomes an attractive joining method. Thus, there is a need to investigate the strength of bonded steel-composite joints.In the present paper, the stress distribution and strength of bonded double-lap steel-composite joints are investigated through theoretical analysis and experimental testing. The strength of a set of joints, with a variety of overlap lengths and two different bonding techniques and environmental conditions, has been measured experimentally. Furthermore, a new elastic–plastic stress analysis, accounting for adhesive shear deformations as well as axial and shear deformations in the adherends, has been derived.Predictions using the new theory derived herein as well as linear models are compared with the reported results. It is clearly demonstrated that linear theories are completely inappropriate for modelling such joints when loaded to failure. On the other hand, the strength predictions obtained by the new nonlinear theory agree well with the experimental results.     

Effect of loading rate on fracture toughness of bonded joints

Feasibility studies in using adhesive bonding to replace conventional fastening methods have been proved successful. At this stage it is essential to furnish the designer with the data required for such type of fabrication. One of the main factors affecting the bonded joint strength is the loading rate. Therefore, the objective of this study is to investigate the role of loading rate on fracture toughness of bonded joints. Cleavage strength tests were carried out at different loading rates using epoxy resin as an adhesive material and two adherend materials, namely aluminium and brass. Tests were carried out to cover seven different loading rates. The results indicate a significant role of the strain rate on the fracture strength.     

An investigation of the strength and tightness of joints based on a modified adhesive

The results are given of an investigation of the strength and tightness of joints in bar specimens and shells of silicate materials based on a modified epoxy adhesive. The promise of use of the studied adhesive for assembly of shells of silicate materials operating in contact with sea water is shown.Institute of Problems of Strength, Academy of Sciences of the Ukrainian SSR, Kiev. Leningrad. Translated from Problemy Prochnosti, No. 9, pp. 79–83, September, 1989.     

Ceramic matrix composite-metal brazed joints

A silicon nitride fibre-reinforced cordierite glass ceramic matrix composite has been brazed to titanium and stainless steel in argon with four different interlayer materials, copper, nickel, tungsten and a metal matrix composite (mmc). Joints were tested in shear and all but one failed in the ceramic composite. The highest strength joint, using a metal matrix interlayer to join cmc to stainless steel failed in the mmc at 106 MPa. Silver-copper eutectic braze and aluminium braze can be used to join metals to titanium-coated cmc, producing joints with low levels of interfacial defects. Some joints, however, show debonding at the edges where residual shear stresses are highest.     

Fatigue behaviour of laser repairing welded joints

This paper presents a fatigue study in Nd-YAG laser surface repairing welded joints in specimens of two base materials used in mould production. The tests were carried out in a servo-hydraulic machine in tension, under constant amplitude loading, with two stress ratios R = 0 and R = 0.4. Welded specimens were prepared with U notches and filled with laser welding deposits. The fatigue results are presented in the form of S–N curves obtained in welded and non-welded conditions. Complementary measurements of hardness and residual stresses profiles were carried out along the surface of laser welded specimens to understand the observed fatigue behaviour. The melted material was the weaker region, with lower values of hardness and higher tensile residual stresses, presenting also a high number of defects that are potential failure sites. The presence of such defects can explain the relatively poor fatigue strength of the laser repairing joints in comparison to base materials.     

The j-integral for augmented double cantilever beams and its application to bonded joints

The J-integral for the double cantilever type specimen is obtained explicitly based on the assumption of a beam on the elastic/plastic foundation. The solution agrees with known solutions as special cases. Also discussed is the path independence of the J-integral for the bonded double cantilever joints where the integral path crosses the boundary of two dissimilar materials. Though contrary to some investigator's previous conclusion, the J-integral for bonded joints is found path independent within the framework of the strength of materials.     

On the behavior of bonded joints in composite material structures

The application of advanced reinforced composite materials in aerospace structures during the remainder of this century is widely predicted. The joining of structural components by adhesive bonding is extremely desirable, because both bolting and riveting result in the cutting of fibers as well as the introduction of stress concentrations, both of which reduce the structural efficiency. R.E. Watson states that, “The next two decades will surely see dramatic advances in structures as compared to those experienced over the last 30 years. Improved titanium alloys and the advanced high strength composites, with more strength per pound than aluminum, will be the principal materials used.” Further he writes, “New bonding techniques will gradually replace riveting in many applications, permitting greater design stresses and more efficient distribution of the materials.”Because in aerospace structures dynamic loads are always present, it is absolutely essential that the fatigue behavior of bonded joints between composite material components be better understood, in order to have available design principles and rationale to take advantage of the desirable characteristics of composite materials.To date the few isolated experimental studies of composite-composite or composite-metal adherend bonded joints have been conducted under static and/or constant amplitude cyclic loading, and no generally accepted cumulative damage theories have evolved.The present research is a systematic, analytical and experimental program of study concentrating on those parameters considered to be the most influential on the static and fatigue life of an adhesive bonded single lap joint. The objectives of the program are to better understand the reasons why certain parameters have such a large influence on the structural integrity of the joint. As a result it is hoped that considerable insight will be gained as to static and fatigue life of more complicated joints such as the doable lap, the scarf, and the stepped lap joints. The analytical as well as the experimental static and fatigue test portions of the program are reported on herein.The following parameters, deemed to be the most important, were selected for study: overlap length, adhesive thickness, orientation of the laminae of the laminated adherends (particularly the lamina immediately adjacent to the adhesive), and the effect on the fatigue life of whether or not the mean value of the fatigue load causes maximum stresses above or below the shear proportional limit of the adhesive material.The determination of stresses in the test specimens is made by an analysis method developed in this program.     

