Materials Testing - Advances in String Technology in Sporting Equipment Materials Testing by Lloyd Instruments

Topics Covered

Background
Introduction
String Performance
Polyester & Co-Poly Monofilament Strings
String Testing
Tensile testing
LFPlus Material Testing Machine
NEXYGENPlus Material Test And Data Analysis Software
Summary

Background

Materials testing is a well-established technique used to determine the physical and mechanical properties of raw materials and components from a human hair to steel, composite materials and ceramics.

To meet the challenges posed in testing a wide diversity of materials, Lloyd Instruments offers a comprehensive range of high performance materials testing machines, designed to make accurate and repeatable force measurements in the range from 0.1 N to 300 kN (0.0225 lbf - 67443 lbf). Depending on the machine, elongations of between 1 micron and 2.5 m (98.4 in) can be measured.

Introduction

Recent epic contests in World Tennis such as the Men’s Wimbledon Final in 2008 and the US Open Final in 2009, both of which went to a gruelling five sets, emphasise the extraordinary stresses and strains experienced by modern tennis rackets and strings. Huge impacts are experienced from serving at speeds up to 140 mph, together with a range of other forces resulting from the breadth of spin shots, drop shots, lobs and smashes that occur during the course of a single game. As racket heads have become larger and frames have become lighter, an even greater demand has been placed on the strings. Material testing of tennis, badminton and squash rackets helps racket manufacturers assess quality during production, with measurements such as impact strength, durability, fatigue, torsion strength and flex being important tests for quality assurance. In addition, tensile testing plays an important role in string production. String Technology UK, a specialist supplier of strings for tennis rackets uses an LFPlus materials testing machine from Lloyd Instruments for checking the tensile strength, elongation at break, knot strength and elongation percentage of strings at given loads.

Features and Performance of String

Whilst the construction of the racket itself is of great importance, string selection can significantly influence the performance of a given racket. Strings are available in a variety of materials and gauges. In general, thick strings are more durable, while thinner strings tend to have a better “feel” and bite the ball better, helping to impart spin. In addition, string tension can be set to optimize the power, spin, lifetime and even the feeling on the arm. Synthetic strings such as polyester, co-poly monofilament and hybrid strings provide an alternative to traditional nylon and gut strings.

Polyester & Co-Poly Monofilament Strings

Polyester and co-poly monofilament strings are the fastest growing sector of the string market primarily due to low prices with high durability and are available in a variety of grades from "cost effective" to "high performance". Polyester strings are made from a single polymer whereas co-poly monofilament is a mix of 2 or more polymers allowing higher specifications to be achieved. They are generally available in different gauges (1.17-1.35mm) for different elasticity/durability levels. Polyesters are quite stiff compared yo nylon or multifilament, but provide significantly better durability, allowing thinner gauges to be used. The new co-poly monofilaments are however getting much softer and easier to string. Multifilament strings consist of many microfibres (which can be of many different materials) twisted together to form a string, which is wrapped with a resistant cover to give a performance more like natural gut.

String Testing

During the manufacturing process (extrusion) of co-polyester monofilament tennis strings, over 30 quality checks are made, starting with the raw material through to the final collection of the material onto drums. String Technology UK has extended the quality checks by adding tensile testing during the processing of the string ready for retail distribution. String Technology UK supplies tennis racket strings worldwide. The company specifies the product then adds value for the retailer by “in-house” branding, package printing and reel design, to make instantly recognisable brands.

Tensile Testing

String Technology UK uses an LFPlus single column materials testing machine from Lloyd Instruments for tensile testing, operating under computer control using NEXYGENPlus material test and data analysis software. The instrument is fitted with a 2kN horn grip. Tests are performed to measure tensile strength, elongation at break, knot strength and elongation percentage at given loads. Elongation is measured at 45kg force to replicate the ball hitting the strings. Knot strength is measured because if this is too low, the string will break. Full testing is carried out at regular intervals during processing (1 in 500 samples) plus product is randomly selected for further analysis. Future tests will include a load then relaxation cycle to allow tension loss over time to be measured. Although there is no specific testing standard for tensile testing of tennis racket strings, the NEXYGENPlus software offers an extensive library of pre-defined test set-ups based on international standards as well as the facility to set up routine tests or create an advanced multi-stage test, with the ability to monitor all aspects of data analysis.

LFPlus Material Testing Machine

The LFPlus single column universal materials testing machine is an easy to use solution for materials testing applications up to 1 kN (225 lbf) capacity. The machine uses high accuracy interchangeable XLC Series load cells. Tension and compression testing with cycling to load and extension limits are standard features. The machine features an innovative design that maximises working space. The LFPlus features an integral control console with membrane multi-function keypad and easy to read backlit LCD to display load and extension information. The control console displays prompts and menus, which guide the user through machine operations. The system is capable of storing up to 600 test results from a choice of 10 programmable test set-ups. Machine performance can be optimised under computer control using NEXYGENPlus material test and data analysis software. Test information can easily be exported to Microsoft® products such as Word, Excel® and Outlook®.

NEXYGENPlus Material Test And Data Analysis Software

NEXYGENPlus is extraordinarily easy to use, flexible, multi-lingual materials testing software. It allows the operator to control and monitor all aspects of the system from a single front end, ensuring fast, reliable and powerful testing and data analysis for tensile testing, as well as compression, peeling, tearing, creep, relaxation and flexural test applications. With NEXYGENPlus software, the user can simply select from the extensive library of pre-defined test set-ups based on international standards to set up routine tests or create an advanced multi-stage test, all using the same program. Still picture and video capture are also included in the software as well as a powerful security and audit trail module to provide 100% traceability, full automation capability for automatic testing and much more.

Summary

A complex set of variables influence the performance of a tennis racket. Players from the top professionals to the weekend player have their preferences. The application of materials testing as part of the quality control procedures brings enhanced levels of repeatability from batch to batch.

Source: Lloyd Instruments

For more information on this source please visit Lloyd Instruments