Case Study: The Use Of Cambridge Engineering Selector Program To Assign The Most Effective Material For A Hip Implant

Introduction

The selection of a material for a product is among one of the most important decisions that an engineer can take on. If material selection is conducted incorrectly it can lead to unpredictable consequences for the product life. So what is material selection and how can it be conducted effectively? Material selection is the systematic process of designing any physical product and it is highly important to the life span of the product. We can make fast, concise and critical decisions on material design informed by programs like CES by data analysis. Material selection truly is important to ensure the integrity of the design. The ability to prioritise certain properties over others to give us a more appropriate selection of materials for our product in question.

Description of the CES

Cambridge Engineering Selector is a program for the evaluation of material choice for engineering design. It is a computer aid that helps a design engineer create a robust design by data analysis. This data analysis is critical to providing decision support since theoretical computations and experiments can be highly time consuming and can often fail to address all issues in question. The Cambridge Engineering Selector consists of two main components, a selector and a constructor. The selector is a user face that allows the designer to search, browse and analyse the data that is stored within the database. This is a systematic methodology for the selection of materials such that the designer can optimise the life span of the product based on specific design criteria. The constructor allows the user to develop new databases within the system. One of the features of the CES is the database which is a table containing material data, manufacturing process data and information on structural section.

1. Summarising the Brief

The objective of the case study was to utilise the CES program in order to assign the most effective material for the product. In this given assignment, I chose the hip implant to research a suitable material. Certain characteristics were analysed and charts were made with CES to get the perfect combination of the chosen characteristics in order to select the most suitable material for the hip implant.

2. Aim of the Report

The aim of this case study is to find the most suitable material based on attributes we wish the material to possess and constraints such as price that are going to restrict the choice of material. Research, intuition along with the CES charts and graphs were used to choose an appropriate material for the hip implant. central issue for a hip replacement candidate We have a client that requires a hip joint replacement. The area that we are targeting is located in the hip socket region and will involve the extraction of the upper femur and potentially damaged bone and cartilage. Looking at the age range for hip replacement within Ireland it ranges from 60 to 80-year-old age group. These patients experience many symptoms ranging from pain that prevents walking or bending of the joint, pain while at rest and stiffness in the hip.

I will not be considering high impact loads for this hip implant i. e. athlete, due to the expected lifestyle that that age group will have and the longer life span that the product would require. Methodology With my primary aim in check for the age range that I have chosen I have identified some potential mechanical, thermal and chemical properties that I wish to consider for my material selection. The identification of these materials has stemmed from research into the mechanics and the makeup of compact bone and the wish to closely match the characteristic of the bone to the hip implant. These are:

1. Static Load – the strength of the material
2. Youngs modulus
3. Manufacturing
4. Cost
5. Density
6. Cyclical loading
7. Toxicity
8. Corrosion resistance of the material in the human body.

Having carried out extended research into each property I have identified four properties that demonstrate a more pressing issue for the target age group of over 60’s.

The implimentation of CES in my material selection

The first two properties that I will discuss is young’s modulus and density. Young’s modulus is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression and density is simply the measurement that compares mass of an object to its volume. These two properties were of increased importance for my study due to my concern for selecting a material that was long lasting and would aim to require no additional surgical procedures or replacement. In addition, it was important to match the density of real compact bone to the manufactured implant so that we are matching the characteristics of bone.

When researching the data for the Young’s modulus and density for bone it was found to be between 0. 1 – 25 GPa for the Young’s modulus and average of 1760 kg/m^3 for the density of bone. With these figures in mind a bubble chart on CES was created of Young’s modulus versus density with density being weighted as more critical to the material hence the slope of two was implemented. It was made apparent that bone is not exactly in the known range of many existing engineering materials which proves difficult to selecting an appropriate material for the job.

The other two properties that were considered in the report were toxicity and fatigue strength. Toxicity is evidently a factor that is important. When introducing a metal into the body it is important that it is non-toxic and biocompatible. This is to reduce the risk of blood poising, infections or rejection of the implant. Toxicity rating was compared to the fatigue strength of the metal. Fatigue strength is the highest stress that the metal can withstand for a given number of cycles without breaking. In our case we are looking for a high fatigue strength as the hip implant will be used each day for various manoeuvres.

The best matches for the bone material are listed as follows:

1. Age-hardening wrought Al-alloys
2. Cast Al-alloys
3. Cast magnesium alloys
4. Commercially pure titanium
5. Non age-hardening wrought Al-all
6. Titanium alloys
7. Wrought magnesium alloys.

Using the data analysis on CES I was able to access numerical data for the various materials that could be used on the hip implant. A table was constructed in order to compare the numerous materials and to evaluate which would be the most suitable.

Evidentially the cast magnesium alloy appears to be very close in characteristics to bone with bone having a range of 3-30 for the Young’s modulus and cast magnesium alloy having a range of 42-47 GPa. Density is the same for cast magnesium alloy and bone and fatigue strength is similar. Unfortunately cast magnesium alloy has a fast degradation rate, therefore the implant would most likely cause poisoning to the patient. Titanium demonstrates itself as a strong contender as a possible material for the hip implant. One of the major benefits of using titanium is the fact that it is a biologically inert biomaterial meaning that once placed within the body it will remain unchanged. Hence it is a non-toxic material meaning that it will cause no harm to the patient and will present no risks of potential blood poisoning if the hip implant were to break within the body. This combats part two of my primary aims for the material selection.

When analysing the fatigue strength for titanium, it came out as the top performer compared to the other materials. Fatigue strength is a highly important factor to take into account when choosing a material for a hip implant. Fatigue strength looks at the capability of the material to withstand stress for a given number of cycles without it breaking – in this case it is 1200 MPa. This implies that the material will be able to endure the consistent force on it in everyday life. The density of titanium is higher than bone by a factor of two and a half, making it heavier in the body. Although comparing it to other metals it has a significantly low density. Having a lower young’s modulus is more desirable as it will tend to behave more like bone. The young’s modulus for titanium is high but still within range of being suitable for a hip implant.

Aluminium has many characteristics that match those of bone. The density of aluminium is in the same range as bone with the density being (2. 5 e3) and (1. 75 e3) respectively. The Young’s modulus is around 70GPa for aluminium and around 40 GPa for bone. The Young’s modulus for aluminium is relatively good for a metal. If the modulus is substantially larger than the bone modulus the implant will begin to take more of the load than the bone and the bone will begin to remodel itself around the implant causing loosening of the implant. This is prevented if the Young’s modulus is close to the bones.