YIELD & TENSILE STRENGTH OF STEEL & ALUMINIUM USING MICROINDENTATION Prepared by Duanjie Li, PhD & Pierre Leroux 6 Morgan, Ste156, Irvine CA 9618 P: 949.461.99 F: 949.461.93 nanovea.com Today's standard for tomorrow's materials. 014 NANOVEA
INTRODUCTION The yield strength (YS) is the stress at which a material begins to deform plastically and will not returns to its original shape when the applied stress is removed. The ultimate tensile strength (UTS) is the maximum stress that a material can withstand while being pulled before breaking. These two properties indicate the upper limit to the load that can be applied to mechanical parts and structure, and play important roles in materials production including annealing, forging, rolling and pressing. IMPORTANCE OF YS and UTS MEASUREMENT USING INDENTATION TECHNIQUE Traditionally the YS and UTS have been tested using a tensile testing machine, a large instrument requiring enormous strength to pull apart the test specimen. It is both costly and time-consuming to properly machine many test coupons for a material each sample can only be tested once. Small defects in the sample could possibly create noticeable variance in the test result. The different configuration and alignment of the tensile testers in the market often result in substantial variations in the mechanics of testing and their outcomes. Nanovea s innovative indentation method directly provides the YS and UTS values comparable to that measured by conventional tensile tests. This measurement opens up a whole new realm of possibilities for all industries. The simple experimental setup significantly cuts the time and cost for sample preparation, compared to the coupons with a complex shape for tensile tests. The small indentation size makes it possible to perform multiple measurements on one single sample. It can prevent the influence of defects in the tensile test coupons created during sample machining. Moreover, it allows YS/UTS measurements on small samples and localized areas, vital for YS/UTS mapping and local defect detection of pipelines or auto structure. MEASUREMENT OBJECTIVE In this application, the Nanovea Mechanical Tester, in indentation mode is used to measure the YS and UTS of metal alloy samples, including stainless steel SS304 and aluminum Al6061. The test samples were chosen for their commonly recognized YS and UTS values to show the reliability of the indentation method. Fig. 1: Indenter and depth sensor on the test sample.
MEASUREMENT PRINCIPLE Nanoindentation is based on the standards for instrumented indentation, ASTM E546 and ISO 14577. It uses an established method where an indenter tip with a known geometry is driven into a specific site of the material to be tested, by applying an increasing normal load. When reaching a pre-set maximum value, the normal load is reduced until complete relaxation occurs. The load is applied by a piezo actuator and the load is measured in a controlled loop with a high sensitivity load cell. During the experiment the position of the indenter relative to the sample surface is precisely monitored with high precision capacitive sensor. The resulting load/displacement curves provide data specific to the mechanical nature of the material under examination. Established models are used to calculate quantitative hardness and modulus values for such data. Nanoindentation is especially suited to load and penetration depth measurements at nanometer scales and has the following specifications: Maximum displacement (Dual Range) : 50 m or 50 m Depth Resolution (Theoretical) : 0.003 nm Depth Resolution (Noise Level) : 0.15 nm Maximum force : 400 mn Load Resolution (Theoretical) : 0.03 N Load Resolution (Noise Floor) : 0.3 N Analysis of Indentation Curve Following the ASTM E546 (ISO 14577), hardness and elastic modulus are determined through load/displacement curve as for the example below. Pmax S = dp/dh ht hc hmax Fig. : Load-displacement curve of nanoindentation. 3
Hardness The hardness is determined from the maximum load, P max, divided by the projected contact area, A c : Pmax H A c Young s Modulus The reduced modulus, E r, is given by: E r S A c Which can be calculated having derived S and A C from the indentation curve using the area function, A C being the projected contact area. The Young s modulus, E, can then be obtained from: 1 1 1 E E E r i i Where E i and i are the Young s modulus and Poisson s ratio of the indenter and the Poisson s ratio of the tested sample. How are these calculated? A power-law fit through the upper 1/3 to1/ of the unloading data intersects the depth axis at h t. The stiffness, S, is given by the slope of this line. The contact depth, h c, is then calculated as: 3P h c h max max 4S The contact Area A c is calculated by evaluating the indenter area function. This function will depend on the diamond geometry and at