Effect of residual stresses on fatigue crack growth behavior of aluminum substrate repaired with a bonded composite patch

Composite patches bonded to cracked metallic aircraft structures have been shown to be a highly cost-effective method for extending the service life of the structures. The fatigue crack growth behavior of pre-cracked 7075-T6 aluminum substrate with the 12.7-mm V-notch crack repaired with boron/epoxy composite patches was investigated. 1-ply, 2-ply, 3-ply and 4-ply composite patches were studied. The residual stresses due to mismatch of the coefficients of thermal expansion between the aluminum plate and boron/epoxy composite patch were calculated based on the classical equation. The effects of the residual stresses and patch layers on fatigue lifetime, fatigue crack growth rate, and fatigue failure mode of the repaired plates were examined experimentally. A modified analytical model, based on Rose's analytical solution and Paris power law, was developed for this research. This model considered the residual stress effect and successfully predicted the fatigue lifetime of the patched plates. Results showed that the composite patch had two competing impacts on the structure. The composite patch could cause residual tensile stress in the aluminum substrate, which could consequently increase the crack growth rate. Moreover, reinforcement with the composite patch could also retard the crack propagation in the aluminum plate. If a 4-ply composite patch was used, it resulted in high residual stresses and effectively would not extend the fatigue lifetime of cracked aluminum plates.     

Effect of residual austenite on properties of tool steel following shortened treatments in lower bainitic phase

Abstract

Recent investigations show the possibility of shortened low-cost treatment of chromium alloyed tool steels in the lower bainitic state. But with these treatments just above the martensite start temperature remains still an amount of residual austenite. For better understanding of the reactions due to the transformation of austenite into lower bainite metallographic, studies completed by dilatometric tests have been carried out. Structural characteristics due to applied heat treatment processes are discussed and special changes of specific properties are discussed by example of the steel 100Cr6 (SAE 52100).     

Young's modulus degradation in fatigue of filled polymers for household appliances

Abstract

Monotonic and fatigue tests in simple bending were carried out on a particulate composite made of polypropylene charged with CaCO₃ particles, fixing the maximum displacement. Under monotonic conditions, the material underwent a considerable loss in modulus with increasing the displacement, without exhibiting a typical fracture in two pieces. The same phenomenon was found in fatigue, suggesting that a failure criterion should be based on the modulus degradation, rather than on fracture. To model the modulus variation as a function of the number of cycles and the maximum displacement, an exponential law was assumed. From the experimental results, the parameters appearing in the model could be found having available two sets of data, concerning two distinct values of the maximum displacement. The analysis of the model revealed that a simplified form of the exponential law, needing only two experimental constants, is effective in predicting the modulus degradation. This finding considerably shortens the characterisation stage, yet allowing a satisfactory correlation with the experimental results.     

Mechanical Properties of Weldbonded Joints of Advanced High Strength Steels

In lightweight car body shell mass production, due to requirements on vehicle weight reduction and carbon dioxide emissions, joining of advanced high strength steels (AHSS) with different joining procedures and especially hybrid bonding techniques is becoming more and more important. One of these hybrid bonding techniques is the combination of resistance spot welding and adhesive bonding called weldbonding. One of the important advantages of weldbonded joints in comparison to resistance spot welded joints are the enhanced mechanical properties. To guarantee sufficiently high quality conditions regarding the strength of the weldbonded joints, the influences of the applied adhesive systems and of different base metal combinations are studied. This is carried out for both non-corrosive and corrosive environments and for the choice of different joining parameters settings. In particular, the mechanical behaviour of the weldbonded joints is investigated under quasi-static, impact and fatigue loads. Furthermore, the energy absorption of the weldbonded joints for both non-corrosive and corrosive environments is studied. It is shown that the weldbonded joints possess higher mechanical strengths in all load cases (quasi-static, impact and fatigue). Corrosive attack affects weldbonded joints, and the quasi-static strength is reduced. Resistance spot welded joints are not affected by the corrosive attack, but even after several weeks of corrosive attack, the quasi-static strength of weldbonded joints remains higher than that of resistance spot welded joints.     

