Failure Analysis of rear destination board bracket at Tata Marcopolo

This is the project I did during my intern at Tata Marcopolo Motors pvt ltd,Lucknow as a part of my academics in B.Tech.

One of the semi low floor Bus Model was dealing with problem of failure of rear destination board bracket and I was given the job to find the reason behind that.

This project epitomise a report on- Analyis of bracket to find out the cause of its failure. Suggestion for its improvement/rectification.

Rear Destination Board Bracket- Supportive Structure to Rear Destination Board. Transfers load from Destination Board( + Self Weight) to bus body structure. From Bus body Structure load is transferred to Chassis then suspension then wheel. It absorbs some impact loading energy and have some stiffness for same. It should be of sufficient tensile and yield strength (both design+ material) to sustain a good life.

The usual causes of mechanical failure in the component or system are:• Misuse or abuse• Assembly errors• Manufacturing defects• Improper or inadequate maintenance• Design errors or design deficiencies• Improper material or poor selection of materials• Improper heat treatments• Unforeseen operating conditions• Inadequate quality assurance• Inadequate environmental protection/control,Casting discontinuities.

Failure Image:

THEORITICAL APPROACH Maximum Stress Concentration is to be checked at every region. Welded parts strength is almost 1/3rd of material strength. Combined shear stress and principal stress is to be checked. Von-mises(theory of failure) is used. Calculation of all forces is approximation(not exactly equals practical forces but are major forces of real time conditions). Type of failure is to be identified (from listed above). Total deformation is to be finded out. Improvement methods to be suggested which is practically possible and cost saving. Welding Analysis to be done to check the thickness of weld. ConsideredMIG WELD have 230MPa yield strength(generally).

TYPES OF FORCES ACTING ON BRACKETVertical loading: Due to weight of destination board on bracket through nut bolt. Bracket Self weight:A gravitational load on cog of each bracket is there due to self weight. Acceleration force:A force due to change of state of vehicle is there on every part of bus as per Newton’s Law. Deacceleration force:Same description as acceleration force but its direction is reversed. Centrifugal Force:Due to turning, a radial force is there on every part of vehicle. Gyroscopic Force’s:It’s a inertial force couple ,generally generated on turning so as act as couple in every part of vehicle. Vibrations:Due to dynamic conditions,random vibrations occur continuously on every part of bus. Various other inertial and non inertial forces,etc.

Cannot share the Force calculation data here as per the Company's policies.

Welding Analysis

Also did the Static,Dynamic Analysis other than this to ensure the proper step by step analysis is done but MOS comes out to be more than 1 in both cases.

Fatigue Analysis shown some interesting results:

Its quite clear that it’s a type of Fatigue Failure. Min No. of cycles=190000(loading and unloading cycles) from the results(cannot show the data here)

SUGGESTION FOR IMPROVEMENT  GUSSET THICKNESS HAS TO INCREASE FROM 4.25mm to 7mm .  ANOTHER ALTERNATIVE IS TO REDESIGN THE PRODUCT(COSTLIER AND TIME CONSUMING).  WELDING RADIUS SHOULD BE INCREASED TO 4mm FOR THE AREA’S MENTIONED IN WELDING ANALYSIS.  WELDING SHOULD BE DONE UNIFORMLY IN EACH BRACKET SO THAT CHANCES OF ANY COUPLE FORMATION IS REDUCED.

Proof of Suggestion It can be proved by doing FEA/SIMULATION with the increased gusset thickness.  IN REDESIGNED MODEL GUSSET THICKNESS IS CHANGED FROM 4.25mm to 7mm.  For bending and breaking near slots,Welding is to be done properly .ANALYSIS OF SUGGESTED MODEL1.With Increases rib thickness to 7mm. MAX STRESS=76.52 MPa  FOS=85/76.52=1.11  MOS=1.11-1=0.11 AS SEEN MOS > 0,SO ITS DESIGN IS IMPROVED IN TERMS OF STRENGH AS MAX STRESS IS REDUCED. THIS MODEL WAS PREVIOUSLY CHECKED FOR STATIC AND DYNAMIC LOADING SO NO NEED FOR RE-ANALYSING IT AGAIN.