UNIVERSITY OF TORONTO

FACULTY OF APPLIED SCIENCE AND ENGINEERING

FINAL EXAMINATION, April 2017

MSE431H1 S— FORENSIC ENGINEERING

Exam Type: X

Examiner - D.D. Perovic

Duration: 2 /2 hours

Question 1:

In the context of forensic engineering and failure analysis, describe the

difference between: (i) imperfections (ii) flaws and (iii) defects. (6 marks)

Differentiate between the safe-l/e and fail-safe design methodologies used to

prevent failure of an engineered component. (4 marks)

A bridge truss component to be designed against failure is exposed to repeated

tensile-compressive cyclic loading in which the mean stress is 25 MPa. If the

initial and critical through-thickness crack lengths (a) are 0.25 and 5.0 mm

respectively, and the values of the fatigue crack growth rate constants A and m

are 5 x 10 15 and 4 respectively (for stress range (Ad) in [MPa] and a in [in]

under steady-state, plane-strain crack growth conditions), calculate the maximum

tensile stress resulting in a fatigue life of 3.2 x 105 cycles. (15 marks)

Question 2:

You are investigating the collapse of a stressed component that failed at some

point during the crash of a commercial aircraft. By considering modes and

mechanisms of fracture, briefly discuss how you would differentiate whether the

component failed by: (I) catastrophic fracture during flight or (ii) impact with the

ground. (10 marks)

An office chair suffered a structural failure of the cast aluminum bracket that

supports the plastic chair seat causing injuries to the plaintiff. The plaintiff

retained an expert to examine the chair using only non-destructive inspection

techniques. A visual examination was performed and an expert report was

prepared containing only photographs of the subject chair components with no

further information provided. You are asked to critique the following

conclusions made in the opposing expert's report: (10 marks)

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in a 'new condition, the cast aluminum housing, even with some of the voids noted in our inspection, would have been able to safely withstand expected loading from people sitting on the chair.

Since the cast aluminum housing would not have failed due to a single (first) application of load, and based on the grease that appeared to have dripped into/onto one of the fracture surfaces. as well as the chevron pattern for some of the fracture surfaces, the failure had to be due to cyclic/repeated/fatigue loading.

Describe one type of statute employed in Canada that can affect product liability litigations? ('5 marks)

Question 3:

In determining the origin of afire, explain the significance of drop-down burning

and what techniques you would employ to investigate whether it played a role in the cause of the fire in the kitchen of a home. ('10 marks)

You are a police services collision reconstructionist who has been called out to the scene of a single vehicle collision, on a rural stretch of isolated highway, with a posted speed limit of 80 km/h. A Porsche 911 has gone off the road and has struck a tree with the left door area (see diagram attached). The driver of the Porsche states that he was swerving to avoid a deer that darted out onto the roadway. You discover fresh skid markings leading to the tires of the Porsche that are 13 metres in length on the paved roadway, and 28 metres in length in the grass ditch. The Porsche has come to rest at its impact position against the tree. You download the information from the airbag control module of the Porsche and learn that it was travelling at 25 km/h at the time of the tree impact. You are

contemplating a Highway Traffic Act (HTA) charge of 'speed in excess of the

posted limit' against the driver. Should you lay the charge?

(Note: Assume that the tire/roadway coefficient of friction is 0.8, and the

tire/grass coefficient of friction is 0.4). (15 marks)

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Question 4:

A pressure vessel employed for dehydrogenation of butane gas to isobutylene gas suffered a catastrophic failure by cracking through one of the circumferential seam welds. The vessel wall was manufactured using austenitic stainless steel and operated at 90 kPa and 550 °C.

You have been retained to investigate the cause of failure. Starting with a fishbone diagram, outline in sufficient detail the logical steps that you would take to investigate this failure as a forensic engineer. Your investigation framework should incorporate details relevant to the case outlined above. You are expected to perform the background work, relevant testing (non-destructive and destructive) and produce an expert report with a view to determining the origin and cause of failure and ultimately who is liable. (25 marks)

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