Fatigue behavior of bonded composite repairs

In this study, recent efforts into the research and development of composite repairs bonded to defective aircraft structures are discussed. The fatigue crack growth (FCG) behavior of pre-cracked Al 7075/T6 substrates with bonded composite patches was investigated experimentally and analytically. Boron-epoxy patches with 2-, 4- and 6-plies were installed on Al substrates with single-side-crack. Tension-tension fatigue tests were also conducted on Al substrates to establish their fatigue behavior for comparing with the repaired specimens. A considerable increase in the fatigue life and a decrease in the stress intensity factor (SIF) were observed as the number of plies increased. An analytical model, based on Rose's analytical solution and Paris' power law, was developed to predict the FCG behavior of the repaired substrates. The analytical and experimental results are found to be in good agreement.     

Fatigue initiation in adhesively-bonded single-lap joints

This paper presents detailed experimental fatigue data obtained from a selection of joints tested under cyclic loads. Some were tested to failure to determine fatigue load-life data. The other specimens were tested to induce different levels of damage. Multiple strain gauges (SGs) were used to record the change in backface strain during the testing, allowing measurement of damage in different locations. The overlap region was then sectioned and polished so it could be inspected using microscopy. The experimental results showed that damage first appeared in the fillet, appearing as a change in adhesive colour in a specific area. Cracks were observed in one specimen, which was tested for a higher number of cycles. A damage scale based on area affected, change in colour and size of cracks is proposed to classify the damaged sections. A result of this work is to tie the backface strain readings with the damage scale allowing damage to be assessed in a non-destructive manner.     

Mathematical modelling and computer simulation of nitriding

Abstract

Mathematical modelling of matter transmission during gas nitriding has been deduced by a numerical calculation in the present study. The diffusion coefficient of nitrogen in 38CrMoAl steel and the matter transmission coefficent in the interfacial reaction have been measured. Owing to the large difference between the nitrogen activity at the surface of the workpiece and that of the gas phase during the nitriding process, it is very difficult to control the nitrogen potential and balance the nitrogen activity. In order to solve this problem, is it proposed that the nitrogen potential is dynamically controlled by computer. Under conditions of high nitriding speed, the computer controlled technique used in practical manufacture shows good reproducibility and can control the nitrogen potential accurately, thereby reducing the brittleness of the nitrided layer.     

Technical note Fabrication of alumina dispersion strengthened copper strips by internal oxidation and hot roll bonding

Abstract

Alumina dispersion strengthened copper strips were fabricated by internal oxidation and hot roll bonding of Cu–Al alloy strips. Cu–Al alloy strips were internally oxidised without using any oxidant powders by a surface oxidation method. Several of the internally oxidised alloy strips were stacked and bonded by rolling at high temperatures. The bonded strip was cold rolled to achieve tensile strengths of 484–539 MPa and yield strengths of 472–522 MPa with thermally stable mechanical properties.     

Fatigue and failure behaviors of silver-filled electronically-conductive adhesive joints subjected to elevated humidity

Conductive adhesives are used in electronics packaging applications for hybrid, die-attach and display assemblies. There are a number of issues of concern in the design of joints bonded using electronically-conductive adhesives (ECAs). An important issue is the cyclic fatigue behavior of conductive adhesive joints under elevated humidity environments, in which failures may occur due to cyclic mechanical and/or thermal stresses. This paper addresses the effect of elevated humidity levels on the fatigue and failure behaviors of ECAs. For this purpose, joints were prepared using stainless-steel adherend specimens and a commercial ECA, and tested under monotonic and cyclic fatigue conditions, at two humidity levels, namely 20% and 90% relative humidity at 28°C. Furthermore, joint failure mechanisms were analyzed using optical techniques, and joint conductivity measurements. Load versus number of cycles (P–N) curves were generated using these specimens at three different load ratios (R), namely 0.1, 0.5 and 0.9, at a cyclic frequency of 150 Hz. The P–N curves were parallel and the failure modes were found to be predominantly interfacial, accompanied by a significant decrease in joint conductivity.     

Short fatigue crack growth behaviour of a ferritic steel weld metal

Abstract

The present study deals with the short fatigue crack growth behaviour of a ferritic steel (nuclear grade SA 333 Grade 6 steel) weld metal at two load ratios R of 0.1 and –1. Single edge notched weld joint specimens were tested at different stress levels. The acceleration and deceleration in short crack growth with increasing stress intensity factor range was explained in terms of microstructure of weld metal. Acicular ferrite and grain boundary ferrite acted as barriers to crack growth. Non-propagating cracks were observed due to multiple cracks and blocking by ferrite grain boundaries. Transition crack length, the limiting crack length above which the crack exhibited a typical long crack behaviour, was determined to be 1 mm at R=0.1 and 1.5 mm at R=2 1.