Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

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You should now watch the video sequence 'An eye for an eye?'. Is based on a fictional court case taken from real case, which involved failure of materials.

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Miss Craddock
My clients manufacture teddy bears and similar toys. We claim that due to the negligence of the defendant a large number of these teddy bears have been going blind, and some have been losing eyes entirely. A very distressing condition, I'm sure we all agree.
Judge
Going blind Miss Craddock! Teddy bears.
Miss Craddock
It would seem so my lord to a child that happened to own one. I appear in this case for the plaintiffs trading as Fortescue Furries Limited. My learned friend Mr Sharp appears for the defendants, Williams Washers Limited.
Judge
Mr Sharp, do you accept this proposition that the teddy bears are going blind.
Mr Sharp
Yes my lud, that is not in contention.
Mr Fortescue
I swear by Almighty God that the evidence I give to the court shall be the truth, the whole truth and nothing but the truth.
Miss Craddock
Mr Fortescue, you are managing director of Fortescue Furries Limited.
Mr Fortescue
I am.
Miss Craddock
Will you describe the effect that the unfortunate phenomenon has had on your trade.
Mr Fortescue
It's been a disaster. We've had ‘em returned from all over the world.
Miss Craddock
With complaints from customers.
Mr Fortescue
I should say so, who wants a blind teddy bear, unless we sell ‘em wearing sun glasses aye. (laughs). Yes well, the retail outlets are up in arms about it, they get the stick before we do. We've had to withdraw whole batches, home markets, exports the lot. Just when our Christmas trade was beginning to build up too, I tell you we've lost a packet over this and I shan't be satisfied until we …
Miss Craddock
Quite so, Mr Fortescue. Details appear in the Statement of Claim my lud. When did these defects begin to occur in your product.
Mr Fortescue
Around er July last year.
Miss Craddock
Will you explain to the Court what the eyes consist of and how they are fitted to the toys.
Mr Fortescue
Er you can see the samples over there.
Miss Craddock
Exhibit D my lud.
Mr Fortescue
But you can see better on diagram. The eye is a polystyrene injection moulding with a rounded front and a shaft at the back. The material is brown by tradition. A spot of black paint is put on the base of the shaft to form a pupil when you look at it from the front. It's attached to the toy by pushing the shaft through the fabric, and also through this retaining washer.
Miss Craddock
Please describe the washer in more detail.
Mr Fortescue
It's got prongs that stick out in t'middle. The eye shaft is forced past the prongs so that they're bent outwards. From then on they grip the shaft.
Judge
To prevent the eye from falling out again.
Mr Fortescue
Yes my lord.
Miss Craddock
Who supply you with these washers.
Mr Fortescue
Williams Washers Limited
Miss Craddock
And what happened in January of last year.
Mr Fortescue
They changed the washers, that's what happened.
Miss Craddock
Did you know they were different.
Mr Fortescue
No, I didn't. The new ones were the same colour and the same shape, it was only a few weeks ago that we found out that they're the cause of all the trouble and the …
Mr Sharp
I object my lud.
Judge
Yes quite so. Miss Craddock your witness should not attempt to pre-judge the case.
Miss Craddock
As your Lordship pleases. Do confine yourself to answering the questions, Mr Fortescue. Did you ever have any trouble with the old washers.
Mr Fortescue
No. You look at that ted on right.
Miss Craddock
Exhibit A, my lord.
Mr Fortescue
Ten years old is that, and the eyes are as good as new.
Miss Craddock
How often have defects occurred since the new washers were supplied.
Mr Fortescue
Ooh ninety per cent of them have gone wrong in the teds, one or t'other or both, getting glazed or falling out.
Miss Craddock
Thank you Mr Fortescue.
Mr Sharp
You say you were unaware that the washers had been changed, do you make no quality control checks on your supplies.
Mr Fortescue
We check the dimensions are right.
Mr Sharp
No metallurgical tests on the material itself.
Mr Fortescue
No, why should we. Our specification, agreed by your people, is that the washers should be suitable for the job and if they're not suitable.