low loads by an area correction. For a perfect Berkovich and Vickers indenters, the area function is A c =4.5h c. For Cube Corner indenter, the area function is A c =.60h c. For Spherical indenter, the area function is A c =πrh c, where R is the radius of the indenter. The elastic components, as previously mentioned, can be modeled as springs of elastic constant E, given the formula: where σ is the stress, E is the elastic modulus of the material, and ε is the strain that occurs under the given stress, similar to Hooke's Law. The viscous components can be modeled as dashpots such that the stress-strain rate relationship can be given as the viscosity of the material, and dε/dt is the time derivative of strain., where σ is the stress, η is Since the analysis is very dependent on the model that is chosen, Nanovea provides the tool to gather the data of displacement versus depth during the creep time. The maximum creep displacement versus the maximum depth of indent and the average speed of creep in nm/s is given by the software. Creep may be best studied when loading is quicker. Spherical tip is a better choice. Other possible measurements by Nanovea Mechanical Tester: Stress-Strain & Yield Stress, Fracture Toughness, Compression strength, Fatigue testing and many others. 4
TEST CONDITIONS & PROCEDURES The YS/UTS tests were performed on the Nanovea Mechanical Tester in the Microindentation mode. The tip used was a cylindrical flat diamond tip of 00 μm diameter. SS304 and Al6061 alloys were selected for their extensive industrial application and commonly recognized YS and UTS values, in order to show the great potential and reliability of the indentation method. They had been mechanically polished to a mirror-like surface finish prior to the tests so as to avoid the influence of surface roughness or defects to the test results. Depth versus load was recorded during the test. The test conditions are listed in Table 1. More than ten tests were performed on each sample to ensure repeatability of the test values. RESULTS AND DISCUSSION Material SS304 Al6061 Applied Force (N) 40 30 Loading rate (N/min) 80 60 Unloading rate (N/min) 80 60 Table 1: Test conditions of YS & UTS for SS304 and Al6061. The load-displacement curves of the SS304 and Al6061 alloy samples are shown in Fig. 3 with the flat indenter imprints on the test samples inset. Through analysis of the S shape of the loading curve using a set of special algorithm developed by Nanovea, the YS and UTS are automatically calculated by the software as summarized in Table 1. The YS and UTS values obtained by conventional tensile tests are listed for comparison. Fig. 3: Load-displacement curves of SS304 and Al6061 samples. The flat indenter imprints on the test samples are inset. 5
Material Yield strength (MPa) Ultimate tensile strength (MPa) Indentation Tensile test Indentation Tensile test SS304 77±1 75 609±16 60 Al6061 31±18 41 301±9 300 Table : YS and UTS of SS304 and Al6061 measured using indentation and tensile tests. The results show that the YS and UTS values of the SS304 and Al6061 alloys measured using the indentation and tensile techniques are in a good agreement. The YS and UTS tested by indentation mode of Nanovea Mechanical Tester exhibit superior precision and repeatability. Compared to conventional tensile test procedure, the indentation technique for YS and UTS evaluation in this study is substantially more time- and cost-effective. It does not require expensive complicated sample machining, enables fast quantitative mapping of YS and UTS values on small samples, and provides a solution of detecting localized mechanical defects of mechanical parts in service. CONCLUSION In this study, we showcased the capacity of Nanovea Mechanical Tester in evaluating YS & UTS of stainless steel and aluminum alloy sheet samples. The simple experimental setup significantly cuts the time and cost for sample preparation required for tensile tests. The small indentation size makes it possible to perform multiple measurements on one single sample. This method allows YS/UTS measurements on small samples and localized areas, providing a solution for YS/UTS mapping and local defect detection of pipelines or auto structure. The Nano, Micro or Macro modules of the Nanovea Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system. Nanovea's unmatched range is an ideal solution for determining the full range of mechanical properties of thin or thick, soft or hard coatings, films and substrates, including hardness, Young s modulus, fracture toughness, adhesion, wear resistance and many others. In addition, optional 3D non-contact profiler and AFM Module are available for high resolution 3D imaging of indentation, scratch and wear track in addition to other surface measurements such as roughness. To learn more about Nanovea Mechanical Tester or Lab Services. 6