陕科大包装技术基础双语课件14产品脆值试验方法.ppt

Lesson 14 Test Method for Product Fragility第第14课课 产品脆值试验方法产品脆值试验方法 Test Method for Product Fragility lA shock machine is used to generate a damage boundary curvelA vibration system is used to map out the natural frequencies of a product.Shock:Damage BoundaryShock damage to products results from excessive internal stress induced by inertia forces -Since F=ma,shock fragility is characterized by the maximum tolerable acceleration level,i.e,how many gs the item can withstand.-Why damaged?-How to reduce gs?The packaging material changes the shock pulse delivered to the product so that the maximum acceleration is greatly reduced(and the pulse duration is many times longer).-The package designers goal:To be sure that the g-level transmitted to the item by the cushion is less that the g-level which will cause the item to fail.Shock:Damage BoundaryThe damage boundary theory is used to determine which shock inputs will cause damage to a product and which will not.-Two parts of a shock can cause damage:1.the acceleration level A2.the velocity change V(the area under the acceleration-time history of the shock,thought as the energy contained in a shock)-The critical velocity change(Vc):a minimum velocity change which must be achieved before damage to the product can occur.1.Below Vc,no damage occurs regardless of the input A 2.Exceeding Vc,does not necessarily imply that damage results.a.If V occurs in a manner which administers acceptable doses of acceleration to the product,the velocity change can be very large without causing damage.b.If Vc and Ac are both exceeded,damage occurs.Figure 14.1:Typical damage boundary curve Shock:Damage BoundaryImplications of Fig.14.1:a.if the input Vthe products Vc,However,the only way to avoid damage is to limit the input A the products Ac.This is usually one of the functions that a cushioned package performs:it translates the high acceleration events experienced on the outside of the container to lower acceleration events experienced inside at the unit.Shock:Damage BoundaryFigure 14.1 Typical damage boundary curve Shock:Damage Boundaryc.For V Vc,area where damage does not occur even with very high accelerations.Here V(drop height)is so low that the item acts as its own shock isolator.d.Ac,damage does not occur,even for large V.Thats because the forces generated(F=ma)are within the strength limits of the products.-From Fig.14.2,a.Vc boundary(vertical boundary line),is independent of the pulse wave shape.b.Ac(to the right of the vertical line)for half sine and sawtooth pulses depends upon V.Shock:Damage BoundaryFigure 14.2 Damage boundary for pulses of same peak acceleration and same velocity change Shock:Damage Boundaryc.The damage boundary generated with use of a trapezoidal pulse encloses the damage boundaries of all the other waveforms.-Fragility testing is the process used to establish damage boundaries of products.a.It is usually conducted on a shock testing machine.The procedure has been standardized(ASTM D3332,Mechanical-Shock Fragility of Products,Using Shock Machines).b.Procedure:the item to be tested is fastened to the top of a shock machine table and the table is subjected to controlled velocity changes and shock pulses.The shock table is raised to a preset drop height.It is then released,free falls and impacts against the base of the machine;it rebounds from the base and is arrested by a braking system so that only one impact occurs.Shock:Damage Boundaryc.For trapezoidal pulses,the programmer is a constant force pneumatic cylinder.The g-level of the trapezoidal pulse is controlled simply by adjusting the compressed gas pressure in the cylinder.The V is controlled by adjusting drop height.A Shock Testing Machine(1)Conducting a fragility test To determine a damage boundary requires running two sets of tests.-A step velocity test is used to determine the products Vc and a step acceleration test is used to determine Ac.1.Step Velocity Test(Figure 14.3)to determine the vertical line of the damage boundary.2.Step Acceleration Test(Figure 14.4)to determine the horizontal line of the damage boundary.a.A new test specimen be attached to the shock table.b.The drop height is set at a level which will produce a velocity change at least 1.57 x Vc.c.The programmer compressed gas pressure is adjusted to produce a low g-level shock.Conducting a fragility test Figure 14.3 Velocity damage boundary development Conducting a fragility test A Shock Testing Machine(2)Conducting a fragility test 3.Plot the damage boundary curve by connecting the vertical velocity boundary line and the horizontal acceleration boundary line.4.Notes:In a rigorous testing program,damage boundary curves are generated for each orientation of the unit.Compromises are often made to limit the number of units which must be damaged.Figure 14.4 Damage boundary line development Vibration:Resonance Search&DwellIt is generally accepted that the steady-state vibration environment is of such low acceleration amplitude that failu。