Mr Sharp
Thank you Mr Fortescue, no more questions my lud.
Judge
Tell me Mr Fortescue, in what way exactly are your teddy bears suffering.
Mr Fortescue
Well first the eyes become glazed
Miss Craddock
Exhibit B, my lud.
Mr Fortescue
What you might call cataracts, or er glaucoma.
Judge
Glaucoma.
Mr Fortescue
Then they start dropping out.
Miss Craddock
I intend to call an expert witness to explain precisely what is happening.
Judge
I shall be most grateful Miss Craddock.
Miss Craddock
After that Dr. Daniels you joined the Faculty of East Sussex University, where you have continued your research into industrial materials, and for six weeks you conducted tests on samples of my clients products, manufactured both before and after January of last year.
Dr Daniels
That is correct.
Miss Craddock
Will you explain to the Court what in your opinion causes the appearance of blindness and the subsequent loss of the eyes.
Dr Daniels
It's due to the phenomenon of environmental stress cracking.
Miss Craddock
Could you put that a little more simply.
Dr Daniels
Cracks have grown across the shafts of the eyes. Polystyrene is normally a glassy transparent polymeric material which gives the eyes a clear look, but when it has a crack in it, the light is reflected from the crack, giving the eyes a glazed look like glaucoma.
Judge
Ah, cracks in the material.
Dr Daniel
And when a crack has spread far enough across the shaft, it fractures and the front part of the eye drops out.
Miss Craddock
Where exactly do these cracks appear?
Dr Daniels
They originate at the point of contact with the prongs of the washers.
Miss Craddock
So, what in your opinion is the cause of the cracking.
Dr Daniels
In my opinion it's caused by the stress imposed on the polymer by the washers.
Miss Craddock
But we understand that it didn't happen with the old washers, only with the new ones, what is the difference.
Dr Daniels
The old washers were made of bronze, I noticed that the new ones although they were still bronze in colour, were magnetic. This indicated that instead of being solid bronze, they were plated steel.
Miss Craddock
And what is the effect of this.
Dr Daniels
Much greater stress. Those arms, the prongs which protrude into the centre of the washer, can be regarded as mini cantilever beams. Now when such a beam is elastically deflected from its position of rest by the fixed amount needed, the force developed depends on E and I, the Young's modulus of the material, and the second moment of area of the beam section which depends only on the dimensions and the way in which the beam is bent …
Judge
Your witness is just blinding us with science Miss Craddock.
Miss Craddock
May we take it Dr Daniels that the force you refer to is caused by the prongs pressing against the shaft when they are pushed through the washer.
Dr Daniels
Yes exactly, the stress is directly proportional to the Young's modulus, and the modulus of steel is approximately twice that of copper based alloys.
Judge
Are you telling us Dr Daniels that a steel washer can apply twice as much force as a copper alloyed one.
Dr Daniels
Yes my lord.
Miss Craddock
And finally, what will be the result of this additional stress.
Dr Daniels
It will increase the likelihood of environmental stress cracking to a level such that the combination of high stress and normal environmental conditions will cause the eye fractures.
Miss Craddock
Thank you.
Judge
Just a moment. The previous witness told us that the defects began to appear last July, but it seems that the new washers were supplied from January.
Dr Daniels
Yes my lord it would take about six months under stress before the cracks began to show.
Judge
I see. You may begin your cross examination after lunch Mr. Sharp. The Court will adjourn until 2 o'clock.
End transcript: Video 1
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Mr Hailey
Well.
Mr Sharp
Daniels has just finished his evidence in chief.
Mr Hailey
And?
Mr Sharp
He's making great play on the increased stress of steel washers.
Mr Hailey
I did warn you.
Mr Sharp
More than twice that of bronze, according to Young's module, or whatever it is.
Mr Hailey
Young's modulus.
Mr Sharp
Whatever that is.
Mr Hailey
It's the relationship between stress and strain for any given material. Here, I'll show you.