Metallurgical and mechanical characterization of electron beam welded DP600 steel joints

Several new commercial advanced high-strength steels exhibit high strength and enhanced formability. These materials have the potential to affect cost and weight saving while improving performance. However, welding, by modifying the microstructure of the steel, has in general a detrimental effect on the mechanical properties of structural components. If high power density technologies are used, the result is that the mechanical properties of such kind of joints can be improved. This article presents a metallurgical and mechanical characterization of electron beam welded joints in advanced high-strength steel DP600. The experimental analysis was supported by a thermal numerical model obtained through the Sysweld^? code. Results show that mechanical properties of the electron beam welded joints are comparable with those of parent metal both in terms of static strength and ductility.     

Adhesively bonded joints composed of pultruded adherends: Considerations at the upper tail of the material strength statistical distribution

The Weibull distribution, used to describe the scaling of strength of materials, has been verified on a wide range of materials and geometries; however, the quality of the fitting tended to be less good towards the upper tail. Based on a previously developed probabilistic strength prediction method for adhesively bonded joints composed of pultruded glass fiber-reinforced polymer (GFRP) adherends, where it was verified that a two-parameter Weibull probabilistic distribution was not able to model accurately the upper tail of a material strength distribution, different improved probabilistic distributions were compared to enhance the quality of strength predictions. The following probabilistic distributions were examined: a two-parameter Weibull (as a reference), m$m$-fold Weibull, a Grafted Distribution, a Birnbaum–Saunders Distribution and a Generalized Lambda Distribution. The Generalized Lambda Distribution turned out to be the best analytical approximation for the strength data, providing a good fit to the experimental data, and leading to more accurate joint strength predictions than the original two-parameter Weibull distribution. It was found that a proper modeling of the upper tail leads to a noticeable increase of the quality of the predictions.     

The role of surface morphology on the strength and failure mode of polymer fibre reinforced single lap joints

The present study shows the relation between the surface properties of composite materials, treated with common surface preparation methods, and the mechanically measured bond strengths as quoted from lap-shear tests. The surface properties are studied by roughness measurements, surface free energy assessment, X-ray photoelectron spectroscopy and scanning electron microscopy. The procedures followed, reveal the measure of significance of the mechanical interlocking, kinetics of wetting, chemical reactivity and intermolecular adhesion of the interfaces. It is shown that the governing adhesion qualities determine significantly the fragmentation process and the strength of the joints alongside the load transfer mechanism that is analysed by a simple finite element model. Based on the results, an emphasis is given on elucidating the difference between the intrinsic interfacial adhesion strength and the measured bond strength.     

Adhesively bonded joints under cyclic loading spectra

Abstract Current designs which involve the use of composite materials in primary aircraft structures are often conservative. This, in turn, significantly lowers the weight advantage that composites have over established metallic airframe materials. Strain restrictions are often applied because the failure mechanism(s) in (fibre) composite joints and stiffener runouts where the stress state is often complex, are not fully understood. Nevertheless, from the airworthiness perspective it is essential that both the static strength and the fatigue behaviour of the components subjected to complex multiaxial stress conditions are both understood and predicted. This topic is extremely complex, and numerous criteria ranging from the purely empirical to the theoretical have been proposed. In both cases, it is necessary to know the localised stress–strain history. One common design methodology is to keep the stresses so low that fatigue will not be an issue. However, this can lead to an overly conservative design. On the other hand, while a detailed (nonlinear) finite element analysis can be performed it is often both resource‐intensive and time‐consuming. The present paper shows that Glinka's hypothesis can be used in order to calculate the localised stresses and strains for a bonded joint subjected to cyclic loading. This is a new result and has not previously been noted. It has the potential to extend the Hart‐Smith design methodology to the adhesively bonded joints in order to encompass durability considerations. This formulation also raises the possibility of enabling the degree of conservatism inherent in traditional joint design to be relaxed provided that failure occurs in the adhesive. This paper also addresses the problem of variable adhesive thickness. We show that while variable adhesive thickness can change the stress and the energy fields, the peak in the strain energy density is relatively insensitive to the stress–strain relationship for the adhesive and that Glinka's hypothesis still appears to be true. This means that, for the present class of problems, even if there is variability in the thickness of the adhesive bond the energy field and, hence, the strength of the joint can be estimated from a purely linear elastic analysis of the joint, provided that failure occurs in the adhesive.     

