Driven by testing
Ian McEnteggart examines all stages of materials testing for the automotive industry.
The urgent need to reduce harmful emissions is now a key driver for the automotive industry. Manufactures are employing a wide range of advanced materials to produce more efficient, green power generation systems and reduce the mass of vehicles. Globally, the drive to reduce the weight of cars has led to a huge increase in R&D across material industries, requiring more mechanical testing of materials. These include new high-strength steel and aluminium alloys, along with a huge range of polymers and composites. The characterisation of the mechanical properties of these materials during all phases of life from processing during manufacture – pressing or moulding, normal use (stiffness, dent resistance and fatigue) to abnormal use (crash) and end-of-life recycling – is essential to provide data for modelling and quality control. Furthermore, when assembling structures made from a wide variety of different materials, the performance of the joints between dissimilar materials is a major consideration.
Traditionally, automotive bodies have been manufactured from press formed sheet metal, and the vast majority of vehicles continue to be manufactured in this way. Advances in formable high-strength steels and aluminium alloys are helping to reduce the mass of vehicles while still using traditional pressing processes and equipment. Tensile testing is widely used to determine the mechanical properties of sheet metals appropriate to press forming operations during manufacturing and end use.
A tensile test on sheet metal can be used to determine properties such as offset yield, ultimate tensile strength, elongations, plastic strain ratio (r-value) and strain hardening exponent (n-value). These values are the properties that define the formability of the material. The r-value is the ratio of the true strain in the width direction to the true strain in the thickness direction when a sheet material is plastically deformed in uniaxial tension and it is a measure of sheet metal's formability. The n-value is a measure of the increase in the strength of the material due to cold working. This is important for the behaviour during the forming process and the strength of the final product.
Injection moulded thermoplastic parts
Polymers are used in almost all parts of a vehicle’s construction, including under bonnet components and body panels. Characterisation of thermoplastic polymer properties is critical in the development of materials and manufacturing processes to determine the functionality of a finished product in quality control.
The quality of injection moulded thermoplastic parts is dependent on the flow behaviour of the polymer. Melt flow testers and capillary rheometers are used to characterise the flow properties of thermoplastics over a range of temperatures. The melt flow index (MFI) measures the ease-of-flow of the melt of a thermoplastic – it is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length under a defined pressure.
The performance of plastic materials at high temperatures are compared by measuring the heat deflection temperature (HDT) or the Vicat softening temperature. The HDT test identifies the temperature at which the stiffness of a plastic material starts to reduce rapidly. In the HDT test, a specimen is placed in a three-point flexure test fixture and subject to a specified flexural stress. The temperature of the specimen is then increased at a uniform heating rate until the specimen deflection reaches a defined amount at the heat deflection temperature. The Vicat test identifies the temperature at which a plastic material begins to soften rapidly. This is similar to the HDT test, but the specimen is loaded using a needle-shaped indenter. HDT and Vicat testing instruments incorporate multiple stations to allow several specimens to be tested in parallel. Other mechanical tests such as long-term creep tests on materials and components over a range of temperatures may be required to establish the dimensional stability of a thermoplastic part.
Composites in cars
Out of all the new material types being adopted by the automotive industry, continuous carbon fibre polymer composites offer the greatest potential for producing lightweight vehicles. However, there are many barriers to their widespread introduction. Currently, the cost and process times of producing composite parts is significantly higher than that of traditional metal parts and the unique nature of composite materials presents designers and engineers with new challenges.
The successful use of composite materials requires a thorough understanding of their mechanical properties. The properties of most metals and plastics are isotropic (independent of direction) and homogenous – a single uniform phase. In consequence, their mechanical properties can be described by a small number of material constants obtained from simple tests. Describing the properties of anisotropic and inhomogeneous composite materials requires many material constants obtained from a range of different mechanical test modes.
Determination of the static bulk properties of composite materials requires tension, compression and shear tests. Other tests are used to characterise properties related to inhomogeneity such as inter-laminar fracture toughness – a measure of resistance to delamination. Creep and fatigue testing is needed to predict the long-term durability of a material in service, with fatigue testing of composite materials generally performed using tension-tension cyclic loading of rectangular specimens. Typically, a number of cyclic tests are performed at various stress amplitudes to produce an S-N curve that plots the stress amplitude against the number of cycles to failure. Fatigue testing of composites is time consuming because the test frequency must be limited to prevent the specimen from overheating. Fatigue loading cycles, including compression loading, are not common because of the difficulty in preventing specimen bucking. Tests often need to be conducted over a range of temperatures, as the operating temperatures of the polymer matrix material may be significant to its glass transition temperature.
Joining and impact conditions
The wide range of materials being employed in automotive structures places an increasing importance on the performance of the joints between the various materials. Common joining methods include welding, fasteners and adhesives. The choice of joining method can have a significant effect upon the weight and perceived quality (noise, vibration and harshness) of the vehicle. Mechanical testing plays a vital role in the development and validation of joint designs as well as production quality control. The type of test should match the type of joint being considered. Static property mechanical tests for joints are tensile pull off, lap shear and peel. In addition, fatigue tests are often conducted on critical joints to assess their durability in service. The strength of adhesive joints can be influenced by the testing speed and these effects can be investigated by performing high loading rate tests such as a wedge impact test on a drop tower. This type of test is used to determine the high rate cleavage resistance of an adhesive bond under impact loading conditions.
High-rate testing of materials is required to provide data for modelling vehicle behaviour in the event of a crash. Common examples of high-rate testing of materials include impact testing and high-rate tension/compression testing. In a high-rate impact test, a material or part is subject to an impact from a drop weight or hydraulically driven indenter, and the resultant damage is examined. In the case of composite laminates, the damage may be hidden. In this case, it is evaluated by either visual or ultrasonic inspection and/or the determination of the residual strength in compression after impact (CAI) test. In a CAI test, an impact-damaged composite panel is mounted in a support to prevent buckling and loaded in compression until it fails. The reduction in the failure force caused by a given level of impact energy gives a comparative measure of the material’s damage resistance.
High-rate tension or compression testing can be performed using either a drop tower with a high-speed force sensor or with a high-rate servo-hydraulic testing machine. Typically, the aim is to determine stress-strain curves for a material over a range of strain rates, as different parts of a vehicle will be subject to different strain rates during a crash. In performing this type of work, the servo-hydraulic testing machine provides a more flexible platform as the test can, over a wider speed range, maintain a constant strain rate.
Driven by demanding environmental regulations and intense competition, the automotive industry is at the forefront in adopting new materials and processes. Mechanical testing is a vital part of materials development and selection, materials modelling, process development and quality control. Testing machine suppliers continue to develop solutions for the wide range of new materials.
Ian McEnteggart is Composites Market Manager at Instron, UK, having worked for more than 35 years in developing electronic controllers, software and transducers, including video extensometers for material testing applications. He has participated in the development of international standards for materials testing, transducer calibration, composite testing fixtures and software testing.