M. Costa

Environmental effect on the fatigue degradation of adhesive joints: A review

ABSTRACT Environmental factors, such as temperature and moisture, are known to have a degrading effect on the mechanical properties and performance of adhesive joints, which may be perceived as a non-problem because various works have shown that the static response of an adhesive is normally unaffected by slight moisture and temperature variations that occur in real-world applications. While this may be true, performance under purely static conditions is rarely found in commercial uses and most adhesive joints are subjected to cyclic loadings throughout their life. Interestingly, not much work has been done on the effects of the environment on cyclically loaded adhesive joints, but the consensus is that the fatigue response is much more affected by environmental changes than the static response, which is arguably the most important analysis. The general trend is that hygrothermal ageing decreases the number of cycles the joint can withstand and also decreases the threshold fracture toughness value, which translates to cracks initiating sooner, but exceptions to these behaviours also exist.

A review on the temperature and moisture degradation of adhesive joints

Despite offering very attractive advantages over traditional joining methods, one of the setbacks of adhesive bonding is its long-term strength in aggressive environments, such as environments with high moisture and extreme temperatures. With the rise of new lightweight materials and their recent use in everyday vehicles, transportation industries have been very interested in determining the long-term behavior of adhesive joints. The aim is to build durable, lighter vehicles, which consume less energy and emit less pollution. The two main factors that affect the strength of vehicle adhesive joints are exposure to moist environments and high and low temperatures. There are some works concerning the effect of these two factors separately and some predictive models have been developed, which help the engineer to design reliable, safe, and efficient adhesive joints. However, the combined effect of temperature and moisture is not yet totally understood. This paper presents a review on the temperature and moisture degradation of adhesive joints.

Behaviour of environmentally degraded epoxy adhesives as a function of temperature

ABSTRACT Structural adhesives are increasingly being used in the aerospace and automotive industries. They allow for light weight vehicles, fuel savings, and reduced emissions. However, the environmental degradation of adhesive joints is a major setback in its wide implementation. Moisture degradation of adhesive joints includes plasticization, attacking of the interface, swelling of the adhesive and consequent creation of residual stresses. This may lead to reversible and irreversible damage. The main factors affecting the strength of adhesive joints under high and low temperatures are the degradation of the adhesive mechanical properties and the creation of residual stresses induced by different coefficients of thermal expansion (between the adhesive and the adherends). The effect of the combined effect of moisture and temperature is not yet fully understood. The aim of this study is to shed light on this subject. In this work bulk water absorption tests were conducted at different moisture conditions in order to assess the diffusion coefficient, maximum water uptake, and glass transition temperature. Aged and unaged small dogbone tensile specimens were tested under different temperature conditions. The glass transition temperature of the adhesives as a function of the water uptake was assessed. The aim is to determine the evolution of the properties of two epoxy adhesives as a function of two variables (environmental temperature and moisture).

Effect of humidity on the mechanical properties of adhesively bonded aluminium joints

The presented work focuses on the effects of water degradation on the long-term behaviour of adhesive joints. The objective of this study is to measure the evolution of various mechanical properties such as tensile stress and fracture toughness as a function of humidity for two distinct adhesives, using bulk adhesive and double cantilever beam specimens in unaged and aged conditions in order to understand the influence of humidity on the adhesive properties. A mathematical equation that allows the prediction of each property degradation as a function of water is proposed and validated, which takes into account various parameters such as the diffusion coefficient, resulting in a general equation for mechanical property degradation prediction of potentially any adhesive. It was also found that the distinct adhesive properties such as strength, stiffness and fracture toughness all decreased due to water degradation with the exception of the strain that increased, concluding that water reduces the joint strength and lifespan of the studied adhesives, although in different ways.

Effect of the size reduction on the bulk tensile and double cantilever beam specimens used in cohesive zone models

Cohesive zone elements used in finite element analysis are a reliable way to design and predict the behaviour of the joint. The characterisation of the traction separation law used in these models is done using tensile and fracture tests, and the parameters of such laws depend on humidity and temperature. Water diffusion tests are therefore necessary, which are dependent on specimen geometry, meaning a bigger specimen takes longer to fully saturate. To solve this problem and increase the efficiency of the ageing process, smaller tensile bulk and double cantilever beam (DCB) specimens are necessary. Another advantage of smaller DCB specimens is that they can be tested in smaller high-temperature chambers, where normal DCB specimens do not fit. Smaller geometries of the bulk tensile and DCB tests are analysed, and a proposed geometry for each test is shown to produce very satisfactory results, validating the use of these specimens.

Effect of Humidity on The Fatigue Behaviour of Adhesively Bonded Aluminium Joints

THE PRESENT WORK FOCUSES ON THE EFFECTS OF WATER DEGRADATION ON THE FATIGUE BEHAVIOUR OF ADHESIVE JOINTS BONDED WITH ALUMINIUM ADHERENDS. THE OBJECTIVE OF THIS STUDY IS TO MEASURE THE INFLUENCE THAT HUMIDITY HAS ON THE FATIGUE CRACK GROWTH VELOCITY OF TWO DISTINCT ADHESIVES CHARACTERIZED USING THE PARIS LAW, USING DOUBLE CANTILEVER BEAM (DCB) SPECIMENS IN UNAGED AND VARIOUS AGED CONDITIONS LOADED IN MODE I IN ORDER TO UNDERSTAND THE INFLUENCE THAT WATER CONTENT HAS ON THE PARIS LAW CONSTANTS. IT WAS FOUND THAT THE SLOPE OF THE PARIS LAW CURVE IS NOT HEAVILY CHANGED WITH THE PRESENCE OF WATER, BUT A SHIFT IN THE CURVES DOES OCCUR, GENERALLY RESULTING IN A CRACK INITIATING AT A LOWER THRESHOLD THAN IN THE UNAGED ADHESIVE. BASED ON THIS BEHAVIOUR, IT CAN BE CONCLUDED THAT AN INCREASE IN WATER CONTENT REDUCES THE FATIGUE JOINT STRENGTH AND LIFESPAN OF ADHESIVE JOINTS BONDED WITH THE STUDIED ADHESIVES.

Strain rate dependence of adhesive joints for the automotive industry at low and high temperatures

Abstract In this study the impact and quasi-static mechanical behaviour of single lap joints (SLJ) using a new crash resistant epoxy adhesive has been characterized as a function of temperature. Single lap adhesive joints were tested using a drop weight impact machine (impact tests) and using a universal test machine. Induction heating and nitrogen gas cooling was used in order to achieve a homogeneous distribution of temperature along the overlap of + 80 °C and −20 °C, respectively. Adherends made of mild steel, similar to the steel used in automobile construction, were chosen in order to study the yielding effect on the strength of the SLJ. Results showed that at room temperature (RT) and low temperature (LT), failure was dictated by the adherends due to the high strength of the adhesive. At high temperature (HT), a decrease was found in the maximum load and energy absorbed by the joint due to the reduced strength of the adhesive at this temperature. The results were successfully modelled using the commercially available finite element software Abaqus®. Good correlation was found between experimental and numerical results, which allows the reduction of experimental testing.

Water Diffusion in Double Cantilever Beam Adhesive Joints

STRUCTURAL ADHESIVES ARE INCREASINGLY BEING USED IN THE AEROSPACE AND AUTOMOTIVE INDUSTRIES. THEY ALLOW FOR LIGHT WEIGHT VEHICLES, FUEL SAVINGS AND REDUCED EMISSIONS. HOWEVER, THE ENVIRONMENTAL DEG-RADATION OF ADHESIVE JOINTS IS A MAJOR SETBACK IN ITS WIDE IMPLEMEN-TATION. MOISTURE DEGRADATION OF ADHESIVE JOINTS INCLUDES PLASTICIZA-TION, ATTACKING OF THE INTERFACE, SWELLING OF THE ADHESIVE AND CONSE-QUENT CREATION OF RESIDUAL STRESSES. THIS MAY LEAD TO REVERSIBLE AND IRREVERSIBLE DAMAGE. IN THIS WORK DOUBLE CANTILEVER BEAM (DCB) SPECIMENS USING TWO DIFFERENT ADHESIVES FOR THE AUTOMOTIVE INDUSTRY WERE SUBJECTED TO TWO DIFFERENT AGEING ENVIRONMENTS. THEY WERE TESTED PERIODICALLY UNTIL THE TOUGHNESS OF THE ADHESIVES STABILIZED, WHICH MEANS THAT THEY WERE FULLY DEGRADED. AN ASSOCIATION WAS MADE BETWEEN THE TOUGHNESS OF THE ADHESIVE AND THE AMOUNT OF WATER THAT IT HAD ABSORBED. THIS WAY IT WAS POSSIBLE TO INDIRECTLY MEASURE THE WATER UPTAKE IN AN ADHESIVE JOINT TAKING INTO ACCOUNT THE WATER UPTAKE PROPERTIES OF THE ADHESIVES STUDIED, WHICH HAD BEEN DETERMINED IN ANOTHER STUDY. IT WAS FOUND THAT DIFFUSION OF WATER INTO THE STUDIED ADHESIVE JOINTS WAS FASTER THAN DIFFUSION THROUGH THE BULK ADHESIVE ALONE. A MODEL THAT TAKES INTO ACCOUNT DIFFUSION THROUGH THE INTERFACE BETWEEN THE ADHESIVE AND THE ADHERENDS WAS PROPOSED.

Moisture and temperature degradation of double cantilever beam adhesive joints

Abstract In this work, the double cantilever beam (DCB) test is analysed in order to evaluate the combined effect of temperature and moisture on the mode I fracture toughness of adhesives used in the automotive industry. Very few studies focus on the combined effect of temperature and moisture on the mechanical behaviour of adhesive joints. To the authors’ knowledge, the simultaneous effect of these conditions on the fracture toughness of adhesive joints has never been determined. Specimens using two different adhesives for the automotive industry were subjected to two different ageing environments (immersion in distilled water and under 75% of relative humidity). Once they were fully degraded, they were tested at three different temperatures (−40, 23 and 80 °C), which covers the range of temperature an adhesive for the automotive industry is required to withstand. The aim is to improve the long term mechanical behaviour prediction of adhesive joints. The DCB substrates were made of a high strength aluminium alloy to avoid plastic deformation during test. The substrates received a phosphoric acid anodisation to improve their long term adhesion to the adhesive. Results show that even though a phosphoric acid anodization was applied to the adherends, when the aged specimens were tested at room temperature and at 80 °C, they suffered interfacial rupture. At −40 °C, however, cohesive rupture was observed and the fracture toughness of the aged specimens was higher.