Prediction and optimization of friction welding parameters for super duplex stainless steel (UNS S32760) joints

Friction welding finds widespread industrial use as a mass production process for joining materials. Friction welding process allows welding of several materials that are extremely difficult to fusion weld. Friction welding process parameters play a significant role in making good quality joints. To produce a good quality joint it is important to set up proper welding process parameters. This can be done by employing optimization techniques. This paper presents a multi objective optimization method for optimizing the process parameters during friction welding process. The proposed method combines the response surface methodology (RSM) with an intelligent optimization algorithm, i.e. genetic algorithm (GA). Corrosion resistance and impact strength of friction welded super duplex stainless steel (SDSS) (UNS S32760) joints were investigated considering three process parameters: friction force (F), upset force (U) and burn off length (B). Mathematical models were developed and the responses were adequately predicted. Direct and interaction effects of process parameters on responses were studied by plotting graphs. Burn off length has high significance on corrosion current followed by upset force and friction force. In the case of impact strength, friction force has high significance followed by upset force and burn off length. Multi objective optimization for maximizing the impact strength and minimizing the corrosion current (maximizing corrosion resistance) was carried out using GA with the RSM model. The optimization procedure resulted in the creation of nondominated optimal points which can aid the process operator to fix the input control variables. The selection of a point from the Pareto front will always be a trade-off between the corrosion resistance and impact strength of the weld depending on the application.     

End effects in scarf joints

Scarf joints with small scarf angles are especially sensitive to stiffness mismatch between adherends and to adherend tip bluntness. Pre-assembly breakage of an adherend tip where it is only a few microns thick can cause significant reduction in joint strength. Mathematically, the reason for such sensitivity is that the solutions to the governing differential equation develop boundary layer character when the scarf angles are small. The boundary layers are regions with large adhesive stresses. Experimental strength data for laminated composite adherends agree with the results of this analysis.     

Computational analysis of geometric nonlinear effects in adhesively bonded single lap composite joints

It is well known that geometric nonlinear effects have to be taken into account when the ultimate strength of single lap composite joints are studied. In the present paper we investigate for which level of loads or prescribed end displacements nonlinear effects become significant and how they appear. These aspects are studied by comparing finite element results obtained from geometric nonlinear models with the results from the linear ones. The well-known software package ANSYS is applied in the numerical analysis together with a self-implemented module in the C++ library Diffpack. Some of the results are also compared with classical analytical theories of idealized joints showing significant differences.The joints examined are made of cross-ply laminates having 0 or 90° surface layers. A combined cross-ply/steel joint and an isotropic joint made of steel are also studied. All the models except the all-steel one are assembled with adhesives, while the latter is welded.Through the investigation a considerable departure from linear behavior has been detected for a large regime of prescribed end displacements or external loads. Geometric nonlinear effects begin to develop for external loads that produces stresses which are far below ultimate strength limits and for average longitudinal strains that are less than 0.5%. It has also been detected that the distribution of materials within the joint has some influence on the nonlinear behavior. Thus, geometric nonlinear methods should always be applied when single lap (or other non-symmetric) composite joints are analyzed.     

A parametric study on the failure of bonded single-lap joints of carbon composite and aluminum

A parametric study on adhesively bonded carbon composite-to-aluminum single-lap joints was experimentally conducted. FM73m, a high strength adhesive produced by Cytec, was used for bonding. The primary objective of this study is to investigate the effects of various parameters, such as bonding pressure, overlap length, adherend thickness, and material type, on the failure load and failure mode of joints with dissimilar materials. While metal bonded joints generally fail at the adhesive, the final failure mode of all the tested bonded joints with dissimilar materials was delamination of the composite adherend. Bonding strengths of the tested joints were lower than the metal-to-metal bonded joint strength. The specimens bonded under pressure of 4 and 6 atm yielded higher failure loads than under pressure of 3 atm, which is within the range of the manufacturer-recommended bonding pressure. Failure loads of the joint increased slightly at an overlap length larger than 30 mm. Increasing adherend thickness resulted in an increase of the failure load, but was not linearly proportional to the failure load.     

Parametric study of riveted joints

As one of the most reliable fasteners, solid riveted joints are widely utilized in many industrial areas. In the present work, the authors recalled some results on the riveting process and the strength of one kind of riveted joints obtained by simulation and experimental investigations in a previous paper. The numerical results were in very good agreement with the experimental results, allowing us to validate our simulation approach and its use for further studies. We selected several engineering parameters for the riveted joint: initial assembly, friction coefficient, rivet’s geometry and sheets’ geometry, in order to carry out a parametric study and determine their relative importance. These were conducted in FEA software. The results showed the impact on riveting process and the strength of the riveted joint by varying each parameter which was interesting for the industry.