cracks can cause the degradation of matrix-dominated  properties. For laminates designed to transmit loads with their  fibers (fiber dominant), only a slight reduction of properties  is observed when the matrix is severely damaged. Matrix  cracks, or microcracks, can significantly reduce properties  dependent on the resin or the fiber-resin interface, such  as interlaminar shear and compression strength. Micro- cracking can have a very negative effect on properties of  high-temperature resins. Matrix imperfections may develop  into delaminations, which are a more critical type of damage. Delaminations form on the interface between the layers in the  laminate. Delaminations may form from matrix cracks that  grow into the interlaminar layer or from low-energy impact.  Debonds can also form from production nonadhesion along  the bondline between two elements and initiate delamination  in adjacent laminate layers. Under certain conditions,  delaminations or debonds can grow when subjected to  repeated loading and can cause catastrophic failure when  the laminate is loaded in compression. The criticality of  delaminations or debonds depend on: In general, impact events cause combinations of damages.  High-energy impacts by large objects (e.g., turbine blades)  may lead to broken elements and failed attachments. The  resulting damage may include significant fiber failure, matrix  cracking, delamination, broken fasteners, and debonded  elements. Damage caused by low-energy impact is more  contained, but may also include a combination of broken  fibers, matrix cracks, and multiple delaminations.  Improper hole drilling, poor fastener installation, and missing  fasteners may occur in manufacturing. Hole elongation can  occur due to repeated load cycling in service.  The repair of parts due to liquid ingression can vary  depending on the liquid, most commonly water or Skydrol  (hydraulic fluid). Water tends to create additional damage in  repaired parts when cured unless all moisture is removed from  the part. Most repair material systems cure at temperatures  above the boiling point of water, which can cause a disbond  at the skin-to-core interface wherever trapped water resides.  For this reason, core drying cycles are typically included prior  to performing any repair. Some operators take the extra step  of placing a damaged but unrepaired part in the autoclave to  dry to preclude any additional damage from occurring during  the cure of the repair. Skydrol presents a different problem.  Once the core of a sandwich part is saturated, complete  removal of Skydrol is almost impossible. The part continues  to weep the liquid even in cure until bondlines can become  contaminated and full bonding does not occur. Removal of  contaminated core and adhesive as part of the repair is highly  recommended.  [Figure 7-21] Many honeycomb structures, such as wing spoilers, fairings,  flight controls, and landing gear doors, have thin face  sheets which have experienced durability problems that  could be grouped into three categories: low resistance to  impact, liquid ingression, and erosion. These structures have  adequate stiffness and strength but low resistance to a service  environment in which parts are crawled over, tools dropped,  and service personnel are often unaware of the fragility of  thin-skinned sandwich parts. Damages to these components,  such as core crush, impact damages, and disbonds, are quite  often easy to detect with a visual inspection due to their  thin face sheets. However, they are sometimes overlooked  or damaged by service personnel who do not want to delay  aircraft departure or bring attention to their accidents, which  might reflect poorly on their performance record. Therefore,  damages are sometimes allowed to go unchecked, often  resulting in growth of the damage due to liquid ingression  into the core. Nondurable design details (e.g., improper core  edge close-outs) also lead to liquid ingression. In-Service Defects Delamination and Debonds Combinations of Damages Flawed Fastener Holes 7-14 •  Loads—behavior of delaminations and debonds  depend on loading type. They have little effect  on the response of laminates loaded in tension.  Under compression or shear loading, however,  the sublaminates adjacent to the delaminations or  debonded elements may buckle and cause a load  redistribution mechanism that leads to structural  failure.  •  Location—in the thickness of laminate, in the  structure, proximity to free edges, stress concentration  region, geometrical discontinuities, etc. •  Number of delaminations at a given location. • Dimensions. •  Environmental degradation •  Impact damage • Fatigue •  Cracks from local overload • Debonding • Delamination •  Fiber fracturing • Erosion In-service defects include: