J.J.M. Machado

Adhesives and adhesive joints under impact loadings: An overview

ABSTRACT The study of the behaviour of adhesive joints under impact loadings is a very active field of research, driven by significant industrial interest. Many industries, such as the automotive industry, are currently employing adhesive joints extensively, making use of the inherent properties of adhesive joints to improve the mechanical behaviour, reduce weight, and simplify manufacturing. Reduced structural weight is achieved by combining multiple lightweight materials, which is made possible by using adhesive joints. Impact strength is also a major factor, as vehicles must be able to provide adequate safety levels for their occupants during collisions. Another example of industrial application is the defence industry, which uses bonded structures to withstand ballistic impacts, with extremely high impact velocities. Understanding the behaviour of adhesive joints under impact is, therefore, crucial for designing stronger and safer structures. This document aims to review the research that has been previously undertaken in this field. Discussed research topics include high strain rate property determination, adhesive joint testing, effects of coupling environmental conditions with impact loads, and sections on numerical and constitutive modelling procedures. The final sections describe some practical applications of adhesive joints under large strain rates and relate them to the fundamental concepts previously discussed.

Mode I fracture toughness of CFRP as a function of temperature and strain rate

Composite structures currently used in the automotive industry must meet strict requirements for safety reasons. They need to maintain strength under varied temperatures and strain rates, including impact. It is therefore critical to fully understand the impact behaviour of composites. This work presents experimental results regarding the influence of a range of temperature and strain rates on the fracture energy in mode I, GIC, of carbon fibre reinforced plastic plates. To determine GIC as a function of temperature and strain rate, double cantilever beam specimens were tested at 20, 80 and −30℃, with strain rates of 0.2 and 11 s−1. A complementary numerical study was performed with the aim of predicting strength using the measured values. This work has demonstrated a significant influence of the strain rate and temperature on GIC of the composite materials, with higher strain rates and lower temperatures causing a decrease in the GIC values.

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.

A strategy to reduce delamination of adhesive joints with composite substrates

The use of bonding for joining composite materials in high-performance structures has increased significantly, as this joining method offers improved stress distributions and capability of joining dissimilar materials. However, the use of adhesive bonding for this purpose might lead to delamination failure, caused by peel stresses acting on the generally weaker transverse direction of the composite adherends. This work focused on improving the resistance to delamination of composite adhesive joints by using a novel composite with a reinforced high toughness resin on the surfaces. Single-lap joints using the novel composite material as adherends, were found to have 22% higher failure loads when compared with the specimens using carbon fiber reinforced polymer only adherends, with the failure mode changing from delamination of the adherends to cohesive failure in the adhesive. The lap shear strength was also close to that attained when using high strength steel adherends. A finite element analysis, using cohesive elements, was performed with the objective of reproducing the experimental results and better understanding the failure mechanism. Using this model, it has been determined that the change of failure mode and the plasticity on the surface layers are the two key factors underlying the increase in strength obtained with the novel adherends.

Mechanical behaviour of adhesively bonded composite single lap joints under quasi-static and impact conditions with variation of temperature and overlap:

The use of adhesively bonded joints in structural components for the automotive industry has significantly increased over the last years, supported by the widespread integration of composite materials. This synergy allows vehicle manufacturers to offer a significant weight reduction of the vehicle allowing for fuel and emissions reduction and, at the same time, providing high mechanical strength. However, to ensure vehicle safety, the crashworthiness of these adhesive joints must be assessed, to evaluate if the structures can sustain large impact loads, transmitting the load and absorbing the energy, without damaging the joint. The novelty of this work is the study of the strain rate dependent behaviour of unidirectional composite adhesive joints bonded with a ductile epoxy crash resistant adhesive, subjected to low and high testing temperatures and using different overlap lengths. It was demonstrated that joints manufactured with this type of adhesive and composite substrates can exhibit excellent quasi-static and impact performance for the full range of temperatures tested. Increasing the overlap length, and independently of the testing temperature, it was observed an increase of energy absorbed for both quasi-static and impact loads, this is of considerable importance for the automotive industry, demonstrating that composite joints exhibit higher performance under impact.