Uniaxial tensile testing is a method used throughout the world to measure the strength and ductility of materials. An important aspect of uniaxial tensile testing which often goes unrecognized is test system alignment. Poor alignment can significantly influence test results at small strains, especially the fracture strengths of materials in a brittle state. The purpose of this review paper is to enable a reader to identify sources of misalignment, recognize the effects of misalignment on tests results, evaluate the extreme surface bending strains and stresses, and become acquainted with some techniques for reducing misalignments to within tolerable limits. Numerous references are made to the literature which describes how misalignment may be influenced by couplings in the loading train and by specimen design. A quantitative assessment of the devices and techniques discussed in this literature is made in those cases where sufficient data have been provided. The literature surveyed indicates that misalignment in carefully designed and precisely machined testing systems ranges between 3 and 15% bending. The need for reporting the misalignment at which a given test result is obtained is pointed out.

Alignment Problems in the Tensile Test

Bearing deflection behaviour of a loaded hole in CFRP

A new uniaxial compression test, developed for high strength ceramics, has successfully measured the strength of 94% alumina (Al2O3). Testing was done using a procedure developed specifically for obtaining strengths to 10 GPa on very difficult to machine ceramics. Strain gage analysis and fractographic observations confirmed that a uniaxial compressive stress state existed. Two existing test methods were also employed to measure the compressive strength of the alumina. Results from these tests are compared with the new methodology.

A Compression Test for High Strength Ceramics

It is common for aircraft design studies to begin with low-fidelity tools and move to higher-fidelity tools at later stages. After early conceptual design stages, designers can take advantage of developments in high-fidelity aerodynamic shape optimization, and more recently, coupled aerostructural optimization to improve their designs. Over the past few years, our research group has developed a framework that allows carrying out high-fidelity aerostructural optimization by coupling a RANS CFD solver to an FEM solver that uses shell elements. In addition, we have recently developed OpenAeroStruct, a light-weight and open-source tool for low-fidelity aerostructural optimization that couples a VLM code to an FEM code that uses spatial beam elements. Due to their low cost, such low-fidelity tools remain useful for design studies. In this paper, we present results from OpenAeroStruct for the optimization of a transport aircraft wing and compare them to results from our group’s high-fidelity framework. Additionally, we describe the simplified wingbox model developed and implemented with OpenAeroStruct for this work.

Low-Fidelity Aerostructural Optimization of Aircraft Wings with a Simplified Wingbox Model Using OpenAeroStruct

A tensile testing technique utilizing a powder-cushion between the specimen/grip surfaces has been devised and tested. Bending strains associated with this gripping technique have been determined as a function of load with a buttonhead specimen instrumented with nine strain gages. Experiments with eight different powders showed that lubricating powders such as graphite and boron nitride are more effective in producing good alignment, with graphite considered superior. Alignment obtained with the graphite powdercushion is comparable with that obtained with more complex devices and procedures.

Powder-Cushion Gripping to Promote Good Alignment in Tensile Testing

Axiality of loading is very important in the tensile test, particularly under creep conditions. An experimental evaluation of bending of the test section using resistance strain gauges has been devised. This test showed that excessive bending was present in spite of the adoption of a coupling design recommended in the literature. The cause of bending was found to be solely due to a friction moment exerted by the pin joints which formed the essential feature of the couplings. A crossed knife-edge design was substituted for them with satisfactory results. Analysis of the pin joint showed it to be unsuitable for specimen alignment in the tensile test.

Axiality of loading in the tensile test

Shear centre is an important geometric property of thin-walled sections that can be difficult to determine in practice. A computer based solution is developed for sections comprising an arb...

Shear centre of thin-walled sections:

A creep laboratory for variable-load and variable-temperature testing in uniaxial tension has been established at the University of Newcastle upon Tyne. The philosophy behind the specification and design of the equipment is described, considerable attention having been paid to accurate control of the test environment. The method of processing experimental data, obtained with a data acquisition system, is outlined.

C Grant

Papers

Axiality of loading in the tensile test

Shear centre of thin-walled sections:

Equipment for creep testing at variable load and temperature