Element C6.2 Generic Hazards and Modes of Failure

Transcription

1 .2 Generic Hazards and Modes of Failure

2 Common Machinery Hazards Procedures / permits Consider Operation Maintenance Cleaning

3 Common Machinery Hazards Drills Entanglement Poorly guarded drive pulleys Chuck ejection Material ejection (swarf)

4 Common Machinery Hazards Circular saws Cuts Entanglement Poorly guarded drive pulleys Material ejection

5 Common Machinery Hazards Guillotines Shear Cuts from contact with blade Crush from clamp devices

6 Common Machinery Hazards Paper shredders Cuts Drawing in from action of cutters Photocopiers Drawing in Entanglement Hot surfaces

7 Sanders Common Machinery Hazards Abrasion Entanglement Abrasive wheels Friction / abrasion Entanglement Drawing in Rupture and ejection of wheel

8 Common Machinery Hazards Lathes Entanglement Friction burns or cuts Drawing in of hands Ejection of swarf Ejection of work piece insecure on chuck Ejection of chuck key

9 Common Machinery Hazards Presses Crushing Shearing Impact hazards from moving parts Ejection of material

10 Common Machinery Hazards Robotics Setting up / maintenance Sudden / unpredictable movements Trapped energy

11 Modes of Failure Stress and strain Properties relate to strength and stiffness of a material STRESS Load / force per unit area Newton per sq m / sq mm Measure of strength How hard atoms are being pulled apart STRAIN Stiffness of material How far atoms are being pulled apart

12 Modes of Failure Tensile Internal forces that act to pull member apart Compressive Internal forces that push member together Shear stress Measure of tendency for one part of solid material to slide past neighbouring part

13 Modes of Failure Metal fatigue Accumulating effects of fluctuating loads promote slow changes in structure Tiny cracks extend slowly until they reach critical length

14 Ductility Modes of Failure Amount of deformation that can take place before fracture Brittle failure Fast fracture Without warning or evidence of distress Lack of toughness Buckling Excessive compressive forces bending moment scaffold out of true Forces placed on it magnified Buckle failure will occur

15 Corrosion Modes of Failure Deterioration through chemical interaction with its environment Rust oxides Choice of materials Protective coatings Wear Loss of material due to sliding contact between two materials Metal to metal Lack of lubrication, presence of abrasives - grit

16 Modes of Failure Creep Deformation over time

17 Failure Prevention Strategies Identifying potential failure modes Designer understands conditions of use Fracture mechanics Test residual strength of materials that contain cracks or flaws Critical crack length, beyond which failure will result Predict future development of cracks

18 Failure Prevention Strategies Environmental factors Offensive environment Selection of materials Working load limits QA during manufacture ISO 9001 Specified requirements ITM s

19 Failure Prevention Strategies Testing during manufacture Final testing and inspection Ensures previous tests and inspections have been carries out Equipment to be calibrated, maintained Forensic examination of failed components Determine modes and causes of failure

20 Non-destructive Testing Techniques NDT Test integrity of materials without causing damage Detect surface flaws Radiography for latent / internal defects Visual inspection Unaided or with microscope Weld inspection Metrology Cheap and fast

21 Non-destructive Testing Techniques Liquid dye penetrant inspection Enhances visual inspection Dye penetrates surface cracks or cavities Low cost Aerospace fatigue cracking Construction welds are defect free

22 Non-destructive Testing Techniques Magnetic particle testing Detection of surface and near surface defects in ferromagnetic materials Magnetic field induced in the item Iron filings mixed with paraffin sprinkled on surface Defects indicated by anomalies in distribution of the particles Cracks, seams, voids, holes High sensitivity Large areas, quick Material needs to be capable of being magnetised High power equipment required

23 Non-destructive Testing Techniques Eddy current testing Current passed through coil of electrically conducting materials surrounding test item Electromagnetic field around item Eddy current within item Impedance changes in coil indicate defects Test item rejected Complex shapes can be tested Low cost, portable Material must be conductive Limited penetration only surface or near surface Testing steel wire Automotive components

24 Non-destructive Testing Techniques Radiography Internal defects X rays or gamma rays through material onto film Cracks and flaws show as dark patches Most materials Exposure health issues Welds, aircraft structures Pressure vessels Pipes Bridge girders

25 Non-destructive Testing Techniques Ultrasonic testing Ultrasonic = beyond human detection > 20kHz Ultrasonic probe detects cracks Waves reflected and observed High sensitivity High penetrating High accuracy Aerospace, nuclear Rocket fuselage- sequenced multi-probe equipment