If you plot the amount of stress that appears in a metal, according to the amount of strain imposed on it, if that metal is bronze you'll get a result roughly like that. The slope of that line represents Young's modulus for bronze. The more the strain the more the stress in a straightforward linear relationship. If say you apply that amount of strain to a bronze washer it'll produce that much stress in return.
Mr Sharp
What if it's steel not bronze.
Mr Hailey
Well in that case your graph line would go more like that. So, put the same amount of strain you would produce around twice as much stress.
Mr Sharp
Just what he said your steel washers are too strong for the polystyrene eyes.
Mr Hailey
Well whose side are you on?
Mr Sharp
One can't ignore facts.
Mr Hailey
You'll have to knock his evidence down in other ways, discredit his investigations.
Mr Sharp
Of course I'll do that if I can but, one can't get away from this can one.
Mr Hailey
Wait a minute. There's more to it than this you know. There's the yield point. If you impose enough strain there comes a point where the metal ceases to behave elastically. After that point it doesn't return to its original shape any more, it shows plastic behaviour, till eventually it'll break. The same thing happens with steel, or any other metal.
Mr Sharp
How does that help us?
Mr Hailey
Well suppose our steel washers don't behave the way Dr. Clever Dick thinks they do. What if the yield point were not up here, but back down here. Now, if the yield point were down here the steel will begin to give way plastically, and it won't exert the same level of stress.
Mr Sharp
And does it give way like that.
Mr Hailey
It does if I say so.
You are still on oath Dr Daniels.
Mr Sharp
Dr Daniels, you conducted your investigations over a period of six weeks, is that right?
Dr Daniels
Yes.
Mr Sharp
How can you properly investigate in six weeks a phenomenon which takes six months to develop.
Dr Daniels
I accelerated the test procedures by suitably changing the environment.
Mr Sharp
Changing it! In what way?
Dr Daniels
By immersing the eye assemblies in various organic solvents.
Mr Sharp
So your tests were carried out in a very aggressive environment?
Dr Daniels
You could put it that way.
Mr Sharp
Can you establish beyond doubt that under those conditions there's no change in the mechanism or the factors governing the control of cracks.
Dr Daniels
No I can't. But we know the same phenomenon occurred in the normal environment.
Mr Sharp
But not necessarily for the same causes.
Dr Daniels
I was unable to find any other causes.
Mr Sharp
Indeed! Are you aware it is not only the washers that were changed at about the same time.
Dr Daniels
Pardon!
Mr Sharp
Are you aware that only a few weeks earlier the plaintiffs altered the design of the eyeball itself.
Dr Daniels
Oh that.
Mr Sharp
Yes Doctor that. I put it to you Dr. Daniels, the damage to the eyes could have been caused by the alteration to the eyeballs, not the washers.
Dr Daniels
Oh yes the curvature of the eyeball was increased.
Mr Sharp
In a way which would have entailed the introduction of new injection moulds etc.
Dr Daniels
Yes.
Mr Sharp
Suppose for example, the new moulds were not being properly fed. Couldn't that cause weakness in the material and subsequent cracking.
Dr Daniels
Yes it could.
Mr Sharp
Thank you Doctor.
Dr Daniels
That was the first thing I checked. I found no indication of faults in the polymers produced in the new mould, so I proceeded to examine other factors.
Judge
Other factors!
Dr Daniels
Yes my lord I investigated whether the eyeballs were being treated in the same manner during storage, assembly and dispatch from the firm's factory and outlets but I found no change. I then looked for the presence of say fingerprints, chemicals in the fabrics of the toys, residual chemicals from the manufacturing technique, harmful vapours in the warehouse, these are all possible culprits that have been known to enhance environmental cracking in polymers. But my results were negative.
Judge
Any further questions Mr Sharp?
Mr Sharp
No my lud.
Miss Craddock
That is our case, my lud.
Judge
Very well.
Mr Sharp
I call Mr. Arthur Hailey.
Usher
Mr Arthur Hailey.
Mr Sharp
And during this period you produced several monographs on the subject.
Mr Hailey
I did.
Mr Sharp
So you are an expert on the subject of industrial metals.
Mr Hailey
I hope so.
Mr Sharp
You gave up your academic career some eight years ago to move into industry, and you have been production manager for Williams Washers Ltd for the past five years.
Mr Hailey
That's right.
Mr Sharp
You are therefore familiar with the firm's manufacturing processes and the materials used.
Mr Hailey
Of course.
Mr Sharp
Your firm has produced the retaining washers for a very long time now.
Mr Hailey
For more than ten years.
Mr Sharp
Did you, in January last year, substitute steel washers for the bronze ones.
Mr Hailey
Yes we did.
Mr Sharp
Why?
Mr Hailey
Well with the price of copper going through the roof it was the only way to keep the costs down.
Mr Sharp
In your expert opinion are the new washers any less suitable for the purpose than the old ones.
Mr Hailey
They're better. They'll stand up to more rough treatment, and we all know what kids are like.
Mr Sharp
Let us come to the allegations made by Dr Daniels. His point as I understand it is that the stress or force exerted on the shafts by the steel washers may be twice that exerted by the bronze washers causing the shafts to crack. Do you accept that view.
Mr Hailey
No I don't.
Judge
Are you saying Mr. Hailey that steel does not exert more stress than bronze?
Mr Hailey
No my lord I don't deny that, providing the steel behaves elastically, ours doesn't.
Mr Sharp
And why not?
Mr Hailey
Because we anneal it.
Mr Sharp
What is the effect of annealing?
Mr Hailey
It reduces the yield point of the steel to such an extent that the imposed assembly strain causes the yield stress to be exceeded.
Judge
I er, I don't quite follow.
Mr Hailey
Well I means my lord, that when the prongs are bent back by pushing the shaft through them, they deform plastically. They stay bent. To be more precise they impose only partial stress on the material, only enough to hold the eye in place.
Mr Sharp
Would you say that your steel washers apply more or less force than the bronze washers.
Mr Hailey
About the same, more or less.
Mr Sharp
In that case the steel washers are not exerting the force that Dr. Daniels referred to.
Mr Hailey
Not at all.
Mr Sharp
Thank you Mr Hailey.
Dr Daniels
I tell you, the man is lying.
Miss Craddock
Well, can you prove it.
Dr Daniels
I need to make more tests.
Mr Fortescue
But there isn't time.
Judge
Miss Craddock, do you wish to cross examine this witness.
Miss Craddock
My lord there seems to be some disagreement between the experts on this point. Such being the case I submit that the onus of proof lies within the defendants to prove beyond doubt.
Mr Sharp
I object my lud. If the plaintiffs fail to prove ..
Miss Craddock
I refer my lord to the case of Appleby Limited and Cronshaw 1953.
Mr Sharp
My lord I refer to Morris & Sons and Ingleby Engineering, 1972.
End transcript: Video 2
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Transcript: Video 3

Mr Fortescue
What's the progress.
Dr Daniels
I've no idea they're still at it.
Mr Fortescue
Two hours. More like three one and another and the next and the next.
Dr Daniels
I rather think Miss Craddock is playing for more time.
Mr Fortescue
Whose time, who's paying for it. No wonder these lawyers make such a good living.
Dr Daniels
I think she's hoping for another adjournment till tomorrow so that I'll have time for more tests.
Mr Fortescue
You said Hailey was lying.
Dr Daniels
I believe so.
Mr Fortescue
Then why don't you get up and say so and have done with it.
Dr Daniels
Well they'll say I conducted my tests on the assumption that the washers behave elastically which doesn't prove they do. I want to make some tests they can't challenge.
Mr Fortescue
Look Doctor, I'm not a a technical man but, what's this yield point they're on about.
Dr Daniels
I could draw it for you. Good heavens, someone's drawn it already. Here you are. Hailey claims he's reduced the yield point of the steel below this critical point here by annealing it. We have to show it's still above that point but it could be close.
Mr Fortescue
Close. You mean that even if you do more tests it might turn out that...
Dr Daniels
That Hailey is right, I suppose it's possible. There must be some doubt until the tests are done otherwise there'd be no need for them.
Mr Fortescue
But, but you can't say he's right, you're my witness.
Dr Daniels
I'm qualified I hope to be an expert witness. If I conduct a test I am bound to state the result whatever the consequences. You surely don't expect me to falsify a scientific observation! Miss Craddock.
Miss Craddock
Well, we've done it. Adjourned till tomorrow morning, 10 o'clock.
Dr Daniels
Should just manage to catch the 5.30 I'll see you tomorrow.
Miss Craddock
I er think we can rely on Dr Daniels.
Mr Fortescue
Don't be too sure of that. What's the results?
Dr Daniels
Fraid I can't...
Miss Craddock
I er beg your pardon my lord. Dr Daniels was delayed.
Judge
Oh very well. Please continue.
Miss Craddock
In view of the fresh evidence submitted by Mr Hailey, I should like to recall Dr. Daniels.
Mr Fortescue
You're quite sure.
Dr Daniels
I said so didn't I.
Miss Craddock
You are still under oath Dr Daniels. You heard Mr Hailey claim yesterday that the steel from which the new washers are made is annealed to enable it to show plastic behaviour instead of elastic, would you agree.
Dr Daniels
I suppose it's possible. That is I can't prove there's been no attempt at annealing, but if there was any such process it was singularly ineffective. All my observations from the start of my enquiry have indicated that the washers behaved elastically not plastically. I have recently confirmed they have a hardness of three hundred and fifty vicars which suggests to me they have been highly cold worked, or heat treated. Both those treatments give the material an improved yield stress, making it more resistant to plastic deformation not less.
Miss Craddock
Are you satisfied Dr. Daniels, that however these washers are produced, the constant stress resulting from their elastic behaviour is sufficient to cause the defects in my clients product.
Dr Daniels
Most certainly.
Mr Sharp
Dr Daniels, when did you measure the hardness of the steel?
Dr Daniels
Last night.
Mr Sharp
And when was your appliance last tested for accuracy.
Dr Daniels
Possibly er, well yes it would be about two years ago.
Mr Sharp
Two years ago. How can you guarantee to the Court that it is still accurate.
Dr Daniels
Well there's no guarantee.
Mr Sharp
No guarantee.
Dr Daniels
That's why I conducted further tests.
Judge
Last night Dr Daniels.
Dr Daniels
Yes, my lord. Here are three standard photographs which show the microstructure of steel. The first, number one, shows the structure of such a steel after it has been heat treated. The second, after being cold worked, and the third shows it after annealing. Here is a photograph of a section taken from the steel washers supplied by Williams Washers Ltd. If you compare it with the others I think you will agree that it is very similar to the second photograph suggesting that the steel has been cold worked and not annealed.
Judge
No doubt to an expert like yourself Dr. Daniels these pictures would be crystal clear but, for my own part I would be reluctant to determine the case on this evidence alone.
Mr Sharp
Perhaps you have further evidence to offer us Doctor?
Dr Daniels
Yes I have.
Mr Sharp
You have.
Dr Daniels
I conducted a third test to determine directly how the metal behaves. I fitted a washer to eye shaft, causing the prongs to deform in the usual way. Later, I removed the shaft and measured carefully to establish that the prongs had returned to their original position.
Mr Sharp
You fitted a washer and then removed it.
Dr Daniels
Yes.
Mr Sharp
But surely the force which you applied in order to remove the shaft must have distorted the prongs yet again, that is bending them back to their original position.
Dr Daniels
No sir that is not possible.
Mr Sharp
And why not?
Dr Daniels
Because I didn't use any force. After fitting the washer I immersed the assembly in one of the solvents we spoke about yesterday, a solvent sufficiently aggressive to dissolve the polystyrene.
Judge
Dissolve it!
Dr Daniels
So that I could remove the washer without using any force at all. There is no question that the prongs behaved elastically, more than enough to blind a teddy bear.
Judge
Furthermore, I find Dr Daniels a reliable and fair minded witness, and I feel bound to accept his evidence. I find for the plaintiffs in this case. On the question of damages, I take it Miss Craddock that this...
End transcript: Video